Membrane proteins as ion channels. Selective and non-selective channels

Ion channels represented by integral membrane proteins. These proteins are capable, under certain influences, of changing their conformation (shape and properties) in such a way that the pore through which any ion can pass opens or closes. Sodium, potassium, calcium, and chlorine channels are known; sometimes a channel can pass two ions, for example, sodium-calcium channels are known. Only passive transport of ions occurs through ion channels. This means that for an ion to move, not only an open channel is required, but also a concentration gradient for that ion. In this case, the ion will move along a concentration gradient - from an area with a higher concentration to an area with a lower concentration. It must be remembered that we are talking about ions - charged particles, the transport of which is also determined by charge. Situations are possible when movement along the concentration gradient can be directed in one direction, and existing charges counteract this transfer.

Ion channels have two important properties: 1) selectivity (selectivity) towards certain ions and 2) ability to open (activate) and close. When activated, the channel opens and allows ions to pass through (Fig. 8). Thus, the complex of integral proteins that form the channel must necessarily include two elements: structures that recognize “their” ion and are able to let it through, and structures that allow you to know when to let this ion through. The selectivity of the channel is determined by the proteins that form it; the “own” ion is recognized by its size and charge.

Channel activation possible in several ways. First, channels can open and close as the membrane potential changes. The change in charge leads to a change in the conformation of protein molecules, and the channel becomes permeable to the ion. To change the properties of the channel, a slight fluctuation in the membrane potential is sufficient. Such channels are called voltage dependent(or electrically controlled). Second, the channels may be part of a complex protein complex called a membrane receptor. In this case, the change in the properties of the channel is caused by a conformational rearrangement of proteins, which occurs as a result of the interaction of the receptor with a biologically active substance (hormone, mediator). Such channels are called chemodependent(or receptor-gated ) . In addition, the channels can open under mechanical influence - pressure, stretching (Fig. 9). The mechanism that provides activation is called channel gating. Based on the speed at which the channels open and close, they can be divided into fast and slow.

Most channels (potassium, calcium, chloride) can be in two states: open and closed. There are some peculiarities in the operation of sodium channels. These channels, like potassium, calcium, and chloride, tend to be either in an open or closed state, however, the sodium channel can also be inactivated, this is a state in which the channel is closed and cannot be opened by any influence (Fig. 10).

Figure 8. Ion channel states

Figure 9. Example of a receptor-gated channel. ACh – acetylcholine. The interaction of the ACh molecule with the membrane receptor changes the conformation of the gate protein in such a way that the channel begins to allow ions to pass through.

Figure 10 Example of a potential-dependent channel

The voltage-gated sodium channel has activation and inactivation gates (gates). Activation and inactivation gates change conformation at different membrane potentials.

When considering the mechanisms of excitation, we will be mainly interested in the work of sodium and potassium channels, however, let us briefly dwell on the features of calcium channels, we will need them in the future. Sodium and calcium channels differ in their properties. Sodium channels are fast and slow, while calcium channels are only slow. Activation of sodium channels only leads to depolarization and the occurrence of either LO or AP; activation of calcium channels can additionally cause metabolic changes in the cell. These changes are due to the fact that calcium binds to special proteins that are sensitive to this ion. The calcium-bound protein changes its properties in such a way that it becomes capable of changing the properties of other proteins, for example, activating enzymes, triggering muscle contraction, and releasing mediators.

According to modern concepts, biological membranes form the outer shell of all animal cells and form numerous intracellular organelles. The most characteristic structural feature is that membranes always form closed spaces, and this microstructural organization of membranes allows them to perform essential functions.

Structure and functions of cell membranes.

1. The barrier function is expressed in the fact that the membrane, using appropriate mechanisms, participates in the creation of concentration gradients, preventing free diffusion. In this case, the membrane takes part in the mechanisms of electrogenesis. These include mechanisms for creating a resting potential, generation of an action potential, mechanisms for the propagation of bioelectric impulses across homogeneous and heterogeneous excitable structures.

2. The regulatory function of the cell membrane is the fine regulation of intracellular contents and intracellular reactions due to the reception of extracellular biologically active substances, which leads to changes in the activity of enzyme systems of the membrane and the launch of mechanisms of secondary “messengers” (“intermediaries”).

3. Conversion of external stimuli of a non-electrical nature into electrical signals (in receptors).

4. Release of neurotransmitters in synaptic endings.

Modern methods of electron microscopy determined the thickness of cell membranes (6-12 nm). Chemical analysis showed that the membranes are mainly composed of lipids and proteins, the amount of which varies among different cell types. The difficulty of studying the molecular mechanisms of the functioning of cell membranes is due to the fact that when isolating and purifying cell membranes, their normal functioning is disrupted. Currently, we can talk about several types of cell membrane models, among which the liquid mosaic model is the most widespread.

According to this model, the membrane is represented by a bilayer of phospholipid molecules, oriented in such a way that the hydrophobic ends of the molecules are located inside the bilayer, and the hydrophilic ends are directed into the aqueous phase. This structure is ideal for the formation of a separation between two phases: extra- and intracellular.

Globular proteins are integrated into the phospholipid bilayer, the polar regions of which form a hydrophilic surface in the aqueous phase. These integrated proteins perform various functions, including receptor, enzymatic, form ion channels, are membrane pumps and transporters of ions and molecules.

Some protein molecules diffuse freely in the plane of the lipid layer; in the normal state, parts of protein molecules emerging on different sides of the cell membrane do not change their position.

Electrical characteristics of membranes:

Capacitive properties are mainly determined by the phospholipid bilayer, which is impermeable to hydrated ions and at the same time thin enough (about 5 nm) to allow efficient separation and accumulation of charges and electrostatic interaction of cations and anions. In addition, the capacitive properties of cell membranes are one of the reasons that determine the time characteristics of electrical processes occurring on cell membranes.

Conductivity (g) is the reciprocal of electrical resistance and is equal to the ratio of the total transmembrane current for a given ion to the value that determined its transmembrane potential difference.

Various substances can diffuse through the phospholipid bilayer, and the degree of permeability (P), i.e., the ability of the cell membrane to pass these substances, depends on the difference in concentrations of the diffusing substance on both sides of the membrane, its solubility in lipids and the properties of the cell membrane.

The conductivity of a membrane is a measure of its ionic permeability. An increase in conductivity indicates an increase in the number of ions passing through the membrane.

Structure and functions of ion channels. Na+, K+, Ca2+, Cl- ions penetrate into the cell and exit through special fluid-filled channels. The channel size is quite small.

All ion channels are divided into the following groups:

1. By selectivity:

a) Selective, i.e. specific. These channels are permeable to strictly defined ions.

b) Low-selective, non-specific, without specific ion selectivity. There are a small number of them in the membrane.

1. According to the nature of the ions passed through:

a) potassium

b) sodium

c) calcium

d) chlorine

1. According to the rate of inactivation, i.e. closing:

a) quickly inactivating, i.e. quickly turning into a closed state. They provide a rapidly increasing reduction in MP and an equally rapid recovery.

b) slow-acting. Their opening causes a slow decrease in MP and its slow recovery.

4. According to opening mechanisms:

a) potential-dependent, i.e. those that open at a certain level of membrane potential.

b) chemo-dependent, opening when the cell membrane chemoreceptors are exposed to physiologically active substances (neurotransmitters, hormones, etc.).

It has now been established that ion channels have the following structure:

1. Selective filter located at the mouth of the channel. It ensures the passage of strictly defined ions through the channel.

2. Activation gates that open at a certain level of membrane potential or the action of the corresponding PAS. The activation gates of potential-dependent channels have a sensor that opens them at a certain MP level.

3. Inactivation gate, ensuring the closure of the channel and the cessation of ion flow through the channel at a certain MP level (Fig).

Nonspecific ion channels do not have a gate.

Selective ion channels can exist in three states, which are determined by the position of the activation (m) and inactivation (h) gates:

1.Closed when the activation ones are closed and the inactivation ones are open.

2. Activated, both gates are open.

3. Inactivated, the activation gate is open and the inactivation gate is closed

Functions of ion channels:

1. Potassium (at rest) – generation of resting potential

2. Sodium – generation of action potential

3. Calcium - slow action generation

4. Potassium (delayed rectification) – ensuring repolarization

5. Potassium calcium-activated – limiting depolarization caused by Ca+2 current

The function of ion channels is studied in various ways. The most common method is voltage clamp, or “voltage-clamp”. The essence of the method is that, with the help of special electronic systems, the membrane potential is changed and fixed at a certain level during the experiment. In this case, the magnitude of the ionic current flowing through the membrane is measured. If the potential difference is constant, then, in accordance with Ohm's law, the current magnitude is proportional to the conductivity of the ion channels. In response to stepwise depolarization, certain channels open and the corresponding ions enter the cell along an electrochemical gradient, i.e., an ion current arises that depolarizes the cell. This change is detected by a control amplifier and an electric current is passed through the membrane, equal in magnitude but opposite in direction to the membrane ion current. In this case, the transmembrane potential difference does not change.

Studying the function of individual channels is possible using the method of local fixation of the “path-clamp” potential. A glass microelectrode (micropipette) is filled with saline solution, pressed against the surface of the membrane and a slight vacuum is created. In this case, part of the membrane is sucked to the microelectrode. If an ion channel appears in the suction zone, then the activity of a single channel is recorded. The system of irritation and recording of channel activity differs little from the voltage recording system.

The current through a single ion channel has a rectangular shape and is the same in amplitude for channels of different types. The duration of the channel's stay in the open state is probabilistic, but depends on the value of the membrane potential. The total ion current is determined by the probability of a certain number of channels being in the open state in each specific period of time.

The outer part of the canal is relatively accessible for study; studying the inner part presents significant difficulties. P. G. Kostyuk developed a method of intracellular dialysis, which allows one to study the function of the input and output structures of ion channels without the use of microelectrodes. It turned out that the part of the ion channel open to the extracellular space differs in its functional properties from the part of the channel facing the intracellular environment.

It is ion channels that provide two important properties of the membrane: selectivity and conductivity.

The selectivity, or selectivity, of the channel is ensured by its special protein structure. Most channels are electrically controlled, that is, their ability to conduct ions depends on the magnitude of the membrane potential. The channel is heterogeneous in its functional characteristics, especially with regard to the protein structures located at the entrance to the channel and at its exit (the so-called gate mechanisms).

Let's consider the principle of operation of ion channels using the sodium channel as an example. It is believed that the sodium channel is closed at rest. When the cell membrane is depolarized to a certain level, the m-activation gate opens (activation) and the flow of Na+ ions into the cell increases. A few milliseconds after the m-gate opens, the h-gate located at the exit of the sodium channels closes (inactivation). Inactivation develops very quickly in the cell membrane and the degree of inactivation depends on the magnitude and time of action of the depolarizing stimulus.

When a single action potential is generated in a thick nerve fiber, the change in the concentration of Na+ ions in the internal environment is only 1/100,000 of the internal Na+ ion content of the squid giant axon.

In addition to sodium, other types of channels are installed in cell membranes that are selectively permeable to individual ions: K+, Ca2+, and there are varieties of channels for these ions.

Hodgkin and Huxley formulated the principle of “independence” of channels, according to which the flow of sodium and potassium across the membrane is independent of each other.

The conductivity properties of different channels are not the same. In particular, for potassium channels, the inactivation process does not exist, as for sodium channels. There are special potassium channels that are activated when the intracellular calcium concentration increases and the cell membrane depolarizes. Activation of potassium-calcium-dependent channels accelerates repolarization, thereby restoring the original value of the resting potential.

Calcium channels are of particular interest. The incoming calcium current is usually not large enough to normally depolarize the cell membrane. Most often, calcium entering the cell acts as a “messenger”, or secondary messenger. Activation of calcium channels is achieved by depolarization of the cell membrane, for example by an incoming sodium current.

The process of inactivation of calcium channels is quite complex. On the one hand, an increase in the intracellular concentration of free calcium leads to inactivation of calcium channels. On the other hand, proteins in the cytoplasm of cells bind calcium, which makes it possible to maintain a stable calcium current for a long time, albeit at a low level; in this case, the sodium current is completely suppressed. Calcium channels play an essential role in heart cells. Electrogenesis of cardiomyocytes is discussed in Chapter 7. The electrophysiological characteristics of cell membranes are studied using special methods.

All channels present in living tissues, and now we know several hundred types of channels, can be divided into two main types. The first type is rest channels, which spontaneously open and close without any external influences. They are important for generating the resting membrane potential. The second type is the so-called gate channels, or portal channels(from the word "gate") . At rest, these channels are closed and can open under the influence of certain stimuli. Some types of such channels are involved in the generation of action potentials.

Most ion channels are characterized selectivity(selectivity), that is, only certain ions pass through a certain type of channel. Based on this feature, sodium, potassium, calcium, and chloride channels are distinguished. The selectivity of channels is determined by the size of the pore, the size of the ion and its hydration shell, the charge of the ion, as well as the charge of the inner surface of the channel. However, there are also non-selective channels that can pass two types of ions at once: for example, potassium and sodium. There are channels through which all ions and even larger molecules can pass.

There is a classification of ion channels according to activation method(Fig. 9). Some channels specifically respond to physical changes in the neuron's cell membrane. The most prominent representatives of this group are voltage-activated channels. Examples include voltage-sensitive sodium, potassium, and calcium ion channels on the membrane, which are responsible for the formation of the action potential. These channels open at a certain membrane potential. Thus, sodium and potassium channels open at a potential of about -60 mV (the inner surface of the membrane is negatively charged compared to the outer surface). Calcium channels open at a potential of -30 mV. The group of channels activated by physical changes includes

Figure 9. Methods for activating ion channels

(A) Ion channels activated by changes in membrane potential or membrane stretch. (B) Ion channels activated by chemical agents (ligands) from the extracellular or intracellular side.

Also mechanosensitive channels that respond to mechanical stress (stretching or deformation of the cell membrane). Another group of ion channels open when chemicals activate special receptor binding sites on the channel molecule. Such ligand-activated channels are divided into two subgroups, depending on whether their receptor centers are intracellular or extracellular. Ligand-activated channels that respond to extracellular stimuli are also called ionotropic receptors. Such channels are sensitive to transmitters and are directly involved in the transmission of information in synaptic structures. Ligand-activated channels, activated from the cytoplasmic side, include channels that are sensitive to changes in the concentration of specific ions. For example, calcium-activated potassium channels are activated by local increases in intracellular calcium concentration. Such channels play an important role in repolarizing the cell membrane during the termination of an action potential. In addition to calcium ions, typical representatives of intracellular ligands are cyclic nucleotides. Cyclic GMP, for example, is responsible for the activation of sodium channels in the retinal rods. This type of channel plays a fundamental role in the operation of the visual analyzer. A separate type of modulation of channel operation by binding an intracellular ligand is the phosphorylation/dephosphorylation of certain sections of its protein molecule under the action of intracellular enzymes - protein kinases and protein phosphatases.

The presented classification of channels by activation method is largely arbitrary. Some ion channels can only be activated by a few stimuli. For example, calcium-activated potassium channels are also sensitive to changes in potential, and some voltage-activated ion channels are sensitive to intracellular ligands.

The excitable membrane model according to the Hodgkin-Huxley theory assumes the controlled transport of ions across the membrane. However, the direct transition of an ion through the lipid bilayer is very difficult, and therefore the ion flow would be small.

This and a number of other considerations gave reason to believe that the membrane must contain some special structures - conducting ions. Such structures were found and called ion channels. Similar channels have been isolated from various objects: the plasma membrane of cells, the postsynaptic membrane of muscle cells and other objects. Ion channels formed by antibiotics are also known.

Basic properties of ion channels:

1) selectivity;

2) independence of operation of individual channels;

3) discrete nature of conductivity;

4) dependence of channel parameters on membrane potential.

Let's look at them in order.

1. Selectivity is the ability of ion channels to selectively allow ions of one type to pass through.

Even in the first experiments on the squid axon, it was discovered that Na+ and Kt ions have different effects on the membrane potential. K+ ions change the resting potential, and Na+ ions change the action potential. The Hodgkin-Huxley model describes this by introducing independent potassium and sodium ion channels. It was assumed that the former allow only K+ ions to pass through, and the latter only pass through Na+ ions.

Measurements have shown that ion channels have absolute selectivity towards cations (cation-selective channels) or anions (anion-selective channels). At the same time, various cations of various chemical elements can pass through cation-selective channels, but the conductivity of the membrane for the minor ion, and therefore the current through it, will be significantly lower, for example, for the Na + channel, the potassium current through it will be 20 times less. The ability of an ion channel to pass different ions is called relative selectivity and is characterized by a selectivity series - the ratio of channel conductivities for different ions taken at the same concentration. In this case, for the main ion, selectivity is taken as 1. For example, for the Na+ channel this series has the form:

Na + : K + = 1: 0.05.

2. Independence of the operation of individual channels. The flow of current through an individual ion channel is independent of whether current flows through other channels. For example, K + channels can be turned on or off, but the current through the Na + channels does not change. The influence of channels on each other occurs indirectly: a change in the permeability of some channels (for example, sodium) changes the membrane potential, and this already affects the conductivity of other ion channels.

3. Discrete nature of the conductivity of ion channels. Ion channels are a subunit complex of proteins that span the membrane. In its center there is a tube through which ions can pass. The number of ion channels per 1 μm 2 membrane surface was determined using a radioactively labeled sodium channel blocker - tetrodotoxin. It is known that one TTX molecule binds to only one channel. Then measuring the radioactivity of a sample with a known area made it possible to show that there are about 500 sodium channels per 1 µm2 squid axon.

Those transmembrane currents that are measured in conventional experiments, for example, on a squid axon 1 cm long and 1 mm in diameter, that is, an area of ​​3 * 10 7 μm 2, are due to the total response (change in conductivity) of 500 3 10 7 -10 10 ion channels. This response is characterized by a smooth change in conductivity over time. The response of a single ion channel changes over time in a fundamentally different way: discretely for Na+ channels, and for K+-, and for Ca 2+ channels.

This was first discovered in 1962 in studies of the conductivity of lipid bilayer membranes (BLMs) when microquantities of a certain excitation-inducing substance were added to the solution surrounding the membrane. A constant voltage was applied to the BLM and the current I(t) was recorded. The current was recorded over time in the form of jumps between two conducting states.

One of the effective methods for experimental study of ion channels was the method of local fixation of membrane potential (“Patch Clamp”), developed in the 80s (Fig. 10).

Rice. 10. Method of local fixation of membrane potential. ME - microelectrode, IR - ion channel, M - cell membrane, SFP - potential clamp circuit, I - single channel current

The essence of the method is that the ME microelectrode (Fig. 10), with a thin end having a diameter of 0.5-1 μm, is suctioned to the membrane so that the ion channel enters its inner diameter. Then, using a potential-clamp circuit, it is possible to measure currents that pass only through a single channel of the membrane, and not through all channels simultaneously, as happens when using the standard potential-clamp method.

The results of experiments performed on various ion channels showed that the conductivity of an ion channel is discrete and it can be in two states: open or closed. Transitions between states occur at random times and obey statistical laws. It cannot be said that a given ion channel will open at exactly this moment in time. You can only make a statement about the probability of opening a channel in a certain time interval.

4. Dependence of channel parameters on membrane potential. Nerve fiber ion channels are sensitive to membrane potential, such as the sodium and potassium channels of the squid axon. This is manifested in the fact that after the start of membrane depolarization, the corresponding currents begin to change with one or another kinetics. This process occurs as follows: The ion-selective channel has a sensor - some element of its design that is sensitive to the action of the electric field (Fig. 11). When the membrane potential changes, the magnitude of the force acting on it changes, as a result, this part of the ion channel moves and changes the probability of opening or closing the gate - a kind of damper that operates according to the “all or nothing” law. It has been experimentally shown that under the influence of membrane depolarization, the probability of the sodium channel transitioning to the conducting state increases. The voltage surge across the membrane created during potential clamp measurements causes a large number of channels to open. More charges pass through them, which means, on average, more current flows. It is important that the process of increasing channel conductivity is determined by an increase in the probability of the channel transitioning to an open state, and not by an increase in the diameter of the open channel. This is the modern understanding of the mechanism of current passage through a single channel.

Smooth kinetic curves of currents recorded during electrical measurements on large membranes are obtained due to the summation of many stepwise currents flowing through individual channels. Their summation, as shown above, sharply reduces fluctuations and gives fairly smooth time dependences of the transmembrane current.

Ion channels can also be sensitive to other physical influences: mechanical deformation, binding of chemicals, etc. In this case, they are the structural basis, respectively, of mechanoreceptors, chemo-receptors, etc.

The study of ion channels in membranes is one of the important tasks of modern biophysics.

Structure of the ion channel.

The ion-selective channel consists of the following parts (Fig. 11): immersed in the bilayer of the protein part, which has a subunit structure; a selective filter formed by negatively charged oxygen atoms, which are rigidly located at a certain distance from each other and allow ions of a certain diameter to pass through; gate part.

The gate of the ion channel is controlled by the membrane potential and can be in either a closed state (dashed line) or an open state (solid line). The normal position of the sodium channel gate is closed. Under the influence of an electric field, the probability of an open state increases, the gate opens and the flow of hydrated ions is able to pass through the selective filter.

If the ion fits in diameter, it sheds its hydration shell and jumps to the other side of the ion channel. If the ion is too large in diameter, such as tetraethylammonium, it is not able to fit through the filter and cannot cross the membrane. If, on the contrary, the ion is too small, then it has difficulties in the selective filter, this time associated with the difficulty of shedding the hydration shell of the ion.

Ion channel blockers either cannot pass through it, getting stuck in the filter, or, if they are large molecules like TTX, they sterically match some entrance to the channel. Since blockers carry a positive charge, their charged part is drawn into the channel to the selective filter as an ordinary cation, and the macromolecule clogs it.

Thus, changes in the electrical properties of excitable biomembranes are carried out using ion channels. These are protein macromolecules that penetrate the lipid bilayer and can exist in several discrete states. The properties of channels selective for K + , Na + and Ca 2+ ions may depend differently on the membrane potential, which determines the dynamics of the action potential in the membrane, as well as the differences in such potentials in the membranes of different cells.

Rice. 11. Cross-sectional diagram of the structure of the sodium ion channel of the membrane

Feedback.

For various substances and, in particular, for mineral ions, it is extremely important in the life of the cell and especially in the mechanisms of perception, transformation, transmission of signals from cell to cell and to intracellular structures.

The determining role in the state of cell membrane permeability is played by their ion channels, which are formed channel-forming proteins. The opening and closing of these channels can be controlled by the magnitude of the potential difference between the outer and inner surfaces of the membrane, a variety of signaling molecules (hormones, neurotransmitters, vasoactive substances), secondary messengers of intracellular signal transmission, and mineral ions.

Ion channel- several subunits (integral membrane proteins containing transmembrane segments, each of which has an α-helical configuration) that ensure the transport of ions across the membrane.

Rice. 1. Classification of ion channels

Modern understanding of the structure and function of ion channels has become possible thanks to the development of methods for recording electrical currents flowing through an isolated section of the membrane containing single ion channels, as well as through the isolation and cloning of individual genes that control the synthesis of protein macromolecules capable of forming ion channels. This made it possible to artificially modify the structure of such molecules, integrate them into cell membranes, and study the role of individual peptide regions in performing channel functions. It turned out that the channel-forming protein molecules of all ion channels have some common structural features and usually are represented by large transmembrane proteins with molecular masses above 250 kDa.

They consist of several subunits. Usually the most important channel properties their a-subunit. This subunit takes part in the formation of the ion-selective hole, the sensor mechanism of the transmembrane potential difference - the gate of the channel, and has binding sites for exogenous and endogenous ligands. Other subunits included in the structure of ion channels play an auxiliary role, modulating the properties of the channels (Fig. 2).

The channel-forming protein molecule is represented by extramembrane amino acid loops and intramembrane helical domain regions that form the subunits of ion channels. The protein molecule folds in the plane of the membrane so that the ion channel itself is formed between the domains in contact with each other (see Fig. 2, bottom right).

The channel-forming protein molecule is located in the cytoplasmic membrane so that its three-dimensional spatial structure forms the mouths of the channel facing the outer and inner sides of the membrane, a pore filled with water, and a “gate.” The latter are formed by a section of the peptide chain that can easily change its conformation and determine the open or closed state of the channel. The selectivity and permeability of the ion channel depend on the size of the pore and its charge. The permeability of a channel for a given ion is also determined by its size, charge, and hydration shell.

Rice. 2. Structure of the Na+ -ion channel of the cell membrane: a - two-dimensional structure of the α-unit of the ion channel of the cell membrane; b - on the left - a sodium channel, consisting of an a-subunit and two P-subunits (side view); on the right is the sodium channel from above. In numbers I. II. III. IV marked domains of the a-subunit

Types of Ion Channels

More than 100 types of ion channels have been described, and various approaches are used to classify them. One of them is based on taking into account differences in the structure of channels and functioning mechanisms. In this case, ion channels can be divided into several types:

• passive ion channels, or resting channels;
• slot contact channels;
• channels, the state of which (open or closed) is controlled by the influence on their gate mechanism of mechanical factors (mechanosensitive channels), potential differences on the membrane (voltage-dependent channels) or ligands that bind to the channel-forming protein on the outer or inner side of the membrane (ligand-gated channels).

Passive channels

A distinctive feature of these channels is that they can be open (active) in resting cells, i.e. in the absence of any influence. This predetermines their second name - passive channels. They are not strictly selective, and through them the cell membrane can “leak” several ions, for example K+ and CI+ K+ and Na+. Therefore, these channels are sometimes called leak channels. Due to these properties, resting channels play an important role in the emergence and maintenance of the resting membrane potential on the cytoplasmic membrane of a cell, the mechanisms and significance of which are discussed below. Passive channels are present in the cytoplasmic membranes of nerve fibers and their endings, striated cells, smooth muscles, myocardium and other tissues.

Mechanosensitive channels

The permeability state of these channels changes under mechanical influences on the membrane, causing disruption of the structural packing of molecules in the membrane and its stretching. These channels are widely represented in mechanoreceptors of blood vessels, internal organs, skin, striated muscles, and smooth myocytes.

Voltage-dependent channels

The state of these channels is controlled by the forces of the electric field created by the magnitude of the potential difference across the membrane. Voltage-gated channels can be in inactive (closed), active (open) and inactivated states, which are controlled by the position of activation and inactivation gates, depending on the potential difference across the membrane.

In a resting cell, a voltage-gated channel is usually in a closed state, from which it can be opened or activated. The probability of its independent opening is low, and at rest only a small number of these channels in the membrane are open. A decrease in the transmembrane potential difference (membrane depolarization) causes activation of the channel, increasing the likelihood of its opening. It is assumed that the function of the activation gate is performed by an electrically charged amino acid group that closes the entrance to the mouth of the channel. These amino acids are a sensor of the potential difference on the membrane; when a certain (critical) level of membrane depolarization is reached, the charged part of the sensor molecule shifts towards the lipid microenvironment of the channel-forming molecule and the gate opens the entrance to the mouth of the channel (Fig. 3).

The channel becomes open (active) for ions to move through it. The opening speed of the activation gate can be low or very high. According to this indicator, voltage-gated ion channels are divided into fast (for example, fast voltage-gated sodium channels) and slow (for example, slow voltage-gated calcium channels). Fast channels open instantly (μs) and remain open for an average of 1 ms. Their activation is accompanied by a rapid avalanche-like increase in the permeability of the channel for certain ions.

Another part of the peptide chain, which is an amino acid sequence in the form of a dense ball (ball) on a thread, located at the exit of the other mouth of the channel, has the ability to change its conformation. When the sign of the charge on the membrane changes, the ball closes the exit from the mouth, and the channel becomes impenetrable (inactivated) for the ion. Inactivation of voltage-gated ion channels can be accomplished through other mechanisms. Inactivation is accompanied by the cessation of ion movement through the channel and can occur as quickly as activation, or slowly - over a period of seconds or even minutes.

Rice. 3. Gate mechanism of voltage-gated sodium (top) and potassium (bottom) channels

To restore the original properties of ion channels after their inactivation, it is necessary to return the original spatial conformation of the channel-forming protein and the position of the gate. This is achieved by restoring the membrane potential difference (repolarization) to a level characteristic of the cell's resting state or some time after inactivation with a strong effect on the membrane. The transition from the inactivation state to the original (closed) state is called channel reactivation. Once reactivated, the ion channel returns to a state of readiness for its re-opening. Reactivation of voltage-gated membrane channels can also be fast or slow.

Voltage-gated ion channels are usually highly selective and play a crucial role in the occurrence of excitation (generation of action potentials), the transmission of information along nerve fibers in the form of electrical signals, and the initiation and regulation of muscle contraction. These channels are widely represented in the membranes of afferent and efferent nerve fibers, in the membranes of striated and smooth myocytes.

Potential-dependent ion channels are built into the membrane of the nerve endings of sensory nerves (dendrites) innervating the dental pulp and oral mucosa, where their opening ensures the conversion of the receptor potential into a nerve impulse and its subsequent transmission along the afferent nerve fiber. With the help of these impulses, information about all types of sensory sensations that a person experiences in the oral cavity (taste, temperature, mechanical pressure, pain) is transmitted to the central nervous system. Such channels ensure the emergence of nerve impulses on the membrane of the axon hillock of neurons and their transmission along efferent nerve fibers, the conversion of postsynaptic potentials into action potentials of postsynaptic effector cells. An example of such processes is the generation of nerve impulses in motor neurons of the trigeminal nerve nucleus, which are then transmitted along its efferent fibers to the masticatory muscles and provide the initiation and regulation of chewing movements of the lower jaw.

When studying the subtle mechanisms of the functioning of voltage-gated ion channels, it was revealed that there are substances that can block the operation of these channels. One of the first of these to be described was the substance tetrodotoxin, a powerful poison produced in the body of puffer fish. Under its influence, blockade of voltage-gated sodium channels was observed in the experiment, and when it was introduced into the body of animals, loss of sensitivity, muscle relaxation, immobility, respiratory arrest and death were noted. Such substances are called ion channel blockers. Among them lidocaine, novocaine, procaine - substances, when introduced into the body in small doses, blockade of voltage-dependent sodium channels of nerve fibers develops and the transmission of signals from pain receptors to the central nervous system is blocked. These substances are widely used in medical practice as local anesthetics.

The movement of ions through ion channels is not only the basis for the redistribution of charges on membranes and the formation of electrical potentials, but can also influence the course of many intracellular processes. This effect on the expression of genes that control the synthesis of channel-forming proteins is not limited only to the cells of excitable tissues, but occurs in all cells of the body. A large group of diseases has been identified, the cause of which is a violation of the structure and function of ion channels. Such diseases are classified as “channelopathies”. Obviously, knowledge of the structure and functions of ion channels is necessary to understand the nature of “channelopathies” and search for their specific therapy.

Ligand-gated ion channels

They are usually formed by protein macromolecules that can simultaneously serve as ion channels and receptor functions for certain ligands. Since the same macromolecule can simultaneously perform these two functions, different names have been assigned to them - for example, synaptic receptor or ligand-gated channel.

Unlike a voltage-dependent ion channel, which opens when the conformation of the activation gate changes under conditions of a decrease in the transmembrane potential difference, ligand-dependent ion channels open (activate) upon interaction of the peptide (receptor) chain of a protein molecule with a ligand, a substance for which the receptor has a high affinity ( Fig. 4).

Rice. 4. Ligand-dependent ion channel (nicotine-sensitive acetylcholine receptor - n-ChR): a inactive; 6 - activated

Ligand-gated ion channels are usually localized in the postsynaptic membranes of nerve cells and their processes, as well as muscle fibers. Typical examples of ligand-gated ion channels are postsynaptic membrane channels activated by acetylcholine (see Fig. 4), glutamate, aspartate, gamma-aminobutyric acid, glycine and other synaptic neurotransmitters. Typically, the name of the channel (receptor) reflects the type of neurotransmitter that is its ligand under natural conditions. So, if these are channels of the neuromuscular synapse in which the neurotransmitter acetylcholine is used, then the term “acetylcholine receptor” is used, and if it is also sensitive to nicotine, then it is called nicotine-sensitive, or simply n-acetylcholine receptor (n- cholinergic receptor).

Typically, postsynaptic receptors (channels) selectively bind to only one type of neurotransmitter. Depending on the type and properties of the interacting receptor and neurotransmitter, the channels selectively change their permeability to mineral ions, but they are not strictly selective channels. For example, ligand-gated channels can change the permeability to Na+ and K+ cations or to K+ and CI+ anions. This selectivity of ligand binding and changes in ionic permeability is genetically fixed in the spatial structure of the macromolecule.

If the interaction of the mediator and the receptor part of the macromolecule that forms the ion channel is directly accompanied by a change in the permeability of the channel, then within a few milliseconds this leads to a change in the permeability of the postsynaptic membrane for mineral ions and the value of the postsynaptic potential. Such channels are called fast and are localized, for example, in the postsynaptic membrane of axo-dendritic excitatory synapses and axosomatic inhibitory synapses.

There are slow ligand-gated ion channels. Unlike fast channels, their opening is mediated not by direct interaction of the neurotransmitter with the receptor macromolecule, but by a chain of events including activation of the G protein, its interaction with GTP, an increase in the level of secondary messengers in the intracellular transmission of the neurotransmitter signal, which, by phosphorylating the ion channel, lead to a change in its permeability for mineral ions and a corresponding change in the value of the postsynaptic potential. The entire described chain of events takes place within hundreds of milliseconds. We will encounter such slow ligand-dependent ion channels when studying the mechanisms of regulation of the heart and smooth muscles.

A special type are channels localized in the membranes of the endoplasmic reticulum of smooth muscle cells. Their ligand is the second messenger of intracellular signal transduction, inositol tri-phosphate-IFZ.

Ion channels are described that are characterized by certain structural and functional properties inherent in both voltage-gated and ligand-gated ion channels. They are voltage-insensitive ion channels, the state of the gate mechanism of which is controlled by cyclic nucleotides (cAMP and cGMP). In this case, cyclic nucleotides bind to the intracellular COOH terminal of the channel-forming protein molecule and activate the channel.

These channels are characterized by less selectivity of permeability for cations and the ability of the latter to influence each other’s permeability. Thus, Ca 2+ ions, entering through activated channels from the extracellular environment, block the permeability of channels for Na 2+ ions. One example of such channels is the rod ion channels of the retina, the permeability of which to Ca 2+ and Na 2+ ions is determined by the level of cGMP.

Ligand-gated ion channels are widely represented in membrane structures that provide synaptic transmission of signals from a number of sensory receptors in the central nervous system; transmission of signals at synapses of the nervous system; transmission of nervous system signals to effector cells.

It has already been noted that the direct transmission of commands from the nervous system to many effector organs is carried out with the help of neurotransmitters that activate ligand-gated ion channels in postsynaptic membranes. However, their ligands (agonists or antagonists) can also be substances of exogenous nature, which in some cases are used as medicinal substances.

For example, after the introduction of the substance diplacin into the body, which is similar in structure to the neurotransmitter apetylcholine, there will be a prolonged opening of ligand-dependent ion channels at neuromuscular synapses, which stop transmitting nerve impulses from nerve fibers to muscles. Relaxation of the skeletal muscles of the body occurs, which may be necessary during complex surgical operations. Diplacin and other substances that can change the state of ligand-gated ion channels and block signal transmission at neuromuscular synapses are called muscle relaxants.

Rice. 5. Gap junction channels between two tightly contacting cells

In medical practice, many other medicinal substances are used that affect the state of ligand-dependent ion channels of cells of various tissues.

Cell gap (tight) junction channels

Gap junction channels are formed in the area of ​​contact between two neighboring cells that are very close to each other. In the membrane of each contacting cell, six protein subunits, called connexins, form a hexagonal structure, in the center of which a pore or ion channel is formed - a connexon (Fig. 5).

A mirror structure is formed at the point of contact in the membrane of an adjacent cell, and the ion channel between them becomes common. Through such ion channels, various mineral ions, including Ca 2+ ions, as well as low molecular weight organic substances, can move from cell to cell. Channels of gap junctions of cells ensure the transfer of information between cells of the myocardium, smooth muscles, retina, and nervous system.

Sodium channels

Voltage-dependent, voltage-independent (ligand-dependent, mechanosensitive, passive, etc.) sodium channels are widely represented in the cells of the body.

Voltage-gated sodium channels

They consist of one α-subunit, which forms the channel, and two β-subunits, which modulate the ion permeability and inactivation kinetics of sodium channels (Fig. 6).

Rice. 6. Two-dimensional structure of the α-subunit of the voltage-gated sodium channel. Description in the text

As can be seen from Fig. 6, the a-subunit is represented by four domains of the same type, consisting of six helical transmembrane segments connected by amino acid loops. The loops connecting the 5th and 6th segments surround the channel pore, and the 4th segment contains positively charged amino acids, which are sensors of the potential difference on the membrane and control the position of the gate mechanism during shifts in the transmembrane potential.

In voltage-gated sodium channels there are two gate mechanisms, one of them - activation (with the participation of the 4th segment) ensures the opening (activation) of the channel upon membrane depolarization, and the second (with the participation of the intracellular loop between the 3rd and 4th domains) - its inactivation upon membrane recharging. Since both of these mechanisms rapidly change the position of the channel gate, voltage-gated sodium channels are fast ion channels and are critical for the generation of action potentials in excitable tissues and for their conduction across the membranes of nerve and muscle fibers.

These channels are localized in the cytoplasmic membranes of the axon hillock of neurons, in dendrites and axons, in the membrane of the perisynaptic region of the neuromuscular synapse, in the sarcolemma of fibers of striated muscles and contractile myocardium. The distribution density of sodium channels in these structures is different. In myelinated nerve fibers they are concentrated mainly in the area of ​​nodes of Ranvier, where their density reaches about 10,000 channels per square micron of area, and in unmyelinated fibers the channels are more evenly distributed with a density of about 20 channels per square micron of area. These channels are practically absent in the structure of the membranes of the nerve cell body, in the membrane of nerve endings that directly form sensory receptors, and in the postsynaptic membranes of effector cells.

Among the voltage-gated sodium channels, more than nine subtypes are already distinguished, differing in the properties of the α-subunits, having a specific tissue affiliation and differing in different sensitivity to the action of blockers. For example, a subtype of channel formed by a channel-forming protein, the synthesis of which is controlled by the SCN4A gene, is present in the sarcolemma of fully differentiated and innervated skeletal muscles and its blockers are tetrodotoxin, saxitoxin and c-conotoxins. In most cases, the α-subunits are sensitive to the action of tetrodotoxin, which in micromolar concentrations blocks the pores and thereby the entrance to sodium channels.

Toxins of sodium channels are known to slow down the rate of their inactivation. For example, sea anemone toxin (ATX) and scorpion a-toxin (ScTX) cause a delay in inactivation by binding to amino acid residues of the S3-S4 loop of segment 4.

Substances called anesthetics (novocaine, dicaine, lidocaine, sovcaine, procaine and etc.). Anesthesia when they block sodium channels is achieved by eliminating the possibility of generating nerve impulses in afferent nerve fibers and thereby blocking the transmission of signals from sensory pain receptors to the central nervous system.

It has been discovered that changes in the structure of sodium channels can lead to the development of a number of diseases. For example, a change in the structure of the channel controlled by the SCNlb gene leads to the development of generalized forms of epilepsy and seizures with increased body temperature (febrile seizures).

Many microorganisms form toxins in the human body—substances that block ion channels in the affected cells, which can be accompanied by an imbalance in the ion balance and cell death. Other microorganisms, on the contrary, use their toxins (perforins) to form ion channels in the cell membrane. In particular, the toxin of the anthrax bacillus, which causes a particularly dangerous infection in humans, attacks the cell and forms new pores (channels) in its membrane through which other toxins penetrate into the cell. The action of these toxins causes the death of attacked cells and high mortality in this disease. Scientists have synthesized a substance β-cyclodextrin, which is close in spatial structure to the shape of the resulting channel. This substance blocks the channels formed by the microorganism's toxin, prevents the entry of toxins into cells and saves experimental animals infected with anthrax from death.

Voltage-independent sodium channels

Ligand-gated sodium channels. Their general structure and properties are discussed above in the description of ligand-gated ion channels. This type of sodium channels is widely represented in the body by sodium channels of the nicotine-sensitive cholinergic receptor of the postsynaptic membrane of the neuromuscular synapse, interneuron synapses of the central nervous system and the autonomic nervous system (preganglionic and ganglionic neurons). Ligand-gated sodium channels are localized in the postsynaptic membranes of other excitatory (glutamate- and aspartatergic) synapses of the central nervous system. They play a crucial role in the generation of excitatory postsynaptic potential at synapses and the transmission of signals between neurons and between neurons and effector cells.

Ligand-gated sodium channels of the postsynaptic membrane are not strictly selective and can be permeable simultaneously to several ions: sodium and potassium, sodium and calcium.

Voltage-independent sodium channels gated by second messengers. The state of these sodium channels can be controlled by cGMP (photoreceptors), cAMP (olfactory receptors) and by G protein subunits (myocardium).

Mechanosensitive sodium channels. Present in the mechanoreceptors of the walls of blood vessels, the heart, hollow internal organs, proprioceptors of striated muscles, and the membrane of smooth myocytes. With their participation in sensory receptors, the energy of mechanical action is converted into an oscillation of the potential difference - the receptor potential.

Passive sodium ropes. Contained in the cytoplasmic membranes of excitable cells. The permeability of these channels for Na+ ions is small, but through them Na ions diffuse along a concentration gradient from the extracellular spaces into the cells and somewhat depolarize the membrane. The sodium channels of the cytoplasmic membrane of smooth myocytes are more permeable. They depolarize it by a greater amount (resting potential about 50 mV) than the membrane of myocytes of striated muscles (resting potential about 90 mV). Thus, passive sodium channels are involved in the formation of the resting membrane potential.

Sodium exchangers. The sodium-calcium exchanger, or sodium-calcium exchanger, has been previously described and plays an important role in the removal of calcium ions from contractile cardiomyocytes.

Sodium proton exchanger. It is a special type of channel-forming protein that removes hydrogen protons from intracellular spaces in exchange for sodium ions entering the cell. The removal of protons is activated when the pH in the cell decreases.

The synthesis of proteins that form sodium exchange channels is controlled by five genes, designated NAH1 -NAH5.

Potassium channels

There are voltage-gated and voltage-insensitive potassium channels. Among the latter, passive, ligand-dependent and other types of potassium channels are distinguished. As a rule, potassium channels are found in the membranes of the same cells and tissues that contain sodium channels. One of the reasons for such parallelism in the arrangement of these ion channels is that Na+ and K+ ions are the most important cations, the nature of the distribution and movement of which determines the emergence and change of electrical potentials as one of the most important forms of information signal transmission in the body.

There is a whole superfamily of potassium ion channels, which are divided according to structural features, localization and properties of the channels into separate families, types and subtypes. More than three dozen potassium channels have been described, and it is not possible to give their detailed characteristics. Therefore, as examples, descriptions of those families and types of ion channels that are primarily related to signaling pathways and control mechanisms of nervous and muscle processes will be given.

Passive potassium channels

It is known that in the resting state, the membranes of excitable cells are relatively permeable to K ions and poorly permeable to Na+ ions. Since the carriers of transmembrane electric currents are ions, by measuring the electric current flowing through the cell membrane, one can judge the state of ion channels. It turned out that the transmembrane electric current, caused by the diffusion of K ions along the concentration gradient from the cell, is about two picoamperes and has a pulsating character, and the average duration of the pulsation is several milliseconds. From this observation, it was concluded that potassium channels in a resting cell can spontaneously open and close, providing conditions for the diffusion of K ions through them from the cell and the formation of a resting potential on the membrane.

Voltage-gated potassium channels

The existence of voltage-gated potassium channels in the cell membranes of excitable tissues became known after it was found that their activation kinetics differs from that of voltage-gated sodium channels and, moreover, they are selectively blocked by other blockers. Potassium channels are activated in the same way as sodium channels, when the cell membrane is depolarized to a critical level, but at the same time, the rate of exit of K+ ions from the cell increases much more slowly than the rate of entry of Na+ ions into the cell.

The selective filter of the potassium channel is located on the inside of the pore mouth, in contrast to the external location of a similar filter in sodium channels (Fig. 7). The existence of selectivity of these channels in relation to Na+ and K+ cations and various specific blockers - tetrodotoxin (for sodium) and tetraethylammonium (for potassium) - indicates the different structure of these channels.

Voltage-gated potassium channels are tetramers and consist of four subunits forming a pore in the center.

Voltage-gated potassium channels are localized in the membranes of both excitable and non-excitable cells. They play an important role in the rate of recovery (repolarization) of the membrane potential after its depolarization and, thus, in controlling the shape and frequency of generation of action potentials. Slow potassium channels are blocked by traethylammonium, 4-aminopyridine, phencyclidine, and 9-aminoacridine.

Rice. 7. Potassium channel: a - left - two-dimensional structure of the a-subunit; on the right is a diagram of the channel; b — electron diffraction diagram of potassium channels in the cytoplasmic membrane.

In addition to slow potassium channels, fast voltage-gated potassium channels have been described, the opening kinetics of which are similar to those of fast voltage-gated sodium channels. These potassium channels open rapidly upon depolarization, then are completely inactivated, and their reactivation requires not only repolarization of the membrane, but hyperpolarization for a while.

In accordance with the names of genes encoding the synthesis and assembly of channel-forming molecules, six KCN types with subtypes KCN A, B, C, E and one family of KCNQ ion channels are distinguished. Channels of the latter family are expressed in the myocardium.

Ligand-gated potassium channels

They are represented by a large number of channels sensitive to the action of various ligands.

One type of numerous ligand-gated potassium channels is the muscarine-sensitive acetylcholine receptor-associated channel. These channels are activated by acetylcholine. The channels can be blocked by bradykinin and barium ions. There are two subtypes of these channels: those inactivated by muscarine and those activated by it. The latter is localized in the pacemaker cells of the heart.

The properties of a ligand-dependent potassium channel are possessed by non-selective voltage-independent cation channels that combine the characteristics of channels and nicotine-sensitive acetylcholine receptors of the postsynaptic membrane of the neuromuscular synapse. When the channel-forming protein interacts with acetylcholine, this non-selective channel opens, through which Na+ ions enter the muscle cell, and K ions exit it. The different rates of movement of these ions ensure the occurrence of depolarization of the postsynaptic membrane, which does not develop into an action potential directly on this membrane.

ATP-sensitive potassium channels, which are inhibited and activated by the action of ATP, have been identified.

A separate family of potassium channels consists of the so-called input rectifying potassium channels (gates), or input rectifiers (inwardrectifying; inwardrectifier). There is no voltage sensor in the rectifying potassium channel gate mechanism. The functional significance of these channels lies in their influence on the excitability of pacemaker cells, muscle cells and neurons.

The family of rectifying incoming potassium channels, according to the names of the genes encoding them, is divided into more than 15 types. An example of the specific significance of rectifying input potassium channels and, in particular, KCNJ channels 3, 5, 6 and 9 (another designation Kir channels) may be their specific role in the regulation of heart rate through the association of these channels with G protein and muscarine-sensitive acetylcholine receptors of cells - heart pacemakers.

Voltage-insensitive sodium-activated potassium channels are known.

Special voltage-insensitive potassium channels, sensitive to changes in pH, are described, which are present in the β-cells of the pancreatic islets and act as a glucose sensor in them. Potassium channels are also known to be sensitive to changes in cell volume.

Calcium channels

The calcium channel family is widely represented in the cells of nerve and muscle tissue. The main places of their localization are the membranes of the presynaptic terminals of the sarcoplasmic and endoplasmic reticulum of muscles, the sarcolemma of cardiomyocytes and the membranes of cells of other tissues.

Based on the methods of controlling permeability, calcium channels are divided into voltage-dependent, passive, ligand-dependent, mechanosensitive, etc.

Calcium channels are divided according to the rate of inactivation into T-type channels ( transient- transient), L-type (slow). Depending on the tissue affiliation and sensitivity to toxins, B-type channels are distinguished (brain- brain), N-type (neuronal- neuronal), P-type (purkinjecell- Purkinje cell) and R-type (resistant to toxins).

Voltage-gated calcium channels

They are formed by an oligomeric protein, usually consisting of five subunits a1, a2, β, y and δ. The ion channel itself is formed by the α-subunit, which has a high degree of similarity in amino acid composition and structure with a similar subunit of voltage-gated sodium and potassium channels (see Fig. 6, Fig. 7).

The voltage-gated calcium channel is selectively permeable to Ca 2+ ions. Selectivity is ensured by the presence of a pore that forms a selective filter.

It's time formed by segments of the a 1 subunit, therefore, given the similarity of its structure to that of monovalent cation channels, one would expect that the calcium channel should be permeable to Na+ and K+ ions. This property actually occurs when calcium is removed from the extracellular environment.

Under natural conditions, selectivity towards calcium is ensured in the channel by the presence of two calcium binding sites in the channel pore. One of them is formed by a group of glutamate residues, and at a low calcium concentration it becomes strongly bound to this site of the channel pore and the channel for calcium becomes weakly permeable. As calcium concentration increases, the likelihood of calcium occupying a second binding site increases; the resulting electrostatic repulsion forces between Ca 2+ ions greatly reduce the residence time of the ions at the binding sites. The released calcium diffuses through the activated channel into the cell along an electrochemical gradient.

Voltage-gated calcium channels differ in the threshold values ​​of potential difference shifts at which they are activated. T-type channels are activated by small voltage shifts on the membrane, L- and P-types are characterized by high voltage shift thresholds that cause their activation.

Voltage-gated calcium channels play an important role in a number of vital processes in the body. Their activation and the entry of calcium into the presynaptic terminal are necessary for synaptic signal transmission.

The entry of calcium through calcium channels into the pacemaker cell is necessary to generate action potentials in the pacemaker cells of the heart and ensure its rhythmic contraction. Voltage-dependent calcium channels regulate the flow of calcium into the sarcoplasm of myocardial fibers, skeletal muscles, smooth myocytes of blood vessels and internal organs, controlling the initiation, speed, strength, duration of their contraction and thereby movement, the pumping function of the heart, blood pressure, respiration and many other processes in body.

Passive calcium channels

Found in the cytoplasmic membranes of smooth myocytes. They are permeable to calcium at rest, and calcium, along with K+ and Na+ ions, is involved in the creation of the transmembrane potential difference or resting potential of smooth myocytes. Calcium entering the smooth myocyte through these channels is a source of replenishment of its reserves in the endoplasmic reticulum and is used as a secondary messenger in the transmission of intracellular signals.

Resting calcium can move from cell to cell through gap junction channels. These channels are not selective for calcium, and intercellular exchange of other ions and organic substances of small molecular weight can simultaneously occur through them. Calcium entering cells through gap junction channels plays an important role in the occurrence of excitation, initiation and synchronization of contractions of the myocardium, uterus, sphincters of internal organs, and maintaining vascular tone.

Ligand-gated calcium channels

When studying the mechanisms of triggering and regulating myocardial and smooth muscle contractions, it turned out that they depend on the supply of calcium to the myocyte both from the extracellular environment and from its intracellular stores. In this case, the entry of calcium into the sarcoplasm can be controlled by a change in the potential difference on the sarcolemma and the activation of voltage-dependent calcium channels and (or) the action of a number of signaling molecules on the sarcoplasmic reticulum membrane.

Ligand-gated calcium channels are localized in the cytoplasmic membranes of smooth myocytes. The ligands of their receptors can be hormones: vasopressin, oxytocin, adrenaline; neurotransmitter norepinephrine; signaling molecules: angiotensin 2, endothelium 1 and other substances. Binding of the ligand to the receptor is accompanied by activation of the calcium channel and the entry of calcium into the cell from the extracellular environment.

In cardiomyocytes, to initiate muscle contraction, it is necessary to initially activate voltage-gated calcium channels of the T-type, then the L-type, the opening of which ensures the entry of a certain amount of Ca 2+ ions into the cell. Calcium entering the cell activates the ryanodine receptor (RYR), a channel-forming protein embedded in the membrane of the sarcoplasmic reticulum of the cardiomyocyte. As a result of activation of the channel, its permeability to calcium increases and the latter diffuses into the sarcoplasm along the concentration gradient. Thus, Ca 2+ ions act as a kind of ligands that activate ryanodine receptors and thereby calcium channels. As a result, extracellular calcium entering the cell acts as a trigger for the release of calcium from its main intracellular storage.

Calcium channels can simultaneously be sensitive to changes in potential differences across the cytoplasmic membrane and to the action of ligands. For example, L-type voltage-gated calcium channels are sensitive to dihydropyridine (nifedipine), phenylalkylamines (verapamil), and benzothiazepines (diltiazem). This type of channel is often called dihydropyridine receptor. This name suggests that the L-calcium channel is ligand-gated, although in reality it is a voltage-gated channel.

P-type channels are resistant to the action of conogoxins and drugs to which other types of calcium channels are sensitive.

The functional properties of the α1 subunits of voltage-gated calcium channels can be modulated by their phosphorylation, and thus the state of ion permeability of calcium channels, for example, in the myocardium, can be regulated.

A special type of ligand-gated calcium ion channels are channels localized in the membranes of the endoplasmic reticulum of smooth muscle cells, the permeability state of which is controlled by the intracellular level of the secondary messenger - IPG. Using these channels as an example, we encounter a case where an extracellular signaling molecule-agonist, activating the receptor of the plasma membrane of the target smooth muscle cell, turns on the inositol phosphate pathway of intracellular signal transmission, which in turn, through the action of IPE, activates the next channel-forming protein in the membrane of the cell organelle. This entire chain of signal transmission events ends with the release of Ca 2+ ions from intracellular stores, which trigger and control the molecular mechanism of smooth muscle cell contraction.

Mechanosensitive calcium channels

They are localized in the plasma membrane of smooth myocytes of the walls of blood vessels, myoitis of internal organs, vascular endothelium, and bronchial epithelium. These channels may be associated with glycoprotein mechanoreceptors. In response to mechanical stress (for example, stretching of the vessel wall by blood pressure), permeability to Ca 2+ ions increases. Mechanosensitive channels do not have high selectivity and change their permeability simultaneously for a number of cations. The entry of calcium and sodium into a smooth muscle cell causes depolarization of its membrane, the opening of voltage-gated calcium channels, an increase in calcium entry and contraction of the smooth myocyte.

These events form part of the mechanism of adaptation of vascular tone and regulation of blood flow to changing values ​​of blood pressure in the vessel and blood flow velocity (myogenic regulation). In addition, mechanosensitive calcium channels are involved in the implementation of vascular stress-relaxation mechanisms during prolonged increases in blood pressure.

Chlorine channels

Chloride channels are present in the plasma membranes of most cells. They play an important role in maintaining the transmembrane potential difference in a resting cell and their shifts when the functional activity of cells changes. Chloride channels are involved in the regulation of cell volume, transepithelial transport of substances, and fluid secretion by secretory cells.

In accordance with the mechanisms of activation, three superfamilies of chlorine channels are distinguished: voltage-gated, ligand-gated and other voltage-insensitive chlorine channels.

Potential dependent chloride channels. Localized in the membranes of excitable and epithelial cells. The permeability state of these channels is controlled by the magnitude of the transmembrane potential difference.

Potential dependent permeability of chloride channels varies in different tissues. Thus, in the axonal membrane, the dependence of the permeability of chloride channels on the potential difference is insignificant and does not significantly affect the change in the magnitude of the action potential during excitation, and in skeletal muscles this dependence of the permeability of chloride channels is higher.

The CLC1 channel is a typical representative of the chloride channels of the sarcolemmal muscle fiber of skeletal muscle. The channel exhibits permeability over the entire range of changes in transmembrane voltage at rest, is activated upon depolarization, and inactivated upon membrane hyperpolarization.

Ligand-gated chloride channels. Predominantly expressed in nervous tissue. The permeability state of these chloride channels is controlled primarily by extracellular ligands, but they can be sensitive to intracellular calcium concentrations and activated by G proteins and cAMP. Channels of this type are widely distributed in postsynaptic membranes and are used to carry out postsynaptic inhibition. The state of channel permeability is controlled by activating the channels with ligands—inhibitory neurotransmitters (γ-aminobutyric acid and glycine).

Voltage-insensitive chlorine channels. Includes passive chloride channels, ATP-sensitive channels, and interstitial fibrosis transmembrane conductance regulator (cysticfibrosistransmembraneconductanceregulator- CFTR).

CFTR apparently consists of the chlorine channel itself and a regulatory channel represented by a special regulatory domain (P-domain). The regulation of ion conductance of these channels is carried out by phosphorylation of the regulatory domain by cAMP-dependent protein kinase. Violation of the structure and function of this channel leads to the development of a serious disease accompanied by dysfunction of many tissues - interstitial fibrosis.

Aquaporins

Aquaporins(from lat. aqua- water, Greek porus- channel, pore) - proteins that form water channels and ensure transmembrane water transfer. Aquaporins are integral, tetrameric membrane proteins, the monomer of which has a mass of about 30 kDa. Thus, each aquaporin forms four water channels (Fig. 8).

A special feature of these channels is that water molecules in them can move under isosmotic conditions, i.e. when they are not affected by the forces of the osmotic gradient. The abbreviation AQP is used to refer to aquaporins. A number of types of aquaporins have been isolated and described: AQP1 - in the epithelial membranes of the proximal renal tubules, the descending limb of the loop of Henle; in the membranes of the endothelium and smooth myocytes of blood vessels, in the structures of the vitreous body; AQP2 - in the membranes of the epithelium of the collecting ducts. This aquaporin was found to be sensitive to the action of antidiuretic hormone, and on this basis it can be considered as a ligand-gated water channel. The expression of the gene that controls the synthesis of this aquaporin is regulated by antidiuretic hormone; AQP3 is found in the membranes of corneal cells; AQP4 - in brain cells.

Rice. 8. Structure of the AQP1 water channel: a - peptide chains forming the channel; b — assembled channel: A, B, C, D, E — sections of the protein chain

It turned out that AQP1 and AQP4 play an important role in the formation and circulation of cerebrospinal fluid. Aquaporins are found in the epithelium of the gastrointestinal tract: AQP4 - in the stomach and small intestine; AQP5 - in the salivary glands; AQP6 - in the small intestine and pancreas; AQP7 - in the small intestine; AQP8, AQP9 - in the liver. Some aquaporins transport not only water molecules, but also organic substances soluble in it (oxygen, glycerol, urea). Thus, aquaporins play an important role in water metabolism in the body, and disruption of their function may be one of the reasons for the formation of cerebral and pulmonary edema and the development of renal and heart failure.

Knowledge of the mechanisms of ion transport through membranes and methods of influencing this transport is an indispensable condition not only for understanding the mechanisms of regulation of vital functions, but also for the correct choice of drugs in the treatment of a large number of diseases (hypertension, bronchial asthma, cardiac arrhythmias, water-salt disorders exchange, etc.).

To understand the mechanisms of regulation of physiological processes in the body, it is necessary to know not only the structure and permeability of cell membranes for various substances, but also the structure and permeability of more complex structural formations located between the blood and tissues of various organs.

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