The cell membrane barrier separates two aqueous media of different composition, the extracellular and intracellular spaces, regulating their composition. The most lipophilic drugs and solutes when not ionized travel directly through the cellular membrane via a passive diffusion process, which facilitates the passage of a substance from an area of high concentration to an area with lower concentration. The difference of concentration between the two areas is often termed as the concentration gradient, and diffusion will continue until this gradient has been eliminated. The speed of this process will be more rapid the higher the concentration gradient and the lipophilicity of the molecule and smaller the size of it (Fick's law).
Hydrophilic molecules, such as ions, are immiscible in lipids and to cross the membrane require specific transport mechanisms. In some cases, the ions pass through specialized hydrophilic pores called ion channels, but they can also be following the concentration gradient by binding to carrier proteins. Both transport systems are passive and therefore do not consume energy. The great advantage is that the channels allow ions to flow there through at a rate much higher than that of any other biological system (10 8 ions/sec vs 10 3 ions/sec when a carrier is used). The ion flow through each channel can be measured as an electrical current, which is able to produce rapid changes in the membrane potential. O ther times, the ion transport takes place against an electrochemical gradient, from the most diluted to the most concentrated space using proteins called ion pumps. This form of transportation is active and requires expenditure of energy from the energy metabolism of the cell, which is usually obtained by hydrolysis of ATP. The active transport mechanisms are responsible for the asymmetric distribution of ions on both sides of the cellular membrane.
The cellular action potential which the beating heart to contract, is the result of a series of sequential reversible changes in the conductance of the membrane to different ions produced in response to changes in the electrical potential between cell and the surrounding environment. Ions are hydrophilic molecules that cross the hydrophobic lipid bilayer through specialized structures called ion channels.
What is an ion channel?. Ion channels are heteromultimeric complex formed by the assembly of several proteins that are wholly or partially embedded in the membrane called subunits. Ion channels are responsible for the transmission of electrical impulses throught the heart. The channels comprise a hydrophilic a subunit that, in response to a suitable stimulis, form a hydrophilic pore which connects the intracellular and extracellular spaces and allows the rapid passage therethrough of certain ions down their electrochemical gradient generating an ionic current. Under physiological conditions, Na+ ([Na+]o = 160 mM vs [Na+]i = 6 mM) and Ca2+ ([Ca2+]o= 2.5 mM vs [Ca2+]i= 0.0001 mM) move into the cardiac cell, generating inward currents that depolarize the membrane potential. Conversely, K+ ions ([K+]o = 4 mM, [K+]i = 155 mM) move out of the cell generating outward currents that repolarize the AP and maintain the resting membrane potential.
Figure. Ion channels are pore-forming proteins that provide pathways for the controlled transmembrane movement of ions
However, the ion channel expression is not restricted to the sarcolemma of excitable cells, but they can also be expressed in the membranes of some intracytosolic organelles (i.e., the sarcoplasmic reticulum, mitochondria). Channels located in the sarcoplasmic reticulum play a key role in cellular Ca2+ handling. Gap junctions are aggregates of intercellular channels that allow direct communication between adjacent cells through the diffusion of ions, metabolites, and small cell signaling molecules. The intercellular channels are formed by head-to-head docking of hexameric assemblies (connexons) of tetraspan integral membrane proteins, the connexins (Cx) Cell-cell communication mediated by connexins is crucial in the propagation of cardiac impulses.
Cardiac Na+, Ca2+ and K+ result from the coassembly of a pore-forming a subunit with other"accesory or auxiliary" subunits. However, the ion channels are not simple aqueous pores conductors, but, present (Figure):
A selectivity filter, which determines which ion moves through. In general, the pore voltage-gated channels is highly selective for a particular ion (i.e., K+ channels are 10,000 times more permeable to K+ than to Na+), whereas receptor-gated channels present a lower selectivity and in many cases, they can conduct both cations or anions through.
Gates which are opened or closed depending on external stimuli to control the permeability of the membrane. In response to a stimulus, the channel proteins are able to adopt different states or structural conformations. The conformational changes of the protein between different states occur very rapidly (<10 m s) and are called channel gating.
The opening and closing of ion channels is controlled by an electric, chemical or mechanical sensor. In the voltage-gated channels this sensor is determined by several positively charged amino acids located in the S4 segment, which acts as an electric dipole, and segments S1-S3. During the cellular depolarization S4 moves through the membrane and creates a movement of free energy changes (the so-call gate current or gating) which modifies the tertiary structure of the channel opening it or closing it.
According to the stimulus that induces the opening of cardiac ion channels are classified as:
Voltage-gated channels. They modulate their gating [opening (activation) or closing (by deactivation or inactivation)] that mediate ion flux across cellular membranes in response to changes in voltage across the cell membrane. Its main function is the generation and propagation of action potentials.
Receptor- or ligand-activated channels. Thay are activated upon the interaction of an agonist (neurotransmitters, drugs or hormones) with its specific receptor located on the cell membrane surface, which may or may not be associated with the channel, causing the channel opening.
Channels activated by i ntracellular mediators (Ca2+, ATP, G proteins, cyclic nucleotides, protein kinases, arachidonic acid and its derivatives).
Channels activated by p hysical factors (stretching or deformation of the membrane, changes in pressure, temperature or pH, cell volume increase). The sensor mechanism of these channels is unknown, although perhaps fatty acids of the membrane or the cytoskeleton may be involved.
Leak channels that open and close spontaneously.
However, quite often this division of the ion channels is artificial, because the depolarization of the membrane can also induce the release of endogenous ligands and open channels activated by receptors or intracellular mediators, whereas many endogenous ligands can also modify the membrane potential and activate voltage-gated channels.
The voltage-gated channels present, at least, a conductive state (open-O or active state) and two non-conductors (inactive-I and resting-R states). The R state does not allow passage of ions, but channels can open from the R state in response to specific stimuli. The O state allows passage of ions across the cell membrane which generates an ionic current. When we apply a depolarizing pulse voltage-gated chanels move from the R to the O state, i.e. channel activation. However, if the depolarization is maintained, the open channel probability decreases as a result of the inactivation process initiated simultaneously by the activation process. The channel then goes into an I-closed state from which the channel can not be reopened until it returns to the R state. For the opening takes place the channel should return to idle. This step in the inactive state of rest, is called channel reactivation and occurs during cell repolarization. Therefore, the magnitude of the current which crosses the membrane depends on the density of channels, open channel conductance and how long the channel remains in the open state.
Until recently, it was considered that the cardiac ion channels were heteromultimeric complexes were formed by the assembly of the a subunit with one or more auxiliary subunits. However, in recent years it has become evident that even when the coassembly of these subunits can form a functional channel, in most cases the correct operation of the channel requires its specific location in a given area of the sarcolemma, its anchoring to the cytoskeleton and/or its binding to scaffold and signalling proteins that associate the channel with other channels, receptors or enzymes. This set of proteins constitute the channelosome and represent the structural and functional unit of the ionic channel. The sodium channelosome is shown in Figure.