Calcium channels in the cell membrane.

The development of the intracellular perfusion technique made isolated nerve cells an extremely convenient object for the detailed study of calcium channels, which allow the corresponding ions to enter the cell through the surface membrane during excitation. A wide range of investigations conducted in this object has shown that calcium channels, allowing the passage of bivalent cations in the order of preference Ba greater than Sr greater than Ca greater than Mg, bind these ions with the aid of a binding group located inside the channel. Other bivalent cations (Co, Ni, Mn, Cd), which bind too strongly with this group, become competitive channel blockers. In the absence of bivalent cations in the extracellular medium the calcium channels lose their selectivity and begin to transmit monovalent cations effectively; the reason for this transformation is detachment of the bound calcium ions from a special regulating group at the mouth of the calcium channels. Calcium channels can exist in two functional states: conducting and nonconducting. The transition between these states is accompanied by movement of charges inside the membrane ("gating currents"). The statistical kinetics of this transition, like the kinetics of gating currents, can be described by a modified Hodgkin-Huxley equation, with an activation variable raised to the power of 2. During long-term membrane depolarization the calcium channels pass into an inactivated state, which is connected with the recurrent blocking action of calcium ions, entering the cell, on the channels. Meanwhile, for some types of calcium channels, potential-dependent activation analogous to that in sodium or potassium channels is observed.