[Inactivation of calcium channels in the somatic membrane of mollusk neurons].

The time-dependent decrease of calcium inward currents (inactivation) at long (about 1s) depolarizing membrane potential shifts was studied on isolated dialysed neurons of the mollusc Helix pomatia. It is shown that the time course of this decrease can be approximated by two exponents with corresponding time constants 20-70 and 250-350 ms. The fast component of the decay disappeared completely after pH increase of the intracellular solution to 8.5. The kinetics of the slow component did not change. A conclusion is made that the fast decay of the recorded current does not represent variations in the calcium current but is due to parallel activation of a nonspecific outward current whereas the slow decay represents true inactivation of the calcium current. It is shown that the inactivation velocity of this current is related to the maximal value of the latter but not to the level of membrane depolarization, and depends on conditions of accumulation of calcium ions at the inner surface of the cell membrane.