Calcium currents in bullfrog sympathetic neurons. II. Inactivation.

Calcium currents in bullfrog sympathetic neurons inactivate slowly and partially during depolarizations lasting 0.5-1 s. There is also a slower (minutes) inactivation process with a broad voltage dependence. An irreversible loss of current (rundown) is prominent with low concentrations of intracellular Ca2+ buffers, with either Ca2+ or Ba2+ as the charge carrier. The extent and rate of the more rapid inactivation process are maximal near the voltage at which the peak inward current is generated, suggesting that inactivation might be Ca2+ dependent. However, inactivation occurs with either Ca2+ or Ba2+ as the charge carrier, is not prevented by strong buffering of intracellular Ca2+ with 10 mM BAPTA, and varies little as the peak current is changed 10-fold by changing the divalent ion concentration. That is, rapid inactivation is not explained by simple versions of voltage, Ca2+- or current-dependent inactivation models. A model in which ion binding within the channel allows a slower, rate-limiting inactivation process fits some but not all of the observed features of inactivation. A purely voltage-dependent three-state cyclic model fits the data if microscopic inactivation is favored by hyperpolarization.