Abstract
Fast, single cell, measurement of the average cytosolic [Ca 2+] i with the Fura-2 technique suggests that the depolarization induced [Ca 2+] i rise is entirely due to entry through the voltage-activated Ca 2+ channels. Involvement of a Ca 2+-induced Ca 2+-release process is not evident. Under physiological cytosolic buffering the current-induced [Ca 2+] i rise persists for seconds and decays exponentially (τ = 7 s). Analysis of the [Ca 2+] i changes during two-pulse protocols indicates that the purely voltage-dependent inactivation of the high voltage-activated (HVA) channels, in the range −80 +70 mV , is a slow process (0.2 – 1 s) which removes at most 40% of the current. On the contrary, Ca 2+-dependent inactivation acts in a fast way and it is therefore responsible for the fast inactivating phase of the current; this phase disappears under sustained [Ca 2+] i loads, and reappears when redistribution of free Ca 2+ takes place. A suitable correction may be devised to compensate for the Ca 2+-dependent inactivation.
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