[Ca2+]i recordings and the inactivation of the high-voltage activated Ca2+ currents in the adult rat sensory neuron.

Fast, single cell, measurement of the average cytosolic [Ca2+]i with the Fura-2 technique suggests that the depolarization induced [Ca2+]i rise is entirely due to entry through the voltage-activated Ca2+ channels. Involvement of a Ca(2+)-induced Ca(2+)-release process is not evident. Under physiological cytosolic buffering the current-induced [Ca2+]i rise persists for seconds and decays exponentially (tau = 7 s). Analysis of the [Ca2+]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 [Ca2+]i loads, and reappears when redistribution of free Ca2+ takes place. A suitable correction may be devised to compensate for the Ca(2+)-dependent inactivation.