Large Depolarization Induces Long Openings of Calcium Channels in Adrenal Chromaffin Cells Voltage-Dependent

Single Ca*+-channel currents in bovine adrenal chromaffin ceils were studied with the patch-clamp technique using Ba2+ as the charge carrier. Depolarizing pulses to voltages less than + 10 mV from holding voltage of -60 mV elicited short openings with a mean life time of less than 1 msec. Depolarization to more positive voltages elicited longer openings with a mean life time of about 3 msec in addition to the short openings similar to those observed at less positive voltages. Following large depolarizing prepulses, 2 types of “tail” openings, one with a mean duration of less than 1 msec and the other with a mean duration of 4 msec, were observed. In the presence of a dihydropyridine BAY K 8644, openings with a mean duration of more than 12 msec were present. Depolarization-induced long openings and BAY K 8644-produced long openings differed in the first latency and open-time properties. The results could be explained in terms of multiple open states of one type of Ca*+ channel. A kinetic model with at least 2 open states is required to explain activation of Ca*+ channels in chromaffin cells.