CATECHOLAMINE SECRETION, CALCIUM LEVELS AND CALCIUM INFLUX IN RESPONSE TO MEMBRANE DEPOLARIZATION IN BOVINE CHROMAFFIN CELLS

In this paper, we show that exposure of chromaffin cells to high K+ (7 5 mM) for 5 min releases about 15% of total norepinephrine and 8% of t otal epinephrine contained in chromaffin cells. The measured resting m embrane potential of these cells was -55 mV. Long (10 s) depolarizing electrical pulses applied from a holding potential of -55 mV to 5 mV, that would produce a depolarization similar to exposure to high K+ (75 mM), induced an inward Ca2+ current that inactivated with a time cons tant of about 0.8s and promoted the influx of about 1 fmol of Ca2+ int o the cell. Both high K+ and electrically-induced depolarization incre ased intracellular Ca2+ levels to a similar value (about 350 nM). Extr apolation would indicate that total Ca2+ influx in high K+ (75 mM)-sti mulated 10(6) chromaffin cells would amount to 1 nmol which would prom ote the secretion of about 4.9 nmol of norepinephrine and 3.5 nmol of epinephrine from 10(6) chromaffin cells. The results indicate that Ca2 + influx in response to depolarization is short-lived, likely due to C a2+-dependent inactivation of voltage-dependent Ca2+ channels. However , intracellular Ca2+ levels remain high as long as depolarization is p resent and long after Ca2+ influx has ceased. This would suggest that some processes related to either Ca2+ buffering or extrusion from the cell may be voltage dependent.