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+ (75 mM) for 5 min releases about 15% of total norepinephrine and 8% of total epinephrine contained in chromaffin cells. The measured resting membrane 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 constant of about 0.8 s and promoted the influx of about 1 fmol of Ca2+ into the cell. Both high K+ and electrically-induced depolarization increased intracellular Ca2+ levels to a similar value (about 350 nM). Extrapolation would indicate that total Ca2+ influx in high K+ (75 mM)-stimulated 10(6) chromaffin cells would amount to 1 nmol which would promote 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 Ca(2+)-dependent inactivation of voltage-dependent Ca2+ channels. However, intracellular Ca2+ levels remain high as long as depolarization is present 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.