Multiple calcium-dependent processes related to secretion in bovine chromaffin cells

We have used the caged calcium compound DM-nitrophen to investigate the kinetics of calcium-dependent secretion in bovine chromaffin cells. Perfusion with partially calcium-loaded nitrophen often caused a loading transient--slow secretion for up to 1 min due to displacement of Ca2+ by cytoplasmic Mg2+. Flash photolysis elicited 100 microM [Ca2+]i steps that evoked intense secretion, lasting a few seconds. In cells experiencing a loading transient, [Ca2+]i steps evoked an especially fast secretion. A persistent, slow secretion often followed these fast phases. Distinct kinetic components may reflect secretion from pools that are differentially capable of release. Both secretion and movement of vesicles between pools appear to be [Ca2+]i sensitive. Later [Ca2+]i steps sometimes evoked a rapid capacitance decrease, indicating a fast, [Ca2+]i-dependent phase of endocytosis.