Extracellular calcium has distinct effects on fast and slow components of the depolarization-induced secretory response from chromaffin cells

An increase in extracellular Ca²⁺ concentration from 0.26 to 10 mM enhanced secretion of norepinephrine and epinephrine induced by a high extracellular K⁺ concentration (76 mM). The increment in extracellular Ca²⁺ concentration also increased the observed peak inward Ca²⁺ current in response to long (10-s) depolarizing pulses from a holding potential of - 55 mV to +5 mV, from about -26 to -400 pA. However, the total amount of Ca²⁺ influx into the celt only increased when the extracellular Ca²⁺ concentration was raised from 0.25 to 1 mM and then remained constant up to 10 mM extracellular Ca²⁺. ATP is cosecreted with catecholamines following a depolarizing stimulus. Kinetic studies indicated that ATP secretion had two components with time constants, in the presence of 2.5 mM extracellular Ca²⁺, of ~4 and 41 s, being the fast component of secretion produced by the exocytosis of ~220 chromaffin granules. The results suggest that, for a given depolarizing stimulus, the size and rate of release for the fast and slow components of secretion are dependent on extracellular Ca²⁺ concentration.