Sodium-dependent calcium efflux from adrenal chromaffin cells following exocytosis: Possible role of secretory vesicle membranes

Although cytosolic Ca²⁺ transients are known to influence the magnitude and duration of hormone and neurotransmitter release, the processes regulating the decay of such transients after cell stimulation are not well understood. Na⁺-dependent Ca²⁺ efflux across the secretory vesicle membrane, following its incorporation into the plasma membrane, may play a significant role in Ca²⁺ efflux after stimulation of secretion. We have measured an enhanced ⁴⁵Ca²⁺ efflux from cultured bovine adrenal chromaffin cells following cell stimulation with depolarizing medium (75 mM K⁺) or nicotine (10 μM). Such stimulation also causes Ca²⁺ uptake via voltage-gated Ca²⁺ channels and secretion of catecholamines. Na⁺ replacement with any of several substitutes (N-methyl-glucamine, Li⁺, choline, or sucrose) during cell stimulation inhibited the enhanced ⁴⁵Ca²⁺ efflux, indicating an Na⁺-dependent Ca²⁺ efflux process. Na⁺ deprivation did not inhibit ⁴⁵Ca²⁺ uptake or catecholamine secretion evoked by elevated K⁺. Suppression of exocytotic incorporation of secretory vesicle membranes into the plasma membrane with hypertonic medium (620 mOsm) or by lowering temperature to 12 °C inhibited K⁺-stimulated ⁴⁵Ca²⁺ efflux in Na⁺-containing medium but did not inhibit the stimulated ⁴⁵Ca²⁺ uptake. Enhancement of exocytotic secretion with pertussis toxin resulted in an enhanced ⁴⁵Ca²⁺ efflux without affecting calcium uptake. The combined results suggest that Na⁺-dependent Ca²⁺ efflux across secretory vesicle membranes, following their incorporation into the plasma membrane during exocytosis, plays a significant role in regulating calcium efflux and the decay of cytosolic Ca²⁺ in adrenal chromaffin cells and possibly in related secretory cells.