Sodium-calcium exchange affects local calcium signal decay and the rate ofexocytotic secretion in single chromaffin cells

The effects of Na+ deprivation on local calcium signal decay and the rate o f exocytotic secretion were measured in single bovine chromaffin cells to d etermine whether Na-Ca exchange influences the local cytosolic Ca2+ signal for neurohormone release. Na+ replacement with N-methylglucamine caused a m arked slowing of the decay of the local Ca2+ signal near points of its init iation, as measured by high-resolution fluorescent Ca2+ imaging in the conf ocal laser scanning microscope. Na+ replacement also resulted in a doubling of the rate and magnitude of exocytotic secretion measured in single cells by high-resolution microamperometry, Release rates provide an independent measure of local active zone Ca2+, Five repetitive stimulations of the same cell in Na+-free, but not in Na+-containing, medium resulted in a progress ively increasing rate of catecholamine release, suggesting an increasing le vel of active zone Ca2+ and a role of Na-Ca exchange activity in Ca2+ clear ance between stimulations. As secretory activity and its triggering Ca2+ si gnals are known to be co-localized in active zones along the plasma membran e, the results suggest that Na-Ca exchange may influence the decay of the l ocal Ca2+ signal for exocytotic secretion. This would be consistent with a contribution to local Ca2+ clearance by a novel mechanism utilizing the ins ertion of secretory vesicle Na-Ca exchangers into the plasma membrane durin g exocytosis.