PRESENCE OF NA+/CA2+ EXCHANGE ACTIVITY AND ITS ROLE IN REGULATION OF INTRACELLULAR CALCIUM-CONCENTRATION IN BOVINE ADRENAL CHROMAFFIN CELLS

The presence of a Na+/Ca2+ exchanger in bovine adrenal chromaffin cells was demonstrated by measuring the efflux of Ca-45(2+) which had been preloaded into cells by a brief depolarization. The efflux of Ca-45(2+) was dependent on extracellular Na+ (Na+o); Ca-45(2+) efflux was significantly decreased by replacing Na+o with N-methylglucamine (NMG), or Li+. Replacement of Na+o by NMG increased the resting intracellular Ca2+ concentration ([Ca2+]i) of freshly isolated chromaffin cells. This could be reversed by adding Na+, suggesting that Na+/Ca2+ exchanger activity was involved in maintaining [Ca2+]i at its resting level. The initial rate of Na+-dependent [Ca2+]i recovery after Ca2+ loading by depolarization was dependent on the level of [Ca2+]i. There was an apparent linear relationship between the activity of the Na+/Ca2+ exchanger and [Ca2+]i both in the presence and absence of Na+o. When cells were treated with other stimuli, including 10-mu-M DMPP or 40 mM caffeine, the ability of the stimulated cells to decrease [Ca2+]i was significantly reduced upon replacing Na+o with NMG. Our data show that the Na+/Ca2+ exchanger is one of the major pathways for regulating [Ca2+]i in chromaffin cells in both resting and stimulated states.