The caffeine-sensitive Ca 2+ store in bovine adrenal chromaffin cells; an examination of its role in triggering secretion and Ca 2+ homeostasis

Abstract

The effect of caffeine on catecholamine secretion and intracellular free Ca 2+ concentration ([Ca 2+] i) in bovine adrenal chromaffin cells was examined using single fura-2-loaded cells and cell populations. In cell populations caffeine elicited a large (∼200 nM) transient rise in [Ca 2+] i that was independent of external Ca 2+. This rise in [Ca 2+] i triggered little secretion. Single cell measurements of [Ca 2+] i showed that most cells responded with a large (> 200 nM) rise in [Ca 2+] i, whereas a minority failed to respond. The latter, whose caffeine-sensitive store was empty, buffered a Ca 2+ load induced by a depolarizing stimulus more effectively than those whose store was full. The caffeine-sensitive store in bovine chromaffin cells may be involved in Ca 2+ homeostasis rather than in triggering exocytosis.