Neurotransmitter release from bovine adrenal chromaffin cells is modulatedby capacitative Ca2+ entry driven by depleted internal Ca2+ stores

Two potential mechanisms by which the intracellular Ca2+ stores might modul ate catecholamine release from bovine adrenal chromaffin cells were investi gated: (i) that the cytosolic Ca2+ transient caused by Ca2+ release from th e intracellular stores recruits additional chromaffin granules to a readily releasable pool that results in augmented catecholamine release when this is subsequently evoked, and (ii) that the Ca2+ influx that follows depletio n of intracellular stores (i.e. store-operated Ca2+ entry) triggers release per se thereby augmenting evoked catecholamine release. When histamine or caffeine were applied in Ca2+-free perfusion media, a transient elevation o f intracellular free Ca2+ occurred owing to mobilization of Ca2+ from the s tores. When Ca2+ was later readmitted to the perfusing fluid there followed a prompt and maintained rise in intracellular Ca2+ concentrations of magni tude related to the degree of store mobilization. In parallel experiments, increased catecholamine secretion was measured under the conditions when Ca 2+ influx following store-mobilization occurred. Furthermore, the size of t he catecholamine release increment correlated with the degree of Ca2+ influ x. Store-operated Ca2+ entry evoked by mobilization with histamine and/or c affeine did not augment nicotine-evoked secretion per se; that is, it augme nted evoked catecholamine release only to the extent that it increased basa l catecholamine release. The nicotine-evoked catecholamine release was sens itive to cytosolic BAPTA, which, at the concentration used (50 muM BAPTA-AM ), reduced release by approximately 25%. However, the increment in basal ca techolamine release which followed Ca2+ influx triggered by Ca2+ store mobi lization was not reduced by intracellular BAPTA. This finding is inconsiste nt with the hypothesis that the elevated cytosolic Ca2+ from store mobiliza tion recruits additional vesicles of catecholamine to the sub-plasmalemmal release sites to augment subsequently evoked secretion. This position is su pported by the observation that histamine (10 muM) in Ca2+-free medium caus ed a pronounced elevation of cytosolic free Ca2+, but this caused no greate r catecholamine release when Ca2+ was re-introduced than did prior exposure to Ca2+-free medium atone, which caused no elevation of cytosolic free Ca2 +. It is concluded that intracellular Ca2+ stores can modulate secretion of catecholamines from bovine chromaffin cells by permitting Ca2+ influx thro ugh a store-operated entry pathway. The results do not support the notion t hat the Ca2+ released from intracellular stores plays a significant role in the recruitment of vesicles into the ready-release pool under the experime ntal conditions reported here. (C) 2001 Harcourt Publishers Ltd.