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

• Published in: Cell calcium (Edinburgh) Vol. 29; no. 1; pp. 49 - 58
• Main Authors: Zerbes, M., Clark, C.L., Powis, D.A.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-01-2001
• ISSN: 0143-4160; 1532-1991
• DOI: 10.1054/ceca.2000.0160

Two potential mechanisms by which the intracellular Ca2 stores might modulate catecholamine release from bovine adrenal chromaffin cells were investigated: (i) that the cytosolic Ca2+transient caused by Ca2+release from the 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 depletion 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 of intracellular free Ca2+occurred owing to mobilization of Ca2+from the stores. When Ca2+was later readmitted to the perfusing fluid there followed a prompt and maintained rise in intracellular Ca2+concentrations of magnitude related to the degree of store mobilization. In parallel experiments, increased catecholamine secretion was measured under the conditions when Ca2+influx following store-mobilization occurred. Furthermore, the size of the catecholamine release increment correlated with the degree of Ca2+influx. Store-operated Ca2+entry evoked by mobilization with histamine and/or caffeine did not augment nicotine-evoked secretion per se; that is, it augmented evoked catecholamine release only to the extent that it increased basal catecholamine release. The nicotine-evoked catecholamine release was sensitive to cytosolic BAPTA, which, at the concentration used (50μM BAPTA-AM), reduced release by approximately 25%. However, the increment in basal catecholamine release which followed Ca2+influx triggered by Ca2+store mobilization was not reduced by intracellular BAPTA. This finding is inconsistent with the hypothesis that the elevated cytosolic Ca2+from store mobilization recruits additional vesicles of catecholamine to the sub-plasmalemmal release sites to augment subsequently evoked secretion. This position is supported by the observation that histamine (10μM) in Ca2+-free medium caused a pronounced elevation of cytosolic free Ca2+, but this caused no greater catecholamine release when Ca2+was re-introduced than did prior exposure to Ca2+-free medium alone, 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 through a store-operated entry pathway. The results do not support the notion that the Ca2+released from intracellular stores plays a significant role in the recruitment of vesicles into the ready-release pool under the experimental conditions reported here.