Neurotransmitter release from bovine adrenal chromaffin cells is modulated by capacitative Ca(2+)entry driven by depleted internal Ca(2+)stores.
Two potential mechanisms by which the intracellular Ca(2 stores might modulate catecholamine release from bovine adrenal chromaffin cells were investigated: (i) that the cytosolic Ca(2+)transient caused by Ca(2+)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 Ca(2+)influx that follows depletion of intracellular stores (i.e. store-operated Ca(2+)entry) triggers release per se thereby augmenting evoked catecholamine release. When histamine or caffeine were applied in Ca(2+)-free perfusion media, a transient elevation of intracellular free Ca(2+)occurred owing to mobilization of Ca(2+)from the stores. When Ca(2+)was later readmitted to the perfusing fluid there followed a prompt and maintained rise in intracellular Ca(2+)concentrations of magnitude 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 the catecholamine release increment correlated with the degree of Ca(2+)influx. Store-operated Ca(2+)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 (50 microM BAPTA-AM), reduced release by approximately 25%. However, the increment in basal catecholamine release which followed Ca(2+)influx triggered by Ca(2+)store mobilization was not reduced by intracellular BAPTA. This finding is inconsistent with the hypothesis that the elevated cytosolic Ca(2+)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 (10 microM) in Ca(2+)-free medium caused a pronounced elevation of cytosolic free Ca(2+), but this caused no greater catecholamine release when Ca(2+)was re-introduced than did prior exposure to Ca(2+)-free medium alone, which caused no elevation of cytosolic free Ca(2+). It is concluded that intracellular Ca(2+)stores can modulate secretion of catecholamines from bovine chromaffin cells by permitting Ca(2+)influx through a store-operated entry pathway. The results do not support the notion that the Ca(2+)released from intracellular stores plays a significant role in the recruitment of vesicles into the ready-release pool under the experimental conditions reported here.
Zerbes
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The Neuroscience Group, Faculty of Medicine and Health Sciences, The University of Newcastle, New South Wales, 2308, Australia.
Clark
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Powis
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D000313:Adrenal Medulla / Q000166:cytology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002417:Cattle; D002478:Cells, Cultured; D002614:Chelating Agents / Q000494:pharmacology; D019439:Chromaffin Cells / Q000166:cytology / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004837:Epinephrine / Q000378:metabolism*; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology; D005456:Fluorescent Dyes; D016257:Fura-2; D006632:Histamine / Q000494:pharmacology; D009638:Norepinephrine / Q000378:metabolism*; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D011978:Receptors, Nicotinic / Q000378:metabolism
D002614:Chelating Agents; D005456:Fluorescent Dyes; D010726:Phosphodiesterase Inhibitors; D011978:Receptors, Nicotinic; D002110:Caffeine; D004533:Egtazic Acid; D006632:Histamine; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium; D016257:Fura-2; D009638:Norepinephrine; D004837:Epinephrine