Ji_1993_Cell.Calcium_14_649

Muscarinic acetylcholine (ACh) receptors activate the phospholipase C signal transduction pathway to promote the formation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and the consequent elevation of cytoplasmic calcium (Ca2+). The inositol phosphate and Ca(2+)-mobilization responses to ACh were analyzed in Xenopus oocytes possessing endogenous receptors, and in oocytes expressing exogenous receptors from injected muscarinic RNA transcripts, to evaluate the patterns of signal transduction mediated by native and expressed receptors. Activation of native ACh receptors elicited dose- and time-dependent increases in Ins(1,4,5)P3 and inositol bisphosphate (InsP2) production. ACh-induced Ins(1,4,5)P3 production increased rapidly within the first 2 min and continued to rise over the next 20 min. ACh was a much more effective stimulus of inositol phosphate production at native (up to 35-fold) than at expressed receptors (less than 2-fold). In contrast, measurements of Ca(2+)-mobilization in oocytes injected with the Ca(2+)-specific photoprotein, aequorin, revealed that ACh stimulation of expressed receptors evoked up to 200-fold increase in light emission, whereas ACh stimulation of native receptors elicited less than a 2-fold response. These observations indicate that the oocyte possesses functionally distinct agonist-sensitive Ca2+ pools which differ markedly in their sensitivity to Ins(1,4,5)P3 production and suggest that these pools are mobilized by different effector mechanisms. The finding that the magnitude of the intra-oocyte Ca2+ response is not necessarily determined by the degree of Ins(1,4,5)P3 production, but rather by another aspect of the signal transduction pathway (e.g. the nature and/or location of the Ins(1,4,5)P3 releasable Ca2+ pool), reveals an additional level of complexity in the transduction mechanisms responsible for intracellular Ca2+ signaling.