DIFFERENTIAL ACTIVATION OF INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE CALCIUM POOLS BY MUSCARINIC RECEPTORS IN XENOPUS-LAEVIS OOCYTES

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 cytoplas mic calcium (Ca2+). The inositol phosphate and Ca2+-mobilization respo nses to ACh were analyzed in Xenopus oocytes possessing endogenous rec eptors, and in oocytes expressing exogenous receptors from injected mu scarinic RNA transcripts, to evaluate the patterns of signal transduct ion mediated by native and expressed receptors. Activation of native A Ch 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 o f inositol phosphate production at native (up to 35-fold) than at expr essed receptors (less than 2-fold). In contrast, measurements of Ca2+- mobilization in oocytes injected with the Ca2+-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 24 old response. These observati ons indicate that the oocyte possesses functionally distinct agonist-s ensitive Ca2+ pools which differ markedly in their sensitivity to Ins( 1,4,5)P3 production and suggest that these pools are mobilized by diff erent effector mechanisms. The finding that the magnitude of the intra -oocyte Ca2+ response is not necessarily determined by the degree of I ns(1,4,5)P3 production, but rather by another aspect of the signal tra nsduction 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 t he transduction mechanisms responsible for intracellular Ca2+ signalin g.