We have previously shown that inositol trisphosphate (IP3) releases Ca2+ from a nonmitochondrial pool of permeabilized rat pancreatic acinar cells (Streb, H., Irvine, R. F., Berridge, M. J., and Schulz, I. (1984) Nature 306, 67-69). This pool was later identified as endoplasmic reticulum (Streb, H., Bayerdorffer, E., Haase, W., Irvine, R. F., and Schulz, I. (1984) J. Membr. Biol. 81, 241-253). As IP3 is produced by hydrolysis of phosphatidylinositol bisphosphate on activation of many "Ca2+-mobilizing receptors," our observation supported the proposal that IP3 functions as a second messenger to release Ca2+ from the endoplasmic reticulum. We have here used the same preparation of permeabilized acinar cells to study the relationship of secretagogue-induced Ca2+ release and IP3 production. We show that: 1) secretagogue-induced Ca2+ release in permeabilized cells is accompanied by a parallel production of inositol trisphosphate. 2) When the secretagogue-induced increase in intracellular free Ca2+ concentration was abolished by ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid buffering, secretagogue-induced IP3 production was unimpaired. 3) When secretagogue-induced IP3 production was reduced by inhibiting phospholipase C with neomycin, secretagogue-induced Ca2+ release was also abolished. 4) When the IP3 breakdown was reduced either by lowering the free Mg2+ concentration of the incubation medium or by adding 2.3-diphosphoglyceric acid, the rise in IP3 and the release of Ca2+ induced by secretagogues were both increased. These results further support the role of IP3 as a second messenger to induce Ca2+ mobilization.
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