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Abstract

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.