Role of calcium in cholecystokinin-stimulated phosphoinositide breakdown in exocrine pancreas

In dispersed acini from guinea pig pancreas cholecystokinin octapeptide (CCK-OP) stimulated breakdown of the phosphoinositides phosphatidylinositol (PI) and its phosphorylated derivative, phosphatidylinositol 4,5-bisphosphate (PI-P2), as measured by a decrease in the mass of PI and decreases in the content of [3H]PI and [32P]PI-P2 in acini prelabeled with myo-[2-3H]inositol or H3(32)PO4. The breakdown occurred in the absence of extracellular Ca2+ and when the CCK-OP-induced rise in free intracellular Ca2+ ([Ca2+]i) was ablated by loading the acini with the Ca2+-selective indicator and chelator quin-2 in the absence of extracellular Ca2+. In contrast to CCK-OP, the calcium ionophore A23187 caused breakdown of PI and PI-P2 in the presence but not in the absence of extracellular Ca2+, although like CCK-OP A23187 stimulated 45Ca outflux, a measure of cellular Ca2+ mobilization, and amylase release during the first 5-10 min of incubation independent of extracellular Ca2+. In the absence of extracellular Ca2+ A23187 did not inhibit the ability of CCK-OP to cause PI breakdown. These results indicate that CCK-OP stimulates breakdown of PI and PI-P2 and that this breakdown is independent of extracellular Ca2+, mobilization of intracellular Ca2+, and the CCK-OP-induced rise in [Ca2+]i. These findings suggest that one of the initial events resulting from CCK-OP interaction with its receptor is phosphoinositide breakdown.