Effect of cholecystokinin on the accumulation of inositol phosphates in isolated pancreatic islets.
Sulfated cholecystokinin octapeptide (CCK-8S) potentiated glucose-induced secretion in isolated pancreatic islets with a maximal effect at 12 mM glucose, whereas no effect was observed at 3.3 and 25 mM glucose. This effect of CCK-8S was maximal at 10(-7) M. Anion-exchange fast-protein liquid chromatography analysis of [3H]inositol phosphates derived from islets prelabeled with myo-[3H]inositol showed that glucose induced accumulation of the 1,4,5-isomer of inositol trisphosphate and of inositol tetrakisphosphate. At 3.3 mM glucose, CCK-8S stimulated accumulation of inositol trisphosphate and inositol tetrakisphosphate to levels induced by 25 mM glucose alone. The net effect of CCK-8S on the accumulation of the inositol phosphates was maximal at 12 mM glucose and decreased at higher glucose concentrations. At 12 mM glucose the accumulation of inositol phosphates increased gradually up to 10(-7) M CCK-8S. This study indicates that CCK-8S potentiates glucose-induced insulin secretion through a mechanism involving the hydrolysis of polyphosphoinositides and the generation of inositol phosphates. However, activation of the inositol cycle per se did not seem to induce insulin secretion.