Cholecystokinin-induced alterations in beta-cell sensitivity. Duration, specificity, and involvement of phosphoinositide metabolism.

Prior exposure of isolated perifused rat islets to the sulfated gut hormone cholecystokinin-8 (CCK-8S) dramatically increased their insulin secretory response to 7.5 mM glucose, 10 mM arginine, and 10 mM alpha-ketoisocaproate. In the case of glucose, the heightened secretory response was still apparent 60-80 min after CCK-8S removal from the perifusion medium. Prior exposure of perifused islets to arginine (10 mM), tolbutamide (25 microM), or forskolin (1.0 microM) did not sensitize them to 7.5 mM glucose. CCK-8S exposure increased 3H efflux from islets prelabeled with [3H]inositol, and the increase in 3H efflux was sustained after CCK-8S removal from the perifusion medium. The duration of this increase in 3H efflux paralleled the temporal characteristics of this sensitization process and was significantly attenuated by 25 microM asperlicin, a competitive antagonist of CCK binding to its membrane receptor. Arginine, tolbutamide, or forskolin treatment of islets did not increase 3H efflux from [3H]inositol-prelabeled islets. The results suggest that the turnover of membrane phosphoinositides induced by CCK-8S is largely responsible for this heightened state of secretory responsiveness to various stimulants. Second-messenger molecules generated during phosphoinositide turnover may be responsible for the phenomenon of sensitization displayed by islet tissue to CCK-8S addition.