Cholecystokinin receptor occupation and cholecystokinin-induced calcium mobilization in the early phase in rat pancreatic acini

Abstract

We examined receptor occupation, calcium mobilization and amylase release for cholecystokinin octapeptide (CCK-8) within a 3-min incubation period at 37°C using dispersed acini from rat pancreas. Analysis of competitive binding inhibition data obtained after a 3-min incubation revealed the presence of only a single class of CCK receptors, while two classes of CCK receptor, i.e., high-affinity and low-affinity CCK receptors, were detected when binding reached a steady-state after a 60-min incubation. The IC 50 of CCK receptors calculated from the 3-min binding data was 19.0 ± 0.5 nM (mean ± S.D.), close to the K d of the low-affinity CCK receptors determined by equilibrium binding studies. Exposure of fura-2-loaded acini to 10–1000 pM CCK-8 caused an immediate and dose-dependent increase in [Ca 2+] i followed by a gradual decrease in [Ca 2+] i. The CCK-stimulated amylase release after 3 min of incubation was biphasic; amylase release increased over the dose range of 3–300 pM CCK-8, peaked at 300 pM CCK-8 and decreased with supramaximal concentrations of CCK-8. Our data suggest that occupation of the low-affinity, but not the high-affinity, CCK receptors is more directly associated with calcium mobilization and subsequent stimulation of amylase release in rat pancreatic acini.