Down-regulation and recycling of high affinity cholecystokinin receptors on pancreatic acinar cells.

First incubating dispersed acini from rat pancreas with monensin, a cation ionophore that can inhibit recycling of receptors, inhibited binding of 125I-cholecystokinin 8 (125I-CCK-8) measured during a second incubation by as much as 50%. A maximal effect of monensin required 90 min of first incubation. Detectable inhibition of binding of 125I-CCK-8 occurred with 300 nM monensin, and inhibition increased progressively with concentrations of monensin up to 25 microM. Pancreatic acini possess two classes of receptors that bind 125I-CCK-8. One class has a high affinity (Kd = 461 pM) and a low capacity for CCK (512 fmol/mg DNA); the other class has a low affinity (Kd = 47 nM) and a high capacity for CCK (18 pmol/mg DNA). First incubating acini with monensin caused an 84% decrease in the number of high affinity CCK receptors with no change in the number of low affinity CCK receptors or the values of Kd for either class of receptors indicating that there is recycling of high affinity CCK receptors but not low affinity CCK receptors. First incubating acini with monensin did not alter CCK-stimulated amylase secretion indicating that in contrast to previous conclusions, occupation of low affinity CCK receptors mediates CCK-stimulated enzyme secretion. Moreover, the biphasic dose-response curve for CCK-stimulated enzyme secretion from monensin-treated acini suggests that pancreatic acini also possess a third, previously unrecognized class of very low affinity CCK receptors.