Carbachol acts through protein kinase C to modulate cholecystokinin receptors on pancreatic acini.

Cholecystokinin (CCK) and cholinergic agonists are both major stimulants of pancreatic enzyme secretion and both utilize a common calcium-phosphoinositide-mediated receptor coupling system. In this study we investigated the modulation of pancreatic acinar CCK receptors by the muscarinic agonist carbachol (CCh) and investigated the intracellular mechanisms involved in the modulation. Acini were isolated from rat pancreas and dispersed in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-Ringer solution. Preincubation with 0.1 mM carbachol for 60 min reduced the CCK octapeptide (CCK-8; 100 pM)-stimulated amylase release by 43 +/- 5%. Binding of 125I-Bolton-Hunter-labeled CCK-8 (125I-BH-CCK-8) revealed two classes of CCK receptors, a high affinity with a dissociation constant (Kd) of 20 pM and a low affinity with a Kd of 2.3 nM. Pretreatment with 100 microM CCh decreased total binding by 35 +/- 6%, affecting the binding capacity of the high-affinity site, without change in the maximal binding capacity of the low-affinity site and no change in the Kd of either site. Preincubation of acini with 12-O-tetradecanoylphorbol 12,13-acetate (TPA, 1 microM), an activator of protein kinase C (PKC), decreased subsequent CCK-8-stimulated amylase release, and total binding of 125I-BH-CCK-8 to a similar extent as with pretreatment with CCh. The inhibitory effect of TPA or CCh on CCK-8-stimulated amylase release was reversed by simultaneous preincubation with H-7, an inhibitor of PKC. Pretreatment of acini with the calcium ionophore A23187, vasoactive intestinal peptide, or 8-bromoadenosine 3',5'-cyclic monophosphate had no effect on 125I-BH-CCK-8 binding. After CCh or TPA preincubation, CCK-8-stimulated production of [3H]inositol phosphates was inhibited by at least 49%.(ABSTRACT TRUNCATED AT 250 WORDS)