Mechanism of agonist-induced downregulation of muscarinic receptors in rat pancreatic acini.

Exposure of pancreatic acini to cholinergic agonists reduces their muscarinic acetylcholine receptor content. To investigate the mechanism of this reduction, rat pancreatic acini were cultured in the presence of carbachol and its effects on binding of [N-methyl-3H] scopolamine (NMS), which labels only cell surface receptors, and [3H]scopolamine, which also accesses intracellular receptors, were determined. Carbachol (0.1 mM) caused disappearance of 90% of binding sites for [3H]NMS and [3H]-scopolamine with half-time values of 3.9 +/- 0.4 and 5.7 +/- 0.7 h, respectively. Nocodazole and cytochalasin D, ouabain, ionophore A23187, 12-O-tetradecanoylphorbol 13-acetate (TPA), bombesin, cholecystokinin octapeptide (CCK-8), secretin, and vasoactive intestinal peptide (VIP) had no effect on the carbachol-induced decrease in binding sites for either antagonist, although staurosporine and the calmodulin inhibitor W-7 each caused slight inhibition. By contrast, the lysosomotropic agents methylamine and ammonium chloride caused 80% inhibition of disappearance of binding sites for [3H]scopolamine, although they only slightly inhibited disappearance of [3H]NMS binding sites. These results implicate the endosomal/lysosomal pathway in cholinergic agonist-evoked downregulation of muscarinic receptors in the pancreatic acinar cell.