Regulation of muscarinic acetylcholine receptors in cultured guinea pig pancreatic acini

Regulation of muscarinic receptors in cultured guinea pig pancreatic acini was investigated by assessing the effects of cholinergic agonists on binding of [N-methyl-³H]scopolamine ([³H]NMS) and on amylase release. Freshly dispersed acini bound [³H]NMS with a K(d) of 74 pM and a maximal binding level (B(max)) of 908 fmol/mg DNA. Carbachol (CCh) stimulated amylase secretion and inhibited [³H]NMS binding. Incubation of acini for 30 min with 0.1 mM CCh decreased the subsequent efficacy of CCh in stimulating amylase release by threefold but had no effect on its potency. In contrast, amylase release in response to cholecystokinin octapeptide (CCK-8) was not altered by CCh preincubation. [³H]NMS binding to acini was decreased only 15-20% after 30-min incubation with CCh. However, culture of acini with 0.1 mM CCh decreased [³H]NMS binding by 50% at 3-4 h and by 85-90% at 24 h. This decrease was attributable primarily to a reduction in B(max). [³H]NMS binding also was decreased to a similar extent by the cholinergic agonists bethanechol and metacholine but not by other secretagogues. The decrease in antagonist binding induced by CCh was dose dependent, with the IC₅₀, 5.8 μM, approximating the EC₅₀ for amylase release, 4.3 μM. Culture of acini for 24 h with CCh abolished subsequent amylase release in response to CCh but not to CCK-8. When CCh was removed from the culture medium after 24 h and acini recultured in its absence, [³H]NMS binding increased with a half-time for recovery of 20-24 h; this recovery was blocked by cycloheximide. These results indicate that muscarinic receptor turnover in the pancreatic acinus is regulated by receptor activation and that both a decrease in receptor numbers and sensitivity to agonists follows prolonged cholinergic agonist exposure.