Modulation of rat pancreatic amylase secretion and muscarinic receptor populations by chronic bethanechol treatment

Abstract

F8P6effect of a chronic bethanechol treatment (12 mg X kg-1 X day-1 i.p., for 14 days) was investigated on pancreatic amylase secretion and muscarinic cholinergic receptors in the rat. Dispersed pancreatic acini were used to evaluate enzyme secretion and binding of [3H]N-methylscopolamine, [( 3H]NMS). The bethanechol treatment caused a 4 fold decrease in sensitivity of the pancreas for amylase release in the presence of carbamylcholine, the EC50 being shifted from 0.69 microM to 2.9 microM. Receptor concentration was reduced by 42%, from 3360 to 1930 fmol/mg DNA. The equilibrium dissociation constant (KD) of the receptors for the ligand remained unchanged at 0.17 nM. Binding analysis of carbachol on the muscarinic receptors in terms of two classes of binding sites indicated that the shift in the dose-response curve of amylase secretion was accompanied by modifications to the high and low affinity forms. The maximal number of high affinity sites remained the same while their affinity was greatly decreased from 0.24 to 6.1 microM. The low affinity form showed a moderate decrease in affinity from 34 to 150 microM but a large drop in their numbers from 2620 to 890 fmol/mg DNA. These results suggest that the shift in the amylase dose-response curve to carbamylcholine, noted following bethanechol treatment in vivo, could be coupled with the observed change in affinity of the two agonist forms of muscarinic receptor in the pancreas. However, gradual occupancy or formation of the high affinity form of muscarinic receptors by cholinergic agonist corresponds well with the gradual stimulation of amylase release.