Subtypes of muscarinic receptors in canine pancreatic secretion in vivo and in vitro.

In conscious dogs with esophageal, gastric and pancreatic fistulae, sham-feeding and meat feeding increased the pancreatic protein secretion to a peak, reaching about 39% and 69% of CCK8 maximum, and raised plasma pancreatic polypeptide (PP) levels. Pirenzepine given intravenously (i.v.) (30 nmol.kg-1 or 3 mumol.kg-1) reduced dose-dependently the pancreatic protein and plasma PP responses to sham-feeding and meat feeding, being about 100 times less potent as an inhibitor than atropine. Neither pirenzepine nor atropine affected near-maximal pancreatic bicarbonate and protein responses to secretin (164 pmol.kg-1.h-1) and CCK8 (170 pmol.kg-1.h-1), but both antimuscarinic agents significantly inhibited pancreatic responses to lower doses of these secretagogues. When added to the incubation medium of dispersed canine pancreatic acini, pirenzepine reduced dose-dependently the amylase responses only to urecholine, and not to CCK or gastrin, being about 1000 times less potent as an inhibitor than atropine. This report provides an evidence that pirenzepine inhibits pancreatic secretion in a similar manner to atropine, but that pirenzepine, in both in vivo and in vitro studies, is 2-3 orders of magnitude less potent as an inhibitor than atropine, indicating that the muscarinic pathway of the exocrine pancreas has a low affinity for pirenzepine and may thus involve M2-receptors.