AJP - Gastrointestinal and Liver Physiology, Vol 251, Issue 2 275-G279, Copyright © 1986 by American Physiological Society
Muscarinic receptor subtypes on rat pancreatic acini: secretion and binding studies
D. S. Louie and C. Owyang
Characterization of muscarinic receptor subtypes on rat pancreatic acinar
cells was examined by using specific muscarinic receptor antagonists to
study amylase secretion and binding of [N-methyl-3H]scopolamine ([3H]NMS).
Rat pancreatic acini were dispersed in HEPES-Ringer buffer and incubated
with acetylcholine +/- 4-diphenylacetoxy-N-methylpiperadine-methiodide
(4-DAMP, a specific M2 muscarinic receptor antagonist) or +/- pirenzepine
(a specific M1 muscarinic receptor antagonist). 4-DAMP (10(-9) to 10(-6) M)
caused a progressive parallel rightward shift in the acetylcholine
dose-response curve without a change in maximal amylase release. Only high
concentrations of pirenzepine (10(-6) to 10(-4) M) caused a rightward shift
in the dose-response curve to acetylcholine. Schild analysis of the data
indicated an inhibitory constant (Ki) of 200 pM for 4-DAMP and 183 nM for
pirenzepine. The slope of the Schild regression lines was not different
from unity, suggesting competitive inhibition. Binding of 50 pM [3H]NMS was
specific, rapid, and saturable. [3H]NMS binding was displaced by increasing
concentrations of 4-DAMP or pirenzepine with apparent Ki's of 102 pM and
330 nM, respectively, and similar maximal binding levels of 60 fmol/mg
prot. We have demonstrated that 4-DAMP has an approximately 1,000-fold
greater potency than pirenzepine to inhibit amylase release and binding,
indicating that cholinergic-stimulated amylase release from pancreatic
acini is mediated by M2 muscarinic receptors.
