AJP - Gastrointestinal and Liver Physiology, Vol 261, Issue 1 128-G135, Copyright © 1991 by American Physiological Society
Mechanism of agonist-induced downregulation of muscarinic receptors in rat pancreatic acini
S. R. Hootman, S. M. Valles and S. A. Kovalcik
Department of Physiology, Michigan State University, East Lansing 48824.
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.
