Characterization of muscarinic receptor subtypes on rat pancreatic acini: pharmacological identification by secretory responses and binding studies.

In order to identify subtypes of muscarinic receptor on the rat pancreas, the effects of new muscarinic receptor antagonists, [11-[[2-(diethylamino)-methyl]-1-piperidinyl]acetyl]-5, 11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepine-6-one (AF-DX 116) and 4-diphenylacetoxy-N-methylpiperadine-methiodide (4-DAMP), on amylase secretion stimulated by carbachol and binding of [3H]quinuclidinyl benzilate (QNB) were evaluated using isolated rat pancreatic acini. Atropine, pirenzepine, AF-DX 116 and 4-DAMP inhibited carbachol-stimulated amylase release in a dose-dependent manner. All these antagonists caused a concentration-dependent rightward shift of the dose-response curve for carbachol-stimulated amylase release without altering the maximal response. Schild plots revealed that pA2 values for atropine, pirenzepine, AF-DX 116 and 4-DAMP were 9.15, 6.78, 6.09 and 8.79, respectively. Every slope of Schild plots was not different from unity, suggesting that these antagonists act as competitive inhibitors. These antagonists also inhibited the binding of [3H]QNB in a dose-dependent manner. The inhibition constants were 1.21 x 10(-9) M (atropine), 1.26 x 10(-7) M (pirenzepine), 0.57 x 10(-6) M (AF-DX 116) and 2.75 x 10(-9) M (4-DAMP). Thus, the order of inhibitory potencies was atropine > or = 4-DAMP > pirenzepine > AF-DX 116. These findings suggest that 4-DAMP-sensitive M3 receptor may play an important role in the pancreatic exocrine functions.