Abstract
The regulation of isolated smooth muscle cells from the circular layer of the rabbit cecum by muscarinic receptors was studied in this paper. Binding of N-[3H]methylscopolamine was found to be specific, saturable (maximal binding capacity of about 325,000 sites/cell) and of high affinity (dissociation constant [KD) of 0.52 +/- 0.12 nM] The muscarinic M1-selective antagonist pirenzepine (PRZ), the muscarinic M2-selective AF-DX 116 (11-2[[2-[(diethyl-amino)methyl]-1-piperidinyl]acetyl]-5, 11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one) and the muscarinic M3-selective para-fluoro-hexahydro-sila-difenidol (p-F-HHSiD) inhibited N-[3H]methylscopolamine binding with respective inhibition constants (Ki) of (in nanomolar): 1018 +/- 382, 254 +/- 76 and 916 +/- 305. [3H]inositol phosphates accumulation was increased by carbachol (CCh) (EC50 of 3 +/- 1 microM). Antagonists competitively inhibited the CCh-induced [3H]inositol phosphates accumulation with the following order of potency: atropine > p-F-HHSiD > PRZ > AF-DX 116. In addition, CCh increased inositol-1,4,5-trisphosphate level in a time- and concentration-dependent fashion (EC50 of 1.5 +/- 0.5 microM). CCh inhibited both isoproterenol- and forskolin-induced cyclic AMP accumulation in isolated smooth muscle cells. Moreover, CCh inhibited forskolin-stimulated adenylate cyclase activity in smooth muscle homogenates (EC50 of 10.0 +/- 22.1 microM); the CCh-induced inhibition of forskolin-stimulated adenylate cyclase activity was reversed significantly by atropine and AF-DX 116, whereas PRZ and p-F-HHSiD were ineffective.(ABSTRACT TRUNCATED AT 250 WORDS)
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