Pharmacological coupling and functional role for the muscarinic receptor subtypes in isolated cells from the circular smooth muscle of the rabbit cecum.

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)