Regulation of PI hydrolysis and cAMP formation by muscarinic M3 receptor in guinea pig gallbladder.

Carbachol (10(-8)-10(-3) M) produced two distinct biochemical responses in the guinea pig gallbladder smooth muscle: simulation of phosphoinositide (PI) hydrolysis and inhibition of forskolin-mediated adenosine 3',5'-cyclic monophosphate (cAMP) formation in a dose-dependent manner. The mean effective dose (ED50) concentration (1.6 x 10(-5) M) of carbachol-mediated stimulation of PI hydrolysis was 145 times greater than the ED50 concentration (1.1 x 10(-7) M) of carbachol mediated inhibition of cAMP formation. The inhibitory effect of carbachol on cAMP formation was antagonized by the pretreatment of pertussis toxin. To determine whether these two biochemical responses were mediated by the same or different subtypes of muscarinic receptors, the relative potencies of muscarinic receptor antagonists were calculated by Schild analysis. The M3 muscarinic antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) exhibited inhibitory constant (Ki) values at 0.3 and 1.2 nM in antagonizing the stimulation of PI hydrolysis and the inhibition of cAMP formation, respectively. The corresponding Ki values for pirenzepine, a muscarinic M1 antagonist, were 11 and 130 nM. The corresponding Ki values for AF-DX 116, a muscarinic M2 antagonist, were 34 and 450 nM. Thus 4-DAMP was 37x and 108x more potent than pirenzepine in antagonizing the stimulation of PI hydrolysis and the inhibition of cAMP formation, respectively. In addition, compared with AF-DX 116, 4-DAMP was 113x and 375x more potent in reducing stimulation of PI hydrolysis and inhibition of cAMP formation. Cholecystokinin (CCK) octapeptide (10(-10)-(10-6) M) caused a significant increase of PI hydrolysis but had no inhibitory effects on cAMP formation evoked by forskolin (10(-5) M).(ABSTRACT TRUNCATED AT 250 WORDS)