Regulation of PI hydrolysis and cAMP formation by muscarinic M3 receptor in guinea pig gallbladder
Takahashi, T.; Kurosawa, S.; Owyang, C.
American Journal of Physiology 267(4 Pt 1): G523-G528
1994
ISSN/ISBN: 0002-9513 PMID: 7943317 Document Number: 428805
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 (ED-50) concentration (1.6 times 10-5 M) of carbachol-mediated stimulation of PI hydrolysis was 145 times greater than the ED-50 concentration (1.1 times 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 M-3 muscarinic antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) exhibited inhibitory constant (K-i) values at 0.3 and 1.2 nM in antagonizing the stimulation of PI hydrolysis and the inhibition of cAMP formation, respectively. The corresponding K-1 values for pirenzepine, a muscarinic M-1 antagonist, were 11 and 130 nM. The corresponding K-i values for AF-DX 116, a muscarinic M-2 antagonist, were 34 and 450 nM. Thus 4-DAMP was 37 times and 108 times 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 113 times and 375 times 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). Therefore, we conclude that muscarinic M-3 receptors of the gallbladder smooth muscles are coupled to PI hydrolysis and adenylate cyclase system, whereas CCK receptors are coupled to only PI hydrolysis.