DISTINCT MUSCARINIC RECEPTORS AND SIGNAL-TRANSDUCTION PATHWAYS IN GALLBLADDER MUSCLE

Acetylcholine (ACh) caused a dose-dependent contraction of gallbladder muscle cells in either a normal (1.9 mM) Ca2+, zero-Ca2+ or 4 mM Sr2 medium, with a maximal contraction about 21 +/- 1% at 10(-6) M. Piren zepine, methoctramine and p-flouro-hexahydro-sila-difenidol (the M1, M 2 and M3 antagonist, respectively) alone had no inhibitory effect on A Ch-induced contraction in normal Ca2+ medium, which was blocked by the combination of methoctramine and p-F-HHSiD. In the 4 mM Sr2+ medium, methoctramine dose dependently inhibited ACh-induced contraction and s hifted the ACh dose-response curve to the right. The contraction induc ed by ACh was further blocked by 10(-4) M propranolol (phosphatidic ac id phosphohydrolase inhibitor that prevents the production of diacylgl ycerol from phospholipase D activation), 10(-5) M H-7 and chelerythrin e (the protein kinase C inhibitors) by 64%, 75% and 77%, respectively. In contrast, in the zero-Ca2+ medium, p-flouro-hexahydro-sila-difenid ol dose-dependently inhibited ACh-induced contraction and shifted the ACh dose-response curve to the right. The action of ACh was further bl ocked by 10(-6) M U-73122 (phospholipase C inhibitor) and 10(-5) M CGS 9343B (calmodulin antagonist) by 95% and 77%, respectively. In conclu sion, ACh contracts the gallbladder muscle by stimulating the M2 and M 3 muscarinic receptors. The M2 receptors are linked to Ca2+ influx, ac tivation of phospholipase D and protein kinase C-dependent pathway, wh ereas the M3 receptors are preferentially associated with the activati on of phospholipase C, intracellular Ca2+ release and calmodulin-depen dent pathway.