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BACKGROUND The purpose of this study was to evaluate the muscarinic receptor subtypes expressed in rat bladder smooth muscle and characterize the muscarinic receptor-coupled phosphatidylinositol (PI) hydrolysis in order to clarify the first step of bladder smooth muscle contraction. METHODS Expressions of mRNAs of muscarinic receptor subtypes were examined by Northern blot analysis. Changes in the mass of inositol 1,4,5-trisphosphate (IP3) and the inhibitory effects of muscarinic subtype specific antagonists on PI hydrolysis were determined after carbachol stimulation. RESULTS mRNAs of m2 and m3 genes, encoding M2 and M3 receptors, were expressed in rat bladder smooth muscle. Carbachol produced a rapid increase of IP3, which returned to the basal level within 30 seconds. 4-Diphenylacetoxyl-N-methylpiperidine methiodide (4-DAMP; M1 and M3 antagonist) strongly inhibited the PI hydrolysis, but methoctramine (M2 antagonist) partially inhibited it at 10(-4) mol/L. The IC50 value for atropine was 9.5 x 10(-9) mol/L, for pirenzepine 6.4 x 10(-6) mol/L, and for 4-DAMP 1.5 x 10(-7) mol/L. CONCLUSIONS M2 and M3 receptors are expressed in rat urinary bladder. Only M3 receptor was involved in the production of IP3, which might induce the initial phase of contractile response in rat bladder smooth muscle after carbachol stimulation.
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