Abstract

A pharmacological analysis was carried out in the rat urinary bladder to assess the nature of muscarinic receptors subtypes functionally involved in the negative feedback mechanism regulating acetylcholine (ACh) secretion from postganglionic cholinergic nerve terminals and in smooth muscle contraction. Bladder strips were preincubated with³H-choline, and the electrically evoked [³H]ACh release was detected simultaneously with contraction in the absence of acetylcholinesterase inhibitors. The effects were compared of seven muscarinic antagonists on [³H]ACh secretion (prejunctional effect) and muscle contraction (postjunctional effect). The rank order of postjunctional potencies (−log EC₅₀) for the seven antagonists (atropine > 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > hexahydrosiladiphenidol hydrochloride (HHSiD) > tripitramine > pirenzepine > AF DX-116 > methoctramine) as well as their postjunctional affinity estimates (pA₂) are in keeping with the notion that muscarinic receptors responsible for bladder contraction belong to the M₃ subtype. The M₃ subtype-preferring 4-DAMP and HHSiD did not discriminate between prejunctional and postjunctional effects. The M₂/M₄ subtype-preferring antagonists tripitramine, methoctramine and AF-DX 116 were more potent in facilitating the evoked [³H]ACh release than in inhibiting the contractile response. The rank order of prejunctional potencies was atropine > 4-DAMP > tripitramine > HHSiD > methoctramine > AF-DX 116 > pirenzepine, indicating the involvement of M₄ receptors. Furthermore, when potency relationship was determined by correlating prejunctional−log EC₅₀ values with published constants for cloned and natives muscarinic receptor subtypes, the correlations were significant for both M₄ and M₅ subtypes, but the best correlation found (P < .001) was for the M₄subtype. These findings suggest that the negative feedback mechanism inhibiting the release of ACh in the rat urinary bladder is mediated by prejunctional autoreceptors of the M₄ subtype.