Characterization of the functional muscarinic receptors in the rat urinary bladder

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Abstract
Muscarinic receptors mediating contraction of the rat urinary bladder were characterized functionally in vitro by use of atropine, 4‐diphenylacetoxy‐N‐methylpiperidine methiodide (4‐DAMP methiodide), 4‐diphenylacetoxy‐N‐(2‐chloroethyl)‐piperidine hydrochloride (4‐DAMP mustard), hexahydro‐sila‐diphenidol hydrochloride (HHSiD), the p‐fluoro analogue of hexahydro‐sila‐diphenidol hydrochloride (p‐F‐HHSiD), methoctramine, and pirenzepine. (+)‐cis‐Dioxolane contracted bladder strips in a concentration‐dependent manner with an EC50 of 0.169±0.018 μm and an Emax of 7.84±0.67 g. Concentration‐effect curves to (+)‐cis‐dioxolane were shifted to the right in the presence of the antagonists in a concentration‐dependent manner. The rank order of antagonist affinities against the (+)‐cis‐dioxolane response was (pA2 values in the parentheses) atropine (9.28)≥4‐DAMP methiodide (9.04)>HHSiD (8.01) > p‐F‐HHSiD (7.28)=pirenzepine (7.12)≥ methoctramine (6.77, 7.25). The profile resembles that associated with the M3 receptor subtype. Atropine, 4‐DAMP methiodide, pirenzepine, and methoctramine had no effects on the contractile response to 120 mM KCl. However, HHSiD and p‐F‐HHSiD decreased the response to KCl, and 4‐DAMP mustard increased it. Contractile responses to electrical field stimulation (1–32 Hz, 0.05 ms pulse duration) were biphasic in nature. The tonic response was suppressed more than the phasic response by all antagonists except methoctramine. The suppression was not always concentration‐dependent, and did not seem to be related to antagonism of any one receptor subtype. Our findings are consistent with the minority M3 receptors mediating the contractile response to muscarinic stimulation by (+)‐cis‐dioxolane in the rat bladder.