On a cholinergic motor innervation of the rat urethra
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Choline acetyltransferase activity was demonstrated in the proximal part of the male rat urethra, indicating a cholinergic innervation of this tissue. The cholinergic nerve fibres emanated evidently from the pelvic nerves, since bilateral removal of the pelvic ganglion caused a major fall in the activity of this enzyme. The muscle activity of the circular layer of the proximal urethra was recorded in vitro. The basal activity of this segment was low. The parasympathomimetics acetylcholine and methacholine, evoked rapid and marked contractile responses; the maximal responses to these drugs were 36 and 44%, respectively, of that to potassium. The corresponding figures for phenylephrine and noradrenaline were found to be 79 and 88%, respectively. The responses evoked by the parasympathomimetics were unaffected by the ganglion blocker hexamethonium, the alpha-adrenoceptor blocker dihydroergotamine and the beta-adrenoceptor blocker propranolol. Atropine, however, abolished the responses completely. Following degeneration of adrenergic or cholinergic nerves of the urethra the parasympathomimetics still evoked contractions. Taken together these findings indicate that the parasympathomimetics exert their contractile effect through a direct action on muscarinic receptors. Parasympathectomy but not sympathectomy (caused by 6-hydroxydopamine treatment) gave rise to a supersensitivity to methacholine, as judged by a leftward shift of the dose-response curve for this drug, the ED50-value being ten times less than that of the controls. The observations seem to suggest that the proximal urethra normally is under the influence of cholinergic activity beside that of adrenergic activity previously demonstrated.