Neurotransmission and drug effects in urethral smooth muscle

Urethral smooth muscle tension is one of the factors contributing to the maintenance of intraurethral pressure and to continence. This tension is controlled by nervous mechanisms involving a complex pattern of efferent and afferent signalling in parasympathetic, sympathetic, and somatic nerves, with resulting release of contraction- and relaxation-producing transmitters and mediators. Sympathetic activity, via release of NA and stimulation of urethral smooth muscle alpha1-ARs, is the main contraction-producing factor. Among the three high affinity alpha1-AR subtypes identified in molecular cloning and functional studies, the alpha1A subtype seems to predominate in both the male and female urethra. The physiological role of the urethral beta-adrenoceptors has not been established, and the importance of muscarinic receptors (and subtypes) for urethral function is presently unclear. Nitric oxide, produced by nitric oxide synthase within cholinergic nerves, seems to be the predominant inhibitory neurotransmitter, but there is good evidence for the existence of other, as yet unidentified, non-adrenergic, non-cholinergic inhibitory messengers. Vasoactive intestinal polypeptide, adenosine 5'-triphosphate, and carbon monoxide (CO) all relax urethral smooth muscle, and are potential candidates for the nonadrenergic, noncholinergic relaxation, but their roles for urethral function remain to be defined.