Different effects of neuropeptide Y on electrically induced contractions in the longitudinal and circular smooth muscle layers of the female rabbit urethra

The effects of neuropeptide Y (NPY) on preparations of isolated longitudinal and circular smooth muscle from rabbit urethra were studied. In both types of muscle, electrically induced contractions and relaxations could be abolished by tetrodotoxin, (TTX). In the longitudinal muscle preparations the contraction was slightly reduced by prazosin, but markedly reduced by scopolamine and NPY. The NPY effect was not influenced by pretreatment with rauwolscine. Pretreatment with NPY had no effect on contractions induced by noradrenaline (NA) or carbachol and the peptide did not relax preparations contracted by these agents. In circular muscle an initial, fast response, not sensitive to prazosin or scopolamine was occasionally observed following electrical stimulation. A slow contraction component was regularly seen; this response was abolished by prazosin. Neuropeptide Y did not influence any of these responses. The preparations were concentration-dependently contracted by NA, whereas carbachol had no effect. Pretreatment with NPY did not affect contractions induced by NA, nor did the peptide relax NA-contracted preparations. In neither longitudinal nor circular muscle strips did NPY affect the electrically induced TTX sensitive relaxation of NA-contracted preparations. The results suggest that in the rabbit urethra NPY reduces contractions in the longitudinal muscle layer by selectively inhibiting the release of acetylcholine from cholinergic nerves. Neuropeptide Y did not appear to have any significant postjunctional effects nor to interfere with the release, or effects of NA or other transmitter agents. The physiological importance of the urethral effects of NPY remains to be established.