MEDIATORS AND MECHANISMS OF RELAXATION IN RABBIT URETHRAL SMOOTH-MUSCLE

1 Electrophysiological and mechanical experiments were performed to in vestigate whether the nitric oxide (NO)-mediated relaxation of rabbit urethral smooth muscle is associated with a hyperpolarization of the m embrane potential. In addition, a possible role for vasoactive intesti nal peptide (VIP) and carbon monoxide (CO) as relaxant agents in rabbi t urethra was investigated. 2 Immunohistochemical experiments were per formed to characterize the NO-synthase (NOS) and VIP innervation. Poss ible target cells for NO were studied by using antisera against cyclic GMP. The cyclic GMP-immunoreactivity was investigated on tissues pret reated with 1 mM IBMX, 0.1 mM zaprinast and 1 mM sodium nitroprusside. 3 Intracellular recordings of the membrane potential in the circular smooth muscle layer revealed two types of spontaneous depolarizations, slow waves with a duration of 3-4 s and an amplitude of 30-40 mV, and faster (0.5-1 s), more irregular depolarizations with an amplitude of 5-15 mV. The resting membrane potential was 39+/-1 mV (n=12). Applica tion of NO (30 mu M), CO (30 mu M) or VIP (1 mu M) did not change the resting membrane potential. 4 Both NO (1-100 mu M) and VIP (1 nM-1 mu M) produced concentration-dependent relaxations amounting to 87+/-4% a nd 97+/-2% (n=6), respectively. The relaxant effect of CO (1-30 mu M) amounted to 27+/-4% (n=5) at the highest concentration used. 5 Immunoh istochemical experiments revealed a rich supply of NOS immunoreactive nerve fibres in the smooth muscle layers. Numerous spinous cyclic GMP- immunoreactive cells were found interspersed between the smooth muscle bundles, mainly localized in the outer layer. These cells had long pr ocesses forming a network surrounding the smooth muscle bundles. VIP-i mmunoreactivity was sparse in comparison to NOS-immunoreactive nerves. 6 The rich supply of NOS-immunoreactive nerve fibres supports the vie w that NO is an important NANC-mediator in the rabbit urethra. In cont rast to several other tissues, the relaxant effect of NO in the rabbit urethra does not seem to be mediated by hyperpolarization. The networ k of cyclic GMP-immunoreactive cells may constitute target cells for N O, but their function remains to be established.