THE POSSIBLE EFFECT OF NITRIC-OXIDE ON RELAXATION AND NORADRENALINE RELEASE IN THE ISOLATED RABBIT URETHRA

We evaluated the effects of N-omega-nitro-L-arginine (L-NNA, a nitric oxide (NO) synthase inhibitor) and oxy-2-phenyl-4,4,5,5-tetramethylimi dazoline-1-oxyl 3-oxide (carboxy-PTIO, a NO scavenger) on NO-mediated relaxation and noradrenaline release from adrenergic nerve endings ind uced by electrical field stimulation in the rabbit urethra. Electrical field stimulation caused frequency-dependent relaxation of rabbit ure thral smooth muscles precontracted with phenylephrine. The relaxation responses were significantly inhibited by treatment with L-NNA or carb oxy-PTIO. The inhibitory effect of carboxy-PTIO was significantly weak er than that of L-NNA. Electrical field stimulation caused significant noradrenaline release from adrenergic nerve endings in the rabbit ure thra. Treatment with carboxy PTIO enhanced electrical field stimulatio n-induced noradrenaline release, and simultaneous application of L-NNA and carboxy-PTIO did not further enhance noradrenaline release in the rabbit urethra. As carboxy-PTIO reacts only with the free radical NO, the present results suggest that free radical NO and NO-containing co mpounds are involved in the L-NNA-sensitive nitrergic nerve-mediated r elaxation in the rabbit urethra. At the same time free radical NO has a prejunctional action by which it may inhibit noradrenaline release f rom adrenergic nerves. (C) 1998 Elsevier Science B.V. All rights reser ved.