Nitrergic relaxation in urethral smooth muscle: involvement of potassium channels and alternative redox forms of NO

We examined the contribution of K+ channels to the relaxation responses ind uced by different redox forms of nitric oxide (NO., NO- and NO+) in compari son with those evoked by electrical field stimulation (EFS) of nitrergic ne rves in the sheep urethra. K+ channel blockers with different selectivity p rofile were used. Sodium nitroprusside (SNP) and different S-nitrosothiols were used as NO+ donors, Angeli's salt as an NO- donor and nitroglycerin (G TN) was chosen as a representative compound known to require metabolic acti vation in the target tissue. Pure NO gas was used to prepare NO. solutions. Relaxation evoked by EFS of nitrergic nerves or by exogenous NO. was not in hibited by any of the K+ channel blockers, but was enhanced by 4-aminopyrid ine [inhibitor of voltage-dependent K+ (Kv) channels]. This suggests that, whereas K+ channel activation and hyperpolarization of the postsynaptic mem brane do not contribute to relaxation, prejunctional modulation of the nitr ergic neurotransmission by Kv channels may be relevant. Relaxation induced by NO+ or NO- donors was not affected by K+ channel blockade with the follo wing exceptions: glybenclamide, a blocker of ATP-sensitive K+ channels (K-A TP), enhanced responses to SNP and Angeli's salt, 4-aminopyridine inhibited relaxation evoked by Angeli's salt and GTN, and charybdotoxin, a blocker o f large-conductance, Ca2+-activated K+ channels (BKCa) inhibited those indu ced by the S-nitrosothiol S-nitrosoglutathione. These results do not sugges t the existence of a general mechanism of action on K+ channels for compoun ds releasing either NO+ or NO- in the sheep urethra. None of the K+ channel blockers affected relaxation induced by the membrane-permeable analogue of cGMP, 8-bromo-cGMP. However, the fact that the addition of the phosphodies terase inhibitor zaprinast (0.1 mM) enhanced the relaxation to Angeli's sal t, while preventing the inhibition induced by 4-aminopyridine, suggests tha t involvement of guanylate cyclase activation in the action of NO donors on K+ channels can not be excluded. Accordingly, the guanylate cyclase inhibi tors IH-[1,2,4]-oxadiazole-[4,3-a]-quinoxalin-1-one (ODQ, 10 muM) and 4H-8- bromo-1,2,4-oxadiazolo(3,4-d)benz(b)(1,4)oxazin-1-one (NS 2028, 10 muM) alm ost abolished relaxations to EFS and Angeli's salt. In contrast, ODQ only m oderately inhibited relaxations to NO.. In addition, the NO scavenger 2-(4- carboxyphenyl)-4,4,5,5-tetramethyl imidazoline-1-oxyl 3-oxide (carboxy-PTIO ) effectively inhibited responses to NO. whilst not affecting those to EFS or NO-, suggesting a close similarity between the nitrergic transmitter and nitroxyl ion. We conclude that nitrergic relaxation induced either by the endogenous tran smitter or by exogenous NO donors in the ovine urethra is not mediated by p ostsynaptic alterations in the K+ conductance; only a prejunctional modulat ion through Kv channels seems to be significant. In addition, the productio n and/or release of alternative redox forms of NO, such as NO-, may be invo lved in neurotransmission processes in the urethra.