This study was designed to examine the mechanism of penile erection in adult bull by analyzing the responses of bovine proximal retractor penile muscle strips(BRP) to electrical field stimulation(EFS), exogenous nitric oxide(NO), NO synthesis precursor(L-arginine), NO synthase inhibitors(L-NAME, L-NMMA), guanylate cyclase inhibitor(methylene blue) and nonspecific potassiun channel blocker(tetraethylammonium, TEA) treatments. Isometric tension of BRP was measured using physiograph. Results were summarized as follows:
1. EFS of nonadrenergic noncholinrgic(NANC) nerve in BRP produced frequency-dependent inhibitory responses to the contraction induced by co-treatment of epinephrine, guanethidine and atropine. The inhibitory responses to EFS were blocked by tetrodotoxin(TTX, 1μM).
2. Treatment of L-NAME (10, 20μM) inhibited the relaxation to EFS whereas L-NMMA (100μM) had no effect.
3. Treatment of NO(20, 40μM; as an acidified solution of NaNO₂) induced concentration-dependent relaxation whereas preincubation of TTX(1μM) and L-NAME(20μM) had no effect on the relaxation response.
4. L-arginine treatment(lOmM) blocked the inhibitory effect of L-NAME(20μM).
5. Pretreatment of methylene blue(40μM) reduced the NANC-induced relaxation of BRP.
6. Tetraethylammonium(TEA, 80mM) reduced NANC relaxation.
These results suggest that NO may act as a NANC neurotransmitter in BRP and the effects might be mediated by cGMP and potassium channel.
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