Involvement of nitric oxide and vasoactive intestinal peptide in the nonadrenergic-noncholinergic relaxation of the porcine retractor penis muscle

Neurotransmitters mediating nonadrenergic-noncholinergic (NANC) relaxation were investigated in strips of porcine retractor penis muscle (RPM). Muscle tone was raised by phenylephrine (1 muM) in the presence of atropine (1 mu M) and guanethidine (50 muM). Upon electrical field stimulation (1 ms, 80 V , 1 - 32 Hz for 10 s), the initial fast relaxation was followed by the slow relaxation. Although the fast and the slow relaxation were completely abol ished by tetrodotoxin (1 muM), they showed different pharmacological sensit ivities to the nitric oxide (NO) synthase inhibitor NO-nitro-L-arginine met hyl ester (L-NAME, 0.1 mM). The fast relaxation was markedly inhibited by L -NAME in an L-arginine reversible manner and by oxyhemoglobin (50 muM), whi le the slow relaxation was hardly blocked by L-NAME. L-NAME and alpha -chym otrypsin (alpha -CT, 3 U/ml) selectively inhibited the fast and the slow re laxation, respectively. alpha -CT abolished L-NAME-resistant slow relaxatio n, and L-NAME completely abolished the alpha -CT-resistant fast relaxation. alpha -CT-resistant relaxation was not significantly different from the di gitally calculated L-NAME-sensitive component, and L-NAME-resistant relaxat ion was similar to the digitally calculated alpha -CT-sensitive component. Vasoactive intestinal peptide (VIP, 0.003 - 0.1 muM) relaxed porcine RPM in a concentration-dependent manner. The effect of a VIP was partially inhibi ted by a VIP receptor antagonist, VIP(10-28) (1 and 3 muM). L-NAME-resistan t relaxation was also reduced by VIP(10-28) (3 muM) and by another putative antagonist, VIP(6-28) (1 muM), although the effects of the two antagonists were somewhat inconsistent. From the histochemical staining, it was verifi ed that nerve bundles that showed VIP-like immunoreactivities were also pos itive for the NADPH diaphorase reaction. These results suggest that NO and peptide neurotransmitter(s) including VIP mediate the NANC relaxation in po rcine RPM.