Louis J. Ignarro

The objective of this study was to elucidate the close similarity in properties between endothelium-derived relaxing factor (EDRF) and nitric oxide radical (NO). Whenever possible, a comparison was also made between arterial and venous EDRF. In vascular relaxation experiments, acetylcholine and bradykinin were used as endothelium-dependent relaxants of isolated rings of bovine intrapulmonary artery and vein, respectively, and NO was used to relax endothelium-denuded rings. Oxyhemoglobin produced virtually identical concentration-dependent inhibitory effects on both endothelium-dependent and NO-elicited relaxation. Oxyhemoglobin and oxymyoglobin lowered cyclic guanosine monophosphate (cGMP) levels, increased tone in unrubbed artery and vein, and abolished the marked accumulation of vascular cGMP caused both by endothelium-dependent relaxants and by NO. The marked inhibitory effects of oxyhemoglobin on arterial and venous relaxant responses and cGMP accumulation as well as its contractile effects were abolished or reversed by carbon monoxide. These observations indicate that EDRF and NO possess identical properties in their interactions with oxyhemoproteins. Both EDRF from artery and vein and NO activated purified soluble guanylate cyclase by heme-dependent mechanisms, thereby revealing an additional similarity in heme interactions. Spectrophotometric analysis disclosed that the characteristic shift in the Soret peak for hemoglobin produced by NO was also produced by an endothelium-derived factor released from washed aortic endothelial cells by acetylcholine or A23187. Pyrogallol, via the action of superoxide anion, markedly inhibited the spectral shifts, relaxant effects, and cGMP accumulating actions produced by both EDRF and NO. Superoxide dismutase enhanced the relaxant and cGMP accumulating effects of both EDRF and NO. Thus, EDRF and NO are inactivated by superoxide in a closely similar manner. We conclude, therefore, that EDRF from artery and vein is either NO or a chemically related radical species.

BACKGROUND: Nitric oxide has been identified as an endothelium-derived relaxing factor in blood vessels. We tried to determine whether it is involved in the relaxation of the corpus cavernosum that allows penile erection. The relaxation of this smooth muscle is known to occur in response to stimulation by nonadrenergic, noncholinergic neurons. METHODS: We studied strips of corpus cavernosum tissue obtained from 21 men in whom penile prostheses were inserted because of impotence. The mounted smooth-muscle specimens were pretreated with guanethidine and atropine and submaximally contracted with phenylephrine. We then studied the smooth-muscle relaxant responses to stimulation by an electrical field and to nitric oxide. RESULTS: Electrical-field stimulation caused a marked, transient, frequency-dependent relaxation of the corpus cavernosum that was inhibited in the presence of N-nitro-L-arginine and N-amino-L-arginine, which selectively inhibit the biosynthesis of nitric oxide from L-arginine. The addition of excess L-arginine, but not D-arginine, largely reversed these inhibitory effects. The specific liberation of nitric oxide (by S-nitroso-N-acetylpenicillamine) caused rapid, complete, and concentration-dependent relaxation of the corpus cavernosum. The relaxation caused by either electrical stimulation or nitric oxide was enhanced by a selective inhibitor of cyclic guanosine monophosphate (GMP) phosphodiesterase (M&B 22,948). Relaxation was inhibited by methylene blue, which inhibits cyclic GMP synthesis. CONCLUSIONS: Our findings support the hypothesis that nitric oxide is involved in the nonadrenergic, noncholinergic neurotransmission that leads to the smooth-muscle relaxation in the corpus cavernosum that permits penile erection. Defects in this pathway may cause some forms of impotence.