Biosynthesis of nitric oxide and citrulline from L-arginine by constitutive nitric oxide synthase present in rabbit corpus cavernosum.

The objective of this study was to determine whether a constitutive isoform of nitric oxide (NO) synthase is present in rabbit corpus cavernosum that could account for the involvement of the L-arginine-NO pathway in neurogenically-elicited relaxation of the corpus cavernosum and, therefore, penile erection. Citrulline was determined by monitoring the formation of 3H-citrulline from 3H-L-arginine. NO was determined by monitoring the formation of total NO(x) (NO+nitrite [NO2-]+nitrate [NO3-]) by chemiluminescence after reduction of NO(x) to NO by acidic vanadium (III). Equimolar quantities of NO plus citrulline were generated from L-arginine and the formation of both products was time-dependent at 37 degrees C. NO synthase activity was distributed almost entirely to the cytosolic fraction. Enzymatic activity was completely dependent on NADPH, calmodulin, and calcium. Addition of tetrahydrobiopterin increased NO synthase activity by about 30 percent. The NO synthase inhibitor NG-nitro-L-arginine, abolished enzymatic activity. The Km for L-arginine was 17 microM and the Vmax of the reaction was 18 pmol/min/mg protein. These observations indicate that a cytosolic, constitutive isoform of NO synthase, like that found in brain neuronal tissue, is present in rabbit corpus cavernosum.

Nitric oxide is a potent relaxant of human and rabbit corpus cavernosum.

Nitric oxide (NO) caused a potent, marked, and transient relaxation of precontracted strips of corpus cavernosum isolated from humans and rabbits. The relaxation response elicited by NO was very similar to the relaxation evoked by electrical field stimulation via the nonadrenergic-noncholinergic pathway. Sodium nitroprusside, nitroglycerin, and S-nitroso-N-acetylpenicillamine, which are nitrovasodilators known to generate NO, also caused marked concentration-dependent relaxation of corpus cavernosum. Relaxant responses to NO were enhanced by the cyclic GMP phosphodiesterase inhibitor M&B 22,948 and inhibited by oxyhemoglobin. Similarly, relaxation of corpus cavernosum in response to electrical field stimulation or acetylcholine was enhanced by M&B 22,948 and inhibited by oxyhemoglobin. NO stimulated cyclic GMP formation in corpus cavernosum and a close positive correlation was found between the magnitudes of relaxation and cyclic GMP formation. The data suggest that NO-elicited activation of guanylate cyclase and cyclic GMP formation represents the signal transduction mechanism responsible for relaxation and nonadrenergic-noncholinergic-mediated penile erection. These observations indicate that NO is a potent relaxant of human and rabbit corpus cavernosum and support our hypothesis that endogenous NO is the principal mediator of penile erection caused by nonadrenergic-noncholinergic stimulation.

Nitric oxide synthase from cerebellum catalyzes the formation of equimolar quantities of nitric oxide and citrulline from L-arginine.

This study examined whether constitutive nitric oxide (NO) synthase from rat cerebellum catalyzes the formation of equimolar amounts of NO plus citrulline from L-arginine under various conditions. Citrulline was determined by monitoring the formation of 3H-citrulline from 3H-L-arginine. NO was determined by monitoring the formation of total NOx (NO+nitrite [NO2-] + nitrate [NO3-]) by chemiluminescence after reduction of NOx to NO by acidic vanadium (III). Equal quantities of NO plus citrulline were generated from L-arginine and the formation of both products was linear for about 20 min at 37 degrees C provided L-arginine was present in excess to maintain a zero order reaction rate. Deletion of NADPH, addition of the calmodulin antagonist calmidazolium, or addition of NO synthase inhibitors (NG-methyl-L-arginine, NG-amino-L-arginine) abolished or markedly inhibited the formation of both NO and citrulline. The Km for L-arginine (14 microM; 18 microM) and the Vmax of the reaction (0.74 nmol/min/mg protein; 0.67 nmol/min/mg protein) were the same whether NO or citrulline formation, respectively, was monitored. These observations indicate clearly that NO and citrulline are formed in equimolar quantities from L-arginine by the constitutive isoform of NO synthase from rat cerebellum.

Nitric oxide as a mediator of relaxation of the corpus cavernosum in response to nonadrenergic, noncholinergic neurotransmission.

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.

Nitric oxide and cyclic GMP formation upon electrical field stimulation cause relaxation of corpus cavernosum smooth muscle.

In the presence of functional adrenergic and cholinergic blockade, electrical field stimulation relaxes corpus cavernosum smooth muscle by unknown mechanisms. We report here that electrical field stimulation of isolated strips of rabbit corpus cavernosum promotes the endogenous formation and release of nitric oxide (NO), nitrite, and cyclic GMP. Corporal smooth muscle relaxation in response to electrical field stimulation, in the presence of guanethidine and atropine, was abolished by tetrodotoxin and potassium-induced depolarization, and was markedly inhibited by NG-nitro-L-arginine, NG-amino-L-arginine, oxyhemoglobin, and methylene blue, but was unaffected by indomethacin. The inhibitory effects of NG-substituted analogs of L-arginine were nearly completely reversed by addition of excess L-arginine but not D-arginine. Corporal smooth muscle relaxation elicited by electrical field stimulation was accompanied by rapid and marked increases in tissue levels of nitrite and cyclic GMP, and all responses were nearly abolished by NG-nitro-L-arginine. These observations indicate that penile erection may be mediated by NO generated in response to nonadrenergic-noncholinergic neurotransmission.

Depletion of arterial L-arginine causes reversible tolerance to endothelium-dependent relaxation.

This study examined the influence of lowered arterial levels of L-arginine on endothelium-dependent relaxation of isolated rings of bovine pulmonary artery. Incubation of arterial rings under tension for 24 hr in oxygenated Krebs bicarbonate solution at 37 degrees C resulted in the development of marked or complete tolerance to A23187 (calcium ionophore)- and acetylcholine-elicited relaxation. Relaxant responses to nitric oxide were unaffected. Addition of L-arginine did not relax control rings but did elicit marked endothelium-dependent relaxation of tolerant rings that was inhibited by oxyhemoglobin or methylene blue. L-Arginine also restored acetylcholine-elicited relaxation. Inclusion of L-canavanine in the 24 hr incubations protected against the development of tolerance. The tissue concentration of arginine was 3-fold lower in tolerant than control arterial rings and L-canavanine restored arterial arginine levels to control values. Therefore, depletion of arterial L-arginine causes reversible tolerance to endothelium-dependent relaxation.