A nitric oxide-like factor mediates nonadrenergic-noncholinergic neurogenic relaxation of penile corpus cavernosum smooth muscle

A large proportion of pelvic neurons innervating the corpora cavernosa of the rat penis exhibit NADPH-diaphorase activity

Nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry, which indicates the presence of neural nitric oxide synthase, the enzyme responsible for the generation of nitric oxide, was used in combination with retrograde labelling methods to determine, in whole-mounts and sections of rat major pelvic ganglia, whether neurons destined for the penile corpora cavernosa were able to produce nitric oxide. In whole-mount preparations of pelvic ganglia, among the 607 +/- 106 retrogradely labelled neurons innervating the penile corpora cavernosa, 84 +/- 7% were NADPH-diaphorase-positive, 30 +/- 7% of which were intensely histochemically stained. In serial sections of pelvic ganglia, out of a mean count of 451 retrogradely labelled neurons, 65% stained positively for NADPH-diaphorase. An average of 1879 +/- 363 NADPH-diaphorase positive cell bodies was counted in the pelvic ganglion. In the major pelvic ganglion, neurons both fluorescent for Fluorogold or Fast Blue and intensely stained for NADPH-diaphorase were consistently observed in the dorso-caudal part of the ganglia in the area close to the exit of the cavernous nerve and within this nerve. This co-existence was much less constant in other parts of the ganglion. In the rat penis, many NADPH-diaphorase-positive fibres and varicose terminals were observed surrounding the penile arteries and running within the wall of the cavernous spaces. This distribution of NADPH-diaphorase-positive nerve cells and terminals is consistent with the idea that the relaxation of the smooth muscles of the corpora cavernosa and the dilation of the penile arterial bed mediated by postganglionic parasympathetic neurons is attributable to the release of nitric oxide and that nitric oxide plays a crucial role in penile erection.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of nitric oxide synthase activity in the human lower urinary tract and its correlation with neuroeffector responses

OBJECTIVES

The present study was designed to correlate the localization of nitric oxide synthase (NOS) activity to nerve-induced smooth muscle responses in the human lower urinary tract.

METHODS

Nerve-induced smooth muscle activity was studied in the human lower urogenital tract. NOS activity was studied by measurement of citrulline formation and guanylate cyclase activity.

RESULTS

Nerve-induced contractions in the human detrusor muscle, bladder neck, and prostatic urethra were not significantly enhanced by the NOS inhibitor N omega-nitro-L-arginine methyl ester (L-NAME). In the prostatic urethra, relaxations to transmural nerve stimulation were obtained after increase in tension. The relaxations were abolished by L-NAME and restored by L-arginine. Nerve-induced relaxations were occasionally obtained in the bladder neck, whereas nerve-induced relaxations were never obtained in the detrusor muscle. Citrulline formation was highest in the prostatic urethra, it was intermediate in the bladder neck, and it was less pronounced in the detrusor muscle. Guanylate cyclase activity was also highest in the prostatic urethra, whereas there was no significant difference in guanylate cyclase activity in the bladder neck and detrusor muscle.

CONCLUSIONS

The nerve-induced smooth muscle responses and the localization of NOS activity were in good agreement. Thus, in areas where marked relaxations to nerve stimulation were obtained, there was also a high NOS activity. The data suggest that nitric oxide is a mediator for the neurogenic dilation of the bladder neck and urethra during the micturition reflex.

The roles of nitric oxide in sexual function of male rats

Nitric oxide (NO) may mediate penile erection by inhibiting smooth muscle of the corpora cavernosa, thereby allowing vasodilation of the corpora. In order to test the role of NO in the sexual function of intact male rats, either the precursor of NO (L-arginine, L-Arg) or an inhibitor of its synthesis (NG-nitro-L-arginine methyl ester, NAME) was administered systemically before tests of copulation, ex copula genital reflexes, or sexual motivation/motor activity. NAME impaired copulation in a dose dependent manner. It also decreased the number of ex copula erections, but it increased the number of ex copula seminal emissions and decreased the latency to the first seminal emission. L-Arg marginally increased the number of penile reflexes, but had no other effects. NAME had no effect on sexual motivation or motor activity. The results indicate that nitric oxide promotes erection in intact male rats, probably by mediating filling of the corpora cavernosa. The data also suggest that NO inhibits seminal emission, probably by decreasing sympathetic nervous system activity; this may help prevent premature ejaculation.

Nitric oxide synthase activity in the human urogenital tract

Nitric oxide (NO) has been suggested as a nonadrenergic non-cholinergic neurotransmitter in the urogenital tract and has previously been shown to have a smooth muscle relaxing effect in the urogenital organs both in various animals and in humans. It has been shown that NO is a mediator of the erection and the dilatation of the bladder neck and urethra. The aim of the study was to analyse nitric oxide synthase (NOS) activity in the human urogenital tract. NOS activity was measured by the conversion of L-[U-14C] arginine to L-[U-14C] citrulline. In the upper urinary tract there was Ca(2+)-dependent NOS activity in the renal pelvis, but no significant NOS activity could be found in the ureter. In the lower urinary tract we found high Ca(2+)-dependent NOS activity in the urethra, intermediate activity in the bladder neck and comparatively low activity in the detrusor muscle. In the male genital tract the testis and epididymis had no significant NOS activity. The vas deferens, prostate, seminal vesicle and corpus cavernosum were found to have high levels of Ca(2+)-dependent NOS activity. Ca(2+)-independent NOS activity was not obtained in the urogenital tract. Our results correspond well with previous functional studies indicating NO to be an important nerve-induced mediator of erection and in the micturition reflex, but also suggest that NO may be involved in several other functions in the human urogenital tract.

Regulation of tone in penile cavernous smooth muscle. Established concepts and new findings

Since Benson, in 1983, reported on a potent nonadrenergic, noncholinergic (NANC) transmitter postulated to relax penile vessels and the corpus cavernosum, much new information on the mechanisms of contraction and relaxation of corporeal smooth muscle and penile vasculature has been obtained. The information currently available suggests that NANC transmitters may be involved in both contractile and relaxant responses of penile erectile tissues. There is good experimental evidence to allow the assumption that neurogenic nitric oxide (NO) is a mediator of penile erection, but even if NO probably is the most important factor for relaxation of penile vessels and the corpus cavernosum, this does not exclude the possibility that other agents released from nerves may have a modulatory function in this process. However, the roles of, for example, vasoactive intestinal polypeptide and related peptides as neurotransmitters and/or neuromodulators in the nervous control of penile erection have yet to be established. The restricted availability of human penile erectile tissues has led to the use of cavernous tissue and penile vessels from animals, both for screening and for detailed analysis of mechanisms previously demonstrated to exist also in human tissues. When interpreting the results obtained, it is important to stress that there may be important differences between human and animal tissues, that each of the tissues only gives a piece of information on the complex process of penile erection, and that the physiological and clinical importance of results from such experiments may be limited. The differing responses in different parts of the vasculature within the penis and the multiplicity of putative transmitters present in the corpus cavernosum and in perivascular nerves make further investigations necessary, as do the interactions between transmitters and neuromodulators at the neuromuscular junction, and between the neural and endothelial control of vascular tone.

Adenosine modulation of neurotransmission in penile erection

1. Adenosine inhibited the noradrenaline-induced contraction of rabbit corpus cavernosum in a dose-dependent manner. The effect of adenosine was greater in intact corpus cavernosa than in endothelium-denuded preparations. This finding indicates that the relaxing effect of adenosine is partially endothelium-dependent and involved in the release of endothelium-derived relaxing factors. 2. Adenosine and its analogues relaxed the noradrenaline-induced contractile response as well as inhibited the transmural nerve induced contraction with the potency order: NECA > R-PIA > adenosine. These data indicate that adenosine can modulate both the non-adrenergic non-cholinergic and adrenergic neurotransmission. DMPX, an adenosine antagonist selective for the A2 receptors, abolished the electrically elicited relaxation. However, CGS 21680, selective for A2a receptor, had no effect on relaxation. Therefore, adenosine receptors involved in the modulation of neurotransmission in rabbit corpus cavernosum appear to be A2b subtype. 3. Adenosine also induced an increase in human cavernosal arterial velocity and resistive index measured by colour duplex sonography. The combination of adenosine and 10 micrograms prostaglandin E1 was more effective in resistive index and erection grade than 20 micrograms prostaglandin E1 alone. Our results suggest that adenosine seems to be an important neuromodulator for penile erection and can be an effective and alternative combination in the treatment of impotence.

Corpus cavernosal relaxation in impotent men

OBJECTIVE

To investigate smooth muscle responsiveness in isolated preparations of corpus cavernosum from men with vasculogenic impotence.

PATIENTS AND METHODS

Biopsies of corpus cavernosum were obtained from 63 men undergoing penile surgery. These included a control group of eight men (mean age 59 years, range 38-82) undergoing penile amputation for carcinoma, 47 men with vasculogenic impotence (mean age 58, range 36-72) who were further sub-divided into arterial (n = 9), venous (n = 24) or mixed arterial/venous (n = 14) impotence according to the results of pre-operative haemodynamic investigation, and eight men with non-vasculogenic impotence (mean age 49 years, range 34-66). Smooth muscle contractile responses to alpha-adrenoceptor activation and relaxant responses to stimulation of intrinsic nerves and exposure to papaverine and sodium nitroprusside were recorded in tissue strips prepared from the biopsies. Morphology was assessed histologically using haematoxylin and eosin staining of tissue sections together with immunocytochemical labelling of intrinsic nerves.

RESULTS

Nerve-evoked relaxation was markedly impaired in tissue from men with venous or mixed arterial/venous impotence. A lesser degree of impairment was found in tissue from men with arterial impotence alone. Tissue from men with all types of vasculogenic impotence also showed a decreased contractile response to alpha-adrenoceptor stimulation. The magnitude of relaxant responses to papaverine and sodium nitroprusside in the vasculogenic group was similar to that of the control. There were no differences in smooth muscle content or nerve density between the vasculogenic group and the control. In the non-vasculogenic group responses to relaxant nerve stimulation, alpha-adrenoceptor activation and relaxant drugs were similar to those of the control. Nerve density in this group was similar to the control but smooth muscle content was reduced.

CONCLUSIONS

The results of this study demonstrate a functional impairment of smooth muscle contractility and neurogenic relaxation in corpus cavernosum from impotent men with abnormal penile haemodynamics. Altered smooth muscle responsiveness is likely to be a factor in the aetiology of impotence in such men and may contribute to the relatively poor results of vascular surgery for impotence.

Some pre- and postjunctional effects of castration in rabbit isolated corpus cavernosum and urethra

Pre- and postjunctional effects of castration were investigated in isolated corpus cavernosum (CC) and prostatic and preprostatic urethral preparations obtained from rabbits that had been castrated surgically 14 days before investigation. Preparations obtained from untreated animals were used as controls. Castration did not change the relaxing effects of SIN-1 (NO donor) or papaverine in CC preparations contracted by noradrenaline (NA). Electrical field stimulation of CC preparations contracted by NA or endothelin-1 produced frequency-dependent and tetrodotoxin-sensitive relaxations. As compared with controls, the electrically induced relaxations were increased in preparations from castrated animals. Pretreatment with prazosin increased the electrically induced relaxations in CC from untreated rabbits, but had no effect on preparations from castrated animals. In CC preparations incubated with 3H-NA, castration significantly reduced the electrically evoked release of 3H. L-NOARG, an inhibitor of NO synthase, had no effect on 3H-efflux. In prostatic, but not preprostatic, urethral preparations contracted by NA, the relaxant effects of SIN-1 and vasoactive intestinal polypeptide were significantly smaller following castration. Furthermore, castration significantly reduced electrically evoked relaxations in prostatic urethral preparations contracted by NA, while in preprostatic urethra, no such effect was seen. Castration or L-NOARG had no effect on the electrically induced release of 3H-NA in either of the urethral tissues. The results suggest that the hormonal changes caused by castration may modulate the functional effects in vitro of some parts of the urogenital tract. In penile erectile tissue, the relaxations induced by electrical field stimulation are increased, probably for the most part through a decrease in the neuronal release of NA. In prostatic urethra, on the other hand, electrically evoked relaxations are decreased, possibly as a result of an impaired ability of the smooth muscle itself to respond to relaxant agents. In preprostatic urethra, castration has no obvious functional effects. The physiological consequences of these findings in the in vivo situation remain to be established.

Penile erection in the primate: induction with nitric-oxide donors

The primate model has been used for investigations on the physiology and pharmacology of erection. Recent in vitro investigations indicate that nitric oxide acts as the mediator of penile erection, but in vivo primate studies are needed to corroborate these findings. Penile erections were induced in a primate model using intracavernosal injections of nitric oxide donors s-nitrocysteine (NO-CYS) and sodium nitroprusside (SNP), and acetylcholine (ACh) which stimulates the formation of nitric oxide. Penile length and intracavernosal pressures following agonist injection were compared with baseline (flaccid) and control erections (elicited by injection of a papaverine/phentolamine/PGE1 standard mixture). Dose-response curves for each drug were determined with respect to maximal intracavernosal pressure, duration of effect and penile length, and systemic arterial pressure was monitored. All three agents induced erections, with dose-dependent increases in cavernosal pressure and penile length. The maximal cavernosal pressure attained was similar for all three agents, but the duration of action was significantly shorter with ACh (p < .05). Injection of L-nitro-arginine-methyl-ester (L-NAME), a nitric oxide synthase inhibitor, before injection of the nitric oxide donor shortened the duration of effect but did not alter maximal cavernosal pressure or penile length attained. Although systemic hypotension was induced by each agent, digital compression at the base of the penis at the time of injection prevented such changes. These results suggest that the primate is a useful model to evaluate the action of substances that induce or inhibit penile erection. The findings provide support for the hypothesis that nitric oxide is a mediator of penile erection and that nitric oxide donors may be useful in the treatment of erectile dysfunction.

Metabolism of methylarginines by human vasculature; implications for the regulation of nitric oxide synthesis

1. The metabolism of methylarginines by human cultured endothelial cells and human saphenous vein was studied in vitro. The human endothelial cell line (SGHEC-7), primary cultures of human umbilical vein endothelial cells (HUVEC) and human saphenous vein were incubated with [14C]-monomethyl-L-arginine ([14C]-L-NMMA) and the cytosolic extract analysed by high performance liquid chromatography (h.p.l.c.) with on-line radioisotope detection. 2. SGHEC-7, HUVEC and human saphenous vein metabolized [14C]-L-NMMA to a compound which co-eluted with [14C]-citrulline. A second metabolite which co-eluted with [14C]-arginine was evident on the radiochromatograms of HUVEC cytosol and saphenous vein extracts. 3. The intracellular levels of [14C]-L-NMMA and [14C]-citrulline in SGHEC-7 cells incubated with [14C]-L-NMMA (0.5 microCi ml-1: 8.9 microM) for 1 h were 113 +/- 22 and 67.6 +/- 6.2 pmol mg-1 cell protein respectively (n = 7). Co-incubation with NGNGdimethyl-L-arginine (ADMA; 100 microM) but not NGNGdimethyl-L-arginine (SDMA; 100 microM) reduced the intracellular level of [14C]-citrulline to 26.3 +/- 3.7 pmol mg-1 cell protein (P < 0.01; n = 3) without reducing the intracellular level of [14C]-L-NMMA. 4. The intracellular levels of [14C]-citrulline in SGHEC-7 cells incubated with [14C]-L-NMMA for 1 h were reduced following co-incubation with NGnitro-L-arginine methylester (L-NAME; 1 mM), NGnitro-L-arginine (L-NOARG; 1 mM) and L-canavanine (1 mM) to 47.1 +/- 6.2, 24.7 +/- 3.6 and 12.5 +/- 2.8% of control levels (P < 0.001; n = 9). ADMA (1 mM; n = 3) reduced intracellular [14C]-citrulline levels to4 +/- 4% of control (P<0.01) but SDMA (1 mM; n = 3) had no effect.5. The accumulation of endogenously synthesized ADMA in the culture supernatant of SGHEC-7 cells was increased by co-incubation with L-NMMA (1 mM) from 1.98 +/- 0.08 to 2.74 +/- 0.36 nmol mg- cell protein, an increase of 40%.6. These results demonstrate that human vasculature possesses an enzyme which has similar properties to dimethylarginase; human endothelial cells and human saphenous vein metabolize L-NMMA to citrulline via a process inhibited by ADMA but not SDMA. The increase in endothelium-derivedADMA following co-incubation with L-NMMA is consistent with competition between ADMA and L-NMMA for dimethylarginase. Inhibition of this enzyme might increase the intracellular concentration of ADMA, an endogenously produced compound that inhibits nitric oxide synthesis.

Effects of nicorandil on human isolated corpus cavernosum and cavernous artery

Nitric oxide (NO) released from nonadrenergic-noncholinergic (NANC) nerves seems to be a principal mediator of the relaxation of penile erectile tissue necessary for erection, and drugs acting by release of NO have been shown to produce erection when injected intracorporeally into impotent patients. By producing hyperpolarization, K+ channel openers are effective in relaxing isolated penile erectile tissue from rabbit and man, and can produce tumescence and erection when injected intracorporeally into animals. Nicorandil is classified as a K+ channel opener, but it also acts as a donor of NO. In the present study, the effects of nicorandil on isolated preparations from human corpus cavernosum (CC) and deep cavernous artery (Acc) were compared with those of cromakalim (K+ channel opener) and SIN-1 (NO donor). Nicorandil produced a concentration-dependent relaxation of CC and Acc preparations. The relaxations obtained at the highest nicorandil concentration used (10(-4) M.) were 75 +/- 3% and 66 +/- 4% in CC preparations contracted by noradrenaline and endothelin-1, respectively. The corresponding effects in Acc preparations were 70 +/- 14% and 73 +/- 5%. Glibenclamide (blocking ATP-dependent K+ channels) significantly reduced the nicorandil-induced relaxation in CC, but not in Acc. Methylene blue (believed to block soluble guanylate cyclase) reduced nicorandil's relaxant effect in CC, although statistical significance was not obtained. NG-nitro-L-arginine 10(-4) M. (NO synthase inhibitor) did not significantly influence the effect of nicorandil on precontracted preparations in either tissue. In CC preparations contracted by electrical field stimulation, nicorandil and cromakalim concentration dependently inhibited the responses. This effect was significantly counteracted by glibenclamide. It is concluded that nicorandil is effective in relaxing human CC chiefly by its K+ channel opening action, and to some extent by its ability to release NO. For nicorandil's relaxing effect on Acc, ATP dependent K+ channels seem to be of limited importance. If effective in impotent patients, the drug may represent a new, interesting approach to the treatment of erectile dysfunction.

Nitric oxide-mediated neurotransmission is attenuated in the anococcygeus muscle from diabetic rats

The effect of STZ-induced diabetes of 8-weeks duration was examined on nitric oxide-mediated neurotransmission in the rat anococcygeus muscle. In the presence of noradrenergic blockade and raised tissue tone, relaxant response to nerve stimulation (0.5-5 Hz, for 10 s), sodium nitroprusside (5 and 10 nmol/l) and nitric oxide (1 and 3 mumol/l) were significantly reduced in anococcygeus muscles from diabetic rats compared to responses from control rats (p < 0.05). In contrast, relaxations to papaverine (3 and 10 mumol/l) were not reduced in tissues from diabetic rats. The nitric oxide synthesis inhibitor NOLA (100 mumol/l) abolished relaxant responses to nerve stimulation but had no effect on responses to any of the relaxant agents used. Exposure to NOLA at 10 mumol/l reduced stimulation-induced relaxations; this reduction was significantly greater in tissues from the diabetic group than from the control group (p < 0.05), probably as a consequence of the smaller relaxant responses in muscles from diabetic rats. Contractile responses to nerve stimulation (1-10 Hz, for 10 s), but not noradrenaline (0.03-30 mumol/l), were significantly greater in anococcygeus muscles from diabetic rats than from control rats (p < 0.05). NOLA (100 mumol/l) significantly enhanced stimulation-induced contractions (p < 0.05), however the enhancement was significantly less in tissues from diabetic rats (p < 0.05). The results suggest that STZ-induced diabetes impairs smooth muscle reactivity to nitric oxide in the rat anococcygeus muscle.

Impotence and chronic renal failure: a study of the hemodynamic pathophysiology

A retrospective multi-institutional study was performed to document and characterize the arterial vascular disease in the hypogastric-cavernous arterial bed and/or veno-occlusive dysfunction of the corpora cavernosa in patients with end stage renal disease. We evaluated 20 impotent patients (mean age 40 +/- 9 years) with chronic renal failure using pharmaco-cavernosometry and pharmacocavernosography (4 also underwent pharmaco-arteriography). Patients were divided into groups based on the treatment (14 with renal transplantation and 6 with hemodialysis or peritoneal dialysis), as well as by history of vascular risk factors (16 with and 4 without risk factors). Of the patients 19 revealed abnormal intracavernous pressure responses to repeated intracavenous injections of vasoactive agents implying vascular disease of the penis. Cavernous artery occlusive disease was found in 78% of the patients. All patients who underwent arteriography had diffuse atherosclerotic disease of the distal penile arteries. Corporeal veno-occlusive dysfunction was found in 90% of the patients, of whom 60% had diffuse pan-cavernous leakage involving the dorsal, cavernous and crural veins, glans penis and corpus spongiosum. This renal failure-associated vascular disease of the penis was found to occur independently of the presence of known systemic atherosclerotic vascular risk factors. Patients who underwent early treatment of the uremia by renal transplantation had vasculogenic impotence only in the case of rejection of the renal transplant, suggesting that early renal transplantation may delay or prevent the development of the penile vasculopathy. The most likely pathophysiology of the vascular impairment includes renal failure-associated atherosclerosis, and renal failure-associated hypoxia changes of the contractile (smooth muscle) and structural (collagen/elastin) components of the erectile tissue. Strategies for future research and clinical therapies are suggested.

Intracellular mechanism of penile erection in monkeys

To elucidate the sequence of events between the release of neurotransmitters and cavernous smooth muscle relaxation in erection, we studied the role of the cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) systems. In a well-established simian model, the effects of specific agonists and antagonists of the intracellular sequence for smooth muscle relaxation and potassium channel openers on the intracavernous pressure were examined. Sodium nitroprusside (10(-3) M), a nitric oxide releaser and thus a stimulant of the cGMP system, caused an increase in the intracavernous pressure from 82 to 115 cm H2O for 7 to 19 min and penile diameter from 24.8 +/- 2.28 to 43 +/- 4.87 mm. When nitroprusside was injected after methylene blue (10(-3) M), a specific antagonist of the enzyme guanylate cyclase, intracavernous pressure rise decreased significantly, but cromakalin, a potassium channel opener, provoked excellent increases after the block. A smaller dose of sodium nitroprusside (10(-4) M) caused an increase in intracavernous pressure from 35 to 85 cm H2O for 7 to 11.5 min. When nitroprusside was injected after zaprinast, a phosphodiesterase inhibitor, the increase in pressure ranged from 80 to 116 cm H2O for 15 to 30 min. Prostaglandin E1, an activator of the cAMP system, caused an increase in the intracavernous pressure of 20-80 cm H2O for 5 to 10 min, and an increase in penile diameter from 25 +/- 2.22 to 35 +/- 3.48 mm. The erectile response to PGE1, but not to cromakalin, was nearly abolished by ethylmaleimide, an adenylate cyclase blocker. The response to nitroprusside was significantly greater (P < 0.05) than to PGE1. Both systems, cAMP and cGMP, may be involved in cavernous smooth muscle relaxation, and cGMP is probably the predominant intracellular second messenger in penile erection in monkeys. Stimulants of the cGMP system, such as nitric oxide releasers, could represent a more physiological and effective approach in the treatment of erectile dysfunction.

Nitric oxide mediates penile erection in cats

The present study was undertaken to investigate the in vivo effects of nitric oxide (NO) mediating agents injected intracavernosally on penile erection in cats. All NO donors increased the cavernosal pressure and penile length in a dose-dependent manner. The maximal effects on cavernosal pressure and penile length induced by s-nitrosocysteine (NO-CYS) and s-nitroso-n-acetylpenicillamine (SNAP), respectively, were 8-fold and 5-fold increases in pressure, and 45% and 34% increases in length when compared with baseline values. These changes were comparable to that caused by the control drug combination (papaverine, phentolamine and prostaglandin E1). The effects of acetylcholine (ACh) and substance P on cavernosal pressure and penile length were less than those obtained with the control drug combination, NO-CYS (p < 0.01), or SNAP (p < 0.05). N omega-nitro-l-arginine-methyl-ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, significantly decreased the effects of NO-CYS, ACh and substance P on penile erection. This in vivo study with NO donors and an NOS inhibitor suggests that NO is a mediator of penile erection in cats.

Detection of m2 muscarinic acetylcholine receptor mRNA in human corpus cavernosum by in-situ hybridization

Relaxation and contraction of the smooth muscle of human corpus cavernosum (HCC) of the penis is essential for penile erection and detumescence. Relaxation of the smooth muscle is controlled, locally, by cholinergic, adrenergic and nonadrenergic, noncholinergic neurotransmitters as well as by the vascular endothelium, which lines the lacunar spaces and releases endothelium-derived relaxing factor. Cholinergic neurotransmitters are postulated to act on other neuroeffectors and on the endothelium rather than act directly on the trabecular smooth muscle. We use in-situ hybridization to determine the presence and distribution of muscarinic acetylcholine receptor (mAChR) subtype mRNA in HCC. First, we verified that the riboprobe used for in-situ hybridization is specific for m2 mAChR subtype using Chinese Hamster Ovary cells transfected with m2 mAChR subtype. Then, we validated that the tissue obtained from surgical specimens is adequate for in-situ hybridization. The data demonstrate that m2 mAChR subtype mRNA is expressed in HCC smooth muscle cells. m2 mAChR subtype mRNA expression is not detected in endothelium or nerves. It is possible that this receptor subtype plays a role in smooth muscle contraction.

Effects of cyclic GMP on smooth muscle relaxation

Cyclic GMP levels within smooth muscle are affected then by a number of different pathways. Physiologically NO and ANF are probably the two most important regulators for smooth muscle function, but a variety of other mediators and pharmacological agents may also influence this system. Because of the important role that cyclic GMP plays in the control of smooth muscle tone, which clearly includes vascular smooth muscle, it is now and will continue to be in the future an important physiological and biochemical target for research and a pharmacological target for therapeutic agents.

Role of the L-arginine/nitric oxide pathway in relaxation of isolated human penile cavernous tissue and circumflex veins

In human penile corpus cavernosum strips, pre-contracted by noradrenaline, electrical stimulation of nerves evoked non-adrenergic, non-cholinergic (NANC) relaxant responses which could be inhibited by tetrodotoxin 10(-6) M, NG-nitro-L-arginine (L-NNA) 10(-7)-10(-4) M, and oxyhaemoglobin 10(-5) M, but not by methylene blue (MB) 10(-5) M. Acetylcholine-induced relaxations were also inhibited by L-NNA 10(-4) M and oxyhaemoglobin 10(-5) M, but were unaffected by pyrogallol 10(-4) M, MB 10(-5) M, and tetrodotoxin 10(-6) M. MB 5 x 10(-4)-10(-4) M significantly reduced the responses to both electrical stimulation and to acetylcholine. Nitric oxide (NO) 10(-7)-10(-4) M and sodium nitroprusside 10(-9)-10(-4) M caused concentration-dependent relaxations. The NO-induced relaxations were inhibited by oxyhaemoglobin 10(-5) M, and the concentration-response curve for sodium nitroprusside was shifted to the right by MB 10(-5) M. The response to sodium nitroprusside was unaffected by L-NNA 10(-4) M, oxyhaemoglobin 10(-5) M, and pyrogallol 10(-4) M. In circumflex veins, pre-contracted by noradrenaline, no NANC-mediated relaxation was found in response to electrical stimulation; acetylcholine caused endothelium-dependent relaxations, which were insensitive to L-NNA 10(-4) M and oxyhaemoglobin 10(-5) M. NO and sodium nitroprusside caused concentration-dependent relaxations; the concentration-response curves for NO and sodium nitroprusside were shifted to the right by oxyhaemoglobin 10(-5) M. Removal of the endothelium left the NO- and sodium nitroprusside-induced relaxations unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathophysiology of erectile dysfunction

During the past decade, our knowledge of the hemodynamics, functional anatomy, neurophysiology, and neuropharmacology of erectile function has evolved substantially. The change of smooth muscle tone has emerged as a key factor in erection and detumescence. However, future studies are needed to elucidate the cellular and molecular basis of erectile physiology. With insight into normal physiology we will understand the pathologic process and be able to treat it.

Actions of 3-morpholinosydnonimin (SIN-1) on rabbit isolated penile erectile tissue

The effects of the NO-donor 3-morpholinosydnonimin (SIN-1) on isometric tension, cyclic guanosine 3',5'-monophosphate (cyclic GMP) accumulation and neuronal release of 3H-noradrenaline were investigated in rabbit isolated corpus cavernosum (CC), and compared to the actions of sodium nitroprusside (SNP) and the cyclic GMP-specific phosphodiesterase inhibitor zaprinast. SIN-1, zaprinast and SNP concentration dependently relaxed rabbit CC preparations contracted by 1 microM. phenylephrine. All the drugs were highly effective, and the order of potency was SNP > zaprinast > SIN-1. SIN-1 had a biphasic effect on contractions evoked by electrical field stimulation of nerves: at low concentrations (1 and 10 microM.), SIN-1 inhibited the contractions, while at concentrations > or = 100 microM., the contractions were again increased. There were no changes in baseline tension. Electrically evoked contractions were inhibited by zaprinast in a concentration-dependent manner. Compared with controls, 1 mM. SIN-1 caused a significant (p < 0.05) increase in both the basal efflux and in the electrically induced release of 3H from CC preparations incubated with 3H-noradrenaline. SIN-1, zaprinast and SNP increased tissue levels of cyclic GMP. There was no positive correlation between cyclic GMP accumulation and the relaxant effects of the drugs. The effects of SIN-1 and SNP on the tissue content of cyclic GMP were not significantly affected by methylene blue, an inhibitor of soluble guanylate cyclase. It may be concluded that SIN-1, zaprinast and SNP are effective in relaxing isolated penile erectile tissue, and this effect is associated with an increase in the tissue content of cyclic GMP via pathways not sensitive to methylene blue. However, additional mechanisms beside stimulation of adrenergic neurotransmission and activation of guanylate cyclase in the smooth muscle cell seem to participate in the action of SIN-1 on rabbit penile erectile tissue.

Intracavernous sodium nitroprusside: inappropriate impotence treatment

On the basis of reports describing nitric oxide as a form of endothelium-derived relaxing factor and on our own experience with intracavernous use of nitric oxide-releasing substances in animal models, we undertook an approved human study of intracavernous sodium nitroprusside as a treatment for impotence. We report our early experience in which severe hypotension and only mild tumescence in our first 3 patients caused us to discontinue the trial.

Immunohistochemical localization of nitric oxide synthase in the autonomic innervation of the human penis

An improved understanding of the physiology of penile erection has resulted from recent evidence that implicates nitric oxide as the principal mediator of erectile function. Previously, the neuroanatomy of erection in man was established with descriptions of the autonomic innervation of the pelvic organs and external genitalia. The basis upon which novel physiological concepts of erection relate to earlier neuroanatomical principles remains to be determined. In the present study these relationships were explored with nitric oxide synthase immunohistochemistry and reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemistry of select pelvic tissue specimens obtained from 4 men (3 at radical prostatectomy and 1 at autopsy). Nitric oxide synthase, the enzyme that catalyzes nitric oxide production, was identified in discrete neuronal locations, including the pelvic plexus, cavernous nerves and their terminal endings within the corporeal erectile tissue, branches of the dorsal penile nerves and nerve plexuses in the adventitia of the deep cavernous arteries. This distribution of nitric oxide synthase-containing nerves suggests that nitric oxide neuronally modulates local vascular smooth musculature of the penis. On this basis, nitric oxide is identified as a neuronal mediator of penile erection in man.

Drug-induced male sexual dysfunction. An update

Impotence, defined as the consistent inability to maintain an erect penis of sufficient rigidity for sexual intercourse, has been estimated to affect 10 million American men. An age dependence has been shown to exist, with 25% of men over age 65 affected. A large body of clinical experience and published reports in the literature link many commonly prescribed drugs with sexual dysfunction. Drugs can affect sexual function at a variety of points such as inhibition of ejaculation or sedation/depression leading to reduced libido. Antihypertensive drugs have been most commonly associated with impotence. There have been reports of sexual dysfunction with almost all classes of antipsychotics, but little clinical investigation has been performed. Other drugs associated with sexual dysfunction include digoxin, clofibrate, cimetidine and various hormonal agents and antineoplastics. An important first step in approaching all impotent patients is the taking of a detailed medical, surgical, sexual and drug/substance abuse history. The least invasive form of therapy should be employed. Recent studies have shown intracavernous injections of alprostadil (prostaglandin E1) to be safe and effective for long term use. Vacuum constriction devices may also be of help. Better and more durable prostheses are now available should other treatment be unsuccessful.

Analysis of microsurgical penile revascularization results by etiology of impotence

We reviewed the results of microsurgical penile revascularization, with or without a combined procedure to correct cavernosal venous leakage, in 50 consecutive patients with vasculogenic impotence. All patients underwent an extensive preoperative evaluation, including dynamic infusion cavernosography and cavernosometry, and selective penile arteriography. Overall 48% (24 patients) had an excellent postoperative result, 40% (20 patients) were improved and 12% (6 patients) failed, with a median followup of 24 months (range 19 to 56). Furthermore, these results appear durable with no significant difference in length of followup between groups irrespective of surgical outcome (p > 0.05). Analysis of surgical outcomes by preoperative etiology of impotence (pure arterial versus arterial combined with corporeal venous dysfunction) revealed a statistically significant advantage of an excellent surgical outcome in patients with pure arterial impotence compared to those with mixed etiology with results of 67% and 42%, respectively (p < 0.01). There was no significant difference in outcome when patients were analyzed with respect to age or duration of impotence (p > 0.05). We conclude that in patients with arteriogenic impotence identification of concomitant corporeal veno-occlusive dysfunction diagnosed by preoperative dynamic infusion cavernosography and cavernosometry may be helpful, not only in planning a more physiological surgical procedure but also in predicting long-term postoperative success.

Followup results of a combination of calcitonin gene-related peptide and prostaglandin E1 in the treatment of erectile dysfunction

Recent human and animal studies have shown a possible role for calcitonin gene-related peptide in penile erection and a therapeutic benefit in combination with prostaglandin E1 for autoinjection therapy. The ethical committee approved calcitonin gene-related peptide-prostaglandin E1 combination for cases of nonresponse or cavernous fibrosis to papaverine-phentolamine. Since June 1990, 65 patients (59 nonresponders and 6 with fibrosis) were injected with 5 micrograms. calcitonin gene-related peptide plus 10 micrograms. prostaglandin E1. Of the 59 nonresponders to papaverine-phentolamine 31 and of the 6 patients with fibrosis 5 had full erectile response. Of these 36 patients 2 experienced pain during the pharmacologically induced erection. A total of 39 patients who had had at least 20 autoinjections of calcitonin gene-related peptide plus prostaglandin E1 was available for minimum followup. There were no side effects, such as pain (the 2 patients with pain mentioned previously were not included in the autoinjection therapy group), systemic side effects or (increased) fibrosis. Our results show that a combination of calcitonin gene-related peptide and prostaglandin E1 may be beneficial to the treatment of impotence in carefully selected patients.

Long-term results of penile vein ligation for impotence from venous leakage

Between 1986 and 1991, 46 men with organic impotence documented by dynamic pharmacocavernosometry and cavernosography to have venous leakage underwent penile vein ligation. Despite initial improvement in erections allowing normal intercourse in 34 men (74%) within the first 6 months, long-term (more than 12 months) evaluation revealed sustained potency without adjunctive therapy in only 11 (24%). Of the remaining 35 men 6 (13%) progressed to a penile prosthesis, 8 (17%) required intracavernous vasoactive injection therapy and 21 (46%) have not sought further therapy despite continued impotence. Of the 14 patients who had isolated distal leakage 6 (43%) had sustained erectile function while only 5 of the 32 patients (16%) with proximal leakage maintained potency. Associated complications included penile shortening in 20 (43%) and penile hypoesthesia in 9 men (20%). Therefore, we conclude that the long-term success of penile vein ligation is poor, with only 24% of the patients able to have normal intercourse more than 1 year later, although those patients with distal penile shaft leakage appear to have a greater chance of success than those with more proximal leakage.

Inhibition of nerve stimulation-induced vasodilatation in corpora cavernosa of the pithed rat by blockade of nitric oxide synthase

1. The effect of inhibition of nitric oxide synthase by NG-nitro-L-arginine methyl ester (L-NAME) on nerve stimulation-induced vasodilation in corpora cavernosa was studied in the pithed rat. Corporal vasodilation was estimated by the increase in ratio (corpora cavernosal pressure/systemic blood pressure; CP/BP) following electrical stimulation of the sacral part of the spinal cord. 2. L-NAME (2, 5, 10 and 25 mg kg-1) caused an increase in BP and a dose-dependent inhibition of the rise in the CP/BP ratio following stimulation. 3. The inhibitory effect of L-NAME (25 mg kg-1) on the corporal response to spinal cord stimulation, as well as the pressor response, was partially prevented by prior administration of L- but not D-arginine (400 mg kg-1, i.v.). 4. L-NAME (20 mg kg-1, i.v.) did not inhibit the rise in corporal pressure resulting from direct intracavernosal administration of papaverine (400 micrograms over 2 min). However, this response was inhibited by 5-hydroxytryptamine (20 micrograms kg-1, i.v.). 5. The results are indicative of a role of nitric oxide (NO) in the corporal vasodilator response to erectile stimulation.

The role of cyclic adenosine monophosphate, cyclic guanosine monophosphate, endothelium and nonadrenergic, noncholinergic neurotransmission in canine penile erection

To elucidate the neuropharmacology of erection, we undertook an in vivo canine study to examine the role of cholinergic and nonadrenergic, noncholinergic (NANC) neuroeffectors and the sinusoidal endothelium in erection induced by electrostimulation. We also examined the effect of adenylate cyclase and guanylate cyclase blockers by intravenous injection of N-ethylmaleimide and methylene blue, respectively. In addition, the effects of intracavernous injection of the nitric oxide-releasing substance, nitroprusside, and bromocyclic adenosine monophosphate (AMP) and bromocyclic guanosine monophosphate (GMP) were also studied. In contrast to in vitro results, atropine reduced the increase of intracavernous pressure after neurostimulation (p = 0.029). Intracavernous injection of CHAPS to destroy the sinusoidal endothelium abolished the response to acetylcholine (p = 0.001), but only partially inhibited the response to electrostimulation (mean = 75% pressure increase, p = 0.022), indicating that neuronal nitric oxide plays a major role in penile erection. Methylene blue, a guanylate cyclase inhibitor, significantly inhibited the erectile response to both neurostimulation and sodium nitroprusside (p = 0.000 and 0.017, respectively). However, N-ethylmaleimide, an adenylate cyclase inhibitor, could not reduce the response to neurostimulation (p = 0.078). The erectile response to intracavernous injection of cGMP was significantly better than that induced by cAMP (p = 0.025). Our results suggest that both the cholinergic and NANC neuroeffectors and the sinusoidal endothelium are involved in erection. In addition, our data imply that the neuronal nitric oxide/cyclic GMP system is the most likely pathway for penile smooth muscle relaxation and erection.

Oxygen tension regulates the nitric oxide pathway. Physiological role in penile erection

Relaxation of the trabecular smooth muscle of the corpus cavernosum (the erectile tissue) of the penis is mediated by nitric oxide released by the nerves and endothelium. We have investigated the physiological role of oxygen tension in the regulation of trabecular smooth muscle tone. In human subjects, measurement of intracavernosal PO2 in blood drawn from corpus cavernosum in the flaccid state was comparable to that of venous blood (25-43 mmHg). Vasodilatation of the resistance arteries and trabecular smooth muscle relaxation by intracavernosal injection of papaverine and phentolamine caused oxygen tension to rise rapidly to arterial levels (PO2 approximately 100 mmHg). Isolated human and rabbit corpus cavernosum tissue strips in organ baths, exposed to arterial-like PO2 relaxed to the endothelium-dependent dilator acetylcholine and to electrical stimulation of the autonomic dilator nerves. These nitric oxide-mediated responses were progressively inhibited as a function of decreasing PO2 to levels measured in the flaccid penis. Reverting to normoxic conditions readily restored endothelium-dependent and neurogenic relaxation. Relaxation to exogenous nitric oxide was not impaired in low PO2. In rabbit corpus cavernosum, low PO2 reduced basal levels of cGMP and prevented cGMP accumulation induced by stimulation of dilator nerves. Furthermore, low PO2 inhibited nitric oxide synthase activity in corpus cavernosum cytosol. It is concluded that physiological concentrations of oxygen modulate penile erection by regulating nitric oxide synthesis in corpus cavernosum tissue.

Evidence against vasoactive intestinal polypeptide as the relaxant neurotransmitter in human cavernosal smooth muscle

1. The putative role of vasoactive intestinal polypeptide (VIP) as the relaxant neurotransmitter in human cavernosal smooth muscle has been studied in isolated tissue preparations. 2. Consistent neurogenic relaxations were evoked by electrical field stimulation (EFS; 2-64 pulses/train, 0.8 ms pulse duration, 10 Hz). VIP (0.1-3 microM) relaxed cavernosal smooth muscle in a dose-dependent fashion. Relaxant responses to both EFS and VIP were reduced in tissue from impotent men. 3. Neurogenic relaxant responses were not diminished in the presence of the VIP-inactivating peptidase, alpha-chymotrypsin (alpha-CT, 2 units ml-1). In contrast VIP-induced relaxations were completely abolished. 4. Inhibition of nitric oxide synthase by NG-nitro-L-arginine (30 microM), and of guanylate cyclase by methylene blue (50 microM) caused highly significant reductions of neurogenic relaxant responses whereas VIP-evoked relaxations were unaffected. 5. It is concluded that VIP-evoked relaxations are not mediated by the NO-guanosine 3':5'-cyclic monophosphate (cyclic GMP) pathway and that VIP release is not essential for neurogenic relaxation of human cavernosal smooth muscle. VIP does not therefore act as the major relaxant neurotransmitter in this tissue.

Diabetes mellitus impairs neurogenic and endothelium-dependent relaxation of rabbit corpus cavernosum smooth muscle

The effect of alloxan-induced diabetes on the reactivity of corporeal nerves, endothelium and smooth muscle was studied in the New Zealand white rabbit. Fifteen rabbits were randomly divided into treated (n = 6) and control (n = 9) groups. The treated group was maintained for 6 weeks. Two control groups were studied. One control group (n = 3) was maintained for 6 weeks as littermate controls for diabetic group. The second control group (n = 6) was not maintained but was weight matched with the 6 week diabetic group. The reactivity of corpus cavernosum tissue from the diabetic animals and the control animals was studied in organ chambers. When tissue contraction was produced with phenylephrine for the study of relaxation to various stimuli, the tension induced was similar in the diabetic and the control groups. Relaxation of corpus cavernosum tissue to electrical stimulation of autonomic nerves as well as relaxation to the endothelium-dependent vasodilator acetylcholine were comparably unaffected in the weight matched and littermate control groups while significantly inhibited in the diabetic group. Treatment of the corporeal tissue with the cyclooxgenase inhibitor indomethacin enhanced the relaxation to electrical stimulation and to acetylcholine in the control and in the diabetic groups but did not improve the significant difference in relaxation between the two groups. Relaxation of corporeal tissue to endothelium-independent vasodilators, papaverine and nitroprusside was similar in the control groups and the diabetic groups. It is concluded that diabetes impairs neurogenic and endothelium-mediated relaxation of rabbit corpus cavernosum smooth muscle. These findings are comparable to those described in corpus cavernosum tissue from diabetic men, showing the validity of this experimental animal model. The mechanism for the nerve or endothelial dysfunction does not appear to involve alteration in cyclooxygenase products of arachidonate or the ability of the corporeal smooth muscle to relax via a cGMP-dependent mechanism. Since nitric oxide has been shown to act as the nonadrenergic noncholinergic neurotransmitter as well as endothelium-derived relaxing factor (EDRF) of the trabecular smooth muscle, it is possible that impairment of neurogenic and endothelium-dependent relaxation due to diabetes is mediated by alteration in the synthesis or availability of nitric oxide in corporeal tissue.

Preliminary results with the nitric oxide donor linsidomine chlorhydrate in the treatment of human erectile dysfunction

Recent experimental studies showed an important role of endothelium derived relaxing factor for cavernous smooth muscle relaxation. Since nitric oxide seems to account for the biological actions of endothelium derived relaxing factor, a study was done to examine a possible role of the nitric oxide donor linsidomine chlorhydrate (SIN-1) in the treatment of erectile dysfunction. To determine a therapeutically useful dose 0.1, 0.2, 0.5 and 1 mg. SIN-1 were injected intracavernously in patients with erectile dysfunction. Each dose was given to 2 patients. Then, 63 patients received 1 mg. SIN-1, including 7 who had prolonged erections to minimal doses of papaverine plus phentolamine and 4 who did not respond with a full erection to other pharmacological agents. Intracavernous injection of SIN-1 induced a dose-dependent erectile response by increasing the arterial inflow and relaxing cavernous smooth muscles. Of the patients 29 had a full, 21 an almost full and 13 a moderate erection to 1 mg. SIN-1. There were no systemic or local side effects. In the patients with prolonged erections to papaverine plus phentolamine the mean duration of a full erectile response to SIN-1 was 57 minutes. Compared to the responses to a papaverine (15 mg./ml.) and phentolamine (0.5 mg./ml.) mixture, the erection induced by SIN-1 was superior in 10, comparable in 47 and inferior in 6 patients. Our data suggest a possible role for SIN-1 in the treatment of erectile dysfunction. Possible advantages may be that erection is induced by a mechanism similar to that occurring physiologically, a decreased risk of inducing prolonged erections and low therapy costs.

Characterization and localization of nitric oxide synthase in non-adrenergic non-cholinergic nerves from bovine retractor penis muscles

1. Partially purified soluble nitric oxide (NO) synthase was isolated from the bovine retractor penis muscle (BRP), a tissue in which the inhibitory response to non-adrenergic non-cholinergic nerve (NANC) stimulation appears to be mediated by NO or NO-like material. 2. NO synthase from BRP used L-arginine as a substrate, required NADPH, tetrahydrobiopterin, and FAD as co-factors and was Ca2+/calmodulin-dependent. The activity of NO synthase was inhibited by NG-methyl-L-arginine and NG-nitro-L-arginine, and haemoglobin blocked the effect of NO formed by the enzyme. 3. On reducing SDS polyacrylamide gel electrophoresis the apparent molecular mass of NO synthase from BRP was 160 +/- 2 kDa, which is similar to that of the cerebellar NO synthase. Protein immunoblot and immunoprecipitation showed that NO synthase from BRP cross-reacted with the selective antiserum to neuronal NO synthase from rat cerebellum. 4. Immunohistochemistry using the same antiserum demonstrated that NO synthase in BRP was located exclusively within nerve fibres. Thus, autonomic nerves synthesizing the NANC neurotransmitter seem to contain an isoform of NO synthase which is similar to that from rat cerebellum.

Effect of intracavernous simultaneous injection of acetylcholine and vasoactive intestinal polypeptide on canine penile erection

We investigated the effects of intracavernous injection of a combination of acetylcholine (ACh) and vasoactive intestinal polypeptide (VIP) on the erectile response in eleven adult male dogs. The minimum dose of ACh which increased the intracavernous pressure in eight dogs varied from 0.2 to 40 micrograms, and the minimum dose of VIP varied from 0.2 to 5 micrograms. When the minimum doses of ACh and VIP were injected simultaneously, a strong increase of intracavernous pressure (the mean increase was 102 cm. H2O from the baseline level) and a sustained erection (mean 5 min.) were observed in all eight dogs. The effect of simultaneous injection of both drugs was not additive but synergistic. Pretreatment with VIP-antibody and atropine intracavernously suppressed the erectile response induced by cavernous nerve stimulation. VIP may increase the affinity of muscarinic receptors for ACh in canine corpus cavernosum because pretreatment with atropine alone before the simultaneous injection of ACh and VIP completely abolished the effect of the combination. We conclude that ACh and VIP may play a cooperative role in canine penile erection.

Nitric oxide: a physiologic mediator of penile erection

Nitric oxide (NO) is a cytotoxic agent of macrophages, a messenger molecule of neurons, and a vasodilator produced by endothelial cells. NO synthase, the synthetic enzyme for NO, was localized to rat penile neurons innervating the corpora cavernosa and to neuronal plexuses in the adventitial layer of penile arteries. Small doses of NO synthase inhibitors abolished electrophysiologically induced penile erections. These results establish NO as a physiologic mediator of erectile function.

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.