The effects of long-term oral administration of L-arginine on the erectile response of rabbits with alloxan-induced diabetes

eNOS Function and Dysfunction in the Penis

Endothelial nitric oxide (NO) synthase (eNOS) has an indispensable role in the erectile response. In the penis, eNOS activity and endothelial NO bioavailability are regulated by multiple post-translatlonal molecular mechanisms, such as eNOS phosphorylation, eNOS interaction with regulatory proteins and contractile pathways, and actions of reactive oxygen species (ROS). These mechanisms regulate eNOS-mediated responses under physiologic circumstances and provide various mechanisms whereby endothelial NO availability may be altered in states of vasculogenlc erectile dysfunction (ED), in view of the recent advances in the field of eNOS function in the penis and its role in penile erection, the emphasis in this review is placed on the mechanisms regulating eNOS activity and its interaction with the RhoA/Rho-kinase pathway in the physiology of penile erection and the pathophysiology of ED.

Effects of chronic l-DOPA administration on neurogenic and endothelium-dependent relaxation responses in rabbit corpus cavernosum

BACKGROUND

Dopamine is a crucial central neurotransmitter that plays a fundamental role in the autonomic and somatic components of penile reflexes in animals and humans. Similar to the erectile responses of dopamine, systemic administration of l-DOPA induces yawning and penile erection in some species. The possible effects of l-DOPA on nitric oxide (NO)-dependent and -independent non-adrenergic non-cholinergic (NANC) relaxation responses mediated by electrical field stimulation (EFS) and endothelium-dependent relaxation were investigated in this study.

METHODS

Thirty-two adult albino male rabbits, in two- and four-week-treatment groups, were divided into three subgroups: control group (saline-injected) (n=4), 3mg/kg/day (low dose) l-DOPA-injected groups (n=6) and 12mg/kg/day (high dose) l-DOPA-injected groups (n=6). After the intraperitoneal injection treatments, the corpus cavernosum tissues were placed in organ bath chambers. The EFS-mediated responses, and the concentration-response curve to carbachol, sodium nitroprusside (SNP), sildenafil were assessed.

RESULTS

The two-week treatment with high-dose l-DOPA decreased the NO-dependent NANC relaxation responses, while there was no change in the low-dose two- and four-week treatment groups. The NO-independent NANC relaxation responses in the two-week groups decreased, and the responses in the four-week groups were unchanged when compared to the controls. The relaxation responses to carbachol showed no differences among all groups except for the high-dose four-week l-DOPA group. The relaxation responses of SNP and sildenafil were increased in all of the treatment groups when compared to the controls.

CONCLUSIONS

The observed increases in SNP- and sildenafil-induced responses, along with the decreased EFS-mediated responses, suggest increased sensitivity in the NO-signalling pathway following l-DOPA administration.

Effects of nucleotides adenosine monophosphate and adenosine triphosphate in combination with L-arginine on male rabbit corpus cavernosum tissue

Purines and more specifically adenosine monophosphate (AMP) and adenosine triphosphate (ATP) have a strong relaxant effect on smooth muscle cells of the dog, rabbit and human corpus cavernosum, to approximately the same degree as nitric oxide (NO). However, purines are considered as modulators of erectile function rather than key mediators. This suggests that the use of purines combined with NO donors could be effective to treat some specific erectile disorders. The relaxation induced by the combination of l-arginine (Arg), a natural substrate for NO synthase, was assessed with a purine-nucleotide (AMP, ATP) on a rabbit corpus cavernosum model, to determine if these substances could potentiate each other's effect. When a pre-contraction was induced by phenylephrine, AMP alone induced a 43% CC relaxation rate and ATP alone a 26% rate. The relaxation rate induced by Arg was lower in comparison (8% at 5.10(-4) m vs. 25% at AMP 5.10(-4) m and 15% at ATP 5.10(-4) m). NO synthase inhibitor n-nitro-l-arginine did not modify the relaxing effect provoked by AMP suggesting that the mechanism of action of this nucleotide does not involve the NO pathway. The combination of Arg at 5.10(-4) m with either AMP or ATP at different doses ranging from 5.10(-4) to 10(-3) m significantly enhanced the relaxing response reaching rates of 62 and 80% respectively, leading to a synergistic effect. The present data indicate that a 'NO donor' combined with an 'adenosine donor' could be an effective therapeutic approach.

Vardenafil and resveratrol synergistically enhance the nitric oxide/cyclic guanosine monophosphate pathway in corpus cavernosal smooth muscle cells and its therapeutic potential for erectile dysfunction in the streptozotocin-induced diabetic rat: preliminary findings

INTRODUCTION

Phosphodiesterase type 5 (PDE5) inhibitors are very effective agents for erectile dysfunction; however, specific patient populations are hard to treat. The efficacy of PDE5 inhibitors is limited because a minimum amount of nitric oxide (NO) is necessary. Resveratrol, a plant polyphenol, is reported to activate endothelial NO synthase (eNOS) through activation of sirtuin 1. We previously reported that human corpus cavernosal smooth muscle cells (CCSMCs) express eNOS and synthesize cyclic guanosine monophosphate (cGMP) via the NO/cGMP pathway.

AIM

To investigate the ability of resveratrol and/or vardenafil to increase cGMP in an in vitro model using CCSMCs and to improve erectile function in an in vivo rat model of streptozotocin (STZ)-induced diabetes.

METHODS

CCSMCs were treated with resveratrol and/or vardenafil. Twenty male Sprague-Dawley rats were randomly divided into five groups (N = 4 in each group): age-matched controls, diabetic controls, and diabetic rats treated with resveratrol, vardenafil, or both in combination for the last 4 weeks of an 8-week period of diabetes induction.

MAIN OUTCOME MEASURES

Intracellular cGMP measurement, intracovernous pressure (ICP)/mean arterial pressure (MAP) ratio, and smooth muscle/collagen ratio.

RESULTS

Intracellular cGMP level was elevated by resveratrol treatment in CCSMCs. The combination treatment of resveratrol and vardenafil had a synergistic effect. Diabetic rats showed impairment of erectile function. Treatment with either resveratrol or vardenafil improved ICP/MAP ratio, and combination therapy with resveratrol and vardenafil had a synergistic effect in improvement of ICP/MAP.

CONCLUSIONS

Treatment with either resveratrol or vardenafil elevated cGMP level in CCSMCs and improved erectile function in STZ-induced diabetic rats. Furthermore, a synergistic effect was observed in vitro and in vivo. Resveratrol or combination therapy of resveratrol and vardenafil can improve erectile function in which NO release is impaired, although further study is needed to confirm the results.

Treatment strategies for diabetic patients suffering from erectile dysfunction

Erectile dysfunction is a common complication of diabetes. Clinical practice has no treatment modality specifically designed for the difficult to treat diabetic erectile dysfunction due to the multifactorial and complex pathophysiology of development. PDE type 5 inhibitors are the first-line treatment option. Non-responders should have total and free testosterone checked and testosterone replacement is recommended for hypogonadal patients. For patients who cannot take PDE type 5 inhibitors, or are proven non-responders, the vacuum constriction device continues to serve as a major treatment option. Intracavernosal injection is the most effective medical therapy for diabetic erectile dysfunction despite its high dropout rate. Use of the Medicated Urethral System for Erection to overcome the disadvantages of needle injection is disappointing due to lack of effectiveness. Penile prosthesis will continue to play an important role in diabetic patients with severe erectile dysfunction in coming years.

Reactive oxygen species and erectile dysfunction: possible role of NADPH oxidase

Erectile dysfunction (ED) is a widespread condition, the incidence of which is increasing globally. ED is also indicative of underlying vasculopathy and represents a predictor of more serious cardiovascular disorders. Understanding the aetiology of ED may therefore provide invaluable pointers to the pathobiology of other cardiovascular diseases (CVDs) and syndromes. It follows, too, that therapeutic interventions that are successful in treating ED may, ipso facto, be effective in treating the early stages of conditions that include atherosclerosis, angina, plaque rupture and diabetic angiopathy. One common pathological denominator in both CVD and ED is oxidative stress, that is, the overproduction of reactive oxygen species (ROS), in particular, superoxide (O(2)(*-)) and hydrogen peroxide (H(2)O(2)). In this review, therefore, we consider the aetiology and pathobiology of O(2)(*-) in promoting ED and focus on NADPH oxidase as an inducible source of O(2)(*-) and H(2)O(2). Therapeutic strategies aimed at reducing oxidative stress to improve erectile function are also discussed.

Endothelial dysfunction in diabetic erectile dysfunction

Erectile dysfunction (ED) is highly prevalent in diabetes mellitus. Pathophysiological mechanisms underlying diabetes-associated ED are in large part due to endothelial dysfunction, which functionally refers to the inability of the endothelium to produce vasorelaxing messengers and to maintain vasodilation and vascular homeostasis. The precise mechanisms leading to endothelial dysfunction in the diabetic vasculature, including the penis, are not yet fully understood. Hyperglycemia affects endothelial nitric oxide synthase activity and nitric oxide production/bioavailability, nitric oxide-independent relaxing factors, oxidative stress, production and/or action of hormones, growth factors and/or cytokines, and generation and activity of opposing vasoconstrictors. Considering recent advances in the field of vascular biology and diabetes, the emphasis in this review is placed on the mechanisms of hyperglycemia-induced endothelial dysfunction in the pathophysiology of diabetes-associated ED.

Secondhand tobacco smoke impairs neurogenic and endothelium-dependent relaxation of rabbit corpus cavernosum smooth muscle: improvement with chronic oral administration of L-arginine

The first goal of this study was to examine the effect of secondhand smoking on neurogenic, endothelium- and cGMP-dependent relaxant responses of rabbit corpus cavernosum smooth muscle. Our second goal was to determine whether such an effect can be prevented by oral administration of L-arginine. Male New Zealand rabbits were divided into control, chronic passive cigarette smoking and L-arginine treatment groups. Relaxant or contractile responses in isolated corpus cavernosum smooth muscle strips were determined by using in vitro muscle technique. There was no significant difference in the relaxant response of the strips to papaverine, sodium nitroprusside and contractile response to KCl among the groups. Relaxant responses to acetylcholine and electrical field stimulation and contractile response to phenylephrine were significantly decreased in the strips of the smoking group than that of the control group. The impaired relaxations of strips were markedly improved by treatment of L-arginine, but the contractile responses to phenylephrine were not affected. These data indicate that secondhand smoking may impair both neurogenic and endothelium-dependent relaxation of corpus cavernosum smooth muscle, and may contribute to the etiology of impotence. Chronic dietary supplementation with L-arginine offsets the impairment of neurogenic and endothelial relaxation. Therefore, we suggest that secondhand smoking exposure to cigarette produces selective impairment of neurogenic and endothelium-dependent relaxation of corpus cavernosum smooth muscle via a mechanism related to the decreased production and/or availability of nitric oxide.

Therapeutic strategies for optimizing PDE-5 inhibitor therapy in patients with erectile dysfunction considered difficult or challenging to treat

Phosphodiesterase 5 (PDE5) inhibitors prevent the normal hydrolysis of cGMP. As the resulting cGMP accumulation facilitates penile smooth muscle relaxation, PDE5 inhibitors can partially reverse deficiencies in the nitric oxide (NO)/cGMP pathway to treat erectile dysfunction (ED). However, approximately 30-40% of men with ED do not respond to drug therapy. Patients with severe neurologic damage, diabetes mellitus, or severe vascular disease may be resistant to PDE5 inhibitors. Decreased expression or activity of neuronal or endothelial NO synthase (NOS), impaired NO release, or NO destruction will preclude sufficient cGMP formation to permit PDE5 inhibitor efficacy. This article discusses the possible reasons for unresponsiveness and strategies to overcome it. Therapeutic approaches proposed to increase available NO in penile tissue include facilitating NO release by using alpha-2 antagonists, enhancing NO synthesis by providing more substrate for the reaction, and using antioxidants to inhibit NO breakdown by reactive oxygen species.

Role of arginase in the male and female sexual arousal response

The NO-cGMP pathway plays a key role in the male and female genital sexual arousal response. Nitric oxide synthase (NOS) utilizes L-arginine and oxygen as substrates to produce nitric oxide (NO) and citrulline. Arginase is a metalloenzyme that catalyzes the hydrolysis of L-arginine to produce L-ornithine and urea. It is proposed that arginase competes for L-arginine and reduces NOS activity in genital tissues, thus modulating sexual function. Using 2 transition state analogue inhibitors of arginase, 2(S)-Amino-6-boronohexanoic acid (ABH) and S-(2-boronoethyl)-L-cysteine (BEC), we have characterized arginase activity in penile and vaginal tissue. Neither of these inhibitors has activity against NOS. Thus, ABH and BEC are useful compounds for examining the role of arginase in genital tissue physiology, without directly influencing NOS activity. We present data to suggest that arginase may regulate NO production by competing for endogenous pools of L-arginine. In this fashion, arginase is an indirect regulator of penile and vaginal blood flow and specific arginase inhibitors may improve genital blood flow during sexual arousal. As evidenced by the upregulation of arginase in specific disease states, its distribution in the vagina, and its modulation by sex steroid hormones, this enzyme may also participate in numerous other physiological and pathophysiological processes, such as tissue growth, fibrosis, and immune function.

Melatonin treatment protects against diabetes-induced functional and biochemical changes in rat aorta and corpus cavernosum

Enhanced oxidative stress due to diabetes is accepted to lead to endothelial dysfunction, and this is known to play a key role in the pathogenesis of diabetic vascular diseases and complications. This study was designed to determine the possible protective effect of melatonin and/or insulin treatment on the functional and biochemical changes caused by hyperglycemia in aorta and corpus cavernosum of diabetic rats. Wistar albino male rats were rendered diabetic by injecting streptozotocin (60 mg/kg, intraperitoneally (i.p.)). Melatonin (10 mg/kg, i.p.) and/or insulin (6 U/kg, subcutaneously (s.c.)) were administered for 8 weeks. In the diabetic group, the contractile responses of aortic strips to phenylephrine were significantly impaired (EC(50) 5.5 x 10(-7) M in diabetic and EC(50) 1.47 x 10(-7) M in the control group, P<0.001). Treatment with melatonin (EC(50) 4.6 x 10(-7) M) or insulin+melatonin (EC(50) 1.68 x 10(-7) M, P<0.001) improved the contractile responses. Acetylcholine caused a dose-dependent relaxation response (EC(50) 1.58 x 10(-7) M) which was impaired in the diabetic group (EC(50) 26 x 10(-7) M, P<0.001). There was less impairment in melatonin-, insulin- and insulin+melatonin-treated groups (EC(50) 11.61 x 10(-7), 7.3 x 10(-7) and 1.41 x 10(-7) M, respectively, P<0.01). Contractile responses to phenylephrine were also impaired in the corpus cavernosum strips (EC(50) 2.06 x 10(-5) M in diabetic and 0.94 x 10(-5) M in the control group, P<0.001). In the melatonin- (EC(50) 1.59 x 10(-5) M) and insulin+melatonin-treated (EC(50) 1.53 x 10(-5) M, P<0.5) groups contractile responses were improved. In the diabetic group, the relaxation responses of corpus cavernosum strips to acetylcholine were impaired (EC(50) 24.12 x 10(-5) M, P<0.001), and treatment with melatonin (EC(50) 0.68 x 10(-5) M), insulin (EC(50) 0.53 x 10(-5) M) or insulin+melatonin (0.98 x 10(-5) M, P<0.001) restored the responses to acetylcholine. In diabetic tissues, malondialdehyde levels were increased while glutathione levels were decreased, demonstrating oxidative damage. This was also prevented by treatment with melatonin or the melatonin and insulin combination. The diabetic state enhances the generation of free radicals, and both melatonin and insulin treatments reduced this oxidative stress; however, treatment with the combination was the most efficient in preventing diabetes-induced damage. Thus, our results suggested that giving diabetic patients adjuvant therapy with melatonin may have some benefit in controlling diabetic complications.

Downregulation of cGMP-dependent protein kinase-1 activity in the corpus cavernosum smooth muscle of diabetic rabbits

Increased guanosine 3',5'-cyclic monophosphate (cGMP), induced by nitric oxide release, is crucial for corpus cavernosum smooth muscle (CCSM) relaxation within the penis. This CCSM relaxation (necessary for penile erection) is impaired in men with erectile dysfunction (ED), especially those men with diabetes. One of the effector proteins for cGMP is cGMP-dependent protein kinase-1 (PKG-1). PKG-1 knockout mice exhibit detrusor overactivity (Am J Physiol Regul Integr Comp Physiol 279: R1112-R1120, 2000) and, more relevant to this study, ED (Proc Natl Acad Sci USA 97: 2349-2354, 2000), suggesting an in vivo role for PKG-1 in urogenital smooth muscle relaxation. In the current study, using normal rabbit CCSM, Western blot analysis revealed high expression of PKG-1 at levels almost equivalent to aorta (previously shown to have high PKG-1 expression) and that the two known alternatively spliced isoforms of PKG-1 (alpha and beta) are expressed in nearly equal amounts in the CCSM. However, in response to alloxan-induced diabetes, there was a decrease in expression of both PKG-1 isoforms at the mRNA and protein levels as determined by real-time RT-PCR and Western blotting, respectively, but with the PKG-1alpha isoform expression decreased to a greater extent. Moreover, diabetes was associated with significantly decreased PKG-1 activity of CCSM in vitro, correlating with decreased CCSM relaxation. Immunofluorescence microscopy revealed a diabetes-associated decrease in PKG-1 in the CCSM cells. In conclusion, our results demonstrate for the first time a significant downregulation of PKG-1 expression associated with decreased PKG-1 activity in the CCSM in response to diabetes. Furthermore, these results suggest a mechanistic basis for the decreased efficacy of phosphodiesterase V inhibitors in treating diabetic patients with ED.

Comparative effects of sildenafil, phentolamine, yohimbine and l-arginine on the rabbit corpus cavernosum

Penile erection involves relaxation of smooth muscle of corpus cavernosum and associated arterioles. Sildenafil, a highly selective inhibitor of phosphodiesterase type 5, is effective in the treatment of erectile dysfunction. The aim of this study is to investigate the effect of sildenafil on smooth muscle of the rabbit corpus cavernosum (RCC) and to compare its effect with those of phentolamine, yohimbine and L-arginine. The effects of sildenafil, phentolamine, yohimbine and L-arginine were studied on the response of the RCC to electrical field stimulation (EFS) as well as on the phenylephrine (PE, 3 x 10(-6) M)-induced tone. EFS caused transient, frequency-dependent relaxation of the RCC that was inhibited by the nitric oxide synthase inhibitor NG-nitro-L-arginine (3 x 10(-5) M). Sildenafil (1 x 10(-9)-1 x 10(-6) M) and phentolamine (1 x 10(-9)-1 x 10(-6) M) enhanced the EFS-induced relaxation in a concentration-dependent manner with ED50 of 0.056 +/- 0.004 and 0.572 +/-0.035 microM at 8 Hz, respectively, yohimbine (3 x 10(-7)-3 x 10(-5) M) and L-arginine (3 x 10(-6)-3 x 10(-4) M) did not show significant effects (ED50 at 8 Hz = 35.84 +/-2.24 and 2.164 +/- 0.174 microM, respectively). Sildenafil (1 x 10(-9) and 1 x 10(-8) M) potentiated the EFS-induced relaxation caused by L-arginine (3 x 10(-5) m). Sildenafil, phentolamine, yohimbine and L-arginine reduced the PE-induced tone to different extents; the ED50 values were 0.81 +/- 0.097, 0.49 +/- 0.025 and 13.97 +/- 1.10 microM, respectively. Maximum concentration of L-arginine used failed to produce 50% relaxation (ED20 = 221.82 +/- 15.71 microM). The muscle relaxant effects of different combinations of sildenafil and L-arginine on PE-induced tone did not differ significantly from the sum of the individual effects. The results demonstrate that sildenafil, when compared to other drugs used in penile erection dysfunction, shows the highest potency on the nitrergic transmission in the RCC. On the other hand, phentolamine was found to possess the highest potency in inducing relaxation of RCC proving that its action is independent on the stimulated neurogenic system. In addition, the combination of less effective doses of sildenafil and L-arginine has a potential advantage on erectile functions. The importance of this combination remains to be solved clinically.

Effects of Diabetes on Nitric Oxide-Mediated Relaxations in Male Rat Corpus cavernosum Smooth Muscle

INTRODUCTION

We evaluated the effects of diabetes on nitric oxide-mediated relaxations and nitric oxide synthase activity in male rat corpus cavernosum smooth muscles.

METHODS

Eight-week-old male rats were assigned to three groups: control (injected with the vehicle), DM (diabetes mellitus, induced by injection with 65 mg/kg streptozotocin), and TES (testosterone, testosterone supplemented after induction of diabetes). After 8 weeks, corpus cavernosum smooth muscle strips were mounted in an organ bath for isometric tension recordings. Electrical field stimulation (EFS, 2-ms pulse duration, 0.3-20 Hz and 3 s train) was applied to the strips precontracted with 30 microM phenylephrine. The microdialysis probe was inserted into the strip, and Krebs-Henseleit solution was perfused into the probe. The dialysate during EFS was collected, and the amount of NO(-)(2)/NO(-)(3) (NOx) released in the dialysate was measured by the Greiss method. Sodium nitroprusside (0.1 nM to 10 mM) and carbachol (1 nM to 10 mM) were cumulatively added to the strips precontracted with 30 microM phenylephrine.

RESULTS

EFS caused frequency-dependent relaxations and NOx releases of the strips. Pretreatment with N(omega)-nitro-L-arginine (100 microM) and tetrodotoxin (1 microM) completely inhibited the relaxations and NOx releases. The maximum relaxation was significantly greater in the DM group than in the control or TES group. The release of NOx was significantly greater in the DM group than in the control or TES group. Sodium nitroprusside, the endothelium-independent vasodilator, relaxed the tissues in all three groups. There were no significant differences among control, DM and TES groups in the maximum relaxation to sodium nitroprusside.

CONCLUSION

The present data suggest that diabetes enhances nitric oxide synthase activity and nitric oxide-mediated relaxations in the male rat corpus cavernosum by the reduced testosterone level in the diabetic animals.

Vasculogenic erectile dysfunction: newer therapeutic strategies

PURPOSE

Despite recent advances in therapy, reversal of vasculogenic erectile dysfunction (ED) is rarely possible. A review of vasculogenic ED may further our understanding of the underlying pathophysiology and help develop more effective curative therapy.

MATERIALS AND METHODS

We reviewed the mechanisms of vasculogenic erectile dysfunction and discuss the therapies currently available or being developed for possible future use.

RESULTS

Penile erection is a complex neurovascular phenomenon that may be affected by hypercholesterolemia, atherosclerotic vascular occlusive disease, veno-occlusive dysfunction and cavernosal fibrosis. Animal models of diffuse pelvic atherosclerosis can be maintained by feeding oral cholesterol and injuring the arterial endothelium. Impaired inflow may be addressed by penile revascularization but this strategy is applicable only in select cases. Neovascularization using vascular growth factors has recently been demonstrated to be feasible in animal models. Permanent reversal of impaired cavernosal relaxation requires control of hypercholesterolemia and lifestyle changes, such as smoking cessation. Cavernosal fibrosis may be reversible via some of the same approaches used in treating Peyronie's disease but to date little clinical success has been reported. Venous ligation appears to have a limited role in treating veno-occlusive dysfunction only in highly selected men with minimal cavernosal smooth muscle dysfunction. Hypoxemia, sleep apnea and respiratory failure may also affect erectile dysfunction. However, little attention has been paid to oxygen as therapy for ED.

CONCLUSIONS

Current therapy, while effective in circumventing vasculogenic ED, is relatively ineffective in permanently reversing the condition. Further research aimed at long-term treatment strategies in vasculogenic ED is needed.

Mediation of nitric oxide from photosensitive stores in the photorelaxation of the rabbit corpus cavernosum

Isolated rabbit corpus cavernosum relaxed in response to ultraviolet (UV) light (365 nm). The UV light-induced relaxation (photorelaxation) was diminished on repeated UV irradiation from 30.5+/-4.0% (the first photorelaxation) to 15.5+/-2.7% (the last photorelaxation). Hydroxocobolamine of 100 microM and hemoglobin (Hb) of 10 microM, which are nitric oxide (NO) scavengers, and 10 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a soluble guanylyl cyclase inhibitor, markedly reduced photorelaxation. However, 300 microM 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (carboxy-PTIO) failed to inhibit photorelaxation. NaNO(2) and N(G)-nitro-L-arginine (L-NA) but not 3-nitro-L-tyrosine (3-NT) were found to be photosensitive in that these compounds are photolysed to release NO, as demonstrated by use of an amperometric NO probe; NO signals produced by 500 microM NaNO(2), and 500 microM L-NA were 133.3+/-28.9 and 54.4+/-10.4 pA, respectively. Not 3-NT but the other compounds (all 200 microM) also enhanced photorelaxation of the cavernosal tissue. Based on these findings, the substance, which mediates photorelaxation, could be NO released from putative stores in the rabbit corpus cavernosum, and L-NA as well as NaNO(2) but not 3-NT produce NO under the influence of UV light.

Activation and potentiation of the NO/cGMP pathway by NG-hydroxyl-L-arginine in rabbit corpus cavernosum under normoxic and hypoxic conditions and ageing

1 When nitric oxide synthase (NOS) produces NO from N(G)-hydroxy-L-arginine (OH-arginine) instead of L-arginine, the total requirement of molecular oxygen and NADPH to form NO is reduced. The aim of this work was to evaluate the effects of OH-arginine on the contractility of rabbit corpus cavernosum (RCC) and to compare the capacities of L-arginine and OH-arginine to enhance NO-mediated responses under normoxic and hypoxic conditions and in ageing, as models of defective NO production. 2 OH-arginine, but not L-arginine, was able to relax phenylephrine-contracted rabbit trabecular smooth muscle. OH-arginine-induced relaxation was inhibited by the NOS-inhibitor, L-NNA (300 microM), and by the guanylyl cyclase inhibitor, ODQ (20 microM), while it was not affected by the cytochrome P450 oxygenase inhibitor, miconazole (0.1 mM). Administration of OH-arginine, but not L-arginine, produced a significant increment of cGMP accumulation in RCC tissue. 3 Relaxation elicited by OH-arginine (300 microM) was still observed at low oxygen tension. The increase of cGMP levels induced by ACh (30 microM) in RCC was significantly enhanced by addition of OH-arginine (300 microM) in normoxic conditions, as well as under hypoxia, while L-arginine did not alter the effects of ACh on cGMP accumulation. 4 Endothelium-dependent and nitrergic nerve-mediated relaxations were both significantly reduced in RCC from aged animals (>20-months-old) when compared with young adult rabbits (5-months-old). Treatment with OH-arginine (300 microM) significantly potentiated endothelium-dependent and neurogenic relaxation in corpus cavernosum from aged rabbits, while L-arginine (300 microM) did not have significant effects. 5 Results show that OH-arginine promotes NO-mediated relaxation of RCC and potentiates the NO-mediated responses induced by stimulation of endogenous NO generation in hypoxic and aged tissues. We propose that the use of OH-arginine could be of interest in the treatment of erectile dysfunction, at least in those secondary to defective NO production.

Oxygen free radicals and the penis

Penile erection is dependent upon vascular smooth muscle relaxation in erectile tissue and penile arteries, the principal mediator of relaxation being nitric oxide (NO). Evidence from basic scientific studies indicates that oxidative stress mediated through the superoxide radical (superoxide) and other reactive oxygen species (ROS) may be central to impaired cavernosal function in erectile dysfunction (ED). Increased inactivation of NO by superoxide results in impaired penile NO transmission and smooth muscle relaxation. Furthermore, propagation of endothelial dysfunction by ROS may result in chronic impairment of penile vascular function, a process analogous to early atherogenesis. Indeed, ED and atherosclerosis are closely linked through shared risk factors. Given our current understanding of ED pathophysiology, antioxidants may be of benefit in both the short- and long-term. Evidence supporting the paradigm of antioxidant therapy for the prevention or treatment of ED is presented herein.

Relaxation induced by omeprazole does not change in diabetic rabbit corpus cavernosum

OBJECTIVE

To investigate the changes in relaxation responses to omeprazole in corpus cavernosal strips from rabbits with diabetes induced by alloxan.

MATERIALS AND METHODS

Diabetes was induced in 10 New Zealand white rabbits. After 8 weeks, the reactivity to electrical-field stimulation, carbachol and omeprazole of corporal strips from the penises of the diabetic animals and from 10 untreated controls was assessed in organ chambers.

RESULTS

In the diabetic group, the relaxation responses of corpus cavernosal strips to omeprazole were comparable with those of the control group, whereas the relaxation responses to electrical field stimulation and carbachol were impaired.

CONCLUSION

The relaxant effect of omeprazole in corpus cavernosum was not affected by diabetes, in which neurogenic and endothelium-mediated relaxations were impaired.

Does diabetes mellitus affect the progress of tolerance to isosorbide dinitrate (ISDN) in corporal tissue?

For erection to take place, the penile arteries and sinusoids have to dilate, thereby increasing the blood flow into the penis. There is increasing evidence that release of l-arginine derived nitric oxide (NO) from nonadrenergic-noncholinergic (NANC) nerves and from the sinusoidal endothelium is a major event in penile smooth muscle relaxation and promotes the endogenous formation of cyclic guanosine monophosphate (cGMP). Nitrovasodilators can be attributed to the activation of soluble guanylate cyclase, resulting in an increase in intracellular level of cyclic guanosine monophosphate, but prolonged exposure to high levels of nitroglycerine and other organic nitroesters induces tolerance against the cardiovascular effect. In this study, the aim was to determine the effect of diabetes on the corporal smooth muscle relaxant effect of ISDN and the effect of diabetes on the process of tolerance to the drug. For this purpose, alloxan-induced diabetic rabbits were used to form diabetes group. The responses of the corpus cavernous strips obtained from control and alloxan-induced diabetic rabbit were studied in organ chamber. In conclusion, prolonged in vitro exposure of corpus cavernosum strips obtained from control and diabetic groups to high concentrations of ISDN caused significant desensitization to the relaxant effect the drug. So, prolonged exposure of corporal tissue to the agents like nitroglycerine, used for treatment of impotence, may render ineffective the therapy in diabetic erectile impotence. However, intolerance to nitric oxide provides a rationale for the concept of using nitro oxide agents (like SNP) in the treatment of diabetic erectile dysfunction.