Nitric oxide mediates relaxation in rabbit and human corpus cavernosum smooth muscle

Animal models of erectile dysfunction

Erectile dysfunction (ED) is a prevalent male sexual dysfunction with profound adverse effects on the physical and the psychosocial health of men and, subsequently, on their partners. The expanded use of various types of rodent models has produced some advances in the study of ED, and neurophysiological studies using various animal models have provided important insights into human sexual dysfunction. At present, animal models play a key role in exploring and screening novel drugs designed to treat ED.

Effects of adenosine monophosphate used in combination with L-arginine on female rabbit corpus cavernosum tissue

Introduction

Sexual dysfunction is significantly more prevalent in women than in men. However, to date, no satisfactory oral treatment is yet available.

Aim

The aim of this study was to study the effects of adenosine monophosphate (AMP) alone or its combination with L-Arginine on the relaxation of the female rabbit corpus cavernosum.

Methods

Cylinder strips from the corporal body of the excised clitoris from female New Zealand White rabbits were incubated in Krebs solution. Phenylephrine (PE) precontraction was achieved, then the drugs AMP and L-Arginine were administered either independently or in sequential combinations to the strips under precontracted conditions.

Main Outcome Measures

Contraction percentages were compared.

Results

When precontraction was induced by PE 8 μM or 20 μM, AMP was shown to induce relaxation up to 25% in a dose-dependent manner. The relaxation induced by L-Arginine reached 15.6% at 5.10⁻⁴ M vs. 16.5% at AMP 5.10⁻⁴ M under the same experimental conditions. Nitric oxide (NO) synthase inhibitor N-nitro-L-arginine strongly inhibited the relaxing effect provoked by AMP, suggesting that the action mechanism of this nucleotide is related to the NO pathway. The combination of L-Arginine at 5.10⁻⁴ M with AMP at different doses ranging from 5.10⁻⁴ M to 10⁻³ M significantly amplified the relaxing response up to 40.7% and 58%, respectively.

Conclusions

Our results demonstrate that AMP induces a relaxing effect on the female rabbit corpora. They also show that L-Arginine and AMP can potentiate each other and that a synergistic effect can be obtained by their combined use. Because only slight differences exist between both sexes in response to NO donors and/or nucleotide purines or in their use together, it is very likely that close biochemical mechanisms, although not to the same degree and not quite similar, are involved in the engorgement of the penis and the clitoris of New Zealand White rabbits. Stücker O, Pons C, Neuzillet Y, Laemmel E, and Lebret T. Original research-sexual medicine: Effects of adenosine monophosphate used in combination with L-Arginine on female rabbit corpus cavernosum tissue. Sex Med 2014;2:1–7.

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.

Phosphodiesterase inhibitors: history of pharmacology

The first pharmacological investigations of phosphodiesterase (PDE) inhibitors were developed with the clinical efficacies of drugs isolated from coffee, cacao and tea but only later their relevant ingredients were identified as xanthines that act as PDE. With its diuretic, inotropic and bronchodilating clinical efficacy, use of theophylline anticipated the clinical goals, which were later approached with the first-generation of weakly selective PDE inhibitors in the period from 1980 to 1990. Pharmacological and clinical research with these early compounds provided a vast pool of information regarding desired and adverse actions - although most of these new drugs had to be discontinued due to severe adverse effects. The pharmacological models for cardiac, vascular and respiratory indications were analysed for their PDE isoenzyme profiles, and when biochemical and molecular biological approaches expanded our knowledge of the PDE superfamily, the purified isoenzymes that were now available opened the door for more systematic studies of inhibitors and for generation of highly selective isoenzyme-specific drugs. The development of simple screening models and clinically relevant indication models reflecting the growing knowledge about pathomechanisms of disease are summarised here for today's successful application of highly selective PDE3, PDE4 and PDE5 inhibitors. The interplay of serendipitous discoveries, the establishment of intelligent pharmacological models and the knowledge gain by research results with new substances is reviewed. The broad efficacies of new substances in vitro, the enormous biodiversity of the PDE isoenzyme family and the sophisticated biochemical pharmacology enabled Viagra to be the first success story in the field of PDE inhibitor drug development, but probably more success stories will follow.

Characterization of nitrergic function in monkey penile erection in vivo and in vitro

The nitrergic nerve appears to have a major role in the neuronal regulation of penile erection. Cholinergic innervation has been shown histochemically in penile cavernous tissues, but its functional role is not well understood. This study was aimed at examining the functional properties of the nitrergic nerve and the possible involvement of cholinergic function in the regulation of monkey penile erection in vivo and in vitro. In anesthetized Japanese monkeys, electrical stimulation of the cavernous nerve caused a frequency-dependent increase in intracavernous pressure and penile erection, and atropine enhanced the pressure response. Intravenous injections of N(G)-nitro-L-arginine (L-NA) markedly inhibited the stimulation-induced pressure increase and the erectile response, and L-arginine partially restored the pressure response. In some monkeys, the intracavernous pressure increase caused by nerve stimulation was reversed by treatment with L-NA; however, L-arginine restored the pressor response. In addition, hexamethonium suppressed the pressure increase that resulted from the nerve stimulation. In corpus cavernosum isolated from monkeys, transmural electrical stimulation elicited frequency-dependent relaxation. The relaxation was attenuated by physostigmine, and was potentiated by atropine. Relaxation was markedly inhibited by treatment with L-NA. It appears that nitric oxide (NO) released from inhibitory nerves, even at low frequencies, has a pivotal role in the initiation and maintenance of intracavernous pressure increase and penile erection in monkeys. Prejunctional muscarinic receptors in nitrergic nerves are expected to participate in the impairment of NO release. Nitrergic nerves responsible for penile erection may originate from ganglia close to the corpus cavernosum.

Endothelial function in patients with vasculogenic erectile dysfunction

OBJECTIVES

Erectile dysfunction (ED) commonly coexists with coronary artery disease (CAD) and/or risk factors for atherosclerosis. Because the silent or documented atherosclerosis or vascular risk factors are very frequent, the possibility of endothelial dysfunction in ED patients is expected to be increased. Our aim was to evaluate the endothelial functions in patients with vasculogenic ED with vascular risk factors and compare them with age-matched non-ED patients or healthy controls.

DESIGN

We studied 36 patients with presumed vasculogenic ED, 39 age-matched patients with similar risk factors without ED and 25 age-matched healthy controls without ED, known cardiovascular disease or risk factors. Erectile function was evaluated by the International Index of Erectile Dysfunction (IIEF) scores. Brachial artery flow-mediated dilatation (FMD) and nitroglycerine-mediated dilatation (NMD) were measured.

RESULTS

Baseline demographics were similar except the IIEF score and duration of diabetes in patients with ED. Brachial artery FMD and NMD were significantly reduced in patients with ED (3.2+/-3. vs. 6+/-4, p<0.0001 for FMD, 12.2+/-6 vs. 15.4+/-6 p=0.032 for NMD). In patients with similar risk factors but without ED, FMD was significantly lower but NMD were not different compared with healthy controls (6+/-4 vs. 10.2+/-3, p<0.0001 for FMD and 15.4+/-8 vs. 16.4+/-6, p=0.81). IIEF scores were weakly correlated with FMD (r=0.25, p=0.028) in patients with ED. There were significant correlations between FMD and NMD in patients with ED (r=0.46, p=0.05) and with risk factors (r=0.72, p<0.0001) but not in healthy controls (r=0.54, p=0.792). Vasculogenic ED patients have more markedly impaired endothelial and smooth muscle functions compared with patients with similar risk factors but no ED.

Effects of β-adrenoceptor antagonists in the neural nitric oxide release induced by electrical field stimulation and sodium channel activators in the rabbit corpus cavernosum

Beta-Adrenoceptor antagonists may present receptor-independent mechanisms, such as blockade of voltage-gated sodium channels. This study aimed to investigate the effects of non-selective (propranolol), and selective beta1- (atenolol, metoprolol and betaxolol) and beta2-adrenoceptor (ICI 118,551) antagonists in the nitric oxide (NO)-mediated rabbit corpus cavernosum relaxations induced by either electrical field stimulation (EFS) or activators of voltage-gated sodium channels. The sodium channel blockers tetrodotoxin and saxitoxin abolished the relaxations induced by EFS or sodium channel activators of binding site-2 (aconitine and veratridine), site-3 (Ts3 toxin), site-4 (Ts1 toxin) and site-5 (brevetoxin-3). The beta-adrenoceptor antagonists failed to affect the relaxations induced by EFS, aconitine and veratridine. Relaxations induced by Ts3 and Ts1 toxins, as well as brevetoxin-3, were markedly reduced by prior addition of propranolol, betaxolol and ICI 118,551. During the established relaxation induced by Ts3 toxin, propranolol failed to restore the basal tone. In conclusion, beta-adrenoceptor antagonists may cause an allosteric inhibition at the binding site-3, -4 and -5 of voltage-gated sodium channels, leading to blockade of neural NO release.

Impact of diabetes mellitus on the severity of erectile dysfunction and response to treatment: analysis of data from tadalafil clinical trials

AIMS/HYPOTHESIS

A retrospective analysis of pooled data from twelve placebo-controlled trials was conducted to characterise the efficacy and safety of tadalafil for the treatment of erectile dysfunction in men with diabetes compared with that in men without diabetes.

METHODS

Patients were randomly allocated to tadalafil 10 mg, 20 mg, or placebo, taken as needed for 12 weeks. The study population comprised 637 men with diabetes (mean age 57 years) and 1681 men without diabetes (mean age 56 years).

RESULTS

At baseline, patients with diabetes had more severe erectile dysfunction than patients without diabetes, with mean International Index of Erectile Function (IIEF) erectile function domain scores of 12.6 and 15.0 respectively (p<0.001). Compared with placebo, tadalafil 10 mg and 20 mg improved all primary efficacy outcomes in both patient groups (p<0.001). Men with diabetes receiving tadalafil 20 mg experienced a mean improvement of 7.4 in their IIEF erectile function domain score against baseline versus 0.9 for placebo (p<0.001). This group reported on average that 53% of their attempts at intercourse were successful, compared with 22% for placebo (p<0.001 for the change from baseline). Baseline IIEF erectile function domain scores correlated inversely with baseline HbA(1)c levels. The responses to tadalafil were similar regardless of levels of baseline glycaemic control, diabetic therapy received, or previous use of sildenafil.

CONCLUSIONS/INTERPRETATION

Despite more severe baseline erectile dysfunction in men with diabetes, tadalafil was efficacious and well tolerated in this population. As reported for other phosphodiesterase 5 inhibitors, the response to tadalafil was slightly lower in men with diabetes than in men without diabetes.

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.

The Distribution of Neuronal and Inducible Nitric Oxide Synthase in Urethral Stricture Formation

PURPOSE

The distribution of neuronal (n) and inducible (i) nitric oxide synthase (NOS) may have a role in the maintenance of normal urethral spongiosum and during the development of spongiofibrosis in urethral stricture disease.

MATERIALS AND METHODS

Eight normal and 33 strictured human bulbar urethras were studied by histological and immunohistochemical techniques for the neuronal markers S-100, nNOS and iNOS. The smooth muscle-to-collagen ratio was calculated by morphometric analysis of Masson's trichrome sections. Immunohistochemical staining patterns of the neuronal markers in normal urethral tissue was compared to that in urethral stricture tissue with spongiofibrosis.

RESULTS

The smooth muscle-to-collagen ratio was significantly lower in the strictured urethra compared to that in the control group (p = 0.001). In the strictured bulbar urethra nNOS immunoreactivity was decreased compared to that in normal urethral tissue. The severity of spongiofibrosis corresponded to the loss of nNOS immunoreactivity. iNOS immunoreactivity was found in strictured urethral epithelium and spongiosal tissue, whereas the control group was nonimmunoreactive to iNOS.

CONCLUSIONS

Urethral stricture formation is a fibrotic process associated with significant changes in NOS metabolism. Abnormal collagen synthesis following urethral trauma may be stimulated by inappropriate iNOS activity. A functional nerve supply to the urethral spongiosum seems to be crucial in the maintenance of the unique ultrastructure of the urethral spongiosum.

Contractile role of M2 and M3 muscarinic receptors in gastrointestinal, airway and urinary bladder smooth muscle

Both M(2) and M(3) muscarinic receptors are expressed in smooth muscle and influence contraction through distinct signaling pathways. M(3) receptors interact with G(q) to trigger phosphoinositide hydrolysis, Ca(2+) mobilization and a direct contractile response. In contrast, M(2) receptors interact with G(i) and G(o) to inhibit adenylyl cyclase and Ca(2+)-activated K(+) channels and to potentiate a Ca(2+)-dependent, nonselective cation conductance. Ultimately, these mechanisms lead to the prediction that the influence of the M(2) receptor on contraction should be conditional upon mobilization of Ca(2+) by another receptor such as the M(3). Mathematical modeling studies of these mechanisms show that the competitive antagonism of a muscarinic response mediated through activation of both M(2) and M(3) receptors should resemble the profile of the directly acting receptor (i.e., the M(3)) and not that of the conditionally acting receptor (i.e., the M(2)). Using a combination of pharmacological and genetic approaches, we have identified two mechanisms for the M(2) receptor in contraction: 1) a high potency inhibition of the relaxation elicited by agents that increase cytosolic cAMP and 2) a low potency potentiation of contractions elicited by the M(3) receptor. The latter mechanism may be involved in muscarinic agonist-mediated heterologous desensitization of smooth muscle, which requires activation of both M(2) and M(3) receptors.

Involvement of L-arginine/nitric oxide pathway at the paraventricular nucleus of hypothalamus in central neural regulation of penile erection in the rat

The objective of this study is to investigate whether the L-arginine/nitric oxide pathway is involved in the neurotransmission of paraventricular nucleus of hypothalamus (PVN) activation-induced penile erection in the rat. Male adult Sprague-Dawley rats anesthetized with pentobarbital were used. The femoral artery was cannulated to measure systemic and mean arterial pressure (SAP and MAP), and heart rate (HR). A 26-gauge needle was inserted into corpus cavernosum to measure the intracavernous pressure (ICP) simultaneously with SAP, MAP and HR on a polygraph. Four groups of study were arranged: (1) stereotaxically delivery of L-arginine (500 nmol/500 nl) into PVN; (2) administration of a mixture (1 microl) containing N(G)-Nitro-L-arginine methyl ester (L-NAME) 500 nmol and L-arginine 500 nmol into PVN; (3) microinjection of saline 500 nl into PVN as a vehicle control; and (4) intracavernous injection of L-arginine (100 nmol/50 microl). The ICP, SAP, MAP and HR were monitored for at least 2 h after each administration of the experimental agents. Upon administration of L-arginine into PVN, there was a significant increase of ICP from resting 9.6+/-2.5 mmHg to peaked at 64.4+/-9.8 mmHg after a latency of 3016.0+/-1749.7 s and with a duration of 27.6+/-15.8 min. There was no change of resting ICP after administration of the mixture of L-NAME and L-arginine into PVN. Application of saline to PVN and intracavernous injection of L-arginine failed to increase ICP. Based on elicitation of penile erection upon administration of L-arginine into PVN, and elimination of this L-arginine induced penile erection by co-administration of L-NAME with L-arginine, the results of this study suggest that L-arginine/nitric oxide pathway may be involved in the neurotransmission of PVN activation-induced penile erection in the rat.

General use of animal models for investigation of the physiology of erection

In review, animal models have accounted significantly for the amazing strides made in the field of sexual dysfunction research. Fundamentally, they have offered a unique experimental approach to test many hypotheses regarding sexual function. Since their early use for sexual physiology research, there has been increasing sophistication using animals involving techniques for stimulating and monitoring sexual responses. One specific area that has been advanced is the use of conscious animal models to obtain a better sense of the natural contexts for sexual physiology and to avoid pharmacological interference associated with anesthetics. Another area of interest is the increasing use of simple but valid techniques to record and assess sexual responses. Efforts to develop and evaluate animal models that replicate disorders of sexual function have also been most advantageous. In the future, animal models will remain useful. The expanded applications of animal models include the study of predisposing disease states associated with sexual dysfunction and the study of all aspects of sexual dysfunction, in both male and female subjects. Continued judgment must be applied, understanding the advantages of one or another animal model, to explore questions and provide answers that are most scientifically relevant to the human condition. The promise of advancing therapies in this field indicates the additional prominent role for animals for the purposes of drug development.

Combination of phentolamine and L-arginine or sildenafil synergistically improves neurogenic relaxation of rabbit corpus cavernosum smooth muscle

OBJECTIVES

To analyze the effects of combining an alpha-adrenergic receptor antagonist, phentolamine, with an enhancer of the nitric oxide/cyclic guanosine monophosphate pathway (L-arginine or sildenafil) on neurogenic relaxations of rabbit corpus cavernosum (RCC).

METHODS

Studies were performed on isolated RCC tissue in organ chambers. Transmural electrical stimulation (TES) was applied at increasing frequencies (0.5 to 6 Hz) on endothelin-contracted RCC strips, and the responses were evaluated.

RESULTS

The activation of alpha(2)-adrenergic receptors with UK 14304 (0.3 microM) significantly inhibited the relaxation induced by TES in RCC strips in which adrenergic neurotransmission was blocked with guanethidine (10 microM). The relaxant responses produced by TES application on RCC strips without guanethidine were not significantly affected by the treatment with L-arginine or sildenafil but were significantly augmented by phentolamine (2.7-fold increase in maximum relaxation). Furthermore, the combinations of phentolamine with L-arginine or sildenafil markedly increased the relaxations evoked by the application of TES in RCC tissue, significantly more than those obtained in the presence of phentolamine alone (4.5 or 4.7-fold increase of maximum relaxation, respectively).

CONCLUSIONS

Our results demonstrated a synergistic interaction between the alpha-adrenergic blockade and the potentiation of the nitric oxide/cyclic guanosine monophosphate pathway to increase neurogenic relaxation of trabecular smooth muscle relaxation. This fact suggests that the combination of alpha-adrenergic receptor blockade with L-arginine or sildenafil could represent a therapeutic advantage in the treatment of erectile dysfunction.

The molecular basis of penile erection

Over the past two decades our understanding of the physiology and the various mediators involved in the pathway of penile erection has greatly increased. This has enabled us to develop effective pharmacological treatments for ED.

The effects of diabetes on nitric oxide-mediated responses in rat corpus cavernosum

Nitric oxide (NO)-mediated responses were investigated in corpora cavernosa isolated from 8-week diabetic rats. Relaxations to field stimulation were abolished by N(G)-nitro-L-arginine (NOARG, 100 microM). Responses to stimulation and sodium nitroprusside were reduced in tissues from diabetic rats compared to control rats, when data were expressed as g tension, but not when expressed as g/g tissue. The endothelium-dependent vasodilator, acetylcholine, failed to relax tissues. Stimulation-induced contractions were smaller in the diabetic group compared to the control group when data were expressed as g tension, but not g/g tissue. Contractions were enhanced by NOARG, and inhibited by acetylcholine (300 microM), by a similar degree in both groups. NOARG reduced the inhibitory effect of acetylcholine in tissues from control, but not diabetic rats. The results suggest diabetes caused a general impairment in responsiveness of rat corpus cavernosum, which may be a consequence of tissue weight change. A role for endothelium-dependent NO could not be identified; however, NO-mediated modulation of noradrenergic transmission by acetylcholine, may be defective in diabetes.

Histopathological changes and nitric oxide synthase activity in corpus cavernosum from rats with neurogenic erectile dysfunction

OBJECTIVE

To investigate changes in histology and nitric oxide synthase (NOS) activity in cavernosal tissues from rats with neurogenic erectile dysfunction induced experimentally.

MATERIALS AND METHODS

Twenty-four adult male Sprague-Dawley rats were divided equally into three groups and underwent a sham operation (control, group 1), unilateral (group 2) or bilateral (group 3) cavernosal nerve resection. Three months later they were killed and the cavernosal tissues analysed histologically by light and transmission electron microscopy, with NOS activity detected using an NADPH-diaphorase staining technique.

RESULTS

On light and electron microscopy, while penile nerves and cavernosal smooth muscle cells had a normal morphological appearance in the eight control rats, there were degenerative changes of the myelinated penile nerves and axonal fibrosis in groups 2 and 3. However, these changes were not significant. Using NADPH-diaphorase staining, NOS activity was detected in all three groups in endothelial cells and cavernosal structures. However, the staining was more intense in endothelial cells and cavernosal muscles of rats in group 2 than in the other groups.

CONCLUSION

NOS activity was increased in the cavernosal tissue after cavernosal denervation, but the pharmacological action of nitric oxide may be impaired.

Nitrergic relaxation of the horse corpus cavernosum. Role of cGMP

The involvement of nitric oxide (NO) and the mechanisms mediating neurogenic relaxation were investigated in the horse corpus cavernosum. NADPH-diaphorase activity was expressed in nerve fibres around arteries and muscular bundles in the horse trabecular tissue. Relaxations in response to electrical field stimulation were tetrodotoxin (10(-6) M)-sensitive, indicating their neurogenic origin. The NO synthase inhibitor, L-NO-arginine (L-NO-Arg, 3 x 10(-5) M), abolished the electrically induced relaxations, which were significantly reversed by L-arginine (3 x 10(-3) M). Exogenous NO (10(-6)-10(-3) M) evoked relaxations which were unaffected by L-NO-Arg. 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 5 x 10(-6) M), an inhibitor of guanylate cyclase activation by NO, reduced the relaxations in response to electrical stimulation and exogenous NO. Iberiotoxin (3 x 10(-8) M) or apamin (5 x 10(-7) M), inhibitors of large and small conductance Ca2+-activated K+ channels, respectively, and glibenclamide (3 x 10(-6) M), a blocker of ATP-sensitive K+ channels, failed to modify the relaxations with NO. It is suggested that NO is present in nerve fibres of the horse corpus cavernosum and relaxes smooth muscle through a guanylate cyclase-dependent mechanism. Neither Ca2+-activated nor ATP-sensitive K+ channels seem to be involved in these relaxations.

Effects of sildenafil on the relaxation of human corpus cavernosum tissue in vitro and on the activities of cyclic nucleotide phosphodiesterase isozymes

PURPOSE

Sildenafil, an inhibitor of cGMP-specific phosphodiesterase 5 (PDE5), is currently undergoing evaluation as an oral therapy for penile erectile dysfunction. The aims of this study were to investigate the mechanism of action of sildenafil on the neurogenic relaxation of human corpus cavernosum (HCC) in vitro and to determine the activity of sildenafil against a full range of PDE isozymes.

MATERIALS AND METHODS

Strips of HCC tissue were precontracted with phenylephrine. Relaxation responses resulting from electrical field stimulation (EFS) were then determined in the presence and absence of sildenafil. The effects of sildenafil on PDE1 to 5 prepared from human tissues and PDE6 from bovine retina were determined by measuring the conversion of [3H]-cGMP or [3H]-cAMP to their respective [3H]-5'-mononucleotides.

RESULTS

Sildenafil (0.001 to 1 microM) enhanced the EFS-induced, nitric oxide (NO) dependent, relaxation of HCC in a concentration-dependent manner to a maximum of 3 times the pretreatment level at 1 microM sildenafil. Compared with zaprinast, an early PDE5 inhibitor, sildenafil was approximately 240-fold more potent, inhibiting PDE5 from HCC with a geometric mean IC50 of 3.5 nM. For sildenafil, IC50 values for inhibition of PDE1 to 4 were 80 to more than 8500 times greater than that for PDE5 and the IC50 for PDE6 (33 nM) was approximately 9-fold greater.

CONCLUSIONS

The data support the proposal that enhancement of penile erection by sildenafil in patients with erectile dysfunction involves potentiation of the NO-stimulated cGMP signal mediating relaxation of cavernosal smooth muscle during sexual stimulation. Sildenafil is a potent inhibitor of PDE5 from HCC, with high selectivity for PDE5 relative to other PDE isozymes.

Subtypes of the muscarinic receptor in smooth muscle

Muscarinic receptors are expressed in smooth muscle throughout the body. In most instances, the muscarinic receptor population in smooth muscle is composed of mainly the M2 and M3 subtypes in an 80% to 20% mixture. The M3 subtype mediates phosphoinositide hydrolysis and calcium mobilization, whereas the M2 subtype mediates an inhibition of cAMP accumulation. In addition, a variety of ionic conductances are elicited by muscarinic receptors. Muscarinic agonists stimulate a nonselective cation conductance that is pertussis toxin-sensitive and dependent on calcium. The pertussis toxin-sensitivity of this response suggests that it is mediated by M2 receptors. Following agonist induced depolarization of smooth muscle, voltage dependent calcium channels are activated to enable an influx of calcium. In some instances, muscarinic agonists enhance this conductance through a mechanism involving protein kinase C, whereas in other instances, muscarinic agonists suppress this calcium conductance. Smooth muscle often contains calcium activated potassium channels that tend to repolarize the membrane following calcium influx. Activation of muscarinic receptors suppresses this potassium conductance in some smooth muscles. Under standard conditions, muscarinic agonists elicit pertussis toxin-insensitive contractions through activation of the M3 receptor. When most of the M3 receptors are inactivated, it is possible to measure a pertussis toxin-sensitive contractile response to muscarinic agonists that is most likely mediated through M2 receptors. M2 receptors also cause an indirect contraction by inhibiting the relaxant effects of agents that increase cAMP (e.g., forskolin and isoproterenol).

An in vitro study of nonadrenergic-noncholinergic activity on the cavernous tissue of mouse

The relaxant effects of electrical field stimulation (EFS) and exogenously applied acetylcholine (ACh) or acidified NaNO2 (a-NaNO2) were investigated in the isolated mouse corpus cavernosum precontracted with phenylephrine hydrochloride (PE). Tetrodotoxin (TTX) blocked the relaxant effects of EFS completely, whereas it had no effect on the responses to ACh or a-NaNO2. Guanethidine and indomethacin failed to affect the electrically or ACh-induced relaxations. Atropine completely blocked the effect of ACh; however, it caused a slight reduction in the relaxation evoked by EFS. NG-Nitro-L-arginine (L-NOARG) reduced the effects of EFS and ACh significantly, but it was ineffective on the relaxations induced by a-NaNO2. The inhibitory action of L-NOARG was partly restored by L-arginine, but not by D-arginine. Methylene blue (MB) and hydroxocobalamin (HC) exhibited significant inhibition on the relaxations evoked by EFS, ACh and a-NaNO2. Hydroquinone (HQ) reduced relaxation due to a-NaNO2, but did not affect that of EFS and ACh. Our findings suggest that EFS-induced relaxations of mouse cavernosal tissue are mediated by a transmitter which probably resembles an organic nitrate.

Characterization of in vitro relaxant mechanisms in erectile tissue from rabbits of different ages

In the present study we investigated the in vitro relaxant response of erectile tissue obtained from rabbits of different ages (3, 7 and 24 months) in order to detect the progression with age of cavernosal activity in response to substances acting via endothelium-dependent or -independent mechanisms. Noradrenaline induced a concentration-dependent contraction (0.1 microM-3 mM), with an increase in the contractility in the 24-month-old group. Acetylcholine produced a concentration-dependent relaxant effect in the three age groups, with a reduction of the maximal relaxant effect in older animals. ATP (10 microM-1 mM) and adenosine (10 microM-1 mM) induced a concentration-dependent relaxant effect that was higher in the older group. The presence of the NO2-synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) (0.1 mM) or of the P2-purinoceptor antagonist suramin did not affect ATP relaxation. Relaxation induced by sodium nitrite and nifedipine was reduced in older animals. In conclusion, aging selectively alters the in vitro responsiveness of rabbit erectile tissue. Purinergic system remains more active despite a decrease in the maximal endothelial cholinergic activity and the direct smooth muscle relaxant component.

Nitric oxide donor, linsidomine chlorhydrate (SIN-1), in the diagnosis and treatment of erectile dysfunction: critical appraisal and review of the literature

Recent experimental work has demonstrated that nitric oxide (NO) is the neutrotransmitter responsible for cavernous smooth muscle relaxation. Different studies on the performance of the direct NO-donor linsidomine chlorhydrate (SIN-1) in patients with erectile dysfunction have come to conflicting results ranging from highest praise to complete dismissal. We reviewed all published studies on the use of SIN-1 for intracorporeal injection in erectile failure including our own. To this date, 3 groups published their data. Only the uncontrolled data from Hannover claim good results. The controlled data from Hamburg and Berlin on patients with erectile failure due to venous leakage, to a mixed aetiology in a double-blind fashion and to mixed aetiology with drug increase (1 mg versus 2 mg of SIN-1) showed a significantly worse performance of SIN-1 compared to the standard drug for penile injection, prostaglandin E1. We conclude that there is no place for linsidomine chlorhydrate in either the diagnosis or the treatment of erectile dysfunction.

Diabetes mellitus increases nitric oxide synthase in penises but not in major pelvic ganglia of rats

OBJECTIVE

To determine the effect of diabetes mellitus (DM) on erectile function and evaluate the levels of nitric oxide synthetase (NOS) activity in streptozotocin-induced diabetic rats.

MATERIALS AND METHODS

Rats were studied at 9 weeks and 14 weeks after the induction of DM by streptozotocin and compared with untreated control rats. Erectile potency was assessed physiologically by testing and recording mating behaviour. NOS activity was assayed in penile tissues and major pelvic ganglia (MPG) by conversion of [3H] L-arginine to [3H] citrulline. Histological, ultrastructural and immunohistochemical studies of penile tissues were performed in similar groups of rats.

RESULTS

Diabetes mellitus adversely and significantly degraded all parameters of mating behaviour, thus indicating defective erectile potency. However, NOS activities in penile tissues from both groups of diabetic rats were significantly higher than those in controls (P < 0.01). In MPG, NOS activities were not significantly different between diabetic and control rats (P > 0.05). Histological, ultrastructural and immunohistochemical studies of penile tissues revealed no significant differences between control and diabetic rats, indicating an intact effector organ (smooth muscles) in rats with up to 14 weeks of DM.

CONCLUSION

The impotence frequently observed in diabetic subjects would suggest that despite the increase in NOS activity in the penis, the pharmacological action of nitric oxide is impaired.

Nitroxergic innervation of the human ureterovesical junction

Nitric oxide synthase (NOS) immunohistochemistry and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry were used to investigate the distribution of nitroxergic, i.e., nitric oxide-synthesizing, neuronal perikarya and processes in the human ureterovesical junction (UVJ). Tissue specimens obtained from two cadaver kidney donors and two patients undergoing radical cystectomy for bladder cancer were examined. Clusters of NOS-immunoreactive neurons were localized in extramural ureterovesical ganglia. NOS-containing nerve fibers traveled within large extramural nerve trunks and marched among smooth muscle bundles. Extramural and intramural blood vessels were encircled by varicose NOS-positive axonal processes. The distribution of NOS immunoreactivity paralleled the staining pattern for NADPH-d activity. Urothelium stained strongly for NADPH-d activity but showed no NOS immunolabeling. Specimens from all four patients investigated showed similar staining patterns. Our results suggest that nitric oxide, a potent smooth-muscle-relaxing neurotransmitter in the autonomic nervous system, plays a physiologic role in opening the human UVJ.

Nitric oxide control of lower genitourinary tract functions: a review

It is apparent that evolving concepts of the regulatory basis for functions in the pelvis must take into account the role exerted by nitric oxide. A recently characterized messenger molecule, nitric oxide has been associated with numerous physiologic processes. Intense investigations of this molecule have extended its importance to several genitourinary functions. Penile erection, micturition, peristalsis of the male excurrent duct system, contractile properties of the prostate, and lumbosacral spinal cord neurotransmission are all functions that may transpire under some degree of control by nitric oxide. Impotence, urinary obstruction, or ejaculatory problems, in turn, may represent alterations of nitric oxide production or action. The strategic manipulation of nitric oxide or its mechanism of action, possibly by pharmacologic means, may restore or produce desired functional effects. These possibilities, therefore, suggest that the advancing knowledge of nitric oxide in the genitourinary tract may be of enormous clinical value in the future.

Smooth muscle tone regulation in rabbit cavernosal and spongiosal tissue by cyclic AMP- and cyclic GMP-dependent mechanisms

The relaxing effects of several specific and nonspecific inhibitors of phosphodiesterases (PDE) on rabbit isolated corpus cavernosum (CC) and spongiosum (CS) were investigated. Preparations were mounted in organ baths, and isometric tension was recorded. The results were compared with the effects of direct administration of analogs of the second messenger cyclic nucleotides and the effects of forskolin, a direct stimulator of adenylate cyclase, and the nitric oxide donor 3-morpholinosydnonimine (SIN 1). All drugs relaxed the phenylephrine-induced contractions in CC and CS in a dose-dependent fashion. In CC and CS, type III (SK&F 95654) and type V (zaprinast and dipyramidole) PDE inhibitors, as well as the nonspecific inhibitors papaverine and trequinsin, showed no differences in IC50. The type IV inhibitor rolipram relaxed CC and CS at significantly lower concentrations (p < 0.005) than any other PDE inhibitor, and in CC the type III and IV inhibitor zardaverine was more potent (p < 0.05) than SK&F 95654. SIN 1 stimulates guanylate cyclase and effectively inhibits contractions in CC and CS. Activation of adenylate cyclase by forskolin also was highly effective (p < 0.005). It is concluded that PDE inhibition constitutes an effective relaxing mechanism in rabbit CC and CS. The marked effects of the different types of PDE inhibitors support the importance of cyclic guanosine 3',5'-monophosphate and cyclic adenosine 3',5'-monophosphate in smooth muscle relaxation in erectile tissue.

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.

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.

Risk of radiocontrast nephropathy in patients with and without diabetes mellitus

The present study was designed to test whether altered renovascular reactivity is associated with the increased risk of radio-contrast nephropathy (RCN) in diabetics. We studied 50 patients (24 diabetics, 26 nondiabetics) with chronic renal insufficiency undergoing cardiac catheterization. Patients were randomized to receive either saline, or one of three renal vasodilator/diuretic drugs--dopamine, atrial natriuretic peptide (ANP), or mannitol--by intravenous infusion during cardiac catheterization. Renal blood flow (RBF) was measured by thermodilution at various time points during cardiac catheterization. RCN was defined as an increase in PCr of at least 25% over baseline within 48 hours of cardiac catheterization. Baseline PCr and creatinine clearance were similar in diabetics and nondiabetics (2.6 +/- 0.2 mg/dl vs. 2.4 +/- 0.1 mg/dl, and 32 +/- 3 ml/min vs. 34 +/- 3 ml/min, respectively), but baseline RBF was significantly lower in diabetics (154 +/- 21 ml/min/kidney vs. 277 +/- 36 ml/min/kidney, P < 0.05). Diabetic patients exposed to the three vasodilator/diuretic drugs had the greatest increase in RBF throughout cardiac catheterization. The incidence of RCN among the diabetics receiving those drugs was 83%, 83% and 75%, in the dopamine, ANP and mannitol groups, respectively. In contrast, among the nondiabetics in each of those groups the incidence of RCN was zero (all P < 0.05, diabetics vs. nondiabetics). In the saline control group the rates of RCN in the diabetics and nondiabetics were 43% and 38%, respectively (NS). In conclusion, the increased risk of RCN among diabetics was associated with exaggerated renovascular reactivity: baseline vasoconstriction and enhanced vasodilation with vasodilator/diuretic drugs. These same drugs, however, reduced the risk of RCN in nondiabetic patients.

The major pelvic ganglion is the main source of nitric oxide synthase-containing nerve fibers in penile erectile tissue of the rat

The possible implication of nitric oxide synthase (NOS) in penile erection was examined by utilizing NADPH histochemistry in the rat. NADPH histochemistry indicated that the major pelvic ganglion (MPG), a well-known origin of nerve fibers supplying the external genitalia, contained many NOS-positive neurons. On the other hand, NOS-positive nerve fibers in penile erectile tissue observed in the walls of both arteries and veins, as well as in intrinsic smooth muscles. The retrograde tracing study with Fluoro-Gold (FG) in combination with NADPH histochemistry revealed that almost all MPG neurons which were retrogradely labeled with FG injected into the penile crura were NOS-positive. Thus, the MPG was considered to be the main source of NOS-positive nerve fibers in penile erectile tissue.

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)

Effect of nitric oxide-donor, linsidomine chlorhydrate, in treatment of human erectile dysfunction caused by venous leakage

Recent experimental work has demonstrated that nitric oxide (NO) is the neurotransmitter responsible for cavernous smooth muscle relaxation. We studied the effect of a direct NO-donor, linsidomine chlorhydrate (SIN-1), in 30 patients with venous leakage confirmed by dynamic pharmacocavernosography and pharmacocavernosometry that was refractory to prostaglandin E1 (PGE1) under the assumption that the more physiologic approach might give better results. In all 30 patients, response to SIN-1 was no better, and in 22 cases it was less than the response to PGE1. No systemic or local side effects of SIN-1 were observed. SIN-1 is not superior to PGE1 in the treatment of erectile dysfunction caused by venous leakage, and failure of NO-mediated smooth muscle relaxation does not play a part in the entity, "venous leakage."

Evaluation of vasculogenic impotence by monitoring of cavernous oxygen tension

Color coded duplex sonography, regarded as the gold standard in penile vascular evaluation, does not yield data on cavernous oxygenation itself. In addition to using color coded duplex sonography to measure peak flow velocity in cavernous arteries after injection of 20 micrograms. prostaglandin E1 in 34 unselected patients with impotence, we monitored cavernous oxygen tension with oxygen-sensitive Eppendorf needle electrode. During flaccidity the mean cavernous oxygen tension of 38 mm. Hg increased to 61 mm. Hg after injection of prostaglandin E1. Peak flow shown with color coded duplex sonography and maximal oxygen tension correlated well in 24 men (71%). However, in 10 men (29%) normal peak flow did not result in a cavernous oxygen tension of greater than 65 mm. Hg, so this might have been isolated cavernous perfusion defects. In contrast, there was no case of impaired arterial inflow and high oxygen tension. Monitoring of cavernous oxygen tension allows for characterization of patients with cavernous perfusion deficiency. This new and simple diagnostic method might help to improve diagnosis and followup after penile vascular surgery. However, more data on patients and controls will be required to define normal ranges.