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

Generating comprehensive comparative evidence on various interventions for penile rehabilitation in patients with erectile dysfunction after radical prostatectomy: a systematic review and network meta-analysis

Background

We aim to present a comprehensive comparison of various treatments in the management of penile recovery after radical prostatectomy (RP) and provide recommendations for future research.

Methods

Literature search of electronic databases including PubMed, the Cochrane Library, Embase, PsycInfo, and Web of Science, and manual retrieval were conducted from inception through March 2020. “Erectile dysfunction” and “prostatectomy” were used as the Mesh terms. The patients, intervention, comparison, outcome, and study design (PICOS) approach were used to define study eligibility. Two authors independently selected studies, evaluated the methodological quality, and extracted data using Cochrane Collaboration’s tools. The data analysis was completed by STATA version 14.2.

Results

A total of 24 studies with 3,500 patients were incorporated in the final analysis after screening 6,131 records. Our findings indicated that vacuum constriction devices (VCD) ranked 1st which meant that patients in VCD group had the best effect regarding mean IIEF scores within 3 months after RP, and no significant difference was observed between VCD and VCD with 20 mg/day tadalafil (V20DT) (MD: 5.44; 95% CI: −0.81 to 11.69). VCD and 50 mg/day sildenafil (VC50DS) showed superiority over 50 mg/day sildenafil (50DS) (MD: 3.75; 95% CI: 2.74–4.76) and intraurethral alprostadil 125–250 µg (MD: 3.05; 95% CI: 0.38 to 5.72), respectively. Moreover, V20DT showed significant superiority over the other interventions for ≥6 months mean International Index Erectile of Function (IIEF) scores after RP. Monotherapy appeared to have similar efficacy in terms of mean IIEF scores and proportion of patients return to baseline, and the effect of phosphodiesterase type 5 inhibitors (PDE5is) did not seem to be affected by the patterns of administration (regular or on demand).

Conclusions

The combination therapy showed certain advantages over monotherapy, and we recommended the combination of VCD and PDE5is to be considered in the clinical management of penile rehabilitation after RP.

Stuttering priapism: insights into pathogenesis and management

Priapism is defined as a persistent, painful erection that continues beyond, or is unrelated to, sexual stimulation. It may be categorized as either ischemic (low/absent flow) or nonischemic (high flow). Stuttering priapism is a variant of the ischemic type that is characterized by repetitive, transient, painful, self-limiting episodes of priapism. It is associated with various hematological disorders, including sickle cell disease and pharmacological treatments. The consequences of ineffective treatment of priapism are erectile dysfunction and impaired quality of life due to chronic pain and physical disfigurement. Many of the existing medical therapeutic options for treatment of stuttering priapism are nonmechanistic and associated with significant adverse effects. However, the scientific knowledge of stuttering priapism has transitioned in the past few years, from a condition that is poorly understood to one that has borne a burst of evolving molecular science. In this review, the pathophysiology of priapism is discussed, with particular emphasis on new molecular effectors and mechanisms. Novel treatment methods, as well as potential future agents, based on the emerging molecular evidence are discussed.

Analysis of erectile responses to BAY 41-8543 and muscarinic receptor stimulation in the rat

INTRODUCTION

Soluble guanylate cyclase (sGC) is the receptor for nitric oxide (NO) and in pathophysiologic conditions where NO formation or bioavailability is impaired, erectile dysfunction (ED) occurs.

AIM

The aim of this study was to investigate erectile responses to the sGC stimulator BAY 41-8543 in physiologic and pathophysiologic conditions.

METHODS

Increases in intracavernosal pressure (ICP) in response to intracavernosal (ic) injections of BAY 41-8543 were investigated in the anesthetized rat.

MAIN OUTCOME MEASURES

Increases in ICP/MAP in response to ic injections of BAY 41-8543 and the interaction of BAY 41-8543 with exogenous and endogenously released NO were investigated and the effect of the sGC stimulator on cavernosal nerve injury was assessed. The mechanism of the increase in ICP/MAP in response to ic injection of acetylcholine was investigated.

RESULTS

The ic injections of BAY 41-8543 increased ICP/MAP and the duration of the response. BAY 41-8543 was less potent than sodium nitroprusside (SNP) and ic injections of BAY 41-8543 and SNP produced a larger response than the algebraic sum of responses to either agent alone. Simultaneous ic injection of BAY 41-8543 and cavernosal nerve stimulation produced a greater response than either intervention alone. Atropine and cavernosal nerve crush injury decreased the response to nerve stimulation and ic injection of BAY 41-8543 restored the response.

CONCLUSION

These data show that BAY 41-8543 has significant erectile activity and can synergize with exogenous and endogenously released NO. This study shows that atropine and nerve crush attenuate the response to cavernosal nerve stimulation and that BAY 41-8543 can restore the response. The results with atropine, L-NAME and hexamethonium indicate that the response to ic injection of acetylcholine is mediated by muscarinic receptors and the release of NO with no significant role for nicotinic receptors. These results suggest that BAY 41-8543 would be useful in the treatment of ED.

The future is today: emerging drugs for the treatment of erectile dysfunction

IMPORTANCE OF THE FIELD

Erectile dysfunction (ED) is the most common male sexual dysfunction presented for treatment affecting between 10 and 20% of men. PDE type 5 inhibitors (PDE5I) now account for the largest segment of the ED market. While these drugs are highly efficacious for many men, a relatively large subset of ED patients who do not respond to PDE5I is increasingly recognized.

AREAS COVERED IN THIS REVIEW

In this review, we discuss clinical and preclinical evidence supporting various emerging compounds that regulate penile erection both centrally (clavulanic acid, dopamine and melanocortin receptor agonists) and peripherally (novel PDE5I, soluble and particulate guanylil cyclase activators, rho-kinase inhibitors and maxi-K channel openers).

WHAT THE READER WILL GAIN

The reader will gain a broad understanding of erectile (patho-)physiology and gain insights in the mechanisms of action, efficacy and adverse events of various compounds under development for the treatment of ED.

TAKE HOME MESSAGE

We expect emerging drugs to allow treatment protocols tailored to the specific needs of each individual patient, taking into consideration the efficacy of erectile performance enhancement and the potential for adverse events. This tailored approach may include combination of various emerging drugs to enhance efficacy in difficult-to-treat patients.

Chronic ethanol consumption induces cavernosal smooth muscle dysfunction in rats

OBJECTIVES

To investigate the effects of chronic ethanol consumption on nitric oxide (NO)-mediated relaxation in rat cavernosal smooth muscle (CSM).

METHODS

Male wistar rats were divided into 2 groups: control and ethanol. CSM obtained from both groups were mounted in organ chambers for measurement of isometric tension. Contraction of the strips was induced by electrical field stimulation (EFS, 1-32 Hertz) and phenylephrine. We also evaluated the effect of ethanol consumption on the relaxation induced by acetylcholine (0.01-1000 micromol L(-1)), sodium nitroprusside (SNP, 0.01-1000 micromol L(-1)), or EFS (1-32 Hz) in strips precontracted with phenylephrine (10 micromol L(-1)). Blood ethanol, serum testosterone levels, and basal nitrate generation were determined. Immunoexpression of endothelial NO synthase (eNOS) and inducible NO synthase (iNOS) was also accessed.

RESULTS

Ethanol intake for 4 weeks significantly increased noradrenergic nerve-mediated contractions of CSM in response to EFS. The endothelium-dependent relaxation induced by acetylcholine decreased after the ethanol treatment. Ethanol consumption decreased serum testosterone levels but did not affect the nitrate levels on rat CSM. The mRNA and protein levels for eNOS and iNOS receptors were increased in CSM from ethanol-treated rats.

CONCLUSIONS

Ethanol consumption reduces endothelium-dependent relaxation induced by acetylcholine, but does not affect SNP or EFS-induced relaxation, suggesting that ethanol disrupts the endothelial function. Despite the overexpression of eNOS and iNOS in ethanol-treated rats, the impaired relaxation induced by acetylcholine may suggest that chronic ethanol consumption induces endothelial dysfunction.

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.

Effect of hypothyroidism on the NO/cGMP pathway of corpus cavernosum in rabbits

INTRODUCTION

The incidence of hormonal dysfunction as a cause of impotence remains controversial. However, several recent studies have reported evidence of hormonal abnormalities in 25-35% of impotent men. Hypothyroidism has been reported to occur in 6% of impotent men. There is some evidence suggesting that hypothyroidism may be a cause of impotence.

AIM

We aimed to investigate the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway in hypothyroidism in an experimental rabbit model and compared hypothyroid rabbits with controls to evaluate the possible involvement of the NO/cGMP pathway.

METHODS

The study comprised 20 male New Zealand white rabbits. The rabbits were divided into two equal groups. The first group had hypothyroidism induced surgically by thyroidectomy for 6 weeks. The second group underwent a sham operation.

RESULTS

There was no significant change in the mean body weight of hypothyroid rabbits and controls. Triiodothyronine and thyroxine levels were significantly lower in hypothyroid rabbits. Plasma thyroid-stimulating hormone and prolactin levels were significantly higher in hypothyroid rabbits. Plasma total calcium and parathormone levels remained in the normal range in both groups.

MAIN OUTCOME MEASURES

Papaverine-induced concentration-dependent relaxations were similar in both groups. Carbachol-induced relaxation responses decreased in hypothyroid rabbits. There were significant differences between control and hypothyroid rabbits in frequency-dependent relaxations induced by electrical-field stimulation (EFS). YC-1-induced relaxation responses did not change significantly in hypothyroid rabbits. Concentration-dependent relaxations induced by diethylamine (DEA)/NO were similar in both groups. Amrinone-induced relaxation responses did not change significantly in hypothyroid rabbits.

CONCLUSION

Reductions of relaxant responses to EFS and carbachol in hypothyroid rabbits can depend on the decrease of released or synthesized NO from nitrergic nerves and endothelium.

Physiology of erectile function

INTRODUCTION

There are numerous investigations concerning the balance and interactions between relaxant and contractile factors regulating penile smooth muscle (arterial and trabecular) tone, the determinant of penile flaccidity or erection. Enhanced knowledge of erectile physiology may improve management of men with erectile dysfunction. Aim. To provide state-of-the-art knowledge on the physiology of erectile function.

METHODS

An international consultation in collaboration with the major urology and sexual medicine associations assembled over 200 multidisciplinary experts from 60 countries into 17 committees. Committee members established specific objectives and scopes for various male and female sexual medicine topics. The recommendations concerning state-of-the-art knowledge in the respective sexual medicine topic represent the opinion of experts from five continents developed in a process over a two-year period. Concerning the physiology of erectile function and pathophysiology of erectile dysfunction committee, there were seven experts from five countries.

MAIN OUTCOME MEASURE

Expert opinion was based on the grading of evidence-based medical literature, widespread internal committee discussion, public presentation, and debate.

RESULTS

Key roles in the mechanism determining the tone of penile smooth muscle are played by the rise of the intracellular concentration of free calcium and the sensitivity of the contractile machinery to calcium, endothelial health, endothelium-derived nitric oxide, endothelium-derived hyperpolarizing factor (EDHF), neuronal nitric oxide, cyclic guanosine monophosphate-dependent protein kinase and phosphodiesterase type 5.

CONCLUSIONS

A number of new mechanisms have been identified for the local regulation of penile smooth muscle contractility and therefore penile erection. Molecules participating in these pathways can be considered targets for the development of new treatments to treat erectile dysfunction.

Effect of hypothyroidism on the nitrergic relaxant responses of corpus cavernosal smooth muscle in rabbits

BACKGROUND

The incidence of hormonal dysfunction as a cause of impotence remains controversial. However, several recent studies have reported evidence of hormonal abnormalities in 25-35% of impotent men. Hypothyroidism has been reported to occur in 6% of impotent men.

METHODS

In the present study, we examined nitrergic responses in hypothyroidism in rabbit corpus cavernosum and compared them with controls.

RESULTS

Carbachol-induced relaxation responses and electrical field stimulation (EFS)-induced frequency-dependent relaxations decreased significantly in hypothyroid rabbits. Papaverine and sodium nitroprusside (SNP)-induced relaxation responses did not change significantly in hypothyroid rabbits. The contraction responses of phenylephrine and EFS-induced frequency-dependent contractions were significantly decreased in the hypothyroid group.

CONCLUSIONS

We can speculate that the reduction of relaxant responses to EFS and carbachol in hypothyroid rabbits can depend on a decreased release of nitric oxide (NO) from nitrergic nerves and endothelium or a reduction of muscarinic receptor density. Also, decreases in contraction responses may depend on diminished adrenoceptor density.

Penile erection requires association of soluble guanylyl cyclase with endothelial caveolin-1 in rat corpus cavernosum

Erectile dysfunction is caused by a variety of pathogenic factors, particularly impaired formation and action of nitric oxide (NO). NO released from nerve endings and corpus cavernosum endothelial cells plays a crucial role in initiating and maintaining increased intracavernous pressure, penile vasodilatation, and penile erection. Classically, these effects are dependent on cGMP synthesized during activation of soluble guanylyl cyclase (sGC) by NO in smooth muscle cells. The enzyme NO synthase in endothelial cells has been localized to caveolae, small invaginations of the plasma membrane rich in cholesterol. Membrane cholesterol depletion impairs acetylcholine-induced relaxation in arteries attributed to an alteration in caveolar structure. It has been shown that sGC may be activated in endothelial caveolae contributing to vasodilation. We hypothesized that caveolae are the platform for sGC/cGMP signaling in cavernosum smooth muscle eliciting erection. Methyl-beta-cyclodextrin, a pharmacological tool to deplete membrane cholesterol and disassemble caveolae, impaired rat erectile responses in vivo and cavernosum smooth muscle relaxation induced by the NO donor sodium nitroprusside and the sGC activator 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole in vitro. Methyl-beta-cyclodextrin had no effect on cavernosum smooth muscle relaxation induced by NO released upon nerve stimulation or by exogenous cGMP. Furthermore, sGC and caveolin-1, the major coat protein of caveolae, were colocalized in rat corpus cavernosum sinusoidal endothelium. Electron microscopy indicated caveolae disruption in corpus cavernosum treated with methyl-beta-cyclodextrin. In summary, our results provide evidence of compartmentalization of sGC in the caveolae of cavernosal endothelial cells contributing to NO signaling mediating smooth muscle relaxation and erection.

Plasma levels of endothelin-1, angiotensin II, nitric oxide and prostaglandin E2 in the venous and cavernosal blood of patients with erectile dysfunction

OBJECTIVES

To determine the alterations in the plasma levels of endothelin-1, angiotensin II, nitric oxide (NO) and prostaglandin E(2) (PGE(2)) in the venous and cavernosal blood of patients with organic and psychogenic erectile dysfunction (ED).

PATIENTS, SUBJECTS AND METHODS

The study included 32 patients complaining of ED; they were subdivided into two equal groups with either organic or psychogenic ED. Fifteen healthy potent age-matched male volunteers were enrolled as a control group. For each patient, venous and cavernosal blood samples were obtained, while venous blood was obtained from the controls.

RESULTS

There were significantly greater mean plasma levels of endothelin-1 and angiotensin II, and significantly lower mean plasma levels of NO and PGE(2), in the venous blood of patients with ED than in the controls. Patients with organic ED had significantly higher levels of endothelin-1 and significantly lower levels of NO in both venous and cavernosal blood than had those with psychogenic ED. There were significant positive correlations in both venous and cavernosal blood between endothelin-1 and angiotensin II, and between NO and PGE(2) in all patients with ED and the two subgroups. There were significant negative correlations between venous and cavernosal endothelin-1 and NO, endothelin-1 and PGE(2), angiotensin II and NO, and between angiotensin II and PGE(2).

CONCLUSION

The present results suggest that endothelin-1 could be a clinical marker of diffuse endothelial disease manifested by ED. As angiotensin-converting enzyme (ACE) activity controls angiotensin II there might be a rationale for the use of ACE inhibitors to prevent or treat ED. NO and PGE(2) may provide new strategies for the pharmacological treatment of ED.

Neural control of erection

Penile erection is a vascular event controlled by the autonomic nervous system. The spinal cord contains the autonomic preganglionic neurons that innervate the penile erectile tissue and the pudendal motoneurons that innervate the perineal striated muscles. Sympathetic pathways are anti-erectile, sacral parasympathetic pathways are pro-erectile, and contraction of the perineal striated muscles upon activity of the pudendal nerves improves penile rigidity. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of either eliciting erection or modulating or inhibiting an erection already present. Sensory information from the genitals is a potent activator of pro-erectile spinal neurons and elicits reflexive erections. Some pre-motor neurons of the medulla, pons and diencephalon project directly onto spinal sympathetic, parasympathetic and pudendal motoneurons. They receive in turn sensory information from the genitals. These spinal projecting pathways release a variety of neurotransmitters, including biogenic amines (serotonin, dopamine, noradrenaline, and adrenaline) and peptides that, through interactions with many receptor subtypes, exert complex effects on the spinal network that controls penile erection. Some supraspinal structures (e.g. the paraventricular nucleus and the medial preoptic area of the hypothalamus, the medial amygdala), whose roles in erection have been demonstrated in animal models, may not project directly onto spinal pro-erectile neurons. They are nevertheless prone to regulate penile erection in more integrated and coordinated responses of the body, as those occurring during sexual behavior. The application of basic and clinical research data to treatment options for erectile dysfunction has recently proved successful. Pro-erectile effects of phosphodiesterase type 5 inhibitors, acting in the penis, and of melanocortin agonists, acting in the brain, illustrate these recent developments.

Effects of C-peptide on expression of eNOS and iNOS in human cavernosal smooth muscle cells

OBJECTIVES

To investigate the role of C-peptide alone or in conjunction with insulin on the expression of nitric oxide synthase (NOS) in human corpus cavernosum smooth muscle cells (HCSMCs). Erectile dysfunction, among diabetic patients, is a significant health problem. The specific causes of erectile dysfunction are unknown. It has been suggested that impairment of penile relaxation is related to a reduction of penile NOS. Plasma levels of C-peptide and insulin are decreased in individuals with type 1 diabetes and late-stage type 2 diabetes.

METHODS

Primary cultures were initiated from explants of HCSMCs. Confluent cells at passages 2 to 4 were assigned to one of four groups with the following incubation conditions: (a) 27 mM glucose, (b) 27 mM glucose and insulin, (c) 27 mM glucose and human recombinant (hr)C-peptide, and (d) 27 mM glucose, insulin, and hrC-peptide. After 24 hours, total RNA and protein were extracted from cells and subjected to reverse transcriptase-polymerase chain reaction and Western blot analysis, respectively. Intracellular Ca(2+) was examined under the four conditions, using the Fura 2 method.

RESULTS

The least expression of endothelial NOS (eNOS) and inducible NOS (iNOS) in HCSMCs was observed in cells exposed to 27 mM glucose alone. Increased expression of eNOS and iNOS was found after treatment with insulin or hrC-peptide alone, and the maximal expression of eNOS and iNOS was detected in HCSMCs exposed to both insulin and hrC-peptide. Western blot analyses using eNOS and iNOS antibodies confirmed the RNA data. These effects are likely mediated by the insulin-induced and/or C-peptide-induced increase in intracellular Ca(2+).

CONCLUSIONS

Our results demonstrated that C-peptide, in the presence of insulin, increases the expression of iNOS and eNOS in HCSMCs. These results suggest that C-peptide, especially in conjunction with insulin, may have beneficial effects on cavernosal smooth muscle relaxation.

Relaxation effects of adrenomedullin in isolated rabbit corpus cavernosum smooth muscle

OBJECTIVES

To clarify the pharmacological effects of adrenomedullin, a potent vasodilator and hypotensive peptide isolated from human phaeochromocytoma cells, on corpus cavernosal smooth muscle in vitro, as the intracavernosal injection of adrenomedullin induces penile erection in the anaesthetized cat.

MATERIALS AND METHODS

The effects of adrenomedullin were investigated in isolated muscle strips from New Zealand rabbit corpus cavernosum smooth muscle pre-contracted with phenylephrine alone, in the presence of indomethacin (cyclooxygenase inhibitor), Nomega-nitro l-arginine methyl ester (L-NAME, a nitric oxide synthase inhibitor), and K+-channel blockers.

RESULTS

Adrenomedullin caused relaxation of isolated pre-contracted rabbit corpus cavernosum strips in a concentration-dependent manner. The response of corpus cavernosum was unaffected L-NAME, indomethacin and K+-channel blockers.

CONCLUSION

The relaxation exerted by adrenomedullin in rabbit corporal tissue may arise from the effect of the drug on its specific receptors and/or calcitonin gene-related peptide-1 receptors. The relaxant effect of adrenomedullin might lead to novel clinical applications for erectile dysfunction.

Changes of NADPH-diaphorase reactivity in lumbar spinal cord of short-term streptozotocin induced diabetic rats

Diabetes is an endocrine and metabolic disorder often associated with erectile dysfunction and peripheral neuropathy. Among other factors, penile erection is induced by activation of nitric oxide synthase (NOS). Hypothalamic paraventricular nuclei neurons produce NO and project to spinal cord areas implicated in penile reflexes. These nuclei have shown an increase of NOS in streptozotocin-induced diabetic rats. NOS-containing neurons are identical to the populations of neurons selectively stained for NADPH-diaphorase activity. Using this technique, we have evaluated changes of NOS in the lumbar spinal cord of diabetic rats with or without insulin treatment. Positive staining was found in motoneurons, dorsal horn neurons (layer II), neurons surrounding the ependimus (layer X) and neurons at the intermediolateral cell column (ILCC). Diabetic animals showed significant decrease in reactive area and increase of the histochemical reaction in motoneurons from the sexual dimorphic nuclei and in neurons of the ILCC. A marked decrease of the number of reactive neurons was also observed in layer II. Morphologic alterations were observed in neurons of layer X as an increase in the percentage of multipolar neurons and a decrease in the number and length of secondary processes. The alterations observed in these animals were absent in the insulin treated diabetic animals. These results show the plasticity of lumbar spinal cord neurons, suggesting a direct participation of NO synthesis in the physiopathology of the erection dysfunction in diabetes.

Efficacy of DA-8159, a new PDE5 inhibitor, for inducing penile erection in rabbits with acute spinal cord injury

DA-8159 is a pyrazolopyrimidinone derivative which exhibits potent and selective phosphodiesterase type 5 (PDE5) inhibition. The aim of this study was to investigate the effects of DA-8159 on inducing a penile erection in rabbits with an acute spinal cord injury (ASCI). DA-8159 was given either orally (1, 3, or 10 mg/kg) or intravenously (0.1 or 0.3 mg/kg) to conscious male albino rabbits with a surgical transection of the spinal cord at the L2-L4 lumbar vertebra or ischemic-reperfusion SCI rabbits. Erection was evaluated in a time-course manner by measuring the length of the uncovered penile mucosa. DA-8159 induced a dose-dependent erection in both transection and ischemic-reperfusion ASCI rabbits. The efficacy of DA-8159 was potentiated by an intravenous injection of sodium nitroprusside, a nitric oxide donor. Potentiation of the effect by nitric oxide donor implies that DA-8159 can enhance the erectile activity during sexual arousal. These results suggest that DA-8159 may be useful for treating erectile dysfunction in patients with an SCI.

DA-8159, a new PDE5 Iihibitor, induces penile erection in conscious and acute spinal cord injured rabbits

OBJECTIVES

DA-8159 is a pyrazolopyrimidinone derivative showing potent and selective phosphodiesterase 5 (PDE5) inhibition. In the previous study, DA-8159 induced a dose-dependent increase in the intracavernous pressure (ICP) in anaesthetized dogs. The aim of this study was to investigate the effects of DA-8159 on penile erection in conscious and acute spinal cord injured (ASCI) rabbits.

METHODS

DA-8159 was given orally (0.3 to 10mg/kg) to normal rabbits and ASCI rabbits with a surgical transection of the spinal cord at the L2-L4 lumbar vertebra or ischemic-reperfusion. The erection was evaluated in a time-course manner by measuring the length of the uncovered penile mucosa in the absence or presence of intravenous sodium nitroprusside (SNP), a nitric oxide (NO) donor.

RESULTS

DA-8159 induced a dose-dependent penile erection in both the conscious and ASCI rabbits. The efficacy of DA-8159 was potentiated and the effective doses were significantly decreased by an intravenous injection of SNP. Potentiation of the effect by a nitric oxide donor implies that DA-8159 can enhance the erectile activity during sexual arousal.

CONCLUSION

These results demonstrate that DA-8159 may be a useful treatment option for erectile dysfunction in patients with or without a spinal cord injury, but further evaluation of the effects of DA-8159 on humans must be performed.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

The evaluation of the effects of renal failure on erectile dysfunction in a rabbit model of chronic renal failure

OBJECTIVE

To determine whether chronic renal failure (CRF) reduces nitrergic relaxant responses in a rabbit model.

MATERIALS AND METHODS

Ten rabbits underwent surgery to induce uraemia (CRF rabbits) and a further 10 a sham operation (controls). Corpus cavernosal tissue was prepared and used in organ-chamber experiments, with relaxation assessed against a background of pre-contraction with phenylephrine. At the plateau of contraction, relaxation responses to cumulative concentrations of carbachol or sodium nitroprusside (SNP), to test endothelium-dependent and -independent relaxations, respectively, were assessed. Before electrical-field stimulation (EFS), the tissue was treated with an adrenergic nerve blocker and a muscarinic receptor blocker to eliminate the adrenergic and cholinergic components, and to determine the relaxation responses to the stimulation of nonadrenergic, noncholinergic (NANC) nerves. The relaxation responses in corporal strips obtained from CRF rabbits were compared with those from controls.

RESULTS

When tissues were contracted with KCl, tensions were similar in all groups. The impairment in concentration-dependent relaxation with carbachol was significant in CRF rabbits, but SNP- and papaverine-induced concentration-dependent relaxation responses were no different among the groups. EFS-induced frequency-dependent relaxations were significantly lower in CRF rabbits than in controls.

CONCLUSION

CRF inhibits the NANC-mediated relaxation of rabbit corpus cavernosum smooth muscle. Changes in NANC-mediated and carbachol-induced (endothelium-dependent) relaxation of corporal smooth muscle in the rabbit are probably caused by uraemia and subsequently, hyperthyroidism, hyperparathyroidism or low testosterone levels in CRF. These results also suggest that if vasoactive agents are to be used for treating erectile dysfunction in uraemic patients, direct-acting vasodilators and phosphodiesterase inhibitors will be useful.

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.

Enzymological and pharmacological profile of T-0156, a potent and selective phosphodiesterase type 5 inhibitor

The enzymological and pharmacological properties of 2-(2-Methylpyridin-4-yl)methyl-4-(3,4,5-trimethoxyphenyl)-8-(pyrimidin-2-yl)methoxy-1,2-dihydro-1-oxo-2,7-naphthyridine-3-carboxylic acid methyl ester hydrochloride (T-0156), a new phosphodiesterase type 5 inhibitor, were studied in vitro and in vivo. The inhibitory effects of T-0156 on six phosphodiesterase isozymes isolated from canine tissues were investigated. T-0156 specifically inhibited the hydrolysis of cyclic guanosine monophosphate (cGMP) by phosphodiesterase type 5, at low concentration (IC(50)=0.23 nM), in a competitive manner. T-0156 also inhibited phosphodiesterase type 6 with IC(50) value of 56 nM, which was 240-fold higher than that for inhibition of phosphodiesterase type 5. T-0156 had low potencies against phosphodiesterase types 1, 2, 3, and 4 (IC(50)>10 microM). In the isolated rabbit corpus cavernosum, T-0156 at 10 and 100 nM increased cGMP levels (100 nM T-0156-treated: 6.0+/-1.5 pmol/mg protein, vehicle-treated: 1.1+/-0.4 pmol/mg protein, P<0.05), causing relaxation of the tissue. T-0156 at 1 to 100 nM potentiated the electrical field stimulation-induced relaxation in the isolated rabbit corpus cavernosum in a concentration-dependent manner (100 nM T-0156-treated: 76.9+/-19.8%, vehicle-treated: 12.3+/-10.1%, P<0.05). Intraduodenal administration of T-0156 at 100 to 1000 microg/kg potentiated the pelvic nerve stimulation-induced tumescence in anesthetized dogs (1000 microg/kg T-0156-treated: 279.0+/-38.4%, vehicle-treated: 9.8+/-4.5%, P<0.05). These results suggested that T-0156 enhanced the nitric oxide (NO)/cGMP pathway, probably through blockade of phosphodiesterase type 5 in vitro and in vivo experimental conditions. The present study clearly showed that T-0156 is a potent and highly selective phosphodiesterase type 5 inhibitor, which is a useful tool for pharmacological studies in vitro and in vivo.

Mechanism of erectogenic effect of the selective phosphodiesterase type 5 inhibitor, DA-8159

DA-8159, a new Phosphodiesterase (PDE) 5 inhibitor, has exhibited potent erectogenic potential in a penile erection test in rats and anesthetized dogs. In this study, we investigated the mechanism of its erectogenic activity by measuring the activity of DA-8159 against a various PDE isozymes and assessing cGMP and cAMP formation in a rabbit corpus cavernosum in vitro. DA-8159 inhibited the PDE 5 activity in rabbit and human platelets, which the IC50 was 5.84 +/- 1.70 nM and 8.25 +/- 2.90 nM, respectively. The IC50 of DA-8159 on PDE 1, PDE 2, PDE 3 and PDE 6 were 870+/- 57.4 nM, 101 +/- 15 microM, 52.0 +/- 3.53 microM and 53.3 +/- 2.47 nM, respectively. This suggests that DA-8159 is a potent, highly selective, competitive inhibitor of PDE 5-catalyzed cGMP hydrolysis. The rates of cGMP hydrolysis catalyzed by human platelets-derived PDE 5 as a function of the cGMP concentration (5-100 nM) and two-fixed DA-8159 concentration (11.3 and 18.8 nM) were investigated in order to characterize the mode of PDE 5 inhibition by DA-8159. DA-8159 increased the apparent Km value for cGMP hydrolysis but had no effect on the apparent Vmax, indicating a competitive mode of inhibition. DA-8159 increased the cGMP concentrations in the rabbit corpus cavernosum dose dependently. In the presence of sodium nitroprusside (SNP), DA-8159 significantly stimulated the accumulation of cGMP when compared to the control level. This indicated that the enhancement of a penile erection by DA-8159 involved the relaxation of the cavernosal smooth muscle by NO-stimulated cGMP accumulation. In conclusion, DA-8159 is a selective inhibitor of PDE 5-catalyzed cGMP hydrolysis and the enhancement of a penile erection by DA-8159 is mediated by the relaxation of the cavernosal smooth muscle by the NO-stimulated cGMP accumulation.

KMUP-1 relaxes rabbit corpus cavernosum smooth muscle in vitro and in vivo: involvement of cyclic GMP and K(+) channels

1. In isolated endothelium-intact or denuded rabbit corpus cavernosum preconstricted with phenylephrine, KMUP-1 (0.001 - 10 microM) caused a concentration-dependent relaxation. 2. This relaxation of KMUP-1 was attenuated by endothelium removed, high K(+) and pretreatments with a soluble guanylyl cyclase (sGC) inhibitor ODQ (1 microM), a NOS inhibitor L-NAME (100 microM), a K(+) channel blocker TEA (10 mM), a K(ATP) channel blocker glibenclamide (1 microM), a voltage-dependent K(+) channel blocker 4-AP (100 microM) and Ca(2+)-dependent K(+) channel blockers apamin (1 microM) and charybdotoxin (ChTX, 0.1 microM). 3. The relaxant responses of KMUP-1 (0.01, 0.05, 0.1 microM) together with a PDE inhibitor IBMX (0.5 microM) had additive actions on rabbit corpus cavernosum smooth muscle (CCSM). 4. KMUP-1 (0.01 - 10 microM) induced increase of intracellular cyclic GMP level in the primary cell culture of rabbit CCSM. This increase in cyclic GMP content was abolished in the presence of ODQ (10 microM). 5. Both KMUP-1 and sildenafil at 0.2, 0.4, 0.6 mg kg(-1) caused increases of intracavernous pressure (ICP) and duration of tumescene (DT) in a dose-dependent manner. These in vivo activities of ICP for sildenafil and KMUP-1 are consistent with those of in vitro effects of cyclic GMP. 6. KMUP-1 has the following merits: (1) inhibition of PDE or cyclic GMP breakdown, (2) stimulation of NO/sGC/cyclic GMP pathway, and (3) subsequent stimulation of K(+) channels, in rabbit CCSM. We suggest that these merits play prominent roles in KMUP-1-induced CCSM relaxation-associated increases of ICP and penile erection.

[Nitroxidergic (nitrergic) nerve and erectile dysfunction]

In vascular tissues including the corpus cavernosum, the organ function is reciprocally regulated by noradrenergic and non-adrenergic, non-cholinergic (NANC) nerves. NANC nerves innervating the corpus cavernosum is thought to be nitroxidergic (nitrergic) nerves which liberate nitric oxide (NO) produced by neuronal NO synthase, and liberated NO activates soluble guanylate cyclase (sGC) in cavernous smooth muscle cells. Intracellular increase in cyclic (c) GMP by activation of sGC dilates cavernous smooth muscle and then induces penile erection. Nitroxidergic (nitrergic) vasodilator nerves also innervate cavernous arteries and veins which regulate the blood volume in the corpus cavernosum. The order of potency of nitroxidergic nerve functions in these tissues (cavernosum > artery >> vein) may be suitable for producing the erection. Therefore, obstruction of the arteries and impairment of nitroxidergic (nitrergic) nerve function are speculated to be one of the causes for erectile dysfunction (ED). On the other hand, NO derived from the cavernous endothelium may partly contribute to erectile function. Sildenafil (Viagra) is one of the potent therapeutics for ED. The agent is a selective phosphodiesterase type 5 (PDE-V) inhibitor that inhibits degradation of cGMP elevated by NO mainly derived from the nerves. To develop more selective and safer therapeutics for ED, further systematic investigations are required.

The phosphodiesterase inhibitory selectivity and the in vitro and in vivo potency of the new PDE5 inhibitor vardenafil

We investigated the potency and the selectivity profile of vardenafil on phosphodiesterase (PDEs) enzymes, its ability to modify cGMP metabolism and cause relaxation of penile smooth muscle and its effect on erections in vivo under conditions of exogenous nitric oxide (NO) stimulation. PDE isozymes were extracted and purified from human platelets (PDE5) or bovine sources (PDEs 1, 2, 3, 4 and 6). The inhibition of these PDEs and of human recombinant PDEs by vardenafil was determined. The ability to potentiate NO-mediated relaxation and influence cGMP levels in human corpus cavernosum strips was measured in vitro, and erection-inducing activity was demonstrated in conscious rabbits after oral administration together with intravenous doses of sodium nitroprusside (SNP). The effects of vardenafil were compared with those of the well-recognized PDE5 inhibitor, sildenafil (values for sildenafil in brackets). Vardenafil specifically inhibited the hydrolysis of cGMP by PDE5 with an IC50 of 0.7 nM (6.6 nM). In contrast, the IC50 of vardenafil for PDE1 was 180 nM; for PDE6, 11 nM; for PDE2, PDE3 and PDE4, more than 1000 nM. Relative to PDE5, the ratios of the IC50 for PDE1 were 257 (60), for PDE6 16 (7.4). Vardenafil significantly enhanced the SNP-induced relaxation of human trabecular smooth muscle at 3 nM (10 nM). Vardenafil also significantly potentiated both ACh-induced and transmural electrical stimulation-induced relaxation of trabecular smooth muscle. The minimum concentration of vardenafil that significantly potentiated SNP-induced cGMP accumulation was 3 nM (30 nM). In vivo studies in rabbits showed that orally administered vardenafil dose-dependently potentiated erectile responses to intravenously administered SNP. The minimal effective dose that significantly potentiated erection was 0.1 mg/kg (1 mg/kg). The selectivity for PDE5, the potentiation of NO-induced relaxation and cGMP accumulation in human trabecular smooth muscle and the ability to enhance NO-induced erection in vivo indicate that vardenafil has the appropriate properties to be a potential compound for the treatment of erectile dysfunction. Vardenafil was more potent and selective than sildenafil on its inhibitory activity on PDE5.

Impaired neurogenic and endothelium-dependent relaxant responses of corpus cavernosum smooth muscle from hyperthyroid rabbits

We investigated the effect of hyperthyroidism on the responsiveness of the rabbit corpus cavernosum smooth muscle. In male albino rabbits, hyperthyroidism was established by oral feeding of L-thyroxine at increasing dosages (150-450 microg/kg) over an 8-week period. This treatment produced a stable hyperthyroid state as indicated by the increased serum T4 levels. The reactivity of corpus cavernosum tissue from hyperthyroid animals and euthyroid control animals was studied in organ chambers. Hyperthyroidism caused impaired neurogenic and endothelium-dependent relaxant responses with decreased Emax and pD2 values. However, hyperthyroidism had no effect on both phenylephrine- and KCl-induced contractile responses and sodium nitroprusside- and papaverine-induced endothelium-independent relaxant responses, and there was no change in agonist potency. These data indicate that hyperthyroidism may impair both neurogenic and endothelium-dependent relaxation of corporal smooth muscle, and may contribute to the etiology of impotence.

Priapism: a review

Priapism is persistent penile erection in the absence of sexual stimulation. The pattern of blood flow to the penis that occurs during normal erection is altered so that sustained priapism may result in edema, increased risk of abrasion, tissue drying and necrosis of the penis. Numerous causes have been reported in animals and humans. The prognosis depends on the type of priapism and the amount of time that passes before therapeutic intervention. Surgical methods, such as aspiration and shunting procedures, have traditionally been used to treat priapism but carry a risk of postsurgical complications. Use of alpha-agonists for treatment of priapism in humans is often successful and avoids the risks of impotence and other surgical complications. Investigation of the use of alpha-agonists for treatment of priapism in animals should be considered.

The effect of sildenafil on corpus cavernosal smooth muscle relaxation and cyclic GMP formation in the diabetic rabbit

Sildenafil, a type V phosphodiesterase inhibitor, enhances smooth muscle relaxation in normal human and rabbit corpus cavernosum. We investigated the in vitro effects of sildenafil on non-adrenergic, non-cholinergic and nitric oxide (NO)-mediated cavernosal smooth muscle relaxation in diabetic rabbits, since alterations in this pathway are recognised in diabetic erectile dysfunction. Diabetes mellitus was induced in male New Zealand White rabbits with alloxan. Cavernosal strips from age-matched control, 3- and 6-month diabetic animals were mounted in organ baths. Relaxation responses to electrical field stimulation (1-20 Hz) or sodium nitroprusside (10(-8)-10(-4) M) were assessed in the absence and presence of sildenafil (10(-8) and 10(-7) M). The effect of sildenafil on cGMP formation by the corpus cavernosum was also assessed following stimulation with sodium nitroprusside, A23187 and acetylcholine. Sodium nitroprusside-stimulated relaxations were significantly (P<0.03) impaired in the corpus cavernosum from both diabetic groups, (IC(50)=4.6 x 10(-6) M following 3 months of diabetes mellitus and 4.0 x 10(-6) M following 6 months of diabetes mellitus; compared to 7.5 x 10(-7) M for pooled age-matched controls). Sildenafil (10(-7) M) significantly enhanced sodium nitroprusside-stimulated relaxation in control (P<0.05) and diabetic groups (P<0.03). Electrical field stimulation-mediated relaxations of the corpus cavernosum were significantly impaired after 6-month diabetes mellitus and enhanced by sildenafil (10(-8) M). cGMP formation by the diabetic corpus cavernosum was impaired significantly, but restored towards normal by sildenafil. We suggest that the impairment of NO-mediated relaxation of the corpus cavernosum reflect, at least in part, a defect in guanylyl cyclase activity. These findings support the use of sildenafil as an effective, orally administered, treatment for diabetic erectile dysfunction.

Effects of sildenafil citrate, acetylcholine, and sodium nitroprusside on the relaxation of rabbit cavernosal tissue in vitro

OBJECTIVES

To investigate the effect of sildenafil together with the influence of sodium nitroprusside and acetylcholine and further discuss its mechanism of action in vitro.

METHODS

Isolated strips of rabbit corpus cavernosum were stimulated isometrically with phenylephrine. Graded relaxations were induced using increasing concentrations of acetylcholine, sodium nitroprusside, and sildenafil alone and in combination.

RESULTS

The agents all relaxed the rabbit corpus cavernosal strips in a dose-dependent manner. The inhibitory potentials of the test agents were ranked (from least inhibitory to most inhibitory) as follows: sildenafil, acetylcholine plus sildenafil, sodium nitroprusside, sodium nitroprusside plus sildenafil, acetylcholine. The presence of sodium nitroprusside in the medium reduced the median effective dose for the sildenafil-induced relaxation of the cavernosal tissue from 3.65 x 10(-5) M to 2.73 x 10(-7) M, and the presence of acetylcholine reduced it to 9.79 x 10(-6) M.

CONCLUSIONS

Sildenafil enhances the relaxing effect of both sodium nitroprusside and acetylcholine on the phenylephrine-induced contraction of rabbit cavernosal tissue.

Nonadrenergic, noncholinergic relaxation of human isolated corpus cavernosum induced by scorpion venom

OBJECTIVES

To examine the effects of Tityus serrulatus scorpion venom (TSV) on human corpus cavernosum (HCC) using a bioassay cascade. Priapism is occasionally observed in scorpion envenomation, mostly in children.

METHODS

HCC strips were suspended in a cascade system and superfused with aerated and warmed Krebs' solution at 5 mL/min. Noradrenaline (3 micromol/L) was infused to induce a submaximal contraction of the HCC strips. The release of cyclooxygenase products was prevented by infusing indomethacin (6 micromol/L).

RESULTS

N(omega)-nitro-L-arginine methyl ester (10 micromol/L; n = 10) increased the tone of the preparations and significantly reduced (P <0.01) the acetylcholine (ACh) and TSV-induced relaxations. Subsequent infusion of L-arginine (300 micromol/L) partially reversed the increased tone and significantly restored the relaxations induced by TSV and ACh (P <0.01). The soluble guanylyl cyclase inhibitor ODQ (10 micromol/L; n = 8) markedly reduced (P <0.01) the relaxations induced by TSV, ACh, glyceryl trinitrate, and bradykinin. 7-Nitroindazole (10 micromol/L; n = 8) inhibited the relaxations induced by TSV by 84% (P <0.01) and also caused small, but significant, reductions in the ACh and bradykinin-induced HCC relaxations (P <0.05). Atropine (1 micromol/L; n = 6) abolished the relaxations evoked by ACh (P <0.01), but had no effect on those elicited by TSV. Tetrodotoxin (1 micromol/L; n = 6) abolished the relaxations induced by TSV (P <0.01) and also reversed the established TSV-induced relaxation (n = 4).

CONCLUSIONS

Our results indicate that TSV relaxes HCC through the release of nitric oxide from nonadrenergic, noncholinergic (NANC) nerves. The elucidation of the mechanism responsible for the TSV-induced relaxations might be useful for a better understanding of the development of priapism in cases of scorpion envenomation.

The role of nitric oxide in penile erection

The functional state of the penis, flaccid or erect is governed by smooth muscle tone. Sympathetic contractile factors maintain flaccidity whilst parasympathetic factors induce smooth muscle relaxation and erection. It is generally accepted that nitric oxide (NO) is the principal agent responsible for relaxation of penile smooth muscle. NO is derived from two principal sources: directly from non-adrenergic non-cholinergic parasympathetic nerves and indirectly from the endothelium lining cavernosal sinusoids and blood vessels in response to cholinergic stimulation. The generation of NO from L-arginine is catalysed by nitric oxide synthase (NOS). There has been controversy over the relative prevalence of endothelial or neuronal NOS within the penis of different animal species. This review examines the role of NO in the penis in detail. Established and new treatments for erectile dysfunction whose effects are mediated via manipulation of the NO pathway are also described.

Alpha-adrenoceptor-mediated penile erection in dogs: in vivo and in vitro observations

We have evaluated the role of adrenergic components in the pelvic splanchnic nerve on the erectile function in the dog. Electrical stimulation of pelvic splanchnic nerves increased blood flow in the internal pudendal artery and also elevated the cavernous pressure. These increases were blocked in part by phentolamine or methylene blue, but not by propranolol or atropine. The effects of cholinergic and adrenergic agonists and antagonists on mechanical responses were also examined in muscle strips obtained from various arteries in the intrapelvic region including the internal pudendal artery. Norepinephrine induced contraction in the iliac artery and relaxation in the internal pudendal artery, and both the contraction and relaxation responses were blocked by phentolamine but not by propranolol. These findings suggest that in the dog, alpha-adrenergic components projected through the pelvic splanchnic nerve may contribute to penile erection, together with cyclic GMP-mediated mechanisms.

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.

Parathyroid hormone-related protein in rat penis: expression, localization, and effect on cavernosal pressure

Although PTH-related protein-(1-36) [PTHrP-(1-36)] is known to be expressed in smooth muscle and to exert potent myorelaxant effects, its tonic effects on cavernosal smooth muscle has not yet been explored. Using the RT-PCR technique, the present study establishes that PTHrP messenger RNA is present in microdissected corpus cavernosa in the rat. In immunohistochemical studies using affinity-purified antibodies to middle regions of PTHrP, immunostaining was localized throughout the penile structures, including vessels, cavernosal smooth muscle, and trabecular fibroblasts. Strong immunostaining for PTHrP was also detected in the dorsal nerve bundles. In anesthetized rats, intracavernosally injected boluses of increasing doses of PTHrP-(1-36) (0.3-30 pmol in 100 microl saline) had little effect on intracavernosal pressure. However, they markedly potentiated the dilatory response to papaverine (8-800 nmol), increasing the papaverine-induced intracavernous pressure by 2.5-fold, close to the mean arterial pressure. In conclusion, the cavernosal expression of PTHrP messenger RNA, the distribution of immunoreactive PTHrP throughout the structuro-functional components of the erectile apparatus and its strong potentiating action on papaverine-induced cavernosal relaxation, collectively suggest that PTHrP participates in the control of cavernosal tone.

Evaluation of erectile response by continuous measurement of penile diameter in rats

The present study was performed to determine whether measurement of penile diameter in an in vivo rat model is useful for pharmacologic and physiologic investigations on penile erection. Penile erection induced by electrical stimulation of the cavernous nerve was monitored by measuring the penile diameter sonomicrometrically with a pair of 10-MHz piezoelectric crystals glued to the opposite surfaces of the adventitia of the penile erectile chamber in anesthetized rats. Using this method, we examined the effects of a nitric oxide (NO) synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME), and a well-known phosphodiesterase 5 (PDE5) inhibitor, zaprinast, on the maximal developed penile diameter (D-max) and the time from the maximum response to 50% recovery (T50%) of the maximum response as an index of the duration of penile erection. An intravenous injection of L-NAME at a dose of 10 mg/kg significantly inhibited D-max produced by cavernous electrical stimulation at 5 to 50 V, without affecting T50%. Sequential intravenous infusions of 10, 30, 100, and 300 microg/kg/min of zaprinast at 30-min intervals did not show any effect on D-max, heart rate, and systolic arterial pressure, although doses of 100 and 300 microg/kg/min significantly prolonged T50% and the maximum dose decreased diastolic arterial pressure. Moreover, zaprinast produced a more prominent increase in cyclic guanosine monophosphate (cGMP) levels than cyclic adenosine monophosphate levels in the plasma taken at the end of the maximum dose infusion. Measurement of murine penile diameter with a sonomicrometrical device, indicating that a NO-cGMP-PDE5 pathway plays a pivotal role in the penile diameter increase and its maintenance, would be useful for pharmacologic and physiologic investigations on penile erection.

A nitric oxide synthesis inhibitor administered into the medial preoptic area increases seminal emissions in an ex copula reflex test

Nitric oxide synthesis inhibitors, when administered systemically or into the ventricles of the brain, affect several indices of male sexual behavior. Some of the systemic effects are assumed to be due to local vasoconstriction at the penis. Others are suggested to be mediated within the brain. In these experiments, the nitric oxide synthesis inhibitor L-NMMA, and its less active enantiomer, D-NMMA, were microinjected into the medial preoptic area of male rats. In an ex copula test of genital reflexes, L-NMMA increased the number of seminal emissions, while D-NMMA had no effect. These results are consistent with the hypothesis that nitric oxide is a tonic inhibitor of sympathetic nervous system tone, possibly in part through an influence on dopamine synthesis or release.

Effects of chronic unilateral internal pudendal arterial occlusion on reactivity of isolated corpus cavernosum strips from rabbits

An animal model was developed to elucidate the effect of chronic obstruction of the internal pudendal artery on the responsiveness of the corpus cavernosum. In male albino rabbits, the internal pudendal artery was chronically ligated unilaterally with a silk tie and the occlusion was maintained for 1 month. The control group was sham-operated. The reactivity of corpus cavernosum tissue from the ligated animals and the control animals was studied in organ chambers. Unilateral chronic ligation of the internal pudendal artery caused an impaired contractile response to alpha-adrenoceptor stimulation with decreased Em and pD2 values and an impaired relaxant response to electrical field stimulation but resulted in a marked increase in the endothelium-dependent relaxant response to carbachol with an increased pD2 value. However chronic obstruction of the pudendal artery had no effect on adenosine-, papaverine- and sodium nitroprusside-induced relaxant responses, and there was no change in agonist potency. These data indicate that altered penile hemodynamics have an effect on the reactivity of the corpus cavernosum and may contribute to the etiology of impotence.

Innervation of the fibro-elastic type of the penis: an immunohistochemical study in the male pig

The occurrence and colocalization of several biologically active neuropeptides, catecholamine-, acetylcholine- or nitric oxide-synthesizing enzymes-tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (D beta H), choline acetyl-transferase (ChAT) and nitric oxide synthase (NOS I), respectively, as well as the vesicular acetylcholine transporter (VAChT) were investigated in the penile glans (GP), corpus and crura (CP), as well as in the retractor penis muscle (RPM) of juvenile and adult boars. Immunohistochemistry revealed that nerves immunoreactive (IR) to TH, D beta H, vasoactive intestinal polypeptide (VIP) and somatostatin (SOM) were the most numerous, followed (in decreasing order of density) by nerves IR to NOS, neuropeptide Y (NPY), substance P (SP), calcitonin gene-related peptide (CGRP), galanin (GAL), Leu5-enkephalin (LENK) and ChAT/VAChT. The CP contained the largest number of nerve fibres followed by the RPM, GP and corpus. Enzyme/peptide-containing nerves were associated with both the vascular and non-vascular penile structures. However, differences existed for their density and intrapenile distribution. Nerve terminals IR for different combinations of VIP, GAL or SOM were more frequent than those IR for NOS or CGRP in the non-vascular penile structures while the vasculature and the RPM received a prominent TH/D beta H-, VIP-, SOM- or NOS-IR nerve input. The present data indicate that the porcine penis receives nerve fibres that exhibit diverse chemical codes and that differences in the chemical coding of the nerve fibres may depend on their penile target-structure.