Androgen-dependent nitric oxide release in rat penis correlates with levels of constitutive nitric oxide synthase isoenzymes

Testosterone and erectile function: from basic research to a new clinical paradigm for managing men with androgen insufficiency and erectile dysfunction

OBJECTIVES

Androgens are essential for the development and growth of the penis, and they regulate erectile physiology by multiple mechanisms. Our goal is to provide a concise overview of the basic research and how this knowledge can be translated into a new clinical paradigm for patient management. In addition, this new paradigm may serve as a basis for stimulating constructive debate regarding the use of testosterone in men, and to promote new, innovative basic and clinical research to further understand the underlying mechanisms of androgen action in restoring erectile physiology.

METHODS

A literature review was performed utilizing the US National Library of Medicine's PubMed database.

RESULTS

On the basis of evidence derived from laboratory animal studies and clinical data, we postulate that androgen insufficiency disrupts cellular-signaling pathways and produces pathologic alterations in penile tissues, leading to erectile dysfunction. In this review, we discuss androgen-dependent cellular, molecular, and physiologic mechanisms modulating erectile function in the animal model, and the implication of this knowledge in testosterone use in the clinical setting to treat erectile dysfunction. The new clinical paradigm incorporates many of the consensed points of view discussed in traditional consensed algorithms exclusively designed for men with androgen insufficiency. There are, however, novel and innovative differences with this new clinical paradigm. This paradigm represents a fresh effort to provide mandatory and optional management strategies for men with both androgen insufficiency and erectile dysfunction.

CONCLUSIONS

The new clinical paradigm is evidence-based and represents one of the first attempts to address a logical management plan for men with concomitant hormonal and sexual health concerns.

Modeling and Simulation of Sexual Activity Daily Diary Data of Patients with Female Sexual Arousal Disorder Treated with Sildenafil Citrate (Viagra®)

PURPOSE

To develop a model to explore the dose-response of sildenafil citrate in patients with female sexual arousal disorder (FSAD) based on telephone sexual activity daily diary (TSADD) data obtained in double-blind, placebo controlled clinical studies.

MATERIALS

Data were available on 614 patients with FSAD. A parametric model (Weibull distribution) was developed to describe the probability density function of the time between sexual events. Orgasm satisfaction scores and overall sexual satisfaction scores were simultaneously modeled as ordered categorical variables. Simulations were performed to evaluate the expected clinical response in patients with FSAD.

RESULTS

The expected time between sexual events was approximately 3.5 days. Satisfaction scores increased with time to achieve a plateau after 3 to 4 weeks on treatment. The expected probability of satisfying orgasm (score of 3 and higher) ranged from 34.7% for placebo to 41.6% for 100 mg sildenafil citrate. Treatment effect (difference from placebo) was 6.9% for 100 mg sildenafil citrate, ranging from 0.6 to 24.7% for testosterone levels of 0.1 to 4.0 pg/ml. The treatment effect in postmenopausal women was larger than in premenopausal women.

CONCLUSION

A modeling and simulation framework to support drug development in FSAD was developed. Sildenafil citrate demonstrated a dose-dependent effect in patients with FSAD.

Hypogonadism in the man with erectile dysfunction: what to look for and when to treat

Hypogonadism (low serum testosterone) is commonly associated with erectile dysfunction (ED). However, many urologists may lack appreciation of the relative merits of treating hypogonadism compared with oral phosphodiesterase inhibitors for sexual dysfunction. Testosterone-replacement therapy (TRT) may be the best treatment for men with ED when the presentation includes diminished libido or other sexual symptoms or when non-sexual symptoms such as depressed mood, decreased sense of vitality, and increased fatigue also exist. The health benefits of TRT also include improvements in body composition, bone density, cognition, and sense of well-being. Thus, there may be good reasons to use TRT as first-line therapy for the man with ED. Concerns regarding prostatic and cardiovascular risks of TRT have not been supported by the literature. Nevertheless, men receiving TRT must be monitored at regular intervals with digital rectal examination and blood testing for prostate-specific antigen. Hematocrit or hemoglobin also should be obtained regularly due to the risk of erythrocytosis. Awareness of the benefits of TRT in the man with ED may improve clinical outcomes.

REVIEWS: Are Androgens Critical for Penile Erections in Humans? Examining the Clinical and Preclinical Evidence

Androgens are deemed critical for penile-tissue development, growth, and maintenance of erectile function, however, their role in erection, especially in humans, remains controversial. In this review, we summarize information from clinical and animal model studies to provide a comprehensive and rational argument for the role of androgens, or lack thereof, on penile erection ability in humans. The goal of this review is to present the clinical and preclinical evidence available in the literature with regard to testosterone and erectile physiology and engage the reader in this discussion. Ultimately, each reader will have to form his or her own conclusions based on the existing evidence. In humans, androgen-deficiency manifestations are noted in clinical situations such as: (i) inadequate development of the penis; and (ii) loss of erectile function in prostate cancer and benign prostatic hyperplasia patients managed with medical or surgical castration or antiandrogen therapy. Androgen treatment causes: (i) improvement in sexual function in hypogonadal patients treated with androgen supplementation; (ii) improvement in nocturnal penile tumescence in hypogonadal patients treated with androgens; (iii) improvement in erectile function with androgen supplementation in patients who did not respond to phosphodiesterase type 5 inhibitor therapy initially; and (iv) improvement in the well-being, mood, energy, and sexual function in aging men who have testosterone deficiency treated with androgen therapy. In contrast to animals, especially rodents in which the adrenal cortex does not synthesize androgens, the human adrenal is a source of peripherally circulating androgen precursors, thus, complete androgen insufficiency may not be observed in men at a younger age. Furthermore, in light of the concept that a threshold of androgen levels exists in animals and humans below which sexual function is diminished, further contributes to the complexity of understanding androgens role in erections, especially in humans. Nevertheless, based on the preclinical and clinical data available in the literature, to date, we infer that androgens play a critical role in maintaining erectile physiology in humans.

Testosterone therapy in erectile dysfunction and hypogonadism

INTRODUCTION

Laboratory experiments indicate that the nitric oxide erectile pathway is testosterone-dependent. Castration induces erectile dysfunction (ED) and reduction in nitric oxide synthase and in phosphodiesterase type 5 (PDE5) in the erectile tissue. Furthermore, castration causes apoptosis adversely affecting smooth muscle content and penile hemodynamics leading to veno-occlusive dysfunction. Testosterone therapy reverses these structural, biochemical, and physiological changes. In humans, testosterone therapy improves erectile function in men with hypogonadism. However, the efficacy of testosterone monotherapy may not be adequate because of the multifactorial nature of the pathophysiology of ED.

METHODS

Preliminary data from a number of studies have been reviewed.

RESULTS

There are emerging evidence-based benefits to using the combination of testoterone and PDE5 inhibitors. A recently published multicenter, randomized, placebo-controlled study evaluated the safety and efficacy of testosterone gel 1% plus sildenafil vs. placebo gel plus sildenafil, in producing an erectile response in hypogonadal men who had failed prior sildenafil alone for ED. Screening yielded a prevalence of hypogonadism in ED patients who failed prior sildenafil. Following randomization, the double-blinded treatment phase was 12 weeks. Testosterone therapy with testosterone gel significantly improved erectile function in response to sildenafil. In addition, it significantly improved orgasmic function and patient satisfaction.

CONCLUSION

It is important to screen all men with ED for hypogonadism, especially those with a history of inadequate response to prior PDE5 inhibitors. The combination of testosterone plus PDE5 inhibitors may be considered for the treatment of ED in men with low to low-normal testosterone levels, who had inadequate response to prior treatment with PDE5 inhibitors alone.

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.

Recent insights into androgen action on the anatomical and physiological substrate of penile erection

Erectile response is centrally and peripherally regulated by androgens. The original insights into the mechanisms of action of androgens were that androgens particularly exert effects on libido and that erections in response to erotic stimuli were relatively androgen-independent. It was shown that sexual functions in men required androgen levels at the low end of reference values of testosterone. So it seemed that testosterone was not useful treatment for men with erectile difficulties, particularly following the advent of the phosphodiesterase type 5 (PDE5) inhibitors. However, approximately 50% of those treated with PDE5 inhibitors discontinue their treatment. A number of recent developments shed new light on testosterone treatment of erectile dysfunction (ED) in aging men. (1) A recent insight is that, in contrast to younger men, elderly men might require higher levels of testosterone for normal sexual functioning. (2) Several studies have indicated that PDE5 inhibitors are not always sufficient to restore erectile potency in men, and that testosterone improves the therapeutical response to PDE5 inhibitors considerably. (3) There is growing insight that testosterone has profound effects on tissues of the penis involved in the mechanism of erection and that testosterone deficiency impairs the anatomical and physiological substrate of erectile capacity, reversible upon androgen replacement. The synthesis of PDE5 is upregulated by androgens, and the arterial inflow into the penis is improved by giving androgen. The above invites a re-examination of the merits of giving testosterone to aging men with ED. The beneficial effects of PDE5 inhibitors may only be optimally expressed in a eugonadal environment.

Plasticity of pelvic autonomic ganglia and urogenital innervation

Pelvic ganglia contain a mixture of sympathetic and parasympathetic neurons and provide most of the motor innervation of the urogenital organs. They show a remarkable sensitivity to androgens and estrogens, which impacts on their development into sexually dimorphic structures and provide an array of mechanisms by which plasticity of these neurons can occur during puberty and adulthood. The structure of pelvic ganglia varies widely among species, ranging from rodents, which have a pair of large ganglia, to humans, in whom pelvic ganglion neurons are distributed in a large, complex plexus. This plexus is frequently injured during pelvic surgical procedures, yet strategies for its repair have yet to be developed. Advances in this area will come from a better understanding of the effects of injury on the cellular signaling process in pelvic neurons and also the role of neurotrophic factors during development, maintenance, and repair of these axons.

Erectile dysfunction: pathophysiologic mechanisms pointing to underlying cardiovascular disease

Evidence exists that erectile dysfunction (ED) is analogous to endothelial dysfunction, a known precursor to atherosclerosis in terms of molecular mechanisms and underlying risk factors. These findings are discussed, along with the biologic underpinnings for the clinical observation that ED is an "early warning system" for atherosclerosis. Molecular mechanisms of ED as potential targets of novel therapies are considered, as well as the role of phosphodiesterase 5 inhibitors--currently the most effective treatment of ED--as promising therapies of cardiovascular diseases characterized by endothelial dysfunction.

Transdermal and topical pharmacotherapy for male sexual dysfunction

Transdermal and topical pharmacotherapies have been used for the treatment of male sexual dysfunctions for some time and are well-accepted treatment modalities for these conditions. A Medline search was conducted for transdermal and topical medications, examining published literature over the past two decades. From this search a comprehensive review has been compiled of the available transdermal and topical treatment options for the treatment of male sexual dysfunctions, particularly erectile dysfunction, Peyronie's disease and hypogonadism. It is likely that the transdermal and topical drug armamentarium for sexual dysfunction, male and female, will grow over the next decade.

ORIGINAL RESEARCH—ENDOCRINOLOGY: The Physiological Role of Androgens in Penile Erection: Regulation of Corpus Cavernosum Structure and Function

It is generally accepted that androgens are critical for development, growth, and maintenance of penile erectile tissue. However, their role in erectile function, especially in humans, remains controversial. Clinical and preclinical studies have suggested that venoocclusion is modulated by the tone of the vascular smooth muscle of the resistance arteries and the cavernosal tissue and a balance between trabecular smooth muscle content and connective tissue matrix. In men with erectile dysfunction, venous leakage is thought to be a common condition among nonresponders to medical management and is attributed to penile smooth muscle atrophy. In the animal model, androgen deprivation produces penile tissue atrophy concomitant with alterations in dorsal nerve structure, endothelial morphology, reduction in trabecular smooth muscle content, and increased deposition of extracellular matrix. Further, androgen deprivation results in accumulation of fat-containing cells (adipocytes) in the subtunical region of the corpus cavernosum. Androgen deficiency diminishes protein expression and enzymatic activity of nitric oxide synthases (eNOS and nNOS) and phosphodiesterase type 5 (PDE5). The androgen-dependent loss of erectile response is restored by androgen administration but not by administration of PDE5 inhibitors alone. These data suggest that androgens regulate trabecular smooth muscle growth and connective tissue protein synthesis in the corpus cavernosum. Further, androgens may stimulate differentiation of progenitor cells into smooth muscle cells and inhibit their differentiation into adipocytes. Thus, we conclude that androgens exert a direct effect on penile tissue to maintain erectile function and that androgen-deficiency produces a metabolic and structural imbalance in the corpus cavernosum, resulting in venous leakage and erectile dysfunction. .

Direct regulation of prostate blood flow by vascular endothelial growth factor and its participation in the androgenic regulation of prostate blood flow in vivo

Previous studies on prostate blood flow regulation have indicated that androgen regulates prostate blood flow. However, the mechanism responsible for this regulation is unknown. In the present study, we focused on the effects of vascular endothelial growth factor (VEGF), a key factor responsible for angiogenesis and androgenic blood flow regulation. We examined in vivo the effect of VEGF on prostate blood flow and its participation in the androgenic regulation of this blood flow using a castrated rat model following subcapsular intraprostatic injection method. We found that VEGF is involved in blood flow regulation with an activity equal to that of dihydrotestosterone (DHT). The effect of VEGF on prostate blood flow was already seen at 30 min after the administration. The elevating effect of DHT on castrated rat prostate blood flow was abolished by coadministration of DHT with neutralizing anti-VEGF antibody. The change in VEGF-A mRNA expression in response to androgen stimulation was examined by double-fluorescent probe quantitative PCR (Taqman PCR). The results showed that androgenic regulation of VEGF gene expression occurred shortly after androgen stimulation. VEGF gene up-regulation was abolished or down-regulated by coadministration of neutralizing anti-VEGF antibody. This is the first report on the importance of VEGF in the androgenic regulation signaling pathway that affects prostate blood flow. Alternative treatment targeted toward anti-VEGF activity as a substitute for ordinary antiandrogenic therapy may be effective against prostate diseases, especially those with androgen-independent and hyperhemorrhagic status.

Topical application of a Rho-kinase inhibitor in rats causes penile erection

Studies from this laboratory have demonstrated that RhoA/Rho-kinase signaling mediates vasoconstriction in the penile circulation of the rat and that erection results from inhibition of this activity with Y-27632. In prior animal studies, Y-27632 was administered to the rats by intracavernous injection. To determine if topical application of the Rho-kinase inhibitor is an effective mode of delivery, Y-27632 was applied to the surface of the tunica albuginea or to the glans penis and surrounding skin in intact or castrated rats. Both sites of drug administration resulted in a marked increase in the erectile response both with and without stimulation of the autonomic innervation of the penile vasculature. Although high doses of the drug were found to reduce systemic blood pressure, topical administration of the Rho-kinase inhibitor, in appropriate doses, may have clinical value for the treatment erectile dysfunction.

Effect of testosterone supplementation on sexual function in hypogonadal men with erectile dysfunction

OBJECTIVES

To evaluate the impact of normalization of testosterone levels in men with documented hypogonadism and erectile dysfunction on erectile function and sexual satisfaction. Although the data support the role of testosterone in the preservation of libido and nocturnal erectile function, the evidence supporting the use of testosterone supplementation in hypogonadal men with erectile dysfunction is weak.

METHODS

This analysis used two validated questionnaires, the International Index of Erectile Function and the Erectile Dysfunction Inventory of Treatment Satisfaction, in a serial fashion at baseline (before treatment) and 1, 3, and 6 months after the achievement of a therapeutic testosterone level.

RESULTS

A total of 32 men met all inclusion criteria; 69% of men achieved therapeutic testosterone levels using transdermal testosterone gel, 19% required a transdermal patch, and 12% required intramuscular delivery. Statistical significance was reached for the difference between the baseline and 1-month International Index of Erectile Function erectile function domain score and all post-treatment International Index of Erectile Function libido scores. A steady decrease occurred in the Erectile Dysfunction Inventory of Treatment Satisfaction scores from the 1 to 6-month values that reached statistical significance.

CONCLUSIONS

In men with documented hypogonadism and erectile dysfunction, normalization of serum testosterone levels was associated with only short-term improvement in erectile function and sexual satisfaction. The use of testosterone supplementation in this population for the treatment of erectile dysfunction is questionable.

Phosphorylated Endothelial Nitric Oxide Synthase Mediates Vascular Endothelial Growth Factor-Induced Penile Erection1

The objective of the present study was to evaluate whether vascular endothelial growth factor (VEGF)-induced penile erection is mediated by activation of endothelial nitric oxide synthase (eNOS) through its phosphorylation. We assessed the role of constitutively activated eNOS in VEGF-induced penile erection using wild-type (WT) and eNOS-knockout (eNOS(-/-)) mice with and without vasculogenic erectile dysfunction. Adult WT and eNOS(-/-) mice were subjected to sham operation or bilateral castration to induce vasculogenic erectile dysfunction. At the time of surgery, animals were injected intracavernosally with a replication-deficient adenovirus expressing human VEGF145 (10(9) particle units) or with empty virus (Ad.Null). After 7 days, erectile function was assessed in response to cavernous nerve electrical stimulation. Total and phosphorylated protein kinase B (Akt) as well as total and phosphorylated eNOS were quantitatively assessed in mice penes using Western immunoblot and immunohistochemistry. In intact WT mice, VEGF145 significantly increased erectile responses, and in WT mice after castration, it completely recovered penile erection. However, VEGF145 failed to increase erectile responses in intact eNOS(-/-) mice and only partially recovered erectile function in castrated eNOS(-/-) mice. In addition, VEGF145 significantly increased phosphorylation of eNOS at Serine 1177 by approximately 2-fold in penes of both intact and castrated WT mice. The data provide a molecular explanation for VEGF stimulatory effect on penile erection, which involves phosphorylated eNOS (Serine 1177) mediation.

Sexual Dysfunction in the Older Woman

Sexuality is one of the most important quality of life issues for both men and women. Sexual dysfunction is a highly prevalent, age-related and progressive problem. The various physiological and psychological changes that occur with aging can have a significant impact on sexual function. The complexity of female sexual dysfunction remains distinct from that of a man. Thus, we cannot approach female patients or their sexual function problems in a similar fashion to that of male patients. A woman's motivation and ability to find and respond to sexual stimuli is largely influenced by her emotional intimacy with her partner. Frequently, the emotional and relationship well-being a woman experiences contributes more to her sexual enjoyment than does her physiological response. However, it is imperative to assess for possible physiological barriers a woman may have which impede a healthy and satisfying sexual life. Therefore, a comprehensive approach, addressing both the physiological and psychological factors is instrumental to the evaluation of female patients with sexual complaints. After years of ardent research and recent therapeutic advances in male sexual dysfunction, researchers have begun addressing the intricacy of female sexual complaints. Studies involving both pre- and postmenopausal women have reported that most women do experience some type of sexual dysfunction during their lifetime. The sexual complaints women experience in their younger years may follow them into older adulthood, but often times change considerably because of various age-related changes. In an effort to assist researchers and clinicians in designing studies and implementing appropriate evaluation and treatment options for women with sexual complaints, a classification system for female sexual dysfunction has been designed. The four categories of female dysfunction include: hypoactive sexual desire disorder, sexual arousal disorder, orgasmic disorder and sexual pain disorders. Evaluation of women with sexual complaints should include a detailed psychological, social and medical history and thorough physical examination including a hormonal profile. Current treatment options are dependent on the diagnosis and include physical therapy, psychological counselling, hormonal supplements, medication changes and sexual devices. There has also been a burgeoning interest in investigational medications for female sexual dysfunction, from centrally acting (e.g. serotonin agonists) to peripheral, localised treatment (e.g. vasodilating creams). The area of female sexuality and sexual dysfunction has been undergoing important critical changes within the last 10 years. Researchers and clinicians are continuing to recognise the need to try and understand both the psychological and physiological aspects of the female sexual experience and how they influence one another.

Safety and Efficacy of Sildenafil Citrate for the Treatment of Female Sexual Arousal Disorder: A Double-Blind, Placebo Controlled Study

PURPOSE

We evaluated the efficacy and safety of sildenafil citrate in spontaneously or surgically postmenopausal women with female sexual arousal disorder (FSAD).

MATERIALS AND METHODS

Sildenafil (a 50 mg dose adjustable to 100 or 25 mg) was evaluated in a 12-week, double-blind, placebo controlled study in 202 postmenopausal women with FSAD who had protocol specified estradiol and free testosterone concentrations, and/or were receiving estrogen and/or androgen replacement therapy. Patients were excluded if emotional, relationship or historical abuse issues contributed significantly to sexual dysfunction. Primary end points were questions 2 (increased genital sensation during intercourse or stimulation) and 4 (increased satisfaction with intercourse and/or foreplay) from the Female Intervention Efficacy Index (FIEI). Secondary end points were the remaining questions from this index, the Sexual Function Questionnaire and sexual activity event log questions.

RESULTS

Significant improvements in FIEI questions 2 (p = 0.017) and 4 (p = 0.015) were noted with sildenafil compared with placebo. For women with FSAD without concomitant hypoactive sexual desire disorder (HSDD) sildenafil was associated with significantly greater improvement in 5 of 6 FIEI items compared with placebo (p <0.02). No significant improvements were shown for women with concomitant HSDD. Most adverse events were mild to moderate with headache, flushing, rhinitis, nausea and visual symptoms reported most frequently.

CONCLUSIONS

Sildenafil was effective and well tolerated in postmenopausal women with FSAD without concomitant HSDD or contributory emotional, relationship or historical abuse issues. All patients had protocol specified estradiol and free testosterone concentrations or were receiving estrogen and/or androgen replacement therapy.

Sexual function and dysfunction in women

Multiple factors may affect sexual functioning in women, requiring a thorough assessment of all possible etiologies to guide appropriate treatment. Interventions may also be multifaceted, ranging from sex education to psychotherapy to medical treatment. Restoration of sexual functioning is the goal of treatment, but more research is needed for true success to be realized.

Activational and organizational effects of estradiol on male behavioral neuroendocrine function

Evidence is reviewed that establishes an essential role for estradiol, synthesized locally via aromatization of testosterone in the adult as well as the developing male nervous system of rats, ferrets, and mice, in both the organization and adult activation of neural circuits that control appetitive as well as consummatory components of masculine sexual behavior. Evidence is also reviewed suggesting that perinatal estradiol biosynthesis and action in the female's nervous system may contribute to the development of female-typical sexual behavior.

Improved erectile function after Rho-kinase inhibition in a rat castrate model of erectile dysfunction

Androgens are reported to act as strong modulators of erectile function influencing both nitric oxide and vasoconstrictor signaling. Castration results in a depressed erectile response that is associated with a loss of nitric oxide production and increased responsiveness to constrictive agents. The increased vasoconstrictor response may be a result of an active RhoA/Rho-kinase signaling pathway. We report here results of studies designed to test the hypothesis that inhibition of the Rho-kinase pathway restores erectile function in a castrate model by relaxing the smooth muscle. Mean arterial (MAP) and corpus cavernosal (CCP) pressures were monitored during intracavernosal injection of the Rho-kinase inhibitor Y-27632. Castration reduced the maximal erectile response (CCP/MAP) by 33%, and testosterone replacement restored the response (intact, 0.736 +/- 0.040; castrate, 0.492 +/- 0.022; testosterone, 0.681 +/- 0.073). Injection of Y-27632 increased CCP in all experimental groups; it also left shifted the voltage response curve and increased the maximal CCP/MAP response (intact, 0.753 +/- 0.091; castrate, 0.782 +/- 0.081; testosterone treated, 0.894 +/- 0.033). Y-27632 dose dependently relaxed phenylephrine-stimulated cavernosal tissues. Cavernosal tissues showed increased RhoA and Rho-kinase protein levels after castration. Our data support the hypothesis that an active Rho/Rho-kinase pathway contributes to the reduced erectile response after castration due to an upregulation of RhoA/Rho-kinase protein levels and that inhibition of this pathway may serve as an effective treatment for erectile dysfunction.

Etiology and treatment of erectile failure in diabetes mellitus

Men with diabetes are especially prone to neuronal and endothelial disease and are afflicted with erectile dysfunction (ED) at a much higher incidence and prevalence than normal men, with a consequent reduction in quality of life. ED in diabetes results from combinations of impairments from nearly every step in the production of a penile erection. These include the failed transmission of neural signals to and from the spinal cord due to neuropathy resulting in reduced neural nitric oxide (NO) delivery to cavernosal smooth muscle, impaired sinusoidal endothelial cell NO release because of endothelial dysfunction, reduced arterial and arteriolar inflow due to peripheral vascular disease, and failure of relaxation of the corpora from glycation of the elastic fibers. The evaluation of men with diabetes and ED is similar to that in nondiabetic men. It includes assessment of gonadal, neural, arterial, and on occasion, venous function, along with marital and psychological status. Although treatment options for diabetic men with ED have fortunately expanded, the risk factors for ED are the same as those for cardiovascular disease; a flagging penis should raise a red flag of warning to evaluate and treat the risk factors for coronary, cerebral, and peripheral vascular disease, which are common in persons, of either sex, afflicted by diabetes.

In vivo electrochemical measurement of nitric oxide in corpus cavernosum penis

A wealth of pharmacological studies suggest that nitric oxide (NO) generated in the corpus cavernosum is a main molecular mediator of penile erection. However, the physiological levels of NO in the corpora and their possible changes during penile erection have remained unknown for want of suitable methodologies. We have adapted a voltammetric procedure, derived from Malinski's method, for assessing NO levels in the penis in vivo. Differential normal pulse voltammetry with carbon fiber electrodes (30 microm) coated with a polymeric porphyrin and Nafion was used to measure the NO oxidation current in the corpora cavernosa of urethane-anesthetized rats. The intracavernous pressure was monitored simultaneously. A NO oxidation peak was consistently detected at approximately 650 mV both in NO solutions and in the corpora in vivo. The changes in the NO signals observed in vitro were consistent with the concentration values measured by chemiluminescence. The NO signal recorded in vivo increased following cavernous nerve stimulation and was greatly decreased by intracavernous injections of several inhibitors of the neuronal and endothelial NO synthase isoenzymes. Such results agree with our previous studies using this methodology and substantiate further its validity for monitoring the physiological changes in NO levels in the penis.

Endothelium-dependency of yohimbine-induced corpus cavernosum relaxation

Development and maintenance of penile erection requires the relaxation of the smooth muscle cells in the cavernous bodies and is essentially mediated by nitric oxide (NO). The penile flaccid state is conversely maintained by the alpha adrenergic neuroeffector system and by other vasoconstrictors, such as endothelin-1 (ET-1). In this study we examined the mechanisms involved in yohimbine-induced relaxation in human and rabbit corpora cavernosa (CC). We essentially found that yohimbine not only blocks contractions induced by adrenergic agonists, but also by non-adrenergic substances, such as ET-1. This effect was unrelated to antagonism at the level of ET receptors, because yohimbine did not affect ET-1-induced increase in intracellular calcium in isolated CC cells. Conversely, our data suggest that yohimbine counteracts ET-1-induced contractions by interfering with NO release from the endothelium. In fact, yohimbine-induced CC relaxation was inhibited by the mechanical removing of the endothelium and by blocking NO formation or signalling via guanylate cyclase and cGMP formation. Conversely, yohimbine activity was strongly increased by inhibiting cGMP degradation. In an experimental model of hypogonadism, performed on rabbits by chronic treatment with a long-lasting GnRH agonist, the relaxant yohimbine activity was also decreased, but completely restored by androgen supplementation. This effect was evident only in preparations in which the main source of NO was present (endothelium) or in which NO formation was not impaired by L-NAME. Our data indicate that the relaxant effect of yohimbine is both endothelium and androgen-dependent. This might justify the lack of efficacy of this drug in treatment of some form of organic erectile dysfunction.

The pathophysiology of erectile dysfunction related to endothelial dysfunction and mediators of vascular function

The incidence of erectile dysfunction increases with diabetes, hypertension, hypercholesterolaemia, cardiovascular disease and renal failure. All these conditions are associated with endothelial dysfunction. This review addresses the pathophysiology of erectile dysfunction with a special focus on new insights into nitric oxide (NO)-mediated pathways, oxidative stress and parallels to endothelial dysfunction. NO appears to be the key mediator promoting endothelium-derived vasodilation and penile erection. The possibility is discussed that elevated plasma concentrations of asymmetrical dimethylarginine (ADMA), an endogenous NO synthase inhibitor, may provide an additional pathomechanism for various forms of erectile dysfunction associated with cardiovascular risk factors and disease. Likewise, the role of endothelium-derived factors mediating NO-independent pathways is evaluated.

Changes in mating behavior, erectile function, and nitric oxide levels in penile corpora cavernosa in streptozotocin-diabetic rats

This study assessed whether the in vivo production of nitric oxide (NO) in the penis is impaired in experimental diabetes and whether this phenomenon can be explained by abnormal levels of NO synthase isoenzymes and/or plasma androgens. Adult male Sprague-Dawley rats were injected with streptozotocin (STZ) (40 mg/kg, i.p.) or vehicle. One half of the STZ-treated animals received daily insulin replacement. Twelve weeks later, the animals were tested for mating behavior and erectile reflexes. They were then anesthetized with urethane (1 g/kg), and the NO levels in their corpora cavernosa were monitored electrochemically with porphyrin microsensors before and after electrostimulation of the cavernous nerve. The intracavernous pressure (ICP) was measured simultaneously. The diabetic animals had substantial impairment in the mating and erectile reflexes tests, decreased basal and stimulated NO levels in the corpora, and a reduced ICP response to cavernous nerve stimulation. Insulin replacement fully reversed the effects of diabetes on the mating reflexes, the basal NO signals, and the ICP responses to electrical field stimulation and partially restored the stimulated NO release. Neither diabetes nor diabetes with insulin treatment had significant effects on serum testosterone levels or NOS isoform (nNOS, eNOS, and iNOS) protein content in penile homogenates, indicating that the changes found in erectile function were independent of such variables. These results also suggest that the diabetes-induced reduction in corporeal NO levels could be mainly due to the lack of some essential cofactors for NOS activity rather than to changes in the amount of enzyme proteins.

Repeated PGE1 treatment enhances nitric oxide and erection responses to nerve stimulation in the rat penis by upregulating constitutive NOS isoforms

PURPOSE

To assess whether intracavernosal injections of prostaglandin E1 (PGE1) can influence nitric oxide (NO) release in the corpora in a rat model of penile erection.

MATERIALS AND METHODS

The extracellular levels of NO were monitored at 100 seconds intervals in the corpus cavernosum of anesthetized rats by using differential normal pulse voltammetry with porphyrin-Nafion coated carbon fiber microelectrodes. The intracavernosal pressure (ICP) was simultaneously recorded. PGE1 was given either as a single dose (ranging from 0.2 to 15 microg.) or as repeated 2 microg. injections in alternate days for two weeks. The NO and ICP responses to electrostimulation of the cavernosal nerve (SCN) was studied in the animals in the repeated treatment schedule at 1, 7, 15 and 30 days after its termination. The levels of the three NO synthase (NOS) isoforms in the cavernous tissue were measured by immunoblotting.

RESULTS

Acute PGE1 treatment dose-relatedly increased NO levels in the corpora, with a concomitant ICP increase with the highest dose. Repeated 2 microg. PGE1 injections increased the NO and ICP responses to SCN as compared with intact or vehicle-injected animals. This treatment also increased the penile content of the neuronal and endothelial NOS proteins. The inducible NOS isoform remained unchanged after either vehicle or PGE1 injections. The effects of the repeated PGE1 treatment were greater in the group studied 24 hours after the last injection and decreased progressively thereafter.

CONCLUSIONS

Stimulation of NO release can contribute to the erectogenic effect of intracavernous PGE1 injections. The increased levels of constitutive NOS isoforms in the corpora could contribute to the improvement of the erectile function reported by some patients following repeated treatment with vasorelaxant agents.