Characterization of nitroglycerine-induced relaxation in human corpus cavernosum smooth muscle: implications to erectile physiology and dysfunction

Inhibition of multidrug resistance proteins by MK571 restored the erectile function in obese mice through cGMP accumulation

BACKGROUND

Intracellular levels of cyclic nucleotides can also be controlled by the action of multidrug resistance protein types 4 (MRP4) and 5 (MRP5). To date, no studies evaluated the role of their inhibition in an animal model of erectile dysfunction (ED).

OBJECTIVES

To evaluate the effect of a 2-week treatment with MK571, an inhibitor of the efflux of cyclic nucleotides in the ED of obese mice.

MATERIALS AND METHODS

Mice were divided in three groups: (i) lean, (ii) obese, and (iii) obese + MK571. The corpus cavernosum (CC) were isolated, and concentration-response curves to acetylcholine (ACh), sodium nitroprusside (SNP), and tadalafil in addition to electrical field stimulation (EFS) were carried out in phenylephrine pre-contracted tissues. Expression of ABCC4 and ABCC5, intracellular levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), the protein levels for pVASP^Ser157 and pVASP^Ser239 , and the intracavernous pressure (ICP) were also determined. The intracellular and extracellular (supernatant) ratios in CC from obese and lean stimulated with a cGMP-increasing substance (BAY 58-2667) in the absence and presence of MK571 (20 μM, 30 min) were also assessed.

RESULTS

The treatment with MK571 completely reversed the lower relaxing responses induced by EFS, ACh, SNP, and tadalafil observed in obese mice CC in comparison with untreated obese mice. Cyclic GMP and p-VASP^Ser239 expression were significantly reduced in CC from obese groups. MK571 promoted a sixfold increase in cGMP without interfering in the protein expression of p-VASP^Ser239 . Neither the cAMP levels nor p-VASP^Ser157 were altered in MK571-treated animals. The ICP was ∼50% lower in obese than in the lean mice; however, the treatment with MK571 fully reversed this response. Expressions of ABCC4 and ABCC5 were not different between groups. The intra/extracellular ratio of cGMP was similar in CC from obese and lean mice stimulated with BAY 58-2667.

CONCLUSIONS

The MRPs inhibition by MK571 favored the accumulation of cGMP in the smooth muscle cells, thus improving the smooth muscle relaxation and the erectile function in obese mice.

Development of a novel topical formulation of glyceryl trinitrate for the treatment of erectile dysfunction

Erectile dysfunction (ED), defined as the inability to initiate or maintain an erection sufficient for satisfactory sexual intercourse, is common, particularly in men aged ≥50 years. Existing treatments have significant limitations, and there remains a need for a fast-acting (to facilitate spontaneity during intercourse) and well tolerated local therapy. Topical glyceryl trinitrate (GTN) may meet this need because GTN undergoes rapid metabolism in penile smooth muscle and endothelial cells to produce nitric oxide, which plays a key role in the development of erection. This paper describes the rationale for the development of MED2005, a topical GTN formulation using DermaSys® technology, which is undergoing clinical trials for the treatment of ED. Pharmacokinetic studies have shown that MED2005 provides rapid delivery of GTN following application to the glans penis, and a Phase 2(a) trial in men with ED showed that MED2005 produced significant improvements in erectile function, compared with placebo. MED2005 was well tolerated in this trial, with only 21 cases of headache in 1003 intercourse attempts. It is anticipated that MED2005 will provide an effective therapy for ED, with a fast onset of action, good local tolerability, and fewer contraindications than phosphodiesterase 5 inhibitors, the current cornerstone of ED therapy.

Phenolic extracts and amino acids content from Cucumeropsis mannii naudin and Citrullus lanatus inhibit relevant enzymes of erectile dysfunction in rat's penile tissue

•A focused on the erectogenic potentials of two tropical pumpkin seeds was established.•The antioxidant properties of these seed varieties were also discussed.•The inhibitory effects of the seeds on key enzymes relevant to ED were critically explored.•The potentials of the seeds to manage ED were recommended.

Animal models of erectile dysfunction

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

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.

Pharmacotherapy for Erectile Dysfunction

INTRODUCTION

Advances in understanding of the biochemistry and physiology of penile erection have led to breakthroughs in pharmacotherapy of erectile dysfunction.

AIM

To provide recommendations/guidelines concerning state-of-the-art knowledge for the putative molecular and cellular mechanisms of action of centrally and peripherally acting drugs currently utilized in pharmacotherapy of erectile dysfunction.

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 Pharmacotherapy for Erectile Dysfunction Committee there were 25 experts from 10 countries.

MAIN OUTCOME MEASURE

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

RESULTS

Selective and potent oral PDE5 inhibitors have significantly more affinity than cGMP and form broader molecular interactions with multiple amino acids, thereby blocking access to cGMP in the catalytic sites of the PDE5 enzyme. PDE5 inhibitors, which vary as to biochemical potency, selectivity and pharmacokinetics, lead to cGMP elevation and relaxation facilitation of penile corpus cavernosum smooth muscle cells following sexual stimulation. Various centrally acting drugs influence sexual behaviour. In particular, the dopaminergic substance apomorphine is a central enhancer that acts in the paraventricular nucleus of the hypothalamus as a dopamine (D2) receptor agonist, induces and increases penile erection responses via disinhibition, following sexual stimulation.

CONCLUSIONS

There is a need for more research in the pharmacotherapeutic development of central and peripheral agents for safe and effective erectile dysfunction treatment.

Relaxing effects induced by the soluble guanylyl cyclase stimulator BAY 41-2272 in human and rabbit corpus cavernosum

5-Cyclopropyl-2-[1-(2-fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl]-pyrimidin-4-ylamine (BAY 41-2272) is a potent soluble guanylyl cyclase stimulator in a nitric oxide (NO)-independent manner. The relaxant effect of BAY 41-2272 was investigated in rabbit and human corpus cavernosum in vitro. BAY 41-2272 (0.01-10 microM) relaxed both rabbit (pEC(50)=6.82+/-0.06) and human (pEC(50)=6.12+/-0.10) precontracted cavernosal strips. The guanylyl cyclase inhibitor (ODQ, 10 microM) caused significant rightward shifts in the concentration-response curves for BAY 41-2272 in rabbit (4.7-fold) and human (2.3-fold) tissues. The NO synthesis inhibitor (N-nitro-L-arginine methyl ester (L-NAME), 100 microM) also produced similar rightward shifts, revealing that BAY 41-2272 acts synergistically with endogenous NO to elicit its relaxant effect. The results also indicate that ODQ is selective for the NO-stimulated enzyme, since relaxations evoked by BAY 41-2272 were only partly attenuated by ODQ. The present study shows that both BAY 41-2272 and sildenafil evoke relaxations independent of inhibition of haem in soluble guanylate cyclase. Moreover, there is no synergistic effect of the two compounds in corpus cavernosum.

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

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

The 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 hemodynamics of erection and the pharmacotherapies of erectile dysfunction

The past few years have been witness to a sea of change in understanding the diagnosis and treatment of erectile dysfunction. In the wake of Viagra, effective orally administered therapies are now the expected gold standard. Currently available therapies include both peripherally (at the level of the penis) and centrally (brain, e.g., hypothalamic nuclei) acting compounds. When all modes of pharmacotherapy are considered, ranging from oral, to topical, to intraurethral, to intracavernous injection, it is clear that the vast majority of men with mild to moderate erectile dysfunction can now be effectively treated. However, all forms of pharmacotherapy have the lowest efficacy and greatest side effect profiles (including cardiovascular events) in the patients that need the therapy most, that is, those with relatively severe and longstanding erectile dysfunction. The newer generation of pharmacotherapies, which will likely include gene therapy techniques as well, will therefore have to target this latter group. Improved mechanism-based, perhaps patient-specific therapies are foreseen that will dramatically increase the number of patients seeking treatment, as well as the quality of their lives.

Human and rabbit cavernosal smooth muscle cells express Rho-kinase

Rho-kinase is an enzyme involved in the Ca2+-sensitizing pathway in smooth muscle cells. Inhibition of this enzyme has been recently demonstrated to elicit penile erection by relaxing cavernosal smooth muscle. We aimed to investigate the presence and activity of Rho-kinase in human cavernosal smooth muscle. Primary culture of smooth muscle cells from human and rabbit penile corpus cavernosum was developed, and cells showed characteristic myocyte morphology and alpha-actin immunoreactivity. The presence of Rho-kinase was demonstrated by indirect immunofluorescence and Western blotting. A specific inhibitor of Rho-kinase, Y-27632, inhibited in a concentration-dependent manner the kinase activity of the protein immunoprecipitated with anti-Rho-kinase antibody. These results demonstrate for the first time expression and activity of Rho-kinase in human penile cavernosal smooth muscle cells and suggest that these cells can provide a cellular model for the study of enzymes involved in Ca2+-sensitizing pathways.

Integrative erectile biology. The effects of age and disease on gap junctions and ion channels and their potential value to the treatment of erectile dysfunction

Initiation, maintenance, and modulation of corporal smooth muscle tone are critically dependent upon agonist-induced changes in intracellular calcium levels and mobilization as well as transmembrane calcium flux. The transient control of myocyte excitability and contractility at the cellular level is inextricably linked to membrane potential, which, in turn, is modulated by potassium ion efflux through one of the four known corporeal smooth muscle potassium ion channels. Corporal tissue responses are subsequently coordinated by means of the movement of intracellular second messenger molecules (i.e., IP3, cAMP, cGMP) and ions (i.e., K+ and Ca2+) among the corporal myocytes by means of intercellular communication through gap junction channels. Knowledge of the critical contribution of these interlinking cellular (nonjunctional ion channels [e.g., maxi-K]) and tissue (gap junction channels [e.g., connexin 43]) systems to the modulation of erectile capacity has provided the scientific rationale for the promulgation of the successful preclinical testing of hSlo ion channel gene therapy for the normalization of erectile status in both aged and diabetic rats.

Rapid desensitization of the nitric oxide receptor, soluble guanylyl cyclase, underlies diversity of cellular cGMP responses

A major receptor for nitric oxide (NO) is the cGMP-synthesizing enzyme, soluble guanylyl cyclase (sGC), but it is not known how this enzyme behaves in cells. In cerebellar cells, NO (from diethylamine NONOate) increased astrocytic cGMP with a potency (EC(50) </= 20 nM) higher than that reported for purified sGC. Deactivation of NO-stimulated sGC activity, studied by trapping free NO with hemoglobin, took place within seconds (or less) rather than the minute time scale reported for the purified enzyme. Measurement of the rates of accumulation and degradation of cGMP were used to follow the activity of sGC over time. The peak activity, occurring within seconds of adding NO, was swiftly followed by desensitization to a steady-state level 8-fold lower. The same desensitizing profile was observed when the net sGC activity was increased or decreased or when cGMP breakdown was inhibited. Recovery from desensitization was relatively slow (half-time = 1.5 min). When the cells were lysed, sGC desensitization was lost. Analysis of the transient cGMP response to NO in human platelets showed that sGC underwent a similar desensitization. The results indicate that, in its natural environment, sGC behaves much more like a neurotransmitter receptor than had been expected from previous enzymological studies, and that hitherto unknown sGC regulatory factors exist. Rapid sGC desensitization, in concert with variations in the rate of cGMP breakdown, provides a fundamental mechanism for shaping cellular cGMP responses and is likely to be important in decoding NO signals under physiological and pathophysiological conditions.

Selective nitrergic neurodegeneration in diabetes mellitus - a nitric oxide-dependent phenomenon

1. In vitro and in vivo studies have demonstrated a dysfunctional nitrergic system in diabetes mellitus, thus explaining the origin of diabetic impotence. However, the mechanism of this nitrergic defect is not understood. 2. In the penises of streptozotocin (STZ)-induced diabetic rats, here, we show by immunohistochemistry that nitrergic nerves undergo selective degeneration since the noradrenergic nerves which have an anti-erectile function in the penis remained intact. 3. Nitrergic relaxation responses in vitro and erectile responses to cavernous nerve stimulation in vivo were attenuated in these animals, whereas noradrenergic responses were enhanced. 4. Activity and protein amount of neuronal nitric oxide synthase (nNOS) were also reduced in the penile tissue of diabetic rats. 5. We, thus, hypothesized that NO in the nitrergic nerves may be involved in the nitrergic nerve damage, since only the nerves which contain neuronal NO synthase underwent degeneration. 6. We administered an inhibitor of NO synthase, N(G)-nitro-L-arginine methyl ester (L-NAME), in the drinking water of rats for up to 12 weeks following the establishment of diabetes with STZ. 7. Here we demonstrate that this compound protected the nitrergic nerves from morphological and functional impairment. Our results show that selective nitrergic degeneration in diabetes is NO-dependent and suggest that inhibition of NO synthase is neuroprotective in this condition.

cGMP mediates corpus cavernosum smooth muscle relaxation with altered cross-bridge function

We tested the prevailing paradigm that relaxation of corpus cavernosum smooth muscle (CCSM) and penile erection depends upon nitric oxide-induced elevation of myoplasmic cGMP and reduced Ca2+-dependent myosin regulatory light chain phosphorylation levels. This hypothesis invokes a reversal of normal activation pathways. Upon stimulation with 250 microM phenylephrine, phosphorylation of the 20 kD myosin regulatory light chains of rabbit or human CCSM increased approximately 4-fold coincident with contraction. Removal of the agonist was followed by a slow reduction in cross-bridge phosphorylation and force to basal levels. The NO donor, sodium nitroprusside elicited a dose-dependent increase in tissue [cGMP] associated with a rapid relaxation in the continued presence of phenylephrine, although cross-bridge phosphorylation remained significantly elevated. Thus the NO-cGMP inhibitory pathway in CCSM is not simply a reversal of excitatory signal transduction mechanisms. An unidentified mechanism contributes to relaxation by decreasing the rate of cross-bridge recruitment through phosphorylation.

Opportunities for the treatment of erectile dysfunction by modulation of the NO axis--alternatives to sildenafil citrate

Erectile function in man depends upon a complex interaction of psychogenic, neurologic, hormonal and vascular factors, and therefore the management of erectile dysfunction (ED) reflects this complexity of control. Therapeutic options include psychological and non-pharmacological approaches as well as drug treatments. The effectiveness of the type-5 cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra) confirms the pivotal role of the NO-cGMP axis in promoting and maintaining erection. Although widely acclaimed, sildenafil leaves many questions unanswered, especially regarding its susceptibility to pharmacokinetic drug interactions, and its safety in patients with ischaemic heart disease and those taking nitrates. In view of the epidemiological link between erectile dysfunction and cardiovascular disease in the elderly, this limitation might have much broader implications. The presently available scientific documentation, although less extensive, indicates that NO donors, such as topically applied nitroglycerin (GTN; for example, 1-2 puffs of an ordinary GTN spray applied to the shaft of the penis), might be a reasonable alternative. Further larger-scale research on the efficacy and tolerability of topical GTN is needed to establish its full therapeutic potential in the treatment of erectile dysfunction.