Effects of sildenafil on cAMP and cGMP levels in isolated human cavernous and cardiac tissue

The effect of cardioplegic supplementation with sildenafil on cardiac energetics in a piglet model of cardiopulmonary bypass and cardioplegic arrest with warm or cold cardioplegia

Cardioplegic cardioprotection strategies used during paediatric open-heart surgery remain suboptimal. Sildenafil, a phosphodiesterase 5 (PDE-5) inhibitor, has been shown to be cardioprotective against ischemia/reperfusion injury in a variety of experimental models and this study therefore tested the efficacy of supplementation of cardioplegia with sildenafil in a piglet model of cardiopulmonary bypass and arrest, using both cold and warm cardioplegia protocols. Piglets were anaesthetized and placed on coronary pulmonary bypass (CPB), the aorta cross-clamped and the hearts arrested for 60 min with cardioplegia with or without sildenafil (10 nM). Twenty minutes after removal of cross clamp (reperfusion), attempts were made to wean the pigs from CPB. Termination was carried out after 60 min reperfusion. Throughout the protocol blood and left ventricular tissue samples were taken for analysis of selected metabolites (using HPLC) and troponin I. In both the cold and warm cardioplegia protocols there was evidence that sildenafil supplementation resulted in faster recovery of ATP levels, improved energy charge (a measure of metabolic flux) and altered release of hypoxanthine and inosine, two purine catabolites. There was no effect on troponin release within the studied short timeframe. In conclusion, sildenafil supplementation of cardioplegia resulted in improved cardiac energetics in a translational animal model of paediatric CPB surgery.

Combination of the Phosphodiesterase Inhibitors Sildenafil and Milrinone Induces Cardioprotection With Various Conditioning Strategies

Ischemic preconditioning and postconditioning are strong measures preserving the heart against ischemia-reperfusion injury in experimental setting but are too invasive and impractical for clinical routine. The cardioprotective effects of ischemic preconditioning and postconditioning can be imitated pharmacologically, for example, with the phosphodiesterase inhibitors sildenafil and milrinone. We hypothesize that sildenafil-induced preconditioning is concentration dependent and further that a combined treatment of "nonprotective" versus "protective" concentrations of sildenafil and milrinone leads to a significant infarct size reduction. Experiments were performed on isolated hearts of male Wistar rats, randomized into 12 groups, mounted onto a Langendorff system, and perfused with Krebs-Henseleit buffer. All hearts underwent 33 minutes ischemia and 60 minutes of reperfusion. For determination of a concentration-dependent effect of sildenafil, hearts were perfused with increasing concentrations of sildenafil (0.1-1 µM) over 10 minutes before ischemia. In a second series of experiments, hearts were treated with 0.3 µM sildenafil or 1 µM milrinone as the "protective" concentrations. A higher concentration of respective drugs did not further reduce infarct size. In addition, a combination of "protective" and "nonprotective" concentrations of sildenafil and milrinone was applied. Sildenafil and milrinone in lower concentrations led to significant infarct size reduction, whereas combining both substances in cardioprotective concentrations did not enhance this effect. Sildenafil in a concentration of 0.3 µM induces myocardial protection. Furthermore, treatment with sildenafil and milrinone in lower concentrations had an equally strong cardioprotective effect regarding infarct size reduction compared with the administration of "protective" concentrations.

Cilostazol is an effective causal therapy for preventing paclitaxel-induced peripheral neuropathy by suppression of Schwann cell dedifferentiation

Chemotherapy-induced peripheral neuropathy (CIPN) can lead to discontinuation of chemotherapy and is consequently a serious impediment to effective cancer treatment. Due to our limited understanding of mechanisms underlying the pathogenesis of CIPN, no causal therapy has been approved for relief of this condition. We previously demonstrated that taxanes (paclitaxel and docetaxel) induce Schwann cell dedifferentiation, characterized by increased expression of p75 and galectin-3, ultimately leading to demyelination. These changes appear to be responsible for CIPN pathogenesis. This study was designed to identify a novel candidate therapeutic for CIPN with the ability to suppress paclitaxel-induced Schwann cell dedifferentiation. Given that elevation of cyclic adenosine monophosphate (cAMP) signaling participates in Schwann cell differentiation, we performed immunocytochemical screening of phosphodiesterase (PDE) inhibitors. We found that the PDE3 inhibitor cilostazol strongly promoted differentiation of primary cultures of rat Schwann cells via a mechanism involving cAMP/exchange protein directly activated by cAMP (Epac) signaling. Co-treatment with cilostazol prevented paclitaxel-induced dedifferentiation of Schwann cell cultures and demyelination in a mixed culture of Schwann cells and dorsal root ganglia neurons. Notably, continuous oral administration of cilostazol suppressed Schwann cell dedifferentiation within the sciatic nerve and the development of mechanical hypersensitivity in a mouse model of paclitaxel-related CIPN. Importantly, cilostazol potentiated, rather than inhibited, the anti-cancer effect of paclitaxel on the human breast cancer cell line MDA-MB-231. These findings highlight the potential utility of cilostazol as a causal therapeutic that avoids the development of paclitaxel-related CIPN without compromising anti-cancer properties.

PDE5 inhibition rescues mitochondrial dysfunction and angiogenic responses induced by Akt3 inhibition by promotion of PRC expression

Akt3 regulates mitochondrial content in endothelial cells through the inhibition of PGC-1α nuclear localization and is also required for angiogenesis. However, whether there is a direct link between mitochondrial function and angiogenesis is unknown. Here we show that Akt3 depletion in primary endothelial cells results in decreased uncoupled oxygen consumption, increased fission, decreased membrane potential, and increased expression of the mitochondria-specific protein chaperones, HSP60 and HSP10, suggesting that Akt3 is required for mitochondrial homeostasis. Direct inhibition of mitochondrial homeostasis by the model oxidant paraquat results in decreased angiogenesis, showing a direct link between angiogenesis and mitochondrial function. Next, in exploring functional links to PGC-1α, the master regulator of mitochondrial biogenesis, we searched for compounds that induce this process. We found that, sildenafil, a phosphodiesterase 5 inhibitor, induced mitochondrial biogenesis as measured by increased uncoupled oxygen consumption, mitochondrial DNA content, and voltage-dependent anion channel protein expression. Sildenafil rescued the effects on mitochondria by Akt3 depletion or pharmacological inhibition and promoted angiogenesis, further supporting that mitochondrial homeostasis is required for angiogenesis. Sildenafil also induces the expression of PGC-1 family member PRC and can compensate for PGC-1α activity during mitochondrial stress by an Akt3-independent mechanism. The induction of PRC by sildenafil depends upon cAMP and the transcription factor CREB. Thus, PRC can functionally substitute during Akt3 depletion for absent PGC-1α activity to restore mitochondrial homeostasis and promote angiogenesis. These findings show that mitochondrial homeostasis as controlled by the PGC family of transcriptional activators is required for angiogenic responses.

Probing the Effect of Sildenafil on Progesterone and Testosterone Production by an Intracellular FRET/BRET Combined Approach

Forster resonance energy transfer (FRET)-based biosensors have been recently applied to the study of biological pathways. In this study, a new biosensor was validated for the first time in live HEK293 and steroidogenic MLTC-1 cell lines for studying the effect of the PDE5 inhibitor on the hCG/LH-induced steroidogenic pathway. The sensor improves FRET between a donor (D), the fluorescein-like diarsenical probe that can covalently bind a tetracysteine motif fused to the PDE5 catalytic domain, and an acceptor (A), the rhodamine probe conjugated to the pseudosubstrate cGMPS. Affinity constant ( K_d) values of 5.6 ± 3.2 and 13.7 ± 0.8 μM were obtained with HEK293 and MLTC-1 cells, respectively. The detection was based on the competitive displacement of the cGMPS-rhodamine conjugate by sildenafil; the K_i values were 3.6 ± 0.3 nM (IC₅₀ = 2.3 nM) in HEK293 cells and 10 ± 1.0 nM (IC₅₀ = 3.9 nM) in MLTC-1 cells. The monitoring of both cAMP and cGMP by bioluminescence resonance energy transfer allowed the exploitation of the effects of PDE5i on steroidogenesis, indicating that sildenafil enhanced the gonadotropin-induced progesterone-to-testosterone conversion in a cAMP-independent manner.

Effects of endopeptidase inhibition on the relaxation response of isolated human penile erectile tissue to vasoactive peptides

Peptides, such as CNP, CGRP and VIP, are involved in the function of male penile erectile tissue. Tissue levels of said peptides are controlled by the endopeptidase enzymes. Theoretically, the inhibition of the degradation of CNP, CGRP and/or VIP should result in an enhancement in penile smooth muscle relaxation. The effects were investigated of CNP or VIP (0.1 nm-1 μm), without and following pre-exposure of the tissue to a threshold concentration of the endopeptidase inhibitor KC 12615 (10 μm, for 20 min), on the reversion of tension induced by means of electrical field stimulation. Drug effects on the production of cyclic AMP/GMP were also evaluated. Neither KC 12615, CNP and VIP nor the combination of CNP plus KC 12615 or VIP plus KC 12615 increased the response of the tissue to EFS. While no effects were observed of a pre-exposure of the tissue to KC 12615 on the production of cyclic AMP in the presence of VIP, an enhancement was registered in the accumulation of cyclic AMP in the presence of CNP plus KC 12615. Further studies are indicated to investigate whether endopeptidase inhibitors might tend to be more effective in tissues affected by a decreased local production of vasoactive peptides.

Are aryl hydrocarbon receptor and G-protein-coupled receptor 30 involved in the regulation of seasonal testis activity in photosensitive rodent-the bank vole (Myodes glareolus)?

Within the reproductive system both aryl hydrocarbon receptor (AHR) and G-protein-coupled receptor 30 (GPR30) contribute to estrogen signaling and controlling of reproductive physiology. The specific question is whether and how AHR and GPR30 are involved in regulation of testis function in seasonally breeding rodents. Bank vole testes were obtained from animals reared under 18 hours light:6 hours dark (LD) and 6 hours light:18 hours dark (SD) conditions. Aryl hydrocarbon receptor and GPR30 expression were analyzed by quantitative reverse transcriptase-polymerase chain reaction, Western blot, and immunohistochemistry and/or immunofluorescent staining. In addition, the activity of enzymes involved in the intracellular signal transduction; extracellular signal-regulated kinase (ERK), protein kinase (PKA), matrix metalloproteinase 9 (MMP 9) and the concentrations of cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), and calcium (Ca(2+)) were examined by immunohistochemical, immunoenzymatic, and colorimetric assays, respectively. Aryl hydrocarbon receptor and GPR30 were expressed in testes of actively reproducing voles and regressed ones although their expression at the messenger RNA and AHR also at protein level appeared to be photoperiod-dependent. A specific cellular localization and expression of AHR and GPR30 correlated with the expression of ERK, PKA, and MMP 9. Moreover, we found robust differences in the levels of cAMP, cGMP, and Ca(2+) in testicular homogenates between LD and SD voles. In the testes of LD voles, the levels of second messengers were always higher compared to SD. In vole testis, AHR and GPR30 can induce signaling pathways that involve ERK, PKA, MMP 9 and cAMP, cGMP, Ca(2+). In addition, in AHR, signaling the engagement of both photoperiod and estrogens, whereas in GPR30, signaling only estrogens is reported. It is likely that in vole, because of a differential activity of signaling molecules, signal transduction via AHR rather than through GPR30 plays a role in regulation of seasonal changes of testis physiology.

Relaxant effect and possible mechanism of 17-nor-subincanadine E in rabbit corpora cavernosa

Compounds with dual action on cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) may be a treatment option for erectile dysfunction, as they not only promote penile erection but also prevent the upregulation of phosphodiesterase-5. In this study, we examined the possible relaxant effect and mechanism of 17-nor-subincanadine E (SEC, 0.2-200 µmol l⁻¹), a plant-derived alkaloid, in rabbit corpus cavernosum (RbCC) strips that had been precontracted by exposure to phenylephrine (10 µmol l⁻¹) or a high concentration of K(+) (60 mmol l⁻¹) in vitro. In addition to SEC's effect on cAMP and cGMP levels, electrical field stimulation (EFS) in phenylephrine-precontracted RbCC and calcium chloride (1-100 mmol l⁻¹) evoked responses in depolarized RbCC were analysed. SEC relaxed the phenylephrine-precontracted RbCCs in a concentration-dependent manner. Atropine, guanethidine and N-ω-nitro-l-arginine methyl ester (L-NAME) did not have any effect on the relaxation of RBCCs. When 1H-(1, 2, 4)oxadiazole[4,3-a] quinoxalin-1-one (ODQ) was added, it effectively blocked the relaxant response of SEC. Although SEC enhanced the maximal relaxation produced by sodium nitroprusside (SNP) and forskolin in phenylephrine-precontracted cavernosal smooth muscle, it caused a decrease in the maximal contractile response induced by calcium chloride in depolarized RbCCs. The relaxant effect of SEC was paralleled by an increase in the tissue levels of the cyclic nucleotides cAMP and cGMP. We conclude that SEC promotes the relaxation of RbCC, possibly favouring cAMP and cGMP accumulation and calcium blockade. This novel mechanism could be useful for patients who do not benefit from phosphodiesterase inhibitors and for those with endothelial and nitrergic dysfunction, such as patients with diabetes, hypertension and dyslipidaemias.

Endogenous vasoactive peptides and the human vagina--a molecular biology and functional study

INTRODUCTION

Endogenous peptides, such as vasoactive intestinal polypeptide (VIP), C-type natriuretic peptide (CNP), and bradykinin (BK), have been proposed to play a role in the female sexual arousal response by exerting relaxation of clitoral, labial, and vaginal smooth muscle. While the effects of endogenous peptides on the human male erectile tissue have already been described, only very few studies have been conducted to investigate the peptidergic control of female genital tissues, including the vagina.

AIMS

To elucidate the expression of mRNA specifically encoding for peptide receptors in the human vagina and the effects of VIP, CNP, and BK on the tension induced by endothelin-1 (ET-1) of isolated human vaginal wall smooth muscle. The production of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) in response to exposure of the tissue to the peptides was also measured.

METHODS

The expression of mRNA encoding for receptor proteins specific for VIP, CNP, and BK were investigated by means of molecular biology (reverse transcriptase polymerase chain reaction [RT-PCR] analysis). Using the organ bath technique, the effects of VIP, CNP, and BK (0.1 nM to 1 µM) on the tension induced by 0.1 µM ET-1 of human vaginal strips were investigated. The tissue was also exposed to three different concentrations of VIP, CNP, and BK (0.01 µM, 0.1 µM, 1 µM) and the production of cAMP and cGMP determined by means of radioimmunoassays.

MAIN OUTCOME MEASURES

Characterize the expression of peptide receptors in the human vagina and measure the relaxation exerted by BK, CNP, and VIP on the contraction induced by ET-1 of isolated human vaginal tissue. In addition, the effects of the peptides on the production of cAMP and cGMP were also elucidated.

RESULTS

RT-PCR analysis revealed the expression of mRNA transcripts encoding for the VIP receptors VIP1R/vasoactive intestinal polypeptide receptor type 1 (VPAC1) and VIP2R/VPAC2, CNP receptors natriuretic peptide receptor type A (NPRA), natriuretic peptide receptor type B (NPRB) and natriuretic peptide receptor type C (NPRC), and BK receptor B2R. The tension induced by ET-1 was reversed by the peptides with the following rank order of efficacy: BK (21.7%) > VIP (20.9%) > CNP (13.3%). The relaxing effects of VIP and BK were paralleled by a 4.8-fold and fivefold increase in cAMP, while the production of cGMP was stimulated 38-fold and 119-fold in the presence of CNP or BK, respectively.

CONCLUSION

Our results are in support of the hypothesis that endogenous peptides may contribute to the control of human vaginal smooth muscle tone through the involvement of the cyclic nucleotide-dependent pathways.

Intracavernous delivery of synthetic angiopoietin-1 protein as a novel therapeutic strategy for erectile dysfunction in the type II diabetic db/db mouse

INTRODUCTION

Patients with erectile dysfunction (ED) associated with type II diabetes often have impaired endothelial function and tend to respond poorly to oral phosphodiesterase type 5 inhibitors. Therefore, neovascularization is a promising strategy for curing diabetic ED.

AIM

To determine the effectiveness of a soluble, stable, and potent angiopoietin-1 (Ang1) variant, cartilage oligomeric matrix protein (COMP)-Ang1, in promoting cavernous angiogenesis and erectile function in a mouse model of type II diabetic ED. Methods.  Sixteen-week-old male db/db mice (in which obesity and type II diabetes are caused by a mutation in the leptin receptor) and control C57BL/6J mice were used and divided into four groups (N=14 per group): age-matched controls; db/db mice receiving two successive intracavernous injections of phosphate-buffered saline (PBS) (days -3 and 0; 20 µL); db/db mice receiving a single intracavernous injection of COMP-Ang1 protein (day 0; 5.8 µg/20 µL); and db/db mice receiving two successive intracavernous injections of COMP-Ang1 protein (days -3 and 0; 5.8 µg/20 µL).

MAIN OUTCOME MEASURES

Two weeks later, erectile function was measured by electrical stimulation of the cavernous nerve. The penis was then harvested and stained with antibodies to platelet/endothelial cell adhesion molecule-1 (PECAM-1) (endothelial cell marker), phosphohistone H3 (PH3, a nuclear protein indicative of cell proliferation), phospho-endothelial nitric oxide synthase (eNOS), and eNOS. Penis specimens from a separate group of animals were used for cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) quantification.

RESULTS

Local delivery of COMP-Ang1 protein significantly increased eNOS phosphorylation and cGMP and cAMP expression compared with that in the group treated with PBS. Repeated intracavernous injections of COMP-Ang1 protein completely restored erectile function and cavernous endothelial content through enhanced cavernous neoangiogenesis as evaluated by PECAM-1 and PH3 immunohistochemistry and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling assay, whereas a single injection of COMP-Ang1 protein elicited partial improvement.

CONCLUSION

Cavernous neovascularization using recombinant Ang1 protein is a novel therapeutic strategy for the treatment of ED resulting from type II diabetes.

Role of cGMP and cAMP in the hemodynamic response to intrathecal sildenafil administration

INTRODUCTION

Results from our laboratory have demonstrated that intracerebroventricular administration of sildenafil to conscious rats promoted a noticeable increase in both lumbar sympathetic activity and heart rate, with no change in the mean arterial pressure. The intracerebroventricular administration of sildenafil may have produced the hemodynamic effects by activating sympathetic preganglionic neurons in the supraspinal regions and spinal cord. It is well documented that sildenafil increases intracellular cGMP levels by inhibiting phosphodiesterase type 5 and increases cAMP levels by inhibiting other phosphodiesterases.

OBJECTIVE

To examine and compare, in conscious rats, the hemodynamic response following the intrathecal administration of sildenafil, 8-bromo-cGMP (an analog of cGMP), forskolin (an activator of adenylate cyclase), or dibutyryl-cAMP (an analog of cAMP) in order to elucidate the possible role of the sympathetic preganglionic neurons in the observed hemodynamic response.

RESULTS

The hemodynamic responses observed following intrathecal administration of the studied drugs demonstrated the following: 1) sildenafil increased the mean arterial pressure and heart rate in a dose-dependent manner, 2) increasing doses of 8-bromo-cGMP did not alter the mean arterial pressure and heart rate, 3) forskolin did not affect the mean arterial pressure but did increase the heart rate and 4) dibutyryl-cAMP increased the mean arterial pressure and heart rate, similar to the effect observed following the intrathecal injection of the highest dose of sildenafil.

CONCLUSION

Overall, the findings of the current study suggest that the cardiovascular response following the intrathecal administration of sildenafil to conscious rats involves the inhibition of phosphodiesterases other than phosphodiesterase type 5 that increase the cAMP level and the activation of sympathetic preganglionic neurons.

Exposure to phosphodiesterase type 5 inhibitors stimulates aromatase expression in human adipocytes in vitro

INTRODUCTION

Prolonged tadalafil administration in men with erectile dysfunction is associated with increased testosterone (T): estradiol (E(2)) ratio mainly related to reduction of E(2) levels.

AIM

To investigate the presence of phosphodiesterase type 5 (PDE5) isoenzyme in primary human visceral adipocytes and whether different PDE5 inhibitors (PDE5i) could directly modulate aromatase (ARO) expression in differentiated human visceral adipocytes in culture.

MAIN OUTCOME MEASURES

PDE5 mRNA and protein expression in primary human visceral adipocytes as well as mRNA and protein expression of ARO, with functional activity after selective PDE5 blockade by tadalafil and sildenafil.

METHODS

Purified primary human visceral pre-adipocytes were differentiated ex vivo and were exposed to tadalafil or sildenafil (1 µM) for different intervals of time (6-12-24-96 hours). ARO mRNA content and expression were measured by Western Blot and quantitative reverse transcription-polymerase chain reaction (qRT-PCR), respectively. T and E(2) in supernatants were measured by ELISA also in the presence of letrozole.

RESULTS

Differentiated adipocytes were found to express detectable levels of PDE5 transcripts. Acute exposure (6 hours) to both PDE5i tadalafil and sildenafil increased ARO mRNA expression by 4.7- and 2.8-fold, respectively (P < 0.001). ARO mRNA and protein levels were increased by the treatment with PDE5i in a time- and dose-dependent manner. Such effect was mimicked by 8-bromo-cGMP but was lost after 24 and 96 hours; differently, the PDE3B specific inhibitor milrinone (1 µM), displayed no effect. Accordingly, long-term exposure (24 and 96 hours) to PDE5i caused a significant increase in E(2) concentrations in the supernatant (1.7 and 2 fold, respectively; P < 0.001), with a parallel reduction of T (15% and 30%, respectively; P < 0.001). Such effect was reversed by the co-incubation with the specific ARO-inhibitor letrozole.

CONCLUSIONS

Our results demonstrate that PDE5 is expressed in human visceral adipocytes and that acute exposure to PDE5i selectively stimulates ARO expression, which is related to a specific PDE5 blockade. We speculate that modulation of ARO activity by PDE5i could be one of the mechanisms responsible, at least in part, for the beneficial effects of PDE5i on endothelial and metabolic functions.

Expression of cAMP-dependent protein kinase isoforms in the human prostate: functional significance and relation to PDE4

OBJECTIVES

To investigate the expression of isoforms of the cyclic AMP (cAMP)-dependent protein kinase (cAK) in the transition zone of the human prostate and the functional significance of the enzyme in the control of prostate smooth muscle.

METHODS

Using Western blot analysis and immunohistochemistry, the expression and distribution in the prostate of cAKIalpha, cAKIbeta, cAKIIalpha, and cAKIIbeta in relation to alpha-actin and the phosphodiesterase PDE4 (types A and B) were investigated. The effects of the cAK inhibitor Rp-8-CPT-cAMPS on the reversion of the adrenergic tension of isolated prostate tissue induced by forskolin, rolipram, sodium nitroprusside (SNP), and tadalafil were examined by means of the organ bath technique.

RESULTS

Immunosignals specific for cAKIalpha, cAKIIalpha, and cAKIIbeta were observed in the smooth musculature and glandular structures of the prostate. Double stainings revealed the colocalization of alpha-actin and PDE4 with the cAK isoforms. The expression of the cAK isoforms was confirmed by Western blot analysis. The relaxation of the tension induced by norepinephrine brought about by forskolin, rolipram, SNP, and tadalafil was significantly attenuated by Rp-8-CPT-cAMPS.

CONCLUSIONS

The colocalization of smooth muscle alpha-actin and PDE4 with cAK, as well as the results from the organ bath experiments, provide further evidence for a pivotal role of the cAMP-dependent signaling in the regulation of prostate smooth muscle contractility. Compounds interacting with the cAMP/cAK pathway might represent a new therapeutic avenue to treat symptoms of benign prostatic hyperplasia and lower urinary tract symptomatology.

Signalling pathways involved in sildenafil-induced relaxation of human bladder dome smooth muscle

BACKGROUND AND PURPOSE

The mechanism(s) of action responsible for the beneficial effects of phosphodiesterase 5 (PDE5) inhibitors including sildenafil on lower urinary tract symptoms suggestive of benign prostate hyperplasia are unclear. In particular, the role of the NO-cGMP signalling pathway in regulating human bladder dome smooth muscle relaxation is questionable. Thus, we assessed the ability of a PDE5 inhibitor, sildenafil, to relax such tissue, and identified the signalling pathways involved in this relaxation.

EXPERIMENTAL APPROACH

Human bladder samples were obtained from 20 patients with no overactive bladder undergoing cystectomy for bladder cancer. Detrusor strips were mounted isometrically in Krebs-HEPES solution. Concentration-response curves for sildenafil (10 nM-30 microM) were generated in the presence of various inhibitors on carbachol-induced pre-contraction.

KEY RESULTS

Sildenafil relaxed carbachol-pre-contracted human detrusor strips, starting at 3 microM. This effect was not modified by NO donors, S-nitroso-N-acetylpenicillamine (10 microM) or sodium nitroprusside (300 nM), but was significantly inhibited by inhibition of guanylate cyclase (with ODQ, 10 microM) or adenylyl cyclase (with MDL-12,330A, 10 microM), by the ATP-sensitive potassium channel inhibitor, glibenclamide (10 microM), or inhibition of the large (with iberiotoxin, 30 nM) or small (with apamin, 100 nM) conductance calcium-activated potassium channels.

CONCLUSIONS AND IMPLICATIONS

Sildenafil-induced relaxation of human detrusor smooth muscle involved cGMP-, cAMP- and K(+) channel-dependent signalling pathways, with a minor contribution from NO. The effect of this sildenafil-induced relaxation on the clinical benefit of PDE5 inhibitors on urinary storage symptoms in men deserves further investigation.

Expression of cyclic AMP-dependent protein kinase isoforms in human cavernous arteries: functional significance and relation to phosphodiesterase type 4

INTRODUCTION

The cyclic adenosine monophosphate-dependent protein kinase (cAK) is considered a key protein in the control of smooth muscle tone in the cardiovascular system. There is evidence that erectile dysfunction might be linked to systemic vascular disorders and arterial insufficiency, subsequently resulting in structural changes in the penile tissue. The expression and significance of cAK in human cavernous arteries (HCA) have not been evaluated.

AIMS

To evaluate the expression of cAK isoforms in HCA and examine the role of cAK in the cyclic adenosine monophosphate (cAMP)- and cyclic guanosine monophosphate (cGMP)-mediated control of penile vascular smooth muscle.

METHODS

The expression and distribution of phosphodiesterase type 4 (PDE4) and cAK isoforms in sections of HCA were investigated by means of immunohistochemistry and Western blot analysis. The effects of the cAK inhibitor Rp-8-CPT-cAMPS on the relaxation of isolated preparations of HCA (diameter > 100 µm) induced by rolipram, sildenafil, tadalafil, and vardenafil were studied using the organ bath technique.

MAIN OUTCOME MEASURES

Investigate the expression of cAK in relation to α-actin and PDE4 in HCA and evaluate the effects of an inhibition of cAK on the relaxation induced by inhibitors of PDE4 and PDE5 of isolated penile arteries.

RESULTS

Immunosignals specific for cAKIα, IIα, and IIβ were observed within the wall of HCA. Double stainings revealed colocalization of cAK with α-actin and PDE4. The expression of cAK isoforms was confirmed by Western blot analysis. The reversion of tension induced by inhibitors of PDE4 and PDE5 of isolated penile vascular tissue were attenuated significantly by Rp-8-CPT-cAMPS.

CONCLUSIONS

Our results demonstrate the expression of cAK isoforms in the smooth musculature of HCA and its colocalization with PDE4. A significant role for cAK in the regulation mediated by cAMP and cGMP of vascular smooth muscle tone in HCA can also be assumed.

Phosphodiesterase 5 inhibitors: are they cardioprotective?

A growing body of animal studies provides evidence for potential cardioprotective effects of inhibitors of the enzyme phosphodiesterase isoform 5. Infarct size reduction by administration of phosphodiesterase 5 inhibitors was described in various experimental models of ischaemia and reperfusion. Furthermore, potential beneficial effects were demonstrated in experimental models of congestive heart failure and left ventricular hypertrophy. Some of the observed effects resemble the basic mechanisms of ischaemic pre-conditioning, mimicking both acute and delayed effects. Other effects may be due to action on systemic and cardiac haemodynamics. Mechanisms and signalling pathways, characterized in some of the experimental models, appear to be complex: for instance, the rate of cyclic guanosine monophosphate (cGMP) synthesis and the functional compartmentalization of intracellular cGMP metabolism as well as interaction with ss-adrenergic and nitric oxide signalling may influence effects in different experimental settings. In this review, we discuss mechanisms, signalling pathways, and experimental limitations and touch on considerations for translation into potentially useful applications in the clinical arena.

Pivotal effects of phosphodiesterase inhibitors on myocyte contractility and viability in normal and ischemic hearts

Phosphodiesterases (PDEs) are enzymes that degrade cellular cAMP and cGMP and are thus essential for regulating the cyclic nucleotides. At least 11 families of PDEs have been identified, each with a distinctive structure, activity, expression, and tissue distribution. The PDE type-3, -4, and -5 (PDE3, PDE4, PDE5) are localized to specific regions of the cardiomyocyte, such as the sarcoplasmic reticulum and Z-disc, where they are likely to influence cAMP/cGMP signaling to the end effectors of contractility. Several PDE inhibitors exhibit remarkable hemodynamic and inotropic properties that may be valuable to clinical practice. In particular, PDE3 inhibitors have potent cardiotonic effects that can be used for short-term inotropic support, especially in situations where adrenergic stimulation is insufficient. Most relevant to this review, PDE inhibitors have also been found to have cytoprotective effects in the heart. For example, PDE3 inhibitors have been shown to be cardioprotective when given before ischemic attack, whereas PDE5 inhibitors, which include three widely used erectile dysfunction drugs (sildenafil, vardenafil and tadalafil), can induce remarkable cardioprotection when administered either prior to ischemia or upon reperfusion. This article provides an overview of the current laboratory and clinical evidence, as well as the cellular mechanisms by which the inhibitors of PDE3, PDE4 and PDE5 exert their beneficial effects on normal and ischemic hearts. It seems that PDE inhibitors hold great promise as clinically applicable agents that can improve cardiac performance and cell survival under critical situations, such as ischemic heart attack, cardiopulmonary bypass surgery, and heart failure.

The nitric oxide pathway in the human prostate: clinical implications in men with lower urinary tract symptoms

To date, there is an increasing interest in the nitric oxide (NO) pathway as a potential pharmacological target to treat male lower urinary tract symptomatology (LUTS). In the transition zone of the human prostate, a dense nitrinergic innervation has been shown of the fibromuscular stroma, glandular epithelium and blood vessels. The expression of key proteins of the NO pathway, such as the endothelial and neuronal nitric oxide synthase (eNOS, nNOS), cGMP-degrading phosphodiesterase type 5 (PDE5) and cGMP-binding protein kinase (cGK), has also been demonstrated. The hypothesis that an impaired NO/cGMP-signaling may contribute to the pathophysiology of benign prostatic hyperplasia (BPH) is supported by the results from randomized, placebo-controlled clinical studies, indicating that NO donor drugs and PDE5-inhibitors sildenafil, tadalafil and vardenafil may be useful to treat storage and voiding dysfunctions resulting from LUTS in men. Thus, given a potential role of the NO-pathway in the prostate and/or in other parts of lower urinary tract (e.g. bladder), the enhancement of the NO signaling by NO donor drugs, PDE5 inhibitors or activators of the soluble guanylyl cyclase (sGC) may represent a new therapeutic strategy for the treatment of LUTS. This review serves to focus on the role of NO and the NO-dependent signaling in the control of smooth muscle function in the human prostate. Results from clinical trials in men with LUTS/BPH are also discussed.

Why a combined intracavernosal injection with trimix and oral sildenafil is reliable therapy in the ultrasonographic evaluation of erectile dysfunction

OBJECTIVE

To evaluate prospectively and compare the clinical response and the change in nucleotides correlating with haemodynamic changes in the cavernosal arteries after an intracavernosal injection (ICI) with vasoactive agents with or without oral sildenafil in men with erectile dysfunction (ED).

PATIENTS AND METHODS

In all, 80 patients with ED were prospectively evaluated by clinical assessments, measuring nucleotides in blood plasma and haemodynamics in cavernosal arteries. All patients had colour Doppler ultrasonography (CDU) twice with an interval of 5 h. First, each patient had CDU after ICI with trimix (0.25 mL) or prostaglandin E1 (PGE1, 5 microg), and the second CDU was after ICI trimix given 1 h after oral placebo (group I), sildenafil 25 mg (group II) or 100 mg (group III) and after ICI with PGE1 at 1 h after oral placebo (group IV) or 100 mg sildenafil (group V). Levels of cGMP and cAMP in peripheral venous and penile cavernosal blood plasma were measured at 15 min after ICI.

RESULTS

The mean peak systolic velocity (PSV) at 5, 10, 15 min, and resistive index at 10 min in the second CDU after ICI with trimix, were significantly increased in group III. The mean (sem) levels of cavernosal cGMP were significantly increased in group III and V, from 1130.1 (313.5) to 2056.7 (580.4) and 1017.0 (214.2) to 1905.2 (915.0) fmol/mL, respectively. cAMP was significantly increased in group V, from 9533.1 (2068.4) to 12150 (3684.2) fmol/mL.

CONCLUSIONS

The haemodynamic changes and cGMP and cAMP production in the cavernosum were improved by trimix plus sildenafil more than with than PGE1 plus sildenafil or one ICI with trimix or PGE1. The results suggest that ICI with trimix and sildenafil is the best combination for a pharmacological erection test.

Mechanisms of the relaxant effect of vardenafil in rat penile arteries

The aim of the present study was to investigate the mechanisms underlying the vasorelaxation induced by the selective phosphodiesterase 5 (PDE5) inhibitor vardenafil in rat penile small arteries. Segments of the rat dorsal penile artery were mounted in microvascular myographs for isometric tension recording. Concentration-response curves for vardenafil (1 nM-3 microM) and other PDE inhibitors (sildenafil, rolipram and milrinone) were constructed by adding cummulative concentrations of the drugs to arteries precontracted with phenylephrine. The effect of mechanical endothelial cell removal and of selective blockers of the nitric oxide (NO)/cGMP pathway and K+ channels were evaluated on the vardenafil relaxant responses. Vardenafil was the most potent of the four PDE inhibitors tested that maximally relaxed penile arteries, pD2 and maximum relaxation being 6.96+/-0.08 and 97+/-1% (n=48), respectively. Blockade of guanylate cyclase with ODQ (5 microM), mechanical removal of the endothelium or inhibition of NO synthase with l-NOARG (100 microM) markedly reduced vardenafil-induced relaxations, without altering maximum response. Inhibitors of both the cGMP-dependent (PKG) and the cAMP-dependent (PKA) protein kinases, Rp-8-Br-PET-cGMPS (5 microM) and Rp-8-CPT-cAMPS (50 microM), respectively, both reduced vardenafil relaxant responses and the later abolished that of rolipram. Vardenafil-elicited relaxation was reduced by the selective inhibitor of the large-conductance Ca2+-activated K+ channels (BK(Ca)), iberiotoxin (30 nM) and also by the ATP-sensitive K+ channel (K(ATP)) inhibitor, glibenclamide (1 microM). Vardenafil induces a potent vasodilatation in rat penile arteries that is partially dependent on the endothelium and the NO/cGMP pathway and involves activation of both BK(Ca) and K(ATP) channels.

Does sildenafil cause myocardial infarction or sudden cardiac death?

Sildenafil was the first oral compound to be approved for the treatment of erectile dysfunction. In this paper, we review the current knowledge of the effects of sildenafil on myocardial infarction and sudden cardiac death. The first factor we examine is the sexual activity itself. As several studies have shown, the relative risk for an acute coronary syndrome during intercourse is not very high. Several studies examining the effects of sildenafil on mortality have been published during recent years. The great majority of these studies found that sildenafil is not an extra risk factor for an acute coronary syndrome or sudden cardiac death. In 1997, the rate of myocardial infarction in men 55-64 years of age was 1542 per 1,000000 in the US. According to this, the expected number of deaths as a result of myocardial infarction in patients 55-64 years of age receiving sildenafil, in the 24-hour period after use, from late March 1997 to mid November 1998, should have been 52. Instead, the number of reported deaths were only 15. One very optimistic finding was that sildenafil not only does not increase mortality, but in fact 'preconditions' the heart and has a cardioprotective effect. Besides, many studies have shown that sildenafil does not reduce the exercise tolerance in men with known coronary artery disease. As far as BP is concerned, the differences before and after the use of sildenafil are not clinically significant. The only contraindications for sildenafil are co-administration with alpha-adrenoceptor antagonists or with nitric oxide donors. According to the most recent studies, isoform 5 of phosphodiesterase has also been detected in the myocardium and controls the soluble pool of 3', 5'-cyclic guanosine monophosphate (cGMP). Sildenafil is very specific for cGMP but it may increase cyclic adenosine monophosphate in the myocardium indirectly. This does not occur with small therapeutic doses of the drug. There is some dispute regarding the association of sildenafil with arrhythmias, where the available evidence is not clear. However, there are suspicions that sildenafil may cause sympathetic activation. The overall conclusion is that sildenafil is a safe drug and that its appropriate use does not seem to increase the risk for myocardial infarction or sudden cardiac death.

Physiological regulation of penile arteries and veins

Recent experimental evidence suggests that arterial insufficiency precedes the structural and functional changes in corpora cavernosa (CC) leading to organic erectile dysfunction (ED). The present review gives an overview of the physiological factors involved in the regulation of penile vasculature. Sympathetic nerves maintain flaccidity and tonically released noradrenaline induces vasoconstriction of both arteries and veins through alpha(1)- and alpha(2)-postsynaptic receptors and downregulates its own release and that of nitric oxide (NO) through alpha(2)-presynaptic receptors. The sympathetic cotransmitter neuropeptide Y (NPY) modulates noradrenergic vasoconstriction in penile small arteries by both enhancing and depressing noradrenaline contractions through Y(1)- and Y(2)-postsynaptic and a NO-independent atypical endothelial receptor, respectively. Activation of alpha(1)-adrenoceptors involves both Ca(2+) influx through L-type and receptor-operated Ca(2+) channels (ROC) and Ca(2+) sensitization mechanisms mediated by protein kinase C (PKC), tyrosine kinases (TKs) and Rho kinase (RhoK). In addition, RhoK can regulate Ca(2+) entry in penile arteries upon receptor stimulation. Vasodilatation of penile arteries and large veins during erection is mediated by neurally released NO. The subsequent increased arterial inflow to the cavernosal sinoids and shear stress on the endothelium lining penile arteries activates endothelial NO production through Akt phosphorylation of endothelial NO synthase (eNOS). NO stimulates guanylate cyclase and increased cyclic guanin 3'-monophosphate (cGMP) levels in turn activate protein kinase G (PKG), which enhances K(+) efflux through Ca(2+)-activated (K(Ca)) and voltage-dependent Ca(2+) (K(v)) channels in penile arteries and veins, respectively. PKG-mediated decrease in Ca(2+) sensitivity and its regulation by RhoK remains to be clarified in penile vasculature. Phosphodiesterase type 5 (PDE5) inhibitors are potent vasodilators of penile resistance arteries and increase the content and effects of basally released endothelial NO. Endothelium-dependent relaxations of penile small arteries also include an endothelium-derived hyperpolarizing factor (EDHF)-type response, which is impaired in diabetes and hypertension-associated ED. Locally produced contractile and relaxant prostanoids regulate penile venous and arterial tone, respectively. The latter activates prostaglandin I (IP) and prostaglandin E (EP) receptors coupled to adenylate cyclase and to the increase of cyclic adenosine monophosphate (cAMP) levels, which in turn stimulates K(+) efflux through ATP-sensitive K(+) (K(ATP)) channels. There is a crosstalk between the cGMP and cAMP signaling pathways in penile small arteries. Relevant issues such as the mechanisms underlying the excitation-secretion coupling of the endothelial cells, as well as those involved in cell proliferation and vascular remodeling of the penile vasculature remain to be elucidated. In addition, only few studies have investigated the changes in structure and function of penile arteries in cardiovascular risk situations leading to ED.

Effect of Sildenafil and Rolipram on Adrenergic Responses in Isolated Human and Monkey Corpus Cavernosum

OBJECTIVE

Evaluate and compare effects of phosphodiesterase inhibitors (PDEIs), sildenafil and rolipram, on adrenergic contractile responses of human and monkey cavernosal smooth muscle.

METHODS

Human penises were obtained from patients undergoing gender reassignment surgery. Isolated human and monkey corpus cavernosum (CC) strips were suspended in tissue bath chambers for isometric tension experiments. The effects of the drugs on precontracted monkey and human CC and neurogenic contractions in human CC were investigated.

RESULTS

Both sildenafil and rolipram induced concentration-dependent relaxations of human and monkey CC strips precontracted with noradrenaline (NA). The IC(50) values, determined by reverse regression for nitroglycerin (NTG), isoprenaline, and sildenafil in monkey CC, were, respectively, 1.5+/-0.9x10(-7) M, 3.7+/-0.6x10(-6) M, and 1.7+/-0.7x10(-5) M. Similarly, in human CC muscle, sildenafil was weaker than NTG as a muscle relaxant. Sildenafil, 1.5 microM, reduced neurogenic contractions in human CC due to stimulation of predominantly adrenergic nerves. The suppressant effect of sildenafil on adrenergic transmission was attenuated in CC strips pretreated with N omega-nitro-L-arginine and overcome with a higher stimulus frequency or tetraethylammonium. Rolipram partially inhibited adrenergic excitatory response but without significantly affecting NA-induced contraction.

CONCLUSIONS

Sildenafil and rolipram induced concentration-dependent reversal of human and monkey CC tone mediated by NA. Both PDEIs attenuated contractile adrenergic response of human CC to electrical stimulation. The results also underline the importance of the cyclic adenosine monophosphate-dependent signalling pathway in regulating the tone. PDE4 inhibition in CC is an additional mechanism for erection and potential therapeutic adjunct.

Vardenafil: a selective inhibitor of phosphodiesterase-5 for the treatment of erectile dysfunction

Vardenafil is a selective phosphodiesterase-5 inhibitor approved for the treatment of erectile dysfunction. It was found to be effective in a high percentage of patients and a broad spectrum of underlying conditions. It potentiates the increase in intracellular cGMP in the corpora cavernosa in response to sexual stimuli, resulting in enhanced and prolonged erections. The overall tolerability and safety profile is acceptable, with headache, flushing, rhinitis and dyspepsia being the major reported side effects. Importantly, tolerability and safety in cardiovascular patients seems to be good with no significant increase in cardiovascular events that could be directly attributed to the pharmacologic agent. Only mild blood-pressure lowering effects were observed in healthy individuals, as well as hypertensive patients on multiple antihypertensive agents. However, special caution is mandatory if vardenafil is administered in combination with alpha-blockers, as significant hypotension might occur. Importantly, any drug serving as a nitric oxide donor is contraindicated in combination with vardenafil.

Myocardial phosphodiesterases and regulation of cardiac contractility in health and cardiac disease

Phosphodiesterase (PDE) inhibitors are potent cardiotonic agents used for parenteral inotropic support in heart failure. Contractile effects of these agents are mediated through cAMP-protein kinase A-induced stimulation of I (Ca2+) which ultimately results in increased Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. A number of additional effects such as increases in sarcoplasmic reticulum Ca(2+) stores, stimulation of reverse mode Na(+)-Ca(2+) exchange, direct or cAMP-mediated effects on sarcoplasmic reticulum ryanodine receptor, stimulation of the voltage-sensitive sarcoplasmic reticulum Ca(2+) release mechanism, as well as A(1) adenosine receptor blockade could contribute to positive inotropic responses to PDE inhibitors. Moreover, some PDE inhibitors exhibit Ca(2+) sensitizer properties as they could increase the affinity of troponin C Ca(2+)-binding sites as well as reduce Ca(2+) threshold for thin myofilament sliding and facilitate cross-bridge cycling. Inotropic responses to PDE inhibitors are significantly reduced in cardiac disease, an effect largely attributed to downregulation of cAMP-mediated signalling due to sustained sympathetic activation. Four PDE isoenzymes (PDE1, PDE2, PDE3 and PDE4) are present in myocardial tissue of various mammalian species, of which PDE3 and PDE4 are particularly involved in regulation of cardiac myocyte contraction. PDE cAMP-hydrolysing activity is preserved in compensated cardiac hypertrophy but significantly reduced in animal models of heart failure. However, clinical studies have not revealed any changes in distribution profile as well as kinetic and regulatory properties of myocardial PDEs in failing human hearts. A reduction of PDE inhibitors-induced contractile responses in heart failure has therefore been ascribed to reduced cAMP synthesis due to uncoupling of adenylyl cyclase from beta-adrenoreceptor. In cardiac myocytes, PDEs are targeted to distinct subcellular compartments by scaffolding proteins such as myomegalin, mAKAP and beta-arrestins. Over subcellular microdomains, cAMP hydrolysis by PDE3 and PDE4 allows to control the activity of local pools of protein kinase A and therefore the extent of protein kinase A-mediated phosphorylation of cellular proteins.

Tadalafil, a long-acting type 5 phosphodiesterase isoenzyme inhibitor, improves neurological functional recovery in a rat model of embolic stroke

Sildenafil, a type 5 phosphodiesterase isoenzyme (PDE5) inhibitor with a short half-life, increases brain cyclic guanosine monophosphate (cGMP) levels and improves neurological functional recovery when administered after stroke. In the present study, we investigated the effects of tadalafil (Cialis), a long acting PDE5 inhibitor, on brain cGMP levels, neurogenesis, angiogenesis, and neurological function during stroke recovery in a rat model of embolic stroke. Male Wistar rats (n=28) were subjected to embolic middle cerebral artery (MCA) occlusion. Tadalafil was orally administered every 48 h at a dose of 2 mg/kg or 10 mg/kg for 6 consecutive days starting 24 h after stroke onset. Control animals received the equivalent volume of saline at the same time points. For mitotic labeling, bromodeoxyuridine (BrdU, 100 mg/kg) was administered twice a day at 5, 6, and 7 days after stroke. ELISA assays were performed to evaluate the specificity of the effect of tadalafil on cGMP. Treatment with tadalafil at a dose of 2 or 10 mg/kg significantly improved neurological functional recovery compared with saline-treated rats. In addition, tadalafil treatment increased cerebral vascular density and the percentage of BrdU-positive endothelial cells around the ischemic boundary compared with saline-treated rats. Moreover, tadalafil-treated rats showed greater ipsilateral SVZ cell proliferation than saline-treated rats. However, treatment with tadalafil did not reduce infarct volume when compared to the saline group. Tadalafil selectively increased cGMP but not cyclic adenosine monophosphate (cAMP) in brain. Our data demonstrate that treatment of ischemic stroke with tadalafil improved functional recovery, which was associated with increases of brain cGMP levels and enhancement of angiogenesis and neurogenesis.

Mechanisms of direct relaxant effect of sildenafil, tadalafil and vardenafil on corpus cavernosum

Sildenafil, tadalafil, vardenafil and verapamil induced concentration-dependent relaxation of the rabbit corpus cavernosum muscle precontracted with noradrenaline. The maximal relaxation (%) at 20 microM was 61.4 +/- 6.9, 32.4 +/- 5.4, 100.0 +/- 5.5 and 86.6 +/- 5.1 (n = 5 each) respectively. Pre-incubation of cavernosal muscle strips with N(omega)-nitro-L-arginine or guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) but not adenylate cyclase inhibitor, cis-N-[2-phenylcyclopentyl]-azacyclotridec-1-en-2-amine] (MDL12330A) culminated in only a 20-30% reduction in muscle relaxant action of the 3 phosphodiesterase inhibitors. This suggests that another mechanism of relaxation independent of nitric oxide-cGMP or cAMP pathway was involved. Higher concentrations of sildenafil (100 microM) and vardenafil (10 and 100 microM) produced non-competitive antagonism of noradrenaline-induced contraction characterized by reduced maximal effect. In contrast, tadalafil was devoid of significant effect on noradrenaline. On K(+)-depolarized tissues, sildenafil was as potent as vardenafil whereas tadalafil was the least effective in relaxing K(+)-induced tone. The maximal relaxation (% of K(+)-induced tone) at 20 microM sildenafil, tadalafil and vardenafil was respectively 84.1 +/- 6.5, 9.0 +/- 19.9, and 88.9 +/- 6.2 (n = 5 each). In addition, verapamil, sildenafil and vardenafil were more efficacious than tadalafil in reversing tonic contractions by Ca(2+) channel activator, 1,4,dihydro-2,6-dimethyl-5-nitro-4-[2(triflouromethyl)phenyl]pyridine-3-carboxylic acid methyl ester (BAY K-8644). These results indicate that vardenafil and sildenafil possess direct muscle relaxant potential possibly via inhibiting Ca(2+) influx through both receptor-operated and voltage-dependent Ca(2+) channels whereas tadalafil appears capable of inhibiting receptor-operated transmembrane Ca(2+) entry only.

Phosphodiesterase 5 in the female pig and human urethra: morphological and functional aspects

OBJECTIVE

To characterize the distribution of phosphodiesterase 5 (PDE-5), cGMP and cGMP-dependent protein kinase I (PKG1), and to evaluate the effect of pharmacological inhibition of PDE-5 in isolated preparations of pig and human urethra, as the nitric oxide (NO)/cGMP pathway generates the main inhibitory signals to reduce resistance in the bladder outlet and urethra during emptying of the bladder.

MATERIALS AND METHODS

After obtaining ethics committee approval, urethral specimens were obtained from three female patients during cystectomy, and from young female pigs. The specimens were prepared for immunohistochemical investigations and for functional studies in organ baths. Effects of sildenafil, vardenafil and tadalafil (1 nm to 30 microm) were studied in l-noradrenaline (1 microm)-activated or spontaneously contracted preparations, and on relaxations induced by electrical-field stimulation (EFS). Levels of cGMP were determined by radioimmunoassay.

RESULTS

After stimulation with the NO donor, DETA NONO-ate (1 mm), there was greater cGMP-immunoreactivity (IR) in urethral and vascular smooth muscles. There was a wide distribution of cGMP- and vimentin-positive interstitial cells between pig urethral smooth muscle bundles. There was also cGMP-IR within NO-synthase-IR endothelium. There was PDE-5 IR within the urethral and vascular smooth muscle cells, but also in vascular endothelial cells that expressed cGMP-IR. In pig and human sections, there was strong PKG1-IR in alpha-actin-IR urethral smooth muscle cells that also contained IR for cGMP. Sildenafil, vardenafil and tadalafil caused mean (sem) concentration-dependent relaxations of the pig urethra which, at 30 microm, were 80 (3)% (11 samples), 81 (5)% (12 samples) and 64 (4)% (10 samples) of the spontaneous tone. The relaxation of L-noradrenaline-contracted female human urethra was 100% in response to 10 microm sildenafil, and 85 (15)% and 47 (13)% for 30 microm of vardenafil and tadalafil, respectively (three samples). Vardenafil or sildenafil (30 microm) doubled cGMP levels in pig specimens. There were no effects on cGMP levels with tadalafil. EFS (1-32 Hz) caused l-NG-nitroarginine-sensitive relaxations of pig urethral muscle that were increased in amplitude and duration by PDE-5 inhibition. At 0.1 microm, sildenafil, vardenafil or tadalafil significantly prolonged the mean (sem) duration of the relaxation at 4 Hz by 55 (19)%, 45 (14)% and 51 (12)%, respectively.

CONCLUSIONS

PDE-5-, cGMP- and PKG1-IR is widely distributed in human and pig urethral tissues. Nerve-induced relaxations of urethral preparations were enhanced at low concentrations of sildenafil, vardenafil and tadalafil, whereas there were direct smooth muscle-relaxant actions of the PDE-5 inhibitors at high concentrations. Inhibition of PDE-5 might be an interesting option to facilitate cGMP-mediated relaxation of the outflow region.

Role of nitric oxide in the relaxation elicited by sildenafil in penile resistance arteries

PURPOSE

We investigated the role of the vascular endothelium and the L-arginine/nitric oxide pathway in the vasorelaxant effect of the phosphodiesterase type 5 inhibitor sildenafil in penile resistance arteries.

MATERIALS AND METHODS

Second or third order branches of the horse deep intracavernous penile artery were mounted in microvascular myographs. The vasodilator effects of sildenafil and the NO donor SNAP (S-nitrosoacetyl-D,L-penicillamine) were evaluated in the absence and presence of the endothelium and inhibitors of the NO/cGMP (cyclic guanosine 3',5'-monophosphate) pathway.

RESULTS

In phenylephrine precontracted, endothelium intact arteries sildenafil elicited potent relaxations that were markedly decreased by the blockade of soluble guanylate cyclase with ODQ (1H-[1,2,4]oxadiazolo[4-3a]quinoxalin-1-one). Endothelium removal and the inhibition of NO synthase with N(G)-nitro-L-arginine (L-NOARG) caused pronounced inhibition of sildenafil elicited relaxations but not of SNAP induced responses. Combined treatment with the cyclooxygenase blocker indomethacin and L-NOARG caused significantly greater inhibition of sildenafil relaxations than that produced by L-NOARG alone. Inhibitors of the cGMP (PKG) and the cAMP (cyclic adenosine 3',5'-monophosphate) dependent protein kinases Rp-8-Br-PET-cGMPS (beta-phenyl-1, N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate, Rp-isomer) and Rp-8-CPT-cAMPS (Rp-8-CPT-cAMPS (8-(4-chlorophenylthio) adenosine-3',5'-cyclic monophosphorothioate, Rp-isomer), respectively, inhibited the sildenafil concentration-relaxation curves. The relaxant responses of SNAP were markedly decreased by PKG inhibitor and to a lesser extent by cAMP dependent protein kinase inhibitor.

CONCLUSIONS

The current results demonstrate a potent relaxant effect of sildenafil in penile resistance arteries due in part to cGMP accumulation and to the enhanced effects of basal released, endothelial derived NO acting through PKG activation. Cross-activation of the cAMP signaling pathway by sildenafil is also suggested.

Cyclic nucleotide signaling in cavernous smooth muscle

INTRODUCTION

Penile erection depends on cavernous smooth muscle relaxation that is principally regulated by cyclic nucleotide signaling. It is hoped that a comprehensive review of publications relevant to this subject will be helpful to both scientists and clinicians who are interested in the sciences of erectile function/dysfunction. AIMS. To review the roles of extracellular signaling molecules, their receptors, intracellular effectors, and phosphodiesterases in cyclic nucleotide signaling that leads to cavernous smooth muscle relaxation. The involvement of these molecules in the development of erectile dysfunction and the possibility of using them as therapeutic agents or targets are also discussed.

METHODS

Entrez, the search engine for life sciences, was used to search for publications relevant to the topics of this review. Keywords used in the searches included vascular, cavernous, penis, smooth muscle, signaling molecules (adenosine, nitric oxide, etc.), and key elements in the cyclic nucleotide signaling pathways (cAMP, cGMP, cyclases, PKG, PKA, etc.). Articles that are dedicated to the study of erectile function/dysfunction were prioritized for citation.

RESULTS

More than 1,000 articles were identified, many of which are studies of the vascular system and are therefore reviewed but not cited. Studies on erectile function have identified both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) signaling pathways in cavernous smooth muscle. Many signaling molecules of these two pathways have been shown capable of inducing erection when administered intracavernously. However, for sexually induced erection, nitric oxide (NO) is the responsible signaling molecule and it passes on the signal through soluble guanyl cyclase (sGC), cGMP, and protein kinase G (PKG).

CONCLUSIONS

The NO/sGC/cGMP/PKG pathway is principally responsible for sexually stimulated erection. Detumescence is mainly carried out by the degradation of cGMP by phosphodiesterase 5. Both cAMP and cGMP signaling pathways are susceptible to genetic and biochemical alterations in association with erectile dysfunction. Several key elements along these pathways are potential therapeutic targets.

Ca2+-activated K+ (KCa) channels are involved in the relaxations elicited by sildenafil in penile resistance arteries

The aim of the present study was to evaluate the role of K+ channels in the vasorelaxant effect of the phosphodiesterase 5 inhibitor, sildenafil, in isolated horse penile resistance arteries mounted in microvascular myographs. In phenylephrine-precontracted arteries, sildenafil elicited potent relaxations which were markedly reduced by raising extracellular K+, by the non-selective blocker of Ca2+-activated K+ channels (KCa), tetraethylammonium and by the blocker of large- and intermediate-conductance KCa channels, charybdotoxin. Sildenafil relaxant responses were also reduced by the selective inhibitor of large conductance KCa (BK(Ca)) channels iberiotoxin, but not by the blocker of small conductance KCa channels apamin. The inhibitor of the cGMP-dependent protein kinase (PKG), Rp-8-Br-PET-cGMPS, reduced the relaxations elicited by sildenafil but combined treatment with iberiotoxin and Rp-8-Br-PET-cGMPS did not further inhibit these relaxations, compared to the effect of either blocker alone. Iberiotoxin also shifted to the right the relaxations elicited by both the NO donor, S-nitrosoacetyl-D,L-penicillamine (SNAP) and the adenylate cyclase activator forskolin; treatment with both iberiotoxin and Rp-8-Br-PET-cGMPS did cause an additional inhibition. The present results demonstrate that the relaxant effect of sildenafil and NO in penile resistance arteries is due in part to activation of BK(Ca) channels through a PKG-dependent mechanism.

Phosphodiesterase 5 inhibition in chronic heart failure and pulmonary hypertension

Erectile dysfunction (ED) is highly prevalent in patients with chronic heart failure (CHF) and is among the most distressing symptoms in this patient population. Although the safety and efficacy of phosphodiesterase 5 (PDE5) inhibitors in the management of ED have been evaluated in many cardiovascular disease populations, scant data are available in patients with CHF. In published studies, the short-term safety and efficacy of sildenafil in patients with stable mild-to-moderate CHF with ED appears to be comparable to that observed in other populations with cardiovascular disease. Evidence is not available on the effects of vardenafil or tadalafil in CHF. In addition to their benefits in the treatment of ED, preliminary studies suggest that PDE5 inhibitors enhance endothelial function in patients with CHF and have beneficial effects on pulmonary hemodynamics and exercise capacity in patients with pulmonary hypertension. Additional studies are needed to determine the therapeutic potential of this class of agents in these disease states.

Cyclic AMP-specific and cyclic GMP-specific phosphodiesterase isoenzymes in human cavernous arteries--immunohistochemical distribution and functional significance

OBJECTIVES

It has been well established that male erectile dysfunction is frequently associated with vascular diseases. The normal function of cavernous arteries is considered a prerequisite to maintain sufficient blood flow to the trabecular spaces in order to enable penile erection. Contractility of cavernous arteries is regulated by the peripheral autonomic nervous system and endogenous factors released from the endothelial cell layer. A significant increase of blood flow in the central cavernous arteries is the initial event leading to penile tumescence and rigidity. Besides the significance of the nitric oxide/cyclic guanosine monophosphate (cGMP)-mediated mechanisms, the cyclic AMP (cAMP) signalling pathway is also involved in the regulation of tone of the erectile tissue, and interactions between cGMP- and cAMP-mediated mechanisms have been demonstrated. The aim of the present study was to investigate by means of immunohistochemistry the presence of the phosphodiesterase (PDE) isoenzymes 3, 4 (cAMP-specific PDEs) and 5 (cGMP-specific PDE) in thin sections of human central cavernous arteries (HCA) and their functional significance in the mechanism of vessel tone regulation.

METHODS

Functional experiments were performed using circular segments of HCA and strip preparations of the human corpus cavernosum (HCC). Relaxant effects induced by the cumulative addition of the PDE inhibitors milrinone (PDE3 inhibitor), rolipram (PDE4 inhibitor) and sildenafil (PDE5 inhibitor; 0.01, 0.1, 1 and 10 M) were studied in preparations of HCA and HCC challenged by 1 M norepinephrine (NE). Moreover, immunohistochemistry was carried out in order to evaluate the expression of PDE3, PDE4 and PDE5 in thin sections of HCA.

RESULTS

Milrinone, rolipram and sildenafil dose-dependently reversed the NE-induced tension of the isolated vascular segments and HCC strips with sildenafil being the most effective drug. Neither rolipram nor milrinone reached an EC50 value. Abundant immunoreactivities specific for PDE3, PDE4 and PDE5 were observed in the entire smooth musculature of the wall of HCA and resistance arteries. In addition, immunoreactivity for PDE4 was also detected in the cytoplasm of endothelial cells lining the cavernous arteries.

CONCLUSIONS

The cGMP-dependent relaxation of cavernous arteries is not only dependent on the normal function of the peripheral autonomic nervous system but also on the functional integrity of the vascular endothelium. The expression of the cGMP-specific PDE5 and the ability of the PDE5 inhibitor sildenafil to reverse the adrenergic tension of isolated segments of HCA underline the important role of the NO/cGMP pathway in the control of smooth muscle tone of human trabecular smooth musculature and penile cavernous arteries. Our results also suggest a significance of the cAMP-dependent signaling mechanisms in the regulation of tension of central HCAs. The present findings are also in support of the hypothesis of interactions between the cGMP- and cAMP-mediated signaling pathways in HCAs. Further investigations are indicated in order to outline potential differences between central HCAs and helicine resistance arteries. This may help to understand better the relations between structural and functional changes in the penile erectile tissue in patients with cardiovascular diseases and endothelial dysfunction.

Type 5 phosphodiesterase inhibition in heart failure and pulmonary hypertension

The availability of selective inhibitors of the cyclic guanosine monophosphate (cGMP)-specific type 5 phosphodiesterase (PDE5) has created increasing interest in unlocking the therapeutic potential of PDE5 inhibition in cardiovascular diseases that are marked by dysfunction of nitric oxide (NO)-cGMP signaling. Pulmonary arterial hypertension (PAH) and heart failure (HF) are characterized by pulmonary arterial vasoconstriction that is thought to be caused by relative deficiencies of vasodilators such as NO and exaggerated production of vasoconstrictors such as endothelin. PDE5 is abundant in the pulmonary vasculature where it catabolizes cGMP, the second messenger of NO. Inhibition of PDE5 has been shown to lower pulmonary vascular resistance in PAH and HF by augmenting local cGMP. This review outlines the therapeutic potential of PDE5 inhibition for the treatment of PAH and HF.

Lack of synergistic effect of molsidomine and sildenafil on development of pulmonary hypertension in chronic hypoxic rats

The present study addressed whether combined treatment with a phosphodiesterase type 5 inhibitor, sildenafil, and a nitric oxide donor, molsidomine, prevents development of pulmonary hypertension in chronic hypoxic rats. Two weeks of hypoxia increased right ventricular systolic pressure, and right ventricular and lung weight. Treatment with either sildenafil (10 mg/kg/day) or molsidomine (15 mg/kg/day) in drinking water reduced right ventricular systolic pressure and weight, while lung weight was unchanged. Combining sildenafil and molsidomine did not have additional effects compared to molsidomine alone. The number of muscularized pulmonary arteries with diameters below 50 microm was increased in vehicle and sildenafil-treated, but not in molsidomine-treated hypoxic rats. Acetylcholine relaxation was blunted in arteries from vehicle and molsidomine-treated, but not in sildenafil-treated rats. In conclusion, both sildenafil and molsidomine blunts pulmonary hypertension and right ventricular hypertrophy in chronic hypoxic rats, but no synergistic effects were observed.

Pharmacotherapy of erectile dysfunction: focus on cardiovascular safety

Therapy of erectile dysfunction has been revolutionised in recent years, as specific pharmacological inhibitors of phosphodiesterase 5 (PDE5), such as sildenafil, tadalafil, or vardenafil, were shown to be highly effective in the treatment of erectile dysfunction. They dilate arterial smooth muscle cells of the corpora cavernosa, which express PDE5 abundantly, by inhibiting the breakdown of 3'5'-cyclic guanosine monophosphate. Despite theoretical concerns of a reduced myocardial tolerance to ischaemia or promoting cardiac arrhythmias, randomised trials and retrospective analyses do not support an increased cardiac risk with oral treatment. Therapeutic doses of PDE 5 inhibitors exhibit slight blood pressure lowering effects, and do not appear to compromise coronary blood flow in coronary artery disease. However, the combination of PDE5 inhibitors with any nitric oxide donor is absolutely contraindicated because of potentially life-threatening hypotension. Before prescribing medication for erectile dysfunction, any patient with cardiovascular disease should be evaluated for a potential risk of a cardiovascular event during sexual activity according to the Princeton Consensus Panel. When a stable cardiac condition can be achieved (low risk group), oral treatment for erectile dysfunction may be appropriate.

Sildenafil: Emerging Cardiovascular Indications

The discovery in 1989 of sildenafil, a highly selective inhibitor of phosphodiesterase-5 (PDE-5), was the result of extensive research on chemical agents targeting PDE-5 that might potentially be useful in the treatment of coronary heart disease. Initial clinical studies on sildenafil in the early 1990s were not promising with respect to its antianginal potential. However, the incidental discovery of its antiimpotence effect led to its approval of for the treatment of erectile dysfunction. Thereafter, several reports of adverse cardiac events in patients on sildenafil raised concerns about its safety in cardiovascular disorders. Novel therapeutic indications are emerging for sildenafil with the recent discovery that PDE-5 is expressed in various other tissues such as the arterial vasculature, including pulmonary and coronary arteries, venous vasculature, skeletal muscles, platelets, and visceral and tracheobronchial muscles. In this review we briefly summarize the pharmacology of sildenafil and the current available evidence on its potential therapeutic applications in cardiovascular disorders.

Interactions between cGMP- and cAMP-pathways are involved in the regulation of penile smooth muscle tone

Nitric oxide (NO)/cyclic GMP (cGMP)-mediated mechanisms have a pivotal function in reducing the tone of the penile smooth musculature during normal erectile responses. The cyclic AMP (cAMP) signaling pathway is also involved in the adjustment of smooth muscle contractility, and suggestions for interactions between cGMP- and cAMP-mediated mechanisms have been presented. Using activators of the cGMP- or the cAMP-pathway, as well as inhibitors of protein kinase A (PKA; cAMP-dependent kinase) and protein kinase G (PKG; cGMP-dependent kinase), the present study was undertaken to further delineate the functional relation between these pathways in the penis. In addition, the distribution of PKA and some cAMP-binding phosphodiesterases (cAMP-PDEs) were investigated in human erectile tissue. Functional experiments were performed on isolated human corpus cavernosum (HCC). The effects of an inhibitor of the PKA, Rp-8CPT-cAMPS (10 microM), or the PKG, Rp-8-pCPT-cGMPS (10 microM), on relaxation induced by the cumulative administration of sodium nitroprusside (SNP), forskolin, sildenafil or tadalafil (IC351) were studied in preparations of HCC precontracted with 1 microM norepinephrine (NE). Using immunohistochemical procedures, the presence of immunoreactivity for cAMP-PDEs PDE3, PDE4, and PDE4A, as well as for PKA was investigated in specimens of HCC from which preparations were also used in the functional experiments. Forskolin, SNP, sildenafil, and IC 351 dose-dependently reversed NE-induced tension of isolated HCC preparations. The relaxing effects of SNP were significantly attenuated by Rp-8-pCPT-cGMPS, but not by Rp-8CPT-cAMPS. In contrast, relaxation induced by forskolin, sildenafil and tadalafil were significantly reversed by both Rp-8-pCPT-cGMPS and Rp-8CPT-cAMPS. Abundant immunoreactivity for PDE3 and PKA was observed in the corpus cavernosum smooth muscle cells. Immunoreactivity for PDE4 was also detected in the smooth musculature and in the cytoplasm of endothelial cells lining the cavernous sinusoids, as well as in nerve fibres interspersing the trabecular stroma. The present results support the hypothesis of interactions between cGMP- and cAMP-mediated signals in the HCC, and suggest that the effects of inhibitors of PDE5 on isolated erectile tissue may also partly or indirectly include actions of the cAMP second messenger system. The exact mechanism by which such an interaction occurs is not clear, but it may involve altered activity of the cGMP-inhibited PDE3 brought about by a change in the intracellular levels of cGMP by the inhibition of PDE5. This will in turn lead to increasing levels of cAMP, facilitating the interaction of cAMP with the PKA. The immunoreactivity specific for PDE3, PDE4, PDE4A and PKA registered in HCC section is also in support of an important role for the cAMP/PKA-system for penile smooth muscle function.

Phosphodiesterase 5 Inhibitors in Rapid Ejaculation

Rapid (premature) ejaculation (RE) is a very common sexual disorder. This condition may be primary or secondary to underlying disease. Control of RE has been primarily focused on behavioural therapy, topical anaesthetics, tricyclic antidepressants and selective serotonin reuptake inhibitors; however, an approved treatment does not exist. Recently, a number of clinical trials have studied the potential effectiveness of the phosphodiesterase (PDE)-5 inhibitor sildenafil in the treatment of RE. Results of most of these studies have been encouraging. Available data indicate that there is clinical, anatomical, physiological, pharmacological and genetic evidence to explain the efficacy of PDE5 inhibitors in RE. The rationale for the use of PDE5 inhibitors in the treatment of RE could be due to possible peripheral and central mechanisms. Possible peripheral ejaculation retarding capabilities may include modulation of the contractile response of the vas deferens (VD), seminal vesicles (SV), prostate and urethra, induction of a state of peripheral analgesia, and prolongation of the total duration of erection. Possible central mechanisms may involve lessening of the central sympathetic output. Furthermore, there is evidence from knockout mice to explain the efficacy of PDE5 inhibitors in RE. Mice lacking the gene for endothelial nitric oxide synthase develop a condition similar to RE. On the other hand, mice lacking the gene for heme oxygenase-2 develop a condition similar to delayed ejaculation. This review also discusses the findings against the use of these agents in RE. In conclusion, a review of the literature suggests the potential usefulness of PDE5 inhibitors as a promising line of therapy in RE but further studies are needed.

Current and future trends in the oral pharmacotherapy of male erectile dysfunction

The promising clinical data on the use of the first orally active phosphodiesterase inhibitor sildenafil citrate (Viagra) for treatment of male erectile dysfunction have been accompanied by an increase in research activities on the physiology of the male erectile mechanism. This included both peripheral intracellular signal transduction in the corpus cavernosum as well as central brain and spinal cord pathways that control penile erection. This work provided the basis for the development and introduction of several new therapeutic modalities into the management of erectile dysfunction that is now offered to the patients. Since the concept of 'taking a pill' as a cure for an illness or the relief of symptoms of a disease has become widely accepted by consumers, the pharmacological treatment of erectile dysfunction has primarily focused on selective, orally available drugs that act via influencing intracellular or central regulatory mechanisms, combining a high response rate and the advantage of an 'on-demand' intake. These agents are regarded as more efficacious, have a faster onset of drug action in the target tissue and an improved effect-to-side effect ratio than sildenafil. The purpose of this review is to describe the major novel and evolving pharmacological advances in the field of oral pharmacotherapy for the treatment of male erectile dysfunction.

Effects of sildenafil (viagra) on human myocardial contractility, in vitro arrhythmias, and tension of internal mammaria arteries and saphenous veins

Sildenafil (Viagra) has been proved effective in the therapy for erectile dysfunction. Cardiovascular adverse effects are a matter of continuous debate. The aim of the study was to investigate effects of sildenafil on isolated human cardiovascular tissue directly. Isometric force of contraction was determined in isolated, electrically stimulated (1 Hz, 37 degrees C) human right atrial and left ventricular muscle strips. Vascular tension was determined in rings of human internal mammaria arteries and saphenous veins. Sildenafil (0.0001-10 microM) neither in human atrium (n = 12) nor in failing (n = 8) or nonfailing (n = 5) ventricle exerted a significant inotropic response. Furthermore, no effect on isoprenaline-elicited arrhythmias was observed. Neither addition of isoprenaline (0.1 microM) nor addition of the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) (100 microM) affected myocardial contractility in the presence of sildenafil (10 microM). In precontracted arteries and veins, addition of sildenafil (0.1-10 microM) led to pronounced vasorelaxation (maximal 35.5 +/- 2.2% and 45.6 +/- 6.3%, respectively, in the presence of 10 microM sildenafil). In the presence of SNAP (0.03 microM), this effect was markedly increased in arteries (72.4 +/- 10.1%, n = 4, P < 0.02) as well as in veins (73.5 +/- 6.3%, n = 6, P < 0.02). Sildenafil exerts potent vasodilatory actions but has no direct influence on human myocardial contractility or proarrhythmic effects in vitro.

[Use of oral sildenafil (Viagra) in pulmonary hypertension after cardiac pediatric surgery]

Pulmonary hypertension remains a major cause of morbidity after cardiac surgery, although inhaled nitric oxide (iNO) was shown to have clinical benefit. Some patients are dependent on iNO, increasing the length of hospital stay. The authors report a case of a girl, nine years old (17 kg), with mitral insufficiency, atrial septal defect and pulmonary hypertension (80% of systemic pressure). Following cardiac surgery, pulmonary hypertension persisted and iNO could not be withdrawn. Sildenafil was administered orally (1,5mg x kg(-1), every 4 h) at the 15th postoperative day and iNO could be withdrawn within 24h with clinical improvement.