Involvement of alpha-receptors and potassium channels in the mechanism of action of sildenafil citrate

Sildenafil corrects the increased contractility of rat detrusor muscle induced by alprostadil in vitro

BACKGROUND

Sildenafil (PDE5-inhibitor) and alprostadil (PGE₁) are used in combination clinically for the management of some cases of erectile dysfunction. Despite the roles of prostaglandins (PG) and nitric oxide (NO) pathways in contractility of bladder smooth muscle are frequently studied, the effect of sildenafil/alprostadil combination and the crosstalk between NO/cGMP and PG pathways on bladder activity is not documented.

METHODS

Organ-bath experiments were performed using isolated rat detrusor muscle. Direct and neurogenic contractions were induced using ACh and electric stimulation (EFS, 4Hz, 80V, 1ms), respectively. The contractile responses in absence and presence of the tested drugs at different concentrations were compared. Results are expressed as mean ± SEM (n = 5-7).

RESULTS

Alprostadil (0.01-10 μM) concentration-dependently potentiated ACh (100μM)- and EFS (4 Hz)- induced contraction. Maximum potentiation of ACh-contraction in presence of alprostadil was 40 ± 5%. Sildenafil potentiated ACh-induced contraction at low concentrations (0.01-1 μM), but inhibited it at higher ones (10-100 μM). IBMX (non-selective PDE-inhibitor, 0.01-100μM) and SNP (NO-donor, 1nM-1 mM) produced the same biphasic pattern. The potentiatory phase of sildenafil was inhibited by atropine (0.1μM), L-NAME (non-selective NOS-inhibitor, 100μM), N-PLA (nNOS-inhibitor, 30μM) or MB (nonselective GC-inhibitor, 10μM). In presence of sildenafil (0.1μM), the concentration-response curve of alprostadil (0.01-10μM) on both ACh and EFS-induced contraction was clearly shifted downward.

CONCLUSIONS

A crosstalk between PGE₁ and NO/cGMP pathways may exist. At low concentrations only, the effect of sildenafil on bladder contractility is dependent on NO/cGMP. cGMP intracellularly-elevated by sildenafil, may inhibit the activity of PLC and hence the cascade of EP₁-receptors, thus masking the hyperactivity of bladder caused by alprostadil, which adds to the advantages of this combination.

Inhibition by sildenafil of contractility of isolated non-pregnant human myometrium

Objective:

To investigate the ability of sildenafil to inhibit the contractility of isolated non pregnant human myometrium.

Materials and Methods:

The inhibitory effect of three concentrations (3, 10, and 30 µM) of sildenafil on 55 mM KCl-induced contractility of isolated non-pregnant human myometrium was studied. The ability of the guanylyl cyclase inhibitor ODQ (10 µM), the adenylyl cyclase inhibitor MDL-12,330A (10 µM), the non-specific potassium channel blocker TEA (2 mM), and the calcium-sensitive potassium (BKCa) channel blocker iberiotoxin (100 nM) to reverse the inhibition of 10 µM sildenafil on KCl-induced myometrial contractility was also studied.

Results:

Sildenafil produced a concentration-dependent inhibition of KCl-induced myometrial contractility that was statistically significant at all three concentrations of sildenafil used. The inhibition by 10 µM sildenafil of KCl-induced myometrial contractility was not reversed by the concurrent administration of ODQ or MDL-12,330A. The inhibition of 10 µM sildenafil of myometrial contractility was partially reversed by concurrent administration of TEA and totally and significantly reversed by the concurrent administration of iberiotoxin.

Conclusions:

These results suggest that sildenafil inhibits the contractility of isolated non-pregnant human myometrium. The results suggest that sildenafil does so by opening BKCa channels.

Celecoxib modulates nitric oxide and reactive oxygen species in kidney ischemia/reperfusion injury and rat aorta model of hypoxia/reoxygenation

OBJECTIVE

This study investigated the interaction between COX-2, NO and ROS after ischemia/reperfusion events in the kidney and vascular beds.

MATERIALS AND METHODS

Kidney IRI model in male Sprague-Dawley rats was used and various biochemical and histopathological parameters were examined. The isolated rat aortic rings served as model for hypoxia/reoxygenation.

RESULTS

Celecoxib reduced serum creatinine and urea and kidney malonaldehyde levels, increased kidney superoxide dismutase activity and reduced glutathione level and histopathological scores at 24 and 48 h after reperfusion compared to IRI group. This was associated with a significant increase in NO level to 0.70 ± 0.03 nmol/mg protein compared to 0.37 ± 0.01 nmol/mg protein for IRI group. Unexpectedly, celecoxib reduced COX-2 expression in the kidney. Celecoxib reversed the effect of hypoxia-reoxygenation on ACh and SNP-induced relaxation in aortic rings but failed to potentiate the SNP relaxations in the control rings. Hypoxia-reoxygenation significantly impaired celecoxib's relaxation of aorta (12.69 ± 2.69% vs. 35.84 ± 0.84%) which was significantly inhibited in presence of L-NAME.

CONCLUSIONS

Celecoxib beneficially affects the outcome of renal IRI by lowering the expression of COX-2 and hence reducing oxidative stress and increasing the bioavailability of NO. Direct interaction between celecoxib and NO in associated vascular beds may also be a contributing mechanism.

The potent relaxant effect of resveratrol in rat corpus cavernosum and its underlying mechanisms

The aim of this study was to evaluate the relaxant effect of resveratrol (RVT), one of the most commonly employed dietary polyphenols, in rat corpus cavernosum, and to further investigate the contribution of possible underlying mechanisms. Strips of corpus cavernosum were used in organ baths for isometric tension studies. RVT (10(-6)-10(-4) M) produced concentration-dependent relaxation responses in rat corpus cavernosum precontracted by phenylephrine. The relaxant responses to RVT partially, but significantly inhibited by removal of endothelium. Nitric oxide (NO) synthase (NOS) blocker N-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) or soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10(-5) M) caused a significant inhibition on relaxation response to RVT, whereas cyclooxygenase inhibitor indomethacin (10(-5) M) did not significantly alter relaxant responses of corpus cavernosum strips to RVT. Corpus cavernosum contractions induced by stepwise addition to Ca2+ to high KCl solution with no Ca2+ were significantly inhibited by RVT incubation. The treatment of corpus cavernosum tissues with non-specific potassium channel inhibitor tetraethylammonium (TEA, 10(-2) M) did also significantly affect the relaxant activity of RVT. Otherwise, the relaxation response of corpus cavernosum induced by the phosphodiesterase-5 inhibitor sildenafil increased significantly in the group pretreated with 10(-5) M RVT. These results demonstrated that RVT has a potent relaxant effect on rat corpus cavernosum via endothelium-dependent and -independent mechanisms. Endothelium-dependent relaxation of corpus cavernosum to RVT is thought to be mediated primarily through NO/cGMP signaling pathway, and possibly an additional mechanism, endothelium-dependent hyperpolarization factor (EDHF). The residual endothelium-independent corpus cavernosum relaxation induced by RVT is uncertain but seems to depend on the interactions of RVT with Ca2+ entry mechanism from the extracellular space and also other undefined direct effects in this tissue.

Combined effect of sildenafil and guanethidine, propranolol or verapamil on erectile function in rats

OBJECTIVES

This study aims to further elucidate the role of adrenergic transmission in erection and to highlight whether adrenergic transmission in the penis modulates sildenafil's action.

METHODS

Measurement of intracavernosal pressure in the anesthetized rat model.

KEY FINDINGS

Guanethidine (3 and 6 mg/kg) potentiated intracavernosal pressure/mean arterial pressure (ICP/MAP) rises in response to cavernous nerve stimulation by 4.375 ± 0.425 and 18.375 ± 1.085% respectively. Propranolol did the opposite. In presence of guanethidine, sildenafil (0.01, 0.1 and 1 mg/kg) potentiated ICP/MAP responses by 81.571 ± 4.918%, 147.83 ± 10.864% and 279.285 ± 23.053% at 1 Hz compared to 22.277 ± 2.139%, 123.571 ± 8.443% and 186.25 ± 13.542% respectively in the absence of guanethidine. Propranolol inhibited the effect sildenafil at all frequencies of stimulation. Verapamil exhibited a pro-erectile action and potentiated the effect of sildenafil (0.01, 0.1 and 1 mg/kg) on erectile responses corresponding to 85.25 ± 6.716%, 146 ± 11.288% and 221.571 ± 19.032% respectively compared to 26.011 ± 1.911%, 87.142 ± 8.73% and 182.2 ± 16.921% in its absence.

CONCLUSIONS

This study provides functional evidence that inhibition of sympathetic tone peripherally results in enhancement of erectile function. β-adrenergic receptors seem to play an important role in erection. The combination of sildenafil and guanethidine or verapamil could have a potential advantage on erectile function but propranolol may mask the effect of sildenafil on erectile function.

The effects of the combined use of a PDE5 inhibitor and medications for hypertension, lower urinary tract symptoms and dyslipidemia on corporal tissue tone

ED is closely associated with its comorbidities (hypertension, dyslipidemia and lower urinary tract symptoms (LUTS)). Therefore, several drugs have been prescribed simultaneously with PDE5 inhibitors. If a specific medication for ED comorbidities has enhancing effects on PDE5 inhibitors, it offers alternative combination therapy in nonresponders to monotherapy with PDE5 inhibitors and allows clinicians to treat ED and its comorbidities simultaneously. To establish theoretical basis of choosing an appropriate medication for ED and concomitant disease, we examined the effects combining a PDE5 inhibitor with representative drugs for hypertension, dyslipidemia and LUTS on relaxing the corpus cavernosum of rabbits using the organ-bath technique. The effect of mirodenafil on relaxing phenylephrine-induced cavernosal contractions was significantly enhanced by the presence of 10(-4) M losartan, 10(-6) M nifedipine, 10(-6) M amlodipine, 10(-7) M doxazosin and 10(-9) M tamsulosin (P<0.05). The maximum relaxation effects were 47.2±3.8%, 57.6±2.6%, 64.0±3.7%, 76.1±5.7% and 71.7±5.4%, respectively. Enalapril and simvastatin had no enhancing effects. The relaxation induced by sodium nitroprusside alone (39.0±4.0%) was significantly enhanced in the presence of the 10(-4) M losartan (66.0±6.0%, P<0.05). Tetraethylammonium (1 mM) significantly inhibited the enhancement effects of tamsulosin and doxazosin on mirodenafil-induced relaxation (doxazosin: 76.1±5.7% vs 45.3±2.3%; tamsulosin: 71.7±5.4% vs 48.1±3.5%). On the basis of these findings, losartan seemed to induce synergistic effects through an interaction with nitric oxide. In addition, K(+) channel activation could be one of the mechanisms for the synergistic effect of combining mirodenafil with doxazosin or tamsulosin. We believe that the combination of a PDE5 inhibitor with losartan, nifedipine, amlodipine, doxazosin or tamsulosin could be a pharmacologic strategy for simultaneously treating ED and its comorbidities and increasing response rates to PDE5 inhibitors.

Three phases of corporal tracing elicited by electrical field stimulation on rabbit corpus cavernosum smooth muscle in penile perfusion model

INTRODUCTION

Nitric oxide (NO) has been shown to mediate electrical field stimulation (EFS)-caused smooth muscle relaxation. It is known that the neural control of penile erection involves adrenergic, cholinergic, and non-adrenergic-non-cholinergic (NANC) neuro-effector systems; however, the effects of EFS on adrenergic and cholinergic nerves are not clear.

AIMS

To elucidate EFS-induced signal transductions involved in adrenergic, cholinergic, and NANC neuro-effector systems by using an in vitro penile perfusion model.

METHODS

EFS was performed on penile corpus cavernosum smooth muscle from male New Zealand White rabbits, which was pre-contracted with L-phenylephrine (10 µM). We investigated the penile tracing elicited by EFS on tissues pre-incubated with guanethidine (Guan, 50 µM), tetrodotoxin (TTX, 10 µM), Nω nitro-L-arginine-methyl ester (L-NAME, 1 mM), atropine (50 µM), or eserine (10 µM).

MAIN OUTCOME MEASURES

The time-to-peak of each phase, the percentage of relaxation, and the area under the curve (AUC).

RESULTS

We discovered an extraordinary phenomenon: three distinct phases elicited by EFS. Phase I was abolished by L-NAME. Phase II was decreased by eserine and Guan, but increased by L-NAME. Phase III was abolished by atropine, but enhanced by eserine and Guan. TTX diminished all three phases. The time to reach the top of phase I was delayed by TTX. The time to attain the peak of phase II was shortened by L-NAME, but delayed by TTX and atropine. The time to reach the top of phase III was shortened by L-NAME, eserine, and Guan. AUC was significantly decreased by L-NAME and TTX.

CONCLUSIONS

EFS stimulated adrenergic, cholinergic, and NANC neuro-effector systems simultaneously. Phase I was related to the NO pathway. Phase II was multiply affected by self-recovery properties, and adrenergic and cholinergic nerves. Phase III was related to cholinergic nerves. The corporal tracing elicited by EFS was the balanced result of multiple factors.

NS11021, a novel opener of large-conductance Ca(2+)-activated K(+) channels, enhances erectile responses in rats

BACKGROUND AND PURPOSE

Large-conductance Ca(2+)-activated K(+) channels (BK(Ca)), located on the arterial and corporal smooth muscle, are potential targets for treatment of erectile dysfunction (ED). This study investigated whether NS11021 (1-(3,5-Bis-trifluoromethyl-phenyl)-3-[4-bromo-2-(1H-tetrazol-5-yl)-phenyl]-thiourea), a novel opener of BK(Ca) channels, relaxes erectile tissue in vitro and enhances erectile responses in intact rats. The effects were compared with sildenafil, an inhibitor of phosphodiesterase type 5.

EXPERIMENTAL APPROACH

Patch clamp was used to record whole cell current in rat isolated corpus cavernosum smooth muscle cells (SMCs) and human umbilical vein endothelial cells (HUVECs). Isometric tension was measured in intracavernous arterial rings and corpus cavernosum strips isolated from rats and men, and simultaneous measurements of intracellular Ca(2+) concentration ([Ca(2+)](i)) and tension were performed in intracavernous arteries. Erectile response was measured in anaesthetized rats.

KEY RESULTS

In patch clamp recordings, NS11021 increased currents sensitive to the selective BK(Ca) channel blocker, iberiotoxin (IbTX) in SMCs, but did not modulate K(+) current in HUVECs. NS11021 reduced [Ca(2+)](i) and tension in penile arteries. IbTX inhibited the vasorelaxation induced by NS11021 and sildenafil in human erectile tissue. NS11021 and sildenafil but not vehicle increased erectile responses in anaesthetized rats, an effect which was abolished after pretreatment with tetraethylammonium.

CONCLUSIONS AND IMPLICATIONS

NS11021 leads to relaxation of both intracavernous arteries and corpus cavernosum strips primarily through opening of BK(Ca) channels. It is also effective in facilitating erectile responses in anaesthetized rats. These results suggest a potential for use of BK(Ca) openers in the treatment of ED.

Yohimbine relaxes the human corpus cavernosum through a non-adrenergic mechanism involving the activation of K+ATP-dependent channels

The mechanism by which yohimbine relaxes the human corpus cavernosum remains unclear. Using the human corpus cavernosum strips immersed in isometric baths containing Krebs-Henseleit solution, this study investigates the effect of yohimbine on the relaxation of the human corpus cavernosum through nitrergic pathways involving the activation of ATP-dependent potassium channels (K(ATP)). The maximal relaxation induced by yohimbine in the human corpus cavernosum strips pre-contracted with phenylephrine was 100+/-0% and only 30.5+/-5.0% when they were pre-contracted with 60-mM potassium (K(+)) solution. The maximal relaxation induced by yohimbine in phenylephrine pre-contracted tissues was significantly inhibited by tetrodotoxin, 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or 7-nitroindazole (43.6, 36.1 and 42.6%, respectively). Neither the combination charybdotoxin-apamin nor tetraethylammonium altered the response of the human corpora cavernosa strips to yohimbine. Nevertheless, glibenclamide decreased the maximum relaxant response to yohimbine by 29.8% (P<0.05; n=12). The results suggest that yohimbine relaxes the human corpus cavernosum by a non-adrenergic, non-cholinergic mechanism, probably activating the nitrergic-soluble guanylate cyclase (NO-sGc) pathway and K(ATP).