Effects of sildenafil, a type-5 cGMP phosphodiesterase inhibitor, and papaverine on cyclic GMP and cyclic AMP levels in the rabbit corpus cavernosum in vitro.

Jeremy JY, Ballard SA, Naylor AM, Miller MA, Angelini GD.

Bristol Heart Institute, Bristol Royal Infirmary, University of Bristol, UK.

OBJECTIVE: To investigate further the mechanisms of action of sildenafil, a highly selective and potent inhibitor of type 5 cGMP phosphodiesterase (PDE5) that has proved effective in the treatment of erectile dysfunction, by assessing its effect on the in vitro formation of cGMP and cAMP in the corpus cavernosum of the rabbit. MATERIALS AND METHODS: Male New Zealand White rabbits (2.5 kg) were killed and their penises rapidly excised, cut into segments and pooled. Penile segments were then incubated with various concentrations of sildenafil or papaverine. The formation of cGMP was stimulated with increasing concentrations of sodium nitroprusside (SNP) and the cGMP and cAMP concentrations measured by radioimmunoassay. Responses were compared to those obtained with papaverine, which is used therapeutically as an erectogen. RESULTS: In the presence or absence of SNP, sildenafil increased cGMP concentrations in rabbit penile tissue with increasing dose; the increase was greatest (about 28-fold) when cGMP was stimulated with SNP (up to 10 mumol/L). At all stimulatory concentrations of SNP, the effective concentrations for 50% stimulation (EC50) of sildenafil were 430-520 nmol/L. Concentrations of cAMP were unaltered by sildenafil. Papaverine enhanced cGMP formation in response to SNP, but at much higher concentrations than did sildenafil (> or = 10 mumol/L). CONCLUSIONS: Sildenafil specifically increases cGMP levels in rabbit corpora cavernosa; the increase is greater in the presence of SNP indicating that, in vivo, sildenafil may enhance erection by the augmentation of nitric oxide-mediated relaxation pathways. The erectogenic effect of sildenafil is mediated by a specific enhancement of cGMP accumulation in the corpus cavernosum, consistent with the known activity of sildenafil as a potent and highly selective inhibitor of cGMP-specific PDE.

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The effect of the specific phosphodiesterase (PDE) inhibitors on human and rabbit cavernous tissue in vitro and in vivo.

Stief CG, Uckert S, Becker AJ, Truss MC, Jonas U.

Dept. of Urology, Medizinische Hochschule Hannover, Germany.

PURPOSE: Phosphodiesterases (PDE) are key enzymes in the regulation of the smooth muscle tone. Experimental studies showed PDE III and V-isoenzymes to play an important role in the smooth muscle tone regulation of corpus cavernosum. Recently, a specific PDE III-inhibitor (milrinone) and a PDE V-inhibitor (sildenafil) were introduced in clinical studies. An experimental study was done to examine a potential role of PDE-inhibitors in the treatment of erectile dysfunction. MATERIALS AND METHODS: In the organ bath, strips from human and rabbit corpus cavernosum were precontracted and increasing doses of PDE inhibitors were added. In patients with erectile dysfunction as well as in rabbits, intracavernous injections of milrinone were done. RESULTS: PDE-inhibitors dose-dependently relaxed human and rabbit corpus cavernosum strips. In the precontracted human cavernous tissue, milrinone and sildenafil were equally potent and efficacious in vitro. In the rabbit, milrinone induced slight tumescence but dramatic circulatory side effects. In patients, penile tumescences as well as full erections were observed. CONCLUSIONS: Milrinone strongly relaxes human cavernous smooth muscle cells but it exhibits low relaxant effects in the rabbit cavernous tissue. In human tissue, sildenafil was equieffective with milrinone in vitro. In vivo, milrinone induced a good erectile response in humans but a poor erectile effect in rabbits. Our results support a possible potential for selective PDE-III and -V inhibitors in the treatment of impotence and give further evidence that the rabbit is an animal model of limited value to study the effects of drugs on cavernous smooth muscle tone regulation in vivo.

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Effects of sildenafil on the relaxation of human corpus cavernosum tissue in vitro and on the activities of cyclic nucleotide phosphodiesterase isozymes.

Ballard SA, Gingell CJ, Tang K, Turner LA, Price ME, Naylor AM.

Department of Discovery Biology, Pfizer Central Research, Sandwich, Kent, United Kingdom.

PURPOSE: Sildenafil, an inhibitor of cGMP-specific phosphodiesterase 5 (PDE5), is currently undergoing evaluation as an oral therapy for penile erectile dysfunction. The aims of this study were to investigate the mechanism of action of sildenafil on the neurogenic relaxation of human corpus cavernosum (HCC) in vitro and to determine the activity of sildenafil against a full range of PDE isozymes. MATERIALS AND METHODS: Strips of HCC tissue were precontracted with phenylephrine. Relaxation responses resulting from electrical field stimulation (EFS) were then determined in the presence and absence of sildenafil. The effects of sildenafil on PDE1 to 5 prepared from human tissues and PDE6 from bovine retina were determined by measuring the conversion of [3H]-cGMP or [3H]-cAMP to their respective [3H]-5'-mononucleotides. RESULTS: Sildenafil (0.001 to 1 microM) enhanced the EFS-induced, nitric oxide (NO) dependent, relaxation of HCC in a concentration-dependent manner to a maximum of 3 times the pretreatment level at 1 microM sildenafil. Compared with zaprinast, an early PDE5 inhibitor, sildenafil was approximately 240-fold more potent, inhibiting PDE5 from HCC with a geometric mean IC50 of 3.5 nM. For sildenafil, IC50 values for inhibition of PDE1 to 4 were 80 to more than 8500 times greater than that for PDE5 and the IC50 for PDE6 (33 nM) was approximately 9-fold greater. CONCLUSIONS: The data support the proposal that enhancement of penile erection by sildenafil in patients with erectile dysfunction involves potentiation of the NO-stimulated cGMP signal mediating relaxation of cavernosal smooth muscle during sexual stimulation. Sildenafil is a potent inhibitor of PDE5 from HCC, with high selectivity for PDE5 relative to other PDE isozymes.

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Sildenafil, a novel inhibitor of phosphodiesterase type 5 in human corpus cavernosum smooth muscle cells.

Moreland RB, Goldstein I, Traish A.

Department of Urology, Boston University School of Medicine, MA 02118, USA.

In human corpus cavernosum, release of nitric oxide from the non-adrenergic, non-cholinergic nerves and/or the endothelium activates guanylyl cyclase and increases intracellular cGMP levels. The increase in intracellular cGMP modulates intracellular calcium and in turn regulates smooth muscle contractility and erectile function. Phosphodiesterases play an important physiological role by regulating the intracellular levels of cyclic nucleotides. In this study, we investigated the kinetic parameters of inhibition of phosphodiesterase (PDE) type 5 (E.C. 3.1.4.35 3',5'-cyclic GMP phosphodiesterase) by a novel, high affinity, selective PDE type 5 inhibitor, sildenafil, in soluble extracts of human corpus cavernosum smooth muscle cells. Sildenafil inhibited PDE type 5 cGMP-hydrolytic activity, in the crude extract (Ki=4-6 nM) and in partially purified preparations (Ki=2 nM) in a competitive manner, as determined by Dixon plots. Sildenafil (Ki=2-4 nM) was a more effective PDE type 5 inhibitor than zaprinast (Ki=250 nM). Stimulation of intracellular cGMP synthesis by the nitric oxide donor, sodium nitroprusside, resulted in less than a 5% increase in cGMP levels in the absence of sildenafil and a 35% increase in cGMP levels in the presence of sildenafil, in intact cells at physiological temperatures. These results are in accord with the clinical observations that sildenafil, taken orally, promotes penile erection through increased intracellular cGMP in response to sexual stimulation, potentiating smooth muscle relaxation.

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Sildenafil citrate significantly improves nocturnal penile erections in sildenafil non-responding patients with psychogenic erectile dysfunction.

Abdel-Naser MB, Imam A, Wollina U.

Department of Dermatology, Venereology and Andrology, Ain Shams University, Cairo, Egypt. abdelasermb yahoo.com

Effects of sildenafil citrate on nocturnal penile tumescence and rigidity (NPTR) were evaluated among sildenafil non-responding patients with psychogenic erectile dysfunction. All patients (n=30), equally divided into groups I and II, completed four consecutive nights using the RigiScan Plus device. Sildenafil citrate (50 mg) was given in the third night in group I and in the fourth in group II, whereas a placebo was given in the remaining nights. Additional patients (n=12) receiving only a placebo served as a control group. Results of NPTR recordings revealed neither significant differences between the control and non-sildenafil nights of both test groups, nor between the corresponding values of both groups (P>0.05). On the other hand, when sildenafil citrate nights of groups I and II taken together were compared with placebo nights, a significant increase of total events duration (P<0.001), average rigidity of the tip (P<0.05) and base (P<0.01), and rigidity activity unit (RAU) and tumescence activity unit (TAU) of tip and base (P<0.001) was observed. These results suggest that performance anxiety may be responsible for failure of response during awakening.

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Private prescription costs for sildenafil within the NHS: a telephone survey.

Mostyn PA, Crowley T, Keane FE.

Department of Sexual Health, King's College Hospital, and Victoria Sexual Health Clinic, Chelsea & Westminster Healthcare, UK.

Because of the restrictions on prescribing for impotence within the NHS, doctors routinely write private prescriptions for sildenafil. The aim of this study was to determine the variation in cost of a private prescription of four 100 mg tablets of sildenafil. A selection of different pharmacy types within five areas in England was surveyed. We telephoned a total of 86 pharmacies and we were quoted prices ranging from pounds 28.20 to pounds 42.33. There was a significant difference in price between area and between pharmacy type. Best prices are not necessarily found at the major pharmacy chains or hospital pharmacies, as might be expected. NHS doctors and patients need to be aware of this significant difference in cost.

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Role of androgens in erectile function.

Foresta C, Caretta N, Rossato M, Garolla A, Ferlin A.

Department of Medical and Surgical Sciences, Clinica Medica 3, Center for Male Gamete Cryopreservation, University of Padova, Italy.

PURPOSE: Erectile response in mammals is centrally and peripherally regulated by androgens. Severe hypogonadism in men usually results in loss of libido and potency. The present studies were designed to evaluate the possible influence of circulating androgens in the regulation of sexual function. MATERIALS AND METHODS: A total of 15 men with severe hypogonadism (testosterone less than 2.0 ng/ml) were recruited. The control group consisted of 20 patients with psychogenic erectile dysfunction. All subjects underwent nocturnal penile tumescence (NPT) and rigidity monitoring during 2 consecutive nights, evaluation of cavernous artery flow using penile color duplex ultrasound (P-CDU), and real-time visually stimulated erection evaluation at baseline and after administration of 50 mg sildenafil or 3 mg apomorphine. NPT, P-CDU and visually stimulated erection were evaluated after 6 months of therapy with a 5 mg daily testosterone patch. RESULTS: NPT analysis in subjects with severe hypogonadism showed a significant decrease in sleep related erections. P-CDU showed a partial erectile response with an alteration of the parameters analyzed, and the visually stimulated erections test was pathological. Administration of 3 mg apomorphine and 50 mg sildenafil has no influence on erectile function. Testosterone treatment for 6 months induced normalization of NPT and P-CDU parameters, and of visually stimulated erection effects with the restoration of a normal response to pharmacological stimulation with apomorphine or sildenafil. CONCLUSIONS: Our results show that testosterone has a key role in the central and peripheral modulation of erectile function even if the accurate testosterone plasma level threshold that may influence these processes remains to be established.

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Use of intraurethral alprostadil in patients not responding to sildenafil citrate.

Jaffe JS, Antell MR, Greenstein M, Ginsberg PC, Mydlo JH, Harkaway RC.

Division of Urology, Department of Surgery, Albert Einstein Medical Center, Philadelphia, Pennsylvania 19141, USA.

OBJECTIVES: To determine whether intraurethral alprostadil would be an effective alternative for men with erectile dysfunction who did not respond adequately to sildenafil citrate but desired minimally invasive treatment. METHODS: A total of 44 male patients aged 41 to 74 years with erectile dysfunction refractory to treatment with sildenafil citrate were enrolled in this study. Of the 44 patients, 10 had undergone prior radical retropubic prostatectomy. The patients were evaluated for subjective improvement in an office setting and completed the Sexual Health Inventory for Men questionnaire as an objective assessment of improved erectile ability. Success was defined as subjective improvement in erectile function, as well as an improved Sexual Health Inventory for Men score. RESULTS: Of the 44 men, 13 (29.5%) responded successfully to intraurethral alprostadil, with a follow-up ranging from 2 to 15 months. The remaining 31 men had no response (n = 28, 90%), refused escalating doses (n = 2, 7%), or were lost to follow-up (n = 1, 3%). In the subgroup of 10 men with prior radical retropubic prostatectomy, 5 (50%) reported success with intraurethral alprostadil (500 microg in 2 patients and 1000 microg in 3 patients). CONCLUSIONS: Although sildenafil citrate remains the most common initial therapy in men with erectile dysfunction, intraurethral alprostadil may be a reasonable treatment option for sildenafil nonresponders. This may be especially true in men having undergone prior radical retropubic prostatectomy.

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