Effects of pinacidil on isolated human corpus cavernosum penis

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.

The effects of dopamine receptor agonists on BK Ca channels and signal transduction mechanism in corpus cavernosal smooth muscle cells

In this study, we investigated the effect of dopamine receptor agonists on potassium channels' activity and their signal transduction pathway in corporal smooth muscle cells. We used cultured human corporal smooth muscle cells. The whole cell and cell-attached configuration of the patch-clamp technique were used for electrophysiological recordings, and enzyme immunoassay was used for measuring cyclic AMP (cAMP) and cyclic GMP levels. Extracellular application of 10 microM dopamine and apomorphine significantly increased whole-cell K(+) currents by 283.5+/-55.7% (at +60 mV; n=12, P<0.001), 292.4+/-58.8.0% (at +60 mV; n=9, P<0.005), respectively. We confirmed that the increase in whole-cell currents was mainly due to activation of the tetraethylammonium-sensitive large conductance Ca(2+)-activated K(+) channels (BK(Ca) channels). Enzyme immunoassay indicated that dopamine and apomorphine stimulates cAMP levels in corporal smooth muscle cells in a concentration-dependent fashion. The activation of BK(Ca) channels by dopamine receptor agonists in corporal smooth muscle cells might be one of the mechanisms in inducing penile erection.

Role of adenosine triphosphate-dependent potassium channels in canine penile erection

OBJECTIVES

To define the physiologic role and hemodynamic features of nitric oxide (NO) and the adenosine triphosphate (ATP)-dependent K(+) (K(ATP)) channel in canine penile erection.

METHODS

Mongrel dogs were anesthetized, and penile erection was induced by electrical stimulation of the pelvic nerve. Changes in the intracavernous pressure (ICP) were measured with a transducer.

RESULTS

The basal ICP was 12.8 +/- 5.0 mm Hg. Pelvic nerve stimulation (5 to 20 V, 5 to 15 Hz, for 1-minute intervals) significantly increased the ICP to 86.2 +/- 11.4 mm Hg (n = 5, P <0.05). Treatment with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (10 mg/kg intravenously) abolished this increase (15.4 +/- 5.0 mm Hg, n = 5). Intracavernous injection of the K(ATP) channel opener cromakalim (3 and 10 microg) increased the ICP (103 +/- 14.4 mm Hg and 106 +/- 12.1 mm Hg, respectively; n = 4). This response was abolished by the prior intracavernous injection of the selective K(ATP) channel-specific blocker glibenclamide (10 mg). Glibenclamide did not affect the increase in ICP induced by electric stimulation of the pelvic nerve (88 +/- 24.2 mm Hg).

CONCLUSIONS

Our results indicate that relaxation of canine cavernous smooth muscle and penile tumescence are mediated by NO. The failure of glibenclamide to affect the increase in ICP induced by pelvic nerve stimulation suggests that ATP-dependent K(+) channels probably do not play a physiologic role in canine penile erection.

Potassium channels and erectile dysfunction

The incidence of erectile dysfunction (ED), defined as the persistent inability to achieve or maintain an erection sufficient for satisfactory sexual performance, increases with age and with risk factors for vascular disease, including smoking, diabetes and hypertension. Penile erection results from an arousal-induced synthesis of nitric oxide (NO) in nonadrenergic-noncholinergic nerves (NANC), endothelial cells and cavernosal smooth muscle cells (SMCs). Vasodilation and relaxation of cavernosal SMCs engorges the corpora cavernosa with blood at arterial pressure. The subcellular mechanism by which tumescence occurs involves NO-induced activation of soluble guanylate cyclase, increased cyclic guanosine monophosphate (cGMP) levels and activation of cGMP-dependent protein kinase (PKG). PKG phosphorylates numerous ion channels and pumps, each promoting a reduction in cytosolic calcium. In particular, PKG activates high-conductance Ca2+(-)sensitive K+ (BKCa) channels, which hyperpolarize the arterial and cavernosal SMC membranes, causing relaxation. This mechanism appears to be compromised with age and with vascular disease, leading to ED. Thus, increasing cavernosal nitric oxide synthase (NOS) expression, cGMP levels and/or BKCa channel expression is an effective therapy for experimental ED. Future therapies may involve augmenting K+ channel expression by gene transfer or increasing channel function through the use of Type 5 phosphodiesterase (Type 5 PDE) inhibitors or phosphatase inhibitors.

Effects of sildenafil citrate, acetylcholine, and sodium nitroprusside on the relaxation of rabbit cavernosal tissue in vitro

OBJECTIVES

To investigate the effect of sildenafil together with the influence of sodium nitroprusside and acetylcholine and further discuss its mechanism of action in vitro.

METHODS

Isolated strips of rabbit corpus cavernosum were stimulated isometrically with phenylephrine. Graded relaxations were induced using increasing concentrations of acetylcholine, sodium nitroprusside, and sildenafil alone and in combination.

RESULTS

The agents all relaxed the rabbit corpus cavernosal strips in a dose-dependent manner. The inhibitory potentials of the test agents were ranked (from least inhibitory to most inhibitory) as follows: sildenafil, acetylcholine plus sildenafil, sodium nitroprusside, sodium nitroprusside plus sildenafil, acetylcholine. The presence of sodium nitroprusside in the medium reduced the median effective dose for the sildenafil-induced relaxation of the cavernosal tissue from 3.65 x 10(-5) M to 2.73 x 10(-7) M, and the presence of acetylcholine reduced it to 9.79 x 10(-6) M.

CONCLUSIONS

Sildenafil enhances the relaxing effect of both sodium nitroprusside and acetylcholine on the phenylephrine-induced contraction of rabbit cavernosal tissue.

Relaxation induced by omeprazole does not change in diabetic rabbit corpus cavernosum

OBJECTIVE

To investigate the changes in relaxation responses to omeprazole in corpus cavernosal strips from rabbits with diabetes induced by alloxan.

MATERIALS AND METHODS

Diabetes was induced in 10 New Zealand white rabbits. After 8 weeks, the reactivity to electrical-field stimulation, carbachol and omeprazole of corporal strips from the penises of the diabetic animals and from 10 untreated controls was assessed in organ chambers.

RESULTS

In the diabetic group, the relaxation responses of corpus cavernosal strips to omeprazole were comparable with those of the control group, whereas the relaxation responses to electrical field stimulation and carbachol were impaired.

CONCLUSION

The relaxant effect of omeprazole in corpus cavernosum was not affected by diabetes, in which neurogenic and endothelium-mediated relaxations were impaired.

Evidence of relaxant effect of omeprazole in rabbit corpus cavernosum in vitro

The present experiments were designed to investigate the effects of omeprazole, a H(+)-K+ ATPase inhibitor, on corporal smooth muscle tone in vitro. All spontaneous contractile activity in the corpus cavernosum was blocked following omeprazole (0.1 mM-1 mM) administration. However atropine (1 microM), Nw-nitro L-arginine methyl ester (L-NAME, 30 microM) or indomethacin (10 microM) did not affect the spontaneous contraction. Omeprazole (10 microM-1 mM) concentration-dependently induced relaxation in corporal smooth muscle precontracted with 10 microM phenylephrine or 80 mM KCl. Pretreatment of corporal tissue with L-NAME (30 microM), indomethacin (10 microM), ammonium chloride (7.5 mM), sodium acetate (7.5 mM), tetraethyl ammonium chloride (0.5 mM) or glibenclamide (1 microM) had no effect on the omeprazole induced relaxant responses. Nimodipine, an L-type Ca++ channel blocker, relaxed corporal strips precontracted with 80 mM KCl. Collectively, these results indicate that the inhibition of spontaneous contraction and the relaxation of precontracted corporal smooth muscle by omeprazole is probably mediated by the blockade of calcium channels. Further work is needed to determine the cellular mechanism(s) of action by which omeprazole acts on corpus cavernosum smooth muscle.

A KATP-channel opener as a potential treatment modality for erectile dysfunction

OBJECTIVES

To assess the effects of pinacidil (a KATP-channel opener) for the treatment of penile erectile dysfunction and to examine the role of the K+-channel in cavernosal smooth muscle contractility.

MATERIALS AND METHODS

Using a feline model, the magnitude of penile erection caused by pinacidil was compared with that caused by erectogenic drugs, e.g. acetylcholine, prostaglandin E1 (PGE1) and L-arginine. The effects of K+-channel blockers (4-aminopyridine, glibenclamide and tetraethylammonium) and pinacidil on penile erections induced by the drugs were investigated.

RESULTS

The intra-arterial injection of pinacidil caused a dose-dependent increase in intracavernosal pressure (ICP) and the increase in ICP induced by pinacidil with acetylcholine, PGE1 or L-arginine was more pronounced than with the compounds alone. Furthermore, pinacidil (1 mmol/L) effectively reversed the inhibitory effects of the K+-channel blockers on the cavernosal relaxation induced by acetylcholine, PGE1 or L-arginine (P<0.01). Notably, pinacidil induced cavernosal relaxation after injecting the drugs even in cases refractory to higher concentrations (0.1 mol/L) of the drugs (n=11, P<0.01).

CONCLUSIONS

These results suggest that pinacidil is effective in relaxing feline erectile tissue in vivo, probably via increased K+ permeability and subsequent hyperpolarization. Further comparative studies with erectogenic compounds on human erectile tissue and clinical testing are required to determine whether K+-channel openers can be used in the diagnosis and treatment of erectile dysfunction. However, pinacidil seems promising as an intracavernosal agent combined with PGE1 to produce synergistic effects.

Relaxation of corpus cavernosum and raised intracavernous pressure by berberine in rabbit

1. In the present study, we have investigated the effect of berberine in rabbit isolated corpus cavernosum and measured the intracavernous pressure (ICP) change after intracavernosal injection of berberine in rabbit. 2. Berberine alone suppressed the basal tone and induced a concentration (0.1-100 microM)-dependent relaxation in phenylephrine (PE)-precontracted corpus cavernosum. 3. Tetrodotoxin (0.1 and 1 microM) treatment had no significant effect on the berberine-induced relaxation. Phentolamine (1 and 10 microM), propranolol (1 and 3 microM) and atropine (1 and 3 microM) were also without effect. These results suggest that berberine might cause relaxation of the cavernosal strip by direct action on the corpus cavernosum, not by a neuronal effect. Furthermore, muscarinic- and beta-adrenoceptors were not involved. 4. Berberine-induced relaxations were significantly reduced by endothelium removal and by exposure to L-NG-nitro arginine methyl ester (0.1 and 0.3 mM), but not indomethacin (30 microM). 5. In endothelium-deprived corpus cavernosal tissues, berberine-induced relaxations were significantly reduced in high K+ medium (KCl = 60 mM), by charybdotoxin (ChTX) and 4-aminopyridine (4-AP) but not by glibenclamide and apamin. 6. After intracavernous injection of berberine (1, 2, 3 and 5 mg kg(-1)), the ICP rose from 12.7+/-3.6 to 13.2+/-5.4, 25.3+/-6.1, 46.5+/-8.2, and 63.4+/-10.2 mmHg, respectively. The duration of tumescence ranged from 11.5 - 43.7 min. 7. The results show that berberine possesses a relaxant effect on rabbit corpus cavernosal tissues which is attributable to both endothelium-dependent and-independent properties. While the former component is apparently due to the release of NO from sinusoidal endothelium, the endothelium-independent mechanism involved in berberine relaxation is probably linked to ChTX- and 4-AP-sensitive K+ channel activation in the cavernosal vasculature.

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

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

Intracavernous pharmacotherapy

Intracavernous application of vasoactive substances not only has enhanced our understanding of penile hemodynamics, the physiology of penile erection, and the pathophysiology of erectile dysfunction but also has revolutionized the diagnosis and treatment of erectile dysfunction in the last 15 years. Virag was the first to report on the erectile effect of papaverine in humans, and Brindley later reported the effect of intracavernous application of alpha-receptor-blocking agents on cavernous tissue. These reports led to numerous basic and clinical investigations and ultimately established a new treatment alternative for patients with erectile dysfunction that is now considered to be the treatment of choice for most patients. Changes in penile hemodynamics include the relaxation of cavernous smooth musculature and arteries, which leads to an increase in arterial blood flow and a restriction of venous outflow through a compression of subtunical veins. These hemodynamic changes are the prerequisite for the induction and maintenance of penile erection. With the intracavernous application of vasoactive substances it was possible to influence penile hemodynamics at a local level and to induce an erection despite alterations in the nervous system, penile arterial blood flow, cavernous musculature, or neurotransmitter status. In addition, the local application of pharmacologically active substances directly to the end organ enabled the achievement of high local drug concentrations without severe systemic side effects. The commonly used substances are papaverine the combination of papaverine and phentolamine, and prostaglandin E1 (alprostadil). In addition to these established substances, several other regimens, such as linsidomine (SIN-1), calcitonin gene-related peptide (CGRP), moxisylyte, and various triple- or quadruple-drug mixtures have been described. In addition, several other compounds as well as different routes of administration are on the horizon and may prove to be effective in the future diagnosis and treatment of erectile dysfunction.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

Effects of nicorandil on human isolated corpus cavernosum and cavernous artery

Nitric oxide (NO) released from nonadrenergic-noncholinergic (NANC) nerves seems to be a principal mediator of the relaxation of penile erectile tissue necessary for erection, and drugs acting by release of NO have been shown to produce erection when injected intracorporeally into impotent patients. By producing hyperpolarization, K+ channel openers are effective in relaxing isolated penile erectile tissue from rabbit and man, and can produce tumescence and erection when injected intracorporeally into animals. Nicorandil is classified as a K+ channel opener, but it also acts as a donor of NO. In the present study, the effects of nicorandil on isolated preparations from human corpus cavernosum (CC) and deep cavernous artery (Acc) were compared with those of cromakalim (K+ channel opener) and SIN-1 (NO donor). Nicorandil produced a concentration-dependent relaxation of CC and Acc preparations. The relaxations obtained at the highest nicorandil concentration used (10(-4) M.) were 75 +/- 3% and 66 +/- 4% in CC preparations contracted by noradrenaline and endothelin-1, respectively. The corresponding effects in Acc preparations were 70 +/- 14% and 73 +/- 5%. Glibenclamide (blocking ATP-dependent K+ channels) significantly reduced the nicorandil-induced relaxation in CC, but not in Acc. Methylene blue (believed to block soluble guanylate cyclase) reduced nicorandil's relaxant effect in CC, although statistical significance was not obtained. NG-nitro-L-arginine 10(-4) M. (NO synthase inhibitor) did not significantly influence the effect of nicorandil on precontracted preparations in either tissue. In CC preparations contracted by electrical field stimulation, nicorandil and cromakalim concentration dependently inhibited the responses. This effect was significantly counteracted by glibenclamide. It is concluded that nicorandil is effective in relaxing human CC chiefly by its K+ channel opening action, and to some extent by its ability to release NO. For nicorandil's relaxing effect on Acc, ATP dependent K+ channels seem to be of limited importance. If effective in impotent patients, the drug may represent a new, interesting approach to the treatment of erectile dysfunction.

Clinical pharmacology of potassium channel openers

Opening of K+ channels in cell membranes with resulting increase in K+ conductance, shifts the membrane potential in a hyperpolarizing direction towards the K+ equilibrium potential. Hyperpolarization reduces the opening probability of ion channels involved in membrane depolarization and excitation is reduced. K+ channel openers are believed to hyperpolarize smooth muscle cells by a direct action on the cell membrane. The best known members of the group are cromakalim, nicorandil and pinacidil, but several new compounds are being evaluated. In addition, it has recently been shown that also clinically well-known drugs like, e.g. diazoxide and minoxidil exhibit K+ channel opening properties. Nicorandil and new compounds containing nitro groups have a dual mechanism of action, also activating guanylate cyclase, an effect that contributes to their cardiovascular effect profile. K+ channel openers have a wide range of effects. Some of their properties and actions are summarized, and their present applications and/or potential for future application, in e.g. hypertension, angina pectoris, asthma, bladder instability, and several other disorders are discussed. It is concluded that K+ channel openning represents an interesting pharmacological principle with many potential clinical applications. However, most available drugs do not seem to have a sufficient tissue selectivity to be useful therapeutic alternatives. Before the potential of the new members of the group on clinical trials can be properly evaluated, clinical experiences are needed.

Actions of endothelin on isolated corpus cavernosum from rabbit and man

The effects of endothelin, a vasoconstrictor peptide produced by vascular endothelial cells, were investigated in isolated rabbit and human corpus cavernosum (CC). Preparations from both rabbit and man were potently contracted by endothelin in a concentration-dependent manner. The contractions developed slowly, could not be reversed despite frequent washings, and were only partly inhibited by the Ca2+ channel blocker nimodipine. Even in Ca2(+)-free medium containing the chelator EGTA a small contractile component persisted. In rabbit CC, the contractions in Ca2(+)-free medium were not affected by nimodipine, the Ca2(-)-channel agonist BAY K 86(44), or by depletion of intracellular Ca2(+) stores sensitive to noradrenaline (NA) and caffeine, but were almost abolished by the protein kinase C inhibitor H7. In both rabbit and man, carbachol and vasoactive intestinal polypeptide concentration-dependently relaxed preparations contracted by endothelin. The relaxations induced by carbachol were antagonized by atropine. Endothelin enhanced concentration-dependently the contractions induced by exogenously applied NA in rabbit CC. The enhancement was more pronounced at low concentrations of NA. This study shows that endothelin potently contracts isolated penile erectile tissue. The contraction seems to be mediated mainly by influx of Ca2+ through the cell membrane, which partly occurs through a pathway other than voltage-operated calcium channels. However, involvement of other mechanisms cannot be excluded. The results suggest that endothelin can play a role in penile erectile mechanisms.