Erection and NO override the vasoconstrictive effect of alpha-adrenergic stimulation in the rat penile vasculature

Does Testosterone Salvage PDE5i Non-Responders? A Scoping Review

Erectile physiology, in order to function normally, requires the complex coordination of endocrine, neurocognitive, neuromuscular and vascular mechanisms. Testosterone (T) influences male sexuality as well as penile erections at multiple levels, including a direct influence on the nitric oxide synthase (NOS)/cGMP/phosphodiesterase 5 pathway in the penis. However, the precise role of testosterone replacement (TRT) to “salvage” men with mixed ED failing phosphdiesterase-5 inhibitors (PDE5i) remains unclear. We conducted a scoping review identifying the rationale for TRT in ED failing PDE5i, and we critically discuss clinical trials that have examined TRT in the setting of PDE5i use. Overall, TRT replacement appears to be well tolerated and may enhance the response to PDE5i and quality of life, particularly for men with mixed ED, and particularly among men with very low levels of testosterone. However, most of the available literature examines concurrent TRT alone or simultaneous TRT + PDE5i usage, without necessarily selecting for PDE5i failure cases. The present studies are limited to heterogenous studies with small sample sizes, without an exact predominant etiologic factor causing ED. Furthermore, studies showing the most benefit are non-placebo-controlled trials; however, the correction of more profound hypogonadism may lead to an improved response to PDE5i. Stronger conclusions would require properly selected patient populations and larger placebo-controlled RCTs.

α1A-Adrenergic Receptor Antagonism Improves Erectile and Cavernosal Responses in Rats With Cavernous Nerve Injury and Enhances Neurogenic Responses in Human Corpus Cavernosum From Patients With Erectile Dysfunction Secondary to Radical Prostatectomy

INTRODUCTION

Cavernous nerve injury (CNI) in rats and radical prostatectomy (RP) in men result in loss of nitrergic function and increased adrenergic-neurogenic contractions of cavernosal tissue.

AIM

To evaluate the modulation of the α-adrenergic system as a strategy to relieve erectile dysfunction (ED) and functional cavernosal alterations induced by CNI.

METHODS

A non-selective α-blocker (phentolamine 1 mg/kg daily), a selective α_1A-blocker (silodosin [SILOD] 0.1 mg/kg daily), or vehicle was orally administered for 4 weeks after bilateral crush CNI (BCNI). Erectile and neurogenic responses of the corpus cavernosum (CC) were evaluated. The acute effects of SILOD also were evaluated in vivo (0.03 mg/kg intravenously) and ex vivo (10 nmol/L). The effects of SILOD and tadalafil (TAD) on nitrergic relaxations were determined in human CC from patients with ED with a vascular etiology or ED secondary to RP.

MAIN OUTCOME MEASURES

Erectile responses in vivo in rats and neurogenic contractions and relaxations of rat and human CC.

RESULTS

Long-term treatment with SILOD significantly improved erectile responses and allowed for the potentiation of erectile responses by acute treatment with TAD (0.3 mg/kg intravenously) in rats with BCNI. SILOD partly recovered nitrergic relaxations and normalized neurogenic contractions in CC from rats with BCNI. Long-term treatment with SILOD partly prevented BCNI-induced decreases in neuronal nitric oxide synthase expression. Acute administration of SILOD (0.03 mg/kg intravenously) improved erectile responses in vivo and potentiated nitrergic relaxation and decreased neurogenic contractions ex vivo in CC from rats with BCNI. In human CC from patients with ED with a vascular etiology, TAD (30 nmol/L), SILOD (10 nmol/L), or their combination increased nitrergic relaxations. Potentiation by TAD was lost in human CC from patients with ED after RP but was recovered after co-treatment with SILOD.

CONCLUSION

α-Adrenergic modulation, especially selective α_1A-blockade, improves erectile and cavernosal functions after BCNI. Modulation of the adrenergic system, mainly in combination strategies, could have a role in the management of ED after RP.

Penile erection requires association of soluble guanylyl cyclase with endothelial caveolin-1 in rat corpus cavernosum

Erectile dysfunction is caused by a variety of pathogenic factors, particularly impaired formation and action of nitric oxide (NO). NO released from nerve endings and corpus cavernosum endothelial cells plays a crucial role in initiating and maintaining increased intracavernous pressure, penile vasodilatation, and penile erection. Classically, these effects are dependent on cGMP synthesized during activation of soluble guanylyl cyclase (sGC) by NO in smooth muscle cells. The enzyme NO synthase in endothelial cells has been localized to caveolae, small invaginations of the plasma membrane rich in cholesterol. Membrane cholesterol depletion impairs acetylcholine-induced relaxation in arteries attributed to an alteration in caveolar structure. It has been shown that sGC may be activated in endothelial caveolae contributing to vasodilation. We hypothesized that caveolae are the platform for sGC/cGMP signaling in cavernosum smooth muscle eliciting erection. Methyl-beta-cyclodextrin, a pharmacological tool to deplete membrane cholesterol and disassemble caveolae, impaired rat erectile responses in vivo and cavernosum smooth muscle relaxation induced by the NO donor sodium nitroprusside and the sGC activator 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole in vitro. Methyl-beta-cyclodextrin had no effect on cavernosum smooth muscle relaxation induced by NO released upon nerve stimulation or by exogenous cGMP. Furthermore, sGC and caveolin-1, the major coat protein of caveolae, were colocalized in rat corpus cavernosum sinusoidal endothelium. Electron microscopy indicated caveolae disruption in corpus cavernosum treated with methyl-beta-cyclodextrin. In summary, our results provide evidence of compartmentalization of sGC in the caveolae of cavernosal endothelial cells contributing to NO signaling mediating smooth muscle relaxation and erection.

RhoA-Rho kinase mediates synergistic ET-1 and phenylephrine contraction of rat corpus cavernosum

Maintenance of the detumescent state of the penis is believed to involve the actions of several vasoconstrictors. However, our mechanistic understanding of any synergistic vasoconstrictor influences is extremely limited. We tested the hypothesis that a vasoconstrictor combination of endothelin (ET-1) and phenylephrine (PE) augments the constrictor responses in rat corporal cavernosal tissues by a mechanism involving the RhoA-Rho kinase pathway. Independently, ET-1 (1 nM-30 microM) and PE (100 nM-100 microM) both caused dose-dependent contractions of isolated rat cavernosal tissues. In combination, ET-1 (30 nM) augmented the contractile effect of PE and shifted the calculated EC50 for PE (90 +/- 12 to 45 +/- 5 microM). The active stress generated by cavernosal strips during the ET-1 + PE combined stimulation (4.9 +/- 0.2 mN/mm2) was greater than the combined stress generated with ET-1 (0.4 +/- 0.1 mN/mm2) or PE (3.3 +/- 0.2 mN/mm2) stimulations alone. Blockade of ETA receptors (30 nM; A-127722) reversed the augmented stress generation and the Rho-kinase inhibitor Y-27632 differentially and dose-dependently relaxed the tissue. The combined constrictor effect was associated with a fourfold increase of RhoA in the membrane faction of the tissue homogenates. We conclude that the ET-1 + PE combination potentiate vasoconstriction through mutual activation of the RhoA-Rho kinase pathway. The interactions of these agonists likely play important roles in the maintenance of the flaccid state and contribute to some forms of erectile dysfunction.

Improved erectile function after Rho-kinase inhibition in a rat castrate model of erectile dysfunction

Androgens are reported to act as strong modulators of erectile function influencing both nitric oxide and vasoconstrictor signaling. Castration results in a depressed erectile response that is associated with a loss of nitric oxide production and increased responsiveness to constrictive agents. The increased vasoconstrictor response may be a result of an active RhoA/Rho-kinase signaling pathway. We report here results of studies designed to test the hypothesis that inhibition of the Rho-kinase pathway restores erectile function in a castrate model by relaxing the smooth muscle. Mean arterial (MAP) and corpus cavernosal (CCP) pressures were monitored during intracavernosal injection of the Rho-kinase inhibitor Y-27632. Castration reduced the maximal erectile response (CCP/MAP) by 33%, and testosterone replacement restored the response (intact, 0.736 +/- 0.040; castrate, 0.492 +/- 0.022; testosterone, 0.681 +/- 0.073). Injection of Y-27632 increased CCP in all experimental groups; it also left shifted the voltage response curve and increased the maximal CCP/MAP response (intact, 0.753 +/- 0.091; castrate, 0.782 +/- 0.081; testosterone treated, 0.894 +/- 0.033). Y-27632 dose dependently relaxed phenylephrine-stimulated cavernosal tissues. Cavernosal tissues showed increased RhoA and Rho-kinase protein levels after castration. Our data support the hypothesis that an active Rho/Rho-kinase pathway contributes to the reduced erectile response after castration due to an upregulation of RhoA/Rho-kinase protein levels and that inhibition of this pathway may serve as an effective treatment for erectile dysfunction.

Inhibition of the tonic contraction in the treatment of erectile dysfunction

Erectile dysfunction (ED) reduces the quality of life. It is estimated that 52% of men have some degree of ED, which is associated with ageing. While it is clear that there are a variety of current treatment options for ED, each of these has drawbacks and contraindications. A better understanding of the physiological mechanisms involved in penile erection will provide new ways to treat ED. This review not only focuses on the vasoconstrictors and vasodilators that control the state of contraction and relaxation of the corpora cavernosa smooth muscle, but also presents a novel Ca(2+)-sensitising pathway that contributes to maintaining the penis in the non-erect state. Studies have shown that inhibition of the RhoA/Rho-kinase signalling pathway induces penile erection. Further understanding of this RhoA/Rho-kinase pathway may provide a novel alternative treatment for ED.

Vasoconstriction and vasodilation in erectile physiology

Recent studies have demonstrated that vasoconstriction in the erectile vasculature of the penis is mediated in part by RhoA/Rho-kinase signaling. However, this constrictor activity must be overcome to permit the vasodilation essential for erection. We hypothesize that the primary action of nitric oxide and other agents that cause penile erection is inhibition of the RhoA/Rho-kinase pathway, thereby allowing vasodilation and erection. This hypothesis, as well as experiments using hypogonadal and hypertensive animal models, are discussed in terms of the potential clinical value of Rho-kinase inhibitors for the treatment of erectile dysfunction.

Biochemical and functional characterization of alpha-adrenergic receptors in the rabbit vagina

Vascular and non-vascular smooth muscle within the vagina mediate important physiological changes during sexual arousal in women. In this study, we have characterized alpha-adrenergic receptors (AR) in rabbit vagina by assessment of radioligand binding, contractility of isolated tissue strips and genital hemodynamics. [3H]Prazosin and [3H]RX821002 (alpha-1 and alpha-2 AR selective antagonists) bound to rabbit vaginal membrane preparations with high affinity and limited capacity. Competition binding assays using both non-selective and subtype selective ligands for AR (phentolamine, prazosin, delequamine, rauwolscine and UK14304) further confirmed the presence of alpha-1 and alpha-2 AR in vaginal tissue. In organ bath preparations of vaginal tissue strips, norepinephrine-induced contraction was attenuated by alpha-1 and alpha-2 AR antagonists (prazosin, tamsulosin, delequamine and phentolamine). In anesthetized rabbits, intravaginal injection of the alpha-1 AR selective antagonist REC 15/2615 (50 and 100 microg/kg) caused a 2 to 3-fold increase in genital tissue oxyhemoglobin (OHb) concentration. Similar increases in tissue OHb were observed with intravaginal injection of phentolamine (500 microg/kg) or a tri-mixture of vasodilators (PGE1, papaverine, phentolamine). REC 15/2615, phentolamine or the tri-mixture also enhanced the amplitude and/or duration of change in genital tissue OHb after pelvic nerve stimulation. Thus, vaginal tissue expresses functional alpha-1 and alpha-2 AR, which modulate vaginal smooth muscle contractility and genital engorgement.

RhoA-mediated Ca2+ sensitization in erectile function

A Rho-kinase inhibitor increases corpus cavernosum (CC) pressure in an in vivo rat model (Chitaley, K., Wingard, C. J., Webb, R. C., Branam, H., Stopper, V. S., Lewis, R. W., and Mills, T. M. (2001) Nat. Med. 7, 119-122) suggesting that Rho-mediated Ca(2+) sensitization of CC smooth muscle maintains the flaccid (contracted) state. We directly demonstrate Ca(2+) sensitization of permeabilized rabbit and human CC and identify a highly expressed molecular component of this pathway. Ca(2+) sensitization of force induced by endothelin or GTPgammaS was significantly greater in CC than in rabbit ileum smooth muscle and was accompanied by a 17-fold higher RhoA content. Pull-down assays with the RhoA binding domain of mDia showed the high RhoA content of CC to be available for activation by GTPgammaS. Ca(2+) sensitization induced by endothelin, phenylephrine, or GTPgammaS was completely relaxed by the Rho kinase inhibitor Y-27632. Human and rabbit CC both express the phosphatase inhibitor CPI-17, the myosin phosphatase regulatory (MYPT-1) and catalytic (PP1delta) subunits, and two isoforms of Rho kinase. We suggest that high expression of RhoA contributes, through RhoA-mediated Ca(2+) sensitization, to the flaccid state of CC that can be reversed by a water-soluble, orally active Rho kinase inhibitor suitable for therapy of erectile dysfunction.