Endopeptidase inhibition attenuates the contraction induced by big endothelin-1 of isolated human penile erectile tissue

Peptides, such as C-type natriuretic peptide (CNP), vasoactive intestinal polypeptide (VIP) and endothelin 1 (ET-1), are involved in the control of penile erectile tissue (corpus cavernosum = CC). Inhibiting the degradation of CNP and VIP or conversion of Big ET-1 into ET-1 by endopeptidase enzymes should result in an enhancement of CC smooth muscle relaxation. Using the tissue bath technique, responses of isolated CC, challenged by noradrenaline (NA, 1 μm), to increasing concentrations of the endopeptidase inhibitor KC 12615 (1 nm - 10 μm), CNP and VIP (0.1 nm - 1 μm), were investigated. Effects of CNP, VIP and Big ET-1 (0.1 nm - 100 nm) on the tissue tension were also evaluated following pre-exposure to 10 μm of KC 12615. Big ET-1 induced contraction of the CC amounting to a force generation of 1,200 mg. The contraction was attenuated in the presence of KC 12615 by 35% and 50%, respectively. The tension induced by NA was reversed by VIP and CNP to 38.7% ± 15.8% and 61% ± 13%, respectively, of the initial force. The findings might be of significance with regard to future pharmacological treatment options for male ED, where an endothelial dysfunction exists.

Characterisation of pomegranate juice effects on human corpus cavernosum

Pomegranate (POM) juice may benefit the erectile process, but the scientific evidence is lacking. This study evaluates the molecular characterisation and confirmation of POM's action on human corpus cavernosum (HCC) obtained from patients (n = 16) undergoing penile prosthesis implantation. After phenylephrine contraction, the relaxant effects of POM with various inhibitors in the presence and absence of palmitic acid (PA)-induced acute oxidative stress were investigated. Electrical field stimulation (EFS)- and acetylcholine (ACh)-induced relaxation were performed using organ bath preparation. Expression of neuronal nitric oxide synthase (nNOS), endothelial (eNOS), phosphodiesterase (PDE)-5A and cGMP levels were assessed in cells from ex vivo organ cultures of HCC, using RT-PCR, ELISA and immunohistochemistry techniques. POM induced marked relaxation of HCC (maximum response: 97.0 ± 3.1%) and reversed the PA-induced decrease of EFS (20 Hz). nNOS transcription was increased by 7-fold in POM-treated cells without influencing eNOS and PDE5A expressions. We conclude that POM induced marked relaxation of HCC via: (i) nNOS stimulation, and (ii) downstream relaxation stimulated by nNOS and cGMP and bypassing the NO and PDE5. This action provides a rationale for the therapeutic or preventative use of POM in men with erectile dysfunction who do not respond well to PDE5 inhibitors.

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.

Ca2+ -activated K+ channel (KCa) stimulation improves relaxant capacity of PDE5 inhibitors in human penile arteries and recovers the reduced efficacy of PDE5 inhibition in diabetic erectile dysfunction

BACKGROUND AND PURPOSE

We have evaluated the influence of calcium-activated potassium channels (KCa ) activation on cGMP-mediated relaxation in human penile tissues from non-diabetic and diabetic patients, and on the effects of PDE5 inhibitors on erectile responses in control and diabetic rats.

EXPERIMENTAL APPROACH

Cavernosal tissues were collected from organ donors and from patients with erectile dysfunction (ED). Relaxations of corpus cavernosum strips (HCC) and penile resistance arteries (HPRA) obtained from these specimens were evaluated. Intracavernosal pressure (ICP) increases to cavernosal nerve electrical stimulation were determined in anaesthetized diabetic and non-diabetic rats.

KEY RESULTS

Concentration-dependent vasodilation to the PDE5 inhibitor, sildenafil, in HPRA was sensitive to endothelium removal, NO/cGMP pathway inhibition and KCa blockade. Accordingly, activation of KCa with NS-8 (10 μM) significantly potentiated sildenafil-induced relaxations in HPRA (EC50 0.49 ± 0.22 vs. 5.21 ± 0.63 μM). In HCC, sildenafil-induced relaxation was unaffected by KCa blockade or activation. Potentiating effects in HPRA were reproduced with an alternative PDE5 inhibitor (tadalafil) and KCa activator (NS1619) and prevented by removing the endothelium. Large-conductance KCa (BK) and intermediate-conductance KCa (IK) contribute to NS-8-induced effects and were immunodetected in human and rat penile arteries. NS-8 potentiated sildenafil-induced enhancement of erectile responses in rats. Activation of KCa recovered the impaired relaxation to sildenafil in diabetic HPRA while sildenafil completely reversed diabetes-induced ED in rats only when combined with KCa activation.

CONCLUSIONS AND IMPLICATIONS

Activation of KCa improves vasodilatory capacity of PDE5 inhibitors in diabetic and non-diabetic HPRA, resulting in the recovery of erectile function in diabetic rats. These results suggest a therapeutic potential for KCa activation in diabetic ED.

Calcium dobesilate potentiates endothelium-derived hyperpolarizing factor-mediated relaxation of human penile resistance arteries

1 We have evaluated the participation of endothelium-derived hyperpolarizing factor (EDHF) in the endothelium-dependent relaxation of isolated human penile resistance arteries (HPRA) and human corpus cavernosum (HCC) strips. In addition, the effect of the angioprotective agent, calcium dobesilate (DOBE), on the endothelium-dependent relaxation of these tissues was investigated. 2 Combined inhibition of nitric oxide synthase (NOS) and cyclooxygenase (COX) nearly abolished the endothelium-dependent relaxation to acetylcholine (ACh) in HCC, while 60% relaxation of HPRA was observed under these conditions. Endothelium-dependent relaxation of HPRA resistant to NOS and COX inhibition was prevented by raising the extracellular concentration of K(+) (35 mM) or by blocking Ca(2)(+)-activated K(+) channels, with apamin (APA; 100 nM) and charybdotoxin (CTX; 100 nM), suggesting the involvement of EDHF in these responses. 3 Endothelium-dependent relaxation to ACh was markedly enhanced by DOBE (10 micro M) in HPRA but not in HCC. The potentiating effects of DOBE on ACh-induced responses in HPRA, remained after NOS and COX inhibition, were reduced by inhibition of cytochrome P450 oxygenase with miconazole (0.3 mM) and were abolished by high K(+) or a combination of APA and CTX. 4 In vivo, DOBE (10 mg kg(-1) i.v.) significantly potentiated the erectile responses to cavernosal nerve stimulation in male rats. 5 EDHF plays an important role in the endothelium-dependent relaxation of HPRA but not in HCC. DOBE significantly improves endothelium-dependent relaxation of HPRA mediated by EDHF and potentiates erectile responses in vivo. Thus, EDHF becomes a new therapeutic target for the treatment of erectile dysfunction (ED) and DOBE could be considered a candidate for oral therapy for ED.

Regulation of human penile smooth muscle tone by prostanoid receptors

We have characterized the prostanoid receptors involved in the regulation of human penile arterial and trabecular smooth muscle tone. Arachidonic acid induced relaxation of human corpus cavernosum strips (HCCS) that was blocked by the cyclo-oxygenase inhibitor, indomethacin, and augmented by the thromboxane receptor (TP) antagonist, SQ29548, suggesting that endogenous production of prostanoids regulates penile smooth muscle tone. TP-receptors mediate contraction of HCCS and penile resistance arteries (HPRA), since the agonist of these receptors, U46619, potently contracted HCCS (EC50 8.3+/-2.8 nM) and HPRA (EC50 6.2+/-2.2 nM), and the contractions produced by prostaglandin F(2alpha) at high concentrations (EC50 6460+/-3220 nM in HCCS and 8900+/-6700 nM in HPRA) were inhibited by the selective TP-receptor antagonist, SQ29548 (0.02 microM). EP-receptors are responsible for prostanoid-induced relaxant effects in HCCS because only prostaglandin E1 (PGE1), prostaglandin E2 and the EP2/EP4-receptor agonist, butaprost, produced consistent relaxation of this tissue (EC50 93.8+/-31.5, 16.3+/-3.8 and 1820+/-1284 nM, respectively). In HPRA, both prostacyclin and PGE1 (EC50 60.1+/-18.4 and 109.0+/-30.9 nM, respectively) as well as the selective IP receptor agonist, cicaprost, and butaprost (EC50 25.2+/-15.2 and 7050+/-6020 nM, respectively) caused relaxation, suggesting co-existence of IP- and EP-receptors (EP2 and/or EP4). In summary, endogenous production of prostanoids may regulate penile smooth muscle contractility by way of specific receptors. TP-receptors mediate contraction in HCCS and HPRA, while the relaxant effects of prostanoids are mediated by EP2- and/or EP4-receptors in HCCS and by EP- and IP-receptors in HPRA.

Tachykinin NK1 and NK2 receptors mediate inhibitory vs excitatory motor responses in human isolated corpus cavernosum and spongiosum

Motor effects produced by tachykinins were studied in human isolated corpus spongiosum and cavernosum. In quiescent preparations neurokinin A caused potent contractions (pD(2)=8.3 - 7.9 respectively) prevented by the NK(2) receptor-selective antagonist nepadutant, whereas [Sar(9)]SP sulfone and senktide (NK(1) and NK(3) receptor-selective agonists) produced no effect or spare contractions. In KCl-precontracted corpus spongiosum septide (pD(2)=7.1) and [Sar(9)]SP sulfone (pD(2)=7.7) produced tetrodotoxin-resistant relaxations, abolished by the tachykinin NK(1) receptor-selective antagonist SR 140333. [Sar(9)]SP sulfone (1 microM) produced similar relaxations in precontracted corpus cavernosum. Electrical field stimulation (EFS) elicited tetrodotoxin-sensitive relaxations, which were additive to those produced by [Sar(9)]SP sulfone. N(omega)-nitro-L-arginine (L-NOARG) totally prevented both [Sar(9)]SP sulfone- and EFS-induced relaxations. These results show that tachykinin NK(1) and NK(2) receptors mediate opposite motor effects in human penile tissues, suggesting a possible modulatory role of tachykinins on smooth muscle tone in these organs.