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Introduction Receptors for natriuretic peptides have been demonstrated as potential targets for the treatment of male erectile dysfunction. Aim This study investigates the relaxant effects of the atrial natriuretic peptide (ANP) and uroguanylin (UGN), and expression of natriuretic peptide receptors on strips of human corpora cavernosa (HCC). Main Outcome Measures Quantitative analysis of natriuretic receptor expression and relaxation of precontracted strips were used to assess the membrane-bound guanylate cyclase–cyclic guanosine monophosphate (cGMP) pathway in HCC strips. Methods HCC was obtained from a cadaver donor at the time of collection of organs for transplantation (14–47 years) and strips were mounted in organ baths for isometric studies. Results ANP and UGN both induced concentration-dependent relaxation on HCC strips with a maximal response attained at 300 nM, corresponding to 45.4 ± 4.0% and 49 ± 4.8%, respectively. The relaxation is not affected by 30 µM 1H-[1,2,4]oxaolodiazolo[4,3-a]quinoxalin-1-one (ODQ) (a soluble guanylate cyclase inhibitor), but it is significantly blocked by 10 µM isatin, a nonspecific particulate guanylate cyclase (pGC) inhibitor. UGN was unable to potentiate electrical field stimulation (EFS) or acetylcholine-induced relaxations. The potential role of pGC activation and cGMP generation in this effect is reinforced by the potentiation of this effect by phosphodiesterase-5 inhibitor vardenafil (55.0 ± 7.5-UGN vs. 98.6 ± 1.4%-UGN+vardenafil; P <0.05). The relaxant effect was also partially (37.6%) blocked by the combination iberitoxin-apamin but was insensitive to glybenclamide. The expression of guanylate cyclase receptors (GC-A, GC-B, GC-C) and the expression of the natriuretic peptide “clearance” receptor (NPR-C) were confirmed by real-time polymerase chain reaction. The exposure of HCC strips to ANP (1 µM) and UGN (10 µM) significantly increased cGMP, but not cyclic adenosine monophosphate (cAMP) levels. Conclusions UGN relaxes HCC strips by a guanylate cyclase and Kca-channel-dependent mechanism. These findings obtained in HCC reveal that the natriuretic peptide receptors are potential targets for the development of new drugs for the treatment of erectile dysfunction.

The effect of terpinen-4-ol was studied on rabbit duodenum in-vitro. Terpinen-4-ol induced relaxation of the basal tonus (IC50 170.2 (95% confidence interval, 175-204) microM) with a maximal relaxant response of 180.4+/-3.9% (n=6) of the contraction induced by 60 mM [K(+)]. The maximal relaxation induced in control conditions was not affected (P>0.05) by pretreatment of the tissues with phentolamine (50 microM) or propranolol (10 microM), N(G) nitro-L-arginine methyl ester (L-NAME; 1 mM), 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 100 microM), hexamethonium (1 mM), tetrodotoxin (1 microM), the mixture charybdotoxin-apamin (1 microM), glibenclamide (10 microM), 4-aminopyridine (10 microM) or tetraethyl-ammonium (100 microM). In addition, terpinen-4-ol completely relaxed tissues precontracted with 60 mM [K(+)] solutions (IC50 325.9 (245.1-433.1) microM) and also blocked (IC50 154.7 (117.7-191.7) microM) the phasic component of this contraction. At a concentration of 195 and 650 muM it reduced by 41.3+/-3.4% and 75.4+/-3.1%, respectively the maximal contractile response to Ca(2+) in depolarized duodenum. Terpinen-4-ol completely blocked the component of carbachol-induced contraction, which was resistant to nifedipine (100 microM) pretreatment or to a Ca(2+)-free solution. These data show that terpinen-4-ol relaxes intestinal smooth muscle and suggest that this effect is myogenic in nature and depends on calcium antagonism.

We described earlier that an alkaloid-rich fraction (F 3-5 ) from Aspidosperma ulei (Markgr) induces penile erection-like behavioral responses in mice. This study verified a possible relaxant effect of this fraction on isolated rabbit corpus cavernosum (RbCC) strips precontracted by phenylephrine (1 lM) or K þ 60 mM. F 3-5 (1-300 lg ml À1 ) relaxed the RbCC strips in a concentration-dependent and reversible manner. The relaxant effect of F 3-5 (100 lg ml À1 ) on phenylephrine contraction was unaffected in the presence of atropine, N-x-nitro-L-arginine methyl ester or 1H-[1,2,4]oxadiazole[4,3-a] quinoxalin-1-one and by preincubation with tetrodotoxin, glibenclamide, apamine and charybdotoxin suggesting that mechanisms other than cholinergic, nitrergic, sGC activation or potassium channel opening are probably involved. However, the phasic component of the contraction induced by K þ 60 mM as well as the maximal contraction elicited by increasing external Ca 2 þ concentrations in depolarized corpora cavernosa was inhibited by F 3-5 . We conclude that F 3-5 relaxes the RbCC smooth muscle, at least in part, through a blockade of calcium influx or its function.

Compounds with dual action on cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) may be a treatment option for erectile dysfunction, as they not only promote penile erection but also prevent the upregulation of phosphodiesterase-5. In this study, we examined the possible relaxant effect and mechanism of 17-nor-subincanadine E (SEC, 0.2-200 mmol l 21 ), a plant-derived alkaloid, in rabbit corpus cavernosum (RbCC) strips that had been precontracted by exposure to phenylephrine (10 mmol l 21 ) or a high concentration of K 1 (60 mmol l 21 ) in vitro. In addition to SEC's effect on cAMP and cGMP levels, electrical field stimulation (EFS) in phenylephrine-precontracted RbCC and calcium chloride (1-100 mmol l 21 ) evoked responses in depolarized RbCC were analysed. SEC relaxed the phenylephrine-precontracted RbCCs in a concentration-dependent manner. Atropine, guanethidine and N-v-nitro-L-arginine methyl ester (L-NAME) did not have any effect on the relaxation of RBCCs. When 1H-1,2,4 oxadiazole[4,3-a] quinoxalin-1-one (ODQ) was added, it effectively blocked the relaxant response of SEC. Although SEC enhanced the maximal relaxation produced by sodium nitroprusside (SNP) and forskolin in phenylephrine-precontracted cavernosal smooth muscle, it caused a decrease in the maximal contractile response induced by calcium chloride in depolarized RbCCs. The relaxant effect of SEC was paralleled by an increase in the tissue levels of the cyclic nucleotides cAMP and cGMP. We conclude that SEC promotes the relaxation of RbCC, possibly favouring cAMP and cGMP accumulation and calcium blockade. This novel mechanism could be useful for patients who do not benefit from phosphodiesterase inhibitors and for those with endothelial and nitrergic dysfunction, such as patients with diabetes, hypertension and dyslipidaemias. Asian Journal of Andrology (2011) 13, 747-753; doi:10.1038/aja.2011.41; published online 18 July 2011Keywords: 17-nor-subincanadine E; cyclic nucleotides; electrical field stimulation; nitric oxide; rabbit corpus cavernosum; smooth muscle relaxation INTRODUCTIONThe erect penis has always been a symbol of power, virility and fertility. Erectile dysfunction (ED) is the consistent inability to achieve and maintain an erection sufficient for satisfactory sexual activity. This condition is a complex neurovascular disorder that affects 52% of men aged 40-70 years and causes considerable distress, unhappiness and relationship problems for these men. 1,2 Penile erection occurs as a function of trabecular smooth muscle relaxation and a subsequent increase in blood flow to lacunar spaces, which results in engorgement of the penis and restriction of venous drainage. Basically, the process is mediated by nitric oxide (NO) release from cavernosal nerve varicosities and endothelial cells, and this release activates intracellular guanylate cyclase to produce cGMP. 3 Recently, the selective phosphodiesterase 5 (PDE5) inhibitors sildenafil, tadalafil and vardenafil were shown to enhance the proerectile effect ...