The molecular basis of penile erection

β3 adrenergic receptor activation relaxes human corpus cavernosum and penile artery through a hydrogen sulfide/cGMP-dependent mechanism

Erectile function is a widely accepted indicator of systemic endothelial activity since from a clinical standpoint erectile dysfunction (ED) often precedes cardiovascular events. Recently it has been described a potential role for β₃ adrenoceptor in cardiovascular diseases emphasizing a possible development of new drugs. β₃ adrenoceptor stimulation relaxes human corpus cavernosum (HCC) strips in cyclic guanosine monophosphate (cGMP)-dependent and endothelium/nitric oxide (NO)-independent manner. Hydrogen sulfide (H₂S), along with NO, is another gaseous molecule involved in cardiovascular system and as a consequence also in penile erection. Cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE), the enzymes mainly responsible for H₂S biosynthesis, are constitutively expressed in HCC. CSE rather than CBS is more abundant in human penile tissue. Herein we investigated the involvement of H₂S pathway in β₃ adrenoceptor-induced relaxation in HCC and penile artery. Penile artery expresses both CSE and β₃ adrenoceptor. BRL37344, a β₃ selective agonist, relaxed HCC strips and penile artery rings and this effect was significantly reduced by CSE inhibition. Incubation of HCC and penile artery homogenate with BRL37344 significantly increased H₂S production. This effect was significantly reduced by the inhibition of either CSE or β₃ adrenoceptor. Finally, the BRL37344-induced increase in cGMP was reduced by CSE inhibition in both tissues. Thus, BRL37344-induced relaxation in HCC and penile artery occurs in a H₂S/cGMP-dependent manner. In conclusion, β₃/H₂S/cGMP pathway can act as an alternative to NO. Since about 15% of patients do not respond to phosphodiesterase-5 inhibitors, β₃ agonists could represent a therapeutic alternative or a useful adjuvant therapy to treat these patients.

Diabetes, hypertension and erectile dysfunction

Diabetes mellitus (DM) and hypertension are independent risk factors for erectile dysfunction (ED), macrovascular disease and microangiopathy. ED is very common among diabetic patients. Men with DM have ED at an earlier age and with a significantly higher prevalence (as high as 75%). The prevalence of DM also tends to be higher in patients with Peyronie's disease. DM impairs neurogenic and endothelium-mediated relaxation of penile smooth muscle. It is impossible to separate DM from hypertension and from the other vascular risk factors. Good glycaemic and hypertension control in diabetics is very important since these factors increase the risk of both microvascular and macrovascular complications, possibly including ED.