Erectile dysfunction (ED) is a widespread condition, the incidence of which is increasing globally. ED is also indicative of underlying vasculopathy and represents a predictor of more serious cardiovascular disorders. Understanding the aetiology of ED may therefore provide invaluable pointers to the pathobiology of other cardiovascular diseases (CVDs) and syndromes. It follows, too, that therapeutic interventions that are successful in treating ED may, ipso facto, be effective in treating the early stages of conditions that include atherosclerosis, angina, plaque rupture and diabetic angiopathy. One common pathological denominator in both CVD and ED is oxidative stress, that is, the overproduction of reactive oxygen species (ROS), in particular, superoxide (O 2 KÀ ) and hydrogen peroxide (H 2 O 2 ). In this review, therefore, we consider the aetiology and pathobiology of O 2 KÀ in promoting ED and focus on NADPH oxidase as an inducible source of O 2 KÀ and H 2 O 2 . Therapeutic strategies aimed at reducing oxidative stress to improve erectile function are also discussed.

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OBJECTIVES To determine whether there is an association between vascular phosphodiesterase type 5 (PDE‐5) and NADPH oxidase (NOX) in cavernosal vascular smooth muscle cells (CVSMCs), and to study the actions of the PDE‐5 inhibitor sildenafil; the pro‐erectile actions of nitric oxide (NO) are reduced by PDE‐5 which hydrolyses cGMP to inactive GMP, thus an up‐regulation of PDE‐5 and over‐production of O2‐ derived from NOX might promote erectile dysfunction (ED). MATERIALS AND METHODS To study the effects of nicotine and tumour necrosis factor‐α (TNF‐α) on superoxide (O2‐) production and PDE‐5 expression, CVSMCs from rabbit penis were incubated with nicotine or TNF‐α, and superoxide dismutase (SOD), catalase, sildenafil citrate, or apocynin (NADPH inhibitor) for 16 h. The expression of PDE‐5 and of glyceraldehyde‐3‐phosphate dehydrogenase (internal standard) was assessed using Western blotting. O2‐ was measured spectrophotometrically. RESULTS After a 16‐h incubation, both nicotine (maximal at 10 µm) and TNF‐α (10 ng/mL) significantly increased O2‐ formation in CVSMCs; this effect was blocked by co‐incubating with SOD, catalase, and sildenafil (1 µm). Apocynin also inhibited O2‐ formation when added after 16‐h incubation with nicotine (10 µm) or TNF‐α. PDE‐5 expression was also significantly increased in CVSMCs incubated with nicotine and TNF‐α. This effect was negated by 16‐h co‐incubation with SOD, catalase, apocynin, and sildenafil. CONCLUSIONS Nicotine and TNF‐α up‐regulate PDE‐5 expression in CVSMCs through an a priori up‐regulation of NOX and formation of O2‐. As PDE‐5 hydrolyses cGMP, this effect might ‘blunt’ the pro‐erectile actions of NO. Sildenafil inhibits O2‐ formation, and ‘normalizes’ PDE‐5 expression. This represents a novel pathogenic mechanism underlying ED, and a novel mechanism of action of sildenafil.