Sympathetic Activation by Sildenafil

Background—Sildenafil citrate is an effective and widely prescribed therapy for erectile dysfunction. Little is known about the effects of sildenafil on neural control of the circulation or about the effects of sildenafil on neurocirculatory stress responses. Methods and Results—We studied 14 normal volunteers (age 3267 years) who were randomized in a double-blind crossover fashion to receive a single oral dose of sildenafil 100 mg or placebo on 2 separate study days. Blood pressure, heart rate, forearm vascular resistance, muscle sympathetic nerve activity, and plasma catecholamines were measured at baseline and at 30 and 60 minutes after sildenafil and after placebo administration. The effects of sildenafil and placebo on neural and circulatory responses to stressful stimuli (sustained handgrip, maximal forearm ischemia, mental stress, and the cold pressor test) were also evaluated. Blood pressure, heart rate, and forearm vascular resistance after sildenafil and placebo were similar. However, muscle sympathetic nerve activity increased strikingly after sildenafil (by 141626%, mean6SEM) compared with placebo (368%) (P50.006); plasma norepinephrine levels also increased by 3165 % after sildenafil administration (P50.004). Sympathetic nerve traffic during mental, physical, and cold stresses was 2- to 8-fold higher after sildenafil than with placebo (P,0.05). Conclusions—Sildenafil causes a marked increase in sympathetic activation, evident both at rest and during stressfu