Yohimbine is an alpha 2-adrenoceptor antagonist that is FDA approved for treatment of
impotence. The
drug is an indolalkylamine
alkaloid chemically similar to
reserpine and is believed to act through sympatholysis. We examined effects of oral
yohimbine on blood pressure (BP) and plasma levels of
catechols in patients with
essential hypertension, a condition in which most
drug treatments can produce
impotence. In 25 unmedicated hypertensive subjects, vital signs were measured and blood samples were obtained through an indwelling antecubital venous
catheter at baseline and 1 and 2 h after subjects ingested 4 5.4-mg
tablets of
yohimbine. Mean blood pressure (MBP) increased by an average of 5 mm Hg (p < 0.01), plasma
norepinephrine (NE) levels increased by 66% (p < 0.001), and plasma dihydroxyphenylglycol (
DHPG) levels increased by 25% (p < 0.01) at 1 h after
drug administration. The magnitude of the pressor response was unrelated to baseline MBP but positively correlated with the baseline NE level (r = 0.61, p < 0.01) and with the
yohimbine-induced increment in plasma NE (r = 0.4, p < 0.01). The results indicate that
yohimbine does not inhibit and actually stimulates sympathetically mediated NE release in humans and that the increased NE release produces a pressor response.
Yohimbine should be administered with caution to patients with high BP, especially in individuals with evidence for increased basal sympathetic outflow or those undergoing concurrent treatment with
tricyclic antidepressants or other drugs that interfere with neuronal uptake or metabolism of NE.