In contrast to phenoxybenzamine, phentolamine [fen-TOLE-a-meen] produces a competitive block of α1 and α2 receptors. The drug's action lasts for approximately 4 hours after a single administration. Like phenoxybenzamine, it produces postural hypotension and causes epinephrine reversal. Phentolamine-induced reflex cardiac stimulation and tachycardia are mediated by the baroreceptor reflex and by blocking the α2 receptors of the cardiac sympathetic nerves. The drug can also trigger arrhythmias and anginal pain, and it is contraindicated in patients with decreased coronary perfusion. Phentolamine is also used for the short-term management of pheochromocytoma. Phentolamine is now rarely used for the treatment of impotence (it can be injected intracavernosally to produce vasodilation of penile arteries.

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