Human erectile tissue has a population of membrane receptors that are predominantly of the a-adrenoceptor subtype. Phentolamine (Vasomax) is a nonselective a-adrenoceptor blocking agent (see Chapter 11), and like other such agents, it has been used to treat ED. Nonselective adrenoceptor antagonists may provoke a reflex that increases both sympathetic outflow and the release of norepinephrine.
Phentolamine has been used orally and intracaver-nosally in the treatment of ED. Following oral administration, phentolamine has a plasma half-life of about 30 minutes and a duration of action of 2 to 4 hours. An in-tracavernosal injection of phentolamine results in the drug reaching maximum serum levels in about 20 to 30 minutes. It is rapidly metabolized.
Phentolamine has been used in combination with papaverine, chlorpromazine, and vasoactive peptides in the treatment of ED.
Side effects of phentolamine are dose related. It may cause orthostatic hypotension, reflex tachycardia, cardiac arrhythmias, and rarely, myocardial infarction. Phentolamine also may reduce sperm motility in vitro.
Other a-adrenoceptor receptor antagonists include yohimbine, phenoxybenzamine, and thymoxamine. Yo-himbine is an a2-adrenoceptor antagonist, and thymox-amine is a competitive and relatively selective blocking agent for a1- adrenoceptors. Phenoxybenzamine blocks both a1- and a2-adrenoreceptors, although it has a greater affinity for the a1-subtype. All three of these a-receptor blocking drugs can induce penile erection, but their effects are generally less consistent and less effective than those of phentolamine. Yohimbine is only moderately effective in treating patients with organic impotence, and side effects may include postural hypotension, heart palpitations, fine tremors, and cavernosal fibrosis, especially following intracavernosal injections.
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