The vast majority of pheochromocytomas secrete solely or almost entirely norepinephrine. Successful medical therapy depends on providing sufficient α-adrenergic blockade to diminish BP, prevent paroxysms, and allow intravascular volume expansion. The two agents most commonly used for this purpose are phentolamine mesylate and phenoxybenzamine hydrochloride. Phentolamine is a competitive antagonist to norepinephrine.1It has a short half-life and must be given frequently, with the patient being awakened to receive medication. Nausea, vomiting, and nasal congestion are common side effects. In addition, resistance to the drug frequently develops, making it poorly suited for long-term oral therapy.2The basis for this resistance is unclear, but it may perhaps be explained by the recent observation that there exist two different α-receptors within each synapse,3rather than just one as formerly postulated. See also p 2321. α1-Receptors are postsynaptic and when stimulated result in vasoconstriction, increased vascular resistance, and
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Modlinger Rs (1983) studied this question.