HPLC for urinary catecholamines and metanephrines with alpha-methyldopa.

In five healthy selected volunteers with normal blood pressure and one pheochromocytoma patient, high performance liquid chromatography (HPLC) has been evaluated, with electrochemical detection for quantitation of urinary catecholamines and metanephrines during administration of the antihypertensive, alpha-methyldopa. The clinical usefulness of HPLC is compared with that of the conventional assay method--the trihydroxyindole (THI)-fluorometric procedure. The THI fluorometric method is known to suffer from true false-positive interference as a result of its inability to differentiate between alpha-methyldopa, its primary metabolic derivatives, and the structurally similar endogenous catecholamines. It is shown that the HPLC separation methodology yields accurate, reproducible results devoid of interference from the presence of alpha-methyldopa. Free urinary excretion rates of epinephrine, norepinephrine, and dopamine were elevated by alpha-methyldopa, P less than 0.001, for epinephrine, norepinephrine, and dopamine when measured by the trihydroxyindole technique but not with high performance liquid chromatography. With alpha-methyldopa treatment, urinary normetanephrine excretion rates were slightly increased, P less than 0.05, by fluorometric analysis and slightly decreased. P less than 0.05, when measured by HPLC. Of added interest, the formation of the normetanephrine analog of alpha-methyldopa, previously undetected, is suggested. Slightly elevated metanephrine levels are seen by the THI-fluorometric method in the presence of alpha-methyl metanephrines. Establishing that the HPLC assay procedure is suitable for clinical diagnosis of pheochromocytoma, despite the presence of alpha-methyldopa, makes it unnecessary to discontinue use of this antihypertensive in screening for pheochromocytoma.