Free and Conjugated Dopamine in Pheochromocytoma, Primary Aldosteronism and Essential Hypertension

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SUMMARY Since the hypotensive and natrluretic role recently attributed to dopamine (DA) is difficult to test (free DA is usually undetectable in plasma) we measured free and conjugated DA by a newly developed method of hydrolysis of conjugated catecholamlnes in four pheochromocytoma, four primary aldosteronism and 34 essential hypertensive patients subjected to selective venous catheterization or peripheral venous sampling. The 16 control subjects had plasma DA of 0.98 ± 0.1 ng/ml (SE), almost 100% of which was conjugated, and a urinary excretion of 2 ± 03 fig/min of DA, 79% of which was conjugated.

Patients with a predominantly norepinephrine-secreting pheochromocytoma had elevated conjugated plasma DA (6 ± 0.S vs 2.5 ± 03 ng/ml in non-pheochromocytoma hypertensive patients) even between paroxysms; in a rare case an alternating DA and epinephrine hypersecretion was detected during predominantly normotensive paroxysms, one of which was associated with the flooding of the peripheral circulation by up to 5.5 ng/ml of free DA.

Patients with primary aldosteronism had elevated total plasma DA (3.1 ± 0 3 ng/ml), detectable free DA (0.4 ± 0.2 ng/ml) and an elevated total urinary DA excretion (53 ± 1.5 /ig/min). Following the removal of the aldosteronoma, urinary excretion of total or free DA decreased to normal levels, except in one patient with a bilateral micronodular hyperplasia in whom the hyperaldosteronism persisted after unilateral adrenalectomy. Patients with essential hypertension bad elevated conjugated plasma DA (1.98 ± 03 ng/ml), probably originating in an adrenal outflow of free and conjugated DA, but lower free and total urinary DA excretion than patients with primary aldosteronism (0.19 ± 0.04 vs 0.61 ± 0.01 for free DA and 0.9 ± 0.04 vs 5 3 ± 1.5 Mg/min for total urinary DA). Measurements of conjugated DA, which probably reflects the release of free rapidly conjugated DA, complement the free DA determination in hypertension. Elevated plasma conjugated DA probably mirrors the endogenous autonomous dopamine release in pheochromocytoma and has an adaptive character in primary aldosteronism and essential hypertension. Urinary free and conjugated DA excretion appears to reflect the dopamine modulated natriuresls, modifying the sodium-retaining action of aldosterone.

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