Effect of dexamethasone on plasma free dopamine: dopaminergic modulation in hypertensive patients

To investigate the peripheral dopaminergic modulation in the pathogenesis of human hypertension, we examined the responses of plasma free dopamine (DA) to dexamethasone (Dx) administration, which is suggested to activate dopaminergic activity. We administered Dx 2 mg intravenously to patients with primary aldosteronism (PA), essential hypertension (EH), and normotensive controls (NT). Plasma free DA was increased significantly in all groups and the responses were more remarkable in PA than in EH and NT. Plasma epinephrine (E) showed a gradual increase while plasma norepinephrine (NE) tended to decrease in all groups. The responses of both plasma DA and E were completely blocked by 250 mg of alpha-methyl-p-tyrosine, a tyrosine hydroxylase (TH) inhibitor, suggesting that Dx may stimulate peripheral dopaminergic activity by increasing catecholamine synthesizing enzyme (probably TH) activities. These data suggest that DA itself plays an inherent role in the sympathoadrenal regulation rather than only as a precursor of NE and that dopaminergic hyperresponses may be involved in the pathophysiology of PA.