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The effects of systemic administration of the selective dopamine1 receptor agonist fenoldopam and the selective dopamine2 receptor agonist quinpirole on blood pressure and regional haemodynamics were investigated in anaesthetized normotensive Wistar rats. Both compounds produced dose-dependent reductions in blood pressure. Mesenteric and renal blood flow were enhanced by fenoldopam, but reduced by quinpirole. Hindquarter blood flow was not modified by fenoldopam, but was increased by quinpirole. The calculated vascular resistances were reduced by both compounds in the three vascular beds. The effects of fenoldopam were antagonized by SCH 23390 but SCH 23390 did not affect those of quinpirole. The effects of quinpirole, but not those of fenoldopam, were antagonized by domperidone. Hexamethonium abolished the effects of quinpirole without affecting those of fenoldopam. These results indicate that the hypotensive effects of fenoldopam and quinpirole are due to stimulation of postsynaptic dopamine1 and neuronal dopamine2 receptors, respectively, resulting in differential regional haemodynamic effects.

The effect of fenoldopam, a selective DA1-agonist, on the plasma aldosterone response to metoclopramide was studied in six hypertensive patients included in a multicentre double-blind placebo controlled cross-over study of the antihypertensive effects of fenoldopam. Fenoldopam significantly increased baseline plasma renin activity (PRA); baseline plasma aldosterone levels rose slightly. Baseline PRL and the PRL response to metoclopramide were not altered by fenoldopam. After metoclopramide, a significant increase of plasma aldosterone was observed during treatment with fenoldopam, as well as in the placebo-period. The peak values were not significantly different and occurred at 15 min during both treatment periods. These results indicate that fenoldopam does not reduce metoclopramide-induced aldosterone secretion and therefore suggest that the adrenal dopamine receptor is not identical to the vascular DA1 receptor.