Cardiovascular Characterization of DA-1 and DA-2 Dopamine Receptor Agonists in Anesthetized Rats

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Abstract
This report summarizes studies aimed to characterize pharmacologically, hemodynamically and biochemically DA-1 (fenoldopam) and DA-2 (quinpirole) dopamine receptor agonists in anesthetized rats. Fenoldopam (20 μg/kg/min i.v. over 15 min) and quinpirole (10 μg/kg/min i.v. over 15 min) share the common property of decreasing mean carotid artery blood pressure by lowering peripheral vascular resistance. Fenoldopam increased mesenteric and renal blood flows whereas quinpirole decreased the former blood flow, but enhanced the latter. These effects of quinpirole were antagonized selectively by S-sulpiride, but not SCH 23390; however, with fenoldopam the reverse was found. In chlorisondamine-pretreated rats with blood pressure supported by vasopressin, fenoldopam, but not quinpirole, caused hypotension. In nephrectomized rats, the blood pressure effects of fenoldopam (assessed as area under the infusion time-response curve) were more pronounced than in sham-operated controls. The hypotensive effects due to an i.v. bolus injection of fenoldopam, but not to acetylcholine, histamine, salbutamol or quinpirole, were significantly inhibited in rats pretreated with an infusion of fenoldopam. In pithed rats, quinpirole reduced the pressor responses to electrical stimulation of the spinal cord without affecting those to exogenous norepinephrine, angiotensin 11 or 5-hydroxytryptamine which, on the contrary, were inhibited by fenoldopam. The plasma renin activity (in intact rats) was reduced by quinpirole, but elevated by fenoldopam. The latter effect also occurred in pithed rats and was blocked by SCH 23390. Quinpirole lowered heart rate, whilst fenoldopam produced tachycardia. These effects of quinpirole and fenoldopam were significantly inhibited by S-sulpiride and SCH 23390, respectively. In chlorisondamine-pretreated rats quinpirole failed to change heart rate