Hemodynamic effects of selective dopamine receptor agonists in the rat and dog

The availability of highly selective dopamine receptor agonists has allowed the characterization of the role of DA1 and DA2 receptors in the cardiovascular system. Fenoldopam (SK&F 82526) is a potent agonist at DA1 receptors, with much less agonist activity at the DA2 subtype or at alpha and beta adrenoceptors. In contrast, SK&F 89124 activates only the DA2 subtype. SK&F 85174, the N-allyl derivative of fenoldopam, retains potent DA1 agonist activity but also has moderately potent agonist activity at the DA2 receptor. All three compounds will reduce blood pressure in hypertensive rats. In the anesthetized dog, each agonist will reduce blood pressure and total peripheral resistance. The overall hemodynamic profile is remarkably similar, despite the marked difference in dopamine receptor subtype selectivity. The principal pharmacologic difference is enhanced bradycardia with the compounds having DA2 agonist activity, resulting from activation of neuroinhibitory DA2 receptors on cardiac sympathetic nerve terminals. In the dog, each compound will increase renal blood flow. Studies in the anesthetized rat with fenoldopam and SK&F 89124, using radiolabelled microspheres to measure blood flow to various vascular beds, also show a significant increase in renal flow, with a tendency toward increased blood flow in the splenic and intestinal beds. Hence, dopamine receptor agonists offer a useful approach to cardiovascular therapy via DA1 mediated vascular dilation, DA2 mediated modulation of sympathetic tone or a combination of both activities.