Studies on the mechanisms of the development of tolerance to the hypotensive effects of fenoldopam in rats.

In pentobarbital-anesthetized rats prepared for hemodynamic measurements with Doppler flow probes, intravenous (i.v.) infusions of fenoldopam (2.5 - 160.0 micrograms/kg/min during 15 min) decreased mean carotid artery blood pressure, total peripheral, hindquarter, renal, and mesenteric vascular resistances and increased renal blood flow strongly. The hypotensive effects attained a maximum within the first 3 min of infusion but waned by greater than 30% at the end of fenoldopam administration. This tolerance was observed for calculated total peripheral and hindquarter vascular resistances and to a lesser extent for mesenteric resistance. However, it was absent on the renal vascular bed. Pretreatment with either enalapril, pepstatine, or bilateral nephrectomy significantly increased the hypotensive response to fenoldopam and attenuated the development of tolerance. In conscious spontaneously hypertensive rats (SHR), enalapril potentiated strongly the small blood pressure-lowering activity of fenoldopam. The fall in blood pressure produced by fenoldopam was specifically blocked by SCH 23390, an antagonist of DA-1 dopamine receptors. In normotensive vasopressin-supported pithed rats given phenoxybenzamine plus propranolol, fenoldopam, like SCH 23390, blocked the vasodepressor effects of i.v. bolus injection of dopamine and fenoldopam. In pithed rats, fenoldopam evoked a pressor response that was significantly reduced by enalapril, SCH 23390, or bilateral nephrectomy. In conclusion, fenoldopam exerts DA-1 agonist and antagonist effects. The latter property, together with the activation of the renin-angiotensin system, appears to be responsible for the development of tolerance to the fenoldopam evoked-hypotension. The lack of a tolerance at the level of the renal vascular bed is possibly due to the existence of a large population of DA-1 receptors in this region.