Abstract. Animal studies suggest that endothelin A (ETA) receptor antagonism and angiotensin-converting enzyme (ACE) inhibition may be synergistic. This interaction and the role of ETB receptors and endothelial mediators were investigated in terms of systemic and renal effects in humans in two studies. In one study, six subjects received placebo, the ETA receptor antagonist BQ-123 alone, and BQ-123 in combination with the ETB receptor antagonist BQ-788 after pretreatment with the ACE inhibitor enalapril (E) or placebo. In the other, six subjects who were pretreated with E received placebo, BQ-123, and BQ-123 with concomitant inhibition of nitric oxide (NO) synthase or cyclo-oxygenase (COX). Both were randomized, double-blind, crossover studies. Mean arterial pressure was reduced by BQ-123, an effect that was doubled during ACE inhibition (mean area under curve Ϯ SEM; BQ-123, Ϫ2.3 Ϯ 1.8%; BQ-123ϩE, Ϫ5.1 Ϯ 1.1%; P Ͻ 0.05 versus placebo). BQ-123 increased effective renal blood flow (BQ-123, Ϫ0.1 Ϯ 2.4%; BQ-123ϩE, 10.9 Ϯ 4.2%; P Ͻ 0.01 versus BQ-123), reduced effective renal vascular resistance (BQ-123, Ϫ1.2 Ϯ 3.1%; BQ-123ϩE, Ϫ12.8 Ϯ 3.0%; P Ͻ 0.01 versus placebo and versus BQ-123), and increased urinary sodium excretion markedly (BQ-123, 2.6 Ϯ 12.8%; BQ-123ϩE, 25.2 Ϯ 12.6%; P Ͻ 0.05 versus BQ-123, P Ͻ 0.01 versus placebo and versus E) only during ACE inhibition. These effects were abolished by both ETB receptor blockade and NO synthase inhibition, whereas COX inhibition had no effect. In conclusion, the combination of ETA receptor antagonism and ACE inhibition is synergistic via an ETB receptormediated, NO-dependent, COX-independent mechanism. The reduction of BP and renal vascular resistance and associated substantial natriuresis make this a potentially attractive therapeutic combination in renal disease.Endothelin-1 (ET-1) is a vasoactive peptide that is produced by the vascular endothelium (1) and acts through two receptors to regulate vascular tone. Vascular smooth muscle ETA and ETB receptors (2,3) mediate vasoconstriction, whereas endothelial ETB receptors mediate vasodilation through generation of nitric oxide (NO) and prostanoids (3). Angiotensin II (Ang II) is another powerful vasoconstrictor involved in the regulation of vascular tone, and there is considerable evidence for an interaction between the endothelin and renin-angiotensin systems (4). Ang II increases ET-1 transcription and secretion in vitro in a variety of cell types, including endothelial and vascular smooth muscle cells (5,6), and ET receptor antagonists attenuate the acute hemodynamic effects of Ang II in rats in vivo (7,8). However, this is by no means a uniform finding (9) and has not been replicated in dogs (10) or humans (11). Data in animals suggest that concomitant ET blockade and angiotensin-converting enzyme (ACE) inhibition produce changes greater than those seen with blockade of either system alone (12-15). In addition, clinical studies that demonstrated major hemodynamic effects of ET receptor antagonists have generally been per...