BACKGROUND Endothelin-1 (ET-1) is an endothelium-derived vasoconstrictor peptide. Controversy persists regarding the predominant ET receptor that mediates coronary vasoconstriction at pathophysiological concentrations. The aim of the present study was to test the hypothesis that ET mediates local coronary vasoconstriction via the ET-A receptor at low concentrations of exogenous ET-1 designed to mimic pathophysiological states compared with pharmacological concentrations. RESULTS ET-1 (group 1, n = 5) or sarafotoxin, a specific ET-B receptor agonist (group 3, n = 6) (each at 2 ng/kg per minute), was infused into the left circumflex coronary artery in the anesthetized dog. In group 2 dogs (n = 5), the same dose of ET-1 was infused with 4 micrograms/kg per minute of the specific ET-A receptor antagonist FR-139317. In group 4 (n = 5), the same dose of sarafotoxin was infused with 50 micrograms/kg per minute of the specific inhibitor of nitric oxide formation, NG-monomethyl-L-arginine (L-NMMA). No difference in hemodynamics, coronary blood flow (CBF), coronary vascular resistance (CVR), or coronary artery diameter (CAD) was observed at baseline between the groups. In group 1, intracoronary ET-1 significantly decreased CBF and CAD in association with an increase in CVR. The percentage decrease in CBF and CAD in the group that received ET-1 and the ET-A receptor antagonist (group 2) was significantly less than that in the group that received ET-1 alone (group 1) (-12 +/- 3% versus -48 +/- 6% [P < .001] and -4.6 +/- 0.8 versus 1.0 +/- 0.3 [P < .05], respectively). The administration of the ET-A receptor antagonist (group 2) abolished the ET-mediated increase in CVR (7 +/- 5% versus 105 +/- 21%, P < .005). There was no significant effect on CBF, CVR, or CAD in the group receiving sarafotoxin alone (group 3). The administration of L-NMMA and sarafotoxin (group 3). The administration of L-NMMA and sarafotoxin (group 4) resulted in a significant percentage decrease in CBF compared with the group that received sarafotoxin alone (-28 +/- 7% versus -8 +/- 2% [P < .05]). CONCLUSIONS The present study demonstrates that low concentrations of exogenous ET-1, which may mimic pathophysiological concentrations, result in coronary vasoconstriction mediated predominantly via the ET-A receptor because such vasoconstriction is significantly attenuated by blockade with FR-139317. The ET-B receptor may have a dual vasoconstrictive and vasodilatory effect.