Endothelin B receptor-mediated vasoconstriction induced by endothelin A receptor antagonist

The vasoconstrictor effect of endothelins (ET) is mediated by endothelin A (ETA) and endothelin B (ETB) receptors. Furthermore, ETB receptor stimulation results in release of vasodilators. Hence, ETA receptor antagonists should attenuate ET-mediated vasoconstriction. The aim of the present study was to evaluate and compare the effects of BQ-123, an ETA receptor antagonist, and bosentan, an ETA and ETB receptor antagonist, on coronary vasomotor tone, left ventricular systolic function and ET-1 efflux in the presence or absence of myocardial ischaemia/reperfusion. Isolated rat hearts were perfused using a Langendorff preparation. Global ischaemia was induced on average by 68 +/- 2% (+/- standard error of the mean) reduction of a baseline perfusion flow-rate 10 min after introduction of ET antagonists. Thirty minutes of ischaemia was followed by 30 min reperfusion. ET-1 efflux in coronary perfusate was measured by radioimmunoassay. In non-ischaemic hearts (n = 7), BQ-123 (10(-6) M) perfusion induced a progressive decrease in coronary flow-rate compared with control group. This flow reduction persisted after wash-out of BQ-123. In contrast, bosentan (10(-5) M, n = 7) induced no change in perfusion rate. In the absence of ET antagonists (n = 16), there was a 22 +/- 6% post-ischaemic increase in perfusion flow-rate. BQ-123 (n = 5) but not bosentan (n = 12) abolished this post-ischaemic increase in flow-rate. Neither BQ-123 nor bosentan induced significant variation in force of contraction. In ischaemic hearts, ischaemia per se induced a transient decrease in force of contraction. Bosentan significantly (P < 0.05) accentuated and BQ-123 tended to accentuate (P = 0.06) this decrease in force of contraction during ischaemia. Bosentan but not BQ-123 significantly impaired the recovery of systolic function during reperfusion (P < 0.05). Both BQ-123 and bosentan perfusion increased ET-1 efflux rate to 730 +/- 188% and 315 +/- 81% respectively. This effect was abolished during ischaemia for BQ-123, but not for bosentan. In isolated perfused rat hearts, both BQ-123 and bosentan increased ET-1 efflux, but only BQ-123 exerted vasoconstrictor effects. These results thus generated the hypothesis that: (1) ET-1 release within the coronary vascular bed may be physiologically subject to negative feedback regulation mediated via ETA receptors; (2) ETA receptor antagonists increase ET-1 efflux, which may lead to net vasoconstriction via unopposed ETB stimulation. Furthermore, th