Reversal of hypercapnia induces endothelin-dependent constriction of basilar artery in rabbits with acute metabolic alkalosis

We recently concluded that constriction of basilar artery due to respiration-induced hypocapnia in rabbits with acute metabolic alkalosis and accompanying compensatory hypercapnia was independent of NO and K_ATP channels. Based on reports that endothelin-1-mediated hypocapnic constriction of the rabbit basilar artery in vitro, we further investigated whether the respiration-induced hypocapnic constriction was endothelin-1 mediated. Metabolic alkalosis was induced acutely following ketamine/xylazine injection. The ET_A plus ET_B receptor antagonist, PD145065 (1 μM), and the selective ET_A receptor antagonist, BQ610 (3 μM), completely relaxed the hypocapnic constriction, as determined in a cranial window. Unexpectedly, the ET_B receptor antagonists, BQ788 and RES-701-1 (3 μM), relaxed the constriction by 72.1±2.8% (4) and 77.2±8.7% (5), respectively (means±S.E. (n)). To investigate whether the large magnitudes of relaxation to both ET_A and ET_B receptor antagonists were due to nonselectivity of the antagonists, the effects of the antagonists on the constriction to exogenous endothelin-1 were evaluated. BQ610, BQ788, and RES-701-1 relaxed the 3–5 nM endothelin-1 constriction by only 64.3±7.6% (4), 43.5±8.5% (5), and 26.7±4.8% (3) (means±S.E. (n)), respectively, consistent with the selective blocking action of these antagonists. To investigate whether the greater magnitude of BQ610, BQ788, and RES-701-1 relaxation of hypocapnic constricted versus exogenous endothelin-1-constricted vessels was due to differences between constriction elicited by endogenous versus exogenous endothelin-1, the effects of the endothelin receptor antagonists on constriction to isocapnic alkaline suffusate were evaluated. PD145065 (1 μM) and 0.1 mM phosphoramidon, an endothelin-converting enzyme inhibitor, inhibited the constriction to isocapnic alkaline suffusate by 83.8±7.8% (6) and 74.3±9.7% (8) (means±S.E. (n)), respectively, consistent with the endothelin-1 dependency of the constriction. BQ610, BQ788, and RES-701-1 relaxed the isocapnic alkaline suffusate constriction by 74.9±6.7% (5), 65.5±6.4% (5), and 78.0±6.5% (4) (means±S.E. (n)), respectively. Thus, the relaxation profile to the selective endothelin receptor antagonists in isocapnic alkaline constricted vessels more closely approximated the relaxation profile observed in hypocapnic constricted as compared to endothelin-1-constricted vessels. Hypocapnia did not alter the 5 nM endothelin-1 constriction. These results suggest that, under conditions of acute metabolic alkalosis and accompanying compensatory hypercapnia, subsequent hypocapnic constriction is endothelin mediated. Both ET_A and ET_B receptor activation may mediate the hypocapnic constriction. The hypocapnic constriction is not due to enhanced endothelin-1 constriction and, thus, is due to the release of endothelin-1 and/or additional endothelins.