NO and NO-independent mechanisms mediate ET_B receptor buffering of ET-1-induced renal vasoconstriction in the rat

Vascular endothelin (ET) type B (ET_B) receptors exert dilator and constrictor actions in a complex interaction with ET_A receptors. We aimed to clarify the presence and relative importance of nitric oxide (NO) and other mechanisms underlying the dilator effects of ET_B receptors in rat kidneys. Complete inhibition of NO production with N^ω- nitro-L-arginine methyl ester (L-NAME, 25 mg/kg iv) enhanced the renal vasoconstriction elicited by ET-1 injected into the renal artery from -15 to -30%. Additional infusion of the NO donor nitroprusside (NP) into the renal artery did not reverse this effect (-29%) but effectively buffered ANG II-mediated vasoconstriction. Similarly, ET-1 responses were enhanced after a smaller intrarenal dose of L-NAME (-22 vs. -15%)and were unaffected by subsequent NP infusion (-21%). These results indicate that the responsiveness to ET-1 is buffered by ET_B receptor-stimulated phasic release of NO, rather than its static mean level. Infusion of the ET_B receptor antagonist BQ-788 into the renal artery further enhanced the ET-1 constrictor response to NP + L-NAME (-92 vs. -49%), revealing an NO-independent dilator component. In controls, vasoconstriction to ET-1 was unaffected by vehicle (-27 vs. -20%) and markedly enhanced by BQ-788 (-70%). The same pattern was observed when indomethacin (Indo) was used to inhibit cyclooxygenase (-20% for control, -22% with Indo, and -56% with ET_B antagonist) or methylsulfonyl-6-(2- propargyloxyphenyl)-hexanamide (MS-PPOH) or miconazole + Indo was used to inhibit epoxygenase alone (-10% for control, -11% with MS-PPOH, and -35% with ET_B antagonist) or in combination (-14% for control, -20% with Indo + miconazole, and -43% with ET_B antagonist). We conclude that phasic release of NO, but not its static level, mediates part of the dilator effect of ET_B receptors and that an NO-independent mechanism, distinct from prostanoids and epoxyeicosatetraenoic acids, perhaps ET_B receptor clearance of ET-1, plays a major buffering role.