Role of endothelin B receptors in enhancing endothelium-dependent nitric oxide-mediated vascular relaxation during high salt diet.

High salt diet is often associated with increases in blood pressure, and the state of activation of endothelium-dependent vascular relaxation pathways is critical under these conditions. Basal activation of endothelial endothelin B (ET(B)) receptors by endothelin has been suggested to stimulate the release of factors that promote vascular relaxation. However, whether ET(B) receptors play a role in enhancing endothelium-dependent vascular relaxation during high salt diet is unclear. In this study, we investigated whether chronic treatment with an ET(B) receptor antagonist is associated with impaired endothelium-dependent vascular relaxation and enhanced vascular reactivity particularly during high salt diet. Isometric contraction was measured in aortic strips isolated from male Sprague-Dawley rats on normal sodium (NS, 1%) and high sodium diet (HS, 8%) for 7 days and untreated or treated with the ET(B) receptor antagonist A-192621 (30 mg/kg per day) for 5 days. The mean arterial pressure was (in mm Hg) 122+/-3 in NS, 132+/-3 in HS, 144+/-2 in NS/ET(B) antagonist, and 171+/-12 in HS/ET(B) antagonist rats. In endothelium-intact strips, phenylephrine (Phe, 10(-5) mol/L) increased active stress to 7.6+/-1.0x10(3)N/m(2) in NS rats and 8.2+/-0.9x10(3)N/m(2) in HS rats. Phe (10(-5) mol/L) -induced stress was significantly greater in NS/ET(B) antagonist (11.3+/-0.9x10(3)N/m(2)) than NS and far greater in HS/ET(B) antagonist (14.1+/-0.1.2x10(3)N/m(2)) than HS rats. Also, Phe was more potent in NS/ET(B) antagonist and HS/ET(B) antagonist rats (ED(50)=0.3x10(-7) and 0.15x10(-7) mol/L) than in NS and HS rats (ED(50)=0.8x10(-7) and 0.7x10(-7) mol/L). Removal of the endothelium enhanced Phe-induced contraction significantly in NS and to a greater extent in HS, but not in NS/ET(B) antagonist or HS/ET(B) antagonist rats. In endothelium-intact strips, acetylcholine (ACh) caused relaxation of Phe contraction that was less in NS/ET(B) antagonist than NS and far less in HS/ET(B) antagonist than HS rats. Pretreatment of endothelium-intact strips with L-NAME (10(-4) mol/L), to inhibit nitric oxide (NO) synthase, or with methylene blue (10(-5) mol/L) or 1H-[1,2,4]oxadiazolo[4,3]-quinoxalin-1-one (ODQ, 10(-6) mol/L), to inhibit cGMP production in smooth muscle, inhibited ACh-induced relaxation and enhanced Phe-induced contraction significantly in NS and HS, slightly in NS/ET(B) antagonist, but not in HS/ET(B) antagonist rats. Measurement of basal and ACh-induced nitrite/nitrate production from aortic strips showed a significant reduction in NS/ET(B) antagonist compared with NS, and a greater reduction in HS/ET(B) antagonist compared with HS rats. Relaxation of Phe contraction with sodium nitroprusside was not significantly different among the different groups of rats. Thus, an endothelial ET(B) receptor-mediated pathway of vascular relaxation involving release of NO seems to be active under basal conditions and may protect against excessive vasoconstriction and increased blood pressure particularly during high salt diet.