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

Abstract

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 (ETB) receptors by endothelin has been suggested to stimulate the release of factors that promote vascular relaxation. However, whether ETB 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 ETB 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 ETB 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/ETB antagonist, and 171±12 in HS/ETB antagonist rats. In endothelium-intact strips, phenylephrine (Phe, 10-5 mol/L) increased active stress to 7.6±1.0× 103N/m2 in NS rats and 8.2±0.9× 103N/m2 in HS rats. Phe (10-5 mol/L) -induced stress was significantly greater in NS/ETB antagonist (11.3±0.9×103N/m2) than NS and far greater in HS/ETB antagonist (14.1±0.1.2×103N/m2) than HS rats. Also, Phe was more potent in NS/ETB antagonist and HS/ETB antagonist rats (ED50=0.3×10-7 and 0.15×10-7 mol/L) than in NS and HS rats (ED50=0.8x10-7 and 0.7×10-7 mol/L). Removal of the endothelium enhanced Phe-induced contraction significantly in NS and to a greater extent in HS, but not in NS/ETB antagonist or HS/ETB antagonist rats. In endothelium-intact strips, acetylcholine (ACh) caused relaxation of Phe contraction that was less in NS/ETB antagonist than NS and far less in HS/ETB 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/ETB antagonist, but not in HS/ETB antagonist rats. Measurement of basal and ACh-induced nitrite/nitrate production from aortic strips showed a significant reduction in NS/ETB antagonist compared with NS, and a greater reduction in HS/ETB antagonist compared with HS rats, Relaxation of Pre contraction with sodium nitroprusside was not significantly different among the different groups of rats. Thus, an endothelial ETB 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.