Extracellular alkalinization induces endothelium-derived nitric oxide dependent relaxation in rat thoracic aorta

Aim: To investigate the mechanism through which the extracellular alkalinization promotes relaxation in rat thoracic aorta. Methods: The relaxation response to NaOH-induced extracellular alkalinization (7.4-8.5) was measured in aortic rings pre-contracted with phenylephrine (Phe, 10^-6M). The vascular reactivity experiments were performed in endothelium-intact and -denuded rings, in the presence or and absence of indomethacin (10^-5M), NG-nitro-l-arginine methyl ester (L-NAME, 10^-4M), N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide/HCl (W-7, 10^-7M), 2,5-dimethylbenzimidazole (DMB, 2×10^-5 M) and methyl-β-cyclodextrin (10^-2M). In addition, the effects of NaOH-induced extracellular alkalinization (pH 8.0 and 8.5) on the intracellular nitric oxide (NO) concentration was evaluated in isolated endothelial cells loaded with diaminofluorescein-FM diacetate (DAF-FM DA, 5μM), in the presence and absence of DMB (2×10^-5 M). Results: The extracellular alkalinization failed to induce any change in vascular tone in aortic rings pre-contracted with KCl. In rings pre-contracted with Phe, the extracellular alkalinization caused relaxation in the endothelium-intact rings only, and this relaxation was maintained after cyclooxygenase inhibition; completely abolished by the inhibition of nitric oxide synthase (NOS), Ca²⁺/calmodulin and Na⁺/Ca²⁺ exchanger (NCX), and partially blunted by the caveolae disassembly. Conclusions: These results suggest that, in rat thoracic aorta, that extracellular alkalinization with NaOH activates the NCX reverse mode of endothelial cells in rat thoracic aorta, thereby the intracellular Ca²⁺ concentration and activating the Ca²⁺/calmodulin-dependent NOS. In turn, NO is released promoting relaxation.