pH and nitric oxide synthase activity and expression in bovine aortic endothelial cells

Nitric oxide (NO) plays an important role in the transition from intrauterine to extrauterine life. If this transition fails, a condition called persistent pulmonary hypertension of the neonate (PPHN) may develop. The current treatment modalities for this disease include induction of alkalosis by hyperventilation or alkali infusion, inhaled nitric oxide (iNO) and extracorporeal membrane oxygenation. There is evidence from animal studies that the elevated pH, not the low pCO2 is responsible for the resultant pulmonary vasodilatation. In this study, we examined the effect of pH on the activity and expression of endothelial nitric oxide synthase (eNOS) in cultured bovine aortic endothelial cells (BAEC) as a possible explanation for the pH dependent drop in pulmonary vascular resistance.

BAEC were exposed to a pH gradient of 7.1-7.6 for 4 h (short-term) and 16 h (long-term). Standard Western blotting technique was used to detect expression of eNOS. Activity was measured by an indirect assay using bovine aortic smooth muscle cells (BASM) as reporter cells and measuring cGMP levels as a marker of NO production. The cells were exposed to the pH gradient for a total of 4 h and measurement were made at 30, 60 and 90 min, and 2, 3 and 4 hours.

eNOS activity and expression remained unchanged during the four and sixteen hours of exposure.

In this in vitro experiment, we could not demonstrate an alkalosis-induced increase in eNOS activity and expression. The clinically observed pH dependent vasodilatation does not appear to be directly mediated through the induction of eNOS.