Altered Na+/H+ exchanger isoform 1 activity in immortalized lymphoblasts from women with pre-eclampsia: evidence for an intermediate phenotype

Previous studies have demonstrated a raised Na⁺ content in leucocytes isolated from women with pre-eclampsia. Increased Na⁺/H⁺ exchanger activity is one membrane transport abnormality that may contribute to this phenomenon and may be implicated in the abnormal volume homoeostasis and hypertension associated with the disease. Increased Na⁺/H⁺ exchanger activity has been documented in nucleated white blood cells from both pre-eclamptic and post-partum pre-eclamptic women, and may suggest the importance of genetic influences on exchanger activity. In the present study, we used lymphoblasts from women with pre-eclampsia and from age- and gestation-matched normotensive pregnant controls to determine Na⁺/H⁺ exchanger activity and intracellular resting pH using fluorimetry and the pH-sensitive dye BCECF-AM [bis(carboxyethyl)carboxyfluorescein acetoxymethyl ester]. Determination of Na⁺/H⁺ exchanger protein abundance was performed by Western blotting. Intracellular pH was not significantly different in cells from pre-eclamptic women compared with those from normotensive controls. Na⁺/H⁺ exchanger activity was measured when the intracellular pH was clamped at 6.0, and was found to be significantly higher in cells from pre-eclamptic women (20.77±0.92mmol·min^-1·l^-1) compared with those from normotensive controls (15.22±0.92mmol·min^-1·l^-1; P = 0.001). Na⁺/H⁺ exchanger protein abundance was established to be similar in the two subject groups, suggesting that the turnover number for the Na⁺/H⁺ exchanger is increased in the women with pre-eclampsia. These changes in Na⁺/H⁺ exchanger activity indicate the importance of genetic factors in determining this particular phenotype, since in this cell culture model of pre-eclampsia it is likely that environmental or hormonal influences present in vivo would have declined. Overactivity of the Na⁺/H⁺ exchanger may contribute to the raised intracellular Na⁺ concentration reported previously in white blood cells from women with pre-eclampsia.