ROLE OF NHE3 IN MEDIATING RENAL BRUSH-BORDER NA-H+ EXCHANGE - ADAPTATION TO METABOLIC-ACIDOSIS()

The aims of the present study were to estimate the fraction of renal b rush border membrane Na+-H+ exchange activity mediated by the isoform NHE3 and to evaluate whether the increased brush border Na+-H+ exchang e observed in metabolic acidosis is due to increased expression of NHE 3 protein. Compared with other isoforms, NHE3 is known to have a uniqu e profile of sensitivity to pharmacologic inhibitors, including relati ve resistance to amiloride analogs and HOE694. We therefore assessed t he inhibitor sensitivity of pH gradient-stimulated Na-22 uptake in ren al brush border vesicles isolated from normal rats. The I-50 values fo r amiloride (30 mu M), dimethylamiloride (10 mu M), ethylisopropylamil oride (6 mu M), and HOE694 (>100 mu M) were markedly dissimilar from t hose reported for NHE1 and NHE2 but were nearly identical to reported values for NHE3, Na+-H+ exchange activity in renal brush border vesicl es isolated from rats with 5 days of NH4Cl-induced metabolic acidosis was increased 1.5-fold compared with control rats, with no change in i nhibitor sensitivity. Western blot analysis indicated that NHE3 protei n expression was greater in brush border membranes from acidotic compa red with control rats. We conclude that virtually all measured Na+-Hexchange activity in brush border membranes from control and acidotic rats is mediated by NHE3 and that metabolic acidosis causes increased expression of renal brush border NHE3 protein.