Intracellular Na concentration and Rb uptake in proximal convoluted tubule cells and abundance of Na/K-ATPase α1-subunit in NHE3-/- mice

Proximal solute and fluid absorption is greatly reduced in mice in which the gene encoding the Na/H exchanger isoform 3 has been ablated (NHE3(-/-)). To obtain information on the intracellular functional consequences of such selective NHE3 deficiency, Na, Cl and K concentrations and cell Rb uptake were measured using electron microprobe analysis after a 30-s infusion of Rb (an index of basolateral Na/K-ATPase activity) in proximal convoluted tubule (PCT) cells of NHE3(-/-) and wildtype (NHE3(+/+)) mice. In addition, the relative abundance of the alpha1-subunit of the Na/K-ATPase in the outer cortex was determined by Western blot analysis. PCT cell Na concentration in NHE3(-/-) mice was slightly but significantly lower than in NHE3(-/-) [13.1+/-0.6 (n=64) vs. 14.9+/-0.6 (n=62) mmol/kg wet wt.; means +/-SEM]. The lower intracellular Na concentration was associated with significantly reduced Rb uptake rates [9.7+/-0.6 (n=59) vs. 14.8+/-0.8 (n=50) mmol/kg wet wt./30 s], but the abundance of the alpha1-subunit of the Na/K-ATPase was not different between NHE3(-/-) and NHE3(+/+) mice. Intracellular Cl concentration was higher (14.2+/-0.4 vs. 12.8+/-0.4 mmol/kg wet wt.) and K concentration unchanged (122.7+/-2.7 vs. 121.6+/-2.5 mmol/kg wet wt.) in PCT cells in NHE3(-/-) compared with NHE3(+/+) mice. These findings suggest that the elimination of apical NHE3 in PCT cells of NHE3(-/-) mice reduces apical Na entry and, due to lower cell Na concentrations, Na/K-ATPase activity. The observed changes in intracellular Na concentration did not affect the expression of Na/K-ATPase in the renal cortex of NHE3(-/-) mice. There were no significant changes of cell Na concentration and Rb uptake in distal convoluted tubule, connecting tubule, principal and intercalated cells.