Abstract
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 wild-type (NHE3+/+) mice. In addition, the relative abundance of the α1-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 α1-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.
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Notes
Type-I IC cells are thought to be type-A IC cells [4]. In rabbits, IC cells of the cortical collecting duct have a low membrane potential (−35 mV) and a high basolateral Cl conductance [24]. Comparatively high intracellular Cl concentrations would thus be expected. This type of IC cell is the predominant cell type in the outer medulla and mimics the intrarenal distribution of type-A IC cells [8, 34]. It should be noted that such high intracellular Cl concentrations would compromise the activity of the apical Cl/HCO3 exchanger in type-B IC cells because the concentrations of Cl in the distal tubule fluid may be below 40 mM in this tubule segment [6]. Thus, Cl concentrations in the range of those observed in type-I IC cells would diminish the chemical driving force for apical entry of Cl via electroneutral Cl/HCO3 exchange in type-B IC cells.
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Acknowledgements
These studies were supported by the Deutsche Forschungsgemeinschaft (BE 963/10–1) and NIH grant DK 17433. Collaboration between the authors' laboratories was supported by a NATO collaborative research grant. The authors are indebted to Dr. M. Caplan for the gift of the Na/K-ATPase-specific antibody, to Dr. G. Shull for providing the NHE3+/+ and NHE3−/− mice (NIH grant DK 50594) and to M.L. Fraek and I. Öztürk for excellent technical assistance.
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Beck, FX., Neuhofer, W., Dörge, A. et al. Intracellular Na concentration and Rb uptake in proximal convoluted tubule cells and abundance of Na/K-ATPase α1-subunit in NHE3−/− mice. Pflugers Arch - Eur J Physiol 446, 100–105 (2003). https://doi.org/10.1007/s00424-002-1001-z
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DOI: https://doi.org/10.1007/s00424-002-1001-z