Proximal tubule specific knockout of the Na⁺/H⁺ exchanger NHE3: effects on bicarbonate absorption and ammonium excretion.

The existing NHE3 knockout mouse has significant intestinal electrolyte absorption defects, making this model unsuitable for the examination of the role of proximal tubule NHE3 in pathophysiologic states in vivo. To overcome this problem, we generated proximal convoluted tubule-specific KO mice (NHE3-PT KO) by generating and crossing NHE3 floxed mice with the sodium-glucose transporter 2 Cre transgenic mice. The NHE3-PT KO mice have >80 % ablation of NHE3 as determined by immunofluorescence microscopy, western blot, and northern analyses, and show mild metabolic acidosis (serum bicarbonate of 21.2 mEq/l in KO vs. 23.7 mEq/l in WT, p < 0.05). In vitro microperfusion studies in the isolated proximal convoluted tubules demonstrated a ∼36 % reduction in bicarbonate reabsorption (J HCO3 = 53.52 ± 4.61 pmol/min/mm in KO vs. 83.09 ± 9.73 in WT) and a ∼27 % reduction in volume reabsorption (J v  = 0.67 ± 0.07 nl/min/mm in KO vs. 0.92 ± 0.06 nl/min/mm in WT) in mutant mice. The NHE3-PT KO mice tolerated NH4Cl acid load well (added to the drinking water) and showed NH4 excretion rates comparable to WT mice at 2 and 5 days after NH4Cl loading without disproportionate metabolic acidosis after 5 days of acid load. Our results suggest that the Na(+)/H(+) exchanger NHE3 plays an important role in fluid and bicarbonate reabsorption in the proximal convoluted tubule but does not play an important role in NH4 excretion.