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

Abstract

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 (JHCO3 = 53.52 ± 4.61 pmol/min/mm in KO vs. 83.09 ± 9.73 in WT) and a ∼27 % reduction in volume reabsorption (Jv = 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.

Traditional and emerging roles for the SLC9 Na+/H+ exchangers

“…This results in volume depletion, and hypotension despite increased renin expression [152]. The contribution of a renal sodium leak in the absence of an intestinal defect to the global KO phenotype has been explored by knocking NHE3 into the intestine and by making a conditional renal knockout [118,91]. These experiments confirm a contribution of both systems to the significantly decreased circulating volume in the NHE3 KO mice.…”

“…As one H + is extruded in exchange for the influx of Na + , down its concentration gradient into the cell, NHE3 also participates in the reclaimation of HCO 3 -from the pro-urine. Consistent with this, global and proximal tubular NHE3 KO mice display a metabolic acidosis and alkaline urine [152,91]. Interestingly, NHE3 has been proposed to mediate NH 4 + efflux into the tubular lumen, although recent acid loading experiments on the proximal tubular KO mice are inconsistent with this [91,76].…”

“…Consistent with this, global and proximal tubular NHE3 KO mice display a metabolic acidosis and alkaline urine [152,91]. Interestingly, NHE3 has been proposed to mediate NH 4 + efflux into the tubular lumen, although recent acid loading experiments on the proximal tubular KO mice are inconsistent with this [91,76]. Given the large amount of sodium and water which is not reabsorbed from the proximal tubule in the absence of NHE3 it is surprising the NHE3 KO mice survive at all.…”

Acid-Base Homeostasis

“…The low intracellular Na 1 is maintained by the basolateral Na 1 /K 1 -ATPase. An apical H 1 -ATPase and possibly, NHE8 under some circumstances account for the remaining portion of proximal tubule H 1 secretion and HCO 3 2 reabsorption (17)(18)(19). This H 1 -ATPase shares most subunits with the distal tubule H 1 -ATPase described below.…”

Receptor Protein Tyrosine Phosphatase γ, CO2 Sensing in Proximal Tubule and Acid Base Homeostasis

“…While there are no known loss-of-function mutations for NHE-3, the inactivation of NHE-3 in mouse proximal tubule causes HCO 3 2 wasting and metabolic acidosis. 8,9 Taken together, these studies demonstrate that intact and functional NHE-3 and NBC-e1 are crucial for HCO 3 2 reabsorption in the proximal tubule and systemic acid-base homeostasis. [1][2][3][4][5][6][7][8][9] In order to prevent dramatic changes in intra-or extracellular pH, cells must be capable of sensing and responding to the levels of CO 2 , HCO 3 …”