The AE2 Cl ؊ /HCO 3 ؊ exchanger is expressed in numerous cell types, including epithelial cells of the kidney, respiratory tract, and alimentary tract. In gastric epithelia, AE2 is particularly abundant in parietal cells, where it may be the predominant mechanism for HCO 3 ؊ efflux and Cl ؊ influx across the basolateral membrane that is needed for acid secretion. To investigate the hypothesis that AE2 is critical for parietal cell function and to assess its importance in other tissues, homozygous null mutant (AE2 ؊/؊ ) mice were prepared by targeted disruption of the AE2 (Slc4a2) gene. AE2 ؊/؊ mice were emaciated, edentulous (toothless), and exhibited severe growth retardation, and most of them died around the time of weaning. AE2 ؊/؊ mice exhibited achlorhydria, and histological studies revealed abnormalities of the gastric epithelium, including moderate dilation of the gastric gland lumens and a reduction in the number of parietal cells. There was little evidence, however, that parietal cell viability was impaired. Ultrastructural analysis of AE2 ؊/؊ gastric mucosa revealed abnormal parietal cell structure, with severely impaired development of secretory canaliculi and few tubulovesicles but normal apical microvilli. These results demonstrate that AE2 is essential for gastric acid secretion and for normal development of secretory canalicular and tubulovesicular membranes in mouse parietal cells.

The expression of NHE2 and NHE3 on intestinal-brush border membranes suggests that both Na(+)/H(+) exchangers serve absorptive functions. Studies with knockout mice showed that the loss of NHE3, but not NHE2, causes diarrhea, demonstrating that NHE3 is the major absorptive exchanger and indicating that any remaining absorptive capacity contributed by NHE2 is not sufficient to compensate fully for the loss of NHE3. To test the hypothesis that NHE2 provides partial compensation for the diarrheal state of NHE3-deficient mice, we crossed doubly heterozygous mice carrying null mutations in the Nhe2 and Nhe3 genes and analyzed the phenotypes of their offspring. The additional loss of NHE2 in NHE3-deficient mice caused no apparent reduction in viability, no further impairment of systemic acid-base status or increase in aldosterone levels, and no apparent worsening of the diarrheal state. These in vivo phenotypic correlates of the absorptive defect suggest that the NaCl, HCO, and fluid absorption that is dependent on apical Na(+)/H(+) exchange is due overwhelmingly to the activity of NHE3, with little contribution from NHE2.

To study the role of Na+/H+ exchanger isoform 2 (NHE2) and isoform 3 (NHE3) in sodium-fluid volume homeostasis and renal Na+ conservation, mice with Nhe2 (Nhe2-/-) and/or Nhe3 (Nhe3-/-) null mutations were fed a Na+-restricted diet, and urinary Na+ excretion, blood pressure, systemic acid-base and electrolyte status, and renal function were analyzed. Na+ -restricted Nhe2-/- mice, on either a wild-type or Nhe3 heterozygous mutant (Nhe3+/-) background, did not exhibit excess urinary Na+ excretion. After 15 days of Na+ restriction, blood pressure, fractional excretion of Na+, and the glomerular filtration rate (GFR) of Nhe2-/-Nhe3+/- mice were similar to those of Nhe2+/+ and Nhe3+/- mice, and no metabolic disturbances were observed. Nhe3-/- mice maintained on a Na+-restricted diet for 3 days exhibited hyperkalemia, urinary salt wasting, acidosis, sharply reduced blood pressure and GFR, and evidence of hypovolemic shock. These results negate the hypothesis that NHE2 plays an important renal function in sodium-fluid volume homeostasis; however, they demonstrate that NHE3 is critical for systemic electrolyte, acid-base, and fluid volume homeostasis during dietary Na+ restriction and that its absence leads to renal salt wasting.

The apical Na+‐H+ exchanger NHE3 plays an important role in fluid reabsorption in the proximal tubule. However, whether its deletion alters the salt and water transport in the distal nephron remains unknown. To answer these questions, wild‐type (Nhe3+/+) and NHE3 null mice (Nhe3−/−) were placed in metabolic cages and their water balance and urine osmolality were examined. Nhe3−/− mice demonstrated a significant polydipsia (P < 0.03) and polyuria (P < 0.04), with a lower urine osmolality (P < 0.003) as compared to Nhe3+/+ mice. Northern hybridization and immunoblotting studies indicated that the mRNA expression and protein abundance of the collecting duct (CD) water channel AQP2 decreased by 52 % (P < 0.0003) and 73 % (P < 0.003) in the cortex, and by 53 % and 54 % (P < 0.002) in the inner medulla (IM) of Nhe3−/− vs. Nhe3+/+ mice. The expression of AQP2 in the outer medulla (OM) remained unchanged. Further, the mRNA expression and protein abundance of the medullary thick ascending limb (mTAL) apical Na+‐K+‐2Cl− cotransporter (NKCC2) decreased by 52 % (P < 0.02) and 44 % (P < 0.01), respectively, in the OM of Nhe3−/− vs. Nhe3+/+ mice. The circulating plasma levels of vasopressin as well as the mRNA expression of vasopressin prohormone were significantly increased in Nhe3−/− vs. Nhe3+/+ mice (P < 0.05). Studies in mice treated with acetazolamide indicated that increased bicarbonate and fluid delivery to distal nephron did not alter the expression of NKCC2 in mTAL and decreased AQP2 protein only in OM but not in the cortex or IM. In conclusion, mice lacking the apical NHE3 have impairment in their water balance and urine osmolality, which correlates with the downregulation of AQP2 expression. These defects occur despite increased circulating levels of vasopressin. We propose that an ADH‐independent mechanism is responsible for the downregulation of AQP2 and the resulting polyuria in NHE3 null mice.

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