LAUSR

The renal sodium-hydrogen exchanger isoform 3 (NHE3) has been implicated for maintaining acid-base balance by mediating Na + reabsorption and H + secretion in the proximal tubule, the latter is required to facilitate bicarbonate (HCO 3 - ) reabsorption. In the proximal tubule, HCO 3 - reabsorption requires the enzymatic activities of luminal carbonic anhydrase (CA) IV and intracellular CA II. We hypothesized that NHE3 is required to mediate the effects of CA inhibitors. To test this hypothesis, we studied control (Con, n =8-10) and tubule-specific NHE3 knockout mice (NHE3 KS-KO , n =8-10). In one set of experiments mice were randomized to acute application of vehicle (Veh, 0.85% saline, 0.2% of bw i.v.), acetazolamide (ACZ, cell-permeable CA inhibitor; 50 mg/kg bw i.v.) or C18 (cell-impermeable CA inhibitor; 5 mg/kg bw) and blood gas analysis was performed before (baseline) and 1 hour after administration. In another set of experiments mice were randomized to acute application of Veh or ACZ in combination with an oral gavage of distilled H 2 O (1% of bw). Urine collections were performed in metabolic cages over a 3-hour period and samples analyzed for pH and Cl - . Veh treatment did not significantly alter blood pH, HCO 3 - or Cl - levels in either genotype. Compared to baseline, ACZ decreased blood pH significantly in both, Con (7.39±0.01 vs. 7.26±0.01, P <0.05) and NHE3 KS-KO (7.35±0.01 vs. 7.24±0.01, P <0.05) mice, with no differences between genotypes. Consistent with impaired HCO 3 - reabsorption, plasma HCO 3 - levels significantly decreased in both, Con (23.2±0.5 vs. 19.2±0.5 mmol/L, P <0.05) and NHE3 KS-KO (22.7±0.3 vs. 19.1±0.5 mmol/L, P <0.05) mice with no differences between genotypes. Plasma Cl - levels increased significantly in both Con (114±0.3 vs. 116±0.4 mmol/L, P <0.05) and NHE3 KS-KO (114±0.3 vs. 116±0.4 mmol/L, P <0.05) mice after ACZ treatment without showing differences between genotypes. Neither C18 nor Veh treatment altered blood pH, HCO 3 - or Cl - levels in either genotype compared to baseline. ACZ caused significantly greater urinary flow rates in Con mice (3.8±0.5 vs. 5.6±0.3 µL/min, P <0.05) but not in NHE3 KS-KO (3.8±0.5 vs. 5.0±0.4 µL/min, NS ) mice, compared to Veh treatment. Urine pH was significantly more alkaline with Veh treatment in NHE3 KS-KO compared to Con mice ( P <0.05). Compared to Veh, urinary pH in response to ACZ treatment was significantly more alkaline in both Con (6.9±0.2 vs. 8.5±0.1, P <0.05) and NHE3 KS-KO (7.5±0.2 vs. 8.4±0.1, P <0.05) mice. No effect on urinary Cl - excretion was observed. In summary, our findings demonstrate that luminal blockade of CA IV does not affect blood acid-base parameters. In addition, the effects of ACZ on acid-base status do not require functional NHE3. This work was supported by a VA Merit Review Award IBX004968A (to Dr. Rieg) and an American Heart Association Career Development Award 24CDA1271666 (to Dr. Thomas). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.