The Importance Of Renal Nbce1-B During Metabolic Acidosis

Background The kidney responds to acidosis by increasing ammonium (NH4+) excretion via the metabolic process ammoniagenesis. The electrogenic Na+/HCO3- co-transporter (NBCe1) has an essential role in maintaining overall acid-base balance, and specifically, has been implicated in the regulation of renal ammoniagenesis. Two isoforms of NBCe1 exist in the kidney: NBCe1-A and NBCe1-B. Historically, NBCe1-A was considered the only renal NBCe1 variant and it was further demonstrated that NBCe1-A deletion in mice impairs ammoniagenesis. Recently, NBCe1-B renal expression was demonstrated; however, the physiologic role of NBCe1-B in the kidney remains unknown. Therefore, the goal of this study is to determine the contribution of NBCe1-B to renal ammoniagenesis. We hypothesize that NBCe1-B is essential for the ammoniagenic response to metabolic acidosis. Methods An NBCe1-B/C knockout (KO) mouse model was generated by our laboratory using Crispr/Cas9 gene editing. Since NBCe1-C mRNA is not expressed in the mouse kidney, we refer to this model as the NBCe1-B KO mouse. To determine if NBCe1-B loss alters the renal response to an exogenous acid load, WT and KO mice were housed in metabolic cages for 24-hours and allowed to spontaneously drink a 0.28M NH4Cl solution prepared with 0.5% sucrose for palatability. Control groups of WT and KO mice drank only 0.5% sucrose solution during the same 24-hour period. After the 24-hour experimental period, mice were anaesthetized with isoflurane, sacrificed via cardiac puncture, and blood gases were immediately assessed using the Siemens epoc blood analysis system. The volumes of individual 24-hour urine samples were measured, and urine NH4+ concentrations were determined using the Sigma-Aldrich Ammonia Assay Kit, which together were used to calculate daily NH4+ excretion rates. Results Acid-base characteristics in control and acid-loaded WT and NBCe1-B KO mice are summarized in Table 1. We observed no significant differences between WT and KO mice under control conditions. After 24-hour acid loading, WT and KO mice had significantly lower blood pH and bicarbonate concentrations than control groups, characteristic of metabolic acidosis. As expected, in response to acid-loading, WT mice exhibited a large increase in NH4+ excretion. On the other hand, while acid-loading also induced a significant increase in ammonium excretion in KO mice, it was significantly less than the response observed in WT mice. Conclusion The results of this study demonstrate that the ammoniagenic response is impaired in NBCe1-B KO mice during metabolic acidosis.