Effect of NBCe1 Deletion on Renal Organic Anion Metabolism

The proximal tubule electrogenic, sodium‐coupled bicarbonate transporter, NBCe1, has a critical role in two proximal tubule acid‐base functions, bicarbonate reabsorption and ammonia metabolism. A third component of acid‐base homeostasis involves organic anion metabolism. The purpose of the current study was to determine NBCe1's role in organic anion metabolism by comparing mice with NBCe1 deletion to wild‐type (WT) littermates. Because NBCe1 KO causes early post‐natal mortality, we studied mice at day 8±1. Also, because NBCe1 deletion causes metabolic acidosis, we compared effects of NBCe1 deletion to that of experimental metabolic acidosis in adult WT mice. Both NBCe1 knockout (KO) and acidosis decreased citrate excretion, but their effect on proximal tubule citrate transporters differed. NaDC‐1 is the primary apical citrate transporter, and NBCe1 KO decreased NaDC‐1 mRNA expression and it decreased proximal tubule apical immunolabel. In contrast, acidosis in WT mice increased NaDC‐1 mRNA increased and proximal tubule apical immunolabel. The basolateral citrate transporter is NaDC‐3. NBCe1 deletion decreased NaDC‐3 expression whereas metabolic acidosis increased expression. A second key urinary organic anion is 2‐oxoglutarate. NBCe1 KO increased 2‐oxoglutarate excretion ~15‐fold, whereas metabolic acidosis decreased excretion by ~85%. NBCe1 deletion did not alter expression of the enzyme involved in 2‐oxoglutarate generation (Glud1), whereas metabolic acidosis increased it. There were no substantial changes in expression of the 2‐oxoglutarate dehydrogenase complex components, Ogdh, Dlst or Dld, with either NBCe1 KO or acidosis. Expression of the putative apical 2‐oxoglutarate transporter, OAT10, was decreased similarly by NBCe1 KO and by acidosis. Thus, effects of NBCe1 KO on 2‐oxoglutarate excretion did not correlate with changes in proteins involved in production, metabolism, basolateral transport (NaDC‐3) or its putative apical transporter (OAT10). 2‐oxoglutarate is proposed to stimulate urine alkalinization by activating the Oxgr1 receptor in pendrin‐positive intercalated cells. However, NBCe1 KO causes a urine pH of ~4.2 despite increased 2‐oxoglutarate excretion. Pendrin, the primary mechanism of collecting duct bicarbonate secretion, exhibited significantly decreased expression with NBCe1 KO. Expression of the 2‐oxoglutarate receptor, Oxgr1, was unaffected by NBCe1 KO, whereas it was decreased by metabolic acidosis. In summary: 1) NBCe1 is necessary for normal expression of the proximal tubule citrate transporters, NaDC‐1 and NaDC‐3; 2) NBCe1 is necessary for normal 2‐oxoglutarate metabolism through mechanisms independent of expression of known transport and metabolic pathways; and, 3) the lack of 2‐oxoglutarate urinary alkalinization in NBCe1 KO mice likely results from decreased expression of its target transporter, pendrin. Thus, NBCe1 expression is necessary for normal renal metabolism of the key organic acids, citrate and 2‐oxoglutarate. We conclude that NBCe1 expression is necessary for normal regulation of each of the three major components of proximal tubule acid‐base homeostasis, filtered bicarbonate reabsorption, ammonia metabolism and organic anion transport. Support or Funding Information Funds from the NIH (R01–DK045788 to IDW, R01‐EY017732 to MFR and R01‐DK095879 to KHS) and from the Department of Veterans Affairs (1I01BX000818 to IDW) supported these studies.