Citrate Transport in NaDC1 Knockout Mice

Abstract

Urinary citrate is the major endogenous inhibitor of calcium nephrolithiasis. Regulation of urinary citrate has been presumed to occur solely by the sodium dependent dicarboxylate cotransporter (NaDC1) located on the apical membrane of the proximal tubule. Using NaDC1 knockout (KO) and heterozygous (Het) mice (originally generated by Ho et al Kidney Int 2007) along with their wild type litter (WT) mates, we studied citrate and other Krebs cycle intermediates in plasma and urine with mice on either a normal diet (Nl) or on a 72 hour acid loading protocol (AL). Plasma citrate concentrations did not differ substantially in any of the conditions. On Nl diets, fractional excretion (FE) of citrate was higher in KO animals (0.24±0.02) than in WT (0.13±0.05); Hets were intermediate (0.18±0.04). Therefore, although KO of NaDC1 increases citrate excretion, substantial reabsorption of citrate still occurs in these animals (fractional reabsorption = 1-FE). With acidosis FE in WT was nearly abolished (0.003±0.001) but did not change in the KO animals (0.24± 0.02 Nl vs 0.25±0.0.02 AL diet). Surprisingly Het animals responded somewhat to acidosis but not completely, FE of 0.10±0.06 with AL and 0.18±0.04 with Nl diet. Urinary excretion of other dicarboxylates paralleled that of citrate except that urinary excretion of each decreased significantly in KO animals with acidosis. Thus, in examining FE in NaDC1 KO mice we found that most of renal reabsorption of citrate is not via NaDC1; although NaDC1 appears to account for the response to acidosis for citrate. Interestingly, the reabsorption of other Krebs cycle intermediates did respond significantly to acidosis. Also and surprisingly, Het animals do not have normal citrate reabsorption indicating that two normal alleles are important for normal citrate transport. NIH/NIDDK