Citrate transport in rabbit nephron.

Citrate is an important renal metabolic substrate and urinary inhibitor of calcium stone formation. Our purpose was to characterize citrate reabsorption in the rabbit nephron using isolated perfused tubules. Citrate reabsorption, measured by luminal disappearance of [14C]citrate, was found only in the proximal tubule, not in the cortical thick ascending limb or the cortical collecting tubule. In the proximal convoluted tubule, the collected fluid was also analyzed by thin-layer chromatography and by measurements of chemical citrate concentration using an ultramicroassay. Luminal disappearance of [14C]citrate was determined to accurately represent citrate reabsorption; no significant citrate secretion was found. The absolute magnitude of citrate reabsorption was approximately 3.4 pmol X mm-1 X min-1 using either 1 or 3 mM citrate in the perfusate. This rate of citrate reabsorption in the rabbit proximal tubule could account for all of renal citrate reabsorption but was severalfold lower than glucose reabsorption, which was studied for comparative purposes. In contrast, the magnitude of succinate transport (which probably occurs via the same transport system as citrate) was comparable with that of citrate. Citrate reabsorption was inhibited approximately 80% by 10(-5) M ouabain. This characterization of citrate transport in the intact proximal tubule should provide a useful model to study regulation of urinary citrate excretion.