Renal tubular transport of citrate: relations with calcium

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Abstract
The renal excretion of citrate and calcium was studied in the dog during the infusion of these substances alone or together by the stop-flow method. Citrate per se was found to be reabsorbed only in the proximal tubular area. This reabsorption occurred against marked concentration gradients. Spontaneous variations in the urine pH from 6.11 to 7.11 units did not affect the reabsorption in any way. Infusion of bicarbonate decreased the capacity of the proximal tubule to lower the citrate concentration but did not alter the general pattern of its transport along the nephron length. Calcium without citrate was actively reabsorbed only in the distal tubule. Calcium when given with citrate, did not affect the pattern of citrate transport along the length of the nephron. Citrate did not affect the distal reabsorption of calcium. However, it did produce a proximal reabsorption of calcium. It was postulated that the proximal reabsorption of citrate is active. The molecular requirement for reabsorption of citric acid is discussed.