Effects of citrate on urinary calcium excretion

Thyroparathyroidectomized rats were studied during control, citrate infusion, and recovery periods to analyze the effects of citrate on urinary calcium excretion. The rats were infused with human PTH-(1-34) at a constant rate throughout the experiments that produced physiologic circulating PTH activity. Ultrafiltrable calcium was measured with a micropartition system while maintaining constant the pH and PCO2 of the sample. The plasma acid-base status, filtered load of calcium, and urinary sodium excretion rate did not vary during citrate infusion. The urinary calcium excretion rate increased from 71 +/- 9 nmol/min/g kidney during control period to 122 +/- 22 nmol/min/g kidney (p less than 0.05) during citrate infusion, and then returned below control levels during recovery period. A strong positive linear relationship was observed between the urinary excretion rates of calcium and citrate (r = 0.88; p less than 0.001). We conclude that tubular fluid citrate inhibits renal calcium reabsorption probably by forming calcium-citrate complexes.