Citrate excretion: a window on renal metabolism.

The rate of intracellular metabolism of citrate plays a major role in determining the amount of citrate excreted in the urine. Fractional excretion of citrate can be increased either by increasing intracellular citrate synthesis from precursors or by inhibiting mitochondrial citrate metabolism. Increased excretion secondary to increased synthesis of citrate occurs when citric acid cycle precursors such as malate or succinate are infused. Increased excretion resulting from inhibition of citrate metabolism occurs when malonate, maleate, or fluorocitrate is administered. Systemic acid-base changes cause striking changes in citrate clearance and metabolism. Recent evidence suggests that the effects of acid-base changes are mediated by alteration in the pH gradient across the inner mitochondrial membrane. Metabolic alkalosis causes cytoplasmic pH and bicarbonate to increase, resulting in a decrease in the mitochondrial pH gradient. This change inhibits the tricarboxylate carrier, slowing entry of citrate into the mitochondrial matrix compartment. The level of citrate in the cytoplasm increases, tubular and peritubular citrate uptake are reduced, and citrate clearance increases. Opposite changes occur in acidosis. Change in the mitochondrial pH gradient provides a sensitive mechanism for regulating renal substrate metabolism.