Excretion of l-malic acid in the chicken

¹ Department of Physiology, College of Medicine, University of Cincinnati, Cincinnati, Ohio

The mechanism of excretion of malic acid has been studied during infusion of sodium salts of l-malic acid, succinic acid and malonic acid in the chicken. Infusion of preformed l-malate results in its filtration and reabsorption in the nephron; no renal tubular secretion is seen under these circumstances. However, when succinate is infused in the renal portal circulation, malate is excreted in larger amounts on the ipsilateral side than on the other. Malonate, a succinoxidase inhibitor, not only fails to abolish this excretion but actually enhances it. Infusion of malonate alone does not produce any excretion of malic acid. It is concluded that the renal tubular secretion of l-malate is limited to that which is synthesized in the tubular cells and does not occur even when large amounts of infused l-malate are present in the peritubular circulation. The enhancement of malate excretion by malonate during succinate infusion may occur from either malonate decarboxylation or malonate suppression of a malate reabsorptive limb in the nephron.