THE INFLUENCE OF ANION PENETRATING ABILITY ON URINARY ACIDIFICATION AND THE EXCRETION OF TITRATABLE ACID *

Abstract

The influence on acid excretion of anions differing in penetrating ability has been studied in dogs whose Na and chloride excretion had been reduced by dietary restriction. It was demonstrated that administration of the Na salt of phosphate, a poorly reabsorbable buffer anion, leads to a marked reduction in urine pH and the excretion of near theoretical maximum amounts of titratable acid. These results indicate that, over the range of phosphate excretion rates which has been studied, maximal excretion of acid can be achieved without the stimulus of acidosis. It has been suggested that a disproportion between Na and anion reabsorption was the critical factor in determining not only the pH gradient between blood and urine but also the total excretion of acid. This hypothesis is supported by the finding that the percentage of theoretical maximum titratable acid excreted in the urine is decreased by the subsequent infusion of the readily reabsorbable anions, chloride or thiocyanate, and is sustained or increased by the administration of the poorly reabsorbable anions, sulfate or ferrocyanide. As a possible mechanism to account for these findings, it is proposed that the various anions affected urine pH and acid excretion through their influence on the electrical gradient established by active Na transport across the renal tubular cells. According to this hypothesis, the magnitude of the gradient is determined by the relative penetrating ability of the available anions, those which penetrate the tubular epithelium more readily having a greater tendency to follow Na and thus to reduce the potential difference. Changes in urinary pH and titratable acid excretion might thus be attributed to the passive movement of hydrogen ions in response to variations in the transtubular electrical gradient.