Mechanism by which neutral phosphate infusion elevates urine Pco₂

Currently, the ability to elevate the urine Pco₂ above that of the arterial blood is employed as an estimate of distal hydrogen ion secretion. Therefore, it is important to establish the mechanisms by which various factors affect the urine CO₂ tension. This paper examines the physiologic process by which phosphate elevates the urine Pco₂ in the dog. The rise in urine Pco₂ due to phosphate could be the result of either (1) a distal mechanism, by affecting the delayed dehydration of carbonic acid, or (2) an increase in the medullary Pco₂. The phosphate-induced elevation of urine Pco₂ was abolished by carbonic anhydrase infusion. This indicates that a distal mechanism is a major factor in the phosphate effect. Since acid phosphate infusion did not result in an elevated urine Pco₂, it is unlikely that changes in the medullary Pco₂ occur as a result of phosphate infusion. The rise in urine Pco₂ due to phosphate was accompanied by an increase in net acid excretion, indicating an increase in hydrogen ion secretion. The increased hydrogen ion secretion and rise in urine Pco₂ were reproduced by infusion of the buffer, Tris-(hydroxymethyl)-aminomethane, but not by sodium sulfate administration. These findings suggest that the phosphate-induced rise in urine Pco₂ is due to the buffer properties of phosphate rather than to its nonreabsorbable anion characteristics.