On the mechanisms responsible for the phosphaturia of bicarbonate administration

Experiments were carried out in normal dogs to characterize the mechanisms by which sodium bicarbonate administration results in increased excretion of phosphate. Infusion of sodium bicarbonate alone increased fractional phosphate excretion from 0.8 to 29.3%. During bicarbonate administration, ionized calcium fell and mean parathyroid hormone values increased from 59.6 to 230.4 muleq/ml. In the same group of dogs, administration of sodium bicarbonate plus calcium prevented the fall in ionized calcium, and parathyroid hormone levels remained unchanged. In these dogs fractional phosphate excretion increased from 2.4 to only 4.9%. Similar results were obtained in thyroparathyroidectomized dogs receiving sodium bicarbonate. In these dogs fractional excretion of phosphate increased from 0.6 to 4.5%. Under all three experimental conditions no differences were observed in sodium or bicarbonate excretion or in urinary or plasma pH. Administration of hydrochloric acid, after phosphaturia had been induced by the infusion of bicarbonate, resulted in a decrease in plasma bicarbonate and an acid urine; however, the phosphaturia persisted even in the presence of an acid urine pH. In five thyroparathyroidectomized dogs infused with parathyroid hormone throughout, administration of identical amounts of sodium as either NaCl or NaHCO3 resulted in a similar degree of phosphaturia despite significant differences in urine pH. These experiments suggest that a rise in parathyroid hormone levels, resulting from a fall in ionized calcium, is the major mechanism by which bicarbonate administration produces phosphaturia. An increased natriuresis per nephron, as a consequence of extracellular fluid volume expansion, contributes to the phosphaturia. On the other hand, alkalinization of the urine does not play a significant role in the phosphaturia seen after bicarbonate administration.