THE RENAL EXCRETION OF INORGANIC PHOSPHATE IN RELATION TO THE ACTION OF VITAMIN D AND PARATHYROID HORMONE 1

Abstract

The mode of action of vitamin D in the preven-tion and cure of rickets is still not understood. In 1921, Howland and Kramer (1) pointed out that the concentration of phosphorus in the serum was usually reduced in human rickets, at times to very low levels, although the concentration of calcium might be within normal limits. This has been confirmed repeatedly and it has been found that the administration of vitamin D results in a prompt rise in the concentration of phosphorus, even to abnormally high levels with excessive dos-age. The mechanism through which vitamin D influences the concentration of phosphorus in the body fluids has not been satisfactorily explained. In balance experiments in man (2, 3) and experi-mental animals (4) it has been shown that during states of vitamin D deficiency absorption of cal-cium and phosphate from the intestinal tract is reduced and that the administration of vitamin D increases the absorption of these ions. In the rat, the studies of Nicolaysen (5) have indicated that in the absence of vitamin D the absorption of calcium is primarily diminished and that the ab-sorption of phosphate is only secondarily affected. Careful examination of the data from metabolic studies in the rachitic infant leads to the conclusion that deficient absorption of phosphate from the in-testinal tract cannot wholly explain the diminished concentration of phosphate in the plasma. In in-fants developing rickets on a cow's milk diet, i.e., a diet high in both calcium and phosphorus, the amount of phosphate absorbed from the intestinal tract would be sufficient for the needs of the in-fant were the phosphate retained rather than ex-creted in the urine.