In Vivo and in Vitro Effects of Vitamin D₃ [1,25-(OH)₂D₃] on the Uptake of Phosphate by Isolated Chick Kidney Cells

Abstract

Harrison and Harrison¹ first proposed that vitamin D acts as an important regulator of phosphate homeostasis. Since then, it has has been demonstrated that the active form of the vitamin, 1,25-dihydroxycholecalciferol [1,25-(OH)₂D₃], enhances, by an unknown mechanism, the intestinal absorption of phosphate^2,3. The effect of 1,25-(OH)₂D₃ on the reabsorption of phosphate in the kidney is less clear. In some studies, infusion of the vitamin has been found to decrease phosphate excretion^4,5, whereas in other investigations 1,25-(OH)₂D₃ has been reported to be Phosphaturie⁶ or without effect on urinary phosphate⁷. Some of the difficulties in resolving the apparent discrepancies stem, in part, from the complex interactions in the in vivo animal of the vitamin D metabolic and phosphate transport systems with various regulatory factors, including parathyroid hormone and other endocrines, dietary status, and calcium^8,9. This prompted us to initiate studies on a simpler model system, namely, the isolated renal cell. In this report we will examine the effects of 1,25-(OH)₂D₃ on the uptake of phosphate by these isolated cells; first, after the vitamin is administered to a D-deficient chick in vivo, and second, when 1,25-(OH)₂D₃ is incubated with the cells in vitro.