Evidence for a direct action of cholecalciferol and 25-hydroxycholecalciferol on the renal transport of phosphate, sodium, and calcium.

Acute clearance studies were performed in stable thyroparathyroidectomized dogs to evaluate the possibility of a direct renal action of vitamin D and its biologically active 25-hydroxylated metabolite. Alterations in renal hemodynamics and serum calcium concentration were minimized and attempts at vitamin D depletion were not undertaken. Steady-state volume expansion of modest degree was employed as the control experimental situation so that an effect of the vitamin to enhance phosphate reabsorption would not go undetected because of an already maximal phosphate reabsorptive rate secondary to parathyroidectomy. Under these experimental circumstances, 10,000 U of cholecalciferol and 25-120 U of 25-hydroxycholecalciferol (25-HCC) produced significant depressions in the percentage of filtered phosphate excreted (mean declines of 39 and 47%, respectively), which were not attributable to alterations in renal hemodynamics or to changes in the levels of serum calcium or phosphate. There was an accompanying decline in sodium and calcium excretion; mean percentage excretion rates for sodium fell by 38% with vitamin D and 26% with 25HCC, and for calcium this measurement declined by 46 and 23%, respectively. Furthermore, parathyroid hormone and 25HCC produced antagonistic effects on phosphate excretion. These observations provide the first conclusive evidence for a direct (proximal) tubular action of vitamin D to promote phosphate (as well as sodium and calcium) transport.