Acute effects of a "physiological" dose of 1,25-dihydroxy vitamin D3 on renal phosphate transport.

The renal phosphate transport response of thyroparathyroidectomized, vitamin D-deficient rats to the infusion of 1,25-dihydroxy vitamin D3 (1,25 D3) was studied with and without the simultaneous administration of a small (or "permissive") non-phosphaturic amount of bovine parathyroid hormone (bPTH). Although phosphate excretion (UPV) was unaltered by the infusion of either 0.1 U (= .0025 microgram or 6 pmoles) of 1,25 D3 or 0.2 U bPTH per hour for 6 hours, their combined administration reduced UPV from 14.8 +/- 1.6 to 10.3 +/- 1.2 microgram/min. (P less than .05). There were no alterations in inulin excretion. These data verify that: 1) 1,25 D3 is antiphosphaturic in this experimental setting in a very low dose which may represent a "physiological" amount of the metabolite; and 2) to enhance phosphate transport, the 1,25 D3 requires the presence of a small ("permissive") amount of PTH.