Parathyroid hormone and 25-hydroxy vitamin D3: synergistic and antagonistic effects on renal phosphate transport.

The effect on phosphate excretion of graded doses of parathyroid hormone (PTH) and the biologically active vitamin D3 metabolite, 25-hydroxycholecalciferol (25-HCC), administered singly and in combination, were studied in the nonexpanded, vitamin D-depleted thyroparathyroidectomized rat. Infusion of 1 unit of 25-HCC per hour for 6 hours induced an antiphosphaturia only when administered with 0.2 units of PTH per hour, while neither agent alone changed phosphate excretion. A dose of 2.0 units of PTH per hour did not cause phosphaturia unless given with 1 unit of 25-HCC per hour. In pharmacologic dosage (5 units per hour), PTH produced phosphaturia in the absence of the metabolite.