Effect of lowered vitamin D binding protein levels on the biological activity and metabolism of 1,25-dihydroxyvitamin D3

The authors studied the effect of lowered vitamin D binding protein levels on the biological activity and metabolism of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in vivo. Estrogen administration to vitamin D-deficient rats resulted in decrease of plasma vitamin D binding protein concentrations by about 20%. The authors administered graded doses of 1,25(OH)2D3 (5 - 5000 pmol intravenously) to vitamin D-deficient rats given estrogen or vehicle, and studied the biological response in intestine and bone. Intestinal calcium transport, following the administration of 1,25(OH)2D3, was similar in the estrogen or vehicle-treated groups. Serum calcium concentrations were lower in the estrogen-treated rats when compared to rats given vehicle. Serum calcium in both groups, however, increased the same amount over the range of 1,25(OH)2D3 given. The uptake of [3H] 1,25(OH)2D3 by the intestine and bone 8 hours after the administration of [3H] 1,25(OH)2D3 was similar in estrogen- and vehicle-treated rats. The amount of [3H] 1,25(OH)2D3 in plasma of estrogen-treated rats was the same as in vehicle-treated rats. The authors conclude that in estrogen-treated rats, lowered vitamin D binding protein levels do not alter the effect of 1,25(OH)2D3 on intestine or bone and do not alter the metabolism of 1,25(OH)2D3