25-hydroxy- and 1 alpha,25-dihydroxyvitamin D. Maternal-fetal relationship and the transfer of 1,25-dihydroxyvitamin D3 across the placenta in an ovine model.

Plasma concentrations of 25-hydroxy- and 1,25-dihydroxyvitamin D in chronically catheterized fetal and maternal sheep preparations and transfer of 3H-1,25-(OH)2-D3 from the fetus to mother and vice versa were studied. We tested the hypothesis that 1,25-(OH)2-D does not cross the placenta in either direction. While the fetal plasma 25-(OH)-D concentration was lower than the corresponding maternal values (n = 9, p less than 0.02), the fetal 1,25-(OH)2-D concentration was higher than the mother's (n = 9, p less than 0.02). 3H-1,25-(OH)2-D3 crossed the placenta from the fetus to the mother (n = 4) and vice versa (n = 4). We speculate, therefore, that maternal vitamin D metabolism may be affected by fetal vitamin D metabolism through the transplacental crossover of its highly active metabolite.