25-Hydroxyvitamin D3 metabolism in the pregnant rat: maternal-fetal relationship

Metabolism of 3H-25OHD3 was studied in pregnant, D-deprived rats and their fetuses at 21 days' gestation. Significant changes in the circulating levels of 3H-24,25(OH)2D3 and 3H-1,25(OH)2D3 in maternal and fetal plasma occurred from 4 to 12 h after the administration of 233 ng of 3H-25OHD3. 3H-1,25(OH)2D3 appeared before 3H-24,25(OH)2D3 in both the mother and fetuses. Maternal plasma contained more 3H-1,25(OH)2D3 than fetal plasma and, contrariwise, fetal plasma contained more 3H-24,25(OH)2D3 than maternal plasma. Maternal nephrectomy studies showed that the in vivo synthesis of 3H-24,25(OH)2D3 at 6 h was dependent on circulating levels of 3H-1,25(OH)2D3. These results revealed aspects of maternal-fetal metabolism of 25OHD3 similar to those in the non-pregnant state as well as others unique to pregnancy.