Transfer of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 across the perfused human placenta

The rates of placental transfer of 25-hydroxyvitamin D3 [25-(OH)D3] and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] were determined in an in vitro perfusion system. Antipyrine was included in each perfusion, and the data are expressed as clearance index, the ratio of hydroxyvitamin D3 to antipyrine clearance. In most experiments, serum, 0.2%, was added to the perfusates as a source of vitamin D3-binding protein. Binding as measured by dextran-coated charcoal assay for 25-(OH)D3 was over 90%, for 1,25-(OH)2D3, only 25% to 50%. The clearance index from the maternal to fetal circulation averaged 0.02 and 0.26 for 25-(OH)D3 and 1,25-(OH)2D3, respectively. When vitamin D3-binding protein was omitted from the perfusate, the clearance indices of 25-(OH)D3 were 0.12 and 0.46 in two experiments. Binding to vitamin D3-binding protein is a major determining factor for the transfer rates of 25-(OH)D3 and 1,25-(OH)2D3.