Transfer Of 3h-1-25 Dihydroxyvitamin D3 (3h-1,25d3) From The Fetus To The Mother In Ovine Model

Maternal plasma 1-25 dihydroxyvitamin D3 (1-25 D3) levels increase during pregnancy; the source of this increase remains unclear and could be the fetus. Transfer of 3H-1,25 D3 from fetus to mother was studied in chronically catheterized ewes (n=5, gestation 125-146 days). Ten μCi of 3H-1,25 D3 was given by bolus in the fetal jugular vein. Fetal and maternal carotid arterial blood was collected at -10, 1, 5, 15, 30, 45, 60, 120, 180 and 240 min and at 24 and 48 hrs. Radioactivity in 0.1 ml of plasma was quantitated as CPM and DPM. Recovery of 3H-1,25 D3 in the fetal and maternal plasma at 4 hrs was assessed by high pressure liquid chromatography (HPLC). Plasma 1,25D (HPLC-protein binding assay) in fetuses and ewes was 29-82 and 26-45 pg/ml respectively. In all fetuses, initial disappearance of 3H-1,25 D3 was rapid (t 1/2 90% of radioactivity recovered at 4 hrs was 1,25 D3. We conclude that 1) clearance of 1,25 dihydroxyvit D3 in the fetus is curvilinear suggesting more than two pools, and 2) 1,25 dihydroxyvit D3 may cross the placenta from the fetus to the mother.