Vitamin D metabolism in pregnant rabbits: differences between the maternal and fetal response to administration of large amounts of vitamin D3.

Maternal and fetal metabolism of vitamin D was examined in term pregnant rabbits fed a normal diet and in those supplemented with a large amount of vitamin D3. Term pregnant rabbits (27--30 days of gestation) fed the normal diet showed lower levels of plasma calcium, 25-hydroxyvitamin D3 (250HD3), and 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] and higher plasma 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25-(OH)2D3] levels than age-matched nonpregnant female rabbits. Kidney homogenates from pregnant rabbits produced mainly 1 alpha 25-(OH)2D3, while those from nonpregnant animals produced 24,25-(OH)2D3 primarily. Plasma concentrations of calcium and phosphorus were significantly higher in fetuses than in mothers. Plasma levels of 250HD3 and 24,25-(OH)2D3 in fetuses were almost identical to those in mothers, whereas 1 alpha,25-(OH)2D3 levels in plasma were significantly higher in mothers than in their fetuses. A daily administration of 650 nmol vitamin D3 for 3 days to term pregnant rabbits caused a significant increase in calcium, phosphorus, 25OHD3, and 24,25-(OH)2D3 in maternal plasma, and in 25OHD3 and 24,25-(OH)2D3, but not calcium and phosphorus in fetal plasma. Treatment with large amounts of vitamin D3 also induced a marked suppression of 1 alpha-hydroxylase activity and a concomitant increase of 24-hydroxylase activity in the maternal but not in the fetal kidney. Plasma concentrations of 1 alpha,25-(OH)2D3 were not affected by treatment with large amounts of vitamin D3 in either the fetuses or the mothers. These results clearly indicate that the renal 25OHD3 metabolism in the fetus is regulated independently of that in the mother.