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Vitamin D Metabolism in Pregnant Rabbits: Differences between the Maternal and Fetal Response to Administration of Large Amounts of Vitamin D₃

Division of Nephrology, Department of Medicine, Juntendo University School of Medicine, Bunkyo-ku, Tokyo 113
Department of Biochemistry (Y.S., T.S.), School of Dentistry, Showa University, Shinagawa-ku, Tokyo 142, Japan

Address correspondence and requests for reprints to: Dr. Tatsuo Suda, Department of Biochemistry, School of Dentistry, Showa University, 1-5-8 Hatanodai, Shinagawa-ku, Tokyo 142, Japan.

Abstract

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 (25OHD₃), and 24,25-dihydroxyyitamin D3 [24,25-(OH)₂D₃] and higher plasma l25-dihydroxyvitamin D₃ [l,25-(OH)2D₃] levels than age-matched nonpregnant female rabbits. Kidney homogenates from pregnant rabbits produced mainly l25-(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 25OHD₃ and 24,25-(OH)2D₃ in fetuses were almost identical to those in mothers, whereas la,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 la-hydroxylase activity and a concomitant increase of 24-hydroxylase activity in the maternal but not in the fetal kidney. Plasma concentrations of l25- (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.

Received September 17, 1981.

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