Cholecalciferol and placental calcium transport.

Transplacental movement of calcium from mother to fetus is essential for normal fetal development. In most species, fetal plasma calcium levels are higher than maternal levels at term. The role of cholecalciferol metabolites, with specific emphasis on 1,25-dihydroxycholecalciferol (1,25(OH)2D), in placental calcium transport and maintenance of the fetomaternal gradient has been extensively investigated. In rats, there is not an absolute demand for 1,25(OH)2D for maintenance of fetal calcium homeostasis in utero, even though it is essential for maintenance of maternal plasma calcium levels. However, in sheep, the absence of 1,25(OH)2D results in disruption of both maternal and fetal calcium homeostasis. It is known that rat and human placentas contain specific cytosolic binding proteins for 1,25(OH)2D that are similar to the well-characterized intestinal receptor. Two calcium-binding proteins (CaBP) have been detected in rat and human placentas: a protein immunologically identical to the vitamin D-dependent CaBP and a calcium-dependent ATPase. The levels of CaBP in rat placenta have been shown to increase in response to exogenously administered 1,25(OH)2D but cannot be obliterated with maternal vitamin D deficiency. No relationship has been shown between 1,25(OH)2D and placental Ca-ATPase in any species. Thus, the mechanism of action of 1,25(OH)2D in maintenance of the transplacental calcium gradient in sheep is unknown. In the pregnant rat (and perhaps human), 1,25(OH)2D is a critical factor in the maintenance of sufficient maternal calcium for transport to the fetus and may play a role in normal skeletal development of the neonate.