Calciotrophic hormone levels and calcium absorption during pregnancy in rats.

The mammalian maternal skeleton stores Ca during pregnancy perhaps for fetal skeletal mineralization in late pregnancy and milk production during lactation. The interrelationships between intestinal Ca absorption and hypertrophy and plasma levels of total Ca, ionized Ca, 25-hydroxyvitamin D3 [25(OH)D], 1,25-dihydroxyvitamin D3 [1,25(OH)2D], and parathyroid hormone (PTH) were determined at different stages of pregnancy in rats. By midpregnancy and before fetal skeletal mineralization, plasma ionized Ca levels, Ca absorption by duodenal tissue in vitro, Ca absorption by the duodenum in situ, and duodenal wet weight were increased and 25(OH)D was decreased. Later in pregnancy, during fetal skeletal mineralization, 1,25(OH)2D and PTH levels were also substantially increased and total serum Ca levels decreased. These data demonstrate changes by midpregnancy, before fetal skeletal mineralization, in maternal mineral homeostasis concomitant with known changes in skeletal metabolism. Some of the early changes in mineral metabolism may occur independent of the vitamin D and PTH endocrine system.