Serum calcium and parathyroid hormone during the reproductive cycle in normal and vitamin D-deficient rats

This study was designed to investigate the ability of the vitamin D-deficient (-D) rat to maintain calcium (Ca) homeostasis during pregnancy. Serum Ca and immunoreactive parathyroid hormone (iPTH) were measured before and during pregnancy and after lactation in normal (+D) and -D rats. Serum Ca increased from 9.4 +/- 0.3 to 10.2 +/- 0.2 mg/dl during the first 3 days of pregnancy in +D animals and then declined throughout the remainder of gestation to 9.7 mg/dl at 17 days of gestation. In contrast, serum Ca rose progressively during the first 17 days of pregnancy in -D rats from 5.5 +/- 0.2 to 6.7 +/- 0.4 mg/dl. As expected, the -D rats had markedly elevated serum iPTH, but there were no changes in circulating iPTH during pregnancy in either animal model, even though serum Ca increased significantly during pregnancy in -D animals and Ca decreased in the +D rats. After lactation, serum Ca of -D rats increased 2 mg/dl before returning to prepregnancy levels. These results suggest that there are physiological mechanisms which appear to be independent of vitamin D and are responsible for elevating serum Ca during pregnancy and lactation.