Serum Calcium and Parathyroid Hormone during the Reproductive Cycle in Normal and Vitamin D-Deficient Rats*

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Abstract
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.