Calcium Metabolism during Lactation: Elevated Blood Levels of Calcitonin*

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Abstract
The relatively low serum Ca found in intact lactating rats (1 mg/100 ml lower than in nonlactating rats) could be due to parathyroid hormone (PTH) deficiency, end-organ insensitivity to PTH or increased calcitonin (CT) secretion or activity. These possibilities were examined by comparing lactating rats with nonlactating rats recently deprived of their litters. Parathyroidectomy caused a 2 mg/100 ml decrease in serum Ca within 3 h in fed lactating rats but no change in fed nonlactating rats. Injection of parathyroid extract (40-460 U[units]/100 g BW [body wt]) did not affect serum Ca level in intact lactating or nonlactating rats 3.5 h later, even though lactating rats were hypocalcemic. After thyroparathyroidectomy, PTH injection produced hypercalcemia (> 11.5 mg/100 ml) in both groups, indicating that hypocalcemia during lactation is not due to PTH deficiency or unresponsiveness. The CT content of thyroid glands as well as the blood level of CT was the same in lactating and nonlactating rats after an overnight fast. After overnight feeding ad lib or refeeding after fasting, the serum CT level was 2- to 3-fold higher in lactating than in nonlactating rats. Intragastric Ca administration (10 mg Ca/100 g BW) significantly increased serum CT level in lactating rats but not in nonlacting rats. When milk secretion was temporarily halted by removal of the litters, eating ad lib for 2 h after fasting caused an increase in serum Ca and serum P in thyroid-deprived rats but not in thyroid-intact rats. CT apparently is secreted continuously in response to feeding in lactating rats, and it opposes the effect of PTH on the serum Ca level. The elevated serum level of CT during lactation may contribute to maintenance of a relatively low serum Ca level, thereby aiding in conservation of Ca.