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Calcium metabolism during lactation: elevated blood levels of calcitonin.

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/100 g BW) did not affect the serum Ca level in intact lactating or nonlactating rats 3.5 h later, even though lactating rats were hypocalcemic. However, 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 overnig...