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Dental Research Center and Department of Pharmacology, University of North Carolina Chapel Hill, North Carolina 27514
Address all correspondence and requests for reprints to: Dr. Svein U. Toverud, Dental Research Center, University of North Carolina, Chapel Hill, North Carolina 27514.
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/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 overnight fast. However, after overnight feeding ad libitum 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) caused a significant increase in the serum CT level in lactating rats but not in nonlactating rats. When milk secretion was temporarily halted by removal of the litters, eating ad libitum for 2 h after fasting caused an increase in serum Ca and serum P in thyroiddeprived rats but not in thyroid-intact rats.
These results indicate that CT is secreted continuously in response to feeding in lactating rats and that it opposes the effect of PTH on the serum Ca level. We suggest that the elevated serum level of CT during lactation contributes to the maintenance of a relatively low serum Ca level, thereby aiding in conservation of Ca.
Footnotes
* This work was presented in part at the 58th Annual Meeting of the U.S. Endocrine Society, San Francisco, CA, June 1976, and at the Sixth Parathyroid Conference, Vancouver, British Columbia, Canada, June 1977. This investigation was supported by NIH Research Grant DE-02668 from the National Institute of Dental Research, by NIH Career Development Award AM-50293 (C.W.C.; 1972–1976), by NIH Research Grants AM-10558 and AM-17743 from the NIAMDD, and by NIH Grants RR-05333 and RR-05406 from the Division of Research Facilities and Resources.
Received August 5, 1977.
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