Calcium metabolism during lactation: enhanced effects of thyrocalcitonin

Lactating rats were compared with nonlactating controls, with regard to the intake and absorption of calcium, serum calcium level, and te protective effect of thyroacalcitonin (TC) against hypercalcemia and hyperphosphatemia. While consuming a commercial diet, intact, nonfasted lactating rats maintained a serum calcium level of approximately 9 mg/100 ml, which was 1 mg/100 ml lower than that of nonlactating controls. The level rose to that of the controls within one day after removal of the litters from the mother. Compared with nonlactating rats, lactating rats had a three-fold higher calcium intake and a six-fold higher rate of net absorption of calcium. After intragastric calcium (10 mg/100 g body wt) the increase in serum calcium was small (1 mg/100 ml) 2 h later in both groups of sham-operated rats but was markedly increased in thyroparathyroidectomized groups, with the lactating rats showing a significantly greater increase than the nonlactating rats. The injection of a small dose of porcine thyrocalcitonin completely counteracted this hypercalcemia in lactating rats, but did not have any effect on nonlactating controls. Protection by the thyroid gland against hyperphosphatemia after intragastric calcium also was significant in both lactating and nonlactating rats. The results show that TC is much more effective in lactating than in nonlactating rats, suggesting that TC may be of particular importance in lactation by restricting elevations of serum calcium and phosphorus levels after eating, thereby aiding in conservation of these ions.