Influence of the Thyroid on the Response of Rats to Administered Calcitonins

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Abstract
The calcitonins are known primarily as hypocalcemic agents and their activity as hypophosphatemic agents has been recognized but not stressed. The studies reported here were undertaken to examine more closely the effects of administration of chicken ultimobranchial extract (CUBE) and rat thyrocalcitonin (RTCT) on the relative drop in plasma calcium and phosphate levels in rats. In the course of the experiments, an unexpected relationship of the thyroids to the response of animals to injected calcitonins was discovered. Groups of rats bearing functional parathyroid autotransplants and maintained under standard conditions were administered either CUBE or RTCT. The hypocalcemic and hypophosphatemic responses in rats with intact thyroids were compared to those of thyroidecto-mized rats maintained on thyroxine. In all cases the hypocalcemic effect was greater in thyroidectomized rats, while the hypophosphatemic effect was increased only in the thyroidectomized animals given RTCT. In extensive studies in which the diet of the animals was the only variable, thyroidectomized animals still showed greater hypocalcemic responses to the extracts, whereas the differences in hypophosphatemic responses between the 2 groups could be eliminated. The conclusion reached is that the changes produced by thyroidectomy in the response of the rat to injected calcitonins are not simply due to an increased sensitivity but rather to a change in the relative effect of the agents on depression of plasma calcium and phosphate levels. Why thyroidectomy, accompanied by thyroxine replacement, should differentially alter the calcium and phosphate responses to calcitonin is not now known, and deserves further attention. (Endocrinology84: 1026, 1969)

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