Thyrocalcitonin Inhibition of Bone Resorption Induced by Parathyroid Extract in Thyroparathyroidectomized Rats1

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Abstract
The mode of action of thyrocalcitonin on bone was examined in vivo in rats. Rats that had been injected 24 hr. previously with 45Ca were thyroparathyroid-ectomized and injected with parathyroid extract (PTE). The resulting hypercalcemia, enhanced by the thyroidectomy, was associated with decreased specific activity of serum calcium, a result that supports the concept that parathyroid hormone mobilizes calcium from "stable" bone. Thyrocalcitonin was injected 2 1/2 hr after the administration of PTE and again 1 1/4 hr later. During the 2 1/4 hr. after thyrocalcitonin treatment began, serum calcium decreased by 33% but serum Ca. decreased by only 17%. The results indicate that thyrocalcitonin inhibited the release of calcium from both "stable" and "labile" bone, but that the inhibition was more marked in unlabeled bone, presumably the same area of bone that was responsive to parathyroid hormone.