Evidence for the Secretion of Thyrocalcitonin at Normal and Subnormal Plasma Calcium Levels

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Abstract
The role of the thyroid in response to Ca challenge was examined. A continuous Ca load of 2 mg/hr. for 4 hr. was delivered by peritoneal lavage using a 12.0-12.5 mg/100 ml Ca lavage rinse in rats made hypocalcemic by prior thyroparathyroidectomy and/or parathyroidectomy. The bones of these animals were labeled 1-2 weeks earlier with appropriate radioisotopes (85Sr, 45Ca). Although only slight differences in both plasma Ca and total transfer of Ca out of lavage fluid could be observed between the 2 groups, a marked difference developed in the amount of radioiso-tope transferred to lavage fluid. After 3-4 hr. animals with functional thyroids transferred 25% less radioactivity into the lavage fluid than animals without these glands. Control studies indicated that thyroxine was not responsible for this particular effect. This evidence was interpreted as a demonstration that thyrocalcitonin secretion was elicited by the Ca challenge. In addition, because plasma Ca levels were never elevated above normal, the data also suggested that thyrocalcitonin was secreted and effective in rats at normal and subnormal plasma Ca levels.