A Study of Thyrocalcitonin Secretion by Direct Measurement ofin VivoSecretion Rates in Pigs1

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Abstract
Surgically isolated pig thyroid glands, without parathyroid tissue, were perfused in situ with blood, the Ca concentration of which could be varied. Complete, measured collections of thyroid venous blood were achieved, and the thyrocalcitonin (TC) content of each sample was determined by intravenous bioassay in young rats. The time course of changes in perfusing Ca concentration was closely paralleled by similar changes in TC secretion rate. There was a linear relationship between these 2 parameters over the range 6-10 mEq Ca/liter. The sensitivity of the assay was such that TC secretion could not be detected with certainty at Ca concentrations below 6 mEq/l, although indirect evidence from other experiments indicated that it does occur. The TC secretion rate was usually small relative to the total TC content of the thyroid gland.