In VitroSecretion of Immunoreactive Rat Thyrocalcitonin12

Abstract

Short-term incubation of thyroid glands from 8 day old rats was employed to study secretion of rat thyrocalcitonin (TCT) in vitro. Thyroid glands were incubated in a chemically defined culture medium for up to 24 h, and TCT in medium and tissue was measured by radioimmunoassay. Test agents were added directly to the medium; most were tested at more than 1 dose, and all were tested at 2 different concentrations of medium Ca (1 between 0-1 mM and 1 between 2-3 mM). TCT release into medium increased progressively during the first 8-12 h of incubation, being related to the amount of tissue incubated per flask. Release of TCT was directly related to the medium Ca concentration over the range 0-3 mM; e.g., levels of TCT found in medium containing 2.5 mM Ca after incubation of thyroid glands for 8 h were 10-fold greater than those found in medium containing 0 or 1 mM Ca. Other than an increase in medium Ca, the only condition tested which increased TCT release in vitro was addition of dibutyryl c[cyclic]AMP to the medium. A variety of conditions inhibited TCT release in vitro, indicating that the in vitro system being studied was reliable; inhibition of metabolic activity by lowering temperature from 37 to 0.degree. C or adding iodoacetate, cyanide or cycloheximide to the incubation medium all produced striking inhibition of TCT release. A variety of agents were added to the medium and found not to affect release of rat TCT in vitro. These ineffective agents included Pi (0-5 mM), porcine gastrin (0.1-200 .mu.g/ml), synthetic secretin (1 U[unit]/ml), isoproterenol (10-4 M), K (25 mM), bovine parathyroid hormone (100 U/ml), TSH [thyrotropin] (50 mU/ml), ovine prolactin (100 .mu.g/ml) and 1,25-(OH)2 vitamin D3 (25 ng/ml). Secretion of rat TCT in vitro is especially sensitive to stimulation by Ca and by cAMP and unresponsive to a variety of other agents tested including P levels. The rat thyroid gland, unlike that of certain other mammalian species, does not appear to be especially responsive to gastrin.