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Kinetics of Dog Thyroid Secretion in Vitro^*

Institute of Interdisciplinary Research and Department of Pathology, School of Medicine, Free University of Brussels Brussels, Belgium

Address reprint requests to: Dr. J. Unger, Institut de Recherche Interdisciplinaire en Biologie Humaine et Nucl éaire, Bd de Waterloo 115, 1000 Bruxelles, Belgium.

Abstract

The time sequence of radioiodine sequestration and secretion (BE¹³¹I) have been compared in dog thyroid slices prelabeled with ^I31I in vivo and incubated in vitro with or without TSH. Sequestration has b0en taken to be the amount of radioiodine present in ph vacuoles or colloid droplets; the TSH or (Bu)₂cAMP stimulation of the basal values was suppressed by endocytosis blocking drugs. TSH induced a sequestrated radioactivity (S) after 5 min and a stimulated secretion after 20 min. The secretion rate was constant: 1%/h (mean ± SD = 1.0 ± 0.4; n = 7) of the total radioactivity of the slices. At equilibrium, S was constant and equal to less than 1% of the total radioactivity. The half-life of S, assuming a disappearance rate proportional to S, was 26 min (26 ± 4; n = 5); assuming a disappearance rate independent of S, the lifetime was 44 min (44 ± 7; n = 6). At the steady state, the limiting step of maximally stimulated secretion was the hydrolysis of the sequestrated radioactivity and endocytosis rate was equal to secretion rate. Without TSH, a constant BEI release (0.23% ± 0.07%/h; n = 7), insensitive to cytochalasin B, was observed, which corresponded to basal secretion.

Footnotes

^* This work was carried out within the framework of the As$ociation Euratom-Universite de Bruxelles-Universite de Pise, and thanks to a grant of the Ministere de la Politique Scientifique.

Received November 22, 1977.

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