Kinetics of Dog Thyroid Secretion in Vitro*

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Abstract
The time sequence of radioiodine sequestration and secretion (BE131I [butanol extractable radioactivity]) have been compared in dog thyroid slices prelabeled with 131I in vivo and incubated in vitro with or without TSH [thyrotropin]. Sequestration was the amount of radioiodine present in phagocytic vacuoles or colloid droplets; the TSH or (Bu)2cAMP [dibutyryl cyclic AMP] 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 S 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.

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