KINETIC ASPECTS OF THE DEPRESSION OF 131IODIDE CONCENTRATION BY THYROTROPHIN IN THYROID TISSUE IN VITRO

Abstract

SUMMARY The changes in the kinetics of 131I131I− accumulation accompanying the thyrotrophin (TSH)-induced decrease in the tissue/medium concentration ratio (T*/M*) in propylthiouracil-blocked guinea-pig thyroid slices in vitro have been investigated. 131I− was eluted in serial 2 ml fractions from prelabelled 10 mg thyroid slices incubated for 10 h with TSH. Retrapping of 131I− from the extrafollicular space was abolished by including ClO4− in the elution medium. In the presence of ClO4−, departures from 1st order kinetics were detected by the analysis of regression of successive estimates of instantaneous exit rate, though not by the analysis of regression of logeTt*/T0*. It was predicted that initial exit data would be more representative of exit rates under equilibrium conditions. TSH gave increases in the fractional exit rate (Ktm) of similar order to those required to account for the decrease in T*/M*. The data support the hypothesis that an increase in 131I− exit rate mediates the decrease in T*/M* induced by TSH.