REVERSIBLE BINDING OF LABELLED AND NON-LABELLED THYROTROPHIN BY INTACT THYROID TISSUE IN VITRO

Abstract

SUMMARY 125 I-Labelled thyrotrophin (TSH) showed saturable binding to hyperplastic guinea-pig thyroid slices in vitro , but not to kidney, adrenal, liver, testis or salivary gland slices. Two populations of binding sites were demonstrated: high order sites showed an affinity constant of 3·8 × 10 8 1/mol and a capacity of 8 × 10 3 molecules of TSH/cell; low order sites, affinity constant of 2·9 × 10 7 1/mol and capacity of 8 × 10 4 molecules/cell. The highorder sites saturate at about 1 mu./ml, approximately the level producing maximal in-vivo and in-vitro responses. Binding of 125 I-labelled TSH was reversible, the released hormone appearing substantially unchanged as assessed by chromatographic behaviour and antiserum binding. Release of unlabelled TSH from pre-incubated thyroid slices was demonstrated by bioassay. Bovine thyroid slices were found to have a lower capacity for binding than hyperplastic guinea-pig slices.