The thyrotrophin receptor in guinea-pig thyroid homogenate: general properties

Particulate fractions of guinea-pig thyroid homogenate contained a single class of receptors which bound 125I-labelled bovine thyrotrophin (TSH) by a saturable, reversible process with an affinity constant of 2 × 109 1/mol. The binding process was specific for TSH, and corresponded with the activation of adenylate cyclase. Cleavage of hormone—receptor bonds by treatment with lyotropic agents resulted in the release of unchanged labelled TSH. The radioligand receptor assay system was sensitive to 0·015 mu. TSH. Bovine or mouse thyroid showed reduced binding affinity with correspondingly reduced sensitivity.

The thyrotrophin receptor in guinea-pig thyroid homogenate: interaction with the long-acting thyroid stimulator

Immunoglobulin G (IgG) prepared from sera containing the long-acting thyroid stimulator (LATS) inhibited the receptor binding of 125I-labelled thyrotrophin (TSH) in a particulate fraction of guinea-pig thyroid homogenate. This inhibition was shown to involve a binding interaction between IgG containing LATS and the receptor with a diminution in the number, but not affinity, of sites available for binding of TSH. Studies of dissociation kinetics and gel filtration of receptor—TSH complexes indicated that IgG containing LATS did not combine with receptors occupied by TSH. The data provide evidence that LATS and TSH bind to the same receptor site.

Thiocyanate ion in serum as an interfering factor in the in-vitro bioassay of thyrotrophin

The presence of thiocyanate ion in human and rat serum has been shown to account entirely for the non-specific activity of sera in an in-vitro bioassay for thyrotrophin. Thiocyanate was identified by its chromatographic behaviour on Sephadex G-10, G-15 and G-25, and by the ferric nitrate and cobalt nitrate tests. Cigarette smoking increased mean serum thiocyanate levels (as NaSCN) from 0·2 to 0·56 mg/100 ml. It is suggested that serum thiocyanate levels are sufficient to inhibit significantly iodide trapping in vivo and that these findings may be relevant to the non-specific responses observed with other in-vitro bioassays based on radioiodine dynamics.

Reversible binding of labelled and non-labelled thyrotrophin by intact thyroid tissue in vitro

125I-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 × 1081/mol and a capacity of 8 × 103 molecules of TSH/cell; low order sites, affinity constant of 2·9 × 1071/mol and capacity of 8 × 104 molecules/cell. The highorder sites saturate at about 1 mu./ml, approximately the level producing maximal in-vivo and in-vitro responses. Binding of 125I-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.

Kinetic aspects of the depression of 131 iodide concentration by thyrotrophin in thyroid tissue in vitro

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.

The influence of methallibure (ICI 33,828) on serum and pituitary thyroid-stimulating hormone levels in the rat

The effect of methallibure (ICI 33,828), a non-steroidal pituitary inhibitor, on serum and pituitary thyroid-stimulating hormone (TSH) levels has been investigated.

A biphasic action of the drug on serum TSH levels was observed, the greatest falls occurring with the lowest doses (2mg/day). Increasing dose and period of administration induced progressive decreases in pituitary TSH content. These results are interpreted in terms of three actions on the thyroid—pituitary system: (1) inhibition of the release of TSH from the pituitary, (2) inhibition of TSH synthesis evident only at higher doses, and (3) a thyroid-blocking action, which is also only observed at the higher dose levels, with consequent pituitary stimulation via the thyroid—pituitary feedback mechanism. Effects upon body weight and weight of endocrine organs are reported, that upon the seminal vesicles being the most marked.