COMPARATIVE EFFECTS OF THYROTROPHIN AND LONG-ACTING THYROID STIMULATOR ON IODIDE TRAPPING IN ISOLATED THYROID CELLS

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Abstract
The effects of thyrotrophin (TSH) and long-acting thyroid stimulator (LATS) in isolated bovine thyroid cells were studied. Both TSH (1 to 100 mU/ml) and LATS-IgG (0.25 to 3.5 mg/ml; mouse bioassay response index = 500 %/mg) stimulated I− trapping in a dose-related manner with a maximum 58 ± 13 % increase and 62 ± 18 % increase over control 131I cell/medium (C/M) ratios, respectively. The stimulatory effects of TSH and LATS-IgG were similarly inhibited by Li+ ion, by membrane stabilizers (chlorpromazine, trifluoperazine) at concentrations > 1 × 10−5 m and by phentolamine, 10−6 to 10−4 m, but were unaffected by propranolol. When ineffective concentrations of either stimulator were combined with effective concentrations of the other, a significant reduction in stimulation of iodide trapping was consistently observed. Combinations of maximally effective concentrations of TSH and LATS had no additive effects on C/M ratio. Using equipotent (based on 4.5 h incubations) concentrations of TSH and LATS-IgG, TSH augmentation of I− trapping was apparent only after 60 min incubation, whereas LATS increased C/M ratios within 15 min. These data provide further evidence that: 1) the thyroidal sites of action of TSH and LATS are similar or identical, and, 2) LATS is not slower in onset of action than TSH in vitro and with equipotent doses may be even faster.

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