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Effects of in vivo triiodothyronine and long acting thyroid stimulator (LATS) administration on the in vitro thyroid cAMP response to thyrotrophin and LATS

Abstract. The present study was undertaken to examine the effects of prolonged in vivo treatment with T₃ and long acting thyroid stimulator (LATS) on in vitro responsiveness of mouse thyroid cyclic AMP to thyrotrophin (TSH) and LATS-immunoglobulin G (IgG). In control mice, thyroid cAMP concentrations after incubation with normal-IgG (10 mg/ml) for 2 h. TSH (10 mU/ml) for 10 min and LATS-IgG (10 mg/ml) for 2 h were 1.25 ± 0.11 (mean ± SE) (n = 5), 15.87 ± 3.47 (n = 6) and 2.17 ± 0.25 pmoles/mg wet weight (n = 6), respectively. In mice given T₃ (5 µg/ml in drinking water for 5 days, thyroid cAMP concentrations after an incubation with TSH were reduced by 50%, as compared to those of the control mice. They were also decreased in mice injected ip with 5 mg of LATS-IgG (1000%/5 mg in the McKenzie bioassay) daily for 5 days. Combined treatment with T₃ and LATS decreased the cAMP response to TSH only to the same extent as did T₃ alone, indicating that the inhibitory effects of T₃ and LATS were not additive. Similar findings were observed with the thyroid cAMP response to LATS-IgG in vitro; either T₃ or LATS treatment in vivo decreased cAMP response to LATS-IgG in vitro, but combined treatment with T₃ and LATS did not cause further inhibition as compared with T₃ or LATS treatment alone.

These results indicate, 1) that prolonged in vivo T₃ treatment inhibits the in vitro thyroid cAMP response not only to TSH but also to LATS-IgG, 2) that prolonged in vivo LATS treatment also suppresses the thyroid cAMP response both to TSH and LATS-IgG and 3) that the inhibitory effects of LATS may not be due to the effects of LATS per se but to increases in circulating thyroid hormone levels induced by prolonged LATS treatment.