Acute effect of thyroxine on pituitary neuromedin B content of hypothyroid rats and its correlation with TSH secretion.
Neuromedin B (NB) is a bombesin-like peptide that has been recently characterized as a physiological paracrine/autocrine inhibitor of thyrotropin (TSH) secretion. We report here the time course of the effect of thyroxine (T4) administration to hypothyroid rats on the anterior pituitary content of NB. Dutch-Miranda male rats weighing 250-300 g received 0.03% methimazole in the drinking water for 3 weeks. T4 (0.8 microgram/100 g body weight, sc) was given 1/2, 1, 3 or 6 h before sacrifice. One group received saline rather than T4 (hypothyroid control). The groups contained 6 to 8 animals each. NB, extracted from tissue by boiling in acetic acid, was measured by radioimmunoassay, using a highly specific antiserum. Pituitary NB content was significantly increased 4-fold, as early as 1/2 h after T4 injection, while serum TSH level was similar to that of the hypothyroid control group. The peak response to T4 was at 3 h after injection, when NB content was increased 8-fold (hypothyroid: 45 +/- 8; 1/2 h, 223 +/- 15; 1 h, 203 +/- 48; 3 h, 383 +/- 31; 6 h, 224 +/- 30 fmol/mg protein) and serum TSH decreased to the level of normal rats (0.93-1.5 ng/ml) generally observed in our laboratory (hypothyroid: 31 +/- 3; 1/2 h, 26 +/- 3; 1 h, 31 +/- 2; 3 h, 1.3 +/- 0.1; 6 h, 3.7 +/- 0.6 ng/ml). These data suggest that NB synthesis is rapidly induced by thyroxine and this might represent a new regulatory path involved in the acute inhibitory effect of thyroid hormones on TSH secretion.
Laboratório de Fisiologia Endócrina, Instituto de Biofísica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, Brasil.
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D000818:Animals; D007037:Hypothyroidism / Q000378:metabolism*; D008297:Male; D015287:Neurokinin B / Q000031:analogs & derivatives* / Q000032:analysis; D010902:Pituitary Gland / Q000737:chemistry* / Q000187:drug effects*; D051381:Rats; D013972:Thyrotropin / Q000378:metabolism*; D013974:Thyroxine / Q000494:pharmacology*
D015287:Neurokinin B; C038393:neuromedin B; D013972:Thyrotropin; D013974:Thyroxine