Tania Maria Ortiga Carvalho
ACUTE EFFECT OF THYROID HORMONES ON PITUITARY NEUROMEDIN B
Carvalho , Ortiga Maria Tania ; Professora adjunta do Instituto de Biofísica - UFRJ ; Instituto de Biofisica Carlos Chagas Filho Universidade Federal do Rio de Janeiro CCS- bloco G - Cidade Universitaria Ilha do Fundão - Rio de Janeiro CEP: 21949-900 - RJ - Brasil.
mRNA Neuromedin B (NB) is a bombesin-like peptide, highly concentrated in the pituitary gland, that has been shown to have an inhibitory action on TSH secretion, probably acting as a paracrine/autocrine regulator. It has been demonstrated taht pituitary NB and its mRNA are decreased in hypothyroid rats. We had shown earlier (Reg Pep 67:47, 1996), that a single dose of thyroid hormones given tp hypothyroid rats requires few hours to increase pituitary NB content above euthyroid levels. Here we evaluated if this fast response to thyroid hormones is time correlated to changes in NB mRNA abundance. Male rats, weighing 250-300 g were divided in groups of 5 animals each. All groups, except one (normal), received 0.03% methimazole in the drinking water during 3 weeks. In experiment I, triiofothyronine (T3)-0.4 µg/100 g B.W. (sc.) was given 3, 1or 1/2 h before sacrifice. In experiment II, thyroxine (T4)- 0.8 µg/100g B.W. was given 24, 6 or 3 h before sacrifice. Rats were decapitated, anterior pituitary were removed and total RNA was extracted. NB mRNA was determined by RNase protection assay. Serum (s)TSH was measured by specific RIA (NIDDK). In experiment I, 3 hours after a single in injection of T3, NB mRNA was almost 50 % higher than euthyroid group and sTSH was significantly decreased when compared with hypothyroid rats (3h: 13.4+/- 1.1; hypo: 26.3+/-2.9 ng/ml, p<0.05). In experiment II the peak response to T4 was at 6 hours after its injection, when NB mRNA content maximally increased (185%) compared to nomarl rats. The time pont of peak mRNA abundance are coincident with those reported to the peptide content, which is a further evidence are coincident with those reported to the peptide content, which is a further evidence of NB importance as a new regulatory path involved in acute inhibitory effect of thyroid hormone on TSH secretion.