title: Evidence for a negative feedback in the control of eel growth hormone by thyroid hormones.
abstract: The regulation of growth hormone (GH) by thyroid hormones (THs) has been shown to present species variation. We investigated the regulation of GH in the eel, a representative of an ancient group of teleosts. In vivo administration of triiodothyronine (T(3)) or thyroxine (T(4)) significantly reduced pituitary and serum GH levels, as measured by homologous RIA. In order to investigate the ability of THs to regulate GH production directly at the pituitary level, we used a long-term, serum-free primary culture of eel pituitary cells. Both T(3) and T(4) inhibited GH release in a concentration-dependent manner, producing up to 50% inhibition at 10 nM, with an ED(50) of <0.2 nM, within the range of their physiological circulating levels. Other hormones also acting via the nuclear receptor superfamily, such as sex steroids (testosterone, estradiol and progesterone) and corticosteroid (cortisol), had no effect on GH release in vitro, underlining the specificity of the regulatory effect of THs on GH. Measurement of both GH release and cellular content for calculation of GH production in vitro indicated that THs not only inhibited GH release but also GH synthesis. Dot-blot assay of GH messenger RNA (mRNA) using an homologous eel cDNA probe showed a decrease in GH mRNA levels in cells cultured in the presence of T(3), as compared with control cells. This demonstrated that the inhibition of T(3) on GH synthesis was mediated by a decrease in GH mRNA steady state levels. In conclusion, we demonstrate inhibitory regulation of eel GH synthesis and release by THs, exerted directly at the pituitary level. These data contrast with the rat, where THs are known to have a stimulatory effect and suggest that the pattern observed here in an early vertebrate and also found in birds, reptiles and some mammals including humans, may represent an ancestral and more generalized vertebrate pattern of TH regulation of pituitary GH.
authors.affiliation: Laboratoire de Physiologie Générale et Comparée, Muséum National d'Histoire Naturelle, Centre National de la Recherche Scientifique, 7 rue Cuvier, 75231 Paris Cedex 05, France.
authors.forename: K
authors.initials: K
authors.lastname: Rousseau
authors.forename: N
authors.initials: N
authors.lastname: Le Belle
authors.forename: M
authors.initials: M
authors.lastname: Sbaihi
authors.forename: J
authors.initials: J
authors.lastname: Marchelidon
authors.forename: M
authors.initials: M
authors.lastname: Schmitz
authors.forename: S
authors.initials: S
authors.lastname: Dufour
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D002478:Cells, Cultured; D003864:Depression, Chemical; D004305:Dose-Response Relationship, Drug; D004524:Eels / Q000378:metabolism*; D025461:Feedback, Physiological; D013006:Growth Hormone / Q000235:genetics / Q000378:metabolism*; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism; D012333:RNA, Messenger / Q000032:analysis; D011863:Radioimmunoassay; D013004:Somatostatin / Q000494:pharmacology; D013045:Species Specificity; D013963:Thyroid Hormones / Q000494:pharmacology*; D013974:Thyroxine / Q000494:pharmacology; D014284:Triiodothyronine / Q000494:pharmacology
chemical_list: D012333:RNA, Messenger; D013963:Thyroid Hormones; D014284:Triiodothyronine; D013004:Somatostatin; D013006:Growth Hormone; D013974:Thyroxine