Thyroidal inhibition of chicken pituitary growth hormone: alterations in secretion and accumulation of newly synthesized hormone

Abstract

Hypothyroidism reduces GH synthesis and release in several mammalian species, in which thyroid hormone directly stimulates GH gene transcription. In contrast, hypothyroidism stimulates GH secretion in birds, in which thyroid hormone directly inhibits pituitary GH release. We have, therefore, investigated the effects of thyroid status on the accumulation of newly synthesized GH in the pituitaries of 8- to 10-week-old Leghorn cockerels in vitro and in vivo. The incorporation of [35S]methionine into immunoprecipitable GH ([35S] GH) was increased, over a 4-h incubation period, in glands from birds made hypothyroid by injections of methimazole (50 mg/kg day for 10 days) in comparison with glands from vehicle-injected controls. Treatment with tri-iodothyronine (T3, 100 micrograms/kg per day for 10 days) in vivo did not significantly alter the accumulation of [35S]GH in vitro but did block the release of [35S]GH in response to a GH secretagogue (thyrotrophin-releasing hormone; exposure to 280 nmol/l for 30 min) and reduced immunoassayable pituitary GH content. Pretreatment of glands from euthyroid birds with T3 (100 nmol/l) in vitro (for 20 h) reduced the basal accumulation of [35S]GH as well as that induced by another GH secretagogue (GH-releasing factor; 100 nmol/l) during a 6-h labelling period. These results show that, unlike the generally stimulatory action of thyroid hormone in mammals, in birds, T3 exerts a direct inhibitory effect on the accumulation of newly synthesized pituitary GH.

Thyroid glands: novel sites of growth hormone action

“…These hormones also appear to be reciprocally regulated in a species-specific manner. For instance, thyroid hormones stimulate GH synthesis in mammals via thyroid hormone-response elements in the GH gene (Iranmanesh et al 1991, Valcavi et al 1992, RodriguezArnao et al 1993, whereas tri-iodothyronine (T3) and thyroxine (T4) inhibit in vivo GH synthesis and release and in vitro thyroid hormone-releasing hormone (TRH)/GHreleasing factor-stimulated GH release in birds (Harvey 1983d, 1990a,b, Harvey et al 1990, Denver & Harvey 1991. This inhibitory effect is thought to be mediated at the hypothalamic level by reduced TRH secretion and (for T3) at the pituitary level by reduced TRH receptors (Harvey & Baidwan 1990).…”