Thyroidal inhibition of growth hormone secretion in fowl: tri-iodothyronine-induced down-regulation of thyrotrophin-releasing hormone-binding sites on pituitary membranes

The number, but not affinity, of binding sites for 3H]3-methyl-histidine2-TRH (3H]Me-TRH) on chicken adenohypophysial plasma membranes was increased in chickens made hypothyroid by goitrogen (methimazole) treatment (50 mg/kg per day for 7 days), which also increased circulating GH concentrations. Daily i.p. injection of thyroxine (T4; 100 micrograms/kg for 7 days) had no effect on 3H]Me-TRH binding to pituitary membranes, although it suppressed endogenous GH secretion. Binding of 3H]Me-TRH to pituitary caudal lobe membranes was, however, suppressed by tri-iodothyronine (T3) injected chronically (100 micrograms/kg per day, i.p., for 7 days) or acutely (100 micrograms/kg, 2 h before being killed). The suppression of 3H]Me-TRH binding and inhibition of GH secretion following T3 administration was dose related. Binding of 3H]Me-TRH to caudal lobe membranes was also suppressed following the incubation of pituitary glands with T3 in vitro, and the response was both dose and time related. These results suggest that T3 inhibits GH secretion in fowl by a down-regulation of pituitary TRH receptors. However, other mechanisms are involved in thyroidal inhibition of GH release in birds, since T4 had no effects on 3H]Me-TRH binding yet suppressed GH secretion in vivo.