Participation of tri-iodothyronine and metabolic clearance rate in the inhibition of growth hormone secretion in thyroxine-treated domestic fowl

Abstract

Surgical thyroidectomy increases basal and TRH-induced GH concentrations in the peripheral plasma of immature domestic fowl. Replacement therapy with thyroxine (T4; 100 micrograms/kg per day for 7 days, i.m.) suppressed the GH responses to thyroidectomy. Bolus administration of T4 (10 micrograms/kg, i.m.) to thyroidectomized birds promptly lowered the circulating GH concentrations, which remained suppressed for at least 4 h. Chronic (daily injections for 7 days) or acute (one injection) pretreatment of thyroidectomized birds with iopanoic acid (IOP; 40 mg/bird, i.m.) before the bolus administration of T4 attenuated, but did not prevent, inhibition of circulating GH levels by T4. Administration of IOP (40 mg/bird i.m.) 24 h and immediately before the administration of tri-iodothyronine (T3; 3 micrograms/kg, i.m.) or T4 (10 micrograms/kg, i.m.) also failed to suppress thyroidal inhibition of circulating GH concentrations in thyroidectomized birds. Administration of IOP alone had no effect on GH concentrations. Circulating T3 concentrations were not enhanced following the administration of T4 to IOP-treated birds, indicating its inhibition of hepatic monodeiodinase activity. The metabolic clearance rate (MCR) of 125I-labelled chicken GH in the plasma of thyroidectomized fowl was less than that in sham-thyroidectomized birds. Following pretreatment with T4 (100 micrograms/kg per day for 7 days) sham-thyroidectomized and thyroidectomized birds did not differ significantly in their MCR. The GH secretion rate in thyroidectomized birds was similar to that in sham-thyroidectomized birds and in both groups was markedly reduced following pretreatment with T4. These results demonstrate thyroidal inhibition of circulating GH concentrations in fowl.(ABSTRACT TRUNCATED AT 250 WORDS)

Triiodothyronine Reduces Growth Hormone Secretion and Pituitary Growth Hormone mRNA in the Chicken, in Vivo and in Vitro

“…In the present studies, T, was observed to reduce GH release in young chickens in vivo as indicated by plasma concen- trations of GH. This is similar to previous reports (e.g., 19,26). Administration of T, in vivo also decreased the GH mRNA in the anterior pituitary gland.…”

Growth hormone response to TRH in male broiler chickens selected for body weight gain or food conversion and reared at either a moderate or a high ambient temperature

“…ues, high temperature enhanced both the GH secretory response to TRH and the GH clearance. The increased magnitude of GH peak following TRH administration at the higher temperature may be explicable in terms of the depressive effects of thyroid hormones on GH secretion (Harvey, 1983;Harvey et al, 1990Harvey et al, , 1991 while the thyroid function is known to be reduced in warmer environments (Decuypere et al, 1981;Kuhn et al, 1984). The reduced thyroid function may then result in an enhanced sensitivity of GH to a secretagogue challenge such as TRH (Harvey et al, 1990).…”

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

“…The inability of T4 to inhibit TRH binding to the avian pituitary gland in vitro is, however, consistent with the inability of T4 to suppress TRH-induced GH release from cultured chicken pituitary cells, which were highly responsive to T3 inhibition (Donoghue, Perez, Diamante et al 1989). T4 can, nevertheless, inhibit GH secretion in chickens in vivo (Harvey, 1983ft;), even in birds pretreated with iopanoic acid, which blocks conversion of T4 to T3 (Harvey, Klandorf & Scanes, 1990). The inhibitory effects of T4 on GH secretion are, therefore, likely to be through effects mediated at sites distant to TRH binding to somatotroph membranes and are probably mediated through the hypothalamus.…”