Effect of Age and Protein Restriction on the Clearance and Secretion of Growth Hormone in the Domestic Fowl

Abstract

Metabolic clearance rates (MCR) and secretion rates for growth hormone (GH) were determined in young (6-wk-old) chicks and adults (1 yr old) as well as in chicks fed low protein diets for 2 wk. Birds were either conscious or anesthetized during these determinations in the latter study. Age-related differences in plasma concentrations of GH reflect greater secretion of GH in younger chicks (3.5-fold increase). The MCR was reduced by 35% in chicks compared with adults, but when expressed per unit metabolic body weight, was higher in the young chickens. The MCR in protein-restricted chicks was 28% lower than in that of controls. However, when the MCR was calculated on a metabolic body weight basis, protein-restricted and control chicks did not differ in MCR. Elevated plasma concentrations of GH in unanesthetized, protein-restricted chicks were due to greater GH secretion (2.4-fold) of these chicks compared with secretions of chicks fed control diets. Basal secretions of GH, determined in the anesthetized chicks, did not differ between dietary treatments. These results indicate that secretion of GH but not clearance was affected by protein restriction in conscious birds.

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

“…5) T4 has little, if any, stimulatory effect on BMR in euthyroid birds (Lam, 1986), and failed to acceler¬ ate GH clearance in the present study. The kinetics of GH secretion and clearance observed in the present study are comparable with previous studies in the fowl (Lauterio & Scanes, 1988a).…”

“…Thyroidectomized fowl, therefore, differ from protein-restricted birds, in which increased plasma GH concentrations are due to an increased GH secretion rate rather than reduced clearance rate (Lauterio & Scanes, 1988a), even though they are characterized by reduced thyroid function.…”

“…This method requires strictly steady-state con¬ ditions, whereas GH release in birds (and mammals) is episodic (Hall, Harvey & Scanes, 1986). This method is, however, still appropriate since the estimated MCR of GH in conscious chickens is very similar to that determined under steady-state conditions in anaes¬ thetized fowl, in which episodic GH release does not occur (Lauterio & Scanes, 1988a). Plasma analysis Circulating concentrations of GH (Harvey & Scanes, 1977a), T3 and T4 ) were deter¬ mined by specific radioimmunoassays, as previously described.…”

“…Lauterio & Scanes, 1988a), the index of GH clearance employed was based on the trapezoid methods for determining MCR under steady-state conditions (Normand & Fortier, 1970), this being the dose of 125I-labelled GH injected divided by the area under the disappearance curve. This method requires strictly steady-state con¬ ditions, whereas GH release in birds (and mammals) is episodic (Hall, Harvey & Scanes, 1986).…”

Half-Life and Metabolic Clearance Rate of Recombinant-Derived Chicken Growth Hormone and Purified Turkey Growth Hormone in Intact and Hypophysectomized Turkeys

“…The difference between studies in the number of exponential decay constants for GH clearance may be due to differences in the regression techniques used to identify compartments. Although the present studies employed anesthetized turkeys, similar results would be expected in conscious birds, because Lauterio and Scanes (1988) have shown that pentobarbital anesthesia does not alter the metabolic clearance rate of cGH in White Leghorn chickens.…”