Characterization of hepatic growth hormone binding in domestic poultry

Specific binding of ¹²⁵I-recombinant chicken growth hormone (r-cGH) to avian hepatic crude membrane preparations was characterized in terms of time, temperature, and pH dependence of binding as well as by demonstration of specificity and dissociability of the binding phenomenon. Crude membranes (100,000g pellet) were prepared from livers of female, 4-week-old broiler-strain chickens and 24-week-old random-bred turkeys. Optimal conditions for binding in a 25 mM Tris-HCl, 10 mM CaCl₂ 0.5% BSA buffer system were determined to be 24 hr at 30°, pH 7.0, for chicken membrane preparations and 36 hr at 30°, pH 7.2, for turkey membrane preparations. Total specific binding was proportional to the concentration of membrane protein, and remained linear over the range of 200 to 1400 μg per tube (500-3500 μg/ml) for both chicken and turkey systems. Binding of ¹²⁵I-r-cGH was reduced by addition of unlabeled, pituitary-derived cGH (p-cGH) (IC₅₀ = 0.42 ng/tube/600 μg membrane protein for turkey; 3.8 ng/tube/600 μg membrane protein for chicken). Bovine GH (bGH) was an even more effective competitor than cGH, such that at approximately the IC₅₀ for p-cGH, bGH displaced 83.5% of ¹²⁵I-r-cGH. Bovine prolactin also competed with ¹²⁵I-r-cGH; however, cross-reactivity was only 1% that of bGH. Rat TSH exhibited negligible and LH and FSH essentially no cross-reactivity in the avian system. Binding was reversible as indicated by dissociation of 75% of bound ¹²⁵I-r-cGH from turkey preparations and complete dissociation from chicken preparations by 12 hr following addition of excess unlabeled hormone.