Optimization of a Radioreceptor Assay for Human Growth Hormone Using Male Rat Liver Membranes

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Abstract
Specific somatogenic receptors reportedly are present in liver membrane preparations from male rats. Specific binding was < 3% of total cpm added, and the radioreceptor assay using such receptor preparations was impractical to use. Livers from female rats bound 25% of 125I-labeled human growth hormone [hGH] added, but binding was not specific in that both prolactin and GH competed. Starting with membrane preparations from male rats, preparation of the fraction, the method of separation of bound and free, time, temperature, and conditions of incubation, the amount of membrane preparation and treatment of rats before sacrifice, and the quality of the labeled GH were studied and improved. As a result, specific binding increased from less than 3 to 60%. This is offered as a model system for optimization of radioreceptor assay systems. 125I-hGH binding to male rat liver membranes was low, as previously reported, because the preparation and assay system had not been optimized, and not because of an inadequate number of receptors in the male liver.