The concept of metabolic clearance rate in endocrinology. A case study of growth hormone in different lines of meat-type chickens.

Disappearance of growth hormone from blood (plasma) was studied in young broiler chickens in two experiments: a) following single injection of GH (50 micrograms or 250 micrograms/kg) to anesthetized chicks; b) injection of GH (50 micrograms/kg) (under anesthesia) to two lines of broilers selected for either rapid growth or feed efficiency. Clearance of GH never followed a single negative exponential curve. Characterization of the disappearance by a single metabolic clearance rate and half-life time were found to be inaccurate and inappropriate on methodological grounds. High doses of hormone might disturb receptor or catabolism equilibria and result in aberrant values of MCR which could probably be because of the very dynamic nature of the GH clearance system, which appeared to be influenced by dose, line (or genetics of the animal) and by some indices of clearance. Therefore, physiological significance of such data must be interpreted carefully.