Studies on Human Growth Hormone. Ii. The Physiological Disposition and Metabolic Fate of Human Growth Hormone in Man*

The metabolic effects of human growth hor- mone have received much attention, but little is known regarding the metabolism of the hormone itself in man. Studies of growth hormone metabolism carried out in experimental animals suggest that both bovine growth hormone and human growth hormone have rapid turnovers. Van Dyke, Simpson, Li and Evans (1) and Gemzell, Heijkenskjdld and Str6m (2) injected rats with large doses of bovine growth hormone and deter- mined plasma levels of growth hormone by the tibial cartilage width assay. The half-lives of disappearance were 26 and 40 minutes, respec- tively, in the two studies, and the volume of distribution of the hormone was roughly equivalent to the extracellular fluid space. Isotopic methods have also been applied to the study of growth hormone metabolism. Sonenberg and co-workers (3) injected rats with radioiodinated bovine growth hormone and, although their work was principally concerned with tissue localization, they published a graph depicting the disappearance of radioactivity from plasma after a single intra- venous injection. The half-life of disappearance was approximately 25 minutes. In the experi- ments of Salmon, Utiger, Parker and Reichlin (4), radioiodinated human growth hormone had a half-life of disappearance from plasma of 17 minutes when injected into rabbits.