Comparison of hypoglycaemic responses to human growth hormone and the synthetic 4-15 fragment between 16-18 day-old and 45-50 day-old rats

Isolation from subcellular preparation of a mediator of hypoglycaemic hGH peptides

This study demonstrated the release into the incubation medium of a cellular mediator from isolated fat adipocytes and hepatocytes after treatment with the hypoglycaemic fragment of human growth hormone, hGH 6-13. The activity of the putative mediator observed in the cell "ghosts" of both liver and rat cells suggests that the active component is likely to be derived from plasma membranes and has an ubiquitous cellular distribution. The hGH fragment-induced release of the mediator from plasma membranes depends upon the physiological status of the animals. Liver plasma membranes of starved rats yield significantly higher levels of the cellular mediator in response to treatment with hGH 6-13. The studies of the physiological factors influencing the release of the material from cellular systems clearly enhance the production of adequate amounts of the cellular mediator for molecular characterization. The precise chemical nature and the physiological role of the hGH cellular mediator are currently unknown.

Modulation of hepatic insulin receptors by a human growth hormone fragment (hGH 6-13)

A synthetic amino-terminal fragment of human growth hormone (hGH) containing the sequence H2N-Leu-Ser-Arg-Leu-Phe-Asp-Asn-Ala-COOH (hGH 6-13) was shown to increase [125I]iodoinsulin binding to rat hepatic receptors in vivo. Analysis of the binding data indicated an increase in the capacity of the insulin receptors or the number of available receptors. In vitro experiments with isolated hepatocytes and hepatic plasma membranes revealed no direct interaction between the hGH 6-13 and the hepatic insulin receptors. When the isolated hepatocytes were preincubated with the synthetic hGH fragment (10 micrograms/ml) at 37 degrees C for 30 min prior to tests for insulin binding, the binding of [125I]iodoinsulin to the hepatic receptors was significantly enhanced. The variance between the in vivo and in vitro findings was considered in terms of the mechanism of hormonal actions mediated through a secondary cellular mediator.