The mechanism of the hyperglycaemic action of synthetic peptides related to the C-terminal sequence of human growth hormone

Stimulation of 2-deoxyglucose uptake in rat adipocytes by a human growth hormone fragment (hGH 4-15)

The in vivo and in vitro effects of a hypoglycaemic fragment of human growth hormone containing the sequence H2N-Ile-Pro-Leu-Ser-Arg-Leu-Phe-Asp-Asn-Ala-Met-Leu-COOH (hGH 4-15) on 2-deoxy-[1-14C]-D-glucose uptake in adipocytes were studied. The isolated cells from rats after a single intravenous injection of hGH 4-15 (1 mg/kg) significantly increased uptake of 2-deoxyglucose (p less than 0.005). Adipocytes from untreated rats pre-incubated with the hGH fragment (10 micrograms/ml) at 37 degrees C for 30 min also clearly showed an elevated uptake of 2-deoxyglucose in the absence and the presence of exogenous insulin. The effect of hGH 4-15 was concentration-dependent, steadily increasing with the maximum effect at 10 micrograms/ml. The present findings suggest that the enhancement of glucose transport in target tissues may be a major contributing factor to the hypoglycaemic action of the amino-terminal fragments of human growth hormone.

Regulation of glycogen synthase activity in muscle by a C-terminal part sequence of human growth hormone

The synthetic peptide hGH 177-191, corresponding to the last 15 residues at the carboxyl terminus of human pituitary growth hormone, promotes the conversion of glycogen synthase a to glycogen synthase b in muscle. When injected, the peptide was found to produce inactivation of glycogen synthase phosphatase activity in rat skeletal muscle. The time course of phosphatase inactivation was closely correlated with that for glycogen synthase. The peptide had no effect either on muscle 3',5'-cyclic AMP levels or on synthase kinase activity. These results can be explained in terms of a dynamic cycle of interconversion of synthase between active and inactive forms, by the simultaneous action of synthase kinases and synthase phosphatases. A decrease in the ratio of phosphatase to kinase activity would result in a decrease in the steady-state level of synthase a activity.