Effect of synthetic C-terminal fragments of hGH on insulin release by isolated islets.

Synthetic fragments representing the C-terminal end of the growth hormone molecule have been tested for their direct in vitro effects on insulin release by isolated rat islets of Langerhans. hGH 177-191 caused a dose-related potentiation of glucose-induced insulin release, whereas the peptide by itself caused no stimulation of insulin release from the islets. The rate curves constructed for insulin secretion as a function of extracellular glucose concentration showed that the Km for glucose is not altered in the presence of the peptide, but that the Vmax of secretion is increased. Significant potentiation of insulin release by the peptide was seen only at high extracellular concentrations of glucose. Measurement of cAMP levels in islets showed that the peptide caused no significant alteration of cAMP levels while still potentiating insulin release. It was therefore concluded that the mechanism of potentiation of insulin release by the peptide may be independent of the changes in cAMP levels in islets. hGH 172-191, too, caused potentiation of glucose-stimulated insulin release from islets, whereas hGH 179-191 was not active in this report.