Characteristics of GLP-1 and exendins action upon glucose transport and metabolism in type 2 diabetic rat skeletal muscle

Exendin-4, a peptide 53% structurally homologous with glucagon-like peptide 1 (GLP-1), is insulinotropic and has an antidiabetic effect even more prolonged than that of GLP-1. Exendin-9 is an antagonist of GLP-1 receptor and action in several cell systems, but shows GLP-1- and exendin-4-agonistic characteristics in human muscle cells and tissue. The action of GLP-1 upon glucose transport and metabolism in muscle is mediated by specific receptors. In this study we investigated the effect of both exendin-4 and -9, relative to that of GLP-1, upon glucose transport and metabolism in the skeletal muscle from a streptozotocin-induced type 2 diabetic rat model, compared to normal. In normal rats, exendin-4, like GLP-1 and insulin, enhanced glucose uptake. This effect, which is mediated to a certain extent by some kinases (PI3K/ PKB, p70s6k and MAPKs), may be caused by the peptide acting, at least in part, through the muscle GLP-1 receptors. Exendin-9 also stimulated the same kinases, except for PKB, but failed to modify basal glucose uptake. Type 2 diabetic rats showed lower than normal basal muscle glucose transport and oxidation value, and higher glycogen synthase a activity and pyruvate release; however, no modification of glucose uptake by GLP-1 or exendin-4 was detected, at variance with insulin, and basal activity of P13K/PKB was lower than normal, while that of p70s6k and MAPKs was higher. GLP-1 failed to affect the activity of any of the kinases, while exendin-4 increased the activity of P13K, p70s6k and MAPKs, but not PKB, suggesting that this enzyme plays a major role in exendin-4 effect upon glucose transport in muscle.