INSULIN RESISTANCE IN TYPE-2 (NON-INSULIN-DEPENDENT) DIABETIC-PATIENTS AND THEIR RELATIVES IS NOT ASSOCIATED WITH A DEFECT IN THE EXPRESSION OF THE INSULIN-RESPONSIVE GLUCOSE TRANSPORTER (GLUT-4) GENE IN HUMAN SKELETAL-MUSCLE

To study whether insulin resistance in Type 2 (non-insulin-dependent) diabetes mellitus is due to a defect in the expression of the insulin-responsive glucose transporter gene (GLUT-4) in human skeletal muscle, we measured the level of GLUT-4 mRNA and (in some of the subjects) its protein in muscle biopsies taken from 14 insulin-resistant patients with Type 2 diabetes, 10 first-degree relatives of the diabetic patients and 12 insulin-sensitive control subjects. Insulin sensitivity was measured with a +45 mU.m2(-1).min-1 euglycaemic insulin clamp in combination with indirect calorimetry and infusion of [3-H-3]glucose. GLUT-4 mRNA was measured using a human GLUT-4 cDNA probe and GLUT-4 protein with a polyclonal antibody specific for the 15 amino acid carboxy-terminal peptide. Both Type 2 diabetic patients and their relatives showed impaired stimulation of total-body glucose disposal by insulin compared with control subjects (29.5 +/- 2.1 and 34.0 +/- 4.8 vs 57.9 +/- 3.1-mu-mol.kg lean body mass-1.min-1; p < 0.01). This impairment in glucose disposal was primarily accounted for by a reduction in insulin-stimulated storage of glucose as glycogen (13.0 +/- 2.4 and 15.6 +/- 3.9 vs 36.9 +/-2.2-mu-mol.kg lean body mass-1.min-1; p < 0.01). The levels of GLUT-4 mRNA expressed both per mu-g of total RNA and per mu-g DNA, were higher in the diabetic patients compared with the control subjects (116 +/- 25 vs 53 +/- 10 pg/mu-g RNA and 177 +/- 35 vs 112 +/- 29 pg/mu-g DNA; p < 0.05,p < 0.01, respectively). The GLUT-4 mRNA levels in the relatives were not significantly different from that observed in the control subjects (90 +/- 16 pg/mu-g RNA and 117 +/- 23 pg/mu-g DNA; p = NS). The GLUT-4 protein levels did not significantly differ between control subjects, diabetic patients and relatives (494 +/- 85, 567 +/- 133 and 323 +/- 80 cpm/100-mu-g protein). No correlation was observed between the level of GLUT-4 mRNA and its protein. However, the level of GLUT-4 mRNA and the rate of total-body glucose disposal correlated positively in the control group and in the relatives (both p < 0.05) but not in the diabetic subjects. A positive correlation between the level of GLUT-4 protein and total-body glucose disposal was also observed in the control subjects (r = 0.759; p < 0.05) and in the relatives (r = 0.794;p < 0.01) but not in the diabetic subjects. We conclude that insulin resistance in Type 2 diabetes is not related to a defect in the expression of the GLUT-4 gene in skeletal muscle. Nevertheless, the levels of GLUT-4 mRNA and GLUT-4 protein are related to the rate of total-body glucose disposal in subjects with normal fasting glucose concentrations.