Differential Regulation of GLUT1 and GLUT4 Glucose Transporters in Skeletal Muscle of a New Model of Type II Diabetes: The Obese SHR/N-cp Rat

The obese diabetic SHR/N-cp rat is a newly developed strain that inherits obesity as an autosomal recessive trait. These rats display early-onset hyperinsulinemia and hyperglycemia, which are hallmarks of type II diabetes. This study was undertaken to determine the expression and the subcellular distribution of the GLUT1 and GLUT4 glucose transporters in skeletal muscle of obese diabetic SHR rats. D-glucose-protectable cytochalasin-B binding to subcellular membrane fractions of hindlimb muscles was used to determine glucose transporter number. GLUT1 and GLUT4 glucose transporter isotypes were detected using antibodies to the COOH-terminal region of the GLUT1 and GLUT4 proteins. Glucose transporter number was significantly lower (–40%) in crude unfractionated membranes of obese diabetic SHR than of lean SHR muscles. When crude membranes were fractionated to separate plasma membranes and the intracellular membranes containing glucose transporters, the number of cytochalasin-B binding sites was found to be markedly lower (–50%) in intracellular membranes and slightly but not significantly reduced (–20%) in plasma membranes of muscle from obese diabetic SHR compared with lean SHR rats. Western blot analysis revealed that a lower GLUT4 protein abundance (–40%) accounts for the reduced glucose transporter number in intracellular membranes of obese diabetic SHR compared with lean SHR muscles. GLUT4 protein content was also reduced by 50% in plasma membranes from obese SHR muscles relative to lean rat muscles. In contrast, GLUT1 glucose transporters were more abundant (40d%) in plasma membranes of obese diabetic SHR rats compared with their lean counterparts. The latter finding may explain the lack of phenotypic difference in plasma membrane total glucose transporter number as measured by cytochalasin-B binding. GLUT4 and GLUT1 mRNA levels were both found to be significantly decreased in obese SHR rats compared with their lean littermates. These results demonstrate that 1) GLUT4 transporter expression is reduced in skeletal muscle of obese diabetic SHR rats, a model of type II diabetes; 2) the decreased GLUT4 abundance was localized to both the sarcolemmal and intracellular membrane fractions; 3) in contrast to the GLUT4 glucose transporter, GLUT1 protein content was greater in plasma membranes of skeletal muscle from obese diabetic SHR rats, suggesting that these transporter isoforms are inversely regulated in muscle of these diabetic animals; and 4) posttranscriptional mechanisms appear to be responsible for the differential expression of GLUT1 and GLUT4 proteins in muscle of obese diabetic SHR rats.