THE EFFECTS OF HYPERINSULINEMIA AND HYPERGLYCEMIA ON GLUT4 AND HEXOKINASE-II MESSENGER-RNA AND PROTEIN IN RAT SKELETAL-MUSCLE AND ADIPOSE-TISSUE

The GLUT4 glucose transporter and type II hexokinase are predominately expressed in skeletal muscle and adipose tissue. The effects of insulin and glucose on the expression of GLUT4 and HKII were studied in vivo by using the euglycemic-hyperinsulinemic and hyperglycemic-hyperinsulinemic clamp methods. The clamps were maintained in conscious rats for 6 or 24 h after a 1-day starvation period. Adipose tissue GLUT4 mRNA was increased 4-fold after 6 h and 23-fold after 24 h of hyperinsulinemia; HKII mRNA was increased by four- and eightfold after 6 and 24 h, respectively. In contrast, GLUT4 mRNA was not significantly changed in skeletal muscle by either the euglycemic- or hyperglycemic-hyperinsulinemic clamps. Each of these treatments resulted in a fourfold induction of HKII mRNA. No changes of GLUT4 protein and hexokinase activity were detected after 6 h of hyperinsulinemia in either skeletal muscle or adipose tissue. After 24 h of hyperinsulinemia, adipose tissue GLUT4 protein had doubled, whereas skeletal muscle GLUT4 was unchanged. In contrast, hexokinase activity increased by two- to eightfold in skeletal muscle and adipose tissue. Hyperinsulinemia alone was sufficient to mediate the effects observed, because no additional effects were seen when hyperglycemia accompanied hyperinsulinemia. These results reveal the lack of coordinate regulation of GLUT4 and HKII in adipose tissue and skeletal muscle. Whereas hyperinsulinemia increases both GLUT4 and HKII mRNA and protein levels in adipose tissue, this treatment increases HKII mRNA and protein in skeletal muscle, but has no effect on GLUT4 in this tissue.