TIME COURSES OF CHANGES IN HEPATIC AND SKELETAL-MUSCLE INSULIN ACTION AND GLUT4 PROTEIN IN SKELETAL-MUSCLE AFTER STZ INJECTION

To determine the relative time courses of changes peripheral and hepatic insulin action and skeletal muscle GLUT4 protein levels after a streptozotocin (STZ) injection in rats, we performed hyperinsulinemic (14-18 nM), euglycemic (7.5 mM) clamps in control (n = 8) and diabetic rats at 1 (n = 7),3 (n = 8), 7 (n = 8), and 14 (n = 6) days after intraperitoneal STZ (65 mg/kg). Basal plasma glucose concentrations increased from 8.1 +/- 0.2 mM in control rats to 23.5 +/- 1.2 mM 1 day after STZ (P < 0.01) and remained constant thereafter. Basal plasma insulin levels were approximately 35% of control levels in all STZ groups (P < 0.01). Insulin-stimulated whole-body glucose uptake decreased significantly as early as one day after STZ injection (P < 0.01), resulting predominantly from a decrease in whole-body glycolysis. Insulin action to suppress hepatic glucose output was normal on day 1 after STZ but impaired markedly on day 3 and thereafter (P < 0.01). Insulin-stimulated glucose uptake in individual skeletal muscles was not altered until day 7 after STZ, and the magnitudes of decreases in skeletal muscle insulin action on days 7 and 14 were not fully accounted for by the decreases in GLUT4 protein level measured from the same muscles. Our data indicate that there is a temporal hierarchy in the development of insulin resistance in STZ-induced diabetes. Insulin resistance is manifested initially (within one day) as a reduction in insulin-mediated glucose uptake and glycolysis in tissues other than skeletal muscle; available data suggest that the liver is a potential site of the initial reduction in insulin-mediated glucose uptake. Resistance of hepatic glucose output to suppression by insulin appears next, by day 3. Insulin resistance in skeletal muscle appears last (between days 3 and 7 after STZ administration), and the resistance in muscle cannot be fully accounted for by reduced GLUT4 expression.