Effects of free fatty acids on glucose transport and IRS-1-associated phosphatidylinositol 3-kinase activity

To examine the mechanism by which free fatty acids (FFA) induce insulin res istance in human skeletal muscle, glycogen, glucose-g-phosphate, and intrac ellular glucose concentrations were measured using carbon-13 and phosphorou s-31 nuclear magnetic resonance spectroscopy in seven healthy subjects befo re and after a hyperinsulinemic-euglycemic clamp following a five-hour infu sion of either lipid/heparin or glycerol/heparin. IRS-l-associated phosphat idylinositol 3-kinase (PI 3-kinase) activity was also measured in muscle bi opsy samples obtained from seven additional subjects before and after an id entical protocol. Rates of insulin stimulated whole-body glucose uptake. Gl ucose oxidation and muscle glycogen synthesis were 50%-60% lower following the Lipid infusion compared with the glycerol infusion and were associated with a similar to 90% decrease in the increment in intramuscular glucose-6- phosphate concentration, implying diminished glucose transport or phosphory lation activity. To distinguish between these two possibilities, intracellu lar glucose concentration was measured and found to be significantly lower in the lipid infusion studies, implying that glucose transport is the rate- controlling step. Insulin stimulation, during the glycerol infusion, result ed in a fourfold increase in PI 3-kinase activity over basal that was aboli shed during the lipid infusion. Taken together, these data suggest that inc reased concentrations of plasma FFA induce insulin resistance in humans thr ough inhibition of glucose transport activity; this may be a consequence of decreased IRS-l-associated PI 3-kinase activity.