From the ‡Departments of Internal Medicine and ¶¶Cellular and Molecular Physiology and the ¶Howard Hughes Medical

Recent studies have demonstrated that fatty acids in-duce insulin resistance in skeletal muscle by blocking insulin activation of insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase (PI3-ki-nase). To examine the mechanism by which fatty acids mediate this effect, rats were infused with either a lipid emulsion (consisting mostly of 18:2 fatty acids) or glyc-erol. Intracellular C18:2 CoA increased in a time-dependent fashion, reaching an 6-fold elevation by 5 h, whereas there was no change in the concentration of any other fatty acyl-CoAs. Diacylglycerol (DAG) also in-creased transiently after 3–4 h of lipid infusion. In con-trast there was no increase in intracellular ceramide or triglyceride concentrations during the lipid infusion. Increases in intracellular C18:2 CoA and DAG concen