The relationship between diet and insulin sensitivity was examined in isolated soleus muscle from 10-wk-old lean Zucker rats. Rats were fed either a high fat or high carbohydrate diet that had 67% of calories as fat or carbohydrate, respectively, for 10 days. Plasma insulin but not plasma glucose concentrations were significantly elevated in high-fat-fed rats, indicating that a state of insulin resistance existed. The mechanisms responsible for the insulin resistance were studied by measuring insulin binding, 2-deoxyglucose uptake, and glucose metabolism in soleus muscle. The soleus muscle from the high-fat-fed rats bound significantly less insulin than high carbohydrate control rats under equilibrium binding conditions. The 35% decrease in insulin binding at maximal insulin concentrations resulted from a decrease in insulin receptor number but no change in receptor affinity. Maximal insulin-stimulated 2-deoxyglucose uptake was reduced in soleus muscle from high-fat-fed rats when compared with high carbohydrate controls.
A decrease in postmembrane basal and insulin-stimulated glucose utilization was produced by high rat feeding and varied depending upon the pathway involved. An estimate of glycolysis (3H2O) formation from [5-3H]glucose) and glucose oxidation (14CO2 production from 14C-glucose) demonstrated a greater decrease in basal and insulin-stimulated utilization than in [5-3H]glucose conversion to [3H]glycogen. These results suggest that multiple sites are responsible for the observed insulin resistance in soleus muscle after high fat feeding.
