Fat feeding produces whole-body insulin resistance and decreased glucose uptake in muscle tissue of rats. To examine the effect of glucocorticoid blockade on the insulin resistance caused by high-fat feeding, four groups of rats were fed diets high in starch (70% of calories) or fat (59% of calories) for 4 weeks with or without the antiglucocorticoid RU486 (69.8 μmol · kg−1 · day−1) in the food. Whole-body insulin action was assessed by the euglycemic clamp technique at an upper physiological insulin level with bolus 2-[3H]deoxyglucose to determine individual tissue insulin-stimulated glucose uptake. Whole-body glucose utilization (clamp glucose infusion rate [GIR]) was decreased by high-fat feeding (GIR 68.3 ± 12.2 vs. 182.6 ± 12.8 μmol · kg−1 · min−1 for the starch-fed group; P < 0.001). Addition of RU486 to the diet significantly improved (GIR 133.9 ± 12.8 μmol · kg−1 · min−1; P < 0.01), but did not fully reverse, the insulin resistance caused by fat feeding. RU486 was without effect in the starch-fed rats. In skeletal muscles, RU486 ameliorated 62 and 68% of the insulin resistance produced by fat feeding in red quadriceps and extensor digitorum longus hindlimb muscles, respectively, but had no effect in heart or white adipose tissue. These results suggest that glucocorticoids play, in a tissue-specific manner, a role in the maintenance and/or production of insulin resistance produced by high-fat feeding.

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