Skeletal Muscle Insulin Resistance Is the Primary Defect in Type 2 Diabetes

Insulin resistance is a characteristic feature of type 2 diabetes and plays a major role in the pathogenesis of the disease (1,2). Although β-cell failure is the sine qua non for development of type 2 diabetes, skeletal muscle insulin resistance is considered to be the initiating or primary defect that is evident decades before β-cell failure and overt hyperglycemia develops (3,4). Insulin resistance is defined as a reduced response of target tissues (compared with subjects with normal glucose tolerance [NGT] without a family history of diabetes), such as the skeletal muscle, liver, and adipocytes, to insulin. Because skeletal muscle is the predominant site of insulin-mediated glucose uptake in the postprandial state, here we will focus on recent advances about the time of onset, as well as the mechanism, of the skeletal muscle insulin resistance. The euglycemic insulin clamp technique (5) is considered to be the gold standard for measuring insulin action in vivo. With this technique, whole-body insulin action is quantified as the rate of exogenous glucose infusion (plus any residual hepatic glucose production) required to maintain the plasma glucose concentration at euglycemic levels in response to a fixed increment in the plasma insulin concentration. Because 80–90% of the infused glucose is taken up by skeletal muscle under conditions of euglycemic hyperinsulinemia, insulin sensitivity measured with the insulin clamp technique primarily reflects skeletal muscle (6). Another advantage of this technique is that it can be combined with indirect calorimetry to measure different substrate oxidation rates and with muscle biopsy to examine the biochemical/molecular etiology of the insulin resistance. Measurement of insulin sensitivity by the frequently sampled intravenous glucose tolerance test reflects both hepatic and peripheral insulin resistance and correlates well with the insulin clamp technique (7). Because insulin clamp studies are not feasible in large …