High plasma free fatty acid (FFA) concentrations impair muscle glucose disposal. Previous reports have implicated increases in intramyocellular diacylglycerol (DAG) content as contributing to impaired muscle insulin signaling and subsequent muscle glucose uptake.
Lean (n=16) and obese (BMI: 30-37, n=19) men and women (age 18-52) underwent a single step, 1.0 mU/kg/min hyperinsulinemic-euglycemic clamp to assess muscle insulin sensitivity. To measure the contribution of FFA to muscle lipids we infused [U-13C]palmitate intravenously for 5 h before the insulin clamp (basal conditions) and changed to a [2H9]palmitate infusion during the insulin clamp. Vastus lateralis biopsies were taken directly prior to clamp start and at the end of the 5 h clamp. Total, subsarcolemmal (SS), and intramyofibrillar (IMF) muscle lipids were assayed. DAG species concentrations and isotopic enrichment of 2 species were quantified by LC/MS/MS. Total and phospho-Akt (S473) protein content was measured in biopsy samples.
IMF and total, but not SS, DAG concentrations were greater in obese than lean participants (P<0.05) and were unchanged between basal and clamp biopsies. In all participants, basal fractional synthesis of 16:0/16:0 was greater in IMF than SS (7.4% ± 0.0 vs 3.6% ± 0.0, P<0.001). Basal fractional synthesis of 16:0/18:1 from plasma palmitate in SS was less in obese (6% ± 0.1), than lean participants (10.2% ± 0.4) (P<0.02). Surprisingly, basal SS 16:0/16:0 DAG concentrations were positively correlated with AKT activation. Neither DAG concentrations nor the relative contribution of plasma palmitate to DAG correlated with muscle insulin sensitivity.
Obesity is associated with greater intramyocellular DAG concentrations and plasma FFA trafficking into fraction specific DAG under fasting conditions, however, these measures did not relate to muscle glucose disposal. We conclude that obesity-related differences in intramyocellular DAG do not explain muscle insulin resistance