Assessment of human muscle glycogen synthesis and total glucose content by in vivo 13 C MRS

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Abstract
Obesity is often accompanied by a decreased ability of insulin to stimulate glucose uptake and glycogenesis in skeletal muscle. The aim of this study was to investigate the rate of glycogen formation and of muscular glucose content in relation to insulin sensitivity under euglycemic conditions. We applied a hyperinsulinemic (430 pmol m−2 min−1) euglycemic clamp with infusion of 20% glucose (30% enriched with 13C-1-glucose) to 8 subjects with a wide range of insulin sensitivities. Glycogen and glucose levels were monitored simultaneously by in vivo13C MRS of the calf muscle on a clinical MR system at 1.5T field strength. Glycogen synthesis rate showed a strong correlation with whole body glucose uptake during the clamp (r = 0.93, P < 0.01). With the use of 13C MRS, total muscular glucose content could be determined in vivo, and showed a positive, linear correlation with glycogen synthesis rate (r = 0.85, P < 0.01). 13C MRS provides important information regarding in vivo insulin action. Preliminary results indicate that the glycogen synthesis rate improves after treatment with troglitazone.