Objective:To examine whether rosiglitazone alters gene expression of some key genes involved in mitochondrial biogenesis and oxidative capacity in skeletal muscle of type 2 diabetic patients, and whether this is associated with alterations in skeletal muscle oxidative capacity and lipid content.Design:Skeletal muscle gene expression, mitochondrial protein content, oxidative capacity and lipid accumulation were measured in muscle biopsies obtained from diabetic patients, before and after 8 weeks of rosiglitazone treatment, and matched controls. Furthermore, whole-body insulin sensitivity and substrate utilization were assessed.Subjects:Ten obese type 2 diabetic patients and 10 obese normoglycemic controls matched for age and BMI.Methods:Gene expression and mitochondrial protein content of complexes I-V of the respiratory chain were measured by quantitative polymerase chain reaction and Western blotting, respectively. Histochemical staining was used to quantify lipid accumulation and complex II succinate dehydrogenase (SDH) activity. Insulin sensitivity and substrate utilization were measured during a hyperinsulinemic-euglycemic clamp with indirect calorimetry.Results:Skeletal-muscle mRNA of PGC-1alpha and PPARbeta/delta - but not of other genes involved in glucose, fat and oxidative metabolism - was significantly lower in diabetic patients (P<0.01). Rosiglitazone significantly increased PGC-1alpha ( approximately 2.2-fold, P<0.01) and PPARbeta/delta ( approximately 2.6-fold, P<0.01), in parallel with an increase in insulin sensitivity, SDH activity and metabolic flexibility (P<0.01). Surprisingly, none of the measured mitochondrial proteins was reduced in type 2 diabetic patients, nor affected by rosiglitazone treatment. No alterations were seen in muscular fat accumulation upon treatment.Conclusion:These results suggest that the insulin-sensitizing effect of rosiglitazone may involve an effect on muscular oxidative capacity, via PGC-1alpha and PPARbeta/delta, independent of mitochondrial protein content and/or changes in intramyocellular lipid.International Journal of Obesity advance online publication, 20 February 2007; doi:10.1038/sj.ijo.0803567.