Metformin selectively attenuates mitochondrial H2O2 emission without affecting respiratory capacity in skeletal muscle of obese rats

Abstract Metformin is a widely prescribed drug for treatment of type 2 diabetes, although no cellular mechanism of action has been established. To determine whether in vivo metformin treatment alters mitochondrial function in skeletal muscle, respiratory O 2 flux and H 2 O 2 emission were measured in saponin-permeabilized myofibers from lean and obese ( fa/fa ) Zucker rats treated for 4 weeks with metformin. Succinate- and palmitoylcarnitine-supported respiration generated greater than twofold higher rates of H 2 O 2 emission in myofibers from untreated obese versus lean rats, indicative of an obesity-associated increased mitochondrial oxidant emitting potential. In conjunction with improved glycemic control, metformin treatment reduced H 2 O 2 emission in muscle from obese rats to rates near or below those observed in lean rats during both succinate- and palmitoylcarnitine-supported respiration. Surprisingly, metformin treatment did not affect basal or maximal rates of O 2 consumption in muscle from obese or lean rats. Ex vivo dose–response experiments revealed that metformin inhibits complex I-linked H 2 O 2 emission at a concentration ~ 2 orders of magnitude lower than that required to inhibit respiratory O 2 flux. These findings suggest that therapeutic concentrations of metformin normalize mitochondrial H 2 O 2 emission by blocking reverse electron flow without affecting forward electron flow or respiratory O 2 flux in skeletal muscle.