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 wks with metformin. Succinate-and palmitoyl-carnitine-supported respiration generated >2-fold 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.

Metformin-treated patients with type 2 diabetes have normal mitochondrial complex I respiration

“…Metformin concentrations below 1.0 mmol/l have no inhibitory effect on complex I respiration in permeabilised skeletal muscle fibres or isolated liver mitochondria from rats [9,13], which is in agreement with results from the present study in which patients with type 2 diabetes were treated with a clinically relevant dose of metformin. An important question is, therefore, what is the metformin concentration in the mitochondria of patients with type 2 diabetes.…”

“…Studies on cells, animals and mitochondria isolated from human skeletal muscle samples have shown conflicting results [9][10][11][12][13]. Studies carried out on rat muscle (permeabilised fibres and isolated mitochondria) or liver or in cell lines may not be representative of skeletal muscle from human patients with type 2 diabetes.…”

“…If the concentration is below 1.0 mmol/l, then studies using metformin concentrations above this threshold are not clinically relevant. A metformin dose of 1,000 mg (taken orally) has been calculated to elicit a metformin concentration in the mitochondria of approximately 0.1-0.5 mmol/l [13,14,17]. This estimate does not take the half-life of metformin (approximate 6.5 h in plasma and 18 h in blood) into consideration.…”

“…Metformin treatment has also been associated with inhibition of complex I in the mitochondrial electron transport chain in studies of rat muscle or liver and in studies using cell lines [9][10][11][12]. A recent study on rat skeletal muscle suggested that a possible inhibitory effect of metformin on mitochondrial respiration was concentration-dependent [13] with no effect of metformin in concentrations up to 1.0 mmol/l in the vicinity of the mitochondria. A therapeutic dose of metformin in humans of 1,000 mg elicits a plasma metformin concentration of approximately 0.1 mmol/l [14,15].…”

“…The peak metformin concentration in skeletal muscle (mitochondria) is lower than the plasma level [16]. The metformin concentration is of importance and concentrations above 0.5-1.0 mmol/l in the mitochondria might not be clinically relevant [9,13,17].…”