1,721,046 research outputs found
Deciphering the impact of fructose metabolism on liver mitochondrial bioenergetics
No full textSince its discovery in 1980, the interest in nonalcoholic fatty liver disease, the liver manifestation of metabolic syndrome, has expanded due to the growing impact on the world's health. Commonly characterized by [greater-than-or-equal-to]5% hepatic lipid accumulation in the absence of a secondary cause, NAFLD is a broad term used to encompass a variety of disease states within the liver. The largest predictor of NAFLD is said to metabolic syndrome itself, with type 2 diabetes being the most prevalent link. Currently, up to 75% of diabetic individuals also have a NAFLD diagnosis. It is also known that individuals diagnosed with NAFLD exhibit lower mitochondrial function overall. While the mechanism is largely unknown, fructose consumption has been specifically linked to the development and progression of NAFLD. Fructose metabolism differs from that of glucose metabolism. Unlike glucose, fructose has been shown to increase glucose, glycogen, lactate, uric acid, and pyruvate as end products, as well as a small increase in diet-induced thermogenesis. One major difference in metabolism is the production of uric acid following fructose metabolism. As fructose is brought into the liver, it is rapidly phosphorylated to fructose-1-phospate. This rapid use of ATP increases the generation of AMP and subsequent flux through the purine degradation pathway, ultimately leading to an increase in uric acid production in the liver. A second finding of note is that this phosphorylation process leads to an immediate drop in available ATP within the liver. Since it is unlikely that the cell's ATP usage outpaces its ability to resynthesize ATP using oxidative phosphorylation, there is a possibility that fructose metabolism leads to a decline in mitochondrial function, the primary site of ATP production. Overall, this project set out to determine if the metabolism of fructose, and subsequent production of uric acid, was the driving factor behind the decline of mitochondrial function in the liver. In Aim 1 of this project, the impact of fructose was directly determined. This was done by gavaging mice with both fructose and glucose to determine the mitochondrial changes associated with each sugar source. It was determined that an acute gavage of both glucose and fructose lead to increases in uric acid production in the liver, accompanied by increases in the oxygen consumption rates of the mitochondria isolated from these mice. It was also found that the measured differences between treatment groups was a transient measurement, in which there was only a difference 15-minutes post gavage, and by 60-minutes, there was no impact on mitochondrial function. Additionally, when treated over a 14-day period, there were no changes in mitochondrial function. This aim found that overall there were increase in oxygen consumption rates associated with increases in uric acid production. To better determine the potential direct impact of uric acid on mitochondrial function, Aim 2 was designed. It was found that the addition of uric acid to the system elicited a dose dependent increase in oxygen consumption rate. Using this dose dependent response, an optimal concentration was determined to carry out the remaining assays. From this aim, it was determined that there was an increase in oxygen consumption rate following the addition of uric acid. This increase was found to be present both in the presence and absence of adenylates. When each complex was examined individually, it was determined that there was no single mitochondrial source linked to the increase in oxygen consumption rate. Considering that rodent models retain urate oxidase, the known inhibitor, potassium oxonate, was used to determine if urate oxidase was in fact the source of residual oxygen consumption rate. Once added, it was determined that potassium oxonate was sufficient to inhibit the residual oxygen consumption. Additionally, the presence of potassium oxonate did not impact mitochondrial function alone. Overall, this project set out to determine the impact of fructose and uric acid on mitochondrial function. It was thought that both fructose and uric acid would lead to a decline in the mitochondrial function, resulting in an inability to produce, and therefore maintain, the ATP concentration. This was not what was found. It was determined that neither fructose nor uric acid was sufficient to lead to a decline in mitochondrial bioenergetic function. Instead, it was found that all oxygen consumption observations were due to the presence of urate oxidase and its consumption of oxygen during the metabolization of uric acid. This is a novel finding in that it does not agree with the current literature, which suggests both fructose and uric acid lead to mitochondrial decline
Unravelling the mechanisms of menopause-induced insulin resistance: novel roles for estradiol in skeletal muscle mitochondrial function
No full textMenopause results in a progressive decline in ovarian production of 17[beta]-estradiol (E2), increased adiposity and a higher risk for type-2 diabetes, but hormone replacement therapies containing E2 reverse most of these effects. However, we still lack a comprehensive understanding of the mechanism(s) by which E2 modulates insulin sensitivity (IS) and susceptibility to metabolic disease. By using a short-term ovariectomized mouse (surgical removal of ovaries, OVX) and pharmacological replacement of E2, here we have developed a model to study the short-term effects of menopause and E2 replacement therapies on mitochondrial function and IS, within a simplified context (i.e. in the absence of obesity). Given the primary role that skeletal muscle and liver play in glucose homeostasis, the aims of this work are two-fold. First, to determine whether decreased mitochondrial and cellular redox function represent the underlying molecular mechanisms for menopause-induced insulin resistance, and how E2 replacement can reverse these effects. Second, to determine the effects of ovarian E2 depletion and replacement on liver mitochondrial and redox functions, with a specific emphasis on NADH:ubiquinone oxidoreductase (mitochondrial complex I) kinetics and its associated H2O2 emitting potential. The studies herein provide evidence that E2 protects mitochondrial function and redox homeostasis in skeletal muscle by increasing complex I and IIII activities, enhancing electron transfer supercomplex assembly, preventing free radical leak, and decreasing lipid packing of mitochondrial membranes. We report, for the first time, the detection of E2 in skeletal muscle mitochondria, thus providing compelling evidence that E2 localizes to mitochondrial membranes and regulates mitochondrial function through the "fine-tuning" of electron transfer efficiency. Finally, while OVX by itself did not induce major changes on mitochondrial function in the liver (at least in a short-term OVX model), E2 treatment had detrimental effects on mitochondrial function, particularly targeted to complex I. This highlights the remarkably opposite effects E2 can exert on such critical organelles as the mitochondria across tissues with a different "bioenergetics signature", as well as at physiological versus pharmacological concentrations. Collectively, this study offers insights into novel molecular mechanisms by which menopause sets, and E2 replacement reverses, a pro-diabetogenic state, and further advances our knowledge on the mechanisms behind tissue-specific effects of estrogens. The present findings reveal new horizons in the development of novel pharmacological interventions to prevent metabolic dysfunction in naturally or surgically-induced post-menopausal women
Effect of Metformin on the Lifespan and Health Span of Drosophila
No full textMetformin has been the most used drug to treat diabetes for nearly 70 years. It reduces the
pressure on the mitochondria from over calorific foods (i.e., fast foods), which termed the
Western Diet. Metformin’s ability to mildly decrease the efficiency of mitochondrial energy
transformation has led to speculation that metformin may also promote healthy aging and extend
longevity in non-diabetic, otherwise healthy individuals. Using the Drosophila model, we
hypothesized that metformin, by decreasing mitochondrial and whole body bioenergetic
efficiency, will increase lifespan in flies on the Western Diet but either decrease or have no
effect in flies on a standard low-fat diet. Initial studies determined that in flight muscle
mitochondria, titration of metformin (1-200 mM) or methyl-triphenylphosphonium (cation;
mTPP; 0.01-0.25 mM) in vitro experiments induced a dose-dependent decrease in ADPstimulated oxygen consumption rate (JO2; normalized to complex IV activity). For in vivo
studies, flies were fasted for 20 hours and then provided food containing mTPP (0-1.0 mM) with
blue dye for two or four hours. Surprisingly, despite evidence of food consumption, no
differences in ADP-stimulated JO2 were detected, suggesting either the flies did not consume
enough food or that mTPP was not absorbed or ineffective in vivo. Results from Western Diet
Studies showed no change in mitochondrial efficiencies for D. melanogaster however, for D.
simulans, there was a lower mitochondrial efficiency in flies on the Western Diet that also
exercised
Linking energy state to redox environment through mitochondrial redox circuits
No full textMitochondria are the primary producers of intracellular H2O2, an oxidant signaling molecule that has gained attention for its role in normal physiological processes and disease etiologies such as skeletal muscle insulin resistance and type 2 diabetes. Understanding the factors that govern the rate of mitochondrial H2O2 emission is critical to developing therapeutic strategies that might mitigate H2O2-induced disease progression while simultaneously enabling cytosolic redox homeostasis and proper cell function. Lipid catabolism through the [beta]-oxidation pathway is a known source of mitochondrial H2O2 generation and implicated in the development of skeletal muscle insulin resistance based on its association with high-fat diets. The goal of this dissertation was 2-fold: 1) To investigate how mitochondrial H2O2 emission due to [beta]-oxidation is regulated by inherent redox circuits with the matrix antioxidant system in skeletal muscle; and 2) To identify the bioenergetic consequences and resulting redox environment associated with increased [beta]-oxidation flux and the development of skeletal muscle insulin resistance. To accomplish these goals, wild-type and genetically altered mouse models were used for experimentation at the whole body, muscle, and mitochondrial level. Multiple redox circuits were discovered in skeletal muscle mitochondria linking [beta]-oxidation-induced H2O2 production to both membrane potential ([Delta][Psi]m)-dependent and independent sources of NADPH, the cofactor ultimately responsible for powering antioxidant activity. Collectively, these redox circuits regulated the rate of mitochondrial H2O2 emission by ensuring sufficient NADPH production to maintain a constant 70-80% antioxidant efficiency, regardless of the [beta]-oxidation-flux rate. Therefore, when flux through [beta]-oxidation was increased, thus elevating the mitochondrial energy state and rate of H2O2 production, redox circuitry enabled proportionally increased NADPH generation in order to reduce the same fraction of H2O2 as with lower energy states. Thus, an increased energy state due to increased [beta]-oxidation-flux resulted in an increased, yet proportional rate of mitochondrial H2O2 emission. In this way, redox circuits regulate the rate of mitochondrial H2O2 emission depending on changes in energy state. At high energy states due to elevated [beta]-oxidation flux, the increased rate of mitochondrial H2O2 emission further induced a more oxidized cytosolic redox environment, consistent with the insulin resistant phenotype observed in genetic models. Hence, redox circuitry further links mitochondrial energy state with activation/deactivation of specific redox-sensitive signaling pathways leading to changes in protein and cellular function
The Influence of Energy Expenditure on Mitochondrial Functions, Oxidative Stress and Insulin Resistance under Metabolic Oversupply Conditions
No full textMitochondrial respiratory capacity and oxidative stress have been implicated in the development of insulin resistance (IR) and type II diabetes. A causative role of mitochondrial oxidative stress in the etiology of diet-induced IR has been suggested. Metabolic oversupply causes mitochondrial oxidative stress and leads to IR; however, how the other side of the metabolic balance equation, energy expenditure, may compensate for oversupply is less appreciated. Based on the principles of bioenergetics, in the condition of substrate oversupply without sufficient energy expenditure, the mitochondrial membrane potential ([delta psi subscript]m) is high and an exponential increase in superoxide generation occurs within a small range of [delta psi subscript]m exceeding about -160mV. The inverse occurs when the mitochondrial energy expenditure rises. In this context, it was hypothesized that a mild increase in energy expenditure can sufficiently attenuate the over-nutrition caused H[subscript]2O[subscript]2 emission and IR. To examine this hypothesis acutely, Sprague-Dawley (S-D) rats received a lipid oral gavage with or without 1h of subsequent low intensity exercise. Mitochondria of permeabilized skeletal muscle fibers were studied. The results show that, without a change in respiratory capacity, a single lipid loading quickly elevated [delta psi subscript]m, mitochondrial H[subscript]2O[subscript]2 emitting potential ([subscript]mE[subscript]H2O2) and reduced calcium retention capacity (an index of the resistance of mitochondrial permeability transition) in state IV and/or under "clamped" physiological state III respiration conditions. These effects can be quickly and sufficiently attenuated by a single bout of postprandial low intensity exercise. These findings provide evidence that mitochondrial H[subscript]2O[subscript]2 production/emission and related effects, but not respiratory capacity, are acutely and dynamically regulated by the metabolic status of skeletal muscle. Further, to examine this hypothesis chronically, S-D rats were high fat diet (HFD, 60%) fed for 7 weeks with or without either low intensity exercise or [beta]-guanidinopropionic acid ([beta]-GPA), which chronically elevates mitochondrial energy turnover. The results show that HFD decreased insulin action and increased [subscript]mE[subscript]H2O2, whereas both were preserved by either exercise or [beta]-GPA. The treatment effects of HFD, exercise or [beta]-GPA were mitochondrial respiratory function and fatty acid oxidation rate independent. However, 5'-AMP-activated protein kinase (AMPK) activity, an energy sensing kinase that increases glucose uptake, was also increased by [beta]-GPA treatment. To determine whether AMPK mediated the [beta]-GPA-induced improvements in insulin action, skeletal and cardiac muscle-specific AMPK [alpha]2 catalytic subunit dominant negative mutated (non-functional) mice and their wild-type littermates were fed a HFD with or without [beta]-GPA for 10 weeks. [Beta]-GPA treatment again prevented the increase in [subscript]mE[subscript]H2O2 and IR in both wild-type and AMPK[alpha]2 dominant negative mice fed a HFD. These findings indicate that AMPK[alpha]2 does not mediate the effects of [beta]-GPA on insulin action, supporting the hypothesis that the reduction in mitochondrial H[subscript]2O[subscript]2 emission is a primary mechanism by which exercise and [beta]-GPA attenuate HFD-induced IR. In the context of both acute and chronic manipulation of positive (oversupply) and negative (expenditure) cellular energy balance, together these findings support the concept that the governance of mitochondrial oxidant production is a primary factor regulating insulin sensitivity in skeletal muscle. Following the principles of bioenergetics, these data demonstrate that a mild increase in energy expenditure can sufficiently attenuate the HFD-induced H[subscript]2O[subscript]2 emission and IR. On the mitochondrial level, the balance of substrate supply and energy expenditure on a daily basis is critical for maintaining a proper cellular redox environment, function and whole body metabolic status. Ph.D
The effects of statins on mitochondrial function
No full textStatins, or 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors, are among the most commonly prescribed medications in the United States. They are commonly used to treat hypercholesterolemia, a condition in which blood cholesterol is elevated above normal levels. Statins serve as an effective means of lowering blood cholesterol, which is associated with a decreased risk for cardiovascular disease. Patients diagnosed with or at risk of coronary artery disease are therefore commonly prescribed statins in order to prevent the development of plaque induced narrowing of coronary arteries. While well tolerated by most patients, reports indicate that approximately 10.5% of patients suffer from statin induced myopathies. Commonly reported side effects include muscular cramps, stiffness, heaviness, weakness, tendonitis, and decreased power during physical activity. Reports have also emerged of statin induced changes in the function of skeletal muscle mitochondria. These reports provided the impetus for an examination of the possibility that statin induced mitochondrial abnormalities play a role in the etiology of the myopathies reported by patients. This study was therefore designed to test the hypothesis that statins interfere with normal mitochondrial function. Specifically, the purpose was to determine whether seven days of in vivo administration of simvastatin or atorvastatin reduces mitochondrial respiratory capacity, decreases mitochondrial calcium retention capacity, and/or increases mitochondrial H₂O₂ emission potential. Twelve female subjects participated in this study. Subjects underwent a 7-day treatment with either 40-mg simvastatin or 80-mg atorvastatin, administered orally. Muscle biopsies were obtained from the rectus femoris muscle at four time points: pre-/post-statin consumption on day one of the study, and pre-/post-statin consumption one week later. Skeletal muscle samples were permeabilized and measurements of mitochondrial respiratory capacity, calcium retention capacity, and H₂O₂ emission potential were performed. The results of this study provide little evidence to support the hypothesis that statins administered in vivo over the course of a seven day treatment interfere with normal mitochondrial function. No significant treatment effect emerged either acutely (2-3 hours following statin treatment) or chronically (after a seven day treatment). Statins seemed to have little to no effect on mitochondrial respiratory capacity, calcium retention capacity, and H₂O₂ emission potential during the seven day treatment. The results of this study suggest that the deleterious effects of statin exposure reported in the literature are not a consequence of acute or short term (7d) changes in mitochondrial function with commencement of statin therapy. However, in vitro studies demonstrate that acute statin exposure can indeed negatively affect mitochondrial respiratory capacity, calcium retention capacity, and H₂O₂ emission potential (C-T Lin & PD Neufer, unpublished data). Further studies will be required to elucidate the exact nature of statin induced mitochondrial abnormalities in humans and their role in the etiology of statin induced myopathies.M.S
Effect of Acute Exercise or Fasting on Mitochondrial Function and High Fat Diet-Induced Insulin Resistance
No full textA high-fat diet leads to an accumulation of lipid in skeletal muscle, and the development of both mitochondrial dysfunction and insulin resistance. Recently, our lab reported that lipid overload leads to elevated H[subscript]2O[subscript]2 emission from muscle mitochondria, and that mitochondrial-targeted scavenging of H[subscript]2O[subscript]2 completely prevents the development of high fat diet-induced insulin resistance. These findings raise the possibility that interventions which acutely restore cellular metabolic balance in muscle may also acutely restore insulin sensitivity. We hypothesized that mitochondrial function and insulin sensitivity can be restored in skeletal muscle of high-fat fed rats by creating an acute deficit in metabolic balance via 2 h low-intensity treadmill exercise or 16 h fasting. Male Sprague-Dawley rats (125-150g) were either maintained on a standard high carbohydrate- diet or fed a high-fat (60%) diet for 6 weeks and divided into three groups the day before the study: one group was maintained on the normal high-fat diet, another group was fasted overnight (16 h), and a third group completed a single 2 h bout of low-intensity treadmill exercise (10 m/min) and then were given normal overnight ad libitum access to the high-fat diet. Oral glucose tolerance tests were administrated to assess insulin action. Red gastrocnemius muscles were harvested and permeabilized fibers prepared for determination of mitochondrial respiratory function and H[subscript]2O[subscript]2 emission. A single 16 h fast significantly (P<0.05) improved insulin sensitivity in rats maintained on a high-fat diet (P<0.05). Oxygen consumption rate in permeabilized fibers in response to submaximal and maximal ADP concentration when supported exclusively with complex I substrates were not different among groups. However, when respiration was supported by fatty acids (palmitoylcarnitine plus malate, complex I + II substrates), high-fat diet plus exercise group showed higher (P<0.05) rates compared with high-fat diet group. There were no significant differences in H[subscript]2O[subscript]2 emission among the 4 groups. In conclusion, a single 16 h overnight fast is sufficient to restore insulin sensitivity in high fat diet-induced insulin resistant rats, providing evidence that insulin action in muscle is acutely sensitive to the metabolic state of cells. A single bout of low-intensity treadmill exercise in high-fat fed rats failed to restore insulin action but increased ADP-stimulated respiratory capacity, providing evidence of an as yet unidentified regulatory mechanism of the respiratory system. Somewhat surprisingly however, neither fasting nor exercise altered the H[subscript]2O[subscript]2 emitting potential in permeabilized fibers, suggesting that further work is required to better understand the factors influencing mitochondrial function and their potential link to insulin sensitivity. M.S
Novel mechanisms governing the regulation of mitochondrial bioenergetics : OXPHOS efficiency and cAMP/PKA signaling
No full textUnderstanding the regulation of cellular metabolism is paramount to treating the growing prevalence of metabolic disease worldwide. In cellular metabolism, mitochondrial oxidative phosphorylation (OXPHOS) plays a key role as it is a primary source of energy and the governor of cellular redox homeostasis. A fundamental aspect of mitochondrial function is that cellular metabolic demand requires a corresponding increase in flux through OXPHOS; however, the regulation of OXPHOS is incompletely understood. Herein, two hypotheses were tested: 1) OXPHOS efficiency increases as a function of metabolic demand to allow mitochondria to maximize ATP synthesis at a given level of O₂ flux and 2) that OXPHOS is regulated by cAMP/PKA signaling within skeletal muscle mitochondria. First, in permeabilized myofibers (PmFBs) from mouse skeletal muscle and myocardium, the data provided herein demonstrate that OXPHOS efficiency increases from ~20% to >70% from resting [ADP] to [ADP] found during exhaustive exercise in skeletal muscle, whereas [ADP] in the myocardium remains static (at ~75-100 [mu]M) regardless of workload. Importantly, in the presence of small changes in [ADP] (e.g. 5-20 [mu]M), ATP synthesis increased independent of an increase in JO₂, suggesting that skeletal muscle mitochondria can accommodate increased metabolic demand without a requisite increase in O₂ flux, suggesting a decrease in proton leak. Second, it was demonstrated that tricarboxylic acid (TCA) cycle flux alone is insufficient to increase cAMP levels in isolated skeletal muscle mitochondria. However, pharmacological inhibition of PKA impairs a multitude of mitochondrial function outcomes in both liver and skeletal muscle that summarily implicate Complex I as a primary target. In conclusion, given the absolute necessity for coupled OXPHOS in the maintenance of energy homeostasis and the variety of diseases linked to decreased Complex I activity, the findings provided herein not only advance our current knowledge of mitochondrial bioenergetics, but provide a multitude of opportunities for future investigations.Ph.D
ESTABLISHMENT OF THE PYRUVATE DEHYDROGENASE COMPLEX AS A CENTRAL REGULATOR OF MITOCHONDRIAL REDOX WITHIN SKELETAL MUSCLE
No full textOnce regarded as "byproducts" of aerobic metabolism, the production of superoxide/H₂O₂ is now understood to be a highly specialized and extensively regulated process responsible for exerting control over a vast number of thiol-containing proteins, collectively referred to as the redox-sensitive proteome. Although disruptions within this process, secondary to elevated peroxide exposure, have been linked to disease, delineation of the sources and mechanisms regulating this increased peroxide burden remain poorly defined and as such difficult to target using pharmacotherapy. Herein we demonstrate a role for pyruvate dehydrogenase (PDH) as a key source of H₂O₂ under physiological constraints in which respiratory chain-dependent electron leak is negligible. PDH is shown to generate H₂O₂ as a function of glutathione content, matrix metabolic balance, as well as antioxidant reductase activity. With respect to the latter, manipulation of matrix redox buffering reveals a novel mechanism whereby H₂O₂ producing NADH-linked dehydrogenases, such as PDH, are functionally linked to the redox buffering network within skeletal muscle through the activity of nicotinamide nucleotide transhydrogenase (NNT). These findings highlight the importance of NNT and the entire redox buffering system in regulating cytosolic peroxide emission and suggest a novel and pivotal role for PDH as a redox-sensitive reporter of matrix redox buffering integrity and nutrient status.Ph.D
A single dose of metformin improves whole body insulin sensitivity and alters cellular redox state in skeletal muscle of Zucker fa/fa rats
No full textEnergy balance is considered a fundamental requirement of life forms from single cell organisms to higher mammals such as humans. Unfortunately, our species has also discovered the detrimental metabolic responses to excess dietary intake: obesity and the accompanying pathologies collectively known as the metabolic syndrome. Central to the metabolic syndrome is insulin resistance, defined as a relative failure of insulin to stimulate glucose transport in peripheral tissues such as skeletal muscle. It is generally accepted that prolonged insulin resistance often results in the onset of type 2 diabetes, which is one of the most common diseases in the world. Current treatment for type 2 diabetes generally begins with dimethylbiguanide, an insulin sensitizing drug also known as metformin. In the last 10 years, scientific discovery has identified mitochondrial function as a key player in a variety of metabolic diseases, including insulin resistance and type 2 diabetes. As such, a variety of investigations have been performed in an attempt to indentify [sic] mechanisms by which altered mitochondrial function or physiology may contribute to the pathogenesis of these diseases. Recent evidence from our laboratory indicates that mitochondria derived oxidant (mROS) generation is a key player in the mitochondrial regulation of insulin sensitivity in vivo. Additionally, recent evidence has demonstrated that acute metformin treatment in vitro decreases liver mROS, and that chronic metformin treatment in vivo decreases skeletal muscle mROS concurrent with improvements in whole body glucose tolerance. Together, this evidence indicates that metformin may alter peripheral insulin sensitivity by decreasing the elevated mROS associated with insulin resistance in the obese population. Therefore, the purpose of the current study was to investigate the effects of a single oral dose of metformin on whole body glucose tolerance and mROS in red and white gastrocnemius of Zucker fa/fa rats, a genetically obese animal model. A single oral dose of metformin resulted in improved whole body glucose tolerance compared to controls independent of alterations in serum insulin. Cellular redox state was significantly more oxidized in animals treated with glucose or metformin alone compared to controls or animals which received both treatments. Succinate and palmitoylcarnitine/malate induced mROS was not altered by glucose and/or metformin in red or white gastrocnemius. Mitochondrial respiration with pyruvate/malate or palmitoylcarnitine/malate was unchanged in response to glucose and/or metformin treatment in red or white gastrocnemius. Akt phosphorylation was significantly elevated in both red and white gastrocnemius in response to glucose or metformin alone, but no additive effect was observed when administered simultaneously, indicating that metformin may act as an insulin mimetic in vivo. AMPK phosphorylation was not elevated in response to metformin treatment in either tissue, which suggests that metformin may act through AMPK-independent mechanisms in skeletal muscle in vivo. The results of this study demonstrate that a single oral dose of metformin can improve whole body glucose tolerance independent of changes in mitochondrial respiration, mROS, or altered AMPK signaling in red and white gastrocnemius of Zucker fa/fa rats, but may be associated with altered cellular redox state. M.S
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