The Influence of Energy Expenditure on Mitochondrial Functions, Oxidative Stress and Insulin Resistance under Metabolic Oversupply Conditions

Abstract

Mitochondrial 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.