Antidiabetic effects caused by inhibition of respiration?

Type 2 diabetes is of increasing prevalence in the industrial world, because it is associated with life style factors like high fat diet, obesity, and physical inactivity. A fundamental defect characterizing this disease is the resistance of target tissues to the action of insulin. Thiazolidinediones (TZDs) and metformin are known to reduce insulin resistance, but their primary locus of action is still unclear. The working hypothesis of the proposed study holds that both TZDs and metformin act by direct inhibition of enzyme complex I of the respiratory chain (complex I), thereby causing reduced cellular respiration and a decrease in the cellular energy charge. We further hypothesize that such disruption of respiration causally contributes to the antidiabetic effects of these compounds. Using tissue homogenates and isolated mitochodria, Part 1 of the intended study will examine, to what extent TZDs and metformin are inhibitors of complex 1. Such inhibitory action has been attributed to metformin, but a potential of TZDs to exert similar actions has never been shown and would be a new and important information. Part 2 is to provide a "proof of principle" for the hypothesized causal relation of inhibition of respiration to antidiabetic action. To find out, whether an inhibition of respiration will in the long-term lead to antidiabetic effects, diabetic rats will be subjected to daily oxygen deficiency or to the complex 1 inhibitor rotenone. Amelioration of the diabetic state would be best evidence that antidiabetic action can result from reduced respiratory activity. Part 3 is to provide evidence for or against the inhibition of respiration by TZDs and metformin in vivo (Part 1 is to demonstrate such action only in vitro). After the administration of a single dose of a TZD or metformin, the expression of UCP-3 mRNA in skeletal muscle will be measured. An immediate increase in UCP-3 mRNA would indicate that the examined compound acutely decreases the energy charge of skeletal muscle in vivo. Taken together, the proposed experiments will improve our understanding of the impact of the cellular energy charge on metabolic regulation, and will support or falsify our hypothesis that inhibition of respiration contributes to the antidiabetic actions of TZDs and metformin.