Skeletal muscle plays a central role in systemic carbohydrate metabolism by oxidizing or storing glucose. The mechanisms of reduced glucose clearance by skeletal muscle, the hallmark of type 2 diabetes, are poorly understood and for this reason a proposed role of mitochondrial dysfunction has attracted considerable interest. According to this picture, reduced fatty acid oxidation by mitochondria leads to buildup of intracellular triglyceride stores, followed by accumulation of byproducts of intracellular fatty acid metabolism that interfere with insulin signaling. However, in spite of intense scientific and public interest in obesity and type 2 diabetes, this hypothesis is difficult to test because of the limitations of standard metabolic studies of muscle. New NMR methods have been developed using instruments operating in the range of 1.5 - 3.0 T to noninvasively probe mitochondrial function and intramyocellular triglycerides, but these methods are difficult to apply because of low signal and relatively poor chemical shift resolution. Our recent observations on healthy volunteers indicate that both limitations will be substantially improved at 7 T. The project will focus on the hypothesis that abnormal function of skeletal muscle mitochondria causes insulin resistance through accumulation of triglycerides. Mitochondrial function will be assessed by two methods, the rate of TCA cycle flux measured by oxidation of [2-13C]acetate and by the rate of ATP synthesis at rest. Intramyocellular lipids will be measured directly by single-voxel 1H NMR spectroscopy. In this project we will examine four populations: patients with type 2 diabetes, lean offspring of patients with type 2 diabetes, patients with type 2 diabetes before and after weight loss, and patients before and after acute weight gain. If the hypothesis is correct, all patients with type 2 diabetes and lean offspring of diabetic parents should have both abnormal mitochondrial function and excess intramyocellular lipids, whereas weight gain should not cause changes in mitochondrial function. This project requires close interaction with Drs. Cohen and Hobbs (Project 7), Drs. Elmquist and Tamminga (Project 2), and Drs. Parks, Browning and Burgess (Project 6).
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