An exercise driven, insulin independent glucose uptake pathway in contracting skeletal muscle

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Diabetes Mellitus (DM) affects more than 10% of the US population and this number is increasing. Skeletal muscles play a major role in glucose homeostasis because of their large capacity to import and store glucose. This makes glucose uptake by skeletal muscle of paramount importance in the development and treatment of DM. Most prior research on glucose uptake in muscle has focused on the insulin-regulated glucose transporter, GLUT4. Quiescent skeletal muscles take up glucose in response to rising plasma insulin, which triggers translocation of GLUT4 from intracellular stores to the plasma membrane. A paradox in the field is that actively contracting muscles take up 50-100 times more glucose than quiescent muscles. However, despite more than 30 years of study, the mechanism by which contracting muscles take up glucose without a requirement for insulin remain incompletely understood. Non-insulin-dependent glucose uptake has been explained in part by signaling mechanisms that bypass the insulin receptor to trigger GLUT4 translocation. However, this model has not accounted for all the glucose taken up by contracting muscle or the glucose uptake that remains in GLUT4 knockout models. Notably, it does not account for the huge discrepancy between GLUT4 translocation (2-fold increase in membrane density) and the up to 100-fold increase in glucose uptake. A consensus is growing that additional mechanisms of muscle glucose uptake exist. Our long-term goal is to uncover previously unrecognized mechanisms of insulin-independent glucose uptake by contracting muscles. This goal is motivated by our recent discovery that a significant component of contraction-stimulated glucose uptake requires Na,K-ATPase (NKA) activity, which we term NKA- dependent glucose uptake. Our central hypothesis is that a component of contraction-stimulated glucose uptake is coupled, either directly or indirectly, to the contraction-stimulated activity of Na,K- ATPase. The relationship between glucose uptake and changes in other ions whose transport is also stimulated by contraction is not known. This knowledge gap persists because the available tools for measuring glucose in cells do not readily accommodate co-measurement of other ions and transported species. The Specific Aims of thisproject are to optimize and standardize a new analytical method, based on Inductively Coupled PlasmaMass Spectroscopy (ICP-MS), to measure simultaneous changes in 13C glucose and Rb+ (a congener for K+), and to use this method to characterize NKA- dependent glucose uptake. At the completion of this study, we will have introduced a new analytical method for 13C detection that can be broadly applied to other cell processes that require multi-species detection, and we will have identified a previously unrecognized mechanism of non-insulin-dependent glucose uptake that may provide new therapeutic targets for treating DM.