Non-insulin dependent diabetes mellitus is a common metabolic disorder that affects over ten million Americans and occurs with 3-4 fold increased frequency in Mexican Americans. Treatment of this disorder and related complications presents a large financial burden. Despite numerous investigations into the pathogenesis of NIDDM, the primary metabolic, biochemical, and molecular abnormalities responsible for insulin resistance (characteristic feature of the disease) have yet to be elucidated. In part, this has resulted from lack of knowledge about the early steps, glucose phosphorylation and glucose transport, involved in insulin action in vivo in human muscle, the primary tissue responsible for the disposal of infused or ingested glucose. To our knowledge no previous study has examined either basal or insulin-stimulated glucose phosphorylation in muscle or any other tissue in man. Recently, we have developed a novel triple isotope technique (12C-mannitol, 3-14C-O- methylglucose, 3-3H-glucose injection into the brachial artery) that allows quantitation of glucose phosphorylation, glucose transport, and intracellular free glucose concentration in human forearm muscle. This technique will be combined with muscle biopsy to quantitate hexokinase II and HK I activity/mRNA/protein and GLUT 4/GLUT 1 mRNA/protein levels. This will allow us to relate insulin's effect in vivo to the basic biochemical and molecular events involved in the stimulation of muscle glucose phosphorylation and transport in vivo. Parallel studies will be performed in NIDDM patients to define whether insulin-mediated muscle glucose phosphorylation and/or transport are impaired and, if so, the mechanism(s) responsible for these defect(s). Similar studies will be carried out in the normal glucose tolerant, insulin resistant offspring of two Mexican American NIDDM parents. Such individuals have a 70-80% chance of developing NIDDM in later life. The results in offspring will allow us to define whether defects in glucose phosphorylation/transport occur early in the natural history of NIDDM before the onset of hyperglycemia, insulinopenia, and disturbances in FFA/amino acid metabolism (all of which can impair insulin action). We also will examine the effects of chronic, physiologic hyperinsulinemia and sustained, physiologic hyperglycemia on glucose phosphorylation/transport in healthy subjects, as well as correction of hyperglycemia in NIDDM patients with intensified insulin therapy and phlorizin (an agent which normalizes plasma glucose levels by inducing renal glucosuria). We believe that the present studies are both innovative and novel, will significantly expand our understanding of the regulation of glucose phosphorylation/transport in normal and NIDDM subjects, and may provide new insights into the biochemical and molecular basis of NIDDM. Although studies will be performed in Mexican Americans, we believe that our findings will have generalized importance and will help to define the etiology of NIDDM in other populations. Regardless, Mexican Americans comprise 5% of the US population (12 million people) and definition of the cause of NIDDM in this ethnic group alone would have major scientific, therapeutic, and health care delivery significance.
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