Insulin resistance is associated with many pathological conditions including obesity, insulinopenic diabetes, protein catabolic states (trauma and sepsis); it is the salient feature of the most common form of diabetes in man (Type II). The major defect resides post- insulin-receptor-binding. 1) We observed insulin receptor (IR) structural and functional heterogeneity between muscle and liver, and impaired IR tyrosine kinase (TK) activation in diabetes. The role of IR TK activation in modulating the insulin response will be studied in normal rats (muscle and liver) and in models of insulin resistance (insulinopenic diabetes and diet induced). The molecular basis of impaired IR TK activation will be investigated and its relationship to apparent structural modifications of the IR - subunit studied. The hypothesis that changes in the metabolic milieu may lead to altered processing (glycosylation) of IR will b tested. Functional relevance of altered IR TK activity will be assessed by studies of insulin stimulated phosphorylation of endogenous IR TK substrates in intact cells. 2) Rat muscles develop profound insulin resistance shortly after denervation with intact binding and IR TK activation. Studies of mechanisms of post-receptor insulin resistance will be continued in this model. Glucose transporter number, affinity, subcellular distribution and insulin induced translocation will be studied and possible alterations in the phosphorylation of endogenous IR TK substrates assessed in muscle after denervation. 3) In all conditions studied, accelerated net muscle protein catabolism is associated with accelerated branched chain amino acid (BCAA) oxidation by muscle. We have developed methods to measure the in vivo activation state of the rate limiting enzyme of BCAA catabolism in muscle, branched chain - keto acid dehydrogenase complex (BCKAD), and found that administration of glucorticoids, bacterial endotoxin and activated macrophage secretion products rapidly activate muscle BCKAD. The effect of recombinant monokines, TNF, IL-1 and - interferon on BCKAD activation will be studied in rat muscle in vivo and in a tissue culture model, L-6 myocytes. The regulation of BCAA metabolism and protein turnover by monokines, hormones (glucocorticoids and insulin) and BCAA will be studied in L-6 cells. The mechanism of the apparent association between accelerated BCAA catabolism and muscle protein degradation will be investigated. Understanding of these mechanisms is important, since BCAA supplementation has been advocated as therapy in the catabolic state.
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