Our general hypothesis is that a reduced response of muscle protein anabolism to insulin plays an important role in the loss of muscle mass that takes place with aging. We further hypothesize that this alteration is specific for protein metabolism regardless of glucose tolerance status. Our preliminary results suggest that there is an alteration in the response of muscle anabolism to insulin in the elderly, which can compromise the positive effect of amino acids on postprandial muscle protein gain. The goal of this proposal is to define the age-related changes in insulin stimulated muscle protein anabolism, in order to be able to develop specific interventions to reduce or reverse the loss of muscle mass with aging. We will test the hypothesis that the responsiveness of muscle myofibrillar and mixed protein synthesis and breakdown to insulin is reduced in healthy elderly volunteers and in young healthy controls. Insulin will be infused locally in the femoral artery in order to expose the muscle tissue to specific insulin concentrations and avoids as much as possible the systemic effects of hyperinsulinemia (i.e. hypoaminoacidemia and hypoglycemia). Since the defect of muscle protein anabolism in the elderly might be either generalized for any insulin concentration, or dose-dependent, i.e. specifically localized to a certain insulin level (e.g. prandial), we will measure muscle protein kinetics at graded insulin infusion rates that will increase insulin concentration in the femoral vein to approximatley 30 muU/ml (low dose), approximately 80 muU/ml (prandial dose), approximately 200 muU/ml (high dose). To address the issue as to whether the age-related alteration in the response of muscle protein metabolism to insulin is independent or glucose tolerance, we will measure muscle and whole body glucose kinetics simultaneously with protein kinetics not only in the healthy young and elderly volunteers, but also in young and elderly type 2 diabetes patients. The inclusion of these two additional groups of type 2 diabetic patients will allow us to test if the effects of insulin resistance (type 2 diabetes) and age on muscle protein and glucose metabolism are indpendent.
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