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Contractile weakness and loss of muscle mass are critical features of the aging process in mammalians. Age-related fibre wasting has a profound effect on muscle metabolism, fibre type distribution and the overall physiological integrity of the neuromuscular system. This study has used mass spectrometry-based proteomics to investigate the fate of the aging rat muscle proteome. Using nonionic detergent phase extraction, this report shows that the aged gastrocnemius muscle exhibits a generally perturbed protein expression pattern in both the detergent-extracted fraction and the aqueous protein complement from senescent muscle tissue. In the detergent-extracted fraction, the expression of ATP synthase, isocitrate dehydrogenase, enolase, tropomyosin and beta-actin was increased. Different isoforms of creatine kinase and prohibitin showed differential changes. In the aqueous fraction, malate dehydrogenase, sulfotransferase, triosephosphate isomerase, aldolase, cofilin-2 and lactate dehydrogenase showed increased levels. Interestingly, differential effects on dissimilar 2-D spots of the same protein species were shown for Cu/Zn superoxide dismutase, albumin, annexin A4 and phosphoglycolate phosphatase. Mitochondrial Hsp60, Hsp71 and nucleoside diphosphate kinase B exhibited a reduced abundance in aged muscle. The majority of altered proteins were found to be involved in mitochondrial metabolism, glycolysis, metabolic transportation, regulatory processes, the cellular stress response, detoxification mechanisms and muscle contraction., (Copyright 2010 Elsevier B.V. All rights reserved.)