The hypothesis that the accumulation of electron transport system (ETS) abnormalities and sarcopenia are linked was investigated. Vastus lateralis, soleus, and adductor longus muscles were studied in 5-, 18-, and 36-mo-old male Fischer 344 x Brown Norway F(1) hybrid rats. A significant decrease in soleus and vastus lateralis muscle mass was observed with age. Adductor longus was resistant to muscle mass loss. Multiple serial sections were analyzed for the activities of cytochrome-c oxidase (COX) and succinate dehydrogenase (SDH). The number of fibers exhibiting a COX(-)/SDH(++) phenotype increased with age in both vastus lateralis and soleus muscles. No ETS-abnormal fibers were identified in adductor longus at any age. Cross-sectional area of ETS-abnormal fibers decreased in the abnormal region (region displaying COX(-)/SDH(++) phenotype), whereas control fibers did not. The vastus lateralis muscle, which undergoes a high degree of sarcopenia, exhibited more ETS abnormalities and associated fiber loss than the soleus and adductor longus muscles, which are more resistant to sarcopenia, suggesting a direct association between ETS abnormalities and fiber loss.

Laser-capture microdissection was coupled with PCR to define the mitochondrial genotype of aged muscle fibers exhibiting mitochondrial enzymatic abnormalities. These electron transport system (ETS) abnormalities accumulate with age, are localized segmentally along muscle fibers, are associated with fiber atrophy and may contribute to age-related fiber loss. DNA extracted from single, 10 microm thick, ETS abnormal muscle fibers, as well as sections from normal fibers, served as templates for PCR-based deletion analysis. Large mitochondrial (mt) DNA deletion mutations (4.4-9.7 kb) were detected in all 29 ETS abnormal fibers analyzed. Deleted mtDNA genomes were detected only in the regions of the fibers with ETS abnormalities; adjacent phenotypically normal portions of the same fiber contained wild-type mtDNA. In addition, identical mtDNA deletion mutations were found within different sections of the same abnormal region. These findings demonstrate that large deletion mutations are associated with ETS abnormalities in aged rat muscle and that, within a fiber, deletion mutations are clonal. The displacement of wild-type mtDNAs with mutant mtDNAs results in concomitant mitochondrial enzymatic abnormalities, fiber atrophy and fiber breakage.

Summary Sarcopenia, the loss of skeletal muscle mass and function with advancing age, is a significant cause of disability and loss of independence in the elderly, and, thus, represents a formidable challenge for the aging population. Nevertheless, the molecular mechanism(s) underlying sarcopenia-associated muscle dysfunction remain poorly understood. In this study, we employed an integrated approach combining top-down targeted proteomics with mechanical measurements to dissect the molecular mechanism(s) in age-related muscle dysfunction. Top-down targeted proteomic analysis uncovered a progressive age-related decline in the phosphorylation of myosin regulatory light chain (RLC), a critical protein involved in the modulation of muscle contractility, in the skeletal muscle of aging rats. Top-down tandem mass spectrometry analysis identified a previously un-reported bis-phosphorylated proteoform of fast skeletal RLC and localized the sites of decreasing phosphorylation to Ser14/15. Of these sites, Ser14 phosphorylation represents a previously unidentified site of phosphorylation in RLC from fast-twitch skeletal muscle. Subsequent mechanical analysis of single fast-twitch fibers isolated from the muscles of rats of different ages revealed that the observed decline in RLC phosphorylation can account for age-related decreases in the contractile properties of sarcopenic fast-twitch muscles. These results strongly support a role for decreasing RLC phosphorylation in sarcopenia-associated muscle dysfunction, and suggest therapeutic modulation of RLC phosphorylation may represent a new avenue for the treatment of sarcopenia.

The effects of aging on cardiovascular function and cardiac structure were determined in a rat model recommended for gerontological studies. A cross-sectional analysis assessed cardiac changes in male Fischer 344 ϫ Brown Norway F1 hybrid rats (FBN) from adulthood to the very aged (n ϭ 6 per 12-, 18-, 21-, 24-, 27-, 30-, 33-, 36-, and 39-mo-old group). Rats underwent echocardiographic and hemodynamic analyses to determine standard values for left ventricular (LV) mass, LV wall thickness, LV chamber diameter, heart rate, LV fractional shortening, mitral inflow velocity, LV relaxation time, and aortic/LV pressures. Histological analyses were used to assess LV fibrotic infiltration and cardiomyocyte volume density over time. Aged rats had an increased LV mass-to-body weight ratio and deteriorated systolic function. LV systolic pressure declined with age. Histological analysis demonstrated a gradual increase in fibrosis and a decrease in cardiomyocyte volume density with age. We conclude that, although significant physiological and morphological changes occurred in heart function and structure between 12 and 39 mo of age, these changes did not likely contribute to mortality. We report reference values for cardiac function and structure in adult FBN male rats through very old age at 3-mo intervals. hybrid rat; echocardiography CARDIAC STRUCTURE AND FUNCTION are remarkably similar among mammalian species, and the use of animal models has been extremely helpful in developing treatment strategies for alleviating heart disease in humans. Extending animal studies beyond young adult ages to very old ages may provide similar benefits to cardiovascular health of the growing aged population. In mammalian tissues, aging manifests as detrimental alterations in structure and function. Structural changes in the aging heart extend from cardiomyocyte cell loss, left ventricular (LV) hypertrophy, changes in ventricle chamber diameter, and collagen deposition (3, 13, 14), leading to overt functional changes such as lengthening of contraction and relaxation times and thus a decrease in heart rate (15), decreases in fractional shortening, decreased LV end-systolic pressure (15, 33), and reduced cardiac output. These normal aging changes do not necessarily contribute to morbidity but are clearly associated with the general decline observed with aging.The Fischer 344 (F344) rat has been a standard model of aging with an extensive database on this genotype (29). Even though the F344 rat has been widely used in research, concerns have been raised whether this specific strain is appropriate for all gerontological studies because of its overuse and some severe age-related pathologies (53). The National Institute on Aging and the National Center for Toxicological Research responded to these concerns and found that, among others, the Fischer 344 ϫ Brown Norway F1 hybrid rat (FBN) would complement the F344 aging studies (53). The FBN rat had significantly fewer pathological lesions compared with the F344 and a greater mean age for 50% mortality...

Mitochondrial DNA (mtDNA) deletion mutations accumulate with age in tissues of a variety of species. Although the relatively low calculated abundance of these deletion mutations in whole tissue homogenates led some investigators to suggest that these mutations do not have any physiological impact, their focal and segmental accumulation suggests that they can, and do, accumulate to levels sufficient to affect the metabolism of a tissue. This phenomenon is most clearly demonstrated in skeletal muscle, where the accumulation of mtDNA deletion mutations remove critical subunits that encode for the electron transport system (ETS). In this review, we detail and provide evidence for a molecular basis of muscle fiber loss with age. Our data suggest that the mtDNA deletion mutations, which are generated in tissues with age, cause muscle fiber loss. Within a fiber, the process begins with a mtDNA replication error, an error that results in a loss of 25–80% of the mitochondrial genome. This smaller genome is replicated and, through a process not well understood, eventually comprises the majority of mtDNA within the small affected region of the muscle fiber. The preponderance of the smaller genomes results in a dysfunctional ETS in the affected area. As a consequence of both the decline in energy production and the increase in oxidative damage in the region, the fiber is no longer capable of self‐maintenance, resulting in the observed intrafiber atrophy and fiber breakage. We are therefore proposing that a process contained within a very small region of a muscle fiber can result in breakage and loss of muscle fiber from the tissue.