Sarcopenia is a common complication of chronic kidney disease. Sarcopenia is defined by decreased muscle mass or function and is central to the frailty phenotype that is associated disability, hospitalization, and death. Sarcopenia is a common and devastating complication of chronic kidney disease (CKD). The investigators and others have demonstrated a high prevalence of physical frailty among CKD patients not treated with dialysis. Furthermore the investigators have demonstrated greater than 30% impairment in lower extremity physical performance measures in persons with CKD compared with predicted values. The aging demographic among CKD patients will result in a substantial U.S. public health burden attributable to sarcopenia. In the year 2000, direct healthcare costs attributable to sarcopenia in the U.S. were18.5 billion.
Sarcopenia is associated with adverse health-related outcomes. Sarcopenia in older adults is consistently linked with decreased physical functioning, disability, falls, hospitalization, and mortality. Impaired lower extremity physical performance, as measured by objective testing, is associated with all-cause mortality in CKD patients not treated with dialysis and that these associations are stronger in magnitude than those for traditional risk factors. Skeletal muscle is major site of peripheral glucose utilization ameliorating oxidative stress and endothelial injury associated with acute post-prandial hyperglycemia. Through these mechanisms sarcopenia may contribute to metabolic disturbances of insulin resistance, oxidative stress, and endothelial dysfunction leading to cardiovascular disease.
Mitochondrial dysfunction is central to skeletal muscle dysfunction. Skeletal muscle mitochondria are necessary for the efficient generation of energy (ATP) from oxygen and normal lipid metabolism. Under normal conditions, muscle efficiently utilizes the majority of supplied oxygen such that only 0.2% of mitochondrial oxygen is shuttled into reactive oxidative species (ROS). Under pathologic situations there is uncoupling of oxygen consumption and ATP generation resulting in increased oxygen consumption and decreased ATP production in a process leading to increased ROS and oxidative stress. Uncoupling of oxygen and ATP generation directly affects skeletal muscle function. Decreased efficiency of ATP generation has been linked to decreased muscle strength and more recently associated with decreased gait speed in older adults.
Magnetic resonance spectroscopy and optical spectroscopy (MRS/OS) provide novel, non-invasive and real-time insight into human skeletal muscle mitochondrial function. MRS/OS is a novel technique that measures maximal mitochondrial ATP production in-vivo following acute bouts of ischemia induced by exercise using phosphorous MRS (31P MRS). The rate of recovery of phosphocreatinine after cessation of exercise is used to characterize the rate of aerobic mitochondrial ATP resynthesis above that of basal anaerobic glycolytic ATP production. By combining this technique with optical spectroscopy post-exercise measuring the transport of oxygen from hemoglobin to myoglobin within the muscle the investigators are able to accurately determine the ratio of coupling efficiency between of ATP generation per unit of oxygen consumption. Skeletal muscle mitochondrial dysfunction measured by ATP generating capacity and coupling efficiency has been associated with processes of aging and insulin resistance. Furthermore studies in healthy subjects have shown that muscle performance and fatigue are linked to the ability to resynthesize ATP rather than lactate concentration.
Preliminary MRS/OS findings suggest profoundly altered mitochondrial function in CKD patients. Previous experimental studies suggest that uremia in dialysis patients affects skeletal muscle structure and mitochondrial function. Oxidative damage is highly prevalent in CKD, evidenced by increased biomarkers of oxidative stress and changes in glutathione, an important antioxidant. Prior small studies in non-diabetic CKD patients suggest impairment of ex-vivo mitochondrial function by reductions in mitochondrial enzyme activity. Based on these findings investigators used novel, non-invasive, functional MRS/OS assay to characterize in-vivo mitochondrial function. Preliminary findings in non-diabetic CKD patients indicate markedly reduced ATP to oxygen ratio compared to controls. This finding suggests altered mitochondrial energetics as a candidate central mechanism linking metabolic derangements and impaired physical function in CKD, and motivate the hypothesis that mitochondrial dysfunction is associated with oxidative stress, insulin resistance and impaired physical functioning in CKD.
Exercise may ameliorate mitochondrial dysfunction, insulin resistance and physical functioning in CKD. Studies of exercise in non-CKD patients with diabetes and insulin resistance have demonstrated exercise-induced improvements in mitochondrial biogenesis linked to improved insulin sensitivity as well as decreased mitochondrial oxidative stress. Animal studies suggest that exercise stimulates autophagy resulting in removal of defective and inefficient mitochondrial leading to a healthy mitochondrial network and improved insulin sensitivity.The primary goal of this study is to investigate the impact of combined resistance and aerobic exercise on skeletal muscle dysfunction. Given the investigator's preliminary data demonstrating strong association of sarcopenia and mortality in CKD, effective interventions that can impact physical function are urgently needed to improve patient health outcomes.
