Uremic metabolites impair skeletal muscle mitochondrial energetics through disruption of the electron transport system and matrix dehydrogenase activity.

Chronic kidney disease (CKD) leads to increased skeletal muscle fatigue, weakness, and atrophy. Previous work has implicated mitochondria within the skeletal muscle as a mediator of muscle dysfunction in CKD; however, the mechanisms underlying mitochondrial dysfunction in CKD are not entirely known. The purpose of this study was to define the impact of uremic metabolites on mitochondrial energetics. Skeletal muscle mitochondria were isolated from C57BL/6N mice and exposed to vehicle (DMSO) or varying concentrations of uremic metabolites: indoxyl sulfate, indole-3-acetic-acid, l-kynurenine, and kynurenic acid. A comprehensive mitochondrial phenotyping platform that included assessments of mitochondrial oxidative phosphorylation (OXPHOS) conductance and respiratory capacity, hydrogen peroxide production (JH2O2), matrix dehydrogenase activity, electron transport system enzyme activity, and ATP synthase activity was employed. Uremic metabolite exposure resulted in a ~25-40% decrease in OXPHOS conductance across multiple substrate conditions (P < 0.05, n = 5-6/condition), as well as decreased ADP-stimulated and uncoupled respiratory capacity. ATP synthase activity was not impacted by uremic metabolites; however, a screen of matrix dehydrogenases indicated that malate and glutamate dehydrogenases were impaired by some, but not all, uremic metabolites. Assessments of electron transport system enzymes indicated that uremic metabolites significantly impair complex III and IV. Uremic metabolites resulted in increased JH2O2 under glutamate/malate, pyruvate/malate, and succinate conditions across multiple levels of energy demand (all P < 0.05, n = 4/group). Disruption of mitochondrial OXPHOS was confirmed by decreased respiratory capacity and elevated superoxide production in cultured myotubes. These findings provide direct evidence that uremic metabolites negatively impact skeletal muscle mitochondrial energetics, resulting in decreased energy transfer, impaired complex III and IV enzyme activity, and elevated oxidant production.

Chronic kidney disease exacerbates ischemic limb myopathy in mice via altered mitochondrial energetics.

Chronic kidney disease (CKD) substantially increases the severity of peripheral arterial disease (PAD) symptomology, however, the biological mechanisms remain unclear. The objective herein was to determine the impact of CKD on PAD pathology in mice. C57BL6/J mice were subjected to a diet-induced model of CKD by delivery of adenine for six weeks. CKD was confirmed by measurements of glomerular filtration rate, blood urea nitrogen, and kidney histopathology. Mice with CKD displayed lower muscle force production and greater ischemic lesions in the tibialis anterior muscle (78.1 ± 14.5% vs. 2.5 ± 0.5% in control mice, P < 0.0001, N = 5-10/group) and decreased myofiber size (1661 ± 134 µm2 vs. 2221 ± 100 µm2 in control mice, P < 0.01, N = 5-10/group). This skeletal myopathy occurred despite normal capillary density (516 ± 59 vs. 466 ± 45 capillaries/20x field of view) and limb perfusion. CKD mice displayed a ~50-65% reduction in muscle mitochondrial respiratory capacity in ischemic muscle, whereas control mice had normal mitochondrial function. Hydrogen peroxide emission was modestly higher in the ischemic muscle of CKD mice, which coincided with decreased oxidant buffering. Exposure of cultured myotubes to CKD serum resulted in myotube atrophy and elevated oxidative stress, which were attenuated by mitochondrial-targeted therapies. Taken together, these findings suggest that mitochondrial impairments caused by CKD contribute to the exacerbation of ischemic pathology.