Mice with a malignant hyperthermia mutation (Y522S) in the ryanodine receptor (RyR1) display muscle contractures, rhabdomyolysis, and death in response to elevated environmental temperatures. We demonstrate that this mutation in RyR1 causes Ca(2+) leak, which drives increased generation of reactive nitrogen species (RNS). Subsequent S-nitrosylation of the mutant RyR1 increases its temperature sensitivity for activation, producing muscle contractures upon exposure to elevated temperatures. The Y522S mutation in humans is associated with central core disease. Many mitochondria in the muscle of heterozygous Y522S mice are swollen and misshapen. The mutant muscle displays decreased force production and increased mitochondrial lipid peroxidation with aging. Chronic treatment with N-acetylcysteine protects against mitochondrial oxidative damage and the decline in force generation. We propose a feed-forward cyclic mechanism that increases the temperature sensitivity of RyR1 activation and underlies heat stroke and sudden death. The cycle eventually produces a myopathy with damaged mitochondria.

Malignant hyperthermia (MH) is a life-threatening disorder characterized by skeletal muscle rigidity and elevated body temperature in response to halogenated anesthetics such as isoflurane or halothane. Mutation of tyrosine 522 of RyR1 (the predominant skeletal muscle calcium release channel) to serine has been associated with human malignant hyperthermia. In the present study, mice created harboring this mutation were found to represent the first murine model of human malignant hyperthermia. Mice homozygous for the Y522S mutation exhibit skeletal defects and die during embryonic development or soon after birth. Heterozygous mice, which correspond to the human occurrence of this mutation, are MH susceptible, experiencing whole body contractions and elevated core temperatures in response to isoflurane exposure or heat stress. Skeletal muscles from heterozygous mice exhibit increased susceptibility to caffeine- and heat-induced contractures in vitro. In addition, the heterozygous expression of the mutation results in enhanced RyR1 sensitivity to activation by temperature, caffeine, and voltage but not uncompensated sarcoplasmic reticulum calcium leak or store depletion. We conclude that the heterozygous expression of the Y522S mutation confers susceptibility to both heat- and anesthetic-induced MH responses.

Mitochondrial health is critical to physiological function, particularly in tissues with high ATP turnover, such as striated muscle. It has been postulated that derangements in skeletal muscle mitochondrial function contribute to impaired physical function in older adults. Here, we determined mitochondrial respiratory capacity and coupling control in skeletal muscle biopsies obtained from young and older adults. Twenty-four young (28 ± 7 yr) and thirty-one older (62 ± 8 yr) adults were studied. Mitochondrial respiration was determined in permeabilized myofibers from the vastus lateralis after the addition of substrates oligomycin and CCCP. Thereafter, mitochondrial coupling control was calculated. Maximal coupled respiration (respiration linked to ATP production) was lower in muscle from older vs. young subjects (P < 0.01), as was maximal uncoupled respiration (P = 0.06). Coupling control in response to the ATP synthase inhibitor oligomycin was lower in older adults (P < 0.05), as was the mitochondria flux control ratio, coupled respiration normalized to maximal uncoupled respiration (P < 0.05). Calculation of respiratory function revealed lower respiration linked to ATP production (P < 0.001) and greater reserve respiration (P < 0.01); i.e., respiratory capacity not used for phosphorylation in muscle from older adults. We conclude that skeletal muscle mitochondrial respiratory capacity and coupling control decline with age. Lower respiratory capacity and coupling efficiency result in a reduced capacity for ATP production in skeletal muscle of older adults.

BackgroundCancer cachexia negatively impacts cancer‐related treatment options, quality of life, morbidity, and mortality, yet no established therapies exist. We investigated the anabolic properties of testosterone to limit the loss of body mass in late stage cancer patients undergoing standard of care cancer treatment.MethodsA randomized, double‐blind, placebo‐controlled phase II clinical trial was undertaken to assess the potential therapeutic role of adjunct testosterone to limit loss of body mass in patients with squamous cell carcinoma of the cervix or head and neck undergoing standard of care treatment including chemotherapy and chemoradiation. Patients were randomly assigned in blocks to receive weekly injections of either 100 mg testosterone enanthate or placebo for 7 weeks. The primary outcome was per cent change in lean body mass, and secondary outcomes included assessment of quality of life, tests of physical performance, muscle strength, daily activity levels, resting energy expenditure, nutritional intake, and overall survival.ResultsA total of 28 patients were enrolled, 22 patients were studied to completion, and 21 patients were included in the final analysis (12 placebo, nine testosterone). Adjunct testosterone increased lean body mass by 3.2% (95% confidence interval [CI], 0–7%) whereas those receiving placebo lost 3.3% (95% CI, −7% to 1%, P = 0.015). Although testosterone patients maintained more favourable body condition, sustained daily activity levels, and showed meaningful improvements in quality of life and physical performance, overall survival was similar in both treatment groups.ConclusionsIn patients with advanced cancer undergoing the early phase of standard of care therapy, adjunct testosterone improved lean body mass and was also associated with increased quality of life, and physical activity compared with placebo.

Age-related skeletal muscle loss is thought to stem from suboptimal nutrition and resistance to anabolic stimuli. Impaired microcirculatory (nutritive) blood flow may contribute to anabolic resistance by reducing delivery of amino acids to skeletal muscle. In this study, we employed contrast-enhanced ultrasound, microdialysis sampling of skeletal muscle interstitium, and stable isotope methodology, to assess hemodynamic and metabolic responses of older individuals to endurance type (walking) exercise during controlled amino acid provision. We hypothesized that older individuals would exhibit reduced microcirculatory blood flow, interstitial amino acid concentrations, and amino acid transport when compared with younger controls. We report for the first time that aging induces anabolic resistance following endurance exercise, manifested as reduced (by ∼40%) efficiency of muscle protein synthesis. Despite lower (by ∼40-45%) microcirculatory flow in the older than in the younger participants, circulating and interstitial amino acid concentrations and phenylalanine transport into skeletal muscle were all equal or higher in older individuals than in the young, comprehensively refuting our hypothesis that amino acid availability limits postexercise anabolism in older individuals. Our data point to alternative mediators of age-related anabolic resistance and importantly suggest correction of these impairments may reduce requirements for, and increase the efficacy of, dietary protein in older individuals.