Respiratory muscle dysfunction is implicated in the pathophysiology of obstructive sleep apnea syndrome (OSAS), an oxidative stress disorder prevalent in men. Pharmacotherapy for OSAS is an attractive option, and antioxidant treatments may prove beneficial. We examined the effects of chronic intermittent hypoxia (CIH) on breathing and pharyngeal dilator muscle structure and function in male and female rats. Additionally, we tested the efficacy of antioxidant treatment in preventing (chronic administration) or reversing (acute administration) CIH-induced effects in male rats. Adult male and female Wistar rats were exposed to alternating cycles of normoxia and hypoxia (90 s each; Fi(O(2)) = 5% O(2) at nadir; Sa(O(2)) ∼ 80%) or sham treatment for 8 h/d for 9 days. Tempol (1 mM, superoxide dismutase mimetic) was administered to subgroups of sham- and CIH-treated animals. Breathing was assessed by whole-body plethysmography. Sternohyoid muscle contractile and endurance properties were examined in vitro. Muscle fiber type and cross-sectional area and the activity of key metabolic enzymes were determined. CIH decreased sternohyoid muscle force in male rats only. This was not attributable to fiber transitions or alterations in oxidative or glycolytic enzyme activity. Muscle weakness after CIH was prevented by chronic Tempol supplementation and was reversed by acute antioxidant treatment in vitro. CIH increased normoxic ventilation in male rats only. Sex differences exist in the effects of CIH on the respiratory system, which may contribute to the higher prevalence of OSAS in male subjects. Antioxidant treatment may be beneficial as an adjunct OSAS therapy.

New Findings r What is the central question of this study?The effects of chronic intermittent hypoxia (CIH) on respiratory muscles are relatively underexplored. It is speculated that muscle dysfunction and other key morbidities associated with sleep apnoea are the result of CIH-induced oxidative stress. We sought to investigate the putative role of CIH-induced reactive oxygen species in the development of respiratory muscle dysfunction. r What is the main finding and its importance?The CIH-induced diaphragm muscle fatigue is time and intensity dependent and is associated with a modest oxidative stress. Supplementation with N-acetyl cysteine prevents CIH-induced diaphragm muscle dysfunction, suggesting that antioxidant supplementation may have therapeutic value in respiratory muscle disorders characterized by CIH, such as obstructive sleep apnoea.Respiratory muscle dysfunction documented in sleep apnoea patients is perhaps due to oxidative stress secondary to chronic intermittent hypoxia (CIH). We sought to explore the effects of different CIH protocols on respiratory muscle form and function in a rodent model. Adult male Wistar rats were exposed to CIH (n = 32) consisting of 90 s normoxia-90 s hypoxia (either 10 or 5% oxygen at the nadir; arterial O 2 saturation ß90 or 80%, respectively] for 8 h per day or to sham treatment (air-air, n = 32) for 1 or 2 weeks. Three additional groups of CIH-treated rats (5% O 2 for 2 weeks) had free access to water containing N-acetyl cysteine (1% NAC, n = 8), tempol (1 mm, n = 8) or apocynin (2 mm, n = 8). Functional properties of the diaphragm muscle were examined ex vivo at 35°C. The myosin heavy chain and sarco(endo)plasmic reticulum Ca 2+ -ATPase isoform distribution, succinate dehydrogenase and glyercol phosphate dehydrogenase enzyme activities, Na + -K + -ATPase pump content, concentration of thiobarbituric acid reactive substances, DNA oxidation and antioxidant capacity were determined. Chronic intermittent hypoxia (5% oxygen at the nadir; 2 weeks) decreased diaphragm muscle force and endurance. All three drugs reversed the deleterious effects of CIH on diaphragm endurance, but only NAC prevented CIH-induced diaphragm weakness. Chronic intermittent hypoxia increased diaphragm muscle myosin heavy chain 2B areal density and oxidized glutathione/reduced glutathione (GSSG/GSH) ratio. We conclude that CIH-induced diaphragm dysfunction is reactive oxygen species dependent. N-Acetyl cysteine was most effective in reversing CIH-induced effects on diaphragm. Our results suggest that respiratory muscle dysfunction in sleep apnoea may be the result of oxidative stress and, as such, antioxidant treatment could prove a useful adjunctive therapy for the disorder. DOI

Obstructive sleep apnea is a common disorder associated with upper airway muscle dysfunction. Agents that improve respiratory muscle performance may be useful as an adjunct therapy. The aim of this study was to examine the effects of antioxidants on rat pharyngeal dilator muscle performance. Adult male Wistar rats were killed humanely and isometric contractile properties of isolated sternohyoid muscle strips were examined in physiological salt solution at 35 degrees C in vitro. Muscle strips were incubated in tissue baths under hyperoxic (95%O(2)/5%CO(2)) or hypoxic (95%N(2)/5%CO(2)) conditions in the absence (control) or presence of the antioxidants: N-acetylcysteine (10 mM), Tiron (10 mM), or Tempol (10 mM). Force-frequency relationship was determined in response to supramaximal stimulation (10-100 Hz in increments of 10-20 Hz, train duration: 300 ms). Isometric force was also recorded during repetitive muscle stimulation (40 Hz, 300 ms every 2 s for 2 min). Under hyperoxic conditions, Tiron and Tempol, but not N-acetylcysteine, significantly increased sternohyoid muscle force and caused a left-shift in the force-frequency relationship. In addition, Tempol had a significant positive inotropic effect over the initial 90 seconds of repeated muscle activation. Hypoxia caused a significant decrease in sternohyoid muscle force. Under hypoxic conditions, Tempol-incubated muscles generated significantly higher forces compared with control muscles and showed improved performance in the early phase of the fatigue trial. This study illustrates that superoxide scavengers increase upper airway muscle force and that this effect persists under hypoxic conditions. We conclude that antioxidant treatment may be beneficial as a therapy in obstructive sleep apnea.

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Background: Age, obesity and male sex are risk factors for the development of obstructive sleep apnoea syndrome. Objective: We examined structural and functional properties of the sternohyoid muscle in young lean and aged obese male rats. We hypothesized that the aged muscle would be vulnerable to oxidative stress (hypoxia). Methods: Isometric contractile and endurance properties of the sternohyoid muscle were assessed in vitro with or without the superoxide scavenger Tempol (10 mM). Muscle fibre size and density were determined by myosin heavy chain immunofluorescence. Succinate dehydrogenase (SDH) and glycerol-3- phosphate dehydrogenase (GPDH) enzyme activities were determined. Results: Fibre hypertrophy, increased fast twitch (type 2X) fibre density, decreased SDH activity and increased GPDH activity, together with increased force and fatigue, were observed in aged obese muscles compared to young lean muscles. Tempol treatment increased strength and sensitivity to stimulation. Hypoxic depression of force was ameliorated by antioxidant treatment with equivalent effects in young lean and aged obese muscle. Conclusions: We conclude that the rat sternohyoid exhibits indefinite growth and is protected from oxidative stress as the animal ages.