External anal sphincter fatigue is not improved by N-acetylcysteine in an animal model

Upper airway dilator muscle weakness following intermittent and sustained hypoxia in the rat: effects of a superoxide scavenger

Obstructive sleep apnoea syndrome (OSAS) is a common disorder associated with upper airway muscle dysfunction. Agents that improve respiratory muscle performance may have considerable therapeutic value. We examined the effects of acute exposure to sustained and intermittent hypoxia on rat pharyngeal dilator muscle function. Additionally, we sought to test the efficacy of antioxidant treatment in ameliorating or preventing hypoxia-related muscle dysfunction. Isometric contractile and endurance properties of isolated rat sternohyoid muscle bundles were examined at 35 °C in vitro. Muscle bundles were exposed to one of four gas treatments: hyperoxia (control), sustained hypoxia (SH), intermittent hypoxia (IH) or hypoxia/re-oxygenation (HR), in the absence or presence of the superoxide scavenger--Tempol (10 mM). Stress-frequency relationship was determined in response to electrical stimulation (10-100 Hz in increments of 10-20 Hz, train duration: 300 ms). Muscle performance was also assessed during repetitive muscle stimulation (40 Hz, 300 ms every 2 s for 2.5 min). Compared to control, IH and HR treatments significantly decreased sternohyoid muscle force. The negative inotropic effect of the two gas protocols was similar, but both were of lesser magnitude than the effects of SH. SH, but not IH and HR, increased muscle fatigue. Tempol significantly increased sensitivity to stimulation in all muscle preparations and caused a leftward shift in the stress-frequency relationship of IH and SH treated muscles. Tempol did not ameliorate sternohyoid muscle fatigue during SH. We conclude that Tempol increases upper airway muscle sensitivity to stimulation but only modestly ameliorates respiratory muscle weakness during intermittent and sustained hypoxic conditions in vitro. Respiratory muscle fatigue during sustained hypoxia appears unrelated to oxidative stress.

Structural and functional properties of an upper airway dilator muscle in aged obese male rats

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.

Comparison of the contractile properties, oxidative capacities and fibre type profiles of the voluntary sphincters of continence in the rat

The external urethral sphincter (EUS) and external anal sphincter (EAS) are the principal voluntary striated muscles that sustain continence of urine and faeces. In light of their common embryological origin, shared tonic sphincteric action and synchronized electrical activity in vivo, it was expected that they would exhibit similar physiological and structural properties. However, the findings of this study using paired observations of both sphincters isolated from the rat show clearly that this is not the case. The anal sphincter is much more fatigable than the urethral sphincter. On completion of a fatigue protocol, the amplitude of the last twitch of the EAS had declined to 42 +/- 3% of the first twitch, whereas the last twitch of the EUS was almost identical to that of the first (95 +/- 3%). Immunocytochemical detection of myosin heavy-chain isoforms showed that this difference was not due to the presence of more slow-twitch oxidative type 1 fibres in the EUS compared with the EAS (areal densities 4 +/- 1% and 5 +/- 1%, respectively; P = 0.35). In addition, the fatigue difference was not explained by a greater contribution to force production by fast oxidative type 2A fibres in the urethral sphincter. In fact, the anal sphincter contained a higher areal density of type 2A fibres (56 +/- 5% vs. 37 +/- 4% in the EUS, P = 0.017). The higher oxidative capacity of the EUS, measured histochemically, explained its fatigue resistance. These results were surprising because the fatigue-resistant urethral muscle exhibited faster single-twitch contraction times compared with the anal sphincter (56 +/- 0.87 ms vs. 72.5 +/- 1.16 ms, P < 0.001). Neither sphincter expressed the type 2X myosin isoform but the fast-twitch isoform type 2B was found exclusively in the EUS (areal density 16 +/- 2%). The type 2B fibres of the EUS were small (diameter 19.5 +/- 0.4 mum) in comparison to typical type 2B fibres of other muscles. As a whole the EUS is a more oxidative than glycolytic muscle. In conclusion, analysis of the twitch mechanics and fatigue of two sphincters showed that the EUS contained more fatigue-resistant muscle fibres compared with the EAS.

Superoxide scavengers improve rat pharyngeal dilator muscle performance

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.