Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, PC27
Effects of chronic intermittent hypoxia on rat sternohyoid muscle structure and function
C. Shortt1, K. D. O'Halloran1
1. University College Dublin, Dublin, Ireland.
Chronic intermittent hypoxia (CIH) is a central feature of obstructive sleep apnoea (OSA), a debilitating disorder characterized by recurrent collapse of the upper airway during sleep. We and others have previously shown that CIH can induce skeletal muscle dysfunction1, 2. Stimulus intensity and duration are likely to be important factors driving phenotypic plasticity in skeletal muscle. This study was designed to investigate the effect of different CIH paradigms on the structural and functional properties of rat upper airway dilator muscle. Adult male Wistar rats were exposed to three different CIH treatments. First, we examined the effect of 90s normoxia/ 90s hypoxia [10% oxygen at the nadir], 8h/day for one week. Second, we exposed animals to the same stimulus intensity but increased the duration to two weeks. In a third set of experiments, the animals were treated for one week with an increased stimulus intensity (90s normoxia/ 90s hypoxia [5% oxygen at the nadir], 8h/day). Sham-treated animals served as controls. Contractile and endurance properties were examined in isolated muscle bundles in vitro. Anoxic tolerance was also assessed. Additionally, following all three CIH treatments, sternohyoid muscle was snap frozen in liquid nitrogen and stored for structural analysis. Fibre type analysis was performed by immunofluorescence while enzymatic histochemistry was used to quantify oxidative and glycolytic capacity of muscles. CIH (10% oxygen at the nadir) had no effect on sternohyoid muscle force following a one week exposure (15±1 vs. 17±1 N/c2), mean±SEM, sham (n=8) vs. treated (n=8) at 100 Hz, P>0.05, ANOVA) or two week exposure (19±1 vs. 16±1 N/cm2, mean±SEM, sham (n=6) vs. treated (n=6) at 100 Hz, P>0.05). Fatigue was also unaffected following these two CIH treatments. There was a significant increase in type 2B fibre density (68±3% vs. 77±4% areal density, mean±SEM, sham (n=5) vs. treated (n=5), P<0.05) following 2 weeks of CIH but no change in oxidative or glycolytic capacity. Increasing the stimulus intensity (5% oxygen at the nadir) for one week had no effect on the structural and functional properties of the sternohyoid. In conclusion, these three paradigms of CIH had no effect on upper airway muscle function. We have previously reported respiratory muscle dysfunction in studies where the stimulus duration and/or intensity were greater than in this study. We conclude that respiratory muscle re-modelling in CIH is influenced by duration and intensity of IH exposure. An increase in type 2B fibre density predisposes to a more fatiguable phenotype, which has been reported previously1.
Where applicable, experiments conform with Society ethical requirements