Proceedings of The Physiological Society

University of Manchester (2010) Proc Physiol Soc 19, C102

Oral Communications

Intermittent hypoxia/reoxygenation increases respiratory muscle fatigability in neonatal rats: No role for Reactive Oxygen Species

R. A. O'Connell1, K. D. O'Halloran1

1. university college dublin, Dublin, Ireland.

Intermittent hypoxia/reoxygenation (IHR) is a common feature of several early-life respiratory disorders such as apnoea of prematurity and childhood obstructive sleep apnoea. Reactive oxygen species (ROS) are modulators of skeletal muscle function which are implicated in hypoxia-induced muscle dysfunction in disease states. As the functional activity of the respiratory muscles is essential for adequate ventilation, any impairment will have detrimental effects on respiratory performance. The aim of this study was to examine the effects of acute IHR on respiratory muscle function during development, and to investigate the putative role of ROS in mediating IHR-induced effects on muscle function. Litters of Wistar rats were killed humanely under 5% isoflurane at P10, P20 or P30. The sternohyoid (pharyngeal dilator) muscle was removed and endurance properties were examined in tissue baths containing physiological salt solution (PSS) at 35°C. Muscle strips were exposed to one of two gas treatments as follows: For control experiments, the baths were aerated for 30mins with a hyperoxic gas (95%O2/5%CO2), whereas for IHR, muscles were exposed to alternating 5min periods of hyperoxia and hypoxia (95%N2/5%CO2) for 30mins. Following the gas treatments, fatigue was assessed. Additionally, muscle strips were treated with three different antioxidants during IHR exposures: Tempol, N-acetyl cysteine, and catalase. Snap frozen muscle strips were cryosectioned to 10μm and myosin heavy chain (MHC) isoform composition and enzymatic activities were examined using immunofluorescence and enzymatic histochemistry. IHR caused a significant increase in fatigue of the sternohyoid muscle in early, but not late, development. Fatigue index following control and IHR exposure was: 90±10% and 60±3% (P10, n=6, P<0.05, ANOVA), 45±4% and 51±4% (P20, n=6), and 24±5% and 28±3% (P30, n=6), respectively. None of the antioxidants ameliorated the increased fatigue in P10 muscles following IHR exposure. There was an age-related shift in the sternohyoid muscle from a slow oxidative to a fast glycolytic phenotype; we observed a significant decrease in type 1 and 2A fibres, while type 2B fibre density increased: 9±1%, 37±2% and 25±2% (P10, n=4), 8±1%, 34±2% and 40±3% (P20, n=4), and 6±1%, 30±1% and 45±6% (P30, n=4), respectively. There was also an increase in glycolytic activity with age. In summary, IHR had an age-related deleterious effect on respiratory muscle function. Antioxidant treatment was ineffective in ameliorating this IHR-induced increase in muscle fatigue. We conclude that early development represents a critically vulnerable period of development for hypoxia-induced respiratory muscle impairment. Increased upper airway dilator muscle fatigue in vivo would likely predispose to upper airway obstruction and impaired respiratory homeostasis.

Where applicable, experiments conform with Society ethical requirements