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1 Section of Orthodontics, UCLA School of Dentistry, Los Angeles, CA, USA
2 Neurobiology, David Geffen School of Medicine at UCLA, Los Angeles, CA, USA
* To whom correspondence should be addressed. E-mail: epae{at}dent.ucla.edu.
Patients with obstructive sleep apnea (OSA) often exhibit fatigued or inefficient upper airway dilator and constrictor muscles; an upper airway dilator, the geniohyoid (GH) muscle is a particular example. Intermittent hypoxia (IH) is a frequent concomitant of OSA, and may trigger muscle fiber composition changes that are characteristic of a fatigable nature. We examined effects of short-term IH on diaphragmatic and GH muscle fiber composition and fatigue properties by exposing 24 rats to alternating 10.3% O2, balance N2, and room air every 480 s (240 s duty cycle) for a total duration of 5, 10, 15, 20 or 30 hr. Sternohyoid fiber composition was also examined. Control animals were exposed to room air on the same schedule. Single fiber analyses showed that GH muscle fiber types changed completely from MHC (myosin heavy chain) 2A to MHC 2B after 10 hr exposure, and the conversion was maintained for at least 30 hr. Sternohyoid muscle fibers showed a delayed transition from MHC 2A/2B to MHC 2B. In contrast, major fiber types of the diaphragm were not significantly altered. The GH muscles showed similar tension-frequency relationships in all groups, but an increased fatigability developed, proportional to the duration of IH treatment. We conclude that short-term IH exposure alters GH muscle composition and physical properties toward more fatigable, fast-twitch types, and may account for the fatigable upper airway fiber types found in sleep disturbed breathing.
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