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J Appl Physiol 98: 889-894, 2005. First published November 19, 2004; doi:10.1152/japplphysiol.00978.2004
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Geniohyoid muscle properties and myosin heavy chain composition are altered after short-term intermittent hypoxic exposure

Eung-Kwon Pae,1 Jennifer Wu,1 Daniel Nguyen,1 Ryan Monti,1 and Ronald M. Harper2

1Section of Orthodontics, University of California at Los Angeles School of Dentistry, and 2Department of Neurobiology, David Geffen School of Medicine at University of California at Los Angeles, Los Angeles, California

Submitted 7 September 2004 ; accepted in final form 12 November 2004

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 it 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 h. 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 myosin heavy chain (MHC) type 2A to MHC type 2B after 10 h of exposure, and the conversion was maintained for at least 30 h. Sternohyoid muscle fibers showed a delayed transition from MHC type 2A/2B to MHC type 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 that it may account for the fatigable upper airway fiber types found in sleep-disturbed breathing.

sleep-disordered breathing; upper airway; sleep; fatigue


Address for reprint requests and other correspondence: E.-K. Pae, Sect. of Orthodontics, UCLA School of Dentistry, 10833 Le Conte Ave., Los Angeles, CA 90095-1668 (E-mail: epae{at}dent.ucla.edu)




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