Geniohyoid muscle properties and myosin heavy chain composition are altered after short-term intermittent hypoxic exposure

1 Section of Orthodontics, University of California at Los Angeles School of Dentistry, and 2 Department 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% O 2 -balance N 2 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 )