title: Geniohyoid muscle properties and myosin heavy chain composition are altered after short-term intermittent hypoxic exposure.
abstract: 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.
authors.affiliation: Section of Orthodontics, UCLA School of Dentistry, 10833 Le Conte Ave., Los Angeles, CA 90095-1668, USA. epae@dent.ucla.edu
authors.forename: Eung-Kwon
authors.initials: EK
authors.lastname: Pae
authors.forename: Jennifer
authors.initials: J
authors.lastname: Wu
authors.forename: Daniel
authors.initials: D
authors.lastname: Nguyen
authors.forename: Ryan
authors.initials: R
authors.lastname: Monti
authors.forename: Ronald M
authors.initials: RM
authors.lastname: Harper
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D009119:Muscle Contraction; D018763:Muscle Fatigue; D018485:Muscle Fibers, Skeletal / Q000378:metabolism* / Q000473:pathology; D018995:Myosin Heavy Chains / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012132:Respiratory Muscles / Q000503:physiopathology*; D013997:Time Factors
chemical_list: D018995:Myosin Heavy Chains