Genioglossus and hyoglossus muscle activation during sustained asphyxia in rats

Episodes of apnea caused by pharyngeal airway obstruction produce sustained asphyxia in sleep apnea patients, but little is known of the effects of prolonged asphyxia on muscles which are involved in the control of upper airway caliber. We therefore examined the responses of retractor and protrudor muscles of the tongue (hyoglossus (HG) and genioglossus (GG) muscles, respectively) during sustained asphyxia in anesthetized and vagotomized spontaneously breathing rats (250-300 g). EMG activities of GG and HG muscles were recorded during exposure to 6 min of asphyxia (F₁O₂ = 0.15; F₁CO₂ = 0.07; n=7), and additionally during 6 min of hypoxia (F₁O₂ = 015; n=5). The magnitude and direction of tongue movement was also determined by connecting a suture from a force transducer to the tip of the rat's tongue. EMG activities of both muscles were increased from baseline levels after 30 sec of asphyxia [+48.2 ± 44 % (GG) and +42.4 ± 4.3 % (HG); mean ± SE], and remained elevated after 6 min of asphyxia [+45.6 ± 8.4%(GG); +51.0 ± 7.6%(HG)]. Sustained hypoxia produced a biphasic response for both muscles (stimulation followed by roll-off). Both asphyxia and hypoxia elicited a net retractive movement of the tongue. The time-course of the tongue movement responses mimiced the EMG responses. These results indicate that: 1) retractor and protrudor muscles of the tongue are co-activated during sustained asphyxia and hypoxia in rats; 2) co-activation is associated with tongue retraction; and 3) the GG and HG behave similar to other respiratory muscles during conditions of respiratory stress in rats.