Effects of theophylline on pharyngeal dilator and diaphragm muscle contractile properties

Theophylline alleviates central and obstructive apneas of prematurity, and may improve adult obstructive sleep apnea. One mechanism of action appears to be a stimulatory effect on the motor output to upper airway dilator muscles. The purpose of the present study was to determine whether theophylline might have a second mechanism of action, namely that of improving the force and/or endurance of the pharyngeal dilator musculature. Rat sternohyoid muscle strips were studied in vitro and compared to diaphragm strips. The isometric twitch force and twitch kinetics of neither muscle were altered by theophylline (100 mg/l). Theophylline significantly slowed the rate at which the diaphragm fatigued during intermittent 40-Hz stimulation (p < 0.001). In contrast, theophylline produced no improvement in the fatigue resistance of the sternohyoid muscle. The degree of force potentiation during the early portion of the fatigue protocol was not altered by theophylline for either muscle. These results suggest that the mechanism by which theophylline improves obstructive apnea is unlikely to be due to a beneficial effect on pharyngeal dilator muscle force or endurance.