Force potentiation in respiratory muscles: comparison of diaphragm and sternohyoid
Van Lunteren, E.; Vafaie, H.
American Journal of Physiology 264(6 Pt 2): R1095-R1100
1993
ISSN/ISBN: 0002-9513 PMID: 8322961 Document Number: 411797
Coordinated contraction of thoracic and pharyngeal upper airway respiratory muscles optimizes ventilation, whereas pharyngeal muscle dysfunction may lead to obstructive apneas during sleep. We hypothesized that the force potentiation exhibited by the pharyngeal respiratory muscle, the sternohyoid, in keeping with its faster contractile kinetics, would be greater than that of the thoracic respiratory muscle, the diaphragm. Rat muscles were studied in vitro at 37 degree C with three force-potentiating protocols: posttetanic twitch potentiation, staircase phenomenon (twitch potentiation), and a classic fatigue paradigm. The sternohyoid had a faster isometric contraction time, a more rightward located force-frequency relationship, and both a more rapid onset and a greater degree of fatigue than the diaphragm. During the early portion of the fatigue protocols, the increase in force was significantly greater for the sternohyoid muscle than the diaphragm (e.g., 33 vs. 3% increase at 20 Hz, P lt 0.005). During repetitive twitches at 2, 3, and 5 Hz (staircase test), sternohyoid muscle force increased more than diaphragm force at the higher stimulus frequencies (e.g., by 38 vs. 23% at 5 Hz, P lt 0.01). After brief tetanic stimuli, sternohyoid twitch force increased more than diaphragm twitch force (e.g., 73 vs. 14% increase after 125 Hz tetanus, P lt 0.005). These data indicate that force potentiation is exhibited by both diaphragm and sternohyoid respiratory muscles, but to different extents, when activated repetitively. The large degree of force potentiation noted for the pharyngeal muscle suggests that 1) a unique relationship between muscle electromyographic activity and either muscle force or pharyngeal patency is unlikely to be maintained over even short periods of time, and 2) intrinsic muscle contractile properties may act in concert with dynamic changes in motor output to pharyngeal muscles in producing breath-to-breath variability in pharyngeal patency and ventilation.