Sternohyoid muscle fatigue properties of dy/dy dystrophic mice, an animal model of merosin-deficient congenital muscular dystrophy.

Humans with merosin-deficient congenital muscular dystrophy have both sucking problems during infancy and sleep-disordered breathing during childhood. We hypothesized that merosin-deficient pharyngeal muscles fatigue faster than normal muscles. This was tested in vitro using sternohyoid muscle from an animal model of this disease, the dy/dy dystrophic mouse. Isometric twitch contraction and half-relaxation times were similar for dy/dy and normal sternohyoid. However, rate of force loss during repetitive 25-Hz train stimulation was markedly diminished in dystrophic compared with normal sternohyoid muscle. Furthermore, force potentiation, which occurred during the early portion of the fatigue-inducing stimulation, had a longer duration in dystrophic compared with normal muscle (approximately 60 versus 20 s). As a result of these two processes, at the end of 2 min of stimulation, force of dystrophic muscle had decreased by 8 +/- 5% and that of normal muscle by 69 +/- 4% (p < 0.0001). The potassium-channel blocker, 3,4-diaminopyridine, increased force of dy/dy sternohyoid muscle during twitch and 25-Hz contractions by 148 +/- 20% (p < 0.00001) and 109 +/- 18% (p < 0.00002), respectively. During repetitive 25-Hz stimulation, force of 3,4-diaminopyridine-treated dystrophic muscle remained significantly higher than that of untreated muscle, despite the early force potentiation being eliminated and fatigue being accelerated. Thus, merosin deficiency reduces fatigue and prolongs the duration of force potentiation. The latter alterations may partially preserve the integrity of upper airway muscle function, without which the severity of pharyngeal complications (feeding problems, sleep-related respiratory dysfunction) might be even more pronounced in the human merosin-deficient congenital muscular dystrophies.