|Accessory muscles support respiratory system performance in young dystrophin-deficient mdx mice
|AD Slyne KD O'Halloran DP Burns
|University College Cork
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|Respiratory impairment is a feature of Duchenne muscular dystrophy (DMD), a genetic neuromuscular disorder. Peak inspiratory pressure-generating capacity is preserved in young dystrophin-deficient mdx mice, despite diaphragm muscle weakness and reduced electromyogram (EMG) activity. We hypothesise that accessory muscle compensation limits ventilatory deficit in early dystrophic disease.
Four-month-old male wild-type (n=30) and mdx (n=29) mice were studied. In urethane (1.7g/kg i.p.) anaesthetised mice, diaphragm and scalene EMG activities were recorded during baseline and sustained tracheal occlusion producing maximal respiratory muscle activation. Diaphragm and scalene muscle function were examined ex vivo. Data (mean±SD) were statistically compared using two-way ANOVA with Bonferroni post-hoc test.
Peak diaphragm EMG activity in mdx mice was reduced compared with wild-type (25.8±13.8 vs. 15.7±7.4 mV.s; P=0.0009). Moreover, mdx diaphragm force was considerably lower (significant force loss at 25-150Hz). In comparison, peak scalene EMG activity was equivalent in wild-type and mdx mice (1.74±1.45 vs. 1.48±0.58 mV.s; P=0.5767). Scalene muscle force-generating capacity was preserved in mdx mice.
The early decline in diaphragm performance (decreased activation and intrinsic weakness), suggests that compensation provided by accessory muscles is critical to the support of respiratory performance in mdx mice, which may have relevance to DMD.