Mdx myotubes have normal excitability but show reduced contraction-relaxation dynamics

The pathogenesis of Duchenne muscular dystrophy (DMD), characterised by lac k of the cytoskeletal protein dystrophin, is not completely understood. An early event in the degenerative process of DMD muscle could be a rise in cy tosolic calcium concentration. In order to investigate whether this leads t o alterations of contractile behaviour, we studied the excitability and con tractile properties of cultured myotubes from control (C57BL/10) and mdx mi ce, an animal model for DMD. The myotubes were stimulated electrically and their motion was recorded photometrically. No significant differences were found between control and mdx myotubes with respect to the following parame ters: chronaxy and rheobase (0.33 +/- 0.03 ms and 23 +/- 4 V vs. 0.39 +/- 0 .07 ms and 22 +/- 2 V for C57 and mdx myotubes, respectively), tetanisation frequency (a similar distribution pattern was found between 5 and 30 Hz), fatigue during tetanus (found in 35% of both types of myotubes) and post-te tanic contracture. In contrast, contraction and relaxation times were longe r (P < 0.005) in mdx (36 +/- 2 and 142 +/- 13 ms, respectively) than in con trol myotubes (26 +/- 1 and 85 +/- 9 ms, respectively). Together with our e arlier findings, these results suggest a decreased capacity for calcium rem oval in mdx cells leading, in particular, to alterations of muscle relaxati on.