Long-term study of Ca2+ homeostasis and of survival in collagenase-isolated muscle fibres from normal and mdx mice

Skeletal muscles of the mdx mouse lack dystrophin offering the possibility to study the role of intracellular Ca2+ ions in fibre degeneration. Flexor digitorum brevis muscles of 3-month-old mdx and normal mice were dissociated with collagenase; fibres were maintained in culture for 6 days (do to d5) and their survival was assessed. Cytosolic [Ca2+], passive Mn2+ influx (indicative of Ca2+ influx) and activity of mechanosensitive/voltage-independent Ca2+ channels were studied over the same period. Survival of normal fibres declined steadily from do to d3, but an acceleration of fibre death occurred in mdx fibres from d1 to d2. This could be greatly reduced but not abolished by lowering external [Ca2+] 10-fold. In the d0-d5 period,both mdx and normal fibres showed transient increases of Mn2+ influx and activity of the Ca2+ channels; these peaked at d1 and disappeared by d3-d4. Increases were always significantly larger in mdx fibres. Altogether, over the 6 days, 130 paired measurements of [Ca2+](i) and Mn2+ influx were made on 68 fibres from mdx and 62 fibres from normal mice. In 90 % of the fibres, [Ca2+](i) remained within the 25-85 nM limits while Mn2+ influx varied more than 10-fold. The median for Mn2+ influx was 45 % greater in fibres from mdx mice than in fibres from control C57 mice. However, there was no significant difference between [Ca2+](i) medians in fibres from normal and mdx mice. Addition of 25-75 nm of a Ca2+ ionophore (4-bromo-A23187) to the medium did not affect the level of cytosolic [Ca2+] in both types of fibres, while markedly increasing the rate of Mn2+ influx, as expected. Thus, Ca2+ homeostasis was equally robust in mdx and normal fibres. The remaining 10 % of the fibres showed, at d1, high levels of Mn2+ influx and/or elevated [Ca2+](i) above 100 nm. This did not affect survival of normal fibres but was probably responsible of the increased death rate in mdx fibres.