Intracellular calcium signals measured with indo‐1 in isolated skeletal muscle fibres from control and mdx mice

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Abstract
1 Intracellular free calcium concentration ([Ca2+]i) was measured with the fluorescent indicator indo-1 in single skeletal fibres enzymatically isolated from the flexor digitorum brevis and interosseus muscles of control and dystrophic mdx C57BL/10 mice. Measurements were taken from a portion of fibre that was voltage clamped to allow detection of depolarization-induced changes in [Ca2+]i. 2 The mean (±s.e.m.) initial resting [Ca2+]i from all control and mdx fibres tested was 56 ± 5 nm (n= 72) and 48 ± 7 nm (n= 57), respectively, indicating no significant overall difference between the two groups. However, when comparing a batch of control and mdx fibres obtained from mice older than ∼35 weeks, resting [Ca2+]i was significantly lower in mdx (16 ± 4 nm, n= 11) than in control fibres (71 ± 10 nm, n= 14). 3 Changes in [Ca2+]i elicited by short (5–35 ms) depolarizing pulses from −80 to 0 mV showed similar properties in control and mdx fibres. After a 5 ms duration pulse the mean time constant of [Ca2+]i decay was, however, significantly elevated in mdx as compared to control fibres, by a factor of 1.5–2. For longer pulses, no significant difference could be detected. 4 In response to 50 ms duration depolarizing pulses of various amplitudes the threshold for detection of an [Ca2+]i change and the peak [Ca2+]i reached for a given potential were similar in control and mdx fibres. 5 Overall results show that mdx skeletal muscle fibres are quite capable of handling [Ca2+]i at rest and in response to membrane depolarizations.