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

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, a-hen comparing a batch of control and mdz fibres obtained from mice older than similar to 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 fi om -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 mdz 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 es are quite capable of handling [Ca2+](i) at rest and in response to membrane depolarizations.