MEMBRANE-POTENTIAL, RESTING CALCIUM AND CALCIUM TRANSIENTS IN ISOLATED MUSCLE-FIBERS FROM NORMAL AND DYSTROPHIC MICE

1. Single skeletal muscle fibres were enzymatically isolated from the flexor digitorum brevis muscles (FDB) of dystrophic mdx and control C57BL/10 mice aged 3-9 weeks. In this age range the majority (> 95%) of the mdx fibres were morphologically normal. 2. There was no significant difference between the resting membrane potential (RMP) of mdx and control mice, -71.2+/-1.21 (n = 26) and -70.6+/-1.15 mV (n = 42), respectively. 3. At RMP more negative than -60 mV the resting calcium (recorded with fura-2, free acid ionophoresed into cell) in the dystrophic mdx cells was not significantly different from the normal animals, 45.7+/-4.1 (n = 10) and 46.2+/-3.9 nM (n = 9), respectively. 4. The resting cytosolic calcium concentration was measured simultaneously with the RMP. At RMP between -60 to -17 mV there was an increase in the resting calcium concentration in both mdx and control ranging from 79.3 to 252 nM. This increase was most probably due to the activation of the slow calcium current. 5. Fura-2 calcium transients were produced via single action potential stimulation using an intracellular microelectrode both to stimulate the cell and record potential changes. There was no significant difference between the rise time (T(p)) or half-decay time (T1/2) at 22-degrees-C of the calcium transient in response to a single action potential in mdx compared to normal animals, 5.9+/-0.34 (n = 8) and 5.4+/-0.36 ms (n = 7); 39.5+/-2.9 (n = 8) and 40.75+/-3.7 ms (n = 7), respectively. 6. In conclusion it appears that over the age range 3-9 weeks the absence of dystrophin does not affect the RMP, resting calcium or calcium transient parameters of morphologically normal FDB mdx fibres.