The calcium homeostasis and the membrane potential of cultured muscle cells from patients with myotonic dystrophy.

Using the fluorescence indicator, quin2, we compared the cytoplasmic Ca2+ concentration ([Ca2+]i) of cultured myotubes obtained from control subjects and myotonic dystrophy (MyD) patients. In Ca2(+)-free buffer the [Ca2+]i of the cultured MyD muscle cells was not significantly different from that of the control cells. In the presence of 1 mM external Ca2+ the cultured MyD muscle cells showed a significantly higher [Ca2+]i, which was due to the influx of Ca2+ through voltage-operated nifedipine-sensitive Ca2+ channels. In the presence of external Ca2+, MyD myotubes did not respond to acetylcholine, whereas control myotubes showed a transient increase in [Ca2+]i after addition of acetylcholine. This increase was inhibited by the addition of nifedipine. The differences in Ca2(+)-homeostasis between cultured MyD muscle cells and control cells were not due to differences in the resting membrane potential or the inability of the MyD cells to depolarize as a response to acetylcholine. Therefore, cultured MyD muscle cells exhibit altered nifedipine-sensitive voltage-operated channels which are active under conditions in which they are normally present in the inactive state, and which are unable to respond to depolarization caused by acetylcholine.