Résumé :
|
Communication n° 224 Voltage independent cationic channels in the plasma membrane of skeletal muscle fibers are suspected to play a role in Duchenne dystrophy but the conditions and/or parameters that control their activity remain controversial. Here we used the single channel- and whole-cell- voltage clamp techniques to unravel any possible change in membrane conductance that would depend upon SR Ca2+ release and/or SR Ca2+ depletion. Single skeletal muscle fibers were pressure microinjected with an EGTA- and in some cases calcium dye- containing solution. A portion of fiber was voltage-clamped and intracellular Ca2+ signals and/or single channel activity from a patch pipette in cell-attached mode were simultaneously recorded. Various patterns of whole-cell membrane depolarizations were applied, either in the absence or in the presence of SR Ca2+ uptake inhibitors. Under control conditions, single channel currents carried by the delayed rectifier K+ channels were routinely detected during the whole-cell depolarizing pulses. In addition, the activity of channels carrying an inward current of ~1.5 pA at -80 mV and exhibiting a unitary conductance of ~20 pS, was detected in 13 cell-attached patches. Application of large whole-cell depolarizing pulses did not reproducibly increase this channel activity. A similar channel activity showing no dependence upon the whole-cell depolarizations was also recorded in 21 patches on mdx muscle fibers. Inhibitors of the SR Ca2+ pump produced a slowing of the decay phase of the voltage-evoked calcium transients and a progressive increase in the resting level of the dye saturation, indicating that millimolar levels of Ca2+ remained bound to EGTA and thus outside of the SR at rest. Still, under these conditions there was no reproducible sign of concurrent increase in both the resting whole-cell membrane conductance and in the resting inward single channel activity. Overall results add no support to the possibility that these channels are regulated by the intracellular Ca2+ store content.
|