Fura-2 imaging of spontaneous and electrically induced oscillations of intracellular free Ca2+ in rat myotubes

Rat myotubes have a resting Ca²⁺]_i of about 82 nM. Myotubes 3–5 days old (quiescent myotubes) display electrically induced and spontaneous transients in the intracellular concentration of free Ca²⁺ ions (Ca²⁺]_i) uncoupled to any detectable contraction. By contrast, 1-to 2-day-old myotubes are insensitive to electrical stimuli and, after 6 days in culture, stimulated myotubes always show Ca²⁺]_i transients and twitch contractions. The spatial distribution of Ca²⁺]_i variations in quiescent myotubes is heterogeneous, local increases in Ca²⁺]_i being mainly observed near the periphery of the cell. The small effect of different external Ca²⁺ concentrations and of Cd²⁺ on the amplitude of the Ca²⁺]_i oscillation indicates that the main source of Ca²⁺ may be the sarcoplasmic reticulum. This conclusion is supported by the close similarity between electrically induced and caffeine-induced Ca²⁺]_i maps. These findings suggest that, at an early stage of myotube ontogenesis, a part of the excitation/contraction coupling, as membrane ionic channels, voltage sensors and Ca²⁺ release and reuptake mechanisms, is functional but, apparently, still uncoupled to the contractile machinery.This work was supported by the Centre National de la Recherche Scientifique, the Ministère de la Recherche et de la Technologie, the Conseil Régional de l'Aquitaine and the Association Française contre les Myopathies