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.
Grouselle
M
M
Centre de Recherche Paul Pascal, CNRS, Pessac, France.
Koenig
J
J
Lascombe
M L
ML
Chapron
J
J
Méléard
P
P
Georgescauld
D
D
D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004558:Electric Stimulation; D016257:Fura-2; D009132:Muscles / Q000196:embryology / Q000378:metabolism*; D051381:Rats
D002104:Cadmium; D002110:Caffeine; D002118:Calcium; D016257:Fura-2