Fast calcium removal during single twitches in amphibian skeletal muscle fibres.
Fluorescence signals from the calcium sensitive dyes Fluo-3 or Rhod-2 were obtained simultaneously with isometric tension in single fibres isolated from the anterior tibialis muscle of Leptodactylus insularis (20-22 degrees C). Fluo-3 fluorescence signals were transformed into [Ca2+]i transients as previously described. Most of the decay phase of single twitch transient is well fitted by a single exponential (tau of about 10 ms), followed by a slower declining component lasting tens of milliseconds. During short periods, 10 to 20 s, of low frequency stimulation, between 0.2 and 5 Hz, the basal [Ca2+]i increased slowly from 0.1 to about 0.4 microM, with only minor changes in the exponentially decaying phase. In fibres poisoned with thapsigargin or cyclopiazonic acid (1-2 microM) the tau of decay of fluorescence or Ca2+ transients of single twitches was very similar to that observed in non-poisoned fibres. Nevertheless, in poisoned fibres challenged with repetitive stimulation. the tau of Ca2+ transients decay increased from about 10 ms to >40 ms, while the basal [Ca2+]i increased from 0.1 to 2 microM. Short rest periods (about 5 min) could reverse these effects, indicating that they were not a direct consequence of SR Ca 2+ -ATPase inhibition. The correlation coefficient between tau of decay and basal [Ca2+]i was >0.8 (P<0.0001). Qualitatively similar results were obtained measuring Rhod-2 fluorescence signals. A lumped, two-compartment model could account for these results. Loading the fibres with EGTA-AM, diminished the effects of prolonged stimulation observed in poisoned fibres. Moreover, we show that the Na+ - Ca2+ exchange mechanism does not participate appreciably in fast Ca2+ removal.
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Laboratorio de Biofísica del Músculo, Centro de Biofísica y Bioquímica, Instituto Venezolano de Investigaciones Científicas, IVIC, Caracas, Venezuela.
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D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000814:Aniline Compounds; D000818:Animals; D001001:Anura; D001665:Binding Sites / Q000502:physiology; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism; D004558:Electric Stimulation; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D007537:Isometric Contraction / Q000502:physiology*; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D018485:Muscle Fibers, Skeletal / Q000201:enzymology; D018482:Muscle, Skeletal / Q000166:cytology / Q000502:physiology*; D010320:Parvalbumins / Q000502:physiology; D011930:Reaction Time / Q000502:physiology; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000378:metabolism; D019284:Thapsigargin / Q000494:pharmacology; D014966:Xanthenes
D000814:Aniline Compounds; D002021:Buffers; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D010320:Parvalbumins; D019831:Sodium-Calcium Exchanger; D014966:Xanthenes; C059715:Fluo-3; D019284:Thapsigargin; D012964:Sodium; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium