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Biophysical Journal 86:1436-1453 (2004)
© 2004 The Biophysical Society

Altered Elementary Calcium Release Events and Enhanced Calcium Release by Thymol in Rat Skeletal Muscle

Péter Szentesi *, Henrietta Szappanos *, Csaba Szegedi *, Monika Gönczi {dagger}, István Jona *, Julianna Cseri *, László Kovács * {dagger} and László Csernoch * {dagger}

* Department of Physiology and {dagger} Cell Physiology Research Group of the Hungarian Academy of Sciences, Research Center for Molecular Medicine, Medical and Health Science Center, University of Debrecen, Debrecen, Hungary

Correspondence: Address reprint requests to Dr. László Csernoch, Dept. of Physiology, University of Debrecen, PO Box 22, Debrecen H-4012, Hungary. Tel.: 36-52-416-634; Fax: 36-52-432-289; E-mail: csl{at}phys.dote.hu.

The effects of thymol on steps of excitation-contraction coupling were studied on fast-twitch muscles of rodents. Thymol was found to increase the depolarization-induced release of calcium from the sarcoplasmic reticulum, which could not be attributed to a decreased calcium-dependent inactivation of calcium release channels/ryanodine receptors or altered intramembrane charge movement, but rather to a more efficient coupling of depolarization to channel opening. Thymol increased ryanodine binding to heavy sarcoplasmic reticulum vesicles, with a half-activating concentration of 144 µM and a Hill coefficient of 1.89, and the open probability of the isolated and reconstituted ryanodine receptors, from 0.09 ± 0.03 to 0.22 ± 0.04 at 30 µM. At higher concentrations the drug induced long-lasting open events on a full conducting state. Elementary calcium release events imaged using laser scanning confocal microscopy in the line-scan mode were reduced in size, 0.92 ± 0.01 vs. 0.70 ± 0.01, but increased in duration, 56 ± 1 vs. 79 ± 1 ms, by 30 µM thymol, with an increase in the relative proportion of lone embers. Higher concentrations favored long events, resembling embers in control, with duration often exceeding 500 ms. These findings provide direct experimental evidence that the opening of a single release channel will generate an ember, rather than a spark, in mammalian skeletal muscle.




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