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2 Section of Cell Signaling, Department of Molecular Biophysics and Physiology, Rush University, Chicago, IL, USA1 Department of Physiology, Medical University of Debrecen, Hungary3 Laboratory of Cardiovascular Science, National Institute on Ageing, National Institutes of Health, Baltimore, MD, USA4 Departamento de Biof'sica, Universidad de la República, Facultad de Medicina, Montevideo, Uruguay
Cytosolic [Ca2+] transients elicited by voltage clamp depolarization were examined by confocal line scanning of rat skeletal muscle fibres. Ca2+ sparks were observed in the fibres' membrane-permeabilized ends, but not in responses to voltage in the membrane-intact area. Elementary events of the depolarization-evoked response could be separated either at low voltages (near –50 mV) or at –20mV in partially inactivated cells. These were of lower amplitude, narrower and of much longer duration than sparks, similar to ‘lone embers’ observed in the permeabilized segments. Their average amplitude was 0.19 and spatial half-width 1.3 µm. Other parameters depended on voltage. At –50 mV average duration was 111 ms and latency 185 ms. At –20 mV duration was 203 ms and latency 24 ms. Ca2+ release current, calculated on an average of events, was nearly steady at 0.5–0.6 pA. Accordingly, simulations of the fluorescence event elicited by a subresolution source of 0.5 pA open for 100 ms had morphology similar to the experimental average. Because 0.5 pA is approximately the current measured for single RyR channels in physiological conditions, the elementary fluorescence events in rat muscle probably reflect opening of a single RyR channel. A reconstruction of cell-averaged release flux at –20 mV based on the observed distribution of latencies and calculated elementary release had qualitatively correct but slower kinetics than the release flux in prior whole-cell measurements. The qualitative agreement indicates that global Ca2+ release flux results from summation of these discrete events. The quantitative discrepancies suggest that the partial inactivation strategy may lead to events of greater duration than those occurring physiologically in fully polarized cells.
(Received 3 December 2003;
accepted after revision 24 February 2004;
first published online 27 February 2004)
Corresponding author E. Ríos: Molecular Biophysics and Physiology, Rush University School of Medicine, 1750 W. Harrison St Suite 1279JS, Chicago, IL 60612, USA. Email: erios{at}rush.edu; http://www2.phys.rush.edu/ERios/physiorio.htm
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