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¹ Section of Cellular Signalling, Department of Molecular Biophysics and Physiology, Rush University Medical Center, 1750 W. Harrison Street, Chicago, IL 60612, USA
² School of Biomedical Sciences, University of Queensland, Brisbane, Queensland, 4072, Australia

Store-operated Ca²⁺ entry (SOCE) is activated following the depletion of internal Ca²⁺ stores in virtually all eukaryotic cells. Shifted excitation and emission ratioing of fluorescence (SEER) was used to image mag-indo-1 trapped in the tubular (t) system of mechanically skinned rat skeletal muscle fibres to measure SOCE during intracellular Ca²⁺ release. Cytosolic Ca²⁺ transients were simultaneously imaged using the fluorescence of rhod-2. Spatially and temporally resolved images of t system [Ca²⁺] ([Ca²⁺]_t-sys) allowed estimation of Ca²⁺ entry flux from the rate of decay of [Ca²⁺]_t-sys. Ca²⁺ release was induced pharmacologically to activate SOCE without voltage-dependent contributions to Ca²⁺ flux. Inward Ca²⁺ flux was monotonically dependent on the [Ca²⁺] gradient, and strongly dependent on the transmembrane potential. The activation of SOCE was controlled locally. It could occur without full Ca²⁺ store depletion and in less than a second after initiation of store depletion. These results indicate that the molecular agonists of SOCE must be evenly distributed throughout the junctional membranes and can activate rapidly. Termination of SOCE required a net increase in [Ca²⁺]_SR. Activation and termination of SOCE are also demonstrated, for the first time, during a single event of Ca²⁺ release. At the physiological [Ca²⁺]_t-sys, near 2 mM (relative to t system volume), SOCE flux relative to accessible cytoplasmic volume was at least 18.6 µM s^–1, consistent with times of SR refilling of 1–2 min measured in intact muscle fibres.

(Received 19 April 2007; accepted after revision 11 June 2007; first published online 14 June 2007)
Corresponding author B. S. Launikonis: School of Biomedical Sciences, The University of Queensland, Brisbane, Qld, 4072, Australia. Email: b.launikonis{at}uq.edu.au

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