Figure 2. SR and cytoplasmic Ca²⁺ during prolonged Ca²⁺ release.

The spatially averaged fluorescence values for each frame in an xyt series of SR-trapped fluo-5N fluorescence (blue line) and cytoplasmic rhod-2 fluorescence (red line) from a rat skinned fibre during the lowering of [Mg²⁺]_cyto is plotted in A. The interval of solution change is indicated by the horizontal bars at top and the vertical blue bar on the profile. a–d refer to the Ca²⁺ release and termination of release events. These events are described in the text and analysed in Fig. 6. Lowering [Mg²⁺]_cyto causes Ca²⁺ to initially leak from SR, marked leak time and indicated by the rise in cytoplasmic rhod-2 fluorescence up to the position marked a (see Fig. 5). Images from the rise (B) and decline (C) of the Ca²⁺ transient induced by lowering [Mg²⁺]_cyto are shown. The background of the fluo-5N fluorescence image has been subtracted and the image median filtered. Time stamp in top right-hand corner of fluo-5N fluorescence image corresponds to that of the spatially averaged values of rhod-2 and fluo-5N fluorescence shown in A. A ‘halo’ of Ca²⁺ released from the fibre was observed in C (see Methods). SR, sarcoplasmic reticulum.