Figure 1.

Spatially restricted [Ca²⁺]_c and [Ca²⁺]_m spikes evoked by submaximal activation of the RyR. Fluorescence imaging of [Ca²⁺]_c and [Ca²⁺]_m responses evoked by caffeine in rhod2-loaded permeabilized myotubes was performed. (A) The pseudocolor ratio images (340 nm/380 nm; blue, low [Ca²⁺]; red, high [Ca²⁺]) in Upper show the changes in fura-C18 fluorescence ([Ca²⁺]_c) after the addition of 0.5 mM (ii–vi) and 15 mM (vii) caffeine. The gray images in Lower show the rhod2 fluorescence ([Ca²⁺]_m), and the red overlays show the fluorescence changes from increased [Ca²⁺]_m at each time point, measured simultaneously with [Ca²⁺]_c. (B) Time courses of [Ca²⁺]_c and [Ca²⁺]_m changes for the numbered regions. Regions were selected in the areas displaying local [Ca²⁺]_c spiking in response to 0.5 mM caffeine (1, 3, 5) and in adjacent regions that exhibited a response only during stimulation with maximal caffeine (2, 4, 6). A [Ca²⁺]_m rise was coupled to every localized [Ca²⁺]_c spike in the selected regions (see graphs).