Figure 12. t-system Ca²⁺ during prolonged sarcoplasmic reticulum Ca²⁺ release in low Mg²⁺, 5 mm caffeine and 50 mm EGTA.

Spatially averaged values of t-system trapped fluo-5N fluorescence and cytoplasmic fura-red fluorescence during Ca²⁺ release induced by low Mg²⁺ and 5 mm caffeine in the presence of 50 mm EGTA following exchange from a solution containing 50 mm EGTA, 1 mm Mg²⁺ and 100 nm Ca²⁺ in a rat skinned fibre is shown top right. The lines at the top and pale vertical bar on the spatially averaged profile indicate the solution change. A slow uptake of Ca²⁺ by the t-system followed by a slow depletion is reported by t-system fluo-5N. The fura-red signal shows a peak trailed that of the t-system signal. Note that fura-red fluorescence is inversely proportional to [Ca²⁺]. Selected images of t-system trapped fluo-5N fluorescence are shown with time-stamps corresponding to that in the spatially averaged profile. Below each image are the spatially averaged values of t-system trapped fluo-5N fluorescence versus elapsed time (which maps proportionally to the abscissa of xy scans, see Methods). The horizontal red lines mark the arbitrary baseline for all images. Ca²⁺ increases in the t-system at right as the image is presented and moves to the left, followed by a depletion of Ca²⁺.