Figure 5: Ca²⁺ handling by t-system vacuoles of human skeletal muscle.

(a) Ca²⁺ uptake by vacuoles present in the t-system upon exposure to caffeine or 67 nM free [Ca²⁺]_cyto and 200 nM [Ca²⁺]_cyto. (b) Representative trace of Ca²⁺ uptake by the vacuoles in a human fibre after exposure to 67 nM free [Ca²⁺]_cyto, caffeine and 200 nM [Ca²⁺]_cyto. The vertical blue bars indicate the timing of the internal solution exchanges and the horizontals bars at top indicate the [Ca²⁺]_cyto of the standard solution or the application of caffeine to deplete the SR of Ca²⁺. (c) Ca²⁺ uptake rate of the vacuoles during exposure to the [Ca²⁺]_cyto indicated on the x-axis. (d) Ca²⁺ uptake rate of the vacuoles during caffeine-induced Ca²⁺ release following SR Ca²⁺ loading at the [Ca²⁺]_cyto indicated on the x-axis. The plots in c,d were constructed from 15 and 16 values; 6 and 6 fibres; and 3 and 3 subjects, respectively. The data points are represented as mean±s.e.m. The r² values for the fits in c,d are 0.931 and 0.629, respectively. Scale bar: 25 μm.