Activation of RYR2 by luminal and cytoplasmic Ca²⁺. Cytoplasmic Ca²⁺ dependencies of opening frequency (A) and mean open time (B) in the presence of 2 mM ATP. Data points show the mean ± SE. Labels indicate the number of samples in • (A) and ○ (B; corresponding points in A and B have the same sample numbers). The inset shows the same data with F_o plotted on a linear scale. The solid and dashed curves show theoretical fits to the data of the luminal-triggered feedthrough model as discussed in Results. The model parameters are listed in Table V. (A and B) Dashed curves, 0.01 mM luminal Ca²⁺; solid curves, 0.1 mM luminal Ca²⁺.