Antagonistic effects of cytoplasmic Ca²⁺ and luminal Mg²⁺ on RYR2. The effect of cytoplasmic Ca²⁺ on the luminal Mg²⁺ sensitivity of opening frequency (A) and mean open time (B). 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). Sample numbers for ▪ are displayed in Fig. 3. The solid 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.