Inhibition of RYR2 by cytoplasmic Mg²⁺. The effect of Mg²⁺ (0.22 and 1 mM) on the cytoplasmic Ca²⁺ dependencies of opening frequency (A) and mean open time (B). Mg²⁺ shifts both Ca²⁺ dependencies of channel gating to higher [Ca²⁺]_C. Data points show the mean ± SE. Labels indicate the number of samples in ○ and × (A) and • (B; corresponding points in A and B have the same sample numbers). 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.