The effect of membrane potential on luminal Mg²⁺ inhibition of RYR2 open probability, P_o (A), opening frequency, F_o (B), and mean open time, τ_o (C). Changing the membrane potential from −40 to +40 mV, thereby opposing the flow of Ca²⁺ and Mg²⁺ from luminal to cytoplasmic baths, slightly decreased the sensitivity of P_o and F_o to luminal [Mg²⁺] but had a substantial effect on the Mg²⁺ dependence of τ_o. Data points show the mean ± SE. Labels indicate the number of samples for +40 mV (corresponding points in B and C have the same sample numbers). Sample numbers for −40 mV data are displayed in Fig. 3. The parameters derived from fitting Hill curves to the data are listed in Table I. 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.