Ca²⁺ and Mg²⁺ dependences of [³H]ryanodine binding to microsomes from RyR2-expressing HEK293 cells. (A and B) Dose-dependent action of Ca²⁺ on binding in the absence (A) and presence (B) of 1 mM Mg²⁺. Binding data in WT (black), R2473S (green), and K4750Q (red) are shown in each graph. (C) Dose-dependent effect of Mg²⁺ on binding at pCa 4. Means of triplicate determinations were plotted. Statistical testing was done by one-way ANOVA. *, P < 0.05 (K4750Q vs. WT). ^#, P < 0.05 (K4750Q vs. R2473S). Values shown are mean ± SEM.