Luminal Ca²⁺ of CSQ2-stripped and CSQ2-added RyR2 channels. The CSQ2-added channels here refer to channels that were first CSQ2 stripped before 0.5 μg/ml CSQ2-WT was added to the luminal solution. Cytosolic free Ca²⁺ concentration was constant (1 μM). The luminal Ca²⁺ or Mg²⁺ concentration was titrated from 10 μM to 10 mM. Holding potential was 0 mV. The luminal solution contained 100 mM Cs⁺ and net unit current was always in the lumen-to-cytosolic direction. (A) Example single channel recordings from a CSQ2-stripped channel (left) and a CSQ2-added channel (right). Zero current levels are marked. Bar, 2 pA.(B) Summary Po results. Luminal Ca²⁺ sensitivity of the CSQ2-added data (black circles) was collected from 14 different channels. Luminal Ca²⁺ sensitivity of the CSQ2-stripped data (open diamonds) was collected from six different channels. Dotted line represents control Po result presented in Fig. 1 B.