Kinetics of RyR2 gating from single channel recording. (A–D) Cytoplasmic (A and B) and TSR (C and D) [Ca²⁺] dependencies of RyR2 opening (A and C) and closing rates (B and D) determined from single-channel recordings of bilayer experiments on RyR2 from sheep, human, and rat (mean and SEM). Sheep and rat data are taken from Cannell et al. (2013). (A and B) Cytoplasmic solution contained CsMS (130 mM for sheep and rat or 230 mM for human), 20 mM CsCl (150 mM Cs⁺), 2 mM ATP, 1 mM Mg²⁺, and the indicated [Ca²⁺], and the TSR solution contained CsMS (130 mM for sheep and rat or 230 mM for human), 20 mM CsCl, and 1 mM Mg²⁺. TSR [Ca²⁺] was either 1 mM (filled circles) or 0.1 mM (open circles). (C and D) Cytoplasmic solution contained CsMS (130 mM for sheep and rat or 230 mM for human), 20 mM, 2 mM ATP, 0 mM Mg²⁺, and 100 nM [Ca²⁺], and the TSR solution contained CsMS (130 mM for sheep and rat or 230 mM for human), 20 mM CsCl, 1 mM Mg²⁺, and the indicated Ca²⁺. The solid lines in A and B show the least squares fits of Eqs. 13 and 14 used in the simulations to the data. Their extrapolation down to 0.1 μM is modeled on data from Laver and Honen (2008) (see the text associated with Eq. 15). The dashed line shows values expected for TSR [Ca²⁺] = 0.1 mM derived from Laver and Honen (2008). In B, model values for 0.1 and 1 mM TSR [Ca²⁺] are indistinguishable.