Cardiac RyRs in their native membrane. Selected frames from reconstructions of dyads and RyR tetramers derived from tomograms of rat ventricle. (A) Junctional SR membrane, representative of a resting myocyte, showing a variety of tetramer pairings. (B) Different junction, after exposure to saturating FKBP12.6. (C) After exposure to a phosphorylation cocktail. (D) Phosphorylation plus saturation with FKBP12.6. For a correlation with properties of Ca²⁺ sparks recorded before structural imaging in the same preparations, see text. Frames from videos 1, 4, 2, and 6, respectively, of Asghari et al. (2020), reproduced by courtesy of Edwin Moore.