A) Rate threshold for local SCRs of surface (left), inner coupled (middle) and inner uncoupled (right) CRUs in sparse, intermediate and densely tubulated cells with the impact of altered CSQ-RyR regulation greatest in inner coupled/uncoupled CRUs. B) Cytosolic Ca2+ concentration of surface (left), inner coupled (middle) and inner uncoupled (right) CRUs from cells with dense (i), intermediate (ii), and sparse (iii) tubules following pacing at 3 Hz showing observed SCRs with varying fraction of CSQ RyR-regulation.