A) Rate threshold for local SCRs of surface (left), inner coupled (middle) and inner uncoupled (right) CRUs in dense, intermediate and sparsely tubulated cells. 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 SCRs and alternans for all CRUs at NCX Fraction of 0.2.