Ca _v 1.3 accelerates AP firing rate by interacting with CRUs and I _NCX in the model with square lattice distribution of CRUs. Increasing percentage of Ca_v1.3 in I_CaL increases synchronization and recruitment of CRU to release Ca, resulting in earlier activation of I_NCX and shorter AP firing cycle. (A–D) Shown are representative examples of simulations of V_m (A), number of open CRUs at a given time (B), I_NCX (C), and I_CaL (D). The traces are synchronized at the MDP to clearly see difference in time course during diastolic depolarization.