Low Na+/Ca2+ exchanger current (INCX) promotes fluctuations in diastolic Ca2+ concentration in both in silico populations, whereas high Na+/Ca2+ exchanger conductance (GNCX) and low release current (Jrel) correlate with the presence of a spike-and-dome Ca2+ transient (CaT) in the Grandi-based population only. A–C: proportions of models and examples exhibiting distinct CaT morphologies in the Maleckar population calibrated with action potential (AP) + L-type Ca2+ current (ICaL) + inward rectifier K+ current (IK1) data and parameters underpinning the three CaT phenotypes (C). D–F: showcase analogous results for the Grandi-based population following AP + ICaL + IK1 calibration. In A–C, morphologically normal CaTs, Ca2+ oscillations in diastole, and spontaneous Ca2+ release phenotypes are plotted in black, dark gray, and light gray, respectively. In D–F, morphologically normal CaTs, spike-and-dome CaTs, and spontaneous Ca2+ release models are shown in black, dark gray, and light gray, respectively. Ca2+ oscillations and spontaneous Ca2+ release are referred to as fluctuations in diastolic Ca2+.