Addition of a subpopulation of I _Na with shifted biophysical properties results in earlier AP upstroke and larger I _Na current in single-cell mouse and human models. Transmembrane voltage (V; top and middle; magenta) and I_Na (bottom; blue/red, magenta) are shown as functions of time for a single cell with a 50:50 baseline:shifted I_Na subpopulations for mouse and human ventricular myocyte. The corresponding voltage and I_Na curves are shown for the homogeneous (only baseline I_Na) single cell in black, and I_Na scaled by a factor of 1/2 is shown for comparison with I_Na from the baseline/shifted single cell (dashed black line). The mouse ventricular myocyte model is from Bondarenko et al. (2004), with the Markov model representing I_Na dynamics replaced with the Hodgkin–Huxley formulation from the Morotti et al. (2014) mouse model. The human ventricular myocyte model is from O’Hara et al. (2011).