TRPM4 is a molecular correlate for Ca2+-activated monovalent cation selective channels ubiquitously expressed in both excitable and non-excitable tissues. Recent investigations have revealed that increased expression of this channel protein may be the main causes for certain types of familial atrio-ventricular blocks and other arrhythmogenic propensity associated with cardiac remodeling. However, the apparent Ca2+ sensitivity of this channel evaluated in heterologous systems is almost out of physiological range (typically >10μM), so that it remains still puzzling how exactly it could contribute to arrhythmogenicity. In this study, we re-evaluated the Ca2+ sensitivity of TRPM4 channels in both expression systems and immortalized atrial cardiomyocyte cell line HL-1, and explored their roles in inducing arrhythmic changes by means of mathematical model simulations. In cell-free conditions, the Ca2+ sensitivity of human TRPM4 channels expressed in HEK293 cells rapidly declined requiring more than several-hundred μM Ca2+ for half maximal activation (Kd) near the resting membrane potential. However, a much higher Ca2+ sensitivity was observed when cells were maintained as intact as possible and intracellular Ca2+ concentration was changed via ionomycin-mediated Ca2+ influx (Kd: ~ 0.5μM). A similar estimate of Kd was obtained in experiments evaluating the relationship between the integral of voltage-dependent Ca2+ influx and the magnitude of tail cationic current in HEK cells coexpressing TRPM4 and voltage-dependent Ca2+ channels (VDCC) subunits (α1C/β2/α2δ). Furthermore, repetitive activation of VDCC at high frequencies induced the development of sustained TRPM4 current at -80mV. 4-5 day treatment of HL-1 myocytes with angiotensin II resulted in enhanced expression of TRPM4 channel protein and current density, and induced spontaneous action potentials (APs) with prolongation of AP duration, which could be abolished by a relatively selective concentration (10μM) of a TRPM4 channel blocker, 9-phenanthrol. In silico simulation of cardiac action potentials (APs) based on 2000 Luo-Rudy or Nygren models which incorporated the Ca2+- and voltage-dependent kinetics of TRPM4 channel reproduced the late phase-3 AP prolongation and moderate basal depolarization when increasing the expression level of TRPM4 by several fold. These results suggest that the intact Ca2+ sensitivity of TRPM4 channel would be sufficiently high to induce arrhythmic changes, when the channel expression or function is up-regulated.