G.-R. L i, C.-P. L au and A. S hrier. Heterogeneity of Sodium Current in Atrial vs Epicardial Ventricular Myocytes of Adult Guinea Pig Hearts. Journal of Molecular and Cellular Cardiology (2002) 34, 1185–1194. The different sodium channel currents (I_Na) were reported in myocardium, neuron, and skeletal muscles. To study whether I_Na is homogeneous within the heart, we applied whole-cell voltage clamp technique to evaluate fast voltage-gated I_Na in atrial and ventricular myocytes isolated from guinea pig heart. It was found that the density of inward I_Na was 50% greater at −35mV in atrial (−42.6±2.9 pA/pF) than in ventricular (−27.5±1.8 pA/pF, P<0.01) myocytes. The half activation and inactivation voltages (V_0.5) of I_Na in atrial myocytes were shifted 4.5±0.2 and 9.6±0.3mV negative to those of ventricular myocytes. Time constants forI_Na activation (τ_m) and inactivation (τ_h) were twice as rapid in atrial as in ventricular myocytes. The τ_m and τ_h were 0.34±0.03 and 1.36±0.07ms for atrial myocytes, and 0.69±0.05 and 3.27±0.23ms for ventricular myocytes, respectively. Recovery ofI_Na from inactivation was slower in atrial than in ventricular myocytes, whereas the development of resting state inactivation was more rapid in atrial (τ=67.5±4.3ms) than in ventricular (152.8±7.5ms, P<0.01) myocytes. The results reveal marked heterogeneity of I_Na in the density and biophysical properties in atrial and ventricular myocytes, and the study suggests the potential possibility of tissue specific cardiac sodium channel isoforms.