Kir2.1^E299V increases outward I_K1 in both atria and ventricle, but reduces the slope conductance only in the atria. A, Atrial cardiomyocytes. Current/voltage (IV) relationships for Kir2.1^WT (N = 3, n = 11) and Kir2.1^E299V (N = 3; n = 11). Note the lack of inward-going rectification with increased outward I_K1 at voltages positive to −60 mV (7.19 ± 1.04 pA/pF in E299V vs. 2.62 ± 0.57 pA/pF in WT, results at −14 mV; ****P < 0.0001) and loss of inward current at voltages negative to −120 mV (***P < 0.001, **P < 0.01, *P < 0.05 from −135 mV to −160 mV). B, Ventricular cardiomyocytes. I_K1 IV relationships for both experimental groups Kir2.1^WT (N = 3, n = 10) and Kir2.1^E299V (N = 3; n = 11). Lack of inward-going rectification can be appreciated at voltages positive to −50 mV (4.62 ± 0.89 pA/pF in E299V vs. −0.41 ± 0.14 pA/pF in WT, results at −14 mV; ****P < 0.0001) without changes in inward current. Two-way ANOVA was applied for comparisons.