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: Aims Reduced excitability and gap junction expression are commonly found in electrically remodelled diseased hearts, but their contribution to slow conduction and arrhythmias is unclear. In this study, we have investigated the effect of isolated and combined reductions in membrane excitability and intercellular coupling on impulse propagation and arrhythmogeneity in genetically modified mice. : Methods and results Cx43 and Scn5a1798insD/+ heterozygous (HZ) mice were crossbred to create a mixed offspring: wild-type (WT, n = 15), Cx43 HZ (n = 14), Scn5a1798insD/+ (Scn5a) HZ (n = 17), and Cx43/Scn5a1798insD/+ (Cx43/Scn5a) HZ (n = 15) mice. After ECG recording, epicardial activation mapping (208 recording sites) was performed on Langendorff-perfused hearts. Arrhythmia inducibility was tested by one to three premature stimuli and burst pacing. Conduction velocity longitudinal (CVL) and transverse (CVT) to fibre orientation and dispersion of conduction were determined during S1-S1 pacing (150 ms). Connexin43 (Cx43) and sodium channel Nav1.5 protein expression and myocardial tissue collagen content were determined by immunohistology. Compared with WT animals, P, QRS, and QTc intervals were prolonged in Scn5a HZ and Cx43/Scn5a HZ, but not in Cx43 HZ animals. Scn5a HZ mice showed decreased CVL in right ventricle (RV) but not in left ventricle compared with WT. In the RV of Cx43/Scn5a HZ, CVT was reduced, but CVL was not different from WT. Arrhythmia inducibility was low and not increased in either single- or double-mutant mice. : Conclusion Reduction of both electrical coupling and excitability results in normal conduction velocity parallel to fibre orientation but in pronounced conduction slowing transverse to fibre orientation in RV only, although this does not affect arrhythmogeneity. [ABSTRACT FROM AUTHOR]