SCN4A and atrial fibrillation: 1992). On the mRNA level, the main sodium channel α‐subunits expressed by both SR and AF fibroblasts were Nav1.4, Nav1.5, and Nav1.7 (Fig. 3 and Table S1). The IC50 of TTX for Nav1.4 and Nav1.7 is in the nanomolar range (reviewed in (Catterall et al. 2005)) confirming that the currents observed were most likely conducted through the cardiac‐specific isoform Nav1.5. The main Na+ channel β‐subunits expressed was β1.