Key point• Fibroblasts play a major role in heart physiology. In pathological conditions, they can lead to cardiac fibrosis when they differentiate into myofibroblasts.• This differentiated status is associated with changes in expression profile leading to neo-expression of proteins such as ionic channels.• The present study investigates electrophysiological changes associated with fibroblast differentiation focusing on voltage-gated sodium channels in human atrial fibroblasts and myofibroblasts.• We show that human atrial fibroblast differentiation in myofibroblasts is associated with de novo expression of voltage gated sodium current. Multiple arguments support that this current is predominantly supported by the Na v 1.5 α-subunit which may generate a persistent sodium entry into myofibroblasts.• Our data revealed that Na v 1.5 α-subunit expression is not restricted to cardiac myocytes within the atrium. Since fibrosis is one of the fundamental mechanisms implicated in atrial fibrillation, it is of great interest to investigate how this channel could influence myofibroblasts function.Abstract Fibroblasts play a major role in heart physiology. They are at the origin of the extracellular matrix renewal and production of various paracrine and autocrine factors. In pathological conditions, fibroblasts proliferate, migrate and differentiate into myofibroblasts leading to cardiac fibrosis. This differentiated status is associated with changes in expression profile leading to neo-expression of proteins such as ionic channels. The present study investigates further electrophysiological changes associated with fibroblast differentiation focusing on the activity of voltage-gated sodium channels in human atrial fibroblasts and myofibroblasts. Using the patch clamp technique we show that human atrial myofibroblasts display a fast inward voltage gated sodium current with a density of 13.28 ± 2.88 pA pF −1 whereas no current was detectable in non-differentiated fibroblasts. Quantitative RT-PCR reveals a large amount of transcripts encoding the Na v 1.5 α-subunit with a fourfold increased expression level in myofibroblasts when compared to fibroblasts. Accordingly, half of the current was blocked by 1 μM of tetrodotoxin and immunocytochemistry experiments reveal the presence of Na v 1.5 proteins. Overall, this current exhibits similar biophysical characteristics to sodium currents found in cardiac myocytes except for the window current that is enlarged for potentials between −100 and −20 mV. Since fibrosis is one of the fundamental mechanisms implicated in atrial fibrillation, it is of great interest to investigate how this current could influence myofibroblast properties. Moreover, since several Na v 1.5 mutations are related to cardiac pathologies, this study offers a new avenue on the fibroblasts involvement of these mutations.

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