Contribution of neuronal sodium channels to the cardiac fast sodium current INa is greater in dog heart Purkinje fibers than in ventricles

Abstract: Objective: To determine the presence and the potential contribution of neuronal sodium channels to dog cardiac function. Methods: We used a combination of electrophysiological (patch clamp), RT-PCR, biochemical and immunohistochemical techniques to identify and localize neuronal Na⁺ channels in dog heart and determine their potential contribution to the fast sodium current. Results: In all cardiac tissues investigated, Na_v1.1, Na_v1.2 and Na_v1.3 transcripts were detected. In immunoblots, we found Na_v1.1 and Na_v1.2 proteins in the ventricle (V) and in Purkinje fibers (PF). Na_v1.3 immunoblots suggested strong proteolytic activity against this isoform in the heart. Na_v1.6 was not found in any of the tissues tested. Confocal immunofluorescence on cardiac myocytes showed that Na_v1.1 was predominantly localized at the intercalated disks in V and PF and around the nucleus (V). Na_v1.2 was only present at the Z lines (V). Consistent with the immunoblot data, an intense but diffuse intracellular staining was observed for Na_v1.3. Na_v1.6 fluorescence staining was faint and diffuse. Surprisingly, immunoblots indicated the presence of two Na_vβ2 variants: a 42-kDa protein that co-localized with Na_v1.2 at the Z lines in V and a 34-kDa protein that co-localized with Na_v1.1 at the intercalated disks in PF. In agreement with the biochemical data, electrophysiological results suggest that neuronal sodium channels generate 10±5% and 22±5% of the peak sodium current in dog ventricle and Purkinje fibers, respectively. Conclusions: Our results suggest that neuronal NaChs are more abundant in Purkinje fibers than in ventricles, and this suggests a role for them in cardiac conduction. [Copyright &y& Elsevier]