Mediates the voltage-dependent sodium ion permeability of excitable membranes. Assuming opened or closed conformations in response to the voltage difference across the membrane, forms a sodium-selective channel through which Na(+) ions may pass in accordance with their electrochemical gradient (PubMed:29142310). May contribute to the regulation of serotonin/5-hydroxytryptamine release by enterochromaffin cells (PubMed:29142310). In pancreatic endocrine cells, required for both glucagon and glucose-induced insulin secretion (PubMed:25172946). Heterooligomer of a large alpha subunit and 2-3 smaller beta subunits. Heterooligomer with SCN2B or SCN4B; disulfide-linked. Interacts with NEDD4L. In enterochromaffin cells, localized highly asymmetrically, almost exclusively at the basal side. Expressed in enterochromaffin cells in both colon and small bowel (at protein level) (PubMed:29142310). Expressed in pancreatic alpha and beta cells (PubMed:25172946). Expressed in the hippocampus at late embryonic stages and during the first week after birth. Down-regulated after postnatal day 7. The sequence contains 4 internal repeats, each with 5 hydrophobic segments (S1, S2, S3, S5, S6) and one positively charged segment (S4). Segments S4 are probably the voltage-sensors and are characterized by a series of positively charged amino acids at every third position. May be ubiquitinated by NEDD4L; which would promote its endocytosis. Phosphorylation at Ser-1453 in a highly conserved cytoplasmic loop slows inactivation of the channel and reduces peak sodium currents. Belongs to the sodium channel (TC 1.A.1.10) family. Nav1.3/SCN3A subfamily.