This protein mediates the voltage-dependent sodium ion permeability of excitable membranes. Assuming opened or closed conformations in response to the voltage difference across the membrane, the protein forms a sodium-selective channel through which Na(+) ions may pass in accordance with their electrochemical gradient. It is a tetrodotoxin-resistant Na(+) channel isoform. This channel is responsible for the initial upstroke of the action potential. Channel inactivation is regulated by intracellular calcium levels. Interacts with the PDZ domain of the syntrophin SNTA1, SNTB1 and SNTB2. Interacts with NEDD4, NEDD4L, WWP2 and GPD1L. Interacts with CALM. Interacts with FGF13; the interaction is direct and may regulate SNC5A density at membranes and function. May also interact with FGF12 and FGF14. Interacts with ANK3 (By similarity). RANGRF promotes trafficking to the cell membrane. Strongly expressed in the heart. Also expressed in adult and fetal brain, spinal cord, testis, and at moderate levels in kidney, adrenal gland, lung, skeletal muscle, spleen, stomach and bladder. Isoform 2 is expressed in brain. 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. The IQ domain mediates association with calmodulin. Phosphorylation at Ser-1505 by PKC in a highly conserved cytoplasmic loop slows inactivation of the sodium channel and reduces peak sodium currents. Regulated through phosphorylation by CaMK2D. Ubiquitinated by NEDD4L; which promotes its endocytosis. Does not seem to be ubiquitinated by NEDD4 or WWP2. Lacks the cysteine which covalently binds the conotoxin GVIIJ. This cysteine (position 869) is speculated in other sodium channel subunits alpha to be implied in covalent binding with the sodium channel subunit beta-2 or beta-4. Na(+) channels in mammalian cardiac membrane have functional properties quite distinct from Na(+) channels in nerve and skeletal muscle. Belongs to the sodium channel (TC 1.A.1.10) family. Nav1.5/SCN5A subfamily.