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sodium channel, voltage-gated, type IV, alpha subunit
SCN4A (also known as HYPP; SkM1; HYKPP; NAC1A; HOKPP2; Nav1.4; Na(V)1.4) encodes Nav1.4, a member of the sodium channel alpha subunit gene family. It is expressed in skeletal muscle, and mutations in this gene have been linked to several myotonia and periodic paralysis disorders. http://www.ncbi.nlm.nih.gov/gene/6329
Scn4a : sodium channel, voltage-gated, type IV, alpha subunit
CaM (Calmodulin) regulates the current density of NaV1.4. Association with CaM is important for functional expression of NaV1.4. Disrupting the interaction between CaM and the C terminus of NaV1.4 reduced current amplitude by 99%. Calmodulin also was able to modulate the inactivation kinetics of Nav1.6, but not Nav1.4, currents in a calcium-dependent manner. .
(Calmodulin, a 16.7 kDa protein that is expressed in virtually all eukaryotic cells, can induce changes in target proteins via its binding per se and in response to changes in calcium concentration . For example, the activity of sodium channels in Paramecium tetraurelia is CaM-dependent , and CaM plays a crucial role in the modulation of calcium channels .)
Ranolazine is an antianginal and anti-ischemic drug that is used in patients with chronic angina . Ranzoline blocks Na+ currents of Nav1.4. Both muscle and neuronal Na+ channels are as sensitive to ranolazine block as their cardiac counterparts. At its therapeutic plasma concentrations, ranolazine interacts predominantly with the open but not resting or inactivated Na+ channels. Ranolazine block of open Na+ channels is via the conserved local anesthetic receptor albeit with a relatively slow on-rate. 
Nav1.4 is tetrodotoxin sensitive (non selective). 
The skeletal muscle a subunit functions as ion-conducting channel and consists of large polypeptides (1700–2000 amino acids) that fold into four highly homologous somains (repeats I-IV) containing six transmembrane segments each (S1-S6). The S6 transmembrane segments and the S5- S6 loops form the ion selective pore, and the S4 segments contain positively charged residues conferring voltage dependence to the protein. The repeats are connected by intracellular loops; one of them, the III-IV linker, contains the supposed inactivation particle of the channel. When inserted in the membrane, the four repeats of the protein fold to generate a central pore. ,
Amino acid Ile-1575 in the middle of transmembrane segment 6 of domain IV (DIV-S6) in the adult rat skeletal muscle isoform of the voltage-gated sodium channel (rNaV1.4) may act as molecular switch allowing for interaction between outer and inner vestibules. 
Nav1.4 channels are located in the sarcolemma and T-tubular membranes, with a high density near the endplate of the muscle cell. 
Nav1.4 is expressed at high levels in adult skeletal muscle, at low levels in neonatal skeletal muscle, and not at all in brain or heart.
Relative mRNA expression levels of Nav1.4, analyzed by RT-PCR, were significantly higher in ovarian cancers cells compared with normal ovarian tissues and relative mRNA expression levels Nav1.4 were significantly increased in highly metastatic ovarian cancer cells (Caov-3 and SKOV-3) compared with low-metastatic ovarian cancer cells (Anglne). 
NaV1.4 is responsible for the generation and propagation of action potentials that initiate muscle contraction.
Mutations in NaV1.4 channels increase channel activity by impairing fast and/or slow inactivation causing hereditary sodium channelopathies of skeletal muscle such as [:
VSGC blockers such as mexiletine, flecainide and other lidocaine analogues can reduce repetitive firing of action potential because of their use-depended properties, a mechanism that leads to a preferential action on channels with pathogenic gain-of-function mutations. These blockers reduce muscle stiffness in potassium-aggravated myotonia and paramyotonia congenita by promoting the inactivated state of NaV1.4 by inducing a hyperpolarized shift in steady-state inactivation and by prolonging recovery time from inactivation. Symptoms of muscle weakness are often caused by other pathogenic factors and cannot be treated sufficiently with VGSC blockers [.
Expression of skeletal muscle sodium channel (Nav1.4) prevents reperfusion arrhythmias in murine heart. [
In addition to fast and slow inactivation there is a third type of inactivation named ultra-slow inactivation. This process was described inNav1.4 when the alanine in position 1529 (A1529) is replaced by aspartate (D) in the domain IVP-loop. Binding of the fast inactivating particle inhibits this process. This result demonstrates that there are interactions (mostly, allosteric modulation) among the different inactivation events. ,