The human genome contains a number of almost identical genes coding for slightly different voltage-gated Na+ channels. The different channel genes are specific for expression in the various tissues and this specialization may help restrict the consequence of a mutation to a single type of cell or tissue. The various isoforms of voltage-gated Na+ channels are heterome-tric proteins containing a large, heavily glycosylated a subunit and one or two small 0 subunits. In heart and skeletal muscle, only the subunit has been identified, whereas in the brain, the and a disulfide-linked subunit copurify with the a subunit. Eight different genes (SCN1A-SCN8A) are known so far to encode a sub-units, most of them being expressed in heart, brain, muscle, and peripheral nerve (84). Among them, SCN5A, located on human chromosome 3p21 (85), is expressed in adult cardiac muscle (2, 3) and in fetal skeletal muscle (skM2), and its gene product is characterized by low te-trodotoxin sensitivity. A single gene on human chromosome 19q13.1, SCN1B, encodes the subunit expressed in heart, brain, and skeletal muscle (86-88).
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