A variety of isoforms of mammalian voltage-gated sodium channels have been described. Ten genes encoding sodium channel α subunits have been iden- tified, and nine of those isoforms have been functionally expressed in exogenous sys- tems. The α subunit is associated with accessory β subunits in some tissues, and three genes encoding different β subunits have been identified. The α subunit isoforms have distinct patterns of development and localization in the nervous system, skeletal and cardiac muscle. In addition, many of the isoforms demonstrate subtle differences in their functional properties. However, there are no clear subfamilies of the channels, unlike the situation with potassium and calcium channels. The subtle differences in the functional properties of the sodium channel isoforms result in unique conductances in specific cell types, which have important physiological effects for the organism. Small alterations in the electrophysiological properties of the channel resulting from mutations in specific isoforms cause human diseases such as periodic paralysis, long QT syndrome, and epilepsy.
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