Functional analysis of the rat I sodium channel in Xenopus oocytes

Voltage-gated sodium channels in the mammalian CNS initiate and propagate action potentials when excitatory inputs achieve threshold membrane depolarization. There are multiple sodium channel isoforms expressed in rat brain (types I, II, III, 6, and NaG). We have constructed a full-length cDNA clone encoding type I and compared the electrophysiological properties of type I (Rat1) and II (Rat2) channels in the absence and presence of the two accessory subunits beta(1) and beta(2). Injection into Xenopus oocytes of RNA encoding Rat1 resulted in functional sodium currents that were blocked by tetrodotoxin, with K-app = 9.6 nM. Rat1 sodium channels had a slower time course of fast inactivation than Rat2. Coexpression of beta(1) accelerated inactivation of both Rat1 and Rat2, resulting in comparable inactivation kinetics. Rat1 recovered from fast inactivation more rapidly than Rat2, regardless of whether beta(1) or beta(2) was present. The voltage dependence of activation was similar for Rat1 and Rat2 without the beta subunits, but it was more positive for Rat1 when beta(1) and beta(2) were coexpressed. The voltage dependence of inactivation was more positive for Rat1 than for Rat2, and coexpression with beta(1) and beta(2) accentuated that difference. Finally, sodium current amplitudes were reduced by 7-9% for both Rat1 and Rat2 channels when protein kinase A phosphorylation was induced. It has been suggested previously that Rat1 and Rat6 channels mediate transient and maintained sodium conductances, respectively, in Purkinje cells, and the electrophysiological properties of Rat1 currents are consistent with a role for this channel in mediating the rapidly inactivating, transient current.