FUNCTIONAL-ANALYSIS OF THE RAT-I SODIUM-CHANNEL IN XENOPUS OOCYTES

Voltage-gated sodium channels in the mammalian CNS initiate and propag ate action potentials when excitatory inputs achieve threshold membran e depolarization. There are multiple sodium channel isoforms expressed in rat brain (types I, II, III, 6, and NaG). We have constructed a fu ll-length cDNA clone encoding type I and compared the electrophysiolog ical properties of type I (Rat1) and II (Rat2) channels in the absence and presence of the two accessory subunits beta(1) and beta(2). Injec tion 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 t han Rat2. Coexpression of beta(1) accelerated inactivation of both Rat 1 and Rat2, resulting in comparable inactivation kinetics. Rat1 recove red from fast inactivation more rapidly than Rat2, regardless of wheth er beta(1) or beta(2) was present. The voltage dependence of activatio n 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 diff erence. Finally, sodium current amplitudes were reduced by 7-9% for bo th Rat1 and Rat2 channels when protein kinase A phosphorylation was in duced. It has been suggested previously that Rat1 and Rat6 channels me diate transient and maintained sodium conductances, respectively, in P urkinje cells, and the electrophysiological properties of Rat1 current s are consistent with a role for this channel in mediating the rapidly inactivating, transient current.