Identification of a novel voltage-gated Na+ channel rNa(v)1.5a in the rat hippocampal progenitor stem cell line HiB5

A conditionally immortalised cell line, HiB5, derived from embryonic hippoc ampal precursor cells expressed a voltage-gated Nat channel with electrophy siological characteristics shifted to more negative voltages and a lower se nsitivity to tetrodotoxin [concentration required for half-maximal inhibiti on (IC50) 0.9 muM] compared with endogenous; neuronal Na+ channels. The cha nnel activation and steady-state inactivation occurred at very negative pot entials with the threshold for activation at -55 mV and half-maximal inacti vation at -78.7 mV. The channel was blocked by lamotrigine and sipatrigine voltage and state dependently, with potencies 5-20 times higher (IC50 12 an d 1.8 muM at -80 mV respectively) than the corresponding block of endogenou s Ne channels from neurones and cloned rNa(v)1.2a (rBIIA) alpha -subunits. Both compounds slowed the channel's recovery from inactivation. Whereas lam otrigine and sipatrigine had similar effects on the fast inactivated state, the effect of sipatrigine on the slow inactivation state was more pronounc ed, rendering this compound overall the more effective. The molecular subty pe mainly expressed by HiB5 cells was identified using RT-PCR and was a nov el splice variant of rNa(v)1.5 (rNa(v)1.5a). It differs from the known rNa( v)1.5 version in that it lacks 53 amino acids in the intracellular loop bet ween domains H and III. rNa(v)1.5a was also detected in mRNA derived from r at whole brain.