Abstract
Veratridine (VTD) is an alkaloid toxin found in Liliaceae plants. VTD causes persistent opening of the voltage-gated Na+ channel and reduces its single-channel conductance by 75 %. The mechanisms for these different VTD actions are unknown. Recent reports indicate that the VTD receptor aligns closely with the local anaesthetic (LA) receptor, which resides at D1S6, D3S6 and D4S6 of the Na+ channel alpha-subunit. To study this alignment, we created a mutant with cysteine substitutions at three S6 residues (rNav1.4-N434C/L1280C/F1579C). Under voltage-clamp conditions, amitriptyline and bupivacaine remained as potent blockers of this mutant channel when expressed in human embryonic kidney cells, whereas VTD completely failed to cause persistent opening. Unexpectedly, VTD at 100 microM progressively blocked mutant currents by 90.4 +/- 1.6 % (n = 5), as assayed at 0.1 Hz for 15 min. This VTD block was reversed little during wash-off: approximately 70 % of mutant currents did not return in 30 min. An increase in channel opening either by repetitive pulses at 1 Hz or by the inhibition of the fast inactivation hastened the VTD block. Co-application of amitriptyline or bupivacaine, which targeted the LA receptor, prevented this VTD block. Our data suggest that (a) the VTD receptor and the LA receptor overlap extensively, (b) receptor-bound VTD lies in the inner vestibule, and (c) VTD blocks this mutant channel as a bona fide Na+ channel blocker. We propose that VTD likewise blocks the wild-type open Na+ channel, albeit partially, to decrease the unitary conductance and to stabilize the open conformation for persistent opening.