Inactivation-deficient mutant reveals two conformational states
involved in the use-dependent ttx blockade of human cardiac na+
channel.
Dumaine, Robert, Hali A. Hartmann.
Departments of Molecular Physiology and Biophysics, Baylor College
of Medicine, Houston TX 77030
APStracts 2:0483H, 1995.
We used a fast inactivation deficient mutant (QQQ) of the human heart
sodium channel [alpha]-subunit (hH1a) to assess the influence of the
inactivation gate on TTX use-dependent block (UDB) and post
-repolarization block (PRB). Post-repolarization block had similar
time courses in both channels, suggesting no direct interaction of
the inactivation gate with the TTX binding site. The use-dependent
block in hH1a saturated with high concentrations of TTX while it did
not in QQQ, revealing the modulatory action of fast inactivation on
UDB. TTX did not stabilize the inactivated states of QQQ and the
extra-block developing during long depolarizations suggest a higher
affinity site involved in the gating of the channel. These results
cannot be solely explained by a slow recovery from the block in the
inactivated states. They suggest a common use-dependent block
mechanism for hH1a and QQQ, involving a high affinity site. We
propose that an activated state is primarily responsible for UDB
during short depolarization, in the range of the action potential
plateau, and that fast inactivation modulates the accessibility of
the toxin to this site.
Received 10 November 1994; accepted in final form 12 October
1995.
APS Manuscript Number H1003-4.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95