The Journal of Neuroscience, October 15, 2002, 22(20):8884-8890
Rapid and Reversible Block of N-Type Calcium Channels
(CaV 2.2) by 
-Conotoxin GVIA in the Absence of Divalent
Cations
Haoya
Liang and
Keith S.
Elmslie
Department of Physiology, Tulane University Health Science Center,
New Orleans, Louisiana 70112
-Conotoxin GVIA (CGVIA) has been reported to be an
irreversible blocker of N-type calcium channels (CaV 2.2).
However, recent studies have demonstrated that the CGVIA off-rate is
correlated with divalent cation concentration, because increasing
[Ba2+]o accelerated the recovery from
CGVIA block. This predicts that the dissociation of CGVIA from
N-channels will be negligible in the absence of divalent cations.
Surprisingly, we find that CGVIA block is rapidly reversible in
divalent cation-free (0 Ba2+) external solutions in
which current was carried by MA+. The recovery
followed a single-exponential time course with 
= 31 sec.
Isochronic measurements showed that, at 2 min after the removal of
toxin, current returned to 86% of control in 0 Ba2+
compared with 19% in 3 mM Ba2+. The
off-rate of CGVIA from N-channels was dependent on
[Ba2+]o, because, at an
intermediate concentration (3 µM
Ba2+), N-current recovered with = 64 sec,
significantly slower than that in 0 Ba2+ but faster
than in 3 mM Ba2+. Recovery from
CGVIA block was also observed when Cs+ or
Na+ carried the current in divalent cation-free
conditions. The off-rate was sensitive to
[Ba2+]o only during washout, because
current recovered slowly in the presence of 3 mM
Ba2+, even after it was blocked in 0 Ba2+. Assuming that the toxin is a pore blocker, our
findings are consistent with a model in which Ba2+
interacts at a site on the extracellular surface of the channel to
regulate CGVIA dissociation from N-channels.
Key words:
bullfrog; sympathetic neurons; patch-clamp; monovalent
cations; toxin; off-rate; on-rate
Copyright © 2002 Society for Neuroscience 0270-6474/02/22208884-07$05.00/0
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