Slowed
Inactivation at Positive Potentials in a Rat
Axonal K+ Channel is not Due to Preferential
Closed-State Inactivation
A.
BABES, E. LÖRINCZI, V. RISTOIU, M.-L. FLONTA, G.
REID
Department
of Animal Physiology and Biophysics, Faculty of
Biology, University of Bucharest, Bucharest,
Romania
Received
January 9, 2001
Accepted March 20, 2001
Summary
We
have investigated slow inactivation in a rat
axonal K+ channel, the I channel. Using voltage
steps to potentials between -70 mV and +80 mV,
from a holding potential of -100 mV, we observed
a marked slowing of inactivation at positive
potentials: the time constant was 4.5±0.4 s at
-40 mV (mean ± S.E.M.), increasing to 14.7±2.0
s at +40 mV. Slowed inactivation at positive
potentials is not consistent with published
descriptions of C-type inactivation, but can be
explained by models in which inactivation is
preferentially from closed states (which have
been developed for Kv2.1 and some Ca2+ channels).
We tested two predictions of preferential
closed-state models: inactivation should be more
rapid during a train of brief pulses than during
a long pulse to the same potential, and the
cumulative inactivation measured with paired
pulses should be greater than the inactivation at
the same time during a continuous pulse. The I
channel does not behave according to these
predictions, indicating that preferential
closed-state inactivation does not explain the
slowing of inactivation we observe at positive
potentials. Inactivation of the I channel
therefore differs both from C-type inactivation,
as presently understood, and from the
inactivation of Kv2.1.
Key words
Potassium
channel · Inactivation · Axon · Rat
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Prof.
Gordon Reid, Department of Animal Physiology and
Biophysics, Faculty of Biology, University of
Bucharest, Splaiul Independenţei 91-95,
Bucuresti 76201, Romania. Fax: +40 1 411 39 33.
e-mail: gordon@biologie.kappa.ro
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