Fast and Slow Activation Kinetics of Voltage-Gated Sodium Channels in
Molluscan Neurons.
William F. Gilly1, Rhanor Gillette2, Matthew McFarlane1,3.
1Department of Biological Sciences, Hopkins Marine Station of Stanford
University, Pacific Grove, CA 93950, 2Department of Physiology and Biophysics,
University of Illinois at Urbana- Champaign, Urbana, IL 61801, 3Department of
Molecular & Cellular Physiology, Stanford University School of Medicine,
Stanford, CA 94305.
APStracts 4:0009N, 1997.
ABSTRACT
Whole-cell patch clamp recordings of Na current (INa) were made under
identical experimental conditions from isolated neurons from cephalopod
(Loligo, Octopus) and gastropod (Aplysia, Pleurobranchaea, Doriopsilla)
species in order to compare properties of activation gating. Voltage-
dependence of peak Na conductance (gNa) is very similar in all cases, but
activation kinetics in the gastropod neurons studied are markedly slower.
Kinetic differences are very pronounced only over the voltage range spanned by
the gNa-voltage relation. At positive and negative extremes of voltage,
activation and deactivation kinetics of INa are practically indistinguishable
in all species studied. Voltage-dependent rate constants underlying activation
of the slow-type of Na channel found in gastropods thus appear to be much more
voltage-dependent than are the equivalent rates in the universally fast type
of channel that predominates in cephalopods. Voltage-dependence of
inactivation kinetics show a similar pattern and are representative of
activation kinetics for the two types of Na channels. Neurons with fast Na
channels can thus make much more rapid adjustments in the number of open Na
channels at physiologically relevant voltages than would be possible with only
slow Na channels. This capability appears to be an adaptation that is highly
evolved in cephalopods, which are well known for their high-speed swimming
behaviors. Similarities in slow and fast Na channel subtypes in molluscan and
mammalian neurons are discussed.
Received 24 September 1996; accepted in final form 18 December 1996.
APS Manuscript Number J765-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 21 January 1997