Ion transport and membrane potential in CNS myelinated axons. II: effects
of metabolic inhibition
Lisa Leppanen and Peter K. Stys
Loeb Research Institute, Ottawa Civic Hospital, University of Ottawa,
Ottawa, ON Canada K1Y 4E9
APStracts 4:106N, 1997.
ABSTRACT
Compound resting membrane potential was recorded by the grease gap technique
(37¡ C) during glycolytic inhibition and chemical anoxia in myelinated axons
of rat optic nerve. The average potential recorded under control conditions
(no inhibitors) was -47 3 mV, and was stable for 2-3 hours. Zero glucose
(replacement with sucrose) depolarized the nerve in a monotonic fashion to 55
10% of control after 60 min. In contrast, glycolytic inhibition with
deoxyglucose (10 mM, glucose omitted) or iodoacetate (1 mM) evoked a
characteristic voltage trajectory consisting of four distinct phases. A
distinct early hyperpolarizing response (phase 1) was followed by a rapid
depolarization (phase 2). Phase 2 was interrupted by a second late
hyperpolarizing response (phase 3), which led to an abrupt reduction in the
rate of potential change, causing nerves to then gradually depolarize (phase
4) to 75 9% and 55 6% of control after 60 min, in deoxyglucose and
iodoacetate, respectively. Pyruvate (10 mM) completely prevented iodoacetate-
induced depolarization. Effects of glycolytic inhibitors were delayed by 20-30
min, possibly due to continued, temporary oxidative phosphorylation using
alternate substrates through the tricarboxylic acid cycle. Chemical anoxia
(CN- 2 mM) immediately depolarized nerves, and phase 1 was never observed.
However a small inflection in the voltage trajectory was typical after Å 10
min. This was followed by a slow depolarization to 34 ± 4% of control resting
potential after 60 min of CN-. Addition of ouabain (1 mM) to CN--treated
nerves caused an additional depolarization indicating a minor glycolytic
contribution to the Na+-K+-ATPase, which is preferentially fueled by ATP
derived from oxidative phosphorylation. Phases 1 and 3 during iodoacetate
exposure were diminished under nominally zero Ca2+ conditions, and abolished
with the addition of the Ca2+ chelator EGTA (5 mM). TEA (20 mM) also reduced
phase 1 and eliminated phase 3. The inflection observed with CN- was
eliminated during exposure to zero-Ca2+/EGTA. A Ca2+ activated K+ conductance
may be responsible for the observed hyperpolarizing inflections. Block of Na+
channels with TTX (1 µM) or replacement of Na+ with the impermeant cation
choline significantly reduced depolarization during glycolytic inhibition with
iodoacetate or chemical anoxia. The potential-sparing effects of TTX were less
than those of choline-substituted perfusate, suggesting additional, TTX-
insensitive Na+ influx pathways in metabolically compromised axons. The local
anesthetics, procaine (1 mM) and QX-314 (300 µM), had similar effects to TTX.
Taken together, the rate and extent of depolarization of metabolically
compromised axons is dependent on external Na+. The Ca2+-dependent
hyperpolarizing phases and reduction in rate of depolarization at later times
may reflect intrinsic mechanisms designed to limit axonal injury during
anoxia/ischemia.
Received 16 June 1997; accepted in final form 17 June 1997.
APS Manuscript Number J011-7
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 15 July 1997