Ion Transport and Membrane Potential in CNS Myelinated Axons II. Effects of Metabolic Inhibition
Authors: Leppanen Lisa , Stys Peter K. 
Published: December 2017

Leppanen, Lisa and Peter K. Stys. Ion transport and membrane potential in CNS myelinated axons. II. Effects of metabolic inhibition. J. Neurophysiol. 78: 2095–2107, 1997. 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 (SD) mV and was stable for 2–3 h. 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 depolarize gradually ( 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 ≈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 fueled preferentially 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 ethylene glycol-bis(β-aminoethyl ether)- N,N,N′,N′-tetraacetic acid (EGTA; 5 mM). Tetraethylammonium chloride (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 tetrodotoxin (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.