ION-TRANSPORT AND MEMBRANE-POTENTIAL IN CNS MYELINATED AXONS .2. EFFECTS OF METABOLIC INHIBITION

Compound resting membrane potential was recorded by the grease gap tec hnique (37 degrees C) during glycolytic inhibition and chemical anoxia in myelinated axons of rat optic nerve. The average potential recorde d under control conditions(no inhibitors) was -47 +/- 3 (SD) mV and wa s stable for 2-3 h. Zero glucose (replacement with sucrose) depolarize d the nerve in a monotonic fashion to 55 +/- 10% of control after 60 m in. In contrast, glycolytic inhibition with deoxyglucose (10 mM, gluco se omitted) or iodoacetate (I mM) evoked a characteristic voltage traj ectory consisting of four distinct phases. A distinct early hyperpolar izing response (phase I) 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 iodoa cetate, respectively. Pyruvate (10 mM) completely prevented iodoacetat e-induced depolarization. Effects of glycolytic inhibitors were delaye d by 20-30 min, possibly due to continued, temporary oxidative phospho rylation using alternate substrates through the tricarboxylic acid cyc le. Chemical anoxia (CN- 2 mM) immediately depolarized nerves, and pha se I was never observed. However a small inflection in the voltage tra jectory was typical after approximate to 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 contribut ion to the Na+ - K+ - ATPase, which is fueled preferentially by ATP de rived from oxidative phosphorylation. Phases I and 3 during iodoacetat e exposure were diminished under nominally zero Ca2+ conditions and ab olished with the addition of the Ca2+ chelator ethylene glycol-bis(bet a-aminoethyl ether)-N,N,N',N'-tetraacetic acid(EGTA; 5 mM). Tetraethyl ammonium chloride (20 mM) also reduced phase I and eliminated phase 3. The inflection observed with CN-was eliminated during exposure to zer o-Ca2+/EGTA. A Ca2+-activated K+ conductance may be responsible for th e observed hyperpolarizing inflections. Block of Na+ channels with tet rodotoxin (TTX; 1 mu M) Or replacement of Na+ with the impermeant cati on choline significantly reduced depolarization during glycolytic inhi bition with iodoacetate or chemical anoxia. The potential-sparing effe cts of TTX were less than those of choline-substituted perfusate, sugg esting additional, TTX-insensitive Na+ influx pathways in metabolicall y compromised axons. The local anesthetics, procaine (1 mM) and QX-314 (300 mu M), had similar effects to TTX. Taken together, the rate and extent of depolarization of metabolically compromised axons is depende nt on external Na+. The Ca2+-dependent hyperpolarizing phases and redu ction in rate of depolarization at later times may reflect intrinsic m echanisms designed to limit axonal injury during anoxia/ischemia.