Properties of the resting membrane potential in central myelinated axons during normoxic conditions and metabolic inhibition.

Abstract

Compound resting membrane potential was studied in the rat optic nerve, a representative model of myelinated CNS axons, using the grease gap recording technique which measures a reliable fraction of the true intra-axonal potential. Studies were performed during (1) normoxic conditions, (2) glycolytic inhibition, or (3) chemical anoxia at 37$\sp\circ$C, in order to characterize ions and channels promoting membrane depolarization. Ouabain, an antagonist of Na$\sp+$,K$\sp+$-ATPase, caused strong depolarization, showing that membrane potential is critically dependent on Na$\sp+$,K$\sp+$-ATPase. In addition, inhibiting energy metabolism during ouabain exposure produced further depolarization, suggesting additional ATP-dependent, ouabain-insensitive ion transport systems. Glycolysis was blocked with iodoacetate or deoxyglucose, evoking a response consisting of four distinct phases. An initial transient hyperpolarization (phase 1) preceded a rapid depolarizing response (phase 2). Phase 2 was interrupted by a second brief hyperpolarization (phase 3) which was followed by a gradual depolarization (phase 3). Chemical anoxia (cyanide) immediately depolarized the nerve, with only a small inflection introducing a final slow depolarizing response. Addition of ouabain to cyanide-treated nerves caused an additional depolarization indicating a minor glycolytic contribution to Na$\sp+$,K$\sp+$-ATPase, which seems preferentially fueled by mitochondrial ATP in optic nerve. Hyperpolarizing phases 1 and 3 induced by iodoacetate exposure and the inflection during cyanide treatment were abolished by zero-Ca$\sp{2+}$/EGTA treatment. Block of Na$\sp+$ channels with tetrodotoxin, the local anesthetics, procaine or QX-314, or replacement of Na$\sp+$ with the impermeant cation choline significantly reduced depolarization during iodoacetate or cyanide application, indicating that axonal membrane depolarization requires Na$\sp+$ influx which secondarily allows electroneutral K$\sp+$ efflux and depolarization. (Abstract shortened by UMI.)