Ion transport and membrane potential in CNS myelinated axons I. Normoxic conditions

Abstract

Compound resting membrane potential was recorded by the grease gap technique during normoxic conditions (37°C) in rat optic nerve, a representative CNS myelinated tract. Mean potential was -47 ± 3 (SD) mV and remained, stable for 2-3 h. Input impedance of a single optic nerve axon was calculated to be ≃5 GΩ. Contribution of the Na+ pump to resting axonal potential is estimated at -7 mV. Ouabain (10 μM to 10 mM) evoked a dose- dependent depolarization that was maximal at ≤ 1 mM, depolarizing the nerves to ~35-40% of control after 60 min. Inhibiting energy metabolism (CN- and iodoacerate) during high-dose ouabain (1-10 mM) exposure caused an additional depolarization suggesting additional ATP-dependent ouabain-insensitive ion transport systems. Perfusion with zero-Na+ (choline substituted) caused a transient hyperpolarization, that was greater than with tetrodotoxin (TTX; 1 μM) alone, indicating both TTX-sensitive and -insensitive Na+ influx pathways in resting rat optic nerve axons. Resting probability (P)K:PNa is calculated at 20:1. In contrast to choline-substituted solution, Li+- substituted zero-Na+ perfusate caused a rapid depolarization due to Na+ pump inhibition and the ability of Li+ to permeate the Na+ channel. TTX reduced but did not prevent, ouabain- or zero-Na+/Li+ -induced depolarization. We conclude that the primary Na+ influx path in resting rat optic nerve axons is the TTX-sensitive Na+ channel, with evidence for additional TTX-insensitive routes permeable to Na+ and Li+. In addition, maintenance of membrane potential is critically dependent on continuous Na+ pump activity due to the relatively high exchange of Na+ (via the above mentioned routes) and K+ across the membrane of resting optic axons.