Na(+)-K(+)-
ATPase pump failure during either
anoxia or
ouabain perfusion induces rapid axonal depolarization by dissipating ionic gradients. In this study, we examined the interplay between
cation and
anion transporting pathways mediating axonal depolarization during
anoxia or selective Na(+)-K(+)-
ATPase inhibition. Compound resting membrane (V(m)) potential of rat optic nerve was measured in a grease gap at 37 degrees C. Chemical
anoxia (2 mM NaCN or NaN(3)) or
ouabain (1 mM) caused a loss of resting potential to 42 +/- 11% and 47 +/- 2% of control after 30 min, respectively. Voltage-gated Na(+)-channel blockade was partially effective in abolishing this depolarization. TTX (1 microM) reduced depolarization to 73 +/- 10% (chemical
anoxia) and 68 +/- 4% (
ouabain) of control. Quaternary
amine Na(+) channel blockers
QX-314 (1 mM) or
prajmaline (100 microM) produced similar results. Residual ionic rundown largely representing co-efflux of K(+) and Cl(-) during chemical
anoxia in the presence of Na(+)-channel blockade was further spared with
DIDS (500 microM), a broad-spectrum
anion transport inhibitor (95 +/- 8% of control after 30 min in
anoxia + TTX vs. 73 +/- 10% in TTX alone). Addition of
DIDS was slightly more effective than TTX alone in
ouabain (74 +/- 5%
DIDS + TTX vs. 68 +/- 4% in TTX alone, P < 0.05). Additional Na(+)-entry pathways such as the
Na-K-Cl cotransporter were examined using
bumetanide, which produced a modest albeit significant sparing of V(m) during
ouabain-induced depolarization. Although
cation-transporting pathways play the more important role in mediating pathological depolarization of central axons,
anion-coupled transporters also contribute to a significant, albeit more minor, degree.