Coupling of calcium homeostasis to axonal sodium in axons of mouse optic nerve.

Axonal populations in neonatal and mature optic nerves were selectively stained with calcium dyes for analysis of calcium homeostasis and its possible coupling to axonal Na. Repetitive nerve stimulation causes a rise in axonal Ca(2+) the posttetanus recovery of which is impeded by increasing the number of action potentials in the tetanus. This effect is augmented in 4-aminopyridine (4-AP; 1 mM), which dramatically increases the calcium and presumably sodium load during the tetanus. Increasing axonal Na with the Na-ionophore monensin (4-50 microM) and ouabain (30 microM) retards posttetanus calcium decline, suggesting that efficient calcium clearance depends on a low level of axonal Na. Posttetanus calcium clearance is not affected by K-mediated depolarization. To further examine coupling between axonal Na and Ca(2+), the resting axonal Ca(2+) was monitored as axonal Na(+) was elevated with ouabain, veratridine, and monensin. In all cases, elevation of axonal Na(+) evokes a calcium influx into axons. This influx is unrelated to activation of calcium channels but is consistent with calcium influx via reversal of the Na/Ca exchanger expected as a consequence of axonal Na(+) elevation. In conclusion, this study demonstrates that calcium homeostasis in the axons of the optic nerve is strongly coupled to axonal Na(+) in a manner consistent with the Na/Ca exchanger playing a major role in extruding calcium following nerve activity.