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+)](i) 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](i) 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](i). Posttetanus calcium clearance is not affected by K-mediated depolarization. To further examine coupling between axonal [Na](i) and [Ca(2+)](i), the resting axonal [Ca(2+)](i) was monitored as axonal [Na(+)](i) was elevated with ouabain, veratridine, and monensin. In all cases, elevation of axonal [Na(+)](i) 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(+)](i) elevation. In conclusion, this study demonstrates that calcium homeostasis in the axons of the optic nerve is strongly coupled to axonal [Na(+)](i) in a manner consistent with the Na/Ca exchanger playing a major role in extruding calcium following nerve activity.
Verbny
Yakov
Y
Department of Physiology, University of Wisconsin School of Medicine, Madison 53706, USA.
Zhang
Chuan-Li
CL
Chiu
Shing Yan
SY
D015761:4-Aminopyridine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D000241:Adenosine / Q000494:pharmacology; D000375:Aging; D000818:Animals; D000831:Animals, Newborn; D001369:Axons / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D004558:Electric Stimulation; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D006706:Homeostasis / Q000187:drug effects; D007476:Ionophores / Q000494:pharmacology; D051379:Mice; D008985:Monensin / Q000494:pharmacology; D009900:Optic Nerve / Q000187:drug effects / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D026902:Potassium Channel Blockers / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000494:pharmacology; D014701:Veratridine / Q000494:pharmacology
D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D007476:Ionophores; D026902:Potassium Channel Blockers; D019831:Sodium-Calcium Exchanger; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D010042:Ouabain; D014701:Veratridine; D008985:Monensin; D012964:Sodium; D015761:4-Aminopyridine; D000241:Adenosine; D002118:Calcium