Spatiotemporal gradients of intra-axonal [Na+] after transection and resealing in lizard peripheral myelinated axons.
1. Post-transection changes in intracellular Na+ ([Na+]i) were measured in lizard peripheral axons ionophoretically injected with the Na(+)-sensitive ratiometric dye, sodium-binding benzofuran isophthalate (SBFI). 2. Following axonal transection in physiological saline [Na+]i increased to more than 100 mM in a region that quickly extended hundreds of micrometers from the transection site. This post-transection increase in [Na+]i was similar when the bath contained 5 microM tetrodotoxin, but was absent in Na(+)-free solution. Depolarization of uncut axons in 50 mM K+ produced little or no elevation of [Na+]i until veratridine was added. These results suggest that the post-transection increase in [Na+]i was due mainly to Na+ entry via the cut end, rather than via depolarization-activated Na+ channels. 3. The spatiotemporal profile of the post-transection increase in [Na+]i could be accounted for by movement of Na+ from the cut end with an apparent diffusion coefficient of 1.3 x 10(-5) cm2 s-1. 4. [Na+]i began to decline toward resting levels by 20 +/- 15 min (mean +/- S.D.) post-transection, except in regions of the axon within 160 +/- 85 microns of the transection site, where [Na+]i remained high. The boundary between axonal regions in which [Na+]i did or did not recover probably defines a locus of resealing of the axonal membrane. 5. [Na+]i returned to resting values within about 1 h after resealing, even in axonal regions where the normal transmembrane [Na+] gradient had completely dissipated. The recovery of [Na+]i was faster and reached lower levels than expected by diffusional redistribution of Na+ along the axon. Partial recovery occurred even in an isolated internode, indicating that the internodal axolemma can actively extrude Na+.
David
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Department of Physiology and Biophysics R-430, University of Miami School of Medicine, FL 33101, USA. gdavid@newssun.med.miami.edu
Barrett
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Barrett
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D000818:Animals; D001369:Axons / Q000378:metabolism* / Q000648:ultrastructure; D001572:Benzofurans; D002462:Cell Membrane / Q000502:physiology; D004058:Diffusion; D004988:Ethers, Cyclic; D005453:Fluorescence; D005456:Fluorescent Dyes; D007002:Hypoglossal Nerve / Q000166:cytology / Q000502:physiology; D007091:Image Processing, Computer-Assisted; D007478:Iontophoresis; D008116:Lizards / Q000502:physiology*; D009186:Myelin Sheath / Q000502:physiology; D010525:Peripheral Nerves / Q000166:cytology / Q000378:metabolism*; D015222:Sodium Channels / Q000378:metabolism*; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors
D001572:Benzofurans; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D015222:Sodium Channels; C061665:sodium-binding benzofuran isophthalate; D013779:Tetrodotoxin