Pharmacological characterization of Na+ influx via voltage-gated Na+ channels in spinal cord astrocytes.
Spinal cord astrocytes display a high density of voltage-gated Na+ channels. To study the contribution of Na+ influx via these channels to Na+ homeostasis in cultured spinal cord astrocytes, we measured intracellular Na+ concentration ([Na+]i) with the fluorescent dye sodium-binding benzofuran isophthalate. Stellate and nonstellate astrocytes, which display Na+ currents with different properties, were differentiated. Baseline [Na+]i was 8.5 mM in these cells and was not altered by 100 microM tetrodotoxin (TTX). Inhibition of Na+ channel inactivation by veratridine (100 microM) evoked a [Na+]i increase of 47.1 mM in 44% of stellate and 9.7 mM in 64% of nonstellate astrocytes. About 30% of cells reacted to veratridine with a [Na+]i decrease of approximately 2 mM. Qualitatively similar [Na+]i changes were caused by aconitine. The effects of veratridine were blocked by TTX, amplified by (alpha-)scorpion toxin and usually were readily reversible. Veratridine-induced [Na+]i increases were reduced upon membrane depolarization with elevated extracellular [K+]. Recovery to baseline [Na+]i was unaltered during blocking of K+ channels with 4-aminopyridine. [Na+]i increases evoked by the ionotropic non-N-methyl--aspartate receptor agonist kainate were not altered by TTX. Our results indicate that influx of Na+ via voltage- gated Na+ channels is not a prerequisite for glial Na+,K+-ATPase activity in spinal cord astrocytes at rest nor does it seem to be involved in [Na+]i increases evoked by kainate. During pharmacological inhibition of Na+ channel inactivation, however, Na+ channels can serve as prominent pathways of Na+ influx and mediate large perturbations in [Na+]i, suggesting that Na+ channel inactivation plays an important functional role in these cells.
Rose
C R
CR
Department of Neurology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
Ransom
B R
BR
Waxman
S G
SG
D000157:Aconitine / Q000494:pharmacology; D000818:Animals; D001253:Astrocytes / Q000187:drug effects*; D002478:Cells, Cultured; D006706:Homeostasis; D015640:Ion Channel Gating / Q000187:drug effects*; D008564:Membrane Potentials / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D012604:Scorpion Venoms / Q000737:chemistry; D012964:Sodium / Q000494:pharmacology*; D015222:Sodium Channels / Q000494:pharmacology*; D013116:Spinal Cord / Q000166:cytology / Q000187:drug effects*; D013779:Tetrodotoxin / Q000494:pharmacology; D014118:Toxins, Biological / Q000494:pharmacology; D014701:Veratridine / Q000494:pharmacology
D012604:Scorpion Venoms; D015222:Sodium Channels; D014118:Toxins, Biological; D013779:Tetrodotoxin; D014701:Veratridine; D012964:Sodium; D000157:Aconitine