PHARMACOLOGICAL CHARACTERIZATION OF NA-GATED NA+ CHANNELS IN SPINAL-CORD ASTROCYTES( INFLUX VIA VOLTAGE)

Spinal cord astrocytes display a high density of voltage-gated Na+ cha nnels. To study the contribution of Na+ influx via these channels to N a+ homeostasis in cultured spinal cord astrocytes, we measured intrace llular Na+ concentration ([Na+](i)) with the fluorescent dye sodium-bi nding benzofuran isophthalate. Stellate and nonstellate astrocytes, wh ich display Na+ currents with different properties, were differentiate d. Baseline [Na+](i) was 8.5 mM in these cells and was not altered by 100 mu M tetrodotoxin (TTX). inhibition of Na+ channel inactivation by veratridine (100 mu M) evoked a [Na+](i) increase of 47.1 mM in 44% o f stellate and 9.7 mM in 64% of nonstellate astrocytes. About 30% of c ells reacted to veratridine with a [Na+](i) decrease of similar to 2 m M. Qualitatively similar [Na+](i) changes were caused by aconitine. Th e effects of veratridine were blocked by TTX, amplified by(alpha-)scor pion toxin and usually were readily reversible. Veratridine-induced [N a+](i) increases were reduced upon membrane depolarization with elevat ed extracellular [K+]. Recovery to baseline [Na+](i) was unaltered dur ing blocking of K+ channels with 4-aminopyridine. [Na+](i) increases e voked by the ionotropic non-N-methyl-D-aspartate receptor agonist kain ate were not altered by TTX. Our results indicate that influx of Na+ v ia voltage-gated Na+ channels is not a prerequisite for glial Na+,K+-A TPase activity in spinal cord astrocytes at rest nor does it seem to b e involved in [Na+](i) increases evoked by kainate. During pharmacolog ical inhibition of Na+ channel inactivation, however, Na+ channels can serve as prominent pathways of Na+ influx and mediate large perturbat ions in [Na+](i), suggesting that Na+ channel inactivation plays an im portant functional role in these cells.