Differential modulation of TTX-sensitive and TTX-resistant Na+ channels in spinal cord astrocytes following activation of protein kinase C.
TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) Na+ currents are expressed in high densities (2-8 channels/microns2) in astrocytes cultured from neonatal rat spinal cord. The two Na+ current types differ up to 1000-fold in their TTX sensitivity and additionally have different steady-state activation (g-V) and inactivation (h infinity) curves. Expression of TTX-S and TTX-R Na+ currents is confined to morphologically distinguishable subtypes of astrocytes, allowing characterization of the two types of Na+ currents in isolation: stellate cells express TTX-S Na+ currents and flat pancake cells express TTX-R Na+ currents. Activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) exhibited different effects on TTX-S and TTX-R Na+ currents. PMA reduced peak TTX-S Na+ currents by 25-60%; in contrast, PMA potentiated peak TTX-R Na+ currents by 60-150%. These effects developed within minutes, and were typically not reversible. PMA effects were voltage dependent, and shifted steady-state Na+ current activation of TTX-R and TTX-S currents by 6 and 18 mV, respectively, but without affecting their steady-state current inactivation (h infinity). PMA treatment also changed Na+ current kinetics. TTX-R current activation (tau m) was faster and current inactivation (tau h) changed from a single- to a bi-exponential after PMA exposure, suggesting that PKC phosphorylation may have activated formerly quiescent Na+ channels. In contrast, TTX-S current activation (tau m) was unchanged, and current inactivation (tau h), on average, decreased by 50% following PMA exposure.(ABSTRACT TRUNCATED AT 250 WORDS)
Thio
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Department of Neurology, Yale University, School of Medicine, New Haven, Connecticut 06510.
Sontheimer
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D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000187:drug effects / Q000502:physiology*; D002478:Cells, Cultured; D004789:Enzyme Activation; D007546:Isoquinolines / Q000494:pharmacology; D008433:Mathematics; D008564:Membrane Potentials / Q000187:drug effects; D008959:Models, Neurological; D010879:Piperazines / Q000494:pharmacology; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*; D013116:Spinal Cord / Q000502:physiology*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D013779:Tetrodotoxin / Q000494:pharmacology*
D007546:Isoquinolines; D010879:Piperazines; D015222:Sodium Channels; D013779:Tetrodotoxin; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate