Anemone toxin II unmasks two conductance states in neuronal sodium channels.
Anemone toxin II (ATX)-modified voltage-dependent neuronal sodium channels were studied in planar lipid bilayers. ATX-modified channels displayed two predominant conducting states: a short-lived (ms-s) high-conductance (approximately 65 pS) state and a long-lived (s-min) low-conductance (approximately 10 pS) state. The high-conductance state underwent brief closures (ms) and the low-conductance state underwent long closures (s). The probability of detecting these states was time- and voltage-dependent. The channel's fractional open time (fo) due to the high-conductance state increased with depolarization and had a midpoint potential (Va) of -36 mV and an apparent gating charge (Za) of 2.8. The channel's fo due to the low-conductance state increased with depolarization and had a Va of +13 mV and a Za of 1.4. At positive potentials, ATX-modified channels slowly (minutes) entered an absorbing non-conducting state. The permeability ratio of Na+/K+ was 2 and 4 for the low- and high-conductance states, respectively. The saxitoxin analog C3 blocked ATX-modified sodium channels with high affinity (Kd(60-90 mV) = 410 nM, 0.5 M NaCl). The data suggest that upon a depolarization step, ATX-modified channels enter rapidly (ms) into a high-conductance state and more slowly (s-min) into a low-conductance state. Also as the membrane potential becomes more positive, the equilibrium is shifted from the high- to the low-conductance state and from the conducting states to an absorbing non-conducting state.
Castillo
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C
Instituto de Estudios Avanzados (IDEA), Caracas, Venezuela.
Piernavieja
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C
Recio-Pinto
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E
D000818:Animals; D002462:Cell Membrane / Q000502:physiology; D003064:Cnidarian Venoms / Q000494:pharmacology*; D004141:Dinoflagellida; D004553:Electric Conductivity; D008051:Lipid Bilayers; D008564:Membrane Potentials / Q000187:drug effects; D008121:Nephropidae; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009498:Neurotoxins / Q000494:pharmacology*; D011188:Potassium / Q000378:metabolism; D011336:Probability; D012530:Saxitoxin / Q000031:analogs & derivatives / Q000494:pharmacology; D012615:Sea Anemones; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors
D003064:Cnidarian Venoms; D008051:Lipid Bilayers; D009498:Neurotoxins; D015222:Sodium Channels; D012530:Saxitoxin; D013779:Tetrodotoxin; C028800:toxin II (Anemonia sulcata); C013007:gonyautoxins; D012964:Sodium; D011188:Potassium