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T Gonoi, Y Ohizumi, J Kobayashi, H Nakamura and WA Catterall
Department of Pharmacology, University of Washington, Seattle 98195.
The effects of a polypeptide toxin of 25,000 Da from the marine snail Conus striatus (CsTx) on sodium channels in mouse neuroblastoma cells and rat brain synaptosomes were studied. CsTx slowed sodium channel inactivation without altering the time course of activation of the channels. The voltage dependence of sodium channel inactivation was shifted to more negative membrane potentials and made less steep. Peak sodium currents were increased, and the voltage dependence of activation was shifted to more negative membrane potentials. The action of the toxin was voltage-dependent. Maximum toxin effects were observed at membrane potentials in the range of -100 to -60 mV. Apparent KD values were calculated assuming a one-to-one binding interaction. At more positive membrane potentials, the apparent KD for toxin action increased e-fold for each 19-mV depolarization. Apparent KD also increased at membrane potentials more negative than -100 mV. CsTx did not have significant effects on the binding of saxitoxin or Leiurus alpha-scorpion toxin to their receptor sites on sodium channels. CsTx enhanced the binding of batrachotoxinin A 20-alpha-benzoate to sodium channels in the same concentration range as its physiological effects. It is concluded that CsTx interacts with a new receptor site on the extracellular surface of the sodium channel at which specific effects on channel inactivation can occur.
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