Summary
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1.
Voltage clamp experiments were done on single nodes of Ranvier ofRana esculenta. Equilibrium effects were obtained fromI Na-V curves, the rates of action from changes inI Na on changing solutions during repetitive depolarizing pulses.
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2.
Saxitoxin (STX) exclusively and reversibly blocked Na channels, the effect being fully described by a one-to-one reaction between STX and a receptor at the channel with an equilibrium dissociation constant,K S ,of 1.4 nM; the mean offset rate constant,k 2S ,was 1.76×10−2 sec−1 (16±1°C; pH 7.2), 1.7 times the value for tetrodotoxin (TTX).
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3.
At pH 5.6,k 2S was increased by a factor of 1.33 while the equilibrium STX effect was decreased in a way suggesting competition between STX and protons.
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4.
After pretreatment of nodes with 3.1 nM TTX the extra block on adding 9.0 nM STX as well as its relief on taking out STX of the TTX-STX mixture revealed transients in the time course of receptor occupation.
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5.
These non-monotonic time courses are incompatible with the idea of two independent blocking sites (for STX and TTX) per channel but could be quantitatively fitted by analog-computed curves assuming competition between STX and TTX for the same site.
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Wagner, H.H., Ulbricht, W. The rates of saxitoxin action and of saxitoxin-tetrodotoxin interaction at the node of Ranvier. Pflugers Arch. 359, 297–315 (1975). https://doi.org/10.1007/BF00581441
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DOI: https://doi.org/10.1007/BF00581441