Tityus? toxin, a high affinity effector of the Na+ channel in muscle, with a selectivity for channels in the surface membrane

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Abstract
Toxin γ from the venom ofTityus serrulatus scorpion produces a partial block of the surface Na+ channel in frog muscle. This block occurs with no change in the voltage-dependence or in the kinetics of the remaining surface Na+ current. The partial blockade of Na+ channel activity occurs with no change in tubular Na+ currents nor in twitch tension. The maximum effect of the toxin is attained at concentrations as low as 3×10−10 M. Hyperpolarization to potentials more negative than the resting potential (E=−90 mV) reduces or abolishes the effect of the toxin. Radioiodinated toxin γ binds to frog muscle membranes with a very high affinity corresponding to a dissociation constant of about 1×10−11 M. Data obtained with both rabbit and frog muscle indicate that toxin γ is specific for Na+ channels in surface membranes. Toxin γ does not seem to bind to Na+ channels in T-tubule membranes. The biochemical data are in good agreement with electrophysiological studies and data on contraction. There is oneTityus γ toxin binding site per tetrodotoxin binding site in surface membranes. Competition experiments have confirmed thatTityus γ toxin binds to a new toxin receptor site on the Na+ channel structure. This site is the same that the toxin II fromCentruroides suffusus binding site, but this toxin has 100 times less affinity for the Na+ channel thanTityus γ toxin.

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