Kinetic analysis of the action of Leiurus scorpion alpha-toxin on ionic currents in myelinated nerve.

Abstract

A BS T R A C T The effects of a neurotoxin, purified from the venom of the scorpion Leiurus quinquestriatus, on the ionic currents of toad single myelinated fibers were studied under voltage-clamp conditions. Unlike previous investigations using crude scorpion venom, purified Leiurus toxin IIa at high concentrations (200-400 nM) did not affect the K currents, nor did it reduce the peak Na current in the early stages of treatment. The activation of the Na channel was unaffected by the toxin, the activation time course remained unchanged, and the peak Na current vs. voltage relationship was not altered. In contrast, Na channel inactivation was considerably slowed and became incomplete . As a result, a steady state Na current was maintained during prolonged depolarizations of several seconds. These steady state Na currents had a different voltage dependence from peak Na currents and appeared to result from the opening of previously inactivated Na channels . The opening kinetics of the steady state current were exponential and had rates -100-fold slower than the normal activation processes described for transitions from the resting state to the open state. In addition, the dependence of the peak Na current on the potential of preceding conditioning pulses was also dramatically altered by toxin treatment; this parameter reached a minimal value near a membrane potential of -50 mV and then increased continuously to a "plateau" value at potentials greater than +50 mV. The amplitude of this plateau was dependent on toxin concentration, reaching a maximum value equal to -50% of the peak current; voltagedependent reversal of the toxin's action limits the amplitude of the plateauing effect . The measured plateau effect was half-maximum at a toxin concentration of 12 nM, a value quite similar to the concentration producing half of the maximum slowing of Na channel inactivation . The results of Hill plots for these actions suggest that one toxin molecule binds to one Na channel. Thus, the binding of a single toxin molecule probably both produces the steady state currents and slows the Na channel inactivation . We propose that Leiurus toxin inhibits the conversion of the open state to inactivated states in a voltagedependent manner, and thereby permits a fraction of the total Na permeability to remain at membrane potentials where inactivation is normally complete .

Properties of maintained sodium current induced by a toxin from Androctonus scorpion in frog node of Ranvier.

“…It must be noted that, in contrast with the maintained current induced by toxin II from scorpion Androctonus australis Hector, the maintained current induced by toxin II from scorpion Leiurus quinquedtriatus reverses at the same voltage as the peak current (Wang & Strichartz, 1985). This indicates that the properties of Na channels are differentially modified by various a-scorpion toxins.…”

“…In previous works, the activation of the maintained current, induced by inactivation gate modifiers, was assumed to be faster than the inactivation of the remaining peak current. In consequence, the maintained current, linearly extrapolated to time of depolarization, was subtracted from the total current (Bergman, Dubois, Rojas & Rathmayer, 1976;Neumcke et al 1980;Ulbricht & Schmidtmayer, 1981;Meves et al 1984;Schmidtmayer, 1985;Wang & Strichartz, 1985). This method was applied in Fig.…”

“…The fast and slow channels corresponded to fast and slow inactivation phases of the peak current, respectively. The effects of AaH II on the global Na current can be explained if one assumes that, while the toxin binds to a single class of receptors (Jover et al 1980;Wang & Strichartz, 1985), it exerts two separate types of actions on Na channels. First, it induces an increase in inactivation time constants (Figs.…”

“…(3) A transformation of a fraction of inactivatable channels into non-inactivatable ones (Benoit, Corbier & Dubois, 1985). (4) Quantitative changes in rate constants of channel inactivation (Schmidtmayer, 1985;Wang & Strichartz, 1985). Taking into account that these models are conceptually different, it can be thought that a fine analysis of properties of peak and maintained Na currents may help to define more precisely which of them is the most credible.…”

Inactivation of batrachotoxin-modified Na+ channels in GH3 cells. Characterization and pharmacological modification.

“…This class of toxin can slow the inactivation time course of Na + channels and render the inactivation incomplete (Wang and Strichartz, 1985). As a result, a maintained current is also evident during a prolonged depolarization.…”