Voltage-dependent effect of a scorpion toxin on sodium current inactivation

Abstract

In voltage clamped nodes of Ranvier inactivation of the sodium permeability is slowed by toxin V from the scorpionCentruroides sculpturatus, by sea anemone toxin ATX II or by internally applied KIO₃. The slow decay of the Na inward current is markedly accelerated if the test pulse is preceded by a depolarizing conditioning pulse followed by a 10–500 ms pause. This phenomenon was studied in detail, using conditioning pulses of varying amplitude and up to 15 s duration. In nodes treated with toxin V a 20 ms conditioning pulse to positive potentials was sufficient to produce a clear acceleration of the decay of the Na current and a reduction of the inward current remaining at the end of a 50 ms test pulse, i.e. a weakening of the toxin effect. In nodes treated with ATX II or internal KIO₃ longer conditioning pulses were required. A similar effect of conditioning pulses on the decaying phase of the Na current was also observed in untreated fibres. To study the phenomenon quantitatively we fitted the decaying phase of the inward Na current with the equationI _Na=A exp(-t/τ₁)+B exp(-t/τ₂)+C The effect of depolarizing conditioning pulses could be described as an increase of A, a decrease of B and C and a reduction of the time constants τ₁ and τ₁. I ₅₀/I _peak, the normalised inward current remaining at the end of a 50 ms test pulse, decreased exponentially with increasing duration of the conditioning pulse to a steady-state value. The time constant τ and the steady-state value depended on the potential during the conditioning pulse. For nodes treated with toxin V, τ was 0.24 s at 0 mV and 12° C and half inhibition occurred at −42 mV. The time constant τ was larger for nodes treated with ATX II or internal KIO₃. At positive potentials, I₅₀ was reduced to 20% of the control value in toxin V-treated nodes, but only to 70% in KIO₃-treated nodes. Recovery from the effect of the conditioning pulse was studied by varying the pause between conditioning pulse and test pulse; recovery was 66–100% complete after 500 ms. The results are interpreted by assuming that a sepolarizing conditioning pulse (a) accelerates inactivation of the sodium permeability and (b) causes dissociation of the toxin-receptor complex or transition into an inactive state. The latter effect occurs in toxin V-treated fibres but not in those treated with ATX II or KIO₃.