Pflügers Archiv

, Volume 409, Issue 4–5, pp 381–393 | Cite as

Gating current experiments on frog nodes of Ranvier treated withCentruroides sculpturatus toxins or aconitine

  • H. Meves
  • N. Rubly
  • D. D. Watt
Excitable Tissues and Central Nervous Physiology

Abstract

(1) Gating currents were recorded from frog nodes of Ranvier treated either with toxins III or IV from the venom of the scorpionCentruroides sculpturatus or with the alkaloid toxin aconitine. (2) Toxins III or IV fromCentruroides sculpturatus (which drastically reduce the sodium permeabilityPNa and slightly shift its voltage dependence in the depolarizing direction) caused a small depolarizing shift of the relation between charge (Qon) and membrane potential (E) without affecting the maximum chargeQon max. (3) On nodes treated with toxins III or IV fromCentruroides sculpturatus, a depolarizing conditioning pulse (which transiently shifts the descending branch of theINa(E) curve by up to 60 mV in the hyperpolarizing direction) shifted the midpoint potential (Emid) of theQon(E) curve by −17 mV and slightly increased the slope of the curve; it also decreasedQon max markedly but had little effect onQon measured with small depolarizing pulses. By contrast, massive treatment with aconitine (which irreversibly shifts sodium activation in the hyperpolarizing direction) irreversibly shifted the midpoint potential of theQon(E) curve from −28.5 to −69 mV and significantly increasedQon andQoff measured with small depolarizing pulses; concomitantly, the voltage dependence of the on time constant of the charge movement [τon(E)] was shifted by −44 mV. (4) The sodium currentINa was exponential both in nodes treated with toxins III or IV ofCentruroides sculpturatus and subjected to a depolarizing conditioning pulse and in aconitine-treated nodes; in the latter,INa started after a delay of 30–40 μs. The time constant of the sodium current, τon Na, was larger than the time constant of the charge movement, τon Q; the ratio τon Qon Na was 0.61 and 0.73 in the experiments withCentruroides sculpturatus toxins and aconitine, respectively. (5) The off time constant of the sodium current (τoff Na) was slightly increased in nodes treated withCentruroides sculpturatus toxins and subjected to a depolarizing conditioning pulse, whereas it was markedly increased in aconitine-treated nodes. With the former treatment, the off time constant of the charge movement (τoff Q) was unaffected but with aconitine treatment it was considerably increased although it remained smaller than τoff Na. Consequently, the ratio τoff Qoff Na (which is ≥1 in untreated nodes) became smaller than one, reaching values as low as 0.58 and 0.44 in the experiments withCentruroides sculpturatus toxins and aconitine, respectively. The small τoff Qoff Na ratio suggests that the channels remain open for an appreciable time after most of the gating charges have returned to their resting position. (6) The results obtained with aconitine resemble the findings on batrachotoxin-treated nodes (Dubois and Schneider 1985), except that in the latter the time constants τon Na and τoff Na of the sodium current are smaller than the corresponding time constants τon Q and τoff Q of the charge movement.

Key words

Myelinated nerve fibre Voltage clamp Gating currents Scorpion toxins Aconitine 

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Copyright information

© Springer-Verlag 1987

Authors and Affiliations

  • H. Meves
    • 1
  • N. Rubly
    • 1
  • D. D. Watt
    • 2
  1. 1.I. Physiologisches Institut der Universität des SaarlandesHomburg/SaarGermany
  2. 2.Department of BiochemistryCreighton UniversityOmahaUSA

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