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

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 permeabilityP _Na and slightly shift its voltage dependence in the depolarizing direction) caused a small depolarizing shift of the relation between charge (Q _on) and membrane potential (E) without affecting the maximum chargeQ _{on max}. (3) On nodes treated with toxins III or IV fromCentruroides sculpturatus, a depolarizing conditioning pulse (which transiently shifts the descending branch of theI _Na(E) curve by up to 60 mV in the hyperpolarizing direction) shifted the midpoint potential (E_mid) of theQ _on(E) curve by −17 mV and slightly increased the slope of the curve; it also decreasedQ _{on max} markedly but had little effect onQ _on 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 theQ _on(E) curve from −28.5 to −69 mV and significantly increasedQ _on andQ _off measured with small depolarizing pulses; concomitantly, the voltage dependence of the on time constant of the charge movement [τ_on(E)] was shifted by −44 mV. (4) The sodium currentI _Na 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,I _Na started after a delay of 30–40 μs. The time constant of the sodium current, τ_{on Na}, was larger than the time constant of the charge movement, τ_{on Q}; the ratio τ_{on Q}/τ_{on 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 (τ_{off 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 (τ_{off Q}) was unaffected but with aconitine treatment it was considerably increased although it remained smaller than τ_{off Na}. Consequently, the ratio τ_{off Q}/τ_{off 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 τ_{off Q}/τ_{off 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 τ_{on Na} and τ_{off Na} of the sodium current are smaller than the corresponding time constants τ_{on Q} and τ_{off Q} of the charge movement.