ISSN:
1432-2013
Keywords:
Myelinated nerve fibre
;
Voltage clamp
;
Gating currents
;
Scorpion toxins
;
Aconitine
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
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 (Emid) 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 [τon(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, τon Na, was larger than the time constant of the charge movement, τon Q; the ratio τon Q/τon 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 Q/τoff 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 Q/τoff 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00583792
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