ISSN:
1432-2013
Keywords:
Scorpion toxin
;
Myelinated nerve fiber
;
pH
;
Voltage clamp
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Abstract 1. New toxins, VI and VII, were purified from the venom of the North American scorption,Centruroides sculpturatus, and studied in the voltage-clamped frog node of Ranvier. These toxins reduced peak inward Na currents and caused a transient, depolarization-induced shift in the voltage dependence of Na activation. Their effects were indistinguishable from those of toxins I, III and IV, as previously described by Meves et al. (1982). 2. Toxin VII, 0.2 μg/ml, shifted the voltage dependence of steady-state inactivation (h ∞). In four fibers, the mean shift of theh ∞(E) curve was −17 mV, compared to a mean shift of −28 mV in the descending branch of theI Na(E) curve. Theh ∞(E) curve with toxin VII was monotonic and inactivation was incomplete at positive potentials. 3. Decreasing the pH from 7.4 to 5.7 increased the shift in the voltage dependence of activation with toxin. The increase with toxin VI at pH 5.7 was reversible on returning to pH 7.4, but the increase with toxin VII was not. The lack of reversibility of the effect of pH with toxin VII was quantified in two ways: (a) the mean value ofI Na (measured at −62 mV with a conditioning pulse) at pH 7.4 was 5.1 times larger after treatment than before treatment with 0.1 μg/ml of toxin VII at pH 5.7; (b) the average concentration of toxin VII required for a shift of −30 mV at pH 7.4 was decreased by a factor of 4.3, to 0.19 μg/ml (26 nM), by pretreatment at pH 5.7. In comparison, a shift of −30 mV was produced with 2.31 μg/ml (319 nM) of toxin VI. 4. For both toxin VI and toxin VII (in the latter case, after initial pretreatment with toxin at pH 5.7), the shift in the voltage dependence of activation with a conditioning pulse was 1.8 times larger at pH 5.7 than at pH 7.4. The increase was accounted for by an essentially normal shift (mean = +13.1 mV;n=7) of theI Na(E) curve without a conditioning pulse at pH 5.7, but a minimal shift (mean = +0.4 mV) of theI Na(E) curve with a conditioning pulse. Permeability ratios at the two pH's reached the value of 0.5 at a membrane potential of +28 mV without a conditioning pulse, whereas this value was reached near −62 mV when a conditioning pulse preceded the test pulse. 5. These observations were interpreted as evidence that the binding of both toxin VI and toxin VII is greater at pH 5.7 than at pH 7.4, but that the increase with toxin VI is reversible on increasing the pH whereas the increase with toxin VII is not under the experimental conditions used.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00584030
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