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  • Heloderma horridum  (1)
  • Squid axon  (1)
  • 1
    ISSN: 1432-2013
    Keywords: Squid axon ; Voltage clamp ; K channels ; Scorpion toxin
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract We have studied the selective effects of noxiustoxin (NTX), a fraction of the venom of the scorpionCentruroides noxius, on the K currents of perfused squid giant axons using the voltage-clamp technique. At concentrations below 1.5 μM, NTX blocked K currents in a voltage-independent manner, with little effect on their turning-on and turning-off kinetics. Above 1.5 μM, the block by NTX became voltagedependent and could be partially removed by repetitive pulsing and strong depolarizations. Long repolarizations and more negative holding potentials favoured the slow restoration of channel block. Reduction of K currents by internally perfusing the fibers with solutions of low K+ concentration (200 mM), affected very little the removal of NTX-block during repetitive pulsing, suggesting that block removal depended on membrane potential and not on outward movements of K+ ions through open channels. In high extracellular K+ (300 mM) the blocking action of NTX was reduced and the instantaneous I–V characteristics showed a marked outward rectification. At 20 μM NTX, inward tail currents measured on step repolarizations to −70 mV were fully blocked, suggesting a direct interaction of the toxin with the open channel. The effects of the total venomCentruroides noxius Hoffmann was also studied. External application of 0.25 mg/ml of the venom caused a marked reduction of both Na and K currents, an effect similar to that of other scorpion venoms.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-1424
    Keywords: Cerebellar granule neuron ; Heloderma horridum ; Helothermine ; Potassium current ; Patch clamp
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract Helothermine, a recently isolated toxin from the venom of the Mexican beaded lizard Heloderma horridum horridum was tested on K+ currents of newborn rat cerebellar granule cells. In whole-cell voltageclamp experiments, cerebellar granule neurons exhibited at least two different K+ current components: a first transient component which is similar to an I A-type current, is characterized by fast activating and inactivating kinetics and blocked by 4-aminopyridine; a second component which is characterized by noninactivating kinetics, is blocked by tetraetylammonium ions and resembles the classical delayed-rectifier current. When added to the standard external solution at concentrations ranging between 0.1 and 2 μm helothermine reduced the pharmacologically isolated I A-type current component in a voltage- and dose-dependent way, with a half-maximal inhibitory concentration (IC50) of 0.52 μm. A comparison between control and nelothermine-modified peak transient currents shows a slowdown of activation and inactivation kinetics. The delayed-rectifier component inhibition was concentration dependent (IC50 = 0.86 μm) but not voltage dependent. No frequency-or use-dependent block was observed on both K+ current types. Perfusing the cells with control solution resulted in quite a complete current recovery. We conclude that helothermine acts with different affinities on two types of K+ current present in central nervous system neurons.
    Type of Medium: Electronic Resource
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