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Calcium currents in squid giant axon.

H Meves

    Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences
    |June 10, 1975
    PubMed
    Summary

    Calcium ions carry the inward current through sodium channels in squid giant axons, as demonstrated by voltage-clamp experiments. Tetrodotoxin blocked this calcium-mediated current, confirming its role.

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    Area of Science:

    • Neuroscience
    • Cellular Physiology
    • Ion Channel Function

    Background:

    • The sodium channel is primarily known for its role in action potential propagation.
    • Understanding the permeability of sodium channels to other ions is crucial for comprehending neuronal excitability.

    Purpose of the Study:

    • To investigate the role of calcium ions (Ca2+) in carrying inward currents through the sodium channel in squid giant axons.
    • To determine the relative permeabilities of the sodium channel for calcium, cesium, and sodium ions.

    Main Methods:

    • Voltage-clamp experiments were performed on intracellularly perfused squid giant axons.
    • Axons were bathed in Na-free solutions containing CaCl2 or MgCl2, with varying internal solutions (CsF or RbF with tetraethylammonium chloride).
    • The effects of tetrodotoxin on currents were analyzed.

    Main Results:

    • Depolarizing voltage steps elicited inward currents that reversed at large depolarizations.
    • Tetrodotoxin completely blocked the inward current and partially blocked an outward current.
    • No inward current was observed when MgCl2 replaced CaCl2 in the external solution.
    • Reversal potentials for the tetrodotoxin-sensitive current were measured under different ionic conditions.
    • Relative permeabilities of the sodium channel for Ca2+, Cs+, and Na+ were calculated using constant field equations.

    Conclusions:

    • The inward current observed in these experiments is carried by Ca2+ ions traversing the sodium channel.
    • The sodium channel exhibits significant permeability to Ca2+, in addition to its known permeability to Na+.
    • The findings provide insights into the complex ion selectivity and function of voltage-gated sodium channels.

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