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Calcium-dependent repolarization in Paramecium.

P Brehm, K Dunlap, R Eckert

    The Journal of Physiology
    |January 1, 1978
    PubMed
    Summary
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    Intracellular calcium (Ca) in Paramecium is modulated by a Ca-activated potassium (K) conductance, which normally prevents all-or-none action potentials. EGTA injection reveals slow Ca conductance inactivation and Ca-dependent ciliary reversal.

    Area of Science:

    • Cellular Electrophysiology
    • Ion Channel Function
    • Ciliate Biology

    Background:

    • The electrical behavior of Paramecium is crucial for its motility and response to stimuli.
    • Intracellular calcium ions (Ca) are known to play a role in cellular signaling, but their precise influence on Paramecium's electrical activity requires further elucidation.

    Purpose of the Study:

    • To investigate the role of intracellular Ca in modulating the electrical behavior of the ciliate Paramecium caudatum.
    • To determine how Ca-activated conductances influence action potential generation and membrane potential dynamics.

    Main Methods:

    • Intracellular injection of EGTA (ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid) to buffer intracellular Ca.
    • Recording of membrane potential and current-voltage (I-V) relationships using current-passing methods.

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  • Manipulation of extracellular ion concentrations (Ca, K, Na) and injection of other ions (Cs, TEA).

    • Main Results:

    • EGTA injection converted graded Ca-dependent responses to all-or-none action potentials, indicating a shift from regenerative to spike behavior.
    • EGTA-induced action potentials exhibited slow inactivation of Ca conductance, leading to prolonged plateau potentials.
    • Ca-dependent ciliary reversal occurred at lower intracellular Ca concentrations than those activating the inferred Ca-dependent K conductance.

    Conclusions:

    • A Ca-activated K conductance in Paramecium acts to short-circuit inward Ca currents, preventing all-or-none action potentials under normal conditions.
    • EGTA injection unmasks slow Ca conductance inactivation, contributing to the observed plateau potentials.
    • The findings suggest a complex interplay between Ca influx, Ca-activated K conductance, and Ca-dependent effector systems like ciliary motility.