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Ca2+ homeostasis in unstimulated platelets.

L F Brass

    The Journal of Biological Chemistry
    |October 25, 1984
    PubMed
    Summary

    Unstimulated platelets maintain low calcium levels using influx limits, active efflux, and sequestration in the dense tubular system. This calcium homeostasis is crucial for platelet function.

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    Area of Science:

    • Hematology
    • Cellular Physiology
    • Biochemistry

    Background:

    • Platelets maintain a low cytosolic free Ca2+ concentration and a steep plasma membrane Ca2+ gradient.
    • The precise mechanisms governing this calcium homeostasis in unstimulated platelets are not fully understood.

    Purpose of the Study:

    • To investigate the kinetics of calcium (Ca2+) exchange in intact unstimulated platelets.
    • To identify and characterize intracellular Ca2+ pools involved in maintaining platelet calcium gradients.

    Main Methods:

    • Utilized 45Ca2+ to examine Ca2+ exchange kinetics and Quin2 to measure cytosolic free Ca2+ concentration.
    • Employed differential permeabilization with digitonin to localize Ca2+ pools.
    • Investigated the role of mitochondria and dense tubular system using specific inhibitors and metabolic blockers.

    Main Results:

    • Identified two distinct intracellular exchangeable Ca2+ pools: a rapidly exchanging pool (t 1/2, 17 min) in the cytosol and a slowly exchanging pool (t 1/2, 300 min) associated with the dense tubular system and mitochondria.
    • The size of the slowly exchanging pool varied with extracellular Ca2+ concentration, while the rapidly exchanging pool remained relatively constant.
    • Inhibition of mitochondrial Ca2+ uptake did not affect the slowly exchanging pool, but metabolic inhibitors and trifluoperazine reduced its size and increased cytosolic Ca2+.

    Conclusions:

    • Platelet Ca2+ homeostasis is maintained by limiting Ca2+ influx, active Ca2+ efflux, and sequestration.
    • The dense tubular system, not mitochondria, is the primary internal site for Ca2+ sequestration in unstimulated platelets.
    • These findings elucidate key mechanisms of calcium regulation in platelets, essential for their physiological functions.

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