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Active calcium sequestration by intestinal microsomes. Stimulation by increased calcium load.

M J Rubinoff, H N Nellans

    The Journal of Biological Chemistry
    |July 5, 1985
    PubMed
    Summary
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    The endoplasmic reticulum in rat small intestine actively sequesters calcium, a process enhanced by ATP and modulated by dietary calcium. This suggests a key role for the endoplasmic reticulum in regulating intracellular calcium levels.

    Area of Science:

    • Cell Biology
    • Molecular Physiology
    • Biochemistry

    Background:

    • Intracellular calcium homeostasis is crucial for various cellular processes.
    • The endoplasmic reticulum (ER) is a major intracellular calcium store.
    • Mechanisms of ER calcium uptake and release in the small intestine are not fully elucidated.

    Purpose of the Study:

    • To investigate the characteristics of calcium uptake by the endoplasmic reticulum in rat small intestine.
    • To determine the role of ATP and other factors in ER calcium sequestration.
    • To assess the impact of dietary calcium load on ER calcium transport.

    Main Methods:

    • Isolation of an endoplasmic reticulum-enriched membrane fraction from rat small intestine.
    • Utilizing a rapid filtration technique to measure calcium uptake.

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  • Employing kinetic studies to determine transport parameters (K0.5, Jmax).
  • Assessing the effects of ATP, magnesium, vanadate, calmodulin, and calcium ionophore A23187.

    • Main Results:

    • Calcium sequestration into the ER-enriched fraction was stimulated by ATP and magnesium.
    • Vanadate inhibited ATP-stimulated calcium uptake, while calmodulin had no effect.
    • Calcium release was induced by the calcium ionophore A23187.
    • Kinetic analysis showed a K0.5 of 62.5 nM Ca and Jmax of 1.4 nmol Ca/mg protein/min for ATP-stimulated uptake.
    • A high dietary calcium load increased maximal uptake by 80% without altering affinity.

    Conclusions:

    • The endoplasmic reticulum possesses a high-affinity, ATP-dependent calcium transport system in the rat small intestine.
    • This system plays a significant role in cytosolic calcium sequestration.
    • Extracellular calcium availability modulates the calcium buffering capacity of the endoplasmic reticulum.