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Parotid microsomal Ca2+ transport. Subcellular localization and characterization.

P Kanagasuntheram, T S Teo

    The Biochemical Journal
    |December 15, 1982
    PubMed
    Summary

    Rat parotid gland homogenates show ATP-dependent calcium uptake primarily in the endoplasmic reticulum. This calcium transport system, influenced by sodium and potassium, may regulate intracellular calcium levels.

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    Area of Science:

    • Cell Biology
    • Biochemistry
    • Physiology

    Background:

    • Intracellular calcium homeostasis is crucial for various cellular functions.
    • Understanding the mechanisms of calcium transport in exocrine glands like the parotid gland is important.

    Purpose of the Study:

    • To investigate the localization and characteristics of ATP-dependent calcium (Ca2+) uptake in rat parotid gland subcellular fractions.
    • To identify the specific organelle responsible for this calcium transport.

    Main Methods:

    • Differential centrifugation was used to isolate mitochondrial, heavy microsomal, and light microsomal fractions from rat parotid gland homogenates.
    • ATP-dependent 45Ca2+ uptake assays were performed on these fractions.
    • Enzyme activity (NADPH-cytochrome c reductase) was measured to identify the endoplasmic reticulum fraction.

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  • The effects of oxalate, varying pH, sodium (Na+), potassium (K+), and vanadate on Ca2+ uptake were assessed.

    • Main Results:

    • ATP-dependent 45Ca2+ uptake was highest in the heavy microsomal fraction, which also showed high NADPH-cytochrome c reductase activity, indicating endoplasmic reticulum localization.
    • Calcium uptake was sustained by oxalate and was insensitive to mitochondrial calcium transport inhibitors.
    • Sodium and potassium significantly stimulated Ca2+ uptake, with K+ showing a greater effect (260% stimulation) than Na+ (120% stimulation) at optimal concentrations.
    • The Michaelis constant (Km) for Ca2+ uptake was 3.7 microM free Ca2+ and 0.19 mM-ATP.
    • Vanadate inhibited Ca2+ uptake, with 60 microM vanadate causing a 50% inhibition.

    Conclusions:

    • The ATP-dependent Ca2+ transport system in rat parotid gland homogenates is primarily located on the endoplasmic reticulum.
    • This system plays a potential role in maintaining intracellular free Ca2+ concentrations within a narrow physiological range.
    • The findings highlight the contribution of the endoplasmic reticulum to calcium regulation in exocrine secretion.