Energy-dependent calcium sequestration activity in rat liver microsomes

Area of Science:

• Biochemistry
• Cell Biology
• Physiology

Background:

• Mammalian liver cells regulate intracellular calcium levels.
• Microsomes, particularly the endoplasmic reticulum, are implicated in calcium homeostasis.

Purpose of the Study:

• To characterize the MgATP-dependent calcium sequestering activity of rat liver microsomes.
• To investigate factors influencing this activity, including substrate requirements, inhibitors, and physiological variations.

Main Methods:

• Enzymatic assays measuring calcium uptake in isolated rat liver microsomes.
• Use of specific inhibitors and varying ionic conditions.
• Separation of smooth and rough endoplasmic reticulum.
• Analysis of microsomal phospholipid fatty acid composition.

Main Results:

• Calcium uptake is MgATP-dependent, requires oxalate as a trapping agent, and is temperature-dependent.
• Kinetic parameters (Vmax, Km for Ca2+ and ATP) were determined.
• Activity is higher in male rats and weanling rats compared to adult females, correlating with differences in microsomal phospholipid fatty acid composition (linoleic and palmitic acids).
• The system is insensitive to mitochondrial inhibitors (azide, oligomycin) but sensitive to p-chloromercuribenzoate and mersalyl.

Conclusions:

• Rat liver microsomes possess a distinct MgATP-dependent calcium pump.
• Endoplasmic reticulum and plasma membrane play roles in regulating cytoplasmic calcium.
• Sex- and age-related differences in calcium uptake activity may be linked to microsomal membrane composition.