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Effect of halomethanes on intracellular calcium distribution in hepatocytes
Insights
Halomethanes decrease intracellular calcium in rat hepatocytes, with effects linked to liver metabolism. Some halomethanes, like carbon tetrachloride, paradoxically increase calcium levels.
Area of Science:
- Biochemistry
- Toxicology
- Cell Biology
Background:
- Halomethanes are known hepatotoxins.
- Intracellular calcium (Ca2+) homeostasis is crucial for cell function.
- Hepatic mixed function oxidase system metabolizes xenobiotics.
Purpose of the Study:
- To investigate the effect of hepatotoxic halomethanes on intracellular Ca2+ content in isolated rat hepatocytes.
- To determine the role of halomethane metabolism in Ca2+ dysregulation.
- To elucidate the origin of Ca2+ loss and the paradoxical Ca2+ increase induced by carbon tetrachloride (CCl4).
Main Methods:
- Isolated rat hepatocytes were exposed to various halomethanes (e.g., CBrCl3, CCl4, CHCl3).
- Intracellular Ca2+ content was measured.
- The role of the hepatic mixed function oxidase system was assessed.
- Effects on endoplasmic reticulum and mitochondria were investigated.
Main Results:
- Halomethane exposure caused a 40-60% decrease in intracellular Ca2+ content.
- The potency order (CBrCl3 > CCl4 > CHCl3) suggests metabolism by the hepatic mixed function oxidase system is required.
- Endoplasmic reticulum Ca2+ sequestration was impaired, but mitochondrial Ca2+ appeared to be the primary source of loss.
- High concentrations of CCl4 paradoxically increased cell Ca2+.
- CCl4 acutely increased cytoplasmic free Ca2+ (60 nM to 90 nM) via a non-metabolic, direct plasma membrane action.
Conclusions:
- Halomethanes induce significant intracellular Ca2+ depletion in hepatocytes, dependent on metabolic activation.
- Mitochondria play a key role in halomethane-induced Ca2+ loss.
- Carbon tetrachloride exhibits dual effects on Ca2+ homeostasis: metabolic-dependent depletion and direct, non-metabolic increase at higher concentrations.
Abstract:
Exposure of isolated rat hepatocytes to hepatotoxic halomethanes results in a 40-60% decrease in intracellular Ca2+ content. The order of halomethane potency (CBrCl3 CCl4 CHCl3) suggests that this effect requires halomethane metabolism by the hepatic mixed function oxidase system. Although the Ca2+ sequestering ability of the endoplasmic reticulum is destroyed by CBrCl3 and CCl4, it appears that much of the Ca2+ lost from the cell is mitochondrial in origin. Paradoxically, saturating concentrations of CCl4 cause a marked increase in cell Ca2+. CCl4 also causes an acute increase in cytoplasmic free Ca2+ (from about 60 nM to about 90 nM), but this effect does not appear to require CCl4 metabolism and is probably a result of direct action of CCl4 on the plasma membrane.