Extracellular calcium effects on cell viability and thiol homeostasis.

Studies of chemically induced cell injury and death, which have used as model systems freshly isolated rat hepatocytes and hepatocytes in culture, are discussed. An important model uses the omission of Ca2+ from the medium during rat hepatocyte incubations. Ca2+ omission induces an intense oxidative stress within hepatocytes incubated in a 95% O2 + 5% CO2 atmosphere. The relationship of calcium homeostasis to the parameters of oxidative stress is important to understanding the progression from reversible to irreversible injury. In the Ca2+ omission model, the vitamin E (Vit. E) content of hepatocytes is important for the prevention of cell injury. Recent studies with rat hepatocytes show that ruthenium red (RR) and La3+, which block Ca2+ translocation through the mitochondrial uniport, can prevent malondialdehyde (MDA) formation, reduced glutathione (GSH), and protein-SH loss, Vit. E loss, and LDH leakage induced by Ca2+ omission from the incubation medium. Ca2+ omission promoted a marked loss of mitochondrial transmembrane potential (delta phi) that was prevented by RR, EGTA, Vit. E, and desferrioxamine. The absence of extracellular Ca2+ may cause mitochondrial Ca2+ cycling that contributes to the observed oxidative stress, resultant loss of cell viability, and protein thiol homeostasis. Chemical agents including a glutathione-depleting agent, ethacrynic acid, and a redox cycling agent, adriamycin, increase the loss of cell viability caused by a Ca2(+)-free medium, but they have some additional effects on cellular processes. The demise of cell viability by the agent is also preventable by Vit. E supplementation. Ca2+ has a role in cell injury that appears to uniquely involve mitochondrial homeostasis.