Mechanism of chemical-induced toxicity: II. Role of extracellular calcium

Abstract

Previous studies disagree as to if chemical-induced cell death is caused by the influx and accumulation of extracellular Ca 2+. To determine the role of extracellular Ca 2+ in toxic cell death, the viability (leakage of intracellular K + and lactate dehydrogenase) and total Ca 2+ content of isolated hepatocytes incubated in the presence or absence of extracellular Ca 2+ were determined during a toxic insult with bromobenzene, ethyl methanesulfonate (EMS), Ca 2+ ionophore A23187, and adriamycin (ADR) in combination with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). The present study utilized the dibutyl phthalate separation technique which enabled the analysis of only viable hepatocytes for changes in intracellular Ca 2+ and K + content during toxic cell injury. The three chemical treatments, bromobenzene, EMS, and ADR-BCNU, each caused an accelerated loss of viability in hepatocytes incubated without extracellular Ca 2+ as compared to cells incubated with Ca 2+. Furthermore, the total Ca 2+ content of viable hepatocytes incubated in the presence of extracellular Ca 2+ did not increase during chemically induced cell injury as compared to control cells. In fact, a significant decline in total cellular Ca 2+ was observed in viable hepatocytes incubated in Ca 2+-free medium during toxic cell injury. Treatment with Ca 2+ ionophore A23187 was also toxic to hepatocytes incubated in the presence or absence of extracellular Ca 2+. At high concentrations of ionophore (20 μ m or 4 μg/10 6 cells), cell death was accelerated in hepatocytes incubated with Ca 2+ as compared to cells incubated in Ca 2+-free medium. In contrast, after treatment with lower concentrations of ionophore (10 μ m or 2 μg/10 6 cells), the rate of cell death was reversed with hepatocytes incubated without extracellular Ca 2+ dying first. Thus, depending on the concentration of A23187 and the time of exposure, the presence of extracellular Ca 2+ can be shown either to accelerate or protect against cell death. Surprisingly, reversible and irreversible cell injury were not observed in hepatocytes incubated with extracellular Ca 2+ and 2 μ m A23187 though this treatment resulted in an 800% increase in total intracellular Ca 2+ content. We conclude that chemical-induced hepatic cell death is not caused by an increase in total cellular Ca 2+ resulting from the influx of extracellular Ca 2+.