Role of Na+/Ca2+Exchanger in Preventing Na+Overload and Hepatocyte Injury: Opposite Effects of Extracellular and Intracellular Ca2+Chelation

Abstract

We have previously shown that an increase of intracellular Na+occurs in isolated rat hepatocytes undergoing ATP depletion and that Na+accumulation is associated with an uncontrolled influx of Ca2+through the activation in reverse mode of the Na+/Ca2+exchanger. In the present study we have investigated the relationship between alterations of Na+and Ca2+homeostasis and hepatocyte killing using treatments which differentially chelate extracellular or intracellular Ca2+. Chelation of extracellular Ca2+by ethylene glycol bis-(β-aminoethyl ether) N,N,N′,N′-tetraacetic acid (EGTA) potentiated Na+overload and cell killing induced in isolated rat hepatocytes by hypoxia or menadione. Similar effects were also observed when Na+accumulation was induced by the combined addition of Na+ionophore monensin and the inhibition of plasma membrane Na+/K+ATPase by ouabain. Conversely, the use of the intracellular Ca2+chelator EGTA acetoxymethyl ester (EGTA/AM) reduced Na+overload and hepatocyte death induced by hypoxia or cell treatment with menadione or monensin plus ouabain. The effects of EGTA/AM were reverted in the presence of bepridil, an inhibitor of Na+/Ca2+exchanger. Altogether these results indicated that differential chelation of intracellular or extracellular Ca2+influences in opposite ways hepatocyte killing due to ATP depletion by modulating intracellular Na+levels through the reversed activity of the Na+/Ca2+exchanger.