Investigation of calcium accumulation in mitochondria in cells undergoing apoptosis

One of the earliest features of apoptosis is the induction of the mitochondrial permeability transition (MPT) due to opening of a pore in the mitochondrial membrane. We estimated the Ca2+ capacity of mitochondria (a threshold level of Ca2+ that induces the release of this cation from mitochondria) during apoptosis. Incubation of thymocytes at 37 degrees C for 4 h equally decreased the mitochondrial Ca2+ capacity both in the presence and the absence of dexamethasone, an inducer of apoptosis. At the same time, dexamethasone significantly stimulated internucleosomal DNA fragmentation, which is one of the manifestations of apoptosis. Cyclosporin A prevented the time-dependent decrease in the Ca2+ capacity of mitochondria but did not affect internucleosomal DNA fragmentation. Therefore, induction of apoptosis assessed by internucleosomal DNA fragmentation is not mediated by the mitochondrial permeability transition.