MITOCHONDRIAL MODIFICATIONS DURING RAT THYMOCYTE APOPTOSIS - A STUDY AT THE SINGLE-CELL LEVEL

Apoptosis is an active type of cell death, occurring under several phy siological and pathological conditions, The role of cellular organelle s such as mitochondria in this process is still an open question. We r ecently described a new method to measure mitochondrial membrane poten tial in intact cells using dow cytometry. Using this method we studied alterations of mitochondrial membrane potential in a classical model of apoptosis, i.e., dexamethasone-treated rat thymocytes. Moreover, ap optosis induced by heat shock was also studied in the same cells. Mito chondria are functionally intact during the early phases of apoptosis, when DNA fragmentation occurs, whereas early alteration in their pote ntial and mass takes place after DNA damage. According to flow-cytomet ric analysis, the presence of a hypodiploid peak, an index of nuclear DNA loss, predates depolarization of mitochondrial membrane and decrea se of mitochondrial mass. Loss of plasma membrane integrity, which ind icates cell death and is revealed by the permeability to propidium iod ide, eventually follows. All these phenomena are more evident in dexam ethasone-treated cells than in heat-shocked cells, Thus, in these type s of apoptosis the involvement of mitochondria is apparently not a pri mary event. (C) 1994 Academic Press, Inc.