Changes in mitochondrial matrix volume were studied both on isolated mitochondria and in situ on CHME 5 human microglia and monoblastoid U 937 cells using multiparametric flow cytometric analysis. The use of specific effectors of mitochondrial activity (oligomycin and KCN) allowed the demonstration, on whole cells, of a strict correlation between light scattering and mitochondrial volume changes: mitochondrial swelling induced a concomitant increase in forward scattering, and decrease in side scattering of the cell population.
The technique was applied to the study of the early phases of acetyl-ceramide-induced apoptosis, which has been associated with mitochondrial dysfunction in several cellular systems. Acetyl-ceramide caused a marked swelling of isolated rat liver mitochondria. Scatter modifications were also observed in both cell lines during the first hour of incubation with acetylceramide and were accompanied by an increase in DiOC 6 (3) fluorescence. The results imply that mitochondrial volume changes can be followed using flow cytometry and eventually used to assist in the interpretation of mitochondrial membrane potential variations obtained from fluorescence measurements. By applying this technique to 2 different cell lines, we demonstrated that mitochondrial swelling occurs during the early phases of acetyl-ceramide treatment, but that the induction of apoptosis is cell type-dependent.