The loss of cell volume is a fundamental feature of apoptosis. We have previously shown that DNA degradation and caspase activity occur only in cells which have shrunken as a result of potassium and sodium efflux (Bortner, C. D., Hughes, F. M., Jr., and Cidlowski, J. A. (1997) J. Biol. Chem. 272, 32436-32442). Furthermore, maintaining a normal intracellular potassium concentration represses the cell death process by inhibiting the activity of apoptotic nucleases and suppressing the activation of effector caspases (Hughes, F. M., Jr., Bortner, C. D. Purdy, G. D., and Cidlowski, J. A. (1997) J. Biol. Chem. 272, 30567-30576). We have now investigated the relationship between cell shrinkage, ion efflux, and changes in the mitochondrial membrane potential, in addition to the role of caspases in these apoptotic events. Treatment of Jurkat cells with a series of inducers which act via distinct signal transduction pathways, resulted in all of the cell death characteristics including loss of cell viability, cell shrinkage, K(+) efflux, altered mitochondrial membrane potential, and DNA fragmentation. Interestingly, only cells which shrunk had a loss of mitochondrial membrane potential and the other apoptotic characteristics. Treatment of Jurkat cells with an anti-Fas antibody in the presence of the general caspase inhibitor z-VAD, abrogated these features. In contrast, when Jurkat cells were treated with either the calcium ionophore A23187 or thapsigargin, z-VAD failed to prevent cell shrinkage, K(+) efflux, or changes in the mitochondrial membrane potential, while effectively inhibiting DNA degradation. Treatment of Jurkat cells with various apoptotic agents in the presence of either the caspase-3 inhibitor DEVD, or the caspase-8 inhibitor IETD also blocked DNA degradation, but failed to prevent other characteristics of apoptosis. Together these data suggest that the cell shrinkage, K(+) efflux, and changes in the mitochondrial membrane potential are tightly coupled, but occur independent of DNA degradation, and can be largely caspase independent depending on the particular signal transduction pathway.