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To investigate the involvement of K+ efflux in apoptotic cell shrinkage, we monitored efflux of the K+ congener, 86Rb+, and cell volume during CD95-mediated apoptosis in Jurkat cells. An anti-CD95 antibody caused apoptosis associated with intracellular GSH depletion, a significant increase in 86Rb+ efflux, and a decrease in cell volume compared with control cells. Preincubating Jurkat cells with Val-Ala-Asp-chloromethylketone (VAD-cmk), an inhibitor of caspase proteases, prevented the observed 86Rb+ efflux and cell shrinkage induced by the anti-CD95 antibody. A wide range of inhibitors against most types of K+ channels could not inhibit CD95-mediated efflux of 86Rb+, however, the uptake of 86Rb+ by Jurkat cells was severely compromised when treated with anti-CD95 antibody. Uptake of 86Rb+ in Jurkat cells was sensitive to ouabain (a specific Na+/K(+)-ATPase inhibitor), demonstrating Na+/K(+)-ATPase dependent K+ uptake. Ouabain induced significant 86Rb+ efflux in untreated cells, as well as it seemed to compete with 86Rb+ efflux induced by the anti-CD95 antibody, supporting a role for Na+/K(+)-ATPase in the CD95-mediated 86Rb+ efflux. Ouabain treatment of Jurkat cells did not cause a reduction in cell volume, although together with the anti-CD95 antibody, ouabain potentiated CD95-mediated cell shrinkage. This suggests that the observed inhibition of Na+/K(+)-ATPase during apoptosis may also facilitate apoptotic cell shrinkage.