Inhibition of Na+/K+-ATPase may be one mechanism contributing to potassium efflux and cell shrinkage in CD95-induced apoptosis

To investigate the involvement of K(+) efflux in apoptotic cell shrinkage, we monitored efflux of the K(+) congener, (86)Rb(+), 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 (86)Rb(+) 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 (86)Rb(+) 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 (86)Rb(+), however, the uptake of (86)Rb(+) by Jurkat cells was severely compromised when treated with anti-CD95 antibody. Uptake of (86)Rb(+) in Jurkat cells was sensitive to ouabain (a specific Na(+)/K(+)-ATPase inhibitor), demonstrating Na(+)/K(+)-ATPase dependent K(+) uptake. Ouabain induced significant (86)Rb(+) efflux in untreated cells, as well as it seemed to compete with (86)Rb(+) efflux induced by the anti-CD95 antibody, supporting a role for Na(+)/K(+)-ATPase in the CD95-mediated (86)Rb(+) 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.