p21-IRS increases cytosolic Ca²⁺ but decreases mitochondrial Ca²⁺. U937 cells were incubated with fluorescent dyes fluo-3 (A) or rhod-2 (B), prior to the addition of the indicated compounds. (C) Iberiotoxin, a specific Ca²⁺-dependent K⁺ channel blocker, partially blocks p21-IRS–induced apoptosis. Cells were preincubated with medium (open bar), 10 μM iberiotoxin (black bar), or 15 μM iberiotoxin (striped bar) prior to the addition of p21-IRS.