Dominant-active Cdc42 and Rac enhance calcium signaling, while dominant-negative Cdc42 and Rac dampen calcium signaling. The calcium responses of uninfected RBL-2H3 cells (a and d), cells infected with vaccinia virus expressing wild-type Cdc42 (b and e), wild-type Rac (c and f), Cdc42V12 (g and k), Cdc42N17 (h and l), RacV12 (i and m), and RacN17 (j and n), were compared with those of cells containing empty vector. Cells were infected for 6 h at 37°C and loaded with Indo-1. Overall calcium responses were measured in a fluorescence spectrophotometer in response to stimulation with 100 ng/ml DNP/BSA (a–c and g–j, unfilled arrows). To specifically measure the release of calcium from ER stores (d–f and k–n), loaded cells were treated with 4 mM EDTA (filled arrows) and then stimulated with 100 ng/ml DNP/BSA (unfilled arrows). For each form of Cdc42 and Rac, measurements of the calcium responses in the absence and presence of EGTA were performed concurrently. Traces shown are representative of three independent experiments.