Fig. 4. Capacity of ionomycin and TPA to rescue IGF-I-mediated survival in Null cells. Null cells were placed in suspension culture as described in Fig. 2. 10% FBS, IGF-I (100 ng/ml), or ionomycin (2 µM) and TPA (100 ng/ml) were added as indicated. Caspase 3 activity was measured 20 hours later.