Fig. 2. Involvement of purinergic receptor stimulation in the electrical-field-induced Ca²⁺ response. (A) Tumor spheroids were repetitively treated with 10 µM ATP, which elicited a Ca²⁺ response with declining amplitude during repetitive application of ATP. Subsequent application of an electrical field (750 Vm^-1, 60 seconds) failed to raise [Ca²⁺]_i, indicating interference with ATP-mediated signaling pathways. (B) Pretreatment of tumor spheroids for 60 minutes with 300 µM suramin, which inhibits purinergic receptor activation, blunted the electrical-field-induced Ca²⁺ response. (C) Inhibition of Ca²⁺ elevation was likewise achieved after incubation for 30 minutes with the ATP scavenger apyrase (2 U/ml). The tumor spheroids were treated with electrical field during the time indicated by the horizontal solid bar.