Fig. 3. ATP release upon treatment of multicellular tumor spheroids with DC electrical fields. ATP release after electrical field treatment (750 Vm^-1, 60 seconds) was evalulated by luciferin/luciferase-based bioluminescence. Between 30 and 50 tumor spheroids were treated with an electrical field pulse and subsequently an 150 µl aliquot of the supernatant was analyzed in the bioluminescence apparatus. Pretreatment for 30 minutes with the anion channel inhibitors niflumic acid (200 µM), NPPB (50 µM) and tamoxifen (50 µM) significantly inhibited the electrical-field-induced ATP release, indicating that ATP efflux occurs via anion channels.