CHME-5 cells were washed twice with Krebs-Ringer Hepes (KRH) solution (125 mM NaCl, 5 mM KCl, 1 mM MgSO4, 1 mM KH2PO4, 25 mM HEPES, 1 mM CaCl2, 6 mM glucose) and maintained for 15 min in the presence of ethidium bromide (15 µM, Sigma-Aldrich). ATP (1 mM) was added and the cells were imaged by using an Axiovert A1 microscope (Karl Zeiss) equipped with an Axio Cam ICm1 and a HPX 120 V lamp. Cells treated with ethidium but not with ATP were used as a control.