DFP-mediated relocation of iron between cellular compartments: from nuclei to mitochondria. H9C2 cells were loaded with the nuclear iron sensor H-CALG that had been precomplexed to iron (H-CALG-Fe), followed by loading with the mitochondrial iron sensor RPA. The double-labeled cells were then exposed to 50μM DFP and epifluorescence images were recorded every 5 minutes. Representative fields of cell fluorescence observed under settings for fluorescein (A,B) and rhodamine (C,D). Images are shown at time 0 (A,C) and after incubation for 1 hour in the presence of 50μM DFP (B,D). (E) Mean fluorescence values plus or minus SD in r.u. of 5 cells per field, calculated for each time-point image and normalized to the initial fluorescence (f/f₀), representing cells incubated with 50μM DFP (circles) and with no addition (squares). Fluorescence of H-CALG is indicated by filled symbols (● and ■), and fluorescence of RPA is indicated by open symbols (○ and □). The scheme illustrates entry of DFP into the cytosol (C), nuclei (N) containing H-CALG-Fe (iron donor) and mitochondria (M) containing RPA (iron acceptor), and transfer of iron from nuclei to mitochondria. E refers to endosomes.