Cytoplasmic and mitochondrial Ca2+ signals in Hdh -HET and Hdh -KO cells. MEF cells were transfected with mitochondrial matrix-targeted inverse pericam and were loaded with Fura-2/AM to monitor [Ca2+]m and [Ca2+]c, respectively. A. [Ca2+]c and [Ca2+]m signals in HET5 and KO27 cells. In the images, the inverse pericam fluorescence is shown in the gray scale; the blue overlay indicates the sites of the ATP-induced [Ca2+]m elevations (upper row of images). The Fura2 images are presented as green-red overlay where the [Ca2+]c elevations are indicated by a green to red shift (lower row). The graphs show the calibrated [Ca2+]c signal and the pericam fluorescence response (ΔFmito-pcam, decrease normalized to the initial fluorescence level) to the sequential stimulation by low (loATP, 2 μM) and high (hiATP, 100 μM) ATP in the single cells marked by the numbers. B. Mean [Ca2+]c and [Ca2+]m signals in Hdh-HET and Hdh-KO MEF cells. Traces show the mean of at least triplicate measurements for each cell line. The data are representative of four independent experiments.