Transient increases in rhod-2 fluorescence are localized within mitochondria. A and B: fluorescence images of human ASM cells loaded with MitoTracker green (A) and rhod-2 AM (B). C: overlay of the two fluorescence images with examples of regions of interest (ROIs) chosen within the mitochondria (region 1), in the cytosol (region 2), and near mitochondria (region 3), where some background fluorescence could be detected. Bar = 10 μm. D: measurement of rhod-2 fluorescence expressed as [Ca2+] (in nM) in the ROIs shown in C. A transient increase in [Ca2+] in response to histamine (His; 10 μM) was observed for the ROI chosen within the mitochondria (region 1, solid line) and not for the ROIs placed in either the cytosol (region 2, dotted line) or near the mitochondria (region 3, dashed line). E and F: MitoTracker green-loaded cells excited at 488 nm showed significant specific fluorescence at 501 nm, corresponding to MitoTracker green (E), whereas excitation of the cells at 568 nm (rhod-2 AM excitation wavelength) did not show significant (above background) fluorescence at 590 nm (F). G and H: similarly, excitation of ASM cells loaded with 2.5 μM rhod-2 AM at 488 nm (G; Mitotracker green excitation wavelength) did not result in any nonspecific fluorescence at 501 nm (above background), whereas excitation at 568 nm resulted in specific fluorescence at 590 nm, corresponding to rhod-2 AM (H). Ex, excitation; Em, emission.