α7 nAChR activation induces Ca2+ release from intracellular Ca2+ stores. Human NK cells were stimulated with IL-12/IL-18/IL-15 for 48 h, loaded with Fluo-3/AM, and analyzed by laser scanning microscopy as described under “Experimental Procedures.” The images were obtained immediately before (0 s) and after 80 s (80 s) of exposure to the drugs. The color scale represents the relative fluorescence intensity of Fluo-3/AM-loaded cells, with black representing the lowest and white the highest [Ca2+]i. PNU-282987 (30 μm) was added to cells preincubated with 5 mm EGTA-containing Ca2+-free buffer for 5 min (A and C) or with 5 μm BAPTA-AM for 15 min (B and D). Relative fluorescence values were calculated as F/F0 to represent the relative [Ca2+]i levels. Each curve represents the variations of intracellular Ca2+ in one cell, and the responses of three representative cells are shown for each condition. The arrow indicates the time of drug application. E, bars represent mean ± S.E. of the relative fluorescence intensity of each cell (measured at the maximum value) from each condition. Dashed line represents the mean value relative fluorescence intensity in cells exposed to 30 μm PNU-282987 (control). The data are representative of three independent experiments performed with NK cells from different donors (n = 3). At least 100 cells/condition were analyzed. **, p < 0.01; paired t test.