FIG. 2.

Composite calcium calibration curve. Coverslips with J774 cells were loaded with fluo 3 and SNARF-1 as described in the text. The cells were equilibrated in HEPES-buffered Ringer solution (pH 7.4) containing different concentrations of calcium, 1 mM EGTA, and the Ca²⁺ ionophore A23187 (1 μM) for 30 min at 37°C. The images were collected and analyzed for fluorescence intensity as described in the text. The average fluo 3/SNARF-1 ratio (520/600) was obtained for each concentration of Ca²⁺ and used for calculation of cytosolic Ca²⁺ levels in J774 cells after infection. The results are the mean ± standard error for 13 to 17 cell preparations.