Tobias Jacob, Rebecca. Lee, Joanne Engel and Terry Machen (2002). Modulation of cytosolic Ca2+ concentration in airway epithelial cells by Pseudomonas aeruginosa.  Infection and Immunity. 70, 6399-6408.       Modulation of cytosolic (intracellular) Ca2+ concentration (Cai) may be an important host response when airway epithelial cells are exposed to Pseudomonas aeruginosa. We measured Cai in Calu-3 cells exposed from the apical or basolateral surface to cytotoxic and noncytotoxic strains of P. aeruginosa. Apical addition of either noncytotoxic strains or cytotoxic strains failed to affect Cai over a 3-h time period, nor were changes observed after basolateral addition of noncytotoxic strains. In contrast, basolateral addition of cytotoxic strains caused a slow increase in Cai from 100 nM to 200 to 400 nM. This increase began after 20 to 50 min and persisted for an additional 30 to 75 min, at which time the cells became nonviable. P. aeruginosa-induced increases in Cai were blocked by the addition of the Ca channel blocker La3+ to the basolateral but not to the apical chamber. Likewise, replacing the basolateral but not the apical medium with Ca-free solution prevented P. aeruginosa-mediated changes in Cai. With isogenic mutants of PA103, we demonstrated that the type III secretion apparatus, the type III-secreted effector ExoU, and type IV pili were necessary for increased Cai. We propose that translocation of ExoU through the basolateral surface of polarized airway epithelial cells via the type III secretion apparatus leads to release of Ca stored in the endoplasmic reticulum and activation of Ca channels in the basolateral membranes of epithelial cells.