Fig. 2. Inhibition of PI3K signaling inhibits chemotaxis in shallow gradients but not in steep gradients. (A) Chemotactic activity was measured for pi3k-null cells and wild-type cells in the absence and presence of 50 µM LY294002 using (A) the small-population assay, (B) a Zigmond chamber or (C) the pipette assay. The cAMP gradients to which the cells are exposed were calculated using the equations presented in the Materials and Methods. (D) The chemotaxis index as function of the absolute gradient is shown. The data are the means of three experiments involving about 20 populations or cells analyzed in each experiment.