Leukocyte arrest and adhesion strengthening is critically dependent on PLC, calcium influx, and calmodulin. (A) U937-FPR cells pretreated with the indicated inhibitors were infused at 1 dyne/cm² into parallel plate flow chambers coated with VCAM-1 ± SDF-1. The number of cells arrested after 2 minutes is shown. Significant differences within groups (± SDF-1) are indicated with brackets (P < .05). U73122, SKF96365, and W7 inhibited the response to SDF-1 (asterisks, P < .05). The inset shows data from assays in which the adhesion surface was coated with VCAM-1 at 50% density. (B) U937-FPR cells pretreated with the indicated inhibitors were allowed to adhere for 2 minutes under static conditions to parallel plate flow chambers coated with VCAM-1 ± SDF prior to the introduction of flow at incrementally increasing shear rates. The percentage of cells remaining attached at each shear rate is shown. U73122, SKF96365, and W7 significantly inhibited adhesion strengthening to VCAM-1 alone (*), or in combination with SDF-1 (#) (P < .05). (C) U937-FPR cells pretreated with the indicated inhibitors were infused for 6 minutes at 1 dyne/cm² into parallel plate flow chambers containing monolayers of human aortic endothelial cells. Monolayers were unactivated or treated with IL-1β (30 U/mL) for 24 hours. SDF-1 was adsorbed onto some of the monolayers immediately prior to infusion of leukocytes. The number of leukocytes arrested on the endothelial monolayers is shown (mean ± SEM, n ≥ 3 independent experiments). PTx, U73122, and BAPTA-AM, but not U0126, treatment of U937 cells induced significant differences (asterisks, P < .05) in arrest on monolayers with adsorbed SDF-1.