Role of sphingosine kinase in barrier-enhancing signaling by APC and thrombin. Subconfluent cells were incubated in serum-free growth medium containing 0.4% BSA (A) or in full-growth medium (B) for 3 hours with carrier control (▪) or with APC (□) or 40 pM thrombin (▦). The indicated concentrations of sphingosine kinase inhibitors DHS or DMS were added 10 minutes prior to the agonists (means ± SD, n = 3, *P < .05 and **P < .005 compared to carrier control).