Selective induction of IL-8 by ATP and UDP in A549 cells. (A) A549 cells were incubated for 8 hours in serum-free DMEM containing indicated concentrations of ATP, ADP, UTP, and UDP, and IL-8 levels in cell-culture supernatants were measured with the use of enzyme-linked immunosorbent assay (ELISA). (B) Because only ATP and UDP induce significant IL-8 production, multiple cytokines were measured in the cells stimulated with indicated concentrations of ATP and UDP for 8 hours (n = 3). ATP and UDP concentrations were progressively increased, as indicated in the bar graphs. ^*P < .05; control (0) compared with ATP or UDP at all concentrations, respectively.