Adenosine inhibits fMLP-stimulated adherence and superoxide anion generation by human neutrophils at an early step in signal transduction.

The ability of physiological concentrations of adenosine to inhibit formylmethionylleucylphenylalanine (fMLP)-stimulated superoxide anion (O2-) generation, adherence and degranulation is well established in human neutrophils. However, the mechanism of inhibition remains to be determined. To better understand where adenosine blocks the fMLP signal transduction pathway, we examined the ability of adenosine to inhibit neutrophil adherence stimulated by phorbol myristate acetate (PMA), NaF, and A23187; these agents activate intermediate steps in fMLP signal transduction. Adenosine (0.1-100 microM) did not inhibit adherence mediated by these receptor-independent agonists or NaF- and A23187-mediated O2- production. Additionally, NaF and A23187 completely abrogated adenosine inhibition of fMLP-stimulated neutrophil adherence. We also found that pertussis toxin (5 and 10 microM) completely inhibited fMLP-induced neutrophil adherence and O2- generation, indicating that both processes are G protein mediated. Furthermore, fMLP-stimulated GTPase activity in neutrophil membrane preparations was significantly inhibited by adenosine (1 and 10 microM) or 5'-N-ethylcarboxamidoadenosine (1 microM) (NECA). These data indicate that adenosine inhibits a G-protein-dependent pathway of fMLP stimulation by uncoupling G proteins from the fMLP receptor. This may be a general mechanism of adenosine inhibition of cell-surface receptor-mediated signals as both fMLP- and C5a-stimulated neutrophil adherence were inhibited at similar concentrations.