Inhibition of fMLP-triggered respiratory burst of human monocytes by adenosine: involvement of A3 adenosine receptor

Adenosine (Ado) is a potent anti-inflammatory agent acting oil a variety of cell functions, However, its effects on human monocytes have been less wel l characterized. We investigated the effect of Ado and its receptor-specifi c analogs on NADPH oxidase activity with the use of luminol-enhanced chemil uminescence (CL), Adenosine inhibited fMLP-triggered NADPH oxidase activity with a maximal inhibition of 55 +/- 5%. IB-MECA, a selective A3 Ado recept or agonist reduced fMLP triggered NADPH oxidase activity more potently than the A2 receptor agonist CGS 2180 HCl (CGS) and the A1 Ado receptor agonist N-2-phenylethyl-adenosine (R-PIA), The inhibitory effect of Ado was revers ed by neither the A1 Ado receptor antagonist 1,3-dipropyl-8(2-amino-4chloro phenyl)-xanthine (PACPX) nor the A2 Ado receptor antagonist 3,7-dimethyl-1- (2-propynyl)xanthine (DMPX), It was significantly reversed by the A1/A3 Ado receptor antagonist xanthine amine congener (XAC). Pretreatment of monocyt es by cytochalasin B reversed the effect of Ado but not of dibutyryl cAMP ( dBcAMP) on fMLP-CL response. BT 5720, a specific cAMP-dependent protein kin ase inhibitor completely counteracted the inhibition of NADPH oxidase activ ity by dBcAMP but not by Ado, Using flow cytometry, we observed that Ado di d not inhibit intracellular oxidative metabolism, whereas dBcAMP did, Furth ermore, the inhibition of NADPH oxidase activity by Ado was not mediated by changes in cytosolic calcium, These results demonstrated that Ado inhibite d NADPH oxidase activity via A3 Ado receptor independently of cAMP elevatio n or changes in calcium mobilization.