Adenosine inhibits neutrophil degranulation in activated human whole blood: involvement of adenosine A2 and A3 receptors

Adenosine, acting via A2 receptors, is a potent inhibitor of neutrophil oxidative burst, but its effects and mechanisms of action on neutrophil degranulation have been less well characterized. We, therefore, investigated the effects of adenosine and its receptor-specific analogues on neutrophil degranulation in stimulated human whole blood. Adenosine dose-dependently inhibited the LPS- and TNF-alpha-induced release of the azurophilic granule proteins bactericidal/permeability-increasing protein, elastase, and defensins to approximately the same extent, with a maximum inhibition of 70 to 80% and an IC50 ranging from 14 to 24 microM. The inhibitory effects of adenosine were partially blocked by the A2 receptor antagonist 3,7-dimethyl-1-propargylxanthine, the A1/A2 antagonist 8(p-sulfophenyl)theophyline, and the A1/A3 antagonist xanthine amine congener, but not by the A1 antagonist 1,3-dipropyl-8-cyclopentylxanthine. The highly selective A3 agonist N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide and the nonselective agonist 2-chloroadenosine reduced degranulation more potently than the A1 agonist N6-cyclopentyladenosine. The inhibitory effects of N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide and 2-chloroadenosine were strongly reversed by xanthine amine congener, but were not affected by 8(p-sulfophenyl)theophyline. In addition, the adenosine kinase inhibitor GP515 attenuated degranulation via an adenosine-mediated mechanism. These data indicate that adenosine acts via A2 as well as A3 receptors to inhibit neutrophil degranulation and add to the anti-inflammatory potential of adenosine and adenosine-regulating agents in neutrophil-mediated tissue injury.