Adenosine A 2 receptor‐induced inhibition of leukotriene B 4 synthesis in whole blood ex vivo

1 Engagement of adenosine A 2 receptors suppresses several leukocyte functions. In the present study, we examined the effect of adenosine on the inhibition of leukotriene B4 (LTB4) synthesis in heparinized human whole blood, pretreated with lipopolysaccharide (LPS) and tumour necrosis factor α (TNF‐α) and stimulated with the chemotactic peptide, N‐formyl‐Met‐Leu‐Phe (FMLP). 2 The FMLP‐induced synthesis of LTB 4 in whole blood pretreated with LPS and TNF‐α was dose‐dependently inhibited by adenosine analogues in the following order of potency; 5′(N‐ethyl)carbox‐ amidoadenosine (NECA) ≅ CGS 21680 > 2‐Cl‐adenosine > N 6 ‐cyclopentyladenosine (CPA), indicating the involvement of the adenosine A 2 receptor subtype. The IC 50 values for NECA, CGS 21680, 2‐Cl‐ adenosine, and CPA were 6nM, 9nM, 180nM, and 990 nM, respectively. 3 Dipyridamole, an agent that blocks the cellular uptake of adenosine by red cells and causes its accumulation in plasma, also inhibited the synthesis of LTB 4 in LPS and TNF‐α‐treated whole blood stimulated by FMLP; moreover, this inhibition was reversed upon addition of adenosine deaminase. 4 A highly selective antagonist of the adenosine A 2 receptor, 8‐(3‐chlorostyryl)caffeine (CSC), reversed the inhibition of LTB 4 synthesis by 2‐Cl‐adenosine and dipyridamole in LPS and TNF‐α‐treated whole blood, stimulated by FMLP. 5 LTB4 synthesis in whole blood originates predominantly from neutrophils and to a lesser extent from monocytes. 2‐Cl‐adenosine also inhibited the synthesis of LTB 4 induced by FMLP in these isolated LPS and TNF‐α‐treated cells; however, 2‐Cl‐adenosine was a more potent inhibitor of LTB 4 synthesis in neutrophils than monocytes. 6 The present data demonstrate that adenosine, acting through A 2 receptors, exerts a potent inhibitory effect on the synthesis of LTB 4 and thus contribute to the understanding of its anti‐inflammatory properties.