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 B-4 (LTB(4)) synthesis in heparinized human whole blood, pretreated with lipopolysaccharide (LPS) and tumour necrosis factor alpha (TNF-alpha) 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-alpha was dose-dependently inhibited by adenosine analogues in the following order of potency; 5'(N-ethyl)carboxamidoadenosine (NECA)congruent to CGS 21680 > 2-Cl-adenosine > N-6-cyclopentyladenosine (CPA), indicating the involvement of the adenosine A(2) receptor subtype. The IC50 values for NECA, CGS21680, 2-Cl-adenosine, and CPA were 6 nM, 9 nM, 180 nM, 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-alpha-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-alpha-treated whole blood, stimulated by FMLP. 5 LTB(4) 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-alpha-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.