Adenosine A(3) receptors are reported to couple negatively to adenylyl cyclase (AC) but their mediation of anti-inflammatory effects in human eosinophils prompted us to investigate their coupling to AC. The A(3)-selective agonists IB-MECA and Cl-IB-MECA evoked a concentration-dependent generation of cAMP (EC50, 3.2 and 1.8 muM, respectively) and were more potent than the A,A agonist CGS 21680 (EC50, = 15.4 muM) and adenosine (EC50 = 19.2 muM). The cAMP response was additive to that produced by forskolin (10 muM). The effect of IB-MECA was insensitive to A(1) and A(2A) receptor antagonists. but was antagonized by the A(3)-selective antagonist MRS 1220 (0.1-2.5 muM) in a competitive manner. The estimated K-B of 190 nM was. however, atypical. The cyclo-oxygenase inhibitor. indomethacin, had no effect on the cAMP response. A general inverse relationship between cAMP generation and inhibition of degranulation was seen. We conclude that in human eosinophils. an atypical forth of A, receptors positively coupled to AC may exist. The resulting cAMP generation may underlie the anti-inflammatory actions of A3 agonists in eosinophils. (C) 2002 Elsevier Science (USA). All rights reserved.