Inosine binds to A3 adenosine receptors and stimulates mast cell degranulation

We investigated the mechanism by which inosine, a metabolite of adenosine that accumulates to > 1 mM levels in ischemic tissues, triggers mast cell degranulation. Inosine was found to do the following: (a) compete for [I-125]N-6-aminobenzyladenosine binding to recombinant rat A(3) adenosine receptors (A(3)AR) with an IC50 of 25 +/- 6 mu M; (b) not bind to A(1) or A(2A) ARs; (c) bind to newly identified A(3)ARs in guinea pig lung (IC50 = 15 +/- 4 mu M); (6) lower cyclic AMP in HEK-293 cells expressing rat A(3)ARs (ED50 = 12 +/- 5 mu M); (e) stimulate RBL-2H3 rat mast-like cell degranulation (ED50 = 2.3 +/- 0.9 mu M); and (f) cause mast cell-dependent constriction of hamster cheek pouch arterioles that is attenuated by A(3)AR blockade, Inosine differs from adenosine in not activating A(2A)ARs that dilate vascular smooth muscle and inhibit mast cell degranulation. The A(3) selectivity of inosine may explain why it elicits a monophasic arteriolar constrictor response distinct from the multiphasic dilator/constrictor response to adenosine. Nucleoside accumulation and an increase in the ratio of inosine to adenosine may provide a physiologic stimulus for mast cell degranulation in ischemic or inflamed tissues.