The A3 adenosine receptor is the unique adenosine receptor which facilitates release of allergic mediators in mast cells

Mast cells release the mediators of the immediate hypersensitivity reaction. Adenosine is known to modulate this process, but the receptor responsible for this is not the classical A1 or A2 adenosine receptors. This study was undertaken to determine whether the unique adenosine receptor (AR) previously postulated in a cultured mast cell line (RBL-2H3 cells) is the recently cloned A3AR. The receptors were quantitated by the agonist 125I-labeled APNEA (aminophenylethyladenosine), an A3AR agonist, which yielded Bmax and Kd values of 826 fmol/mg protein and 34 nM, respectively. A variety of adenosine analogs competed for 125I-APNEA binding sites with the following potency series: (R)-phenylisopropyladenosine = 5'-N-ethylcarboxamide adenosine > (S)-phenylisopropyladenosine. 125I-APNEA binding was relatively insensitive to the xanthine amine congener (XAC, 1 microM), a selective antagonist for the A1AR. Functionally, activation of these A3AR stimulated the production of inositol 1,4,5-triphosphate, leading to an increase in the level of intracellular Ca2+. Furthermore, while activation of these receptors alone produced little secretory response in RBL-2H3 cells, it enhanced antigen-induced secretion by 2-2.5-fold. Northern blotting studies using poly(A+) RNA from RBL-2H3 cells detected two transcripts of 2.0 and 3.5 kilobases, which hybridized to an A3AR cDNA but not to the A1 or A2AR cDNA probes. These data indicate that the unique AR that potentiates the secretory response to antigen in RBL-2H3 cells is exclusively the A3AR.