The cross-linking of surface IgE receptors by multi-functional Ags promotes the degranulation of mast cells. Previous studies have indicated that the nucleoside adenosine potentiates this response by activating putative A3 adenosine receptors (AR) coupled to phospholipase C in mast cells or their cultured analogues, rat basophilic leukemia (RBL-2H3) cells. Moreover, it has been shown that exposure of RBL-2H3 cells to dexamethasone attenuated antigen-mediated mast cell degranulation, but potentiated the response elicited by adenosine. To determine whether the A3AR is a potential site of action of dexamethasone, we have assessed the status of these receptors in RBL-2H3 cells treated with and without dexamethasone. Treatment with dexamethasone (100 nM) for 24 h resulted in an increase in the number of A3AR to 217 +/- 50% of control. The increased receptor expression was both time- and concentration-dependent, with optimal increases observed following 16 h of treatment and using 100 nM of dexamethasone. No increase in the level of the A2aAR was detectable following dexamethasone treatment. Northern blotting studies indicated a 2.7 +/- 0.3-fold increase in A3AR mRNA in RBL-2H3 cells treated with dexamethasone for 24 h. Dexamethasone also increased the expression of G protein alpha i2, alpha i3, alpha s, and beta subunits by two- to threefold. Activation of the A3AR by aminophenylethyladenosine (APNEA) following dexamethasone treatment enhanced the production of inositol phosphates and the mobilization of intracellular Ca2+. From these data, it is concluded that dexamethasone increases the expression of both A3AR and G proteins in RBL-2H3 cells which contributes to the enhanced response to adenosine.