Activation of Adenosine A_2a Receptors Inhibits Mast Cell Degranulation and Mast Cell-Dependent Vasoconstriction

Abstract

Objective: Adenosine and inosine accumulate in tissue during periods of ischemia and both molecules have been shown to degranulate mast cells in the hamster cheek pouch via activation of an A₃ receptor. An A₂-mediated inhibitory action of adenosine on mast cell degranulation has also been reported (16), and the objective of this research was to investigate the role of adenosine A₂ receptors in modulating inosine-induced mast cell degranulation and subsequent vasoconstriction of microvessels. Methods: Cheek pouches of the Golden hamster were prepared for in vivo microscopy. Adenosine, inosine, and other agents were applied either globally in the superfusion solution or to selected regions of the tissue by pipette. Results: Micropipette application of 10⁻⁴ M inosine to periarteriolar mast cells caused a vasoconstriction and an associated mast cell degranulation in 71% of the arterioles tested. The average diameter reduction was 29 ± 5%. To establish a modulatory role for the A₂ receptor, low doses of adenosine (100 nM and 10 nM) were applied globally via the superfusion prior to inosine stimulation. This adenosine pretreatment resulted in a decrease in the incidence of the inosineinduced vasoconstriction (17% and 31%), as well as smaller constrictions (0.5 ± 1% and 7 ± 3%). Mast cell degranulation was also reduced by pretreatment with adenosine, as evidenced by a decreased number of mast cells exhibiting ruthenium red dye uptake. The inhibitory effect of adenosine could be eliminated by pretreatment with the nonselective A₁/A₂ antagonist 8-(p-sulfophenyl) theophylline, which restored the inosine-induced responses to control values. To demonstrate that the effect was A_2α-mediated, vessels were pretreated with the selective A_2α agonist 2-[4-(2-carboxyethyl) phenethylamino]-5^'-N-ethylcarboxamidoadenosine (CGS21680). Following this treatment, constriction in response to microapplication of inosine (10⁻⁴ M) occurred in only 11% of the vessels tested; the average constriction was reduced to 2 ± 2% and no mast cell degranulation was observed. Conclusions: We conclude that mast cell degranulation can be inhibited via activation of an adenosine A_2α receptor; which activation occurs at a lower concentration of adenosine than stimulatory A₃ receptor activation. This finding may have implications for the pathology of ischemia. Microcirculation (2000) 7, 129-135.

• Adenosine
• Mast Cell
• Microcirculation
• Arterioles
• Inosine