ADENOSINE ACTIVATES A(2) RECEPTORS TO INHIBIT NEUTROPHIL ADHESION ANDINJURY TO ISOLATED CARDIAC MYOCYTES

Inhibition of neutrophil-myocyte adhesion and adhesion-dependent myocy te injury by adenosine was evaluated using isolated TNF-alpha-activate d canine cells. Adenosine inhibited adhesion of activated neutrophils to cardiac myocytes with an IC50 of 11 +/- 4 nM. inhibition of neutrop hil adhesion (92 +/- 3% by 100 nM adenosine) led to inhibition of myoc yte injury (by 90 +/- 6%, as assessed by dye exclusion). Inhibition of cell adhesion by adenosine was blocked by the A(2) antagonist, 1,3-di methyl-1-propylxanthine, but not by the A(1) antagonist, 8-cyclopentyl -1,3-dipropylxanthine. Moreover, the A(2) agonist, CGS21680 arboxymeth yl)phenethylamino]-5'-N-ethylcarboxamido adenosine), but not the A(1) agonist, N-6-cyclopentyladenosine, mimicked adenosine in preventing ce ll adhesion. These observations implicate the A(2) receptor in the mec hanism of inhibition of cell adhesion. Pretreatment and washing of neu trophils, but not cardiac myocytes, with adenosine or CGS21680 led to inhibition of adhesion, suggesting that the neutrophil A(2) receptor i s the target of adenosine's action. In contrast, inhibition of cell ad hesion by adenosine was potentiated by 8-cyclopentyl-1,3-dipropylxanth ine (IC50 = 4 +/- 1 nM) and attenuated by N-6-cyclopentyladenosine, su ggesting that occupancy of A(1) receptors can conversely increase cell adhesion. Neutrophil-myocyte adhesion was inhibited by acadesine (IC5 0 = 12 +/- 2 mu M) also via an adenosine-dependent mechanism because i t was blocked by 1,3-dimethyl-1-propylxanthine or adenosine deaminase, an enzyme that degrades any adenosine that is formed. Acadesine-induc ed inhibition of cell adhesion (83 +/- 4% by 100 mu M) resulted in inh ibition of myocyte injury (by 76 +/- 6%). Other adenosine-regulating a gents, including the acadesine analogue, GP531 (5-amino-1 ino-5-deoxyr ibofuranosyl)imidazole-4-carboxamide), and inhibitors of adenosine tra nsport and intracellular metabolism also inhibited cell adhesion. Thes e results indicate that exogenous or endogenous adenosine can inhibit neutrophil-myocyte adhesion and injury in cells activated with TNF-alp ha by an A(2)-mediated mechanism. Although the predominant activity of adenosine is to attenuate cell adhesion, stimulation of A(1) receptor s has the opposite effect, i.e., to augment adhesive interactions.