Targeted deletion of adenosine A3 receptors augments adenosine-induced coronary flow in isolated mouse heart

To determine whether adenosine A(3) receptors participate in adenosine-induced changes in coronary flow, isolated hearts from wild-type (WT) and A(3) receptor knockout (A(3)KO) mice were perfused under constant pressure and effects of nonselective and selective agonists were examined. Adenosine and the selective A(2A) agonist 2-[p-(2-carboxyethyl)] phenylethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680) produced augmented maximal coronary vasodilation in A3KO hearts compared with WT hearts. Selective activation of A(3) receptors with 2-chloro-N-6 ( 3-iodobenzyl)-adenosine-5'- N-methyluronamide (Cl-IB-MECA) at nanomolar concentrations did not effect coronary flow, but at higher concentrations it produced coronary vasodilation both in WT and A3KO hearts. Cl-IB-MECA-induced increases in coronary flow were susceptible to both pharmacological blockade and genetic deletion of A(2A) receptors. Because deletion or blockade of adenosine A(3) receptors augmented coronary flow induced by nonselective adenosine and the selective A(2A) receptor agonist CGS-21680, we speculate that this is due to removal of an inhibitory influence associated with the A(3) receptor subtype. These data indicate that the presence of adenosine A(3) receptors may either inhibit or negatively modulate coronary flow mediated by other adenosine receptor subtypes.