A novel mechanism of vasoregulation:: ADP-induced relaxation of the porcine isolated coronary artery is mediated via adenosine release

In this study, we have investigated the mechanism of ADP-induced relaxation of porcine coronary artery (PCA) rings. The P2Y receptor agonists ADP and ADP beta S produced concentration-dependent relaxation of endothelium-denuded PCA smooth muscle with pD(2) values of 5.3 and 4.9, respectively. RT-polymerase chain reaction (RT-PCR) and immunoblotting demonstrated mRNA and protein expression of P2Y(1) and A(2)A adenosine receptors in the PCA. The nonselective P2 antagonist PPADS or the P2Y(1)-selective antagonist MRS2179 failed to alter ADP- or ADP beta S-induced relaxations. Relaxations to ADP were, however, blocked by the A2A adenosine receptor-selective antagonists ZM241385 and SCH58261 (apparent pK(B) values of 9.2 and 8.9, respectively). We excluded roles for direct occupancy of A(2)A adenosine receptors by ADP or ADP beta S as well as metabolism to adenosine as mechanisms for ADP-evoked relaxations. However, ADP responses were significantly enhanced in the presence of the ENT1 nucleoside transporter inhibitors dipyridamole and NBTI and were significantly inhibited by adenosine deaminase, indicating a role for extracellular adenosine. Suprafusion of [H-3]-adenine-labeled PCA segments showed that ADP induced the release of a number of purines, including adenosine. These data suggest that ADP mediates relaxation of the PCA via a novel mechanism that involves adenine nucleotide-evoked adenosine release and the subsequent activation of A(2A) receptors.