Effects of adenosine receptor agonists on guinea-pig isolated working hearts and the role of endothelium and NO.

The hypothesis that the coronary vasodilator effects of adenosine receptor agonists are independent of the vascular endothelium or mediators derived therefrom was examined in guinea-pig isolated working hearts. Adenosine receptor agonists, 5'-(N-ethylcarboxamido)-adenosine (NECA; two-fold selective for A2 over A1 receptors), 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680; A2A selective), N6-cyclopentyl-adenosine (CPA; A1 selective) and N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA; A3 selective), were infused (3 x 10(-7) M) after endothelium removal by passing oxygen through the coronary circulation. In spontaneously beating hearts, CGS21680 and NECA increased, while CPA decreased, coronary flow. NECA and CPA reduced heart rate, left ventricular pressure and aortic output. The nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine (L-NOARG; 3 x 10(-5) M) abolished the vasodilatation by NECA but not CGS21680, indicating that nitric oxide (NO) of a non-endothelial source mediated the NECA response. Coronary vasodilatation by CGS21680 was inhibited bythe A2A receptor antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo [2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385). Indometacin (10(-6) M) attenuated the coronary vasodilatation to CGS21680, suggesting a partial role for cyclooxygenase products. IB-MECA had no effect, indicating no A3 receptor involvement. In paced working hearts, the responses were similar except CPA had no effect on coronary flow or aortic output and CGS21680 increased left ventricular pressure and the maximum rate of ventricular pressure rise. This study has demonstrated functionally effective removal of the endothelium by a novel method of passing oxygen through the coronary vasculature. A coronary vasodilator action of adenosine receptor agonists mediated via A2A receptors is endothelium- and NO-independent, but partially involves cyclooxygenase products.