Evidence that A2 purinoceptors are involved in endothelium‐dependent relaxation of the rat thoracic aorta

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Abstract
1 The effect of adenosine and some adenosine analogues on the isolated thoracic aorta from rats was compared with the effect of adenosine 5′-triphosphate (ATP) and adenosine 5′-diphosphate (ADP). 2 Both ATP and adenosine analogues caused relaxation of the noradrenaline (30 nm)-contracted thoracic aorta. 3 The order of potency for adenosine analogues was 5′-(N-ethyl) carboxamidoadenosine (NECA) > l-N6-phenylisopropyladenosine (l-PIA), adenosine 5′-monophosphate (AMP), adenosine indicating the presence of adenosine A2 receptors. 4 Removal of the endothelium or prior treatment with haemoglobin (10 μm) attenuated relaxant responses to both ATP and NECA, attenuation being greater for ATP than NECA. 5 8-Phenyltheophylline (10 μm) reduced relaxant responses to NECA but not to ATP in the intact tissue. 6 These results provide evidence that there are two components to relaxation of the rat thoracic aorta induced by purinoceptor agonists. The first is an endothelium-dependent mechanism involving release of endothelium-derived relaxant factor (EDRF) and the second is due to a direct effect on smooth muscle.