Relaxation of the mouse isolated aorta and carotid artery in response to adenosine analogues in genetically-modified mice lacking the adenosine A2A receptor

The aim of this study was to characterise the receptor(s) mediating responses to adenosine and/or adenosine analogues in mouse isolated aorta and carotid artery. In addition, since mice lacking the A(2A) adenosine receptor are reported to be hypertensive, the possibility that this gene deletion or the altered phenotype results in alteration of responses mediated via adenosine analogues was investigated. This was achieved by comparing results obtained in parallel within single experiments using tissues from A(2A) knock-out animals and their wild-type littermates. In aortic rings, adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) caused relaxations above 10 muM and 30 muM, respectively, which were unaffected by either 8-sulphophenyltheophylline (8-SPT, 100 muM) or A(2A) receptor knockout. 2-[p-(2-Carbonylethyl)phenylethylamino]5'-N-ethylcarboxamidoadenosine (CGS 21680) was virtually inactive. R-N-6-Phenylisopropyladenosine (R-PIA) induced relaxations which were not inhibited by 8-SPT (100 muM) or altered by A(2A) receptor knockout. No A(1)-mediated contractile responses were observed in wild-type or knock-out tissues in contrast with results in mice of the same strain obtained commercially rather than from our breeding programme. In carotid artery rings NECA contracted at low concentrations (0.1-1 muM) and relaxed at higher concentrations. Curves to NECA were not different in tissues from wild-type and A(2A) receptor knock-out mice and both the contractile and relaxant phases were right-shifted by 8-SPT (100 muM) in tissues from animals of both genotype. 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX, 3 nM) attenuated contractile NECA responses but did not affect relaxant responses. CGS 21680 was inactive in carotid artery rings from both wild-type and A(2A) receptor knock-out mice. In the presence of DPCPX (30 nM) to abolish contractions, R-PIA induced relaxant curves which were not different in tissues from wild-type and A(2A) knock-out mice and were not inhibited by 8-SPT (100 muM). These results confirm the absence of A(2A) or A(2B) receptors in murine aorta and suggest that relaxations to NECA in carotid artery are A(2B) receptor-mediated whilst contractions are A(1) receptor-mediated. They also indicate the presence of an antagonist-resistant site activated by R-PIA in both vascular preparations. There is no evidence for compensatory changes in responses mediated by adenosine and its analogues due to the gene deletion or the reported resulting hypertensive phenotype in either aortic or carotid arterial rings obtained from A(2A) knock-out mice.