Adenosine analogues relax guinea-pig taenia caeci via an adenosine A2B receptor and a xanthine-resistant site.

In this study we have sub-classified the adenosine A2 receptor mediating relaxation in the guinea-pig taenia caecum using the adenosine A2A receptor-selective agonist CGS 21680 (2-[p-(2-carboxyethyl)phenylamino]-5'-N-ethylcarboxamidoadenosine) and the adenosine A2A receptor-selective antagonist ZM 241385 (4-(2-[7-amino-2-(2-furyl) [1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-yl amino]ethyl)phenol). CGS 21680 did not elicit relaxations, and a pKB value of 7.80 was obtained for ZM 241385 against 5'-N-ethylcarboxamidoadenosine suggesting the presence of adenosine A2B receptors. Relaxations are also mediated via a xanthine-resistant site. In this study relaxations to the adenosine A3 receptor agonist IB-MECA (N6-(3-iodo-benzyl)adenosine-5'-N-methyluronamide) were blocked by neither 8-sulphophenyltheophylline (100 microM) nor the adenosine A3 receptor antagonist BW-A1433 (1,3-dipropyl-8-(4-acrylate)phenylxanthine, 100 microM), suggesting that this site is not an adenosine A3 receptor.