Characterization of adenosine receptors on rat ileum, ileal longitudinal muscle and muscularis mucosae.

Adenosine receptors were studied in isolated rat ileum, ileal longitudinal muscle and muscularis mucosae, using a range of agonists and an antagonist. In the rat ileal longitudinal muscle adenosine receptor agonists relaxed the tissues. N6-cyclopentyladenosine (CPA) was more potent than 5'-N-ethylcarboxamidoadenosine (NECA) or adenosine and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (1 nM) gave a 5-fold parallel shift to the right of the concentration-response curves to both CPA and NECA corresponding to an apparent pA2 value of 9.6 suggesting that the agonists relax via adenosine A1 receptors. In the intact ileum adenosine receptor agonists also relaxed the tissue but NECA and CPA were equipotent. DPCPX (3 nM) however inhibited responses to both CPA and NECA with dose-ratios of 8 and 15.6, corresponding to pA2 values of 9.3 and 9.7, respectively. DPCPX (300 nM) gave a much greater shift to the right of the concentration-response curve to NECA with a dose-ratio of 769, corresponding to an apparent pA2 of 9.4. This suggests that the agonists are acting at adenosine A1 receptors to cause relaxation of the whole tissue. Adenosine receptor agonists contracted rat ileal muscularis mucosae with a potency order indicative of an A adenosine receptor. DPCPX (3-100 nM) antagonized responses to CPA giving a linear Schild plot with a slope close to unity and a pA2 of 8.4 suggesting an action on adenosine A1 receptors.