Adenosine A1 receptors are not involved in contraction of canine gastric muscularis mucosae by adenosine analogues

In vitro contractility studies were conducted in canine gastric muscularis mucosae muscle strips with the adenosine analogues 2-chloroadenosine (CIAD), 5'-N-ethylcarboxamidoadenosine (NECA), 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA), R-N6-(2-phenylisopropyl)adenosine (R-PIA), S-PIA, N6-cyclohexyladenosine (CHA) and (2-p-carboxyethyl)phenylamino-5'-N-carboxamidoadenosine (CGS21680) as well as the A1-selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). Adenosine analogues contracted the muscle strips with the following rank order of potency: CPCA > NECA > CIAD > R-PIA > CHA > S-PIA > CGS21680. CPCA, R-PIA, and CHA were partial agonists. At a concentration selective for adenosine A1 receptors (50 nM), DPCPX did not alter the concentration-response curves to CIAD or CHA. However, at higher concentrations (1-10 microM), DPCPX antagonized CIAD-mediated contractions in a competitive manner (pA2 = 6.96; slope = 0.93). CIAD-mediated contraction was not altered by treatment of the muscle strips with tetrodotoxin (1 microgram/ml) or mepyramine (1 microM). Our results indicate that adenosine A1 receptors, nerves or mast cells are not involved in contraction of canine gastric muscularis mucosae by adenosine analogues.