CHARACTERIZATION OF THE ADENOSINE RECEPTOR IN PORCINE CORONARY-ARTERIES

1. Relaxant responses of ring preparations from porcine ventricular coronary arteris to adenosine and various stable adenosine analogues were investigated in vitro. 2. The adenosine analogues did not produce contraction but elicited almost complete relaxation of coronary arteries preconstricted with 3 μM prostaglandin F(2α) (PGF(2α)), even after removal of the endothelium. 3. The order of potency, was 5'-N-ethylcarboxamide-adenosine (NECA) > 2-(2-phenylethyalmino)5'-N-ethylcarboxamide-adenosine (2-PEA-NECA) > 2-phenylamino-adenosine (CV-1808) > N6-[R(-)-1-phenyl-2-propyl]adenosine (R-PIA) > N6-[S(+)-1-phenyl-2-propyl]adenosine (S-PIA) > N6-cyclopentyladenosine (CPA) > adenosine ATP = ADP which suggested the presence of adenosine A2-receptor subtypes. 4. There was an excellent correlation between the calculated pD2 values on coronary arteries and the pK(D) values at adenosine A2 binding sites, whereas no correlation was obtained when the pD2 values were compared to the pK(D) values at adenosine A1-binding sites on membranes from porcine striata. 5. The relaxant effects of adenosine and its analogues were competitively antagonized by 8-(p-sulphophenyl)-theophylline (8-SPT), producing pA2 values similar to the respective pK(D) value of the antagonist at adenosine A2 binding sites. It is suggested that that porcine coronary artery possesses adenosine A2 receptors which seem to be similar to the adenosine A2 binding site in pig striatum, whereas no evidence was obtained for the presence of adenosine A1 receptors.