Characterization of the adenosine receptor in porcine coronary arteries

1 Relaxant responses of ring preparations from porcine ventricular coronary arteries 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 μ m prostaglandin F 2α (PGF 2α ), even after removal of the endothelium. 3 The order of potency, was 5′‐N‐ethylcarboxamide‐adenosine (NECA) > 2‐(2‐phenylethylamino)5′‐N‐ethylcarboxamide‐adenosine (2‐PEA‐NECA) > 2‐phenylamino‐adenosine (CV‐1808) > N 6 ‐[S(−)−1‐phenyl‐2‐propyl]adenosine (S‐PIA) > N 6 ‐[S(+)‐1‐phenyl‐2‐propyl]adenosine (S‐PIA) > N 6 ‐cyclopentyl‐adenosine (CPA) > adenosine > ATP = ADP which suggested the presence of adenosine A 2 ‐receptor subtypes. 4 There was an excellent correlation between the calculated pD 2 values on coronary arteries and the pK D values at adenosine A 2 binding sites, whereas no correlation was obtained when the pD 2 values were compared to the pK D values at adenosine A 1 ‐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 pA 2 values similar to the respective pK D value of the antagonist at adenosine A 2 binding sites. 6 It is suggested that the porcine coronary artery possesses adenosine A 2 receptors which seem to be similar to the adenosine A 2 binding site in pig striatum, whereas no evidence was obtained for the presence of adenosine A 1 receptors.