Pharmacological classification of adenosine receptors in the sinoatrial and atrioventricular nodes of the guinea‐pig

The effects of seven agonist and three antagonist adenosine receptor ligands were compared on the guinea‐pig sinoatrial (SA) node (isolated right atrium) and atrioventricular (AV) node (perfused whole heart). Single agonist concentration‐effect curves were obtained to 5′‐N‐ethylcarboxamidoadenosine (NECA), R(−)‐N6‐(2‐phenylisopropyl)adenosine (R‐PIA), N6‐cyclohexyladenosine (CHA), 2‐chloroadenosine (CADO),),S(+)‐N6‐(2‐phenylisopropyl)adenosine (L‐PIA), 2‐phenylaminoadenosine (CV 1808) and N6‐aminoadenosine (MeAdo). Adenosine and/or NECA curves were obtained in the absence and presence of the antagonists 8‐cyclopentyl‐1,3‐dipropylxanthine (DPCPX), 9‐chloro‐2–(2‐furanyl)‐5,6‐dihydro‐1,2,4‐triazolo[1,5‐c]quinazolin‐5‐imine (CGS15943) and N6‐(endonorbornan‐2‐yl)‐9‐methyladenine (N‐0861). A formal comparison of the agonist and antagonist potency data was made by fitting the data to a straight line using a least squares procedure based on principal components analysis to account for the variance on both axes. The antagonist affinity estimates made on the two assays did not deviate significantly from the line of identity. The agonist p[A]50 data obtained on the two assays did not deviate from the line of identity, indicating that there were no significant differences in potencies between the two assays. The p[A]50 ratio of R‐PIA and S‐PIA was 1.24±0.09 in the SA node and 1.36±0.11 in the AV node, indicating no difference in the stereoselectivity of the PIA isomers between the two tissues. The agonist potency and antagonist affinity data obtained are consistent with previous findings showing that the AV and SA node data are pharmacologically indistinguishable and belong to the adenosine A1‐receptor class. No evidence for the reported A3‐receptor was found.