ALLOSTERIC ENHANCER PD-81,723 ACTS BY NOVEL MECHANISM TO POTENTIATE CARDIAC ACTIONS OF ADENOSINE

The 2-amino-3-benzoylthiophene derivative PD 81,723 is an allosteric enhancer of agonist binding to brain A(1) adenosine receptors. One aim of this study was to characterize and contrast the effects of PD 81,723 on the A(1) receptor-mediated negative dromotropic and A(1) receptor-mediated vasodilatory actions of adenosine and of a nonmetabolizable and unselective N-6-(3-pentyl)adenosine derivative. A second aim was to determine the mechanism of action of PD 81,723. In guinea pig isolated hearts, PD 81,723 potentiated the adenosine and the N-6-(3-pentyl)adenosine derivative-induced prolongations of the stimulus-to-His bundle (S-H) interval in a concentration-dependent manner. PD 81,723 (30 mu mol/L) decreased the EC(50) value for adenosine to prolong the S-H interval by ninefold from 7.4 +/- 1.2 to 0.8 +/- 0.1 mu mol/L but did not increase the content of adenosine in cardiac effluent. PD 81,723 (30 mu mol/L) increased the specific binding of the A(1) agonist [H-3]cyclohexyladenosine ([H-3]CHA) to human atrial and guinea pig atrial and brain membranes by 38%, 78%, and 300%, respectively. PD 81,723 also increased the fraction of A(1) receptors in the high-affinity binding state by an average of 56 +/- 13%. The dissociation rate of [H-3]CKA from guinea pig brain membranes was decreased in the presence of PD 81,723 (10 mu mol/L) from 0.55 +/- 0.01/min to 0.35 +/- 0.01/min. PD 81,723 did not alter the binding of the A(1) antagonist [H-3]cyclopentyldipropylxanthine to guinea pig brain membranes. The IC50 values for 5'-guanylylimidodiphosphate to reduce specific binding of [H-3]CKA to guinea pig cardiac and brain membranes were increased from 1.5 +/- 0.2 and 2.0 +/- 0.2 mu mol/L in the absence of PD 81,723 to 10 +/- 3.3 and 18 +/- 0.5 mu mol/L, respectively, in the presence of PD 81,723 (30 mu mol/L). PD 81,723 did not potentiate the coronary vasodilatory actions of the N-6-(3-pentyl)adenosine derivative. Specific binding of the A(2a) agonist [H-3]CGS 21680 to brain membranes and the nucleoside transporter ligand [H-3]nitrobenzylthioinosine to cardiac membranes was unchanged in the presence of PD 81,723. The results suggest that PD 81,723 specifically potentiates the action of adenosine on A(1) receptors by stabilizing receptor-G protein interactions in the presence of agonists.