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, adenosine receptors. One aim of this study was to characterize and contrast the effects of PD 81,723 on the A, receptormediated negative dromotropic and Al, receptor-mediated vasodilatory actions of adenosine and of a nonmetabolizable and unselective N6-(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 N6-(3-pentyl)adenosine derivative-induced prolongations of the stimulus-to-His bundle (S-H) interval in a concentration-dependent manner. PD 81,723 (30 ,umol/L) decreased the EC50 value for adenosine to prolong the S-H interval by ninefold from 7.4±1.2 to 0.8±0.1 ,umol/L but did not increase the content of adenosine in cardiac effluent. PD 81,723 (30 gmol/L) increased the specific binding of the A1 agonist [3H]cyclohexyladenosine ([3H]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, receptors in the high-affinity binding state by an average of 56±13%. The dissociation rate of [3H]CHA from guinea pig A denosine is an autacoid (or local hormone) that modulates numerous functions of the cardiovascular system."12 In the heart, in addition to being a potent vasodilator, adenosine slows heart rate (negative chronotropic effect), slows atrioventricular (AV) nodal conduction (negative dromotropic effect), and antagonizes the stimulatory effects (ie, inotropic and arrhythmogenic) of catecholamines.' These effects of adenosine, which increase the supply of oxygen and decrease cardiac work, are cardioprotective. Consistent with this cardioprotective action, adenosine, adenosine receptor agonists, and agents that increase the interstitial concentration of adenosine (eg, nucleoside uptake blockers and adenosine deaminase and adenosine kinase inhibitors) have been shown to reduce myocardial cell damage and dysfunction during hypoxia and ischemia (for extended reference lists, refer to References 1, 3, and 4). Adenosine and the nucleoside uptake Received April 19, 1994; accepted August 15, 1994. From the Departments of Medicine (C.K.-B., J.R., L.B.), Pharmacology (L.B.), and Anesthesiology (D.D.), University of Florida, Gainesville; Lilly Research Laboratories (R.F.B.), Eli Lilly & Co, Indianapolis, Ind; and the Departments of Medicine and Molecular Physiology (J.L.), University of Virginia, Charlottesville. Correspondence to Luiz Belardinelli, MD, University of Florida, Department of Medicine, PO Box 100277, Gainesville, FL 32610. C) 1994 American Heart Association, Inc. brain membranes was decreased in the presence of PD 81,723 (10 gmol/L) from 0.55±0.01/min to 0.35±0.01/min. PD 81,723 did not alter the binding of the A1 antagonist [3H]cyclopentyldipropylxanthine to guinea pig brain membranes. The IC50 values for 5'-guanylylimidodiphosphate to reduce specific binding of [3H]CHA to guinea pig cardiac and brain membranes were increased from 1.5±0.2 and 2.0±0.2 ,umol/L in the absence of PD 81,723 to 10±3.3 and 18±0.5 ,umol/L, respectively, in the presence of PD 81,723 (30 ,umol/L). PD 81,723 did not potentiate the coronary vasodilatory actions of the N'-(3-pentyl)adenosine derivative. Specific binding of the A2a agonist [3H]CGS 21680 to brain membranes and the nucleoside transporter ligand [3H]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, receptors by stabilizing receptor-G protein interactions in the presence of agonists. (Circ Res. 1994;75:961-971.)