The A 2 A Adenosine Receptor Mediates Coronary Vasodilation 1

The coronary vasodilation caused by adenosine is due to activation of A2 adenosine receptors (A2AdoRs), but the subtype or subtypes of A2AdoR (A2A and/or A2B) that mediate this action are uncertain. The purpose of this study was to test the hypothesis that A2AAdoRs mediate coronary vasodilation caused by exogenous or endogenous adenosine in the guinea pig isolated perfused heart. The newly described A2AAdoR antagonist SCH58261 was used to selectively block A2AAdoRs. Attenuations by SCH58261 of increases in coronary conductance (A2 response) and of atrioventricular nodal conduction time (A1 response) caused by exogenous and endogenous adenosine and by agonists with relative selectivity for A2A and A1AdoRs were measured. The CGS21680-induced increase of coronary conductance was antagonized by SCH58261 in a concentration-dependent and competitive manner with a KB value of 5.01 nm. Also reversed by SCH58261 (60 nmol/L) were the increases in coronary conductance caused by the relatively selective A1AdoR agonists CCPA (70 nM), and (R)-(2)N -(2-phenyl-isopropyl)adenosine (60 nM) but not those caused by sodium nitroprusside (1.2 mM) and diltiazem (0.4 mM). SCH58261 (#100 nM) did not attenuate the A1AdoR-mediated prolongations of S-H interval caused by either adenosine or CCPA. SCH58261 attenuated the coronary vasodilation caused by 50 nM adenosine with an IC50 value of 6.8 6 0.6 nM. The coronary vasodilations caused by the nucleoside uptake inhibitor draflazine and the adenosine kinase inhibitor iodotubercidin were completely reversed by 60 nM SCH58261 or adenosine deaminase (7 U/ml). Thus, the A2AAdoR plays a major role as mediator of coronary vasodilation caused by exogenous and endogenous adenosine and by AdoR agonists. The A1 and A2 subtypes (A2A and A2B) of cell membrane AdoRs mediate the known cardiovascular effects of adenosine. Responses to activation of A1AdoRs include (1) slowing of heart rate (negative chronotropy) and impulse propagation through the atrioventricular node (negative dromotropy), (2) reduction in atrial contractility (negative inotropy) and (3) inhibition of the stimulatory effects of catecholamines (anti-beta-adrenergic action) (Belardinelli et al., 1989; Olsson and Pearson, 1990). Activation of A2AdoRs causes coronary dilation and increases coronary conductance (Olsson and Pearson, 1990; Mustafa et al., 1995). The subtype of A2AdoR (A2A and A2B) that mediates vasodilation in response to adenosine is uncertain (Mustafa et al., 1995). The nanomolar potency of the selective A2AAdoR agonist CGS21680 [2-p-(2-carboxyethyl)phenethylamino-59-Nethylcarboxamidoadenosine] to cause coronary vasodilation, and the similar rank orders of potency for adenosine analogs to bind to A2AAdoRs in brain membranes and cause coronary vasodilation (Olsson and Pearson, 1990), suggest that A2AAdoRs are mediators of coronary vasodilation. However, specific binding of [H]CGS21680 to coronary artery membrane preparations could not be demonstrated (Mustafa et al., 1995), and thus the evidence for coronary A2AAdoRs remains inconclusive. The absence of radioligand binding data to confirm the functional pharmacological characterization of coronary adenosine receptors was recently overcome by the use of a new selective A2AAdoR antagonist radioligand, [ H]SCH58261 (Belardinelli et al., 1996). SCH58261 [5-amino-7-(2-phenylethyl)-2-(2-furyl)pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine] and its tritiated analog have been used in our laboratories to investigate A2AAdoRs in several tissues (Belardinelli et al., 1996; Dionisotti et al., 1996; Zocchi et al., 1996a, 1996b). Specific high-affinity binding sites for [H]SCH58261 were found to be present in coronary artery membranes in large numbers (900 fmol/mg of protein) (Belardinelli et al., 1996). Received for publication July 2, 1997. 1 This study was supported in part by National Institutes of Health Grant N01-HL-56785. ABBREVIATIONS: A2AdoR, A2 adenosine receptor; ADA, adenosine deaminase; AUC, area under the curve; CCPA, 2-chloro-N -cyclopentyladenosine; CGS21680, 2-p-(2-carboxyethyl)-phenethylamino-59-N-ethylcarboxyamidoadenosine; CPX, 8-cyclopentyl-1,3-dipropylxanthine; DMSO, dimethylsulfoxide; HBE, His bundle electrogram; IC50, concentration of antagonist that causes 50% inhibition; K-H, Krebs-Henseleit; PEG, polyethylene glycol; (R)-PIA, (R)-(2)N-(2-phenylisopropyl) adenosine; S-H, stimulus-to-His bundle; SNP, sodium nitropruside. 0022-3565/98/2843-1066$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 284, No. 3 Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 284:1066–1073, 1998 1066 at A PE T Jornals on Jne 4, 2017 jpet.asjournals. org D ow nladed from The present study uses the A2AAdoR antagonist SCH58261 to test the hypothesis that the A2AAdoR mediates coronary vasodilation caused by either exogenous or endogenous adenosine in the guinea pig. The interpretation of the results depends on the assumption that SCH58261 is a highly selective A2AAdoR antagonist. Accordingly, we first present the results of pharmacological experiments that validate the use of SCH58261 as a selective A2AAdoR antagonist in the guinea pig heart. Next, we use SCH58261 and CPX to distinguish the A1and A2A-AdoR-mediated responses of the heart to CCPA and to CGS21680, which are relatively selective A1AdoR and A2AAdoR agonists, respectively. Last, we use SCH58261 to assess the role of the A2AAdoR in mediating the coronary vasodilation caused by endogenous adenosine.