DISCRIMINATION OF A(1) VERSES A(2) RECEPTOR SUBTYPE SELECTIVITY OF ADENOSINE RECEPTOR AGONISTS IN-VIVO

Previous attempts to discern and quantify the selectivity of agonists for A(1) versus A(2) adenosine receptors in vivo have been confounded by the activation of baroreceptor reflexes and/or simultaneous expression of responses to both A(1) and A(2) receptor activation. In anesthetized, vagotomized rats with isolated in situ constant-flow perfused hindquarters (HQ), bradycardic responses to i.v. agonist injections measured A(1) receptor activation and HQ vasodilation elicited by i.a. agonist injections measured the stimulation of A(2) receptors. Adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) produced A(2) receptor-mediated HQ vasodilation at doses 8- and 4-fold lower (-log ED(50) values, 7.3 +/- 0.04 mol and 8.7 +/- 0.06 mol, respectively) than those required to evoke A(1) receptor-mediated bradycardia (-log ED(50) values, 6.4 +/- 0.01 mol and 8.1 +/- 0.07 mol, respectively). N-6-cyclopentyladenosine (CPA) was approximately 8-fold selective for A(1) receptors (-log ED(50) values, A(1), 8.5 +/- 0.05 mol; A(2), 7.6 +/- 0.16 mol). 2-(Phenylamino)adenosine (CV-1808) and 2[2(4-fluorophenyl)ethoxy]adenosine (FPEA) were at least 125- and 200-fold more potent agonists at A(2) receptors (-log ED(50) values, 7.7 +/- 0.10 mol and 8.0 +/- 0.24 mol, respectively) than at A(1) receptors (-log ED(50) values, 5.6 +/- 0.08 mol and 5.7 +/- 0.01 mol, respectively). These studies demonstrated that stimulation of A(1) and A(2) receptors may be discriminated in vivo and that such responses are selective, reproducible, dose-dependent and quantifiable. A comparison of these in vivo measures with known in vitro data suggests that the A(2a) adenosine receptor mediates vasodilation in the rat HQ and that in vitro assays may predict the orders of potency of adenosine A(1) and A(2a) receptor agonists in vivo but they are less reliable predictors of the absolute potency and, hence, the A(1)/A(2) receptor selectivity of agonists.