Discrimination of A1 versus A2 receptor subtype selectivity of adenosine receptor agonists in vivo

Previous attempts to discern and quantify the selectivity of agonists for A1 versus A2 adenosine receptors in vivo have been confounded by the activation of baroreceptor reflexes and/or simultaneous expression of responses to both A1 and A2 receptor activation. In anesthetized, vagotomized rats with isolated in situ constant-flow perfused hindquarters (HQ), bradycardic responses to i.v. agonist injections measured A1 receptor activation and HQ vasodilation elicited by i.a. agonist injections measured the stimulation of A2 receptors. Adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) produced A2 receptor-mediated HQ vasodilation at doses 8- and 4-fold lower (-log ED50 values, 7.3 +/- 0.04 mol and 8.7 +/- 0.06 mol, respectively) than those required to evoke A1 receptor-mediated bradycardia (-log ED50 values, 6.4 +/- 0.01 mol and 8.1 +/- 0.07 mol, respectively). N6-cyclopentyladenosine (CPA) was approximately 8-fold selective for A1 receptors (-log ED50 values, A1, 8.5 +/- 0.05 mol; A2, 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 A2 receptors (-log ED50 values, 7.7 +/- 0.10 mol and 8.0 +/- 0.24 mol, respectively) than at A1 receptors (-log ED50 values, 5.6 +/- 0.08 mol and 5.7 +/- 0.01 mol, respectively). These studies demonstrated that stimulation of A1 and A2 receptors may be discriminated in vivo and that such responses are selective, reproducible, dose-dependent and quantifiable.(ABSTRACT TRUNCATED AT 250 WORDS)