An experimental paradigm for investigating the role of endogenous adenosine/A1 receptor interactions in vivo

The purpose of the present study was to develop a pharmacological method for determining in the rat in vivo whether endogenous adenosine participates in a given process via activation of A1 adenosine receptors. In anesthetized rats, A1 receptors were activated by infusing the highly selective A1 receptor agonist N6-cyclopentyladenosine, and A2 receptors were stimulated by infusing the highly selective A2 receptor agonist CGS21680C. The bradycardic response to N6-cyclopentyladenosine and the hypotensive response to CGS21680C were used to assess A1 receptor and A2 receptor activation, respectively. After control responses to these purinergic agonists were elicited, animals were given infusions for several hours of either vehicle or one of six dosage levels of FK453 (a potent, selective, nonxanthine A1 receptor antagonist), one of three dosage levels of FR113452 (the S-enantiomer of FK453) or one of seven dosage levels of DPCPX (a potent, selective, xanthine A1 receptor antagonist). Antagonists were infused for > 4 hr, and at various times during the infusions, bradycardic and hypotensive responses to N6-cyclopentyladenosine and CGS21680C, respectively, were reassessed. Both FK453 and DPCPX were highly potent A1 receptor antagonists in vivo, and complete inhibition of bradycardic responses to N6-cyclopentyladenosine were obtained with 3 and 1 micrograms/kg/min, respectively. FR113452 was a very weak antagonist and only slightly reduced bradycardic responses to N6-cyclopentyladenosine at 100 micrograms/kg/min. In vivo FK453 and DPCPX were > 300 and 1000 times selective for the A1 receptor, respectively, compared with the A2 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)