Apparent affinity of 1,3‐dipropyl‐8‐cyclopentylxanthine for adenosine A1 and A2 receptors in isolated tissues from guinea‐pigs

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Abstract
1 The classification of adenosine receptor subtypes (A1 and A2) in intact tissues has been based on the order of agonist potency. In this study the apparent affinity of 1,3-dipropyl-8-cyclopentylxanthine (CPX), an antagonist which has been reported to be A1 selective, and the nonselective antagonist 1,3-dimethyl-8-phenylxanthine (8PT) has been evaluated on isolated tissues from the guinea-pig. 2 The isolated tissues used were atria (bradycardic response, proposed A1 sub-type), aorta and trachea (relaxant response, proposed A2 sub-type). 3 Both the xanthines antagonized responses to adenosine in the three tissues but had little or no effect on responses to carbachol (atria), sodium nitrite (aorta) or isoprenaline (trachea). 4 pA2 values for 8PT were similar on the three tissues (6.3–6.7), however, the pA2 value for CPX on the atria (7.9-8.4) was greater than that on the aorta (6.6) or trachea (6.6). 5 These results support the suggestion that the adenosine receptors which mediate bradycardia in the atrium are of the A1 sub-type and that those which mediate relaxation in the aorta and trachea are of the A2 type.