Adenosine receptor activation in human fibroblasts: nucleoside agonists and antagonists

Abstract

Adenosine (ED₅₀ 15 μM) causes a 50-fold increase in intracellular cyclic AMP in the VA13 human fibroblast line. A total of 128 nucleosides was tested as agonists and antagonists. Eight classes of compounds were found: full agonists (14 compounds), weak agonists (20), high-efficacy partial agonists (16), low-efficacy partial agonists (7), competitive inhibitors (11), noncompetitive inhibitors (3), partial agonist - noncompetitive inhibitors (3), and inactive compounds (54). The noncompetitive inhibitors antagonized the responses to adenosine, isoproterenol, and prostaglandin E₁ and thus may have been adenylate cyclase inhibitors. The most potent noncompetitive inhibitor, 2′,5′-dideoxyadenosine, was a partial inhibitor, reducing the response to isoproterenol by only 77% even at very high concentrations. The most potent agonists, partial agonists, and pure antagonists had apparent affinities of about 5 μM. Although all positions were important for affinity at the adenosine receptor, only the 3′- and 5′-positions and to a much lesser extent the 6- and 8-positions had an effect on efficacy. The receptor tolerated bulky groups at the 6-position of adenosine, had an ethyl-sized "pocket" near the 5′-position, and had little bulk tolerance towards modifications at other positions. Among the full agonists, only one 5′-derivative and one 2-position derivative had higher apparent affinity than adenosine. Studies with conformationally restricted agonists and antagonists showed that adenosine must be in the anti conformation in order to bind to the receptor.