Affinity of various purine nucleosides for adenosine receptors on purified myenteric varicosities compared to their efficacy as presynaptic inhibitors of acetylcholine release.

Isolated myenteric varicosities (autonomic synaptosomes)prepared from the guinea-pig ileum were used as the substrate in competition experiments designed to study the properties of the adenosine receptor present on peripheral nerve endings. Competition experiments using [3H]-N6-[R-(-)-1-methyl-2-phenethyl] adenosine and [3H]-5'-N-ethylcarboxamidoadenosine as the labeled ligands permitted the affinities of several unlabeled adenosine analogs to be examined. In addition, the ability of theophylline, an antagonist at adenosine receptors, to compete for binding was determined. The relative affinities of the nucleosides were compared to their efficacies as inhibitors of acetylcholine release in the stimulated guinea-pig ileum preparation and an excellent correlation was obtained. The identity of the substrate used in the binding studies with the target in the bioassay system suggests that the isolated myenteric varicosity contains the adenosine receptor responsible for the observed biological activity. Similar affinities for theophylline as a competitor of the binding of both labeled ligands paralleled the establishment of similar PA2 values for the antagonist in the bioassay. These findings, together with the similarity of the biological efficacy of N6-[R-(-)-1-methyl-2-phenethyl]adenosine and 5'-N-ethylcarboxamidoadenosine, suggest that the adenosine receptor present on myenteric nerve endings is unitary but do not permit its designation as an A1 or A2 subtype.