Possible heterogeneity of adenosine receptors present on myenteric nerve endings

The presence of more than one adenosine receptor on enteric nerve endings was investigated using the electrically stimulated guinea pig ileum preparation and purified myenteric varicosities obtained from the same source. Competition experiments, using N6-cyclohexyladenosine and 5'-N-ethylcarboxamide adenosine as the labeled ligands allowed the binding characteristics of the adenosine receptor(s) on myenteric nerve endings to be examined. The results showed that both N6-cyclohexyladenosine and 5'-N-ethylcarboxamide adenosine were equieffective as displacers of labeled N6-cyclohexyladenosine binding. In contrast, the binding of labeled 5'-N-ethylcarboxamide adenosine revealed an inability of the A1 ligands N6-[R-1-methyl-2-phenethyl]adenosine and N6-cyclohexyladenosine to displace more than 50% of its specific binding. Competition curves generated using the potent and selective adenosine receptor antagonist 1,3-dipropyl-8-(4-sulfophenyl)xanthine revealed a clear difference between displacement profiles for N6-cyclohexyladenosine and 5'-N-ethylcarboxamide adenosine. These data are indicative of the presence of more than one binding site on enteric nerve endings. Schild analysis of the antagonism of the presynaptic inhibitory effects of the nucleosides on the ileum using theophylline yielded linear isoboles with unit slopes indicating competitive antagonism. Similar analysis using 1,3-dipropyl-8-(4-sulfophenyl)xanthine yielded comparable results for the A1 agonists whereas 5'-N-ethylcarboxamide adenosine gave rise to a curvilinear isobole, a finding consistent with possible receptor heterogeneity. These findings show that data derived from both binding and functional studies support the existence of more than one adenosine receptor on myenteric nerves although they do not permit the subclassification of these sites as A1 or A2 receptor subtypes.