Nature of the N6 region of the adenosine receptor in guinea-pig ileum and rat vas deferens.

This study explored the nature of the purine domain N6 regions of the presynaptic adenosine receptors of guinea-pig ileum and of rat vas deferens. The experimental design tested a model of these receptors which is complementary to the structure of the N6 substituent of the classical A1 adenosine receptor agonist N6-1-phenyl-2R-propyladenosine, (R-PIA). Assays of activity employed ileal segments or the midportions of vasa deferentia under continuous electrical stimulation at 0.2 Hz. Structure activity correlations compared the EC-50s for twitch inhibition. As shown previously, R-PIA was 60-80 times more potent than its S diastereomer, the resultant of the positive contribution of propyl C-3 to activity as well as the negative influence of steric hindrance exerted by propyl C-3 of the S diastereomer. Other pairs of diastereomers having a chiral center adjacent to N6 showed that the stereoselectivity of the PIAs was generalizable. Biological activity appears to reside wholly in the N6 alkyl moiety; the phenyl or aryl groups of similar size actually diminished potency. The receptor subregions interacting with propyl C-1 and C-3 of R-PIA are each large enough to accommodate two - but not three - methylene residues, each methylene contributing additively to activity. Hydrophobicity is a prominent attribute of the propyl C-1 and C-3 subregions. The potencies of these analogs as inhibitors of presynaptic transmission in ileum or vas deferens are covariant with inhibition of [3H]N6-cyclohexyl-adenosine binding to rat cerebral cortical membranes. Singular exceptions to this generalization may represent organ- or species-dependent differences in receptor fine structure.