Structure-activity relationships for N6-substituted adenosines at a brain A1-adenosine receptor with a comparison to an A2-adenosine receptor regulating coronary blood flow.

A series of 145 N6-substituted adenosines have been screened as inhibitors of the binding of [3H]cyclohexyladenosine to an A1-adenosine receptor in rat brain membranes and the results compared to the potencies of these analogs in increasing coronary blood flow via activation of an A2-adenosine receptor. The A1 receptor shows greater stereoselectivity in the N6 region of the receptor towards asymmetric aralkyl substituents, and shows greater bulk tolerance in the N6 region of the receptor such that it retains affinity for certain N6-tertiary alkyladenosines and N6-cycloalkyladenosines that are inactive at the coronary A2 receptor. At the A1 receptor, the most potent analogs have either aliphatic N6-substituents with four or more methylene residues or have an N6-halophenyl substituent. At the A2 receptor, the most potent analogs have an N6-phenethyl or similar heteroarylethyl substituent. Certain sets or series of analogs appear useful for identifying the subtypes of adenosine receptors involved in physiological functions.