Long residence time adenosine A1 receptor agonists produce sustained wash- resistant antilipolytic effect in rat adipocytes

Elevated circulating free fatty acid (FFA) level is closely linked to the pathogenesis of insulin resistance and type 2 diabetes mellitus. Activation of the adenosine A(1) receptor (A(1), R) inhibits lipolysis in adipocytes and hence reduces the concentration of FFA, which represents a potential target for the development of antilipolytic agents. We aimed to assess the binding affinity as well as target binding kinetics of A(1)R agonists and further delineate a possible relationship with their antilipolytic effect in adipocytes. Radioligand binding assays were performed to determine the affinity and kinetics of three representative A(1)R agonists, namely CPA, LUF6944 and LUF6941, on the rat A(1)R. Functional responses to these agonists were examined in both a recombinant cell system and physiologically relevant rat adipocytes. The three A(1)R agonists displayed similar affinity while divergent target binding kinetics on the rat A(1)R. Irrespective of equilibrium binding affinity, temporal analysis of receptor signaling demonstrated persistent functional responses for the long residence time agonist, despite removal of excess agonist, in both a recombinant cell system and in rat adipocytes. By contrast, such effect was less pronounced or even lost for agonists with medium or short receptor residence time, respectively. Our results indicate that ligand receptor binding kinetics rather than their affinity or potency play an essential role in regulating cellular responses. The long residence time A(1)R agonist produces a sustained wash-resistant antilipolytic effect in rat adipocytes and thus may represent a potential antilipolytic alternative for further investigation.