Role of Adenosine A₁ Receptors in Modulating Extracellular Adenosine Levels1

Abstract

The purpose of this investigation was to test the hypothesis that A₁ receptors modulate extracellular levels of adenosine in cardiovascular tissues. Rat cardiac fibroblasts and human aortic vascular smooth muscle cells were cultured to confluence and various pharmacological agents were applied to the cultures. The extracellular fluid was extracted and adenosine concentrations were measured by HPLC. Three selective A₁ receptor antagonists, namely 8-cyclopentyl-1,3-dipropylxanthine, xanthine amine congener, and N-0840, at a concentration of 10 nM significantly increased extracellular levels of adenosine in both rat cardiac fibroblasts and human aortic vascular smooth muscle cells. Further studies in rat cardiac fibroblasts revealed that the effects of A₁receptor blockade on extracellular adenosine levels were concentration dependent and prevented by inhibition of G_i proteins with pertussis toxin or blockade of ecto-5′-nucleotidase with α,β-methyleneadenosine-5′-diphosphate. In cardiac fibroblasts in which the extracellular levels of endogenous adenosine were increased, the ability of A₁ receptor blockade to augment extracellular adenosine was attenuated. A time-course study revealed a time lag of several hours between blockade of A₁ receptors and increases in extracellular adenosine levels. These data suggest that A₁ receptors function to detect the long-term levels of extracellular adenosine, and appropriately adjust extracellular adenosine levels by a slow-onset mechanism involving G_iproteins and ecto-5′nucleotidase.