Role of adenosine A(1) receptors in modulating extracellular adenosine levels

The purpose of this investigation was to test the hypothesis that A(1) rece ptors 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 concentra tions were measured by HPLC. Three selective A(1) receptor antagonists, nam ely 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 smoo th muscle cells. Further studies in rat cardiac fibroblasts revealed that t he effects of A(1) receptor 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 alpha,beta-methyle neadenosine-5'-diphosphate. In cardiac fibroblasts in which the extracellul ar levels of endogenous adenosine were increased, the ability of A(1) recep tor blockade to augment extracellular adenosine was attenuated. A time-cour se study revealed a time lag of several hours between blockade of A1 recept ors and increases in extracellular adenosine levels. These data suggest tha t A1 receptors function to detect the long-term levels of extracellular ade nosine, and appropriately adjust extracellular adenosine levels by a slow-o nset mechanism involving G(i) proteins and ecto-5'-nucleotidase.