TY - JOUR T1 - Role of Adenosine A<sub>1</sub> Receptors in Modulating Extracellular Adenosine Levels JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 76 LP - 80 VL - 291 IS - 1 AU - Bradley T. Andresen AU - Delbert G. Gillespie AU - Zaichuan Mi AU - Raghvendra K. Dubey AU - Edwin K. Jackson Y1 - 1999/10/01 UR - http://jpet.aspetjournals.org/content/291/1/76.abstract N2 - 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 Gi 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 Giproteins and ecto-5′nucleotidase. The American Society for Pharmacology and Experimental Therapeutics ER -