Comparison of the potency of adenosine as an agonist at human adenosine receptors expressed in Chinese hamster ovary cells

The potency of adenosine and inosine as agonists at human adenosine recepto rs was examined in a functional assay using changes in cyclic AMP (cAMP) fo rmation in intact Chinese hamster ovary (CHO) cells stably transfected with the human A(1), A(2A), A(2B), and A(3) receptors. Adenosine increased cAMP formation in cells expressing the A(2A) (Ec(50): 0.7 muM) and A(2B) (Ec(50 ): 24 muM) receptors and inhibited forskolin (0.3-3 muM)-stimulated cAMP fo rmation in cells expressing the A(1) (Ec(50): 0.7 muM) and A(3) receptors ( Ec(50): 0.29 muM). The potency of adenosine at the A(2A) and A(2B) receptor s was not altered by the presence of the uptake inhibitor nitrobenzylthioin osine (NBMPR), whereas it was increased about h-fold by NBMPR at the A(1) a nd A(3) receptors. In the presence of NBMPR, inosine was a potent agonist ( Ec(50): 7 and 0.08 muM at the A(1) and A(3) receptors, respectively), but w ith low efficacy especially at the A(3) receptors. No effect of inosine was seen at the A(2) receptors. Caffeine, theophylline, and paraxanthine shift ed the dose-response curve for adenosine at the A(1), A(2A), and A(2B) rece ptors. These results indicate that adenosine is the endogenous agonist at a ll human adenosine receptors and that physiological levels of this nucleosi de can activate A(1), A(2A), and A(3) receptors on cells where they are abu ndantly expressed, whereas pathophysiological conditions are required to st imulate A(2B) receptors to produce cyclic AMP. (C) 2001 Elsevier Science In c. All rights reserved.