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The potency of adenosine and inosine as agonists at human adenosine receptors was examined in a functional assay using changes in cyclic AMP (cAMP) formation in intact Chinese hamster ovary (CHO) cells stably transfected with the human A₁, A_2A, A_2B, and A₃ receptors. Adenosine increased cAMP formation in cells expressing the A_2A (ec₅₀: 0.7 μM) and A_2B (ec₅₀: 24 μM) receptors and inhibited forskolin (0.3–3 μM)-stimulated cAMP formation in cells expressing the A₁ (ec₅₀: 0.31 μM) and A₃ receptors (ec₅₀: 0.29 μM). The potency of adenosine at the A_2A and A_2B receptors was not altered by the presence of the uptake inhibitor nitrobenzylthioinosine (NBMPR), whereas it was increased about 6-fold by NBMPR at the A₁ and A₃ receptors. In the presence of NBMPR, inosine was a potent agonist (ec₅₀: 7 and 0.08 μM at the A₁ and A₃ receptors, respectively), but with low efficacy especially at the A₃ …