ADENOSINE-A(1) RECEPTORS AND ERYTHROPOIETIN PRODUCTION

N6-cyclopentyladenosine (CPA), a selective adenosine A1 receptor agoni st, in a concentration range of 10(-9) to 10(-7) M, produced a signifi cant decrease in erythropoietin (EPO) levels in a human hepatocellular carcinoma (Hep G2) cell culture (medium levels of EPO, 91.81 +/- 1.61 and 94.36 +/- 0.97% of control, respectively) after 24 h incubation i n a hypoxic atmosphere. CPA, at a concentration of 10(-9) M, also prod uced a significant decrease in Hep G2 cell levels of adenosine 3',5'-c yclic monophosphate (cAMP; 78.13 +/- 3.89% of control) after 2 h incub ation. CPA (10(-9) M) also significantly inhibited forskolin-stimulate d increases in EPO production and cAMP accumulation in Hep G2 cells. O n the other hand, yl)phenethyl-amino]-5'-N-ethylcarboxamidoadenosine ( CGS-21680), a selective adenosine A2-receptor agonist, produced no sig nificant change in EPO production in a dose range of 10(-10) to 10(-6) M but increased cAMP accumulation at 10(-6) M. A1-receptor binding as says using N6-[H-3]cyclohexyladenosine revealed a single type of adeno sine receptor binding site on Hep G2 cell membranes with a dissociatio n constant of 71.4 nM and a binding capacity of 1,530 fmol/mg protein. These results indicate that Hep G2 cells contain high-affinity adenos ine A1 receptors that are linked to decreased cAMP accumulation and EP O production.