Evidence for an A2 adenosine receptor in guinea pig lung

Abstract

Adenosine receptors in guinea pig lung were characterized by measurement of cyclic AMP formation and radioligand binding. 5′-N-Ethylcarboxamidoadenosine (NECA) increased cyclic AMP levels in lung slices about 4-fold over basal values with an EC₅₀ of 0.32 μmol/l. N⁶-R-(−)-Phenylisopropyladenosine (R-PIA) was 5-fold less potent than NECA. 5′-N-Methylcarboxamidoadenosine (MECA) and 2-chloroadenosine had EC₅₀-values of 0.29 and 2.6 μmol/l, whereas adenosine and inosine had no effect. The adenosine receptors in guinea pig lung can therefore be classified as A₂ receptors. Several xanthine derivatives antagonized the NECA-induced increase in cyclic AMP levels. 1,3-Diethyl-8-phenylxanthine (DPX; K _i 0.14 μmol/l) was the most potent analogue, followed by 8-phenyltheophylline (K _i 0.55 μmol/l), 3-isobutyl-1-methylxanthine (IBMX; K _i 2.9 μmol/l) and theophylline (K _i 8.1 μmol/l). In contrast, enprofylline (1 mmol/l) enhanced basal and NECA-stimulated cyclic AMP formation. In addition, we attempted to characterize these receptors in binding studies with [³H]NECA. The K _D for [³H]NECA was 0.25 μmol/l and the maximal number of binding sites was 12 pmol/mg protein. In competition experiments MECA (K _i 0.14 μmol/l) was the most potent inhibitor of [³H]NECA binding, followed by NECA (K _i 0.19 μmol/l) and 2-chloroadenosine (K _i 1.4 μmol/l). These results correlate well with the EC₅₀-values for cyclic AMP formation in lung slices. However, the K _i-values of R-PIA and theophylline were 240 and 270 μmol/l, and DPX and 8-phenyltheophylline did not compete for [³H]NECA binding sites. Therefore, a complete characterization of A₂ adenosine receptors by [³H]NECA binding was not achieved. In conclusion, our results show the presence of adenylate cyclase-coupled A₂ adenosine receptors in lung tissue which are antagonized by several xanthines.