Evidence for an A2 adenosine receptor in guinea pig lung

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 mumol/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 mumol/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; Ki 0.14 mumol/l) was the most potent analogue, followed by 8-phenyltheophylline (Ki 0.55 mumol/l), 3-isobutyl-1-methylxanthine (IBMX; Ki 2.9 mumol/l) and theophylline (Ki 8.1 mumol/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 KD for [3H]NECA was 0.25 mumol/l and the maximal number of binding sites was 12 pmol/mg protein. In competition experiments MECA (Ki 0.14 mumol/l) was the most potent inhibitor of [3H]NECA binding, followed by NECA (Ki 0.19 mumol/l) and 2-chloroadenosine (Ki 1.4 mumol/l). These results correlate well with the EC50-values for cyclic AMP formation in lung slices. However, the Ki-values of R-PIA and theophylline were 240 and 270 mumol/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.(ABSTRACT TRUNCATED AT 250 WORDS)