THE ADENOSINE RECEPTORS PRESENT ON THE PLASMA-MEMBRANE OF CHROMAFFIN CELLS ARE OF THE A2B SUBTYPE

The adenosine receptors in the plasma membrane of chromaffin cells from bovine adrenal medulla were characterized. The presence of A1 receptors was discounted owing to the absence of R-[H-3]phenylisopropyladenosine (R-PIA) and [H-3]8-cyclopentyl-1,3-dipropylxanthine ([H-3]-DPCPX) binding. The binding of the specific A2a ligand CGS-21680 was low. In contrast, the binding of 5'-(N-[H-3]-ethylcarboxamidoadenosine ([H-3]-NECA) was relatively high (1.7 pmol/mg of protein at a ligand concentration up to 90 nM). This binding did not correspond to non-adenosine receptor NECA binding sites because the specific [H-3]NECA binding was similar when unlabeled adenosine, NECA, or R-PIA was used to measure the nonspecific binding. The rank order of potency of different ligands for the displacement of specific [H-3]NECA binding was DPCPX > NECA > chloroadenosine > R-PIA > theophylline = CGS-21680. These results indicate that the receptors present on the plasma membrane of chromaffin cells are exclusively of the A2b subtype.