Functional characterization of an endogenous Xenopus oocyte adenosine receptor

Abstract

1. To investigate the effects of adenosine on endogenous Xenopus oocyte receptors, we analysed defolliculated oocytes injected with mRNAs for the G protein-activated inwardly rectifying K+ (GIRK) channels. 2. In oocytes injected with mRNAs for either GIRK1/GIRK2 or GIRK1/GIRK4 subunits, application of adenosine or ATP reversibly induced inward K+ currents, although ATP was less potent than adenosine. The responses were attenuated by caffeine, a non-selective adenosine receptor antagonist. Furthermore, in uninjected oocytes from the same donor, adenosine produced no significant current. 3. The endogenous receptor was activated by two selective A1 adenosine receptor agonists, N6-cyclopentyladenosine (CPA) and N6-cyclohexyladenosine (CHA), and antagonized by a selective A1 adenosine receptor antagonist, 1,3-dipropyl-8-cyclopenylxanthine (DPCPX) at moderate nanomolar concentrations, but insensitive to micromolar concentrations of selective A2A and A3 adenosine receptor agonists, 2-[p-(2-carbonyl-ethyl)-phenylethylamino]-5′-N- ethylcarboxamidoadenosine (CGS21680) and N6-(3-iodobenzyl)-5′-(N-methylcarbamoyl)adenosine (IB-MECA), respectively. However, the pharmacological characteristics of the receptor were different from those of the cloned Xenopus A1 adenosine receptor and previously proposed adenosine receptors. 4. The adenosine-induced GIRK currents were abolished by injection of pertussis toxin and CPA inhibited forskolin-stimulated cyclic AMP accumulation. 5. We conclude that an adenosine receptor on the Xenopus oocyte membrane can activate GIRK channels and inhibit adenylyl cyclase via Gi/o proteins. Moreover, our results suggest the existence of an endogenous adenosine receptor with the unique pharmacological characteristics. As the receptor was activated by nanomolar concentrations of adenosine, which is a normal constituent of extracellular fluid, the receptor may be involved in some effects through the Gi/o protein signalling pathways in ovarian physiology.