GIRK channels in atrial myocytes are activated by various G-protein-coupled receptors including muscarinic M₂ (M₂AChR) and purinergic A₁ (A₁AdoR). In adult rat atrial myocytes the amplitude of GIRK current evoked by adenosine is limited by the density of A₁AdoR to ~30 % of the current in the presence of ACh at a saturating concentration. Transfection of atrial myocytes with conventional vectors encoding for A₁AdoR increases sensitivity to adenosine (Wellner-Kienitz et al. 2000). In the present study we confirmed these results by means of a more efficient adenovirus-based method of gene transfer (He et al. 1998). Adult rats were anaesthetized and killed by cervical dislocation. The heart was removed for isolation of myocytes. In a fraction of A₁AdoR transfected myocytes, displaying a particularly high fluorescence-intensity of the reporter protein (GFP), activation of GIRK current by Ado (10 mM) was followed by a rapid decay (t_1/2 ~ 300 ms). Upon washout of Ado a rebound of current was observed that was proportional to the degree of the initial decay. In those cells Ado caused an inhibition of M₂AChR-induced current in the continuous presence of ACh. In GTP-λ-S-loaded myocytes stably and irreversibly activated GIRK current was reversibly inhibited by Ado (Fig. 1).

Upon activation of endogenous M₂AChR by a highly saturating concentration of ACh (1 mM), a similar inhibitory component can be detected. These results suggest a novel mechanism of GIRK current inhibition by G-protein-coupled receptors, which requires fast activation of a large number of receptors, and which seems to be independent of GTP cycling of a heterotrimeric G-protein.

This work was supported by Deutsche Forschungsgemeinschaft.

All procedures accord with current National guidelines.