In rat atrial myocytes GIRK (Kir3) channels can be activated by acetylcholine and adenosine via M₂ and A₁ receptors (M₂-R, A₁-R). Due to the lower density of A₁-R, the amplitude of current that can be activated by a saturating concentration (10 mM) of Ado (I_K(Ado)) amounts to 30 - 40% of maximum I_K(ACh). Whereas moderate adenovirus-driven overexpression of A₁-R results in an increase in I_K(Ado), strong overexpression results in a reduction or a complete lack of GIRK channel current activated by ACh and Ado respectively¹. This is paralleled by development of a constitutive background K⁺ current with strong inward-rectifying properties. This current is insensitive to the GIRK channel inhibitor tertiapin (100 nM), but is blocked by a low concentration of Ba²⁺ (2mM), which does not affect GIRK channel current, suggesting this current component to be carried by I_K1 channels composed of Kir2 subunits². Kir2.1 transcripts are increased twofold in myocytes overexpressing A₁-R. siRNA-mediated knockdown of Kir2.1 simultaneous with A₁-R overexpression caused a reduction in I_K1 by ~ 50%. These data demonstrate that manipulation of the expression level of a G protein-coupled receptor has unpredictable effects on functional expression of proteins that are supposed to be unrelated to the pathway controlled by that GPCR. This should be taken into account in experiments using heterologous expression systems to reconstitute signaling pathways. The mechanisms underlying the loss in sensitivity of GIRK channel current to A₁-R/M₂-R activation and the upregulation of functional I_K1 channels needs further investigation.

¹Bösche et al., J.Physiol. 550 (2003); ²Beckmann et al., Cell. Physiol. Biochem. 21 (2008)