Regulation of adenosine-activated GIRK channels by Gq-coupled receptors in mouse atrial myocytes

Adenosine (Ado) is an important mediator of the endogenous defense against ischemia-induced injury in the heart. The action of Ado is mediated by activation of G protein-gated inwardly rectifying K⁺ (GIRK) channels. In turn, GIRK channels are inhibited by reducing phosphatidylinositol 4,5-bisphosphate (PIP₂) through Gq protein-coupled receptors (GqPCRs). We previously found that GIRK channels activated by acetylcholine, a muscarinic M2 acetylcholine receptor agonist, are inhibited by GqPCRs in a receptor-specific manner. However, it is not known whether GIRK channels activated by Ado signaling are also regulated by GqPCRs. Presently, this was investigated in mouse atrial myocytes using the patch clamp technique. GIRK channels were activated by 100 μM Ado. When Ado was repetitively applied at intervals of 5∼6 min, the amplitude of second Ado-activated GIRK currents (I_K(Ado)) was 88.3±3.7% of the first I_K(Ado) in the control. Pretreatment of atrial myocytes with phenylephrine, endothelin-1, or bradykinin prior to a second application of Ado reduced the amplitude of the second I_K(Ado) to 25.5±11.6%, 30.5±5.6%, and 96.0±2.7%, respectively. The potency of I_K(Ado) inhibition by GqPCRs was different with that observed in acetylcholine-activated GIRK currents (I_K(ACh)) (endothelin-1 > phenylephrine > bradykinin). I _K(Ado) was almost completely inhibited by 500 μM of the PIP ₂ scavenger neomycin, suggesting low PIP₂ affinity of I_K(Ado). Taken together, these results suggest that the crosstalk between GqPCRs and the Ado-induced signaling pathway is receptor-specific. The differential change in PIP₂ affinity of GIRK channels activated by Ado and ACh may underlie, at least in part, their differential responses to GqPCR agonists.