GPCR-dependent biasing of GIRK channel signaling dynamics by RGS6 in mouse sinoatrial nodal cells.

How G protein-coupled receptors (GPCRs) evoke specific biological outcomes while utilizing a limited array of G proteins and effectors is poorly understood, particularly in native cell systems. Here, we examined signaling evoked by muscarinic (MR) and adenosine (AR) receptor activation in the mouse sinoatrial node (SAN), the cardiac pacemaker. MR and AR activate a shared pool of cardiac G protein-gated inwardly rectifying K (GIRK) channels in SAN cells from adult mice, but AR-GIRK responses are smaller and slower than MR-GIRK responses. Recordings from mice lacking Regulator of G protein Signaling 6 (RGS6) revealed that RGS6 exerts a GPCR-dependent influence on GIRK-dependent signaling in SAN cells, suppressing MR-GIRK coupling efficiency and kinetics and AR-GIRK signaling amplitude. Fast kinetic bioluminescence resonance energy transfer assays in transfected HEK cells showed that RGS6 prefers Gα over Gα as a substrate for its catalytic activity and that MR signals preferentially via Gα, while AR does not discriminate between inhibitory G protein isoforms. The impact of atrial/SAN-selective ablation of Gα or Gα was consistent with these findings. Gα ablation had minimal impact on MR-GIRK and AR-GIRK signaling in SAN cells. In contrast, Gα ablation decreased the amplitude and slowed the kinetics of MR-GIRK responses, while enhancing the sensitivity and prolonging the deactivation rate of AR-GIRK signaling. Collectively, our data show that differences in GPCR-G protein coupling preferences, and the Gα substrate preference of RGS6, shape AR- and MR-GIRK signaling dynamics in mouse SAN cells.