Influence of RGS6 on Inhibitory G Protein Signaling in Mouse Sinoatrial Nodal Cells

Abstract

IntroductionM2 muscarinic (M2R) activation of G protein‐gated inwardly rectifying K+ (GIRK) channels in sinoatrial nodal (SAN) cells is a key mediator of vagal/parasympathetic slowing of heart rate. The strength of M2R‐GIRK signaling is negatively regulated by Regulator of G protein Signaling 6 (RGS6). Adenosine, used clinically to diagnose and treat certain arrhythmias, exerts these actions through A1 adenosine receptor (A1R) activation of GIRK channels. The extent to which RGS6 regulates A1R‐GIRK signaling, however, is unclear. Here, we investigated the impact of RGS6 on A1R‐GIRK and M2R‐GIRK signaling in mouse SAN cells.Methods and ResultsWe measured the efficacy and potency of M2R‐ and A1R‐GIRK signaling in adult SAN cells from wild‐type and RGS6−/− mice using whole‐cell patch‐clamp electrophysiology. As previously reported, SAN cells from RGS6−/− mice displayed prolonged channel deactivation kinetics and increased channel sensitivity to the non‐selective cholinergic agonist carbachol (CCh), relative to wild‐type counterparts. In contrast, we observed a striking increase in the amplitude of the GIRK response evoked by adenosine (Ado) in RGS6−/− SAN cells compared to wild‐type controls, with no impact on channel deactivation kinetics or sensitivity. Further recordings in wild‐type SAN cells revealed that M2R activation completely occludes the Ado‐induced response, whereas Ado application activates only a smaller fraction of GIRK channels. Both CCh‐ and Ado‐ induced currents were abolished in SAN cells treated by pertussis toxin (PTX), indicating these responses are mediated by Gi/o G proteins. Utilizing a bioluminescence resonance energy transfer (BRET) assay in transfected HEK cells, we found that RGS6 prefers Gαo as a substrate for over Gαi. Additionally, we found that while A1R does not discriminate between Gαo and Gαi, M2R displays a clear preference for Gαo. We next employed a Cre/flox strategy to remove Gαo from atrial tissue, including SAN cells. Loss of Gαo yielded significantly blunted CCh‐induced responses, which also displayed prolonged activation and deactivation kinetics. While Ado‐induced responses had prolonged deactivation kinetics, activation kinetics and response amplitude were unaffected.ConclusionsThese results suggest that RGS6 negatively regulates A1R‐GIRK signaling distinct from its influence on M2R‐GIRK signaling in mouse SAN cells likely due to differing utilization of Gao by M2R and A1R.Support or Funding InformationThis work was supported by NIH grants R01 HL105550 and F31 HL139090.