Differential Impact of RGS6 on GIRK‐dependent Muscarinic and Adenosine Receptor Signaling in Mouse Sinoatrial Nodal Cells

Abstract

IntroductionParasympathetic slowing of heart rate is largely mediated by muscarinic receptor (M2R) activation of the G protein gated inwardly‐rectifying potassium (GIRK) channel on sinoatrial nodal (SAN) cells and atrial myocytes. The strength of M2R‐GIRK signaling is negatively regulated by regulator of G protein signaling 6, RGS6. Genetic ablation of RGS6 in mice gives rise to enhanced M2R‐GIRK signaling in SAN cells, resulting in both exaggerated M2R‐induced bradycardia and increased susceptibility to pacing‐induced atrial fibrillation. Adenosine, used clinically to both diagnose and treat certain arrhythmias, exerts these actions through adenosine receptor (A1R) activation of cardiac GIRK channels. The extent to which RGS6 regulates A1R‐GIRK signaling, however, is not understood. Here, we investigated and compared the impact of RGS6 on A1R‐GIRK and M2R‐GIRK signaling in mouse SAN cells and heart rate.Methods and ResultsUsing whole‐cell patch‐clamp electrophysiology, we measured the efficacy and potency of M2R‐ and A1R‐GIRK channel signaling in adult SAN cells from wildtype and Rgs6−/− mice. SAN cells from Rgs6−/− mice displayed both prolonged channel deactivation kinetics and increased channel sensitivity to CCh‐induced currents as compared to wild‐type SAN cells. Surprisingly, we found no difference in kinetics or channel sensitivity of A1R‐GIRK responses in Rgs6−/− SAN cells. We did observe a striking, significant increase in the amplitude of the A1R‐GIRK response in Rgs6−/− SAN cells compared to wild‐type controls. Recordings from mice lacking cardiac GIRK channels (Girk4−/−) confirm that M2R‐ and A1R‐induced currents are GIRK‐dependent. Furthermore, occlusion studies in wild‐type SAN cells suggest that M2R activates nearly all of the GIRK channels present in SAN cells, while A1R activates a smaller portion of GIRK channels. Intriguingly, RGS6 ablation seems to allow for the activation of a larger proportion of GIRK channels by A1R. Heart rate measurements from isolated Rgs6−/− hearts show exaggerated bradycardia in response to the same doses of CPA, an A1R agonist, as compared to wild‐type controls. Significantly lower heart rates were observed in response to lower doses of CCh in isolated Rgs6−/− hearts as compared to wild‐type hearts, suggesting a shift in sensitivity.ConclusionsOur results suggest that A1R‐GIRK channel signaling displays distinct differences from M2R‐GIRK channel signaling, resulting in distinct negative regulation by RGS6 in mouse SAN cells and the isolated heart.Support or Funding InformationThis work was supported by NIH grants KW (HL105550) and AA (HL139090).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.