Genetic Ablation of G Protein-Gated Inwardly Rectifying K⁺ Channels Prevents Training-Induced Sinus Bradycardia

Background: Endurance athletes are prone to bradyarrhythmias, which in the long-term may underscore the increased incidence of pacemaker implantation reported in this population. Our previous work in rodent models has shown training-induced sinus bradycardia to be due to microRNA (miR)-mediated transcriptional remodeling of the HCN4 channel, leading to a reduction of the “funny” (I_f) current in the sinoatrial node (SAN). Objective: To test if genetic ablation of G-protein-gated inwardly rectifying potassium channel, also known as I_KACh channels prevents sinus bradycardia induced by intensive exercise training in mice. Methods: Control wild-type (WT) and mice lacking GIRK4 (Girk4^–/–), an integral subunit of I_KACh were assigned to trained or sedentary groups. Mice in the trained group underwent 1-h exercise swimming twice a day for 28 days, 7 days per week. We performed electrocardiogram recordings and echocardiography in both groups at baseline, during and after the training period. At training cessation, mice were euthanized and SAN tissues were isolated for patch clamp recordings in isolated SAN cells and molecular profiling by quantitative PCR (qPCR) and western blotting. Results: At swimming cessation trained WT mice presented with a significantly lower resting HR that was reversible by acute I_KACh block whereas Girk4^–/– mice failed to develop a training-induced sinus bradycardia. In line with HR reduction, action potential rate, density of I_f, as well as of T- and L-type Ca²⁺ currents (I_CaT and I_CaL) were significantly reduced only in SAN cells obtained from WT-trained mice. I_f reduction in WT mice was concomitant with downregulation of HCN4 transcript and protein, attributable to increased expression of corresponding repressor microRNAs (miRs) whereas reduced I_CaL in WT mice was associated with reduced Ca_v1.3 protein levels. Strikingly, I_KACh ablation suppressed all training-induced molecular remodeling observed in WT mice. Conclusion: Genetic ablation of cardiac I_KACh in mice prevents exercise-induced sinus bradycardia by suppressing training induced remodeling of inward currents I_f, I_CaT and I_CaL due in part to the prevention of miR-mediated transcriptional remodeling of HCN4 and likely post transcriptional remodeling of Ca_v1.3. Strategies targeting cardiac I_KACh may therefore represent an alternative to pacemaker implantation for bradyarrhythmias seen in some veteran athletes.