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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 Ca 2+  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 v 1.3 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 v 1.3. Strategies targeting cardiac  I   KACh   may therefore represent an alternative to pacemaker implantation for bradyarrhythmias seen in some veteran athletes.

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