The incidence of brady-arrhythmias is known to be higher in athletes. In rodent models of exercise training, bradycardia results from downregulation of f-(HCN4) channels leading to a reduction of the If current in the sino-atrial node (SAN).

To test if genetic ablation of G-protein-gated inwardly rectifying potassium 4 (Girk4) channel prevents sinus bradycardia induced by intensive exercise training in mice.

Control (WT) and homozygous Girk4 knock-out (Girk4-/-) mice were assigned to trained or sedentary groups. Mice in the trained group underwent 1-hour exercise swimming twice a day for 28 days, 7 days per week whereas sedentary mice underwent 5-min swimming daily in the same period. We performed telemetric electrocardiogram recordings in both groups at baseline and during the training period. At training cessation, mice were euthanized and SAN tissues were isolated for patch clamp recordings in isolated SAN cells and transcriptional profiling by quantitative PCR (qPCR).

At day 17 of the swimming regimen, heart rate (HR) reduction in trained WT mice was significantly different vs. WT sedentary mice, whereas Girk4-/- mice failed to develop sinus bradycardia. Action potential recordings in isolated SAN cells from trained WT mice showed lower rates of pacemaking in comparison to cells from mice of the other groups. In line with HR reduction, the density of If was significantly reduced only in SAN cells obtained from WT-trained mice. Correspondingly, qPCR analysis showed mRNA expression of HCN4 was significantly lower in WT-trained SAN relative to WT sedentary animals, but unchanged in Girk4-/- animals. This finding could be attributed to a significant increase in the expression of miR-423-5p (a transcriptional repressor of HCN4) observed in trained WT, but not trained Girk4-/- animals.

Genetic ablation of Girk4 channels prevents sinus bradycardia induced by down regulation of f-HCN4 channels in trained WT mice.