RGS4

The same paper had a genetic version of that finding.

Mice missing a protein called RGS4, which normally dampens vagus nerve signaling in the heart, were more prone to atrial fibrillation when the researchers stimulated that pathway with the drug.

The tempting conclusion here is that backing off exercise fixes it, but no one has done that study in humans.

The closest thing we have is a group of former world-class rowers where atrial fibrillation was actually more common in the ones who had retired, many of them decades earlier than the ones who were still training.