An article from three British researchers makes a good case that vagal tone is not the reason for the athlete's bradycardia in a 2015 commentary in the Journal of Physiology.see footnotes
Traditionally, the slow heart rate of endurance athletes is attributed to increased vagal tone but the authors argue that this increased tone has never been demonstrated at least in part because it is not clear how that could even be measured as the vagus carries both afferent and efferent fibbers. Further experimental blockage of the autonomic nervous system has not lead to mitigation of the bradycardia.
They suggests that the slow heart rate is also not due to SA node fibrosis ( apparently a common explanation for SA node disease) as that also has never been demonstrated. They present evidence that the culprit is remodeling of the ion channels and related molecules in the sinus node-more specifically downregulation of the HCN4 channel (also strangely known as the "funny channel") . They reference a number of articles claiming there is remolding of ion channels in aging and familial bradycardia and pulmonary hypertension.
Endurance exercise is known to cause heart remodeling leading to the features of the so called athletes heart ( so called eccentric hypertrophy). It is also know that cessation of exercise will lead to some degree of "re-remodeling" returning the athletes heart to some degree of that of the sedentary person. Will a similar reversal happen in he electrical system in endurance athletes who quite exercise? or is this remodeling permanent? Baldesberger et al published a long term followup in retired professional cyclists who had stopped training for over thirty years and did report an unimpressive lower heart rate on Holter monitoring in 62 cyclists versus 62 golfers ( 66 +- 9 versus 70 +- 8 but 20% did have a HR less than 40 during the day. versus 6 % in the controls.So is the sinus nodes remodeling to some degree not reversible-these cyclists had not trained for 30 years. (There is more interesting data from this study which is a topic for another blog commentary,here the interest was in the bradycardia not other possible cardiac complications . and it did appear the cyclists had a bit more bradycardia.)
1) D'Souza, A et al . Cross talk opposing view:Bradycardia in the trained athlete is attributable to a downregulation of a pacemaker channel in the sinus node. Journal of Physiology 593 no. 8 1749-1751,April 2015
2) Francesco, D. The Role of the Funny Current in Pacemaker Activity. Circ. Research 2010;106, 434-446
3) Baldesberger, S et al Sinus Node disease and arrhythmias in the long term follow-up of former professional cyclists. Eur hear journal 2008 29, 71-78