Cardiac vagal withdrawal and reactivation during repeated rest-exercise transitions.

It is not known whether subjects that have higher cardiac vagal reactivation (CVR) during repeated exercise transitions also have higher cardiac vagal withdrawal (CVW) at the onset of exercise, which would lead to better heart rate (HR) regulation during exercise transitions. Therefore, our aims were to investigate: (a) the influence of CVR on CVW during repeated rest-exercise transitions; and (b) the influence of the sympathetic activity on CVR and CVW. Fifty-eight healthy men (22 ± 4 years) performed 20 rest-exercise transitions interspaced by 30 s. In addition, nine healthy men (24 ± 3 years) ingested either 25 mg of atenolol or placebo, on a crossover, double-blind, randomized design, then performed 20 rest-exercise transitions interspaced by 30 s. Cardiac vagal reactivation was assessed by a HR variability index (RMSSD) and CVW by the HR increase at the onset of a valid and reliable cycling protocol. The CVR and CVW responses were associated (partial r ranged from 0.60 to 0.66; p < 0.05). Participants with higher CVR over transitions maintained their CVW over repeated transitions [first transition (mean ± SEM) = 1.59 ± 0.04 vs. 20th = 1.50 ± 0.03 (a.u.), p = 0.24], while participants with lower CVR had a CVW decrease over repeated transitions [first transition (mean ± SEM) = 1.38 ± 0.04 vs. 20th = 1.19 ± 0.03 (a.u.), p < 0.01). In addition, the CVR and CVW over the rest-exercise transitions were similar during atenolol and placebo (ANCOVA interaction p = 0.12 and p = 0.48, respectively). In conclusion, the CVR among repeated rest-exercise transitions influenced the CVW at the onset of exercise, which was not affected by a partial β(1) cardioselective adrenoceptor blockade.