Exercise training bradycardia is largely explained by reduced intrinsic heart rate

IntroductionResting heart rate (RHR) declines with exercise training. Possible mechanisms include: 1) increased parasympathetic tone, 2) decreased responsiveness to beta-adrenergic stimulation, 3) decreased intrinsic heart rate or 4) combination of these factors.ObjectiveTo determine whether an increase in resting parasympathetic tone or decrease in response to beta-adrenergic stimulation contributes to the decrease in RHR with training.Methods51 screened healthy subjects aged 18-32 (n = 20, mean age 26, 11 female) or 65-80 (n = 31, mean age 69, 16 female) were tested before and after 6 months of supervised exercise training. Heart rate response to parasympathetic withdrawal was assessed using atropine and beta-adrenergic responsiveness during parasympathetic withdrawal using isoproterenol.ResultsTraining increased VO2 max by 17% (28.7 ± 7.7 to 33.6 ± 9.20 ml/kg/min, P < 0.001). RHR decreased from 62.8 ± 6.6 to 57.6 ± 7.2 beats per minute (P < 0.0001). The increase in heart rate in response to parasympathetic withdrawal was unchanged after training (+ 37.3 ± 12.8 pre vs. + 36.4 ± 12.2 beats per min post, P = 0.41). There was no change in the heart rate response to isoproterenol after parasympathetic blockade with training (+ 31.9 ± 10.9 pre vs. + 31.0 ± 12.0 post beats per min, P = 0.56). The findings were similar in all four subgroups.ConclusionsWe did not find evidence that an increase in parasympathetic tone or a decrease in responsiveness to beta-adrenergic activity accounts for the reduction in resting heart rate with exercise training. We suggest that a decline in heart rate with training is most likely due to decrease in the intrinsic heart rate.