Exercise Training Bradycardia is Largely Explained by Reduced Intrinsic Heart Rate

Abstract Introduction Resting 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. Objective To determine whether an increase in resting parasympathetic tone or decrease in response to beta-adrenergic stimulation contribute to the decrease in RHR with training. Methods 51 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. Results Training 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. Conclusions We did not find evidence that 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 decline in heart rate with training is most likely due to decrease in the intrinsic heart rate.