Abstract 12351: Exercise Training May Reduce Intrinsic Heart Rate

Background: 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, which is the heart rate independent of any nervous or hormonal influences, and 4) a combination of these factors. Objectives: To determine whether an increase in resting parasympathetic tone or a decrease in response to beta-adrenergic stimulation contribute to the decrease in RHR with exercise training. Methods: 51 rigorously screened sedentary 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 endurance exercise training. Heart rate response to parasympathetic withdrawal was assessed using atropine (0.02mg/kg up to 2mg). During continued parasympathetic blockade, beta-adrenergic responsiveness was assessed using isoproterenol (14ng/kg/min x 14min). Results: Training increased VO2 max by 17% (28.7 to 33.6 ml/kg/min, p Conclusions: In healthy subjects, this study did not find evidence that an increase in parasympathetic tone or a decrease in responsiveness to beta-adrenergic stimulation accounts for the reduction in resting heart rate with exercise training, and suggests that a decline in heart rate with training is most likely due to a decrease in the intrinsic heart rate.