title: Exercise training bradycardia is largely explained by reduced intrinsic heart rate.
abstract: 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 or 4) combination of these factors.
  
  
  OBJECTIVE
  To determine whether an increase in resting parasympathetic tone or decrease in response to beta-adrenergic stimulation contributes 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 6months 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±9.20ml/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 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.
authors: Division of Cardiology, Department of Medicine, VA Medical Center and University of Washington, Seattle, WA, United States. Electronic address: samira11@uw.edu.
authors: Samira
authors: S
authors: Bahrainy
authors: Division of Cardiology, Department of Medicine, VA Medical Center and University of Washington, Seattle, WA, United States.
authors: Wayne C
authors: WC
authors: Levy
authors: Department of Radiology, University of Washington, Seattle, WA, United States.
authors: Janet M
authors: JM
authors: Busey
authors: Division of Cardiology, Department of Medicine, VA Medical Center and University of Washington, Seattle, WA, United States; Division of Nuclear Medicine, VA Medical Center and University of Washington, Seattle, WA, United States; Department of Radiology, University of Washington, Seattle, WA, United States.
authors: James H
authors: JH
authors: Caldwell
authors: Division of Cardiology, Department of Medicine, VA Medical Center and University of Washington, Seattle, WA, United States.
authors: John R
authors: JR
authors: Stratton
mesh_terms: D058671:Adrenergic beta-1 Receptor Antagonists / Q000494:pharmacology; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000328:Adult; D000367:Age Factors; D000368:Aged; D001341:Autonomic Nervous System / Q000502:physiology; D001919:Bradycardia / Q000209:etiology / Q000503:physiopathology; D015444:Exercise / Q000502:physiology*; D005260:Female; D064368:Healthy Volunteers; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology*; D008297:Male; D008790:Metoprolol / Q000494:pharmacology*; D011943:Receptors, Adrenergic, beta / Q000502:physiology
chemical_list: D058671:Adrenergic beta-1 Receptor Antagonists; D000318:Adrenergic beta-Agonists; D011943:Receptors, Adrenergic, beta; D008790:Metoprolol; D007545:Isoproterenol
keywords: Autonomic control; Exercise training; Resting heart rate