Exercise training-induced bradycardia: evidence for enhanced parasympathetic regulation without changes in intrinsic sinoatrial node function.
The mechanisms responsible for exercise-induced reductions in baseline heart rate (HR), known as training bradycardia, remain controversial. Therefore, changes in cardiac autonomic regulation and intrinsic sinoatrial nodal (SAN) rate were evaluated using dogs randomly assigned to either a 10- to 12-wk exercise training (Ex, n = 15) or an equivalent sedentary period (Sed, n = 10). Intrinsic HR was revealed by combined autonomic nervous system (ANS) blockade (propranolol + atropine, iv) before and after completion of the study. At the end of the study, SAN function was further evaluated by examining the SAN recovery time (SNRT) following rapid atrial pacing and the response to adenosine in anesthetized animals. As expected, both the response to submaximal exercise and baseline HR significantly (P < 0.01) decreased, and heart rate variability (HRV; e.g., high-frequency R-R interval variability) significantly (P < 0.01) increased in the Ex group but did not change in the Sed group. Atropine also induced significantly (P < 0.01) greater reductions in HRV in the Ex group compared with the Sed group; propranolol elicited similar HR and HRV changes in both groups. In contrast, neither intrinsic HR (Ex before, 141.2 ± 6.7; Ex after, 146.0 ± 8.0 vs. Sed before, 143.3 ± 11.1; Sed after, 141.0 ± 11.3 beats per minute), the response to adenosine, corrected SNRT, nor atrial fibrosis and atrial fibrillation inducibility differed in the Ex group vs. the Sed group. These data suggest that in a large-animal model, training bradycardia results from an enhanced cardiac parasympathetic regulation and not from changes in intrinsic properties of the SAN.
Department of Physiology and Cell Biology, The Ohio State University, Columbus, Ohio; Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, Ohio; and billman.1@osu.edu.
George E
GE
Billman
Department of Physiology and Cell Biology, The Ohio State University, Columbus, Ohio;
Kristen L
KL
Cagnoli
Department of Physiology and Cell Biology, The Ohio State University, Columbus, Ohio;
Thomas
T
Csepe
Department of Physiology and Cell Biology, The Ohio State University, Columbus, Ohio;
Ning
N
Li
Department of Physiology and Cell Biology, The Ohio State University, Columbus, Ohio; Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, Ohio; and.
Patrick
P
Wright
Department of Physiology and Cell Biology, The Ohio State University, Columbus, Ohio; Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, Ohio; and Deaprtment of Internal Medicine, The Ohio State University, Columbus, Ohio.
Peter J
PJ
Mohler
Department of Physiology and Cell Biology, The Ohio State University, Columbus, Ohio; Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University, Columbus, Ohio; and.
Vadim V
VV
Fedorov
D000222:Adaptation, Physiological; D018674:Adrenergic Antagonists / Q000494:pharmacology; D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology; D001919:Bradycardia / Q000209:etiology* / Q000503:physiopathology / Q000628:therapy; D002304:Cardiac Pacing, Artificial; D018680:Cholinergic Antagonists / Q000494:pharmacology; D004285:Dogs; D005260:Female; D006339:Heart Rate; D008297:Male; D010275:Parasympathetic Nervous System / Q000503:physiopathology*; D005082:Physical Exertion; D011897:Random Allocation; D020127:Recovery of Function; D012849:Sinoatrial Node / Q000294:innervation*; D013997:Time Factors
D018674:Adrenergic Antagonists; D000889:Anti-Arrhythmia Agents; D018680:Cholinergic Antagonists
beta-adrenergic receptors; bradycardia; cardiac autonomic regulation; exercise training; heart rate variability; parasympathetic nervous system; sinoatrial node