title: Utilisation of the time constant calculated from heart rate recovery after exercise for evaluation of autonomic activity in horses.
abstract: BACKGROUND
  Heart rate (HR) recovery immediately after exercise is controlled by autonomic functions and the time constant (T) calculated from HR recovery is thought to be an index of parasympathetic activity in man.
  
  
  OBJECTIVE
  To investigate whether it is possible to evaluate autonomic function using the time constant in horses.
  
  
  METHODS
  Five Thoroughbred horses were subjected to a standard exercise test. Following pre-medication with saline, atropine and/or propranolol, the horses ran for 2.5 min at a speed of 8 m/sec at a 10% incline and T was calculated from HR after the exercise. Secondly, 7 Thoroughbred horses were then trained for 11 weeks and T and maximal oxygen uptake (VO2max) measured at intervals of 1 or 2 weeks. In 6 horses, T with atropine pre-medication was also measured before and after the whole training period. Furthermore, the HR variability at rest was evaluated by power spectral analysis at intervals of 3 or 4 weeks.
  
  
  RESULTS
  Time constant was increased by atropine and/or propranolol pre-medication, decreased with the progress of training and inversely correlated with VO2max during training (r = 0.43, P<0.005). Parasympathetic blockade significantly decreased T only after and not before, the training; however, T was lower in post training than in pretraining, irrespective of parasympathetic blockade. On the other hand, parasympathetic activity at rest was attenuated and sympathetic activity became predominant following the training.
  
  
  CONCLUSIONS
  Heart rate recovery is affected by sympathetic withdrawal and parasympathetic reactivation in horses and suggests that physical training hastened HR recovery by improving the parasympathetic function after exercise with aerobic capacity. However, the effects of other factors need to be considered because the training effect appeared on T even under parasympathetic blockade. The parasympathetic activity at rest is in contrast to that after exercise, suggesting that T does not reflect parasympathetic activity at rest.
  
  
  CONCLUSIONS
  If demonstrated how HR recovery is controlled after exercise, its analysis will be important in the evaluation of physical fitness in horses.
authors.affiliation: Equine Science Division, Hidaka Training and Research Center, Japan Racing Association, 535-13 Nischicha, Urakawa-cho, Uraakawagun, Hokkaido, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Hada
authors.forename: H
authors.initials: H
authors.lastname: Ohmura
authors.forename: K
authors.initials: K
authors.lastname: Mukai
authors.forename: D
authors.initials: D
authors.lastname: Eto
authors.forename: T
authors.initials: T
authors.lastname: Takahashi
authors.forename: A
authors.initials: A
authors.lastname: Hiraga
mesh_terms: D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001341:Autonomic Nervous System / Q000187:drug effects / Q000502:physiology*; D005080:Exercise Test / Q000662:veterinary; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006736:Horses / Q000502:physiology*; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology*; D010276:Parasympatholytics / Q000494:pharmacology*; D010805:Physical Conditioning, Animal / Q000502:physiology*; D010809:Physical Fitness / Q000502:physiology; D011433:Propranolol / Q000494:pharmacology
chemical_list: D010276:Parasympatholytics; D001285:Atropine; D011433:Propranolol