title: Parasympathetic neural activity accounts for the lowering of exercise heart rate at high altitude.
abstract: BACKGROUND
  In chronic hypoxia, both heart rate (HR) and cardiac output (Q) are reduced during exercise. The role of parasympathetic neural activity in lowering HR is unresolved, and its influence on Q and oxygen transport at high altitude has never been studied.
  
  
  RESULTS
  HR, Q, oxygen uptake, mean arterial pressure, and leg blood flow were determined at rest and during cycle exercise with and without vagal blockade with glycopyrrolate in 7 healthy lowlanders after 9 weeks' residence at >/=5260 m (ALT). At ALT, glycopyrrolate increased resting HR by 80 bpm (73+/-4 to 153+/-4 bpm) compared with 53 bpm (61+/-3 to 114+/-6 bpm) at sea level (SL). During exercise at ALT, glycopyrrolate increased HR by approximately 40 bpm both at submaximal (127+/-4 to 170+/-3 bpm; 118 W) and maximal (141+/-6 to 180+/-2 bpm) exercise, whereas at SL, the increase was only by 16 bpm (137+/-6 to 153+/-4 bpm) at 118 W, with no effect at maximal exercise (181+/-2 bpm). Despite restoration of maximal HR to SL values, glycopyrrolate had no influence on Q, which was reduced at ALT. Breathing FIO(2)=0.55 at peak exercise restored Q and power output to SL values.
  
  
  CONCLUSIONS
  Enhanced parasympathetic neural activity accounts for the lowering of HR during exercise at ALT without influencing Q. The abrupt restoration of peak exercise Q in chronic hypoxia to maximal SL values when arterial PO(2) and SO(2) are similarly increased suggests hypoxia-mediated attenuation of Q.
authors.affiliation: Copenhagen Muscle Research Center, Rigshospitalet, University of Copenhagen, Denmark. boushel@alcor.concordia.ca
authors.forename: R
authors.initials: R
authors.lastname: Boushel
authors.forename: J A
authors.initials: JA
authors.lastname: Calbet
authors.forename: G
authors.initials: G
authors.lastname: Rådegran
authors.forename: H
authors.initials: H
authors.lastname: Sondergaard
authors.forename: P D
authors.initials: PD
authors.lastname: Wagner
authors.forename: B
authors.initials: B
authors.lastname: Saltin
mesh_terms: D000064:Acclimatization / Q000502:physiology; D000328:Adult; D000531:Altitude; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D002302:Cardiac Output / Q000502:physiology; D002908:Chronic Disease; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D005260:Female; D006024:Glycopyrrolate / Q000494:pharmacology; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology; D007866:Leg / Q000098:blood supply / Q000502:physiology; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology; D010275:Parasympathetic Nervous System / Q000502:physiology*; D012016:Reference Values; D012039:Regional Blood Flow / Q000502:physiology; D014630:Vagus Nerve / Q000187:drug effects / Q000502:physiology
chemical_list: D006024:Glycopyrrolate