title: The Effects of Sympathetic Inhibition on Metabolic and Cardiopulmonary Responses to Exercise in Hypoxic Conditions.
abstract: OBJECTIVE
  Pre-exertion skeletal muscle glycogen content is an important physiological determinant of endurance exercise performance: low glycogen stores contribute to premature fatigue. In low-oxygen environments (hypoxia), the important contribution of carbohydrates to endurance performance is further enhanced as glucose and glycogen dependence is increased; however, the insulin sensitivity of healthy adult humans is decreased. In light of this insulin resistance, maintaining skeletal muscle glycogen in hypoxia becomes difficult, and subsequent endurance performance is impaired. Sympathetic inhibition promotes insulin sensitivity in hypoxia but may impair hypoxic exercise performance, in part due to suppression of cardiac output. Accordingly, we tested the hypothesis that hypoxic exercise performance after intravenous glucose feeding in a low-oxygen environment will be attenuated when feeding occurs during sympathetic inhibition.
  
  
  METHODS
  On 2 separate occasions, while breathing a hypoxic gas mixture, 10 healthy men received 1 hour of parenteral carbohydrate infusion (20% glucose solution in saline; 75 g), after which they performed stationary cycle ergometer exercise (~65% maximal oxygen uptake) until exhaustion. Forty-eight hours before 1 visit, chosen randomly, sympathetic inhibition via transdermal clonidine (0.2 mg/d) was initiated.
  
  
  RESULTS
  The mean time to exhaustion after glucose feeding both with and without sympathetic inhibition was not different (22.7 ± 5.4 minutes vs 23.5 ± 5.1 minutes; P = .73).
  
  
  CONCLUSIONS
  Sympathetic inhibition protects against hypoxia-mediated insulin resistance without influencing subsequent hypoxic endurance performance.
authors.affiliation: Department of Health and Exercise Science, Colorado State University, Fort Collins, CO (Drs Scalzo, Hamilton, and Bell, Mr Peltonen, Binns, and Szallar, and Ms Klochak and Wood).
authors.forename: Rebecca L
authors.initials: RL
authors.lastname: Scalzo
authors.affiliation: Department of Health and Exercise Science, Colorado State University, Fort Collins, CO (Drs Scalzo, Hamilton, and Bell, Mr Peltonen, Binns, and Szallar, and Ms Klochak and Wood).
authors.forename: Garrett L
authors.initials: GL
authors.lastname: Peltonen
authors.affiliation: Department of Health and Exercise Science, Colorado State University, Fort Collins, CO (Drs Scalzo, Hamilton, and Bell, Mr Peltonen, Binns, and Szallar, and Ms Klochak and Wood).
authors.forename: Scott E
authors.initials: SE
authors.lastname: Binns
authors.affiliation: Department of Health and Exercise Science, Colorado State University, Fort Collins, CO (Drs Scalzo, Hamilton, and Bell, Mr Peltonen, Binns, and Szallar, and Ms Klochak and Wood).
authors.forename: Anna L
authors.initials: AL
authors.lastname: Klochak
authors.affiliation: Department of Health and Exercise Science, Colorado State University, Fort Collins, CO (Drs Scalzo, Hamilton, and Bell, Mr Peltonen, Binns, and Szallar, and Ms Klochak and Wood).
authors.forename: Steve E
authors.initials: SE
authors.lastname: Szallar
authors.affiliation: Department of Health and Exercise Science, Colorado State University, Fort Collins, CO (Drs Scalzo, Hamilton, and Bell, Mr Peltonen, Binns, and Szallar, and Ms Klochak and Wood).
authors.forename: Lacey M
authors.initials: LM
authors.lastname: Wood
authors.affiliation: Heart Center of the Rockies, University of Colorado Health, Fort Collins, CO (Drs Larson and Luckasen).
authors.forename: Dennis G
authors.initials: DG
authors.lastname: Larson
authors.affiliation: Heart Center of the Rockies, University of Colorado Health, Fort Collins, CO (Drs Larson and Luckasen).
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Luckasen
authors.affiliation: University of Colorado Denver, Denver, CO (Dr Irwin).
authors.forename: David
authors.initials: D
authors.lastname: Irwin
authors.affiliation: Department of Physical Chemistry, University of Mainz, Mainz, Germany (Dr Schroeder).
authors.forename: Thies
authors.initials: T
authors.lastname: Schroeder
authors.affiliation: Department of Health and Exercise Science, Colorado State University, Fort Collins, CO (Drs Scalzo, Hamilton, and Bell, Mr Peltonen, Binns, and Szallar, and Ms Klochak and Wood).
authors.forename: Karyn L
authors.initials: KL
authors.lastname: Hamilton
authors.affiliation: Department of Health and Exercise Science, Colorado State University, Fort Collins, CO (Drs Scalzo, Hamilton, and Bell, Mr Peltonen, Binns, and Szallar, and Ms Klochak and Wood). Electronic address: christopher.bell@colostate.edu.
authors.forename: Christopher
authors.initials: C
authors.lastname: Bell
mesh_terms: D000328:Adult; D001794:Blood Pressure; D003000:Clonidine / Q000494:pharmacology*; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D006339:Heart Rate; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D010108:Oxyhemoglobins / Q000032:analysis; D010807:Physical Endurance / Q000187:drug effects / Q000502:physiology; D013565:Sympatholytics / Q000494:pharmacology*
chemical_list: D010108:Oxyhemoglobins; D013565:Sympatholytics; D003000:Clonidine
keywords: insulin sensitivity; muscle glycogen; sympathetic nervous system