title: Vasoconstrictor reserve and sympathetic neural control of orthostasis.
abstract: BACKGROUND
  We tested the hypothesis that individual variability in orthostatic tolerance is dependent on the degree of neural and vasomotor reserve available for vasoconstriction.
  
  
  RESULTS
  Muscle sympathetic nerve activity (MSNA) and hemodynamics were measured in 12 healthy young volunteers during 60 degrees head-up tilt (HUT), followed by a cold pressor test (CPT) in HUT. Orthostatic tolerance was determined by progressive lower-body negative pressure (LBNP) to presyncope. The same protocols were performed randomly in normovolemic and hypovolemic conditions. We found that mean arterial pressure increased and stroke volume decreased, whereas heart rate (HR), MSNA, and total peripheral resistance (TPR) increased during HUT (all P<0.01). Application of the CPT in HUT did not increase HR or decrease stroke volume further but elevated mean arterial pressure (P<0.01) and increased MSNA and TPR in some subjects. There was a positive correlation between the time to presyncope from -50 mm Hg LBNP (equivalent to 60 degrees HUT alone) and the changes in MSNA produced by the CPT under both conditions (r=0.442, P=0.039). Those who had greater increases in MSNA had greater increases in TPR during the CPT and longer time to presyncope (both P<0.05). One subject had dramatic increases in MSNA but small increases in TPR during the CPT, which indicates a disassociation between sympathetic activity and the increase in peripheral vascular resistance.
  
  
  CONCLUSIONS
  These results support our hypothesis and suggest that vasoconstrictor capability is a contributor to orthostatic tolerance in humans. Vasoconstrictor reserve therefore may be one mechanism underlying individual variability in orthostatic intolerance.
authors.affiliation: Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas and the University of Texas Southwestern Medical Center at Dallas, Dallas, Tex 75231, USA.
authors.forename: Qi
authors.initials: Q
authors.lastname: Fu
authors.forename: Sarah
authors.initials: S
authors.lastname: Witkowski
authors.forename: Benjamin D
authors.initials: BD
authors.lastname: Levine
mesh_terms: D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D002302:Cardiac Output / Q000502:physiology; D003080:Cold Temperature; D005260:Female; D014644:Genetic Variation; D006339:Heart Rate / Q000502:physiology; D006439:Hemodynamics / Q000502:physiology*; D006801:Humans; D007024:Hypotension, Orthostatic / Q000503:physiopathology; D020896:Hypovolemia / Q000503:physiopathology; D008150:Lower Body Negative Pressure; D008297:Male; D011187:Posture / Q000502:physiology*; D013318:Stroke Volume / Q000502:physiology; D016683:Supine Position / Q000502:physiology; D013564:Sympathetic Nervous System / Q000502:physiology*; D018667:Tilt-Table Test; D014655:Vascular Resistance / Q000502:physiology; D014661:Vasoconstriction / Q000502:physiology*; D014666:Vasomotor System / Q000502:physiology*