Relative role of low- and high pressure reflexes on sympathetic activity in humans during simulated gravitational stress.
In order to determine the relative role of low- and high-pressure reflexes, respectively, on forearm sympathetic nerve activity (fSNA), 10 normal male subjects underwent a 4-step (5 min each) graded lower body negative pressure (LBNP) from -10 to -50 mmHg.  Central venous pressure (CVP) and stroke volume gradually decreased (p<0.05), and arterial pulse pressure (PP) abruptly decreased at LBNP of -50 mmHg.  Mean arterial pressure (MAP) remained unchanged.  Forearm venous plasma norepinephrine concentration (fvNE) increased significantly at LBNP of -35 mmHg (p<0.05) and with a further sharp increase during LBNP of -50 mmHg (p<0.05).  High degrees of intra-individual correlations were observed between changes in Log [fvNE] and CVP (r-values from -0.78 to -0.96, p<0.01).  We conclude that low-pressure reflexes are the major determinants of fSNA during non-hypotensive gravitational stress (MAP and PP unchanged).  When the gravitational stress is more pronounced, a decrease in PP further augments fSNA through inhibition of high-pressure arterial baroreflexes.
Gabrielsen
A
A
Danish Aerospace Medical Centre of Research, Copenhagen, Denmark.
Petersen
H H
HH
Johansen
L B
LB
Foldager
N
N
Christensen
N J
NJ
Norsk
P
P
D000328:Adult; D017704:Baroreflex / Q000502:physiology*; D001794:Blood Pressure / Q000502:physiology; D002320:Cardiovascular Physiological Phenomena; D002346:Carotid Sinus / Q000502:physiology; D002496:Central Venous Pressure / Q000502:physiology; D004837:Epinephrine / Q000097:blood / Q000378:metabolism*; D006339:Heart Rate / Q000502:physiology; D006439:Hemodynamics / Q000502:physiology*; D006801:Humans; D008150:Lower Body Negative Pressure; D008297:Male; D009638:Norepinephrine / Q000097:blood / Q000378:metabolism*; D011311:Pressoreceptors / Q000502:physiology*; D011674:Pulse / Q000502:physiology; D013318:Stroke Volume / Q000502:physiology; D013564:Sympathetic Nervous System / Q000502:physiology
D009638:Norepinephrine; D004837:Epinephrine