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1 Danish Aerospace Medical Centre of Research and Clinic of Aviation Medicine, Rigshospitalet; DK-2200 Copenhagen; 2 Department of Physiology and Pharmacology, University of Southern Denmark, Odense, DK-5000 Odense; and 3 Department of Internal Medicine and Endocrinology, Herlev Hospital, DK-2730 Herlev, Denmark
Plasma vasoactive hormone
concentrations [epinephrine (pEpi), norepinephrine
(pNE), ANG II (pANG II), vasopressin
(pVP), endothelin-1 (pET-1)] and plasma renin
activity (pRA) were measured periodically and
compared during lower body negative pressure (LBNP) to test the
hypothesis that responsiveness of the renin-angiotensin system,
the latter being one of the most powerful vasoconstrictors in the body,
is of major importance for LBNP tolerance. Healthy men on a
controlled diet (2,822 cal/day, 2 mmol · kg
1
· day1 Na+) were exposed to 30 min of LBNP
from 
15 to 50 mmHg. LBNP was uneventful for seven men [25 ± 2 yr, high-tolerance (HiTol) group], but eight men (26 ± 3 yr)
reached presyncope after 11 ± 1 min [P < 0.001, low-tolerance (LoTol) group]. Mean arterial pressure (MAP) did not
change measurably, but central venous pressure and left atrial diameter
decreased similarly in both groups (5-6 mmHg, by 
30%,
P < 0.05). Control (0 mmHg LBNP) hormone
concentrations were similar between groups, however, pRA
differed between them (LoTol 0.6 ± 0.1, HiTol 1.2 ± 0.1 ng
ANG I · ml1 · h1,
P < 0.05). LBNP increased (P < 0.05)
pRA and pANG II, respectively, more in the
HiTol group (9.9 ± 2.2 ng ANG I · ml1
· h1 and 58 ± 12 pg/ml) than in LoTol
subjects (4.3 ± 0.9 ng ANG I · ml1 · h1 and 28 ± 6 pg/ml). In contrast, the increase in
pVP was higher (P < 0.05) in the LoTol
than in the HiTol group. The increases (P < 0.05) for
pNE were nonsignificant between groups, and
pET-1 remained unchanged. Thus there may be a causal
relationship between attenuated activation of pRA and
pANG II and presyncope, with pVP being a
possible cofactor. Measurement of resting pRA may be of
predictive value for those with lower hypotensive tolerance.
hypotensive tolerance; vasopressin; central venous pressure; catecholamines
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