Jw. Williamson et al., HORMONAL RESPONSES DURING ORTHOSTASIS FOLLOWING 4 HOURS OF HEAD-DOWN TILT, Aviation, space, and environmental medicine, 64(7), 1993, pp. 606-611
The purpose of this investigation was to test the hypothesis that endu
rance exercise trained (ET) subjects would demonstrate a greater reduc
tion in orthostatic tolerance as compared to untrained (UT) subjects f
ollowing prior exposure to -6-degrees hood-down tilt (HDT) because vas
oactive hormone and enzyme responses to hood-up tilt (HUT) would be de
creased following HDT so as to predispose ET subjects to orthostatic i
ntolerance. Six ET subjects (VO2peak = 4.52 +/- 0.20 L/min) were match
ed for age and height with six UT subjects (VO2peak = 3.26 +/- 0.22 L/
min; p < 0.01). After 30 min of supine rest, subjects were exposed to
30 min of head-up tilt (HUT) at 70-degrees, then on a separate occasio
n, 4 h of HDT (-6-degrees) followed by 30 min of HUT. The HUT involved
passive standing for 30 min or until subjects become presyncopal. Blo
od sampled from the antecubital vein at min 1, 15, and 30, or at presy
ncope was analyzed for atrial natriuretic peptide (ANP), aldosterone (
PA), arginine vasopressin (AVP), plasma renin activity (PRA), and nore
pinephrine (NE), while BP and HR were continuously monitored. All subj
ects tolerating 30 min of HUT, after either condition, had significant
ly increased (p < 0.05) levels of PRA at the 30th min. Following 4 h o
f HDT, five of six UT subjects and only one of six ET subjects endured
30 min of HUT. The most marked difference during HUT after prior expo
sure to HDT was a significant increase in AVP (p < 0.05) at the onset
of HUT observed only for the UT subjects. Consistent with our hypothes
is, these findings suggest that arginine-vasopressin plays a primary r
ole in the orthostatic responses to head-up tilt following 4 h of head
-down tilt.