Hormonal responses during orthostasis following 4 hours of head-down tilt

The purpose of this investigation was to test the hypothesis that endurance exercise trained (ET) subjects would demonstrate a greater reduction in orthostatic tolerance as compared to untrained (UT) subjects following prior exposure to -6° head-down tilt (HDT) because vasoactive hormone and enzyme responses to head-up tilt (HUT) would be decreased following HDT so as to predispose ET subjects to orthostatic intolerance. Six ET subjects (V̇O₂peak = 4.52 ± 0.20 L/min) were matched for age and height with six UT subjects (V̇O₂peak = 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°, then on a separate occasion, 4 h of HDT (-6°) followed by 30 min of HUT. The HUT involved passive standing for 30 min or until subjects became presyncopal. Blood sampled from the antecubital vein at min 1, 15, and 30, or at presyncope was analyzed for atrial natriuretic peptide (ANP), aldosterone (PA), arginine vasopressin (AVP), plasma renin activity (PRA), and norepinephrine (NE), while BP and HR were continuously monitored. All subjects tolerating 30 min of HUT, after either condition, had significantly increased (p < 0.05) levels of PRA at the 30th min. Following 4 h of 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 exposure 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 hypothesis, these findings suggest that arginine-vasopressin plays a primary role in the orthostatic responses to head-up tilt following 4 h of head-down tilt.