HORMONAL RESPONSES DURING ORTHOSTASIS FOLLOWING 4 HOURS OF HEAD-DOWN TILT

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.