Experiments were undertaken to determine the effects of hydration status on a) orthostatic responses, and on b), relative changes in intravascular volume and protein content, during 70 degrees head-up tilt (HUT). Six men underwent 45 min of HUT, preceded by 45 min supine, first dehydrated, and again 105 min later after rehydration with water. Heart rate was consistently lower following rehydration (p less than 0.01), while supine diastolic pressure was higher (p less than 0.02). Systolic pressure fell during dehydrated HUT (p less than 0.01), but not during rehydrated HUT. Postural haemoconcentration, which was reduced after rehydration (p less than 0.001), was accompanied by a decrease in intravascular albumin content (p less than 0.05). Two subjects experienced severe presyncopal symptoms during dehydrated HUT, but not during rehydrated HUT. Thus, it appears that rehydration after fluid restriction improves orthostatic tolerance. Furthermore, extravascular hydration status may be more important than intravascular hydration status in determining orthostatic tolerance.

Blood pressure, pulse rate (PR), serum osmolality and electrolytes, as well as plasma vasopressin (PVP) and plasma renin activity (PRA), were measured in five men and two women [mean age 38.6 +/- 3.9 (SE) yr] before, during, and after inflation of an antigravity suit that covered the legs and abdomen. After 24 h of fluid deprivation the subjects stood quietly for 3 h: the 1st h without inflation, the 2nd with inflation to 60 Torr, and the 3rd without inflation. A similar control noninflation experiment was conducted 10 mo after the inflation experiment using five of the seven subjects except that the suit was not inflated during the 3-h period. Mean arterial pressure increased by 14 +/- 4 (SE) Torr (P less than 0.05) with inflation and decreased by 15 +/- 5 Torr (P less than 0.05) after deflation. Pulse pressure (PP) increased by 7 +/- 2 Torr (P less than 0.05) with inflation and PR decreased by 11 +/- 5 beats/min (P less than 0.05); PP and PR returned to preinflation levels after deflation. Plasma volume decreased by 6.1 +/- 1.5% and 5.3 +/- 1.6% (P less than 0.05) during hours 1 and 3, respectively, and returned to base line during inflation. Inflation decreased PVP from 6.8 +/- 1.1 to 5.6 +/- 1.4 pg/ml (P less than 0.05) and abolished the significant rise in PRA during hour 1. Both PVP and PRA increased significantly after deflation: delta = 18.0 +/- 5.1 pg/ml and 4.34 +/- 1.71 ng angiotensin I X ml-1 X h-1, respectively. Serum osmolality and Na+ and K+ concentrations were unchanged during the 3 h of standing.(ABSTRACT TRUNCATED AT 250 WORDS)

No abstract

To investigate fluid, electrolyte, and plasma vasopressin (PVP) and renin activity (PRA) responses, six men (20-35 yr) were immersed to the neck (NI) in water at 34.5 degrees C for six h after overnight food and fluid restriction. Diuresis was 1,061 +/- 160 (SE) ml/6 h during immersion and water balance was -1,285 +/- 104 ml/6 h. Preimmersion PVP was 0.7 +/- 0.2 pg/ml and increased to 3.0 +/- 0.6 pg/ml (P less than 0.05) at 6 h. PVP was unchanged at 1.2 +/- 0.1 pg/ml in the 6-h seated nonimmersion experiment at 25 degrees C. Plasma volume increased by 7.8 +/- 1.6% (P less than 0.05) at 60 min of NI and decreased thereafter. Serum osmolality was constant (292 +/- 1 mosmol/kg) throughout NI, whereas PRA decreased progressively from 1.9 to 0.5 ng angiotensin I X ml-1 X h-1 (P less than 0.05) at the end of immersion. In spite of moderate thirst just before NI, thirst sensations were attenuated and no water was consumed ad libitum during immersion. These data indicate that PVP is not suppressed when there is no fluid intake during immersion and suggest that the action of factors other than PVP suppression are necessary to explain the mechanism of immersion diuresis.