Purpose of the project was to investigate the following topics:

• Does moderate dehydration, as when a person refrains from fluid intake in the morning, suffice to lessen orthostatic tolerance, as compared to ad-libitum fluid intake? Criteria of heart rate variability could present evidence—are they differently affected by orthostatic challenge with moderate dehydration as compared to the euhydrated control situation?
• Further, does such moderate dehydration influence relevant endocrine factors of cardiovascular regulation - ie, vasopressin; renin and aldosterone; and atrial natriuretic peptide?
• Is red cell mass density altered under such conditions, potentially confounding estimations of blood volume changes, using hematocrit as well as the mass density of blood plasma?
• Is the output, and thereby the plasma concentration of plasma glycosamino glycanes (GAG), particularly hyaluronic acid (HA)—important determinants of interstitial hydration—affected to any significant extent? Since there is physiological dehydration overnight (resulting in quite hyperosmolal morning urine!), do circadian rhythms of GAG plasma concentration exist at all?
• How is orthostatic regulation influenced by repetitive lower body suction? Could this procedure constitute a sufficiently powerful countermeasure in people suffering from orthostatic “deconditioning”, as astronauts or bedridden patients?

The results indicated that

• Variability criteria (electromechanical systole, RR-intervals, pNN50) responded to LBNP, but not to hydration or test time
• Resting plasma density (PD) increased after each LBNP in rehydrated (RE) conditions, whereas it decreased with moderately dehydrated (DE) conditions
• Red cell density was unaltered in RE but declined with DE
• ANP was reduced LBNP#1 and stayed below control. PRA was increased with each LBNP, with unaltered peak and climbing rest values
• Aldosterone rose and stayed elevated even 1 hour after the final LBNP. These responses were similar in RE and DE. Vasopressin and prolactin did not respond at all

Conclusions:

• DE or LBNP repetition did not alter variability criteria
• With DE, the continuous fall of resting [PD] might indicate decreasing capillary filtration pressures with test time, and the downward shift of [ED] either fluid shift across red cell membranes or increased representation of low-density erythrocytes in the bloodstream
• Consecutive LBNP bouts elevate renin (rest, compared to control), aldosterone (general), and decrease ANP (general, compared to control), without difference of DE vs. RE
• Hormonal patterns are altered by repeated pseudo-orthostatic stimulation

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