A commonly accepted hypothesis is that a chronically high-sodium diet expands extracellular volume and finally reaches a steady state where sodium intake and output are balanced whereas extracellular volume is expanded. However, in a recent study where the main purpose was to investigate the role of natriuretic peptides under day-to-day sodium intake conditions (Heer M, Drummer C, Baisch F, and Gerzer R. Pflügers Arch 425: 390-394, 1993), our laboratory observed increases in plasma volume without any rise in extracellular volume. To scrutinize these results that were observed as a side effect, we performed a controlled, randomized study including 32 healthy male test subjects in a metabolic ward. The NaCl intake ranged from a low level of 50 meq NaCl/day to 200, 400, and 550 meq/day, respectively. Plasma volume dose dependently increased (P < 0.01), being elevated by 315 +/- 37 ml in the 550-meq-NaCl-intake group. However, in contrast to the increased plasma volume, comparable to study I, total body water did not increase. In parallel, body mass also did not increase. Mean corpuscular volume of erythrocytes, as an index for intracellular volume, was also unchanged. We conclude from the results of these two independently conducted studies that under the chosen study conditions, in contrast to present opinions, high sodium intake does not induce total body water storage but induces a relative fluid shift from the interstitial into the intravascular space.

It was the purpose of this study to investigate how the endocrine and renal mechanisms of fluid volume control in humans (n = 4) adapt to microgravity by applying an intravenous isotonic saline infusion. The acute ground-based supine (Sup) and seated (Seat) positions were chosen as references. During microgravity, renal sodium excretion (UNaV) was doubled during the second and third hours after infusion compared with during Seat (P < 0.05) but blunted during the first hour after infusion compared with during Sup, leading to a reduction in cumulative UNaV (59 +/- 15 vs. 108 +/- 12 mmol/5 h; P < 0.05). Plasma norepinephrine (NE) attained the highest value 3 h after infusion during microgravity (31 +/- 5 x 10(-2) ng/ml vs. 19 +/- 1 and 13 +/- 3 x 10(-2) ng/ml for Seat and Sup, respectively; P < 0.05). Inflight levels of plasma renin and aldosterone were very similar to levels during Seat. In conclusion, 1) the microgravity-adapted renal responses to infusion reflected a condition in between that of ground-based Seat and Sup, respectively, and 2) the plasma levels of NE, renin, and aldosterone were elevated inflight and not related to the changes in UNaV and urinary flow rate. These observations are in contrast to results of ground-based simulation experiments and might partly have been caused by a prior inflight reduction in extracellular fluid volume. The high levels of NE during microgravity warrant further investigation.

The data suggest firstly that loss of body mass during space flight is rather a consequence of hypocaloric nutrition. Secondly, microG provokes a sodium retaining hormonal status and may lead to sodium storage without an accompanying fluid retention.

Several hormonal systems participating in body fluid and electrolyte homeostasis were investigated in six healthy volunteers in a supine body position during a period of 9 days and nights. Under strictly controlled conditions, striking circadian rhythms were observed for plasma levels of vasopressin, renin, aldosterone, guanosine 3',5'-cyclic monophosphate, cortisol, and epinephrine. Nocturnal decreases and diurnal increases in urine flow rate and urinary excretion of electrolytes were observed and closely paralleled the urinary excretion of urodilatin. During 48 h after an acute isotonic saline infusion (2 liters within 25 min) and after a 48-h control experiment the urinary excretion of H2O and electrolytes, and simultaneously the alterations in endocrine systems participating in body fluid homeostasis, were determined. Urine flow and urinary electrolyte excretion rates were significantly increased during 2 days after the saline infusion. The largest increase in urinary fluid and electrolyte excretion was observed between 3 and 22 h postinfusion. These long-term changes were paralleled by altered H2O and Na balances and also by elevated body weights that returned to baseline values with an approximate half-life of 7 h. These data suggest that vasopressin, atrial natriuretic peptide, and catecholamines are unlikely to be of major importance for the renal response to this hypervolemic stimulus. The renin-aldosterone system was suppressed during 2 days postinfusion. This suppression correlated with the effects of saline load on Na excretion. However, the closest relation with Na excretion was observed for the kidney-derived member of the atrial natriuretic peptide family, urodilatin, which was considerably increased during the long-term period up to 22 h postinfusion. Thus these data show that the human body in supine position requires approximately 2 days to regulate the amount of Na and H2O provided by an acute saline infusion. The data also suggest that urodilatin and the renin-aldosterone system might participate in the long-term renal response to an acute saline infusion and also in the mediation of circadian urinary excretion rhythms.

Previous results from our laboratory demonstrate that changes in haematocrit (Hct) and haemoglobin concentration (Hb) underestimate the relative (%) change in plasma volume (PV) in seated subjects during simulation of weightlessness by water immersion. Therefore, we examined whether changes in Hct and Hb would accurately reflect the changes in PV in seven subjects during simulation of weightlessness by another model, 6 degrees head-down tilted bed rest (HDBR), for 42 days. Since we have previously observed unexpectedly high plasma levels of noradrenaline (NA) in astronauts during space flight, we also took the opportunity to measure this variable. The measurements were compared with those of the supine horizontal position before and after HDBR. During HDBR, PV measured by the Evans blue dye dilution technique decreased by 6.1 +/- 2.8% (P < 0.05) on day 2 and 9.6 +/- 2.2% (P < 0.05) on the 42nd day compared with that of the supine, horizontal position. Based on changes in Hct and Hb, PV decreased similarly by 8.3 +/- 2.8 and 10.2 +/- 3.2% (P < 0.05) respectively. There were no differences comparing the results of the two methods (P > 0.05). Forearm venous plasma NA was unchanged during the whole course of HDBR compared with that of the pre-HDBR supine position. It is concluded that changes in Hct and Hb reliably reflect the changes in PV comparing prolonged HDBR with the pre- and post-HDBR horizontal, supine position. Thus, changes in Hct and Hb might accurately reflect the change in PV during weightlessness in humans provided that the horizontal supine position is used as the ground-based reference. Furthermore, the results of this study, as well as of previous studies from space, confirm that NA release is unchanged or even increased during weightlessness.