Introduction
It is a classic concept that 5-6° head down bed rest (HDBR) simulates the effects of weightlessness in humans (1-3). Because HDBR augments urine output, the same is expected of weightlessness (4). To test this, we performed an oral water loading (600 ml H#2O) study before, during and after 1) spaceflight on the Russian space station Mir (n=3), and 2) HDBR for 3 weeks (n=8) and for 3.5 months (n=6).
Methods
The same protocols, sodium/fluid intakes, ambient conditions etc. were applied on all occasions.The protocol was approved by the Ethics Comitee of Copenhagen (01-180/94).
Results
Being ground based supine, the urine flow rate in the astronauts increased 7.2-fold during the 2 hrs following the water load from 0.7±0.1 to 4.9±1.1 ml/min (p<0.05). Six to 12 days of spaceflight, however, attenuated the response (0.8±0.1 to 2.3±0.8 ml/min, p>0.05). In contrast, during 2-3 weeks of HDBR urine output increased 5.2- to 5.8-fold following the water load similar to the 5.4 to 5.8-fold increase during supine control studies. Renal responses to repeated water loadings during 5.5 months of spaceflight (2 astronauts) were still attenuated compared with those obtained during 3.5 months of HDBR.
Conclusions
In conclusion, HDBR is not an analog of weightlessness, probably because the tranverse gravitational stress on the ground in supine humans modulates cardiovascular, endocrine, and renal variables. This new concept is the basis for an experiment selected for the International Space Station
References
1. Kakurin, L. I., V. I. Lobachik, V. M. Mikhailov, & Y. A. Senkevich. Antiorthostatic hypokinesia as a method of weightlessness simulation. Aviat. Space Environ. Med. 47: 1083-86, 1976.
2. Nicogossian, A. E. Microgravity simulations and analogues. In: Nicogossian A E, Huntoon C L, Pool S L. Space physiology and medicine. 3. ed. Philadelphia - London: Lea & Febiger, 1993.
3. Fortney, S. M., V. S. Schneider, and J. E. Greenleaf. The physiology of bed rest. In: Handbook of Physiology, Section 4, Environmental Physiology, Volume II, Fregly, M. L., and Blatteis, C. M. (eds.), Am. Physiol. Soc. pp. 889-940, 1996.
4. Norsk, P, and M. Epstein. Manned space flight and the kidney (Review). Am. J. Nephrol. 11: 81-97, 1991.