Water and electrolyte studies during long-term missions onboard the space stations SALYUT and MIR.

This contribution summarizes the results of investigations of water-electrolyte metabolism and its hormonal regulation conducted in cosmonauts who performed long-term space flights (from 18 to 366 days) aboard the space stations Salyut and Mir and compares them with the results obtained during various NASA flights. The role of the kidneys in ion metabolism regulation was assessed by various water-salt load tests before and after flights. In addition, the results of a year-long space flight and of medical experiments performed during the 237- and 241-day missions by the physicians and cosmonaut-researchers Atkov and Polyakov are reviewed in detail. In spite of interindividual variations, metabolic, and endocrine studies during prolonged space flights showed a reduction in body mass, usually with a reduction in body water and electrolytes and considerable changes in blood hormone concentrations and urinary hormone excretion. These changes reflect the processes of extended adaptation to a new environment. It is likely that shifts in electrolyte metabolism in weightlessness are primarily due to metabolic changes that diminish the tissue ability for ion retention and to concomitant changes in the endocrine status. The postflight examinations revealed changes in fluid-electrolyte metabolism and in the function of the kidneys which indicated a hypohydration status and a stimulation of hormonal systems responsible for fluid-electrolyte homeostasis in order to readapt to the normal gravitation. Postflight decline in osmotic concentration of urine in cosmonauts was accompanied by an altered response to antidiuretic hormone and was probably caused by changes in the functional state of the kidneys. We conclude that detailed knowledge of the alterations in water-electrolyte metabolism and its hormonal regulation on different stages of space flight are important prerequisites for the development of countermeasures to space deconditioning and thus for increased human efficiency in space. In addition, these data contribute to an increase in our general knowledge on the regulation of kidney function.