WATER AND ELECTROLYTE STUDIES DURING LONG-TERM MISSIONS ONBOARD THE SPACE STATIONS SALYUT AND MIR

This contribution summarizes the results of investigations of water-el ectrolyte metabolism and its hormonal regulation conducted in cosmonau ts who performed long-term space flights (from 18 to 366 days) aboard the space stations Salyut and Mir and compares them with the results o btained during various NASA flights. The role of the kidneys in ion me tabolism regulation was assessed by various water-salt load tests befo re and after flights. In addition, the results of a year-long space fl ight and of medical experiments performed during the 237- and 241-day missions by the physicians and cosmonaut-researchers Atkov and Polyako v are reviewed in detail. In spite of interindividual variations, meta bolic, and endocrine studies during prolonged space flights showed a r eduction in body mass, usually with a reduction in body water and elec trolytes and considerable changes in blood hormone concentrations and urinary hormone excretion. These changes reflect the processes of exte nded adaptation to a new environment. It is likely that shifts in elec trolyte metabolism in weightlessness are primarily due to metabolic ch anges that diminish the tissue ability for ion retention and to concom itant changes in the endocrine status. The postflight examinations rev ealed changes in fluid-electrolyte metabolism and in the function of t he kidneys which indicated a hypohydration status and a stimulation of hormonal systems responsible for fluid-electrolyte homeostasis in ord er to readapt to the normal gravitation. Postflight decline in osmotic concentration of urine in cosmonauts was accompanied by an altered re sponse to antidiuretic hormone and was probably caused by changes in t he functional state of the kidneys. We conclude that detailed knowledg e of the alterations in water-electrolyte metabolism and its hormonal regulation on different stages of space flight are important prerequis ites for the development of countermeasures to space deconditioning an d thus for increased human efficiency in space. In addition, these dat a contribute to an increase in our general knowledge on the regulation of kidney function.