Calcium metabolism under stress and in repose

Derangement of calcium metabolism, although perhaps not as dramatic as that of the cardiovascular or vestibular systems, constitutes one of the major threats to the health of participants in exploration of space. On the basis of studies in immobilized subjects, the clinical disorders most likely to be encountered during prolonged space flight are primarily the consequence of an imbalance between bone formation and resorption (favoring the latter): (1) loss of skeletal mass, leading to osteoporosis; (ii) hypercalcemia; and (iii) hyper-calciuria, with the attendant risk of nephrolithiasis. By itself, loss of skeletal mass would not be expected to pose an in-flight hazard, but hypercalcemia or nephrolithiasis could jeopardize lives or mission success. Such data as are available from in-flight studies tend to support the use of immobilization as a terrestrial model for alterations in calcium metabolism during space flight. A variety of prophylactic measures have been attempted with this model in an effort to modify the observed disorders. Although there is some evidence that hypercalcemia and hypercalciuria can be reduced or prevented, negative calcium balance has not been completely reversed. Perhaps the most successful prophylactic measure utilized to date has been dietary supplementation of both calcium and inorganic phosphate. With the wide variety of excellent study tools which are currently available for application to this field, significantly increased efforts are clearly required both to define the basic mechanism of immobilization-induced skeletal losses and to devise new prophylactic or therapeutic approaches.