Calcium homeostasis in prolonged hypokinesia.

Calcium metabolism, its hormonal regulation and renal calciuretic function were investigated in 20 healthy volunteers during 120- and 182-day head-down tilt studies. The content of total and ionized calcium and parathyroid hormone was found to increase during the study. The concentration of calcitonin increased at an early stage of exposure and decreased thereafter. This resulted in negative calcium balance which was on the average 5, 6g per month. Measurements of iliac bone biopsies showed calcium decrease and phosphorus increase.