Blood was obtained from all animals at sacrifice and analyzed for Ca, P, total protein, and alkaline phosphatase. The humeri from flight and control rats were tested for breaking strength in three-point bending using an Instron materials testing machine. The load was applied to the medial surface at midshaft halfway between the outer supports, which were 17 mm apart beneath the posterior aspect of the bone. The deformation rate was 1 mm per minute; load deformation curves were analyzed for ultimate load and deformation, work to ultimate load, and initial stiffness. Third lumbar vertebrae were ground to a fine powder in a liquid nitrogen mill, and osteocalcin was extracted from 1 mg portions of bone powder. Because bone and serum osteocalcin in rats are age dependent, flight values were compared to both actual and estimated control values in order to compensate for the age difference between the two groups of rats.

Results:

Vertebral osteocalcin content was decreased in flight animals relative to controls, as were humeral breaking strength and serum osteocalcin levels, even after adjustment for age-related changes. The bone osteocalcin decrement was greater than the decrement in bone mass, indicating that osteocalcin per unit bone mass was reduced. The normal inverse relationship between serum and bone osteocalcin was disrupted in the flight rats, whose serum content was much lower. If the source of serum osteocalcin is new synthesis by the bone cells, then these decreased levels are likely to represent decreased osteoblast activity associated with reduced skeletal growth during space flight. Although a dose-dependent, steroid-induced decrease in serum osteocalcin has been reported, this does not appear to be a factor here since serum corticosterone was the same in all groups and unrelated to serum osteocalcin. While the function of osteocalcin may not yet be known, it appears to be a most sensitive indicator of the early effects of space flight on metabolism.