Evaluation of the usefulness of serum phosphatases and osteocalcin as serum markers in a calcium depletion-repletion rat model.

The present study examined the usefulness of the serum alkaline phosphatase (ALP) activity, osteocalcin, and tartrate-resistant acid phosphatase (TrACP) activity as bone turnover markers in a calcium depletion/repletion rat model. Weanling rats were fed calcium-deficient diet for 4 weeks, followed by 2 weeks of dietary calcium repletion. Serum phosphatases and osteocalcin were determined and compared with those of corresponding age-matched, pair-weighted controls. Rats were sacrificed at the end of each phase of the study, and bone phosphatase activities in tibiae and vertebrae were measured. During calcium depletion, rats developed hypocalcemia and lost significant bone calcium, which were reversed with dietary calcium repletion. During depletion when previously published histologic studies indicated a suppressed bone formation and stimulated bone resorption, serum ALP activity and osteocalcin levels were significantly elevated and serum TrACP activity reduced; at the same time, the bone ALP and TrACP activities were increased. Because the serum level of 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) which has been shown to stimulate the synthesis of skeletal ALP and of osteocalcin, was also significantly increased during depletion, the increased serum ALP and osteocalcin level could be indirect consequences of the hypocalcemia-mediated elevation in 1,25(OH)2D3 level. These effects were reversed upon calcium repletion, during which previously published histologic studies demonstrated a stimulated bone formation and a suppressed resorption in these rats. In conclusion, although there is increasing evidence for the usefulness of these serum proteins as markers of bone metabolism in humans, a great deal more work is required before we can understand the significance of these assays. Until such is accomplished, these assays should not be assumed to be validated.