The metabolic clearance rate of osteocalcin in sheep.

Plasma osteocalcin has been proposed as a useful and convenient biochemical marker of bone formation. However, the effect on plasma osteocalcin due to variations in the rate of its removal from the circulation has been little investigated. We have measured the metabolic clearance rate of plasma osteocalcin in adult oophorectomized sheep. Two methods were used: intravenous bolus injection (six animals) and 6 hour constant intravenous infusion (four animals) of 125I-ovine osteocalcin. Using the bolus injection method, the plasma clearance of osteocalcin was found to be 3.3 liters/h. With the constant infusion method, the calculated value was 2.8 liters/h; based on this value and the mean ovine plasma osteocalcin concentration of 26.9 ng/ml (N = 29), a plasma production rate of 1.8 mg/day was derived. Osteocalcin clearance was relatively constant among animals in a basal state. Hence, the present approach should permit us to evaluate the relative contributions to changes in circulating osteocalcin levels from altered osteocalcin plasma clearance and production under various physiological and pathological conditions.