Dynamics of circulating osteocalcin in rats during growth and under experimental conditions

Osteocalcin is the most abundant non-collagenous protein produced in the process of bone formation. A specific radioimmunoassay has been developed using a rabbit antiserum raised against osteocalcin extracted from rat bone. The sensitivity of the assay was tested in male and female rats under different experimental conditions: ovariectomy led to a mild increase in circulating osteocalcin (70.6 +/- 6.9 vs 51.6 +/- 6.3 ng/ml; p < 0.05) and deprivation of dietary calcium elevated plasma levels further (119 +/- 6.3 ng/ml; p < 0.01). As expected, pharmacological enhancement of bone turnover with calcitriol produced a significant increase in plasma osteocalcin (296 +/- 24.1 vs 89.5 +/- 5.1 ng/ml; p < 0.01), whereas prednisolone, a steroidal compound known to inhibit osteoid mineralization, significantly reduced circulating concentrations of this protein (70 +/- 7.4 vs 100 +/- 6.3 ng/ml; p < 0.05). Plasma kinetics recorded in female rats between birth and the 100th week revealed a highly significant (p < 0.001) elevation peaking at the third week (231 +/- 70.6 ng/ml) and slowly declining to reach values measured at birth (41.3 +/- 9.2 ng/ml) at the 16th week (47 +/- 4.6 ng/ml). Subsequently, a small but significant (p < 0.05) decline towards senescence was recorded. The osteocalcin surge preceded the period of rapid growth (weeks 3 to 11) estimated by vertebral length progression, showing a tendency to stabilize as growth spurt slowed down. A moderate but significant (p < 0.01) increment was observed after mating (87.8 +/- 5.1 vs 69.5 +/- 4.0 ng/ml). Although plasma osteocalcin remained stable during lactation, average levels were elevated in comparison with age-matched non-pregnant controls.(ABSTRACT TRUNCATED AT 250 WORDS)