Dynamics of Circulating Osteocalcin in Rats During Growth and Under Experimental Conditions

Summary

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.

It is concluded that osteocalcin dynamics estimated by a new and specific rat radioimmunoassay constitute a useful tool for the non-invasive follow-up of bone kinetics under physiological and experimental conditions.

Key words

Osteocalcin - Dynamics - Growth - Experimental