Measurement of plasma osteocalcin in sheep: assessment of circadian variation, the effects of age and nutritional status and the response to perturbation of the adrenocortical axis

An assay for plasma osteocalcin (BGP) in ruminants is described. Satisfactory cross-reaction was obtained for bovine BGP in several species, namely ovine, cervine, caprine and human. Sample stability at room temperature, after freezing and thawing and after lyophilization and reconstitution is described. Sheep of both sexes have been used throughout and no significant sex effect has been noted. A series of experiments showed a lack of circadian rhythm of plasma BGP concentration in sheep. Plasma BGP values from sheep ranging in age from 0.25 to 7 years fell exponentially from 300 ng/ml at 0.25 years to approximately 50 ng/ml at 1.25 years of age where it remained until the end of the study at 7 years. Long-term administration of adrenocorticotrophic hormone (ACTH) depressed plasma BGP concentrations, although plasma BGP levels were not affected sufficiently by short-term administration of ACTH to increase plasma cortisol concentrations 10-fold during a period of 4 h. It was confirmed that dietary phosphate deficiency depressed plasma BPG concentrations. It is concluded that neither circadian variation nor the short-term stress of handling and bleeding untrained animals need be considered in sampling sheep for measurement of plasma BGP concentration and that the change with age reflected the pattern of development of skeletal maturity in sheep. Values in both growing and mature sheep were at least 6 times higher than in human subjects, which may reflect the 10-fold higher skeletal growth rate in sheep than in humans. This high skeletal activity in sheep may have been selected for in the development of highly productive breeds.