Circadian and longitudinal variation of serum C-telopeptide, osteocalcin, and skeletal alkaline phosphatase in C3H/HeJ mice.

Inbred strains of mice are increasingly being used as an animal model to investigate skeletal disorders relevant to humans. In the bone field, one of the most convenient endpoints for evaluating genetic, physiological, or pharmaceutical perturbations is the use of biochemical markers. To apply biochemical markers in an effective manner, it is of key importance to establish the biological variation and appropriate sampling time. In this study, we evaluate two components: (i) circadian changes, and (ii) longitudinal variation for three serum markers, osteocalcin, C-telopeptide, and skeletal alkaline phosphatase (sALP), using 6-week-old C3H/HeJ (C3H) mice. To study circadian rhythms, the mice were randomly divided into eight groups of 15 mice each. Blood was collected at 3 h intervals, starting at 9:00 A.M. and continuing until 6:00 A.M. the next day. To determine whether circadian rhythm is intrinsically regulated or influenced by restricted food intake, it was also studied after a 12 h fasting period. Serum osteocalcin and C-telopeptide levels were measured by enzyme-linked immunoassay (ELISA) and skeletal alkaline phosphatase by a kinetic assay. The results demonstrated significant circadian variations in osteocalcin and C-telopeptide levels with a peak value between 0900 and 1200 h during daytime and a nadir between 15:00 and 18:00 h. The peak levels of C-telopeptide and osteocalcin were 26%-66% higher as compared with 24 h mean values. The pattern of the circadian variation of C-telopeptide and osteocalcin was similar in female and male animals and was not significantly affected by restricted food intake. The sALP levels were only marginally affected by the circadian rhythm. Longitudinal variations, expressed as coefficient of variation (CV), for osteocalcin, C-telopeptide, and sALP concentrations were 17%, 14%, and 16%, respectively. In addition, the longitudinal variations were not significantly influenced by the time of blood collection in sALP and osteocalcin levels, whereas C-telopeptide levels showed significantly higher within-subject day-to-day variation in morning samples, as compared with blood samples collected in the afternoon. The results highlight the importance of: (i) the timing of sample collection for appropriate interpretation of the bone marker data; and (ii) using the appropriate number of samples based on the variance obtained herein.