Biochemical markers of bone metabolism in growing thoroughbreds: a longitudinal study

This study describes longitudinal changes in serum levels of biochemical ma rkers of bone cell activity in a group of 24 thoroughbred foals from birth to 18 months of age. The markers of bone formation included the type I coll agen carboxy-terminal propeptide (PICP), the bone-specific isoenzyme of alk aline phosphatase (BAP), and osteocalcin (OC). Levels of the cross-linked t elopeptide of type I collagen (ICTP), a marker of bone resorption, and the N-terminal propeptide of type III collagen (PNIIIP), a marker of soft tissu e turnover, were also measured. Levels of all markers fell significantly be tween birth and 18 months of age (70-80 per cent); this decrease being most marked between 0 and 6 months. However, a transient increase in levels of the markers then occurred between 6 and 14 months of age. The timing of thi s increase was specific for each parameter. ICTP and OC concentrations incr eased between October and December. PICP concentrations increased between D ecember and April whereas the increase in PIIINP was coincident with the pe ak in weight gain between April and June. Changes in BAP concentration were less distinct at this time. Season was shown to have significant effects o n the biochemical markers independent from the effect of age. Concentration s of all markers decreased with increasing body weight and at any given age heavier horses had lower marker levels. These results show that biochemica l markers of bone cell activity and soft tissue turnover follow characteris tic patterns of change in growing thoroughbreds influenced by age, season a nd bodyweight. The demonstration that the reference ranges for the biochemi cal markers change from month to month means that single samples from indiv iduals are of little value for monitoring bone cell activity in growing tho roughbreds. (C) 2001 Harcourt Publishers Ltd.