The influence of exercise on bone morphogenic enzyme activity of immature equine subchondral bone

This study aimed at the determination of the influence of exercise on the levels of a number of bone morphogenic enzymes in subchondral bone and at the comparison of these data with other (subchondral) bone-related parameters that have been investigated in the same experimental population. Forty-three foals were reared until weaning at 5 months of age under similar conditions, except for the type and amount of exercise. Fifteen foals remained at pasture (Pasture group and also control group), 14 foals were kept in box stalls (Box group) and 14 foals were kept in the same box stalls but were subjected daily to an increasing number of gallop sprints (Training group). After weaning 8 foals from each group were euthanised. All remaining 19 animals were housed together in a loose box with access to a small paddock to study a possible reversibility of exercise-induced effects. Post mortem subchondral bone samples were collected from the femoropatellar joint and analysed for the bone morphogenic enzymes alkaline phosphatase (ALP), tartrate resistant acid phosphatase (TRAP) and lysyl oxidase (LO). Data were compared to calcium content, numbers of collagen cross-links, bone mineral density (BMD) and cross-sectional area (CSA) collected in other bone-related studies in the same group of experimental animals. At 5 months of age, ALP levels were significantly lower and TRAP levels higher in both the Box and the Training group, making the ALP : TRAP ratio reversed in relation to the Pasture group. LO levels were lower in the Box group only. The ALP and TRAP data corresponded with changes in CSA, but not with calcium and BMD, the levels of which were the same in the Training and Pasture groups. The LO levels corresponded nicely with hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP) cross-links. At 11 months of age ALP and TRAP levels had reached similar levels in most groups, normalising the ALP : TRAP ratio. TRAP levels in the former Training group lagged somewhat behind. LO levels were still significantly lower in the former Box rest group. It is concluded that the overall increase in bone mass (characterised by the change in CSA) is apparently related to continuous, evenly distributed exercise as in the Pasture group (natural situation). This process seems to be related with ALP and TRAP levels and their ratio. For normal cross-link levels and BMD, short heavy bouts of exercise superimposed on a basic rest regimen seem sufficient. However, both data from this study and from earlier studies suggest that this latter exercise regimen might have a long-term deleterious effect.