Bone Vol. 17, No. 3 September 1995221-227 “J&F&@ ELSEVIER The Response of the Skeleton to Physical Training: A Biochemical Study in Horses J S PRICE ‘,’ B . . A. E. GOOIkHIi3 JACKSON,2 R. EASTELL,2 A. M. WILSON,’ R. G. G. RUSSELL,2 L. E. LANYON,’ and ’ Department of Basic Sciences, The Royal Veterinary College, London, UK ’ Department of Human Metabolism and Clinical Biochemistry. University of Sheffield, Sheffield. UK 3 Comparative Orthopaedics Unit. Department of Anatomy, School of Veterinar?: Science, University of Bristol, Bristol. UK In this study we tested the hypothesis that exercise induces an adaptive response in the developing skeleton which may he monitored in vivo by measuring biochemical markers of bone metabolism. The effects of exercise on two biochemical markers of bone formation were determined; the carboxy- terminal propeptide of type I procollagen (PICP), and the bone-specific isoenzyme of alkaline phosphatase (BAP), and one putative marker of resorption, the pyridinoline crosslinked telopeptide domain of type I collagen (ICTP). All three markers were measured for a year in 2.year-old thor- oughbred horses exercised three times a week on a treadmill, and values compared to a control group of age-matched an- imals. Levels of all three markers fell in both exercised and control groups over the 12.month period reflecting normal age changes. However, there were differences between groups in the pattern of this decrease. When expressed as a percentage of baseline values, BAP was higher @ < 0.05) at 2 months and both BAP and the PICP were higher at 4 months @ < 0.01 and p < 0.05, respectively) in the exercised group, reflecting an increase in bone turnover in this group in the early stages of training. PICP levels were also elevated in the exercised group at 10 months and this result indicates an increase in bone turnover at this time. The changes in ICTP were different; at 2 months, levels were higher in ex- ercised animals than in controls, but there was no significant difference between the two groups at 4 and 6 months. After 8 months, ICTP levels in the exercised group increased re- turning to near baseline values at 10 months. In summary, the results of these assays for bone ALP and metabolites of type I collagen indicate that the treadmill exercise regimen used for this study resulted in a general increase in bone turnover in 2.year-old thoroughbreds. These findings indi- cate that biochemical marker determinations may provide a sensitive, noninvasive method of monitoring skeletal turn- over during athletic training. (Bone 17:221-227; 2995) Key Words: Bone markers; Exercise; Horse; Bone turnover; Bone. Address for correspondence and reprints: Dr. Joanna S. Price, Depart- ment of Veterinary Basic Sciences, The Royal Veterinary College, Lon- don NW1 OTU, UK. Introduction It is bone’s responsiveness to changes in its mechanical environ- ment which ensures that bone mass and architecture are appro- priate for the loads it is required to withstand throughout the life of the animal,‘l.‘i.41.53.M) Failure of this adaptive response to change bone architecture sufficiently to withstand the mechani- cal stresses generated by the physical activity of an animal can lead to injury. In horses, injuries to the skeleton are a major cause of morbidity and mortality. This is a particular problem in the racing thoroughbred because these animals generally begin a rigorous and relatively short training program as 2 year olds when still skeletally immature.“7~52 It is likely that many exer- cise programs used for training horses, if not inappropriate, are at least not optimal for inducing and maintaining the skeletal mass and strength required. The mechanisms for loading-related regulation of bone cell behavior may be studied in vitro by investigating biochemical changes and gene expression. ‘0.24.4X.h4 However, the effect of mechanical influences on bone architecture must be studied ul- timately in vivo. Many in vivo studies have involved surgical intervention to alter the mechanical properties of bone.R.22,5’ Such techniques are useful but may be confounded by effects on bone distinct from mechanical adaptation, that is, disruption of the local microcirculation and increased release of inflammatory mediators. These disadvantages are avoided by using noninva- sive methods for the measurement of the properties of bone quality, for example, ultrasonography, dual-energy X-ray ab- sorptiometry, and photon absorptiometry.‘X.2’.‘2.” It is now possible to supplement such measures of bone struc- ture with biochemical measurements of bone metabolism. In recent years, several assays have been developed for studies in humans (for reviews, see Refs. I, 13, and 50). The biochemical markers used are matrix components or enzymes released into the circulation during bone remodeling by osteoblasts and/or osteoclasts. Studies in man have shown that exercise has effects on various biochemical parameters which reflect bone metabo- lism, including serum calcium, calcitropic hormones, and osteo- calcin.‘.“’ However, there are, to our knowledge, no reports describing the effects of a long-term controlled exercise regimen on bone metabolism as determined by longitudinal measure- ments of bone markers. Furthermore, few studies have examined how exercise influences bone in growing animals. Woo et al.” observed that, in growing pigs, moderate exercise led to in- creases in the area of the femoral cortex. In a more recent study, 0 1995 by Elsevier Science Inc. 221 8756.3282/95/$9.50 SSDI 87%3282(95)00221-X