Five horses were studied during a five-week regime of controlled intensive daily training on a high-speed treadmill followed by five weeks of detraining. Muscle biopsies were taken weekly from both the right and left gluteus muscle and from the sternocephalicus muscle before, and at the end of, the training and detraining periods. Histochemical and biochemical analyses of the sternocephalicus muscle showed no metabolic adaptation with either training or detraining. No significant differences were observed in any of the analysed parameters in the gluteus muscle between contralateral sites. Glycogen levels decreased by 10 to 15 per cent after one to two weeks of training, remained low during the training period and increased to pretraining levels after one week's cessation of training. Citrate synthase activity increased rapidly and was 27 per cent higher after one week and 42 per cent higher after five weeks of training. Lactate dehydrogenase activity decreased by 15 per cent during this period. The changes seen in these enzyme levels persisted during the detraining period. No alterations were seen in fibre type composition but type IIA fibre areas decreased by 19 per cent after five weeks training and capillary density increased by 17 per cent. It is concluded that a period of intensive training will rapidly increase the oxidative capacity and the capillary density in an actively working muscle, and that these metabolic adaptations are well maintained during a subsequent period of detraining.

Summary Venous blood samples and middle gluteal muscle biopsies were obtained from 21 horses taking part in 100 km or 50 km endurance rides. Group A consisted of seven horses competing over 100 km (four horses completed the ride), Group B consisted of the six horses that were among the 10 best over 50 km while the other eight horses of Group C completed 50 km at a slower speed. Blood lactate, glycerol and creatine kinase increased in all groups while aspartate aminotransferase levels were higher only in Group A. No change was found in blood glucose in Groups B and C while horses in Group A had lower levels after the ride. Neither fibre composition, fibre areas nor enzyme activities differed between the groups. Intramuscular glycogen content was similar before the ride in all groups. After the ride glycogen had decreased only half as much in Group C as compared to Groups A and B. Group C had a higher intramuscular triglyceride content at rest than Group B. The greatest decrease in triglyceride content after the ride was found in Group C. In Groups A and B many Type I, IIA and IIB fibres were glycogen depleted after the ride while in Group C mainly Type I and some Type IIA fibres were depleted. The results of this study show that intramuscular carbohydrate and lipid stores are both important fuels during endurance rides.

No abstract

Fibre type composition of skeletal muscle in horses varies due to factors like breed, age and training. Variations due to these factors in muscle fibre type composition of the Swedish cold-blooded trotter, a heavier and slower breed than the Standardbred trotter, have not previously been reported. The objective of this study was therefore to investigate muscle fibre type composition of young and racing cold-blooded trotters using both histochemical and immunohistochemical methods. Muscle biopsies (gluteus medius) were obtained from 2-year-old (n ¼ 18) and 4-8-year-old horses (n ¼ 6). Four of the 2-year-old horses were sampled again during their 4-yearold season. All of the horses were in professional training and the group included superior racehorses. Fibre types (I, IIA and IIB) were visualized with the myosin ATPase technique. Some samples were also stained immunohistochemically to identify MHCI, MHCIIA and MHCIIX fibres using myosin heavy chain (MHC) antibodies. The oxidative capacity of the fibres was subjectively evaluated from NADH-tetrazolium reductase stains. Type I and type IIA fibres were all identified as MHCI and MHCIIA fibres, respectively, whereas type IIB fibre population included both pure MHCIIX and hybrid MHCIIAX fibres. The older racehorses had a higher proportion of type MHCIIA and a lower proportion of type MHCIIX fibres than the 2-year-old horses. Areas of type I fibres were larger and those of type IIB fibres were smaller in racing horses compared with young horses. The proportion of type MHCIIX fibres that stained medium for oxidative capacity increased with age and training. In conclusion, training and racing induce muscular adaptations in cold-blooded trotters as in other breeds. Furthermore, immunohistochemical methods detect hybrid fibres indicating that transitions of fibre types may occur.

Samples were taken at slaughter from heart and both locomotor and nonlocomotor muscles from animals of similar body weight but adapted to different levels of activity: three horses and three steers. All samples were analyzed biochemically to measure the activity of key metabolic enzymes. The skeletal muscles were analyzed histochemically for fibre type composition, fibre area and capillary supply. The general pattern of differences in fibre type composition and metabolic profile between muscle groups was similar in both horses and steers. The hearts of both species had high citrate synthase (CS), 3-OH-acylCoA-dehydrogenase (HAD) and hexokinase (HK) and low lactate dehydrogenase (LDH) activities. In both species, deep portions of muscles and muscles localized deeper in the body had a more oxidative metabolic profile than superficial portions and muscles. Taking all muscles into account, it was found that CS and HAD were higher and LDH lower in horse than in steer muscles. Horse muscles contained more type ΠA fibres and had a higher capillary supply than steer muscles. There was no difference between the two species regarding mean fibre area. The adaptation of the horse to a higher level of activity in comparison with steers was reflected in a higher oxidative capacity, better vascularization and a larger proportion of type IIA fibres. It was also obvious from these results that the ATPase fibre-typing system does not reflect the metabolic profile of a muscle.