SKELETAL-MUSCLE CHARACTERISTICS IN RED-BLOOD-CELL NORMOVOLEMIC AND HYPERVOLEMIC STANDARD-BRED RACEHORSES

Muscle biopsy specimens from the middle gluteal muscle were studied in 16 red blood cell hypervolaemic (Group HV) and 19 normovolaemic (Grou p NV) Standardbred racehorses. All horses were stallions, 4-8 years ol d and having similar mean racing performance values, as described by a n individual selection index value. All horses raced regularly but tho se in Group HV did not perform as expected and were therefore referred to the clinics for exercise tolerance testing. Muscle biopsy specimen s were analysed for fibre type distribution (Type I, IIA and IIB), fib re area and relative fibre area. In addition, oxidative capacity of th e fibres was evaluated by staining for nicotinamide adenine dinucleoti de (NADH) tetrazolium reductase, and the activities of citrate synthas e, 3-OH-acyl CoA dehydrogenase and lactate dehydrogenase were analysed in whole-muscle samples. With the exception of a higher percentage of Type IIB fibres in Group HV having a high oxidative capacity as evalu ated by the NADH stain, no significant difference were found in fibre composition, fibre area or enzyme activity between the Groups HV and N V.