PIEHL: Fibre composition, en zyme activity and concentrations of metabolites and electrolytes in muscles of standardbred horses. Acta vet. scand. 1974, 15, 287-309. -Measurements of metabolites, electrolytes, water, RNA and protein concentrations, the activity of certain muscle enzymes (SDH and PFK) and muscle fibre composition were made on biopsy specimens from the gluteus medius muscle of 68 standardbred horses, % to 8 years old. The muscle fibres were classified in 3 major categories, slow

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... itch (ST), fast twitch and high oxidative (FTH) and fast twitch (FT) fibres. The percentage of FTH fibres was higher after the age of 4 years, averaging 54 %. ST fibres comprised 24 % and this value remained unchanged. Glycogen concentration increased with age and averaged 95 and 126 mmol X kg-I wet muscle in the youngest and oldest age groups, respectively. Lactate and pyruvate concentrations were markedly decreased, whereas ATP, CP, G-6-P and glucose were unaffected with age. Water content averaged 75 % in all age groups, whereas Na t concentration increased, K+ concentration decreased and Mg2+ concentration remained unchanged with increasing age. SDH activity in %and 8-year old horses increased from 6.1 to 13.6 !-Lmol X (g X min.)-I. PFK activity reached a peak at the age of 4 years after which it declined. With the data presented as a background, measurements on muscle biopsies may be a new aid in diagnosing diseases in horses and even in evaluating treatment. Of special interest might be investigations of muscle biopsy specimens as a base in the formation of more adequate training methods in race-horses. e lee t r 0 I Y t e s; fib ret y pes; g lye 0 g ens tor age; h 0 r s e s k e let a I m usc I e; ph 0 s P h 0 f r u c t 0 kin a s e; s u c c ina t e d e h y d r 0 g e n a s e.