MUSCLE-FIBER CHARACTERISTICS IN 4 MUSCLES OF GROWING BULLS - I - POSTNATAL DIFFERENTIATION

Mammalian muscle fibres maturation follows a distinct programme lineag e (I --> IIC --> IIA --> (IID/X) --> IIB). Changes in morphology, prot ein content and metabolic characteristics are related to several facto rs, age being one of the most crucial for muscle fibre type. Similarly , muscle fibre type affects overall muscle growth rate. The objective of this study was to characterise the profile of muscle fibre types an d metabolic enzyme activities in various muscles [semitendinosus (ST), longissimus dorsi (LD), biceps femoris (BF), triceps brachii (TB)] of growing Montbeliard bulls. Groups of four or five animals were slaugh tered at 4, 8, 12 and 16 months of age. Results indicated that contrac tile muscle fibre type maturation occurred similarly across all muscle s investigated. The percentage of slow myosin heavy chain 1 (MHC1) dim inished between 4 and 8 months of age (26.3% vs. 19.0%) and steadily i ncreased until 16 months (34.4%; P < 0.001). The evolution of metaboli c enzyme activities in the muscles studied was related to muscle-speci fic growth patterns: e.g., TB showed enhanced (P < 0.05) glycolytic me tabolism during the final period of increased allometric growth rate ( 1.23). These data show that both contractile fibre categories are invo lved in postnatal muscle fibre differentiation of cattle, which might contribute to alterations in meat tenderness with increasing age. (C) 1998 Elsevier Science B.V.