EFFECTS OF SALBUTAMOL, A BETA-2-ADRENERGIC AGONIST, ON MUSCLES OF GROWING PIGS FED DIFFERENT LEVELS OF DIETARY-PROTEIN .1. MUSCLE-FIBER PROPERTIES AND MUSCLE PROTEIN ACCRETION

Dietary inclusion of salbutamol (3 ppm), a beta2-adrenergic agonist, t o growing pigs fed either a normal or a high level of dietary protein from 25 to 90 kg body weight, resulted in a pronounced increase in the protein accretion rate of skeletal muscles. This effect was two-fold higher when pigs were fed the high level compared to the normal level of dietary protein (16 vs. 7%). Histochemical analysis of slow-twitch (ST) and fast-twitch fibres (FTa and FTb) of biopsy samples taken eith er in vivo by a biopsy needle or post mortem from M. longissimus dorsi (LD) and M. biceps femoris (BF) demonstrated an increase in the relat ive area of the muscles occupied by FTb fibres. This was mainly due to a reduced frequency of the smaller FTa fibres and an increased freque ncy of the larger FTb fibres, which indicates an FTa to FTb fibre type conversion induced by salbutamol. In BF, a reduction in the frequency of and the relative area occupied by ST fibres was also found. The ch anges had already occured at 45 kg body weight in the LD muscle. The e xtent of vascularization of the muscles was not influenced by salbutam ol. The level of dietary protein did not affect muscle fibre propertie s, nor did it interact with salbutamol. The mechanism(s) by which salb utamol induces these related changes in protein metabolism and fibre t ype distribution is (are) complex, and may involve direct beta-adrener gic receptor stimulation as well as interactions with other anabolic o r catabolic hormones.