Alterations in Protein Accretion and Histochemical Characteristics of the M. Longissimus Dorsi in Pigs Caused by Salbutamol (a β-Adrenergic Agonist)

Ractopamine-induced fiber type-specific gene expression in porcine skeletal muscles is independent of growth

Feeding ractopamine (RAC), a β-adrenergic agonist (BAA), to pigs increases type IIB muscle fiber type-specific protein and mRNA expression. However, increases in the abundance of these fast-twitch fiber types occur with other forms of muscle hypertrophy and thus BAA-induced changes in myosin heavy chain (MyHC) composition may simply be associated with increased muscle growth known to occur in response to BAA feeding. The objective of this study was to determine whether RAC feeding could change the MyHC gene expression in the absence of maximal muscle growth. Pigs were fed either an adequate diet that supported maximal muscle hypertrophy or a low nutrient diet that limited muscle growth. RAC was included in diets at 0 or 20 mg/kg for 1, 2, or 4 wk. Backfat depth was less (P < 0.05) in pigs fed the low nutrient diet compared with the adequate diet but was not affected by RAC. Loin eye area was greater (P < 0.05) in pigs fed an adequate diet plus RAC at 1 wk but did not differ among remaining pigs. At 2 and 4 wk, however, pigs fed the adequate diet had greater loin eye areas (P < 0.05) than pigs fed the low nutrient diet regardless of RAC feeding. Gene expression of the MyHC isoforms, I, IIA, IIX, and IIB, as well as glycogen synthase, citrate synthase, β 1-adrenergic receptor (AR), and β 2-AR were determined in longissimus dorsi (LD) and red (RST) and white (WST) portions of the semitendinosus muscles. MyHC type I gene expression was not altered by RAC or diet. Feeding RAC decreased (P < 0.01) MyHC type IIA gene expression in all muscles, but to a greater extent in WST and LD. MyHC type IIX gene expression was lower (P < 0.05) in WST and LD muscles in response to RAC but was not altered in RST muscles. RAC increased (P < 0.05) MyHC type IIB gene expression in all muscles, but to a greater extent in RST. β 1-AR gene expression was unaffected by RAC or diet, whereas the expression of the β 2-AR gene was decreased (P < 0.001) by RAC. No significant RAC * diet interactions were observed in gene expression in this study, indicating that RAC altered MyHC and β 2-AR gene expression in porcine skeletal muscles independent of growth.

Muscle glycogen depletion pattern in halothane-gene-free pigs at slaughter and its relation to meat quality

Muscle fibre type composition and glycogen depletion pattern at slaughter as related to meat quality, were studied in M. longissimus dorsi of halothane-gene-free Swedish Yorkshire pigs fed a high (n=19; 10 entire males and 9 gilts) or a low (n=18; 10 entire males and 8 gilts) protein diet. The muscle consisted of, on average, 8% type I, 9% type IIA and 83% type IIB fibres. Muscle fibre areas were significantly smaller in entire male pigs than in gilts irrespective of fibre type. There were no marked differences in muscle characteristics between pigs fed the low and high protein diets. Low glycogen concentrations were found in most type I and type IIA muscle fibres, while a greater variation in glycogen content was evident in type IIB fibres. The pigs were divided into two groups according to the proportion of glycogen depleted IIB fibres (more or less than 30% depleted IIB fibres). In the group where 30% or more of the type IIB fibres were glycogen depleted, a tendency toward DFD meat (dry, firm, dark) was seen, as the meat had higher ultimate pH (5·62 vs 5·52; p=0·02), lower drip loss (2·7% vs 4·4%; p=0·007) and lower reflectance value (darker meat; 16·5 vs 20·6 EEL units; p=0·0005), compared with the group with less than 30% depleted IIB fibres. No difference in total glycogen content was found between these two groups. This indicates that the distribution of glycogen in different fibre types seems to be of importance for the ultimate meat quality.

Histo- and biochemical characteristics of the Longissimus dorsi muscle in pigs and their relationships to performance and meat quality

Histo- and biochemical characteristics of the longissimus dorsi muscle at 65-70 kg body weight and their relationships to performance and meat quality at 100 kg were examined in Danish Landrace and Danish Large White female and castrated male pigs. Breed differences were observed for feed conversion rate, and a number of histochemical and biochemical traits. The organoleptic traits, flavour (13%), tenderness (15%) and overall acceptability (13%) were rated higher in Large White pigs. Significant correlations between histological and biochemical traits of the live muscle on the one side and performance, meat quality and organoleptic traits on the other side, could be demonstrated. However, these correlations were generally low (r < 0.35), and can thus only explain a small part of the variation in the measured quality traits. Consequently live muscle traits measured at 65-70 kg are poor predictors of meat quality characteristics after slaughter at 100 kg.

Prolonged administration of a β-agonist, salbutamol, to lambs does not impair metabolic or endocrine responses to the stress of dipping

Previous studies in foetal and growing sheep have shown that prolonged administration of β-agonist drugs leads to selective but marked attenuation of cardiovascular, metabolic and endocrine responses to the natural catecholamine hormones adrenaline and noradrenaline, as well as attenuation of responses to the drug itself. The experiment reported here was carried out to determine whether administration of the β-agonist drug, salbutamol, by twice-daily intramuscular injection at rates of 40 or 200 μ/kg per day, for a period long enough to result in substantial attenuation of its metabolic and endocrine effects, might also result in impairment in the ability of treated animals to respond to stressful husbandry procedures which disturb metabolic homeostasis and which depend on increased sympathetic nervous and adrenal activity for the restoration of homeostasis. Serial blood sampling on day 1 and day 14 of salbutamol treatment showed that the large increases in plasma lactate and glucose observed on the 1st day were absent on day 14, while the rapid increases in free fatty acids and insulin observed on day 1 were very greatly attenuated. Daily blood sampling also demonstrated that salbutamol, like cimaterol, significantly decreased both pre- and post-feeding plasma insulin and glucose concentrations.

Metabolic and endocrine changes consequent on herding and immersion of the lambs in a sheep-dip, were unaltered by 14 days of salbutamol treatment, even though the procedure resulted in activation of the pituitary-adrenal axis as evidenced by increased β-endorphin and cortisol concentrations, and a large increase in plasma lactate concentration. These results suggest β-agonist-treated animals can respond normally to physiological stresses but further investigations remain necessary to determine whether responses dependent on increased lipid mobilization and shivering, such as shearing or prolonged exposure to severe cold, remain normal in β-agonist treated animals.

Ractopamine induces differential gene expression in porcine skeletal muscles1

Ractopamine (RAC) improves growth by increasing lean accretion and decreasing fat deposition through repartitioning nutrients from adipose tissue to skeletal muscle. Although the process is not completely understood, RAC alters the proportion of muscle fiber type composition toward a faster-contracting phenotype. Because one of the primary determinants of contractile speed is the relative abundance of myosin heavy chain (MyHC) isoforms and because the genes encoding these isoforms are transcriptionally regulated, RAC likely alters MyHC gene expression. Using real-time PCR, the relative abundance of transcripts of individual type I, IIA, IIX, and IIB, and total MyHC, as well as glycogen synthase, citrate synthase, lactate dehydrogenase, peroxisome proliferator activated receptor alpha, beta1-adrenergic receptor (AR), and beta2-AR were determined in the LM of 44 pigs fed RAC (20 mg/kg) for 0, 1, 2, or 4 wk. In addition, MyHC isoform expression was determined in the LM and red semitendinosus and white semitendinosus muscles of 48 pigs fed RAC (20 mg/kg) for shorter periods of 12, 24, 48, or 96 h. Type I MyHC expression was unaffected (P > 0.73) by RAC administration. Type IIA MyHC expression decreased (P < 0.0001) by 96 h, was lower (P < 0.0001) by 1 wk, and returned to normal by 4 wk. Type IIX MyHC mRNA decreased (P < 0.001) by 2 wk and continued to decrease (P < 0.0001) by 4 wk. Most interesting was an increase (P < 0.0001) in type IIB MyHC by 12 h, which was maintained at an elevated level throughout the 4-wk feeding period. Abundance of glycogen synthase transcript was increased (P < 0.05) by 12 h, but was not different from controls at 2 wk, and was lower (P < 0.01) at 4 wk. Gene expression of beta1-AR was not affected by feeding RAC, whereas beta2-AR gene expression was decreased (P < 0.05) by 2 wk. These data show MyHC genes are differentially regulated by RAC and suggest that the beta adrenergic agonist-induced repartitioning effect is, in part, mediated by changing muscle fiber type-specific gene expression, perhaps through the beta2-AR.

Effect of clenbuterol on growth, carcase and skeletal muscle characteristics in broiler chickens

1. Male and female broiler chickens (144 in total) were given diets supplemented with clenbuterol (CB) at 0 (control) and at 1 mg/kg between 28 and 49 d of age to study the effect of CB on growth, carcase and skeletal muscle. 2. CB improved growth in males by increasing daily weight gain and final live weight and by lowering food conversion ratio. In females it changed the carcase composition by reducing abdominal fat pad and by increasing the proportion of protein. Consequently, carcase protein gain was increased in both sexes (11% and 16%, respectively). 3. Skeletal muscle weights were enhanced by between 6% and 22%. Muscle fibre diameters were increased in extensor hallucis longus (EHL) but not in gastrocnemius (GAS) muscle. This increase was more pronounced in females. EHL total muscle fibre number remained unchanged. The proportion of fast-twitch glycolytic fibres was increased at the expense of fast-twitch oxidative fibres in males only. Nuclear/cytoplasm and DNA/protein ratios tended to be decreased by CB. 4. From the elevated EHL muscle RNA/DNA, unchanged protein/RNA and translation activity it is suggested that CB stimulated protein synthesis at the pretranslational level. Reduced protein degradation is deduced from decreased neutral calcium-dependent proteolytic activity. 5. It is concluded that broiler chickens respond to long-term CB treatment as has been shown in various mammals. However, the sex-specific response in growth, carcase composition and skeletal muscle cellularity is more clearly apparent in broiler chickens.

Dietary salbutamol and level of protein: effects on the acute stress response in pigs

Effect on the acute stress response of dietary inclusion of 3 ppm salbutamol (beta-2-adrenergic agonist) at two levels of protein were investigated in growing pigs (from 25 kg live weight). The trial comprised six litters (blocks) of four females allocated randomly to four treatment groups in accordance with a 2 x 2 factorial arrangement. The response to an open-field test and to an intruder were measured at 50 kg live weight. Salbutamol increased immobility and looking, reduced total exploration, and increased plasma ACTH after test. At high dietary levels of protein, salbutamol also increased the latency to attack. High dietary levels of protein reduced standing still, latency to contact a novel object and an intruder, and level of plasma cortisol before test. Moreover, high protein without salbutamol seemed to reduce the latency to attack an intruder. In conclusion, chronic treatment with salbutamol shifted the acute stress response in pigs toward a passive response, whereas high dietary level of protein provoked an active response, which may have consequences in pig production.

[The effect of the beta-adrenergic agonist clenbuterol on the growth of skeletal muscles of rats]

The effects of clenbuterol (CB) on the weights of 6 hindleg muscles as well as on structural and biochemical characteristics of extensor digitorum longus muscle (EDL) were studied on Wistar rats. Different CB-doses (5 vs. 18 mg/kg diet) were tested and rats of different sex were used. The 12 days treatment accelerated the growth of the 6 hindleg muscles by 13 to 24% without predominant response of fast twitch or slow twitch muscles. The higher muscle weights were achieved by fibre hypertrophy as demonstrated by increased fibre diameters (+13 to +16%), and not by fibre multiplication. There was no evidence for a selective fibre hypertrophy of one of the metabolic fibre types. Fibre type composition shifted to the fast glycolytic fibres (FTG +0.7 to +4.9%-units). There were no significant differences in responses between the sexes of CB-doses. Clenbuterol decreased the nucleus/cytoplasm ratio, measured as nuclei number/mm2 fibre area, between 24 and 28% and the DNA/protein ratio by 17%. RNA content and RNA/DNA ratio were enhanced by 25 and 22%, respectively; the protein/RNA ratio remained unchanged. The results suggest that clenbuterol changes fibre type composition and stimulates fibre growth in a pretranslational stage of protein synthesis without addition of nuclei by satellite cell proliferation.