Protein metabolism in chicken muscle cell cultures treated with cimaterol

Effect of serum from chickens treated with clenbuterol on myosin accumulation, beta-adrenergic receptor population, and cyclic AMP synthesis in embryonic chicken skeletal muscle cell cultures

Broiler chickens at 35 d of age were fed 1 ppm clenbuterol for 14 d. This level of dietary clenbuterol led to 5-7% increases in the weights of leg and breast muscle tissue. At the end of the 14-d period, serum was prepared from both control and clenbuterol-treated chickens, and was then employed as a component of cell culture media at a final concentration of 20% (v/v). Muscle cell cultures were prepared from both the leg and the breast muscle groups of 12-d chick embryos. Treatment groups included control chicken serum to which 10 nM, 50 nM, and 1 microM clenbuterol had been added, as well as cells grown in media containing 10% horse serum. Cultures were subjected to each treatment for 3 d, beginning on the seventh d in culture. Neither the percent fusion nor the number of nuclei in myotubes was significantly affected by any of the treatments. The quantity of myosin heavy chains (MHCs) was not increased by serum from clenbuterol-treated chickens in either breast or leg muscle cultures; however, the MHC quantity was 50-150% higher in cultures grown in control chicken serum to which 10 and 50 nM clenbuterol had also been added. The beta-adrenergic receptor (betaAR) population was 4000-7000 betaARs per cell in cultures grown in chicken serum, with leg muscle cultures having approximately 25-30% more receptors than breast muscle cultures. Receptor population was not significantly affected by the presence of clenbuterol or by the presence of serum from clenbuterol-treated chickens. In contrast, the betaAR population in leg and breast muscle cultures grown in the presence of 10% horse serum was 16,000-18,000 betaARs per cell. Basal concentration of cyclic adenosine 3':5'monophosphate (cAMP) was not significantly affected by the treatments. When cultures grown in chicken serum were stimulated for 10 min with 1 microM isoproterenol, limited increases of 12-20% in cAMP concentration above the basal levels were observed. However, when cultures grown in the presence of horse serum were stimulated with 1 microM isoproterenol, cAMP concentration was stimulated 5- to 9-fold above the basal levels. Thus, not only did cells grown in horse serum have a higher betaAR population, but also each receptor had a higher capacity for cAMP synthesis following isoproterenol stimulation. Finally, the hypothesis that clenbuterol exerts its action on muscle protein content by changes in cAMP concentration was tested. No correlation was apparent between basal cAMP concentration and MHC content.

beta-adrenergic receptor population is up-regulated by increased cyclic adenosine monophosphate concentration in chicken skeletal muscle cells in culture

Skeletal muscle hypertrophy is promoted in vivo by administration of beta-adrenergic receptor (betaAR) agonists. Chicken skeletal muscle cells were treated with 1 microM isoproterenol, a strong betaAR agonist, between days 7 and 10 in culture. betaAR population increased by approximately 40% during this treatment; however, the ability of the cells to synthesize cyclic adenosine monophosphate (cAMP) was diminished by twofold. Neither the basal concentration of cAMP nor the quantity of myosin heavy chain (MHC) was affected by the 3-d exposure to isoproterenol. To understand further the relationship between intracellular cAMP levels, betaAR population, and muscle protein accumulation, intracellular cAMP levels were artificially elevated by treatment with 0-10 betaM forskolin for 3 d. The basal concentration of cAMP in forskolin-treated cells increased up to sevenfold in a dose-dependent manner. Increasing concentrations of forskolin also led to an increase in betaAR population, with a maximum increase of approximately 40-60% at 10 microM forskolin. A maximum increase of 40-50% in the quantity of MHC was observed at 0.2 microM forskolin, but higher concentrations of forskolin reduced the quantity of MHC back to control levels. At 0.2 microM forskolin, intracellular levels of cAMP were higher by approximately 35%, and the betaAR population was higher by approximately 30%. Neither the number of muscle nuclei fused into myotubes nor the percentage of nuclei in myotubes was affected by forskolin at any of the concentrations studied.

Effect of electrical stimulation on beta-adrenergic receptor population and cyclic amp production in chicken and rat skeletal muscle cell cultures

Expression of the beta-adrenergic receptor (betaAR) and its coupling to cyclic AMP (cAMP) synthesis are important components of the signaling system that controls muscle atrophy and hypertrophy, and the goal of this study was to determine if electrical stimulation in a pattern simulating slow muscle contraction would alter the betaAR response in primary cultures of avian and mammalian skeletal muscle cells. Specifically, chicken skeletal muscle cells and rat skeletal muscle cells that had been grown for 7 d in culture were subjected to electrical stimulation for an additional 2 d at a pulse frequency of 0.5 pulses/sec and a pulse duration of 200 msec. In chicken skeletal muscle cells, the betaAR population was not significantly affected by electrical stimulation; however, the ability of these cells to synthesize cyclic AMP was reduced by approximately one-half. In contrast, the betaAR population in rat muscle cells was increased slightly but not significantly by electrical stimulation, and the ability of these cells to synthesize cyclic AMP was increased by almost twofold. The basal levels of intracellular cyclic AMP in neither rat muscle cells nor chicken muscle cells were affected by electrical stimulation.

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.

Effects of dietary supplementation of beta 2-adrenergic agonist clenbuterol on carcase characteristics and some metabolites in ducks

1. Ducks (622 in total) aged 25 d were given diets supplemented with clenbuterol (CL) at 0 (control), 0.5, 1, 2, 3 and 5 mg/kg for 25 d to investigate the effect of dietary CL on muscle and fat deposition and some metabolites in ducks. 2. The mass of the breast muscles was increased by 10 to 31%, while subcutaneous fat plus skin and abdominal fat pad were reduced by 8 to 23% and 20 to 37%, respectively, in the ducks supplemented 1 to 5 mg CL/kg diet. 3. Increased RNA:DNA ratios in the breast muscle, reduced uric acid and increased free fatty acid concentrations in the serum were observed in clenbuterol-fed ducks.

Hind-limb protein metabolism in growing sheep; effects of acute and chronic infusion of clenbuterol by close arterial and systemic routes

1. Close arterial infusion of clenbuterol (0.7 microgram/hr) for a 3 hr period led to decreased protein synthesis and degradation (both P < 0.05) across the hind-limb of sheep. 2. Systemic infusion of the drug (14 micrograms/hr) for a similar period increased blood flow (P < 0.001) through the hind-limb and decreased protein degradation (P < 0.05). 3. Following chronic (7 day) close-arterial infusion of clenbuterol (1.9 micrograms/hr) hind-limb protein synthesis and degradation again declined (P < 0.05) but this was apparently not recovered during 7 days of drug withdrawal. 4. In consequence, during subsequent systemic infusion of the drug (62 micrograms/hr) the lower rates of protein metabolism were not altered significantly, despite improved whole nitrogen retention (+30%, P < 0.01). 5. The results are discussed with reference to the differential actions of beta-agonists on the various tissues of the hind-limb.

Temporal pattern of skeletal muscle changes in lambs fed cimaterol

The objectives of this study were to compare the efficacy of 3-week vs 6-week dietary administration of the beta-adrenergic agonist cimaterol on skeletal muscle growth, and to measure the changes in muscle nucleic acid and protein concentration and content to provide evidence regarding the mechanism(s) by which cimaterol stimulates muscle hypertrophy in growing ruminants. Two groups of 12 Dorset or Dorset-Finn cross ram lambs weighing 36 kg or 33 kg were assigned to treatment intervals of 3 or 6 weeks, respectively. Lambs within each weight group were randomly assigned to receive 0 or 10 ppm cimaterol in a complete mixed diet fed ad libitum. Initial live weights and treatment periods were chosen to achieve similar slaughter weights. Cimaterol increased the mass of three hind leg muscles 30% and 25% on average (both P less than .001) with 3- and 6-week administration, respectively, resulting in identical average muscle weights of treated lambs at both treatment intervals. The mean mass of these 3 muscles, expressed as a percentage of body weight, was increased 18.6% (P less than .001) at both treatment intervals. RNA concentration and content of the semitendinosus muscle were increased 24.8% (P less than .01) and 84.6% (P less than .001), respectively, after 3 weeks of treatment, but neither was significantly different from controls after 6 weeks. DNA concentration in the muscle was reduced 42% (P less than .05) with 3-week cimaterol administration, and was 25% less than controls (P greater than .05) in lambs fed cimaterol for 6 weeks. Total DNA content of the semitendinosus was unchanged at either treatment interval.(ABSTRACT TRUNCATED AT 250 WORDS)