Effects of Synovex-S and recombinant bovine growth hormone (Somavubove) on growth responses of steers: II. Muscle morphology and proximate composition of muscles

MEAT SCIENCE AND MUSCLE BIOLOGY SYMPOSIUM--mechanism of growth hormone stimulation of skeletal muscle growth in cattle

Growth hormone, also called somatotropin (ST), is a polypeptide hormone produced by the anterior pituitary. The major functions of GH include stimulating bone and skeletal muscle growth, lipolysis, milk production, and expression of the IGF-I gene in the liver. Based on these functions, recombinant bovine ST (bST) and recombinant porcine ST (pST) have been used to improve milk production in dairy cows and lean tissue growth in pigs, respectively. However, despite these applications, the mechanisms of action of GH are not fully understood. Indeed, there has been a lot of controversy over the role of liver-derived circulating IGF-I and locally produced IGF-I in mediating the growth-stimulatory effect of GH during the last 15 yr. It is in this context that we have conducted studies to further understand how GH stimulates skeletal muscle growth in cattle. Our results do not support a role of skeletal muscle-derived IGF-I in GH-stimulated skeletal muscle growth in cattle. Our results indicate that GH stimulates skeletal muscle growth in cattle, in part, by stimulating protein synthesis in muscle through a GH receptor-mediated, IGF-I-independent mechanism. In this review, besides discussing these results, we also argue that liver-derived circulating IGF-I should be still considered as the major mechanism that mediates the growth-stimulatory effect of GH on skeletal muscle in cattle and other domestic animals.

Modification of muscle inherent properties through age at slaughter, growth promotants and breed crosses

Girard, I., Aalhus, J. L., Basarab, J. A., Larsen, I. L. and Bruce, H. L. 2011. Modification of muscle inherent properties through age at slaughter, growth promotants and breed crosses. Can. J. Anim. Sci. 91: 635–648. A 24 factorial experiment tested the interactions of slaughter age (12–13 or 18–20 mo), growth hormone use, β-adrenergic agonist (β-AA) use and breed cross [Hereford–Aberdeen Angus (HAA) or Charolais–Red Angus (CRA)] on the composition, fibre types, and connective tissue characteristics of m. semitendinosus (ST) and m. gluteus medius (GM) from 112 crossbred steers. Muscle weights increased with slaughter age, implantation and CRA genetics (P<0.05), but were not affected by ractopamine hydrochloride (RAC) (P>0.10).Animal age increased fast glycolytic (FG) and decreased fast oxidative glycolytic (FOG) fibre percentages by 7.2 and 6.6%, respectively, in the ST and increased slow oxidative (SO) and FOG fibre areas in both muscles (P<0.05). Cross-sectional areas of all fibre types were increased in the ST with implantation. In the GM, implantation increased SO (3.1%) and reduced FOG (3.2%) fibre percentages, while RAC reduced the SO (3.8%) and increased the FG (6.1%) fibre percentages (P<0.05).Only GM total collagen content increased with slaughter age (P<0.05),but collagen solubility decreased with slaughter age for both muscles (P<0.05). CRA genetics increased FG percentage in the GM of yearling-fed steers and increased moisture and protein and reduced fat contents of both muscles (P<0.05). In the muscles studied, IMP, slaughter age and animal genetics induced greater changes in muscle inherent properties than RAC.

Effect of ractopamine-hydrochloride and trenbolone acetate on longissimus muscle fiber area, diameter, and satellite cell numbers in cull beef cows1

The objective of this study was to evaluate the effects of coadministration of ractopamine-HCl (RAC) and trenbolone acetate plus estradiol (TBA) on LM fiber cross-sectional area (CSA), diameter, fiber-associated myonuclei, and satellite cell number. Culled crossbred beef cows (n = 98; 11 +/- 1.8 yr old; BCS 4.3 +/- 0.03) from a single ranch in south Florida were fed a concentrate diet for 92 d in a 2 x 2, randomized block design. Cows were blocked by BW on arrival into light (initial BW = 369.75 +/- 2.68 kg and end BW = 501.96 +/- 6.90 kg) and heavy (initial BW = 418.31 +/- 2.75 kg and end BW = 522.15 +/- 7.09 kg) groups before assignment to treatment. Factors included dietary treatment (0 or 15 ppm) and implant status (0 or 80 mg of trenbolone acetate + 16 mg of estradiol). Ractopamine was provided in the diet to 2 pens or half the treatments during the final 35 d of feeding. Cows were slaughtered on d 92. Forty-eight hours postmortem, the 6th-rib portions of the LM were obtained from 10 randomly selected carcasses from each treatment group (n = 40). Cryosections (12 mum) were immunostained for dystrophin and myosin heavy chain I or II for the measurement of fiber CSA and type, respectively. Fiber-associated nuclei and satellite cell numbers were measured in serial cryosections. There was a RAC x TBA interaction (P < 0.05). Type I fiber CSA and diameter were increased (P < 0.05) by TBA and RAC. Type I CSA and diameter were larger (P < 0.05) in TBA + RAC than RAC only. Type II fiber CSA and diameter were not affected by TBA (P = 0.48), RAC (P = 0.15), or TBA + RAC (P = 0.60). Satellite cell numbers and fiber-associated nuclei were not affected (P > 0.05) by implant status or ractopamine supplementation. These results indicate that TBA and RAC preferentially increase the size of type I fibers in cull cows.

Desempenho de novilhas búfalas terminadas em confinamento em resposta ao uso de promotor de crescimento ou de esferas de chumbo no útero

Avaliaram-se ganho de peso, ingestão e conversão alimentar de matéria seca e rendimento de carcaça quente de 20 novilhas bubalinas, confinadas, com média de peso de 350kg e média de idade de 16 meses. Os animais permaneceram em baias coletivas durante 84 dias e foram alimentadas com silagem de cana-de-açúcar + 1% de uréia e concentrado. Os tratamentos foram: VAZ - novilhas vazias, PRC - novilhas com implante de promotor de crescimento e CHU - novilhas com 100 esferas de chumbo introduzidas no útero. O delineamento foi inteiramente ao acaso com seis repetições no tratamento VAZ e sete repetições em PRC e CHU. O peso final, ganho médio diário e conversão alimentar da matéria seca foram mais elevados para as búfalas do tratamento PRC, respectivamente, 445,0kg; 1,2kg e 9,7kg de MS/kg de ganho de peso vivo, em comparação com as do tratamento VAZ, respectivamente, 412,6kg; 0,9kg e 11,4kg de MS/kg de ganho de peso vivo e as do tratamento CHU, respectivamente, 407,9kg; 0,8kg e 12,2kg de MS/kg de ganho de peso vivo. Peso final, ganho médio diário e conversão alimentar da matéria seca das búfalas dos tratamentos VAZ e CHU não diferiram entre si. Para o rendimento de carcaça quente não foi observado efeito entre tratamentos.

Effects of growth hormone and feeding level on endocrine measurements, hormone receptors, muscle growth and performance of prepubertal heifers

Prepubertal Friesian heifer calves (n = 24, initial BW = 195 +/- 5 kg) were assigned to a 2 x 2 factorial block design and used to evaluate the effects of daily GH treatment (0 or 15 mg/d) at either a low or a high feeding level in a 5-wk treatment period on endocrine measurements, hormone receptors, muscle growth, and overall performance. In the pretreatment period, a low feeding level was employed for all calves. During the treatment period, animals at the low feeding level had free access to a roughage-based mixture, whereas animals at the high feeding level had free access to a concentrate mixture and were offered 2 kg/d of the roughage-based mixture. Blood samples were collected weekly starting 3 wk before treatment. Longissimus (LM) and supraspinatus (SS) muscles were obtained at slaughter. Metabolizable energy intake was 81% higher, digestible CP intake was 140% higher, and ADG was 115% higher (all P < 0.001) at the high vs. low feeding level. Feed (DMI, ME, and protein) intake was not affected by GH treatment, but ADG was 18% higher (P < 0.13) in GH-treated than in control heifers at both feeding levels. Although of different magnitudes, the muscle anabolic effects of GH treatment and high vs. low feeding level were additive, and both treatments increased carcass weights (P < 0.02 and P < 0.001, respectively), LM (P < 0.05 and P < 0.001), and SS (P < 0.06 and P < 0.003). The anabolic effect of GH treatment was similar in both muscles, whereas the effect of feeding level was most pronounced in LM. Overall, GH treatment increased plasma GH, IGF-I (both P < 0.001), and IGFBP-3 (P < 0.02); however, GH treatment increased total IGF-I, free IGF-I, and IGFBP-3, and decreased IGFBP-2 mainly at the high feeding level (GH x feeding level interaction; P < 0.02, 0.01, 0.03, and 0.10, respectively). The high feeding level increased insulin, free and total IGF-I, and IGFBP-3 (all P < 0.001), but decreased GH and IGFBP-2 (both P < 0.001). High feeding increased type-1 IGF receptor density (P < 0.02), mainly in LM, in accordance with the largest anabolic response in this muscle, whereas GH treatment had no effect on type-1 IGF receptors. The results suggest that in skeletal muscle, the anabolic effects of exogenous GH are related to endocrine changes in the GH-IGF axis, whereas the effects of feeding level also seem to rely on IGF receptor density in the muscles.

Differential effects of diminished oestrogen and androgen levels on development of skeletal muscle fibres in hypogonadal mice

Androgen and oestrogen hormones influence skeletal muscle size and the characteristics of skeletal muscle fibre types. These effects have typically been assessed by producing acute shortages (castration/ovariectomy) or by hormone supplementation. Little evidence exists, however, on how sex hormone shortages affect muscle development from early stages through to adulthood. Using the hypogonadal mouse model (hpg) we examined the effects of diminished androgen and oestrogen upon muscle size and fibre type composition in murine gastrocnemius and soleus muscles. Hypogonadal male soleus muscle was significantly smaller than normal males, and approximated the normal and hypogonadal females weight and fibre type characteristics. The hypogonadal male gastrocnemius muscle, however, was significantly small in comparison with normal and hypogonadal female gastrocnemius muscles, with the type IIB fibre diameters decreased most markedly. The hypogonadal female soleus muscle approximated the normal female phenotype, but the gastrocnemius muscle was larger than the normal female, approximating the size of the normal male gastrocnemius muscle. Here too, the type IIB fibres showed the most alteration, with greatly increased fibre diameters. Appropriate amounts of androgens were necessary for gender-specific patterns of growth in male muscles, whilst similar amounts of oestrogen were necessary for female gastrocnemius muscle growth, but not for female soleus muscle. Hypogonadism in this murine model generally retards muscle development in males, but has no apparent influence or enhances muscle development in females. Type IIB fibres are most dependent upon sex hormones for appropriate development, but this relationship is muscle-specific.

Hormone containing growth promoting implants in farmed livestock

Growth promoting implants have been used in the production of cattle and sheep for over 40 years. Implants improve growth rate (+10 to 30%), feed efficiency (+5 to 15%) and carcass leanness (+5 to 8%). The history of this technology is mainly one of optimizing dose and hormone combinations, although matrices to optimize delivery rates of hormones from implants has received some attention. Estrogens are the first requirement for the growth response and in combination with androgens, growth is further enhanced. Several implant matrices are used, affecting pay-out rate and delivery time. The delivery time of most compressed implants is approximately 120 days and reimplantation after 60-120 days gives an additional response. Blood concentrations of implant hormones are increased and there appears to be a threshold blood level below which a growth response is not observed. Several proposed mechanisms are reviewed. The somatotropic axis appears most plausible for estrogens. Androgens may occupy muscle corticosteroid receptors. Regulated and proper use of implants assures their safety.

Absence of growth hormone-induced avian muscle growth in vivo

Growth hormone (GH) clearly has the potential to dramatically enhance skeletal muscle accretion in red meat animals such as swine. It is generally accepted that this anabolic effect is mediated by insulin-like growth factor-I (IGF-I), a potent stimulator of proliferation and differentiation of satellite cells that are important for myofiber hypertrophy and for regeneration in postnatal muscle tissue. All available evidence suggests that the capacity for IGF-I-mediated actions of GH on avian myogenic cells is intact, and recent evidence is accumulating that GH may even have direct effects on avian skeletal muscle satellite cell proliferation and differentiation. However, with little exception, exogenous GH does not improve skeletal muscle mass, carcass protein, or any measure of muscle anabolism in domestic poultry. A primary lesion would appear to be the inability of GH to induce significant increases in circulating IGF-I concentrations in sexually immature, growing poultry. This is the case despite clear evidence of GH binding to hepatic receptors, GH-induced tyrosine phosphorylation of Janus kinase 2 (JAK2), and GH-induced expression of hepatic IGF-I mRNA and protein. Factors that should be explored with respect to this apparent discrepancy are discussed, including the regulation of IGF-I release, uptake, and interaction with cell-associated IGF binding proteins or receptors. In addition to its growth-promoting effects via IGF-I, GH has direct metabolic effects that are expressed as changes in circulating regulatory hormone and metabolite concentrations. The possibility that such changes may influence IGF-I release and action is also proposed.

Biology of somatotropin in growth and lactation of domestic animals

Impressive progress has been made during the past 15 years in our understanding of the biology of somatotropin (ST) in domestic animals. In part, this progress was sparked by advances in biotechnology that made feasible the production of large quantities of recombinant bovine ST (bST) and porcine ST (pST). The availability of recombinant bST and pST resulted in an exponential increase in investigations that explored their role in growth and lactation biology, as well as evaluated their potential for commercial use. Collectively, these studies established that administration of bST to lactating dairy cows increased milk yield, and treatment of growing pigs with pST markedly stimulated muscle growth and reduced fat deposition. In addition to these "efficacy" studies, a substantial number of investigations examined the mechanisms by which ST affects lactation and growth of domestic animals. This review summarizes the diverse physiological effects ST has on growth and lactation and discusses the underlying mechanisms that mediate these effects in domestic animals.