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Biology of Growth
Pages 135-183

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From page 135...
... To enhance growth and improve feed conversion efficiency in agricultural animals, scientists must understand the roles of hormones (pepticle and steroid) and peptide growth factors in these processes and identify the limiting factors so that these processes can be modulated.
From page 136...
... Effects of Chicken GH on Body Weight Gain in Chickens Large quantities of chicken pituitary GH were purified to examine its eject on growth (Leung et al., 1986b)
From page 137...
... There was no difference in the effect of feed conversion efficiency on carcass composition between cGH-treated and control birds. Effects of Human Pancreatic GRF and TRH on Body Weight Gain in Chickens Chicken hypothalamic GRF has not yet been isolated and purified, but a synthetic human pancreatic GRF (hpGRF)
From page 138...
... For example, analysis of the amino acid] and nucleoticle sequences of purified epidermal growth factor receptor (EGF-R)
From page 139...
... also successfully introduced foreign genes in pigs by microinjection. Leung and coworkers have attempted to directly inject foreign DNA into the blastoderm of freshly laid eggs with recombinant DNA technology (unpublished!
From page 140...
... 1986. Insulinlike growth factor I stimulates growth in normal growing rats in viva.
From page 141...
... 1983. The identity of human insulin-like growth factors I and II with somatomedins C and A and homology with rat IGF I and II.
From page 142...
... Muscle fibers contain the major organelles present in most cells. The most striking difference between muscle cells an(1 the majority of other cells is their multinu cleated nature.
From page 143...
... (1984~. These features of muscle cells are common to all skeletal muscle fibers, but specific fibers have cli~erentiated somewhat depending on their purpose.
From page 144...
... As expected, mitochondrial content and glycolytic enzyme content vary among fiber types, as (lo energy substrates such as glycogen and triglyceride. Aspects of fiber type variation that affect muscle growth include the notable differences in fiber size that generally correlate with muscle fiber type.
From page 145...
... In looking only at muscle cells, however, significant variations in cholesterol content have not been seen, even among most of the species used for muscle foods (Reiser, 1975; Watt and Merrill, 1963~. This is also true for the amino acid composition of muscle.
From page 146...
... , as is the in vitro development of fibers from one of the later muscle-colony-forming types (Bonner and Adams, 1982~. An additional class of myogenic cells, or branch of the myogenic lineage, is the satellite cell, which is discussed further in the subsection on postnatal development.
From page 147...
... One class of protein growth factors, the fibroblast growth factor (FGF) , has been shown to be mitogenic for myoblasts in culture and can reduce the tendency to differentiate (Allen et al., 1984; Gospodarowicz et al., 1976; Linkhart et al., 1981~.
From page 148...
... Detailed information about the structure of important muscle-specific genes, including identification of regulatory sequences, will open the door to studies that are critical to un(lerstancling quantitative aspects of muscle protein synthesis regulation, one of the key problems in animal growth research. Postnatal Development Understanding the regulation of postnatal muscle growth requires an appreciation of the cellular events underlying the process.
From page 149...
... Myotubes formed by satellite cells in vitro synthesize musclespecific proteins and spontaneously contract in culture (Allen et al., 1980; Cossu et al., 1980~. Although qualitatively they resemble embryonic myogenic cells, satellite cells may well be a separate type of myogenic cell.
From page 150...
... Consequently, in vitro data clirectly link the action of the IGFs to an authentic target cell in postnatal skeletal muscle. Two additional growth factors active in promoting satellite cell proliferation are the basic (Allen et al., 1984)
From page 151...
... These aspects of muscle protein metabolism are obviously critical to the physiology of the animal, but they are not necessarily directly related to muscle growth. Consequently, this discussion dwells on two broad growth-related processes in muscle: protein synthesis and protein degradation.
From page 152...
... in muscle and are active on myofibrilIar protein substrates (see Goll et al., 1983~. A problem with attributing myofibrillar protein clegradation in skeletal muscle to catheptic proteases is the fact that myofibrils or myofilaments have not been observed in lysosomal structures in muscle.
From page 153...
... (1976~. This protease, with a molecular weight of 110,000 daltons, is located inside skeletal muscle cells, as well as many other cell types, and is active at neutral pH.
From page 154...
... be influencecl, satellite cells may be more receptive to manipulation efforts during certain periods of growth than others. Early postnatal growth is the time of greatest satellite cell activity and would correspond to the period of greatest sensitivity to hormones and growth factors.
From page 155...
... In terms of the biological events responsible for muscle growth, it must either directly or in(lirectly stimulate nuclear accretion, stimulate protein synthesis, or decrease protein degradation. At least one report suggests that protein synthesis is depresses]
From page 156...
... Eviclently, the beta-adrenergic agonists may have multiple sites of action, especially for protein degradation and adipose tissue metabolism, but this conclusion remains highly speculative. These examples of a few of the most interesting agents currently being investigatec3 for use in stimulating muscle growth not only demonstrate that application is leading investigation, they also provi(le striking demonstrations that muscle growth in meat animals can be manipulated to increase protein production and decrease triglyceride deposition beyond the normal physiological limits of a particular animal.
From page 157...
... 1986. Transforming growth factor-beta inhibits the IGF-I-induced proliferation and differentiation of skeletal muscle satellite cells.
From page 158...
... 1986. Effects of cimaterol and fishmeal on performance, carcass characteristics and skeletal muscle growth in lambs.
From page 159...
... 1983. Age-related differences in absolute numbers of skeletal muscle satellite cells.
From page 160...
... 1961. Satellite cell of skeletal muscle fibers.
From page 161...
... 1986. Identification of the fibroblast growth factor receptor of Swiss 3T3 cells and mouse skeletal muscle myoblasts.
From page 162...
... 1978. Identification of a novel form of myosin light chain present in embryonic muscle tissue and cultured muscle cells.
From page 163...
... is stored in adipose tissue in the form of triglyceride, which is a triester composed of three molecules of Tong-chain fatty acids per molecule of glycerol. Stored lipids can be mobilized from the fat cell to meet the energy needs of muscle and other tissues.
From page 164...
... To determine whether growth hormone decreased carbohydrate utilization and fatty acid synthesis in adipose tissue, Goodman (1968b) injected hypophysectomized rats with growth hormone.
From page 165...
... Fat is stored in adipose tissue in the form of triglycerides, which are synthesized continuously from fatty acids, en c! alpha-glycerol phosphate, which is derived from glucose.
From page 166...
... incubated segments of adipose tissue from normal rats in bicarbonate buffer in the presence or absence of epinephrine. To obtain frequent measurements of glycerol production, the tissue segments were transferred to a fresh medium every 5 minutes.
From page 167...
... It is unlikely, however, that this action can explain all the effects of glucocorticoids on adipose tissue. The effects of growth hormone may also involve protein synthesis, but the nature of the inducer]
From page 168...
... investigated growth hormone in adipose tissue from normal rats and from hypophysectomized rats that were either untreated or given growth hormone for 2 days. In normal tissues, glycerol production was nearly doubled in 4 hours of incubation with growth hormone and dexamethasone.
From page 169...
... The ejects of growth hormone and dexamethasone on the response to epinephrine were examined by Goodman (1969~. Eight segments of adipose tissue were taken from each of eight hypophysectomized rats and preincubated for 3 hours.
From page 170...
... The other prominent enclogenous inhibitory agent in adipose tissue is adenosine, which is released from fat cells by the breakdown of cyclic AMP. Therefore, Goodman et al.
From page 171...
... Membranes prepared from a(lipocytes of hypophysectomizec! or normal rats and from hypophysectomized rats treated with growth hormone 3 hours earlier were incubated with 32P-labeled NAD and cholera toxin or a mixture of cholera toxin and pertussis toxin (Goodman et al., 1986~.
From page 172...
... 1976. Interconvertible enzymes in adipose tissue regulated by cyclic AMP-dependent protein kinase.
From page 173...
... In fact, it is generally believed that many if not all of the effects of somatotropin on muscle growth are mediated through somatotropin-clependent plasma factors somatomedins produced in response to somatotropin. In culture, muscle cells do not appear to respond!
From page 174...
... On the basis of this information, it appears likely that insulinlike growth factors are potent stimulators of all aspects of muscle growth and clevelop ment. Insulin The role of insulin in regulating general cell metabolism has been recognized for many years, but its mechanism of action is still not well understood.
From page 175...
... In muscle cell cultures, iron-saturated transferrin stimulates both proliferation and differentiation and is essential for maintenance of healthy myotubes. The effect of transferrin on muscle growth in culture is absolutely dependent on the presence of iron and appears to be class specific (that is, mammalian transferring do not affect avian myoblasts, nor do avian transferring affect mammalian myoblasts)
From page 176...
... The mechanism by which this factor is accumulated in skeletal muscle and the relationship of this accumulation to regulation of muscle growth and regeneration is of interest. BIOASSAYS FOR FACTORS INFLUENCING MUSCLE GROWTH To develop effective strategies for controlling animal growth, a better understanding is needed of the mechanism by which known growth factors regulate proliferation, (differentiation, and protein turnover in muscle cells.
From page 177...
... At present, this is a laborious and inaccurate procedure. In vitro incubation of excised muscle tissue has also been used to study the effects of various peptides on muscle growth, primarily the influence of different substances on the rates of protein synthesis and degradation in skeletal muscle tissue (Fulks et al., 1975~.
From page 178...
... Methods Standardized bioassays for measuring the elect of porcine serum on proliferation in cultured LO muscle cells were done according to procedures described in detail by Kotts et al.
From page 179...
... . The inability of pGH to stimulate proliferation of cultured muscle cells is in agreement with results obtained by others using primary myogenic cultures or L6 myogenic cells (Ewton and Florini, 1980; Gospodarowicz et al., 1976~.
From page 180...
... are responsible for these increases in mitogenic activity observed at supraphysiological concentrations of GH. In contrast to the lack of response observed when pGH was aciclec3 directly to muscle cells, sera from four out of five pigs injected with pGH exhibiter]
From page 181...
... They also provide a valuable tool for use in identifying unknown growth factors that affect muscle growth in meat animals. Identification of these factors ant]
From page 182...
... 1981. Proliferation of chick skeletal muscle cells cultured in a chemically defined medium.
From page 183...
... 1984. There is selective accumulation of a growth factor in chicken skeletal muscle.


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