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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
Department of Animal Sciences, Purdue University, West Lafayette 47907
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, ß1-adrenergic receptor (AR), and ß2-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 ß1-AR was not affected by feeding RAC, whereas ß2-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 ß2-AR.
Key Words: fiber type • myosin heavy chain • ractopamine
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Grant et al., 1993). However, muscle histochemical data show that BAA administration stimulates muscle growth in a fast fiber type-specific manner (Moloney et al., 1990; Wheeler and Koohmaraie, 1992), suggesting that BAA may differentially alter muscle fiber type-specific MyHC gene expression. Specifically, BAA feeding increases the frequency and size of type II fibers, especially type IIB fibers, the fastest contracting, most glycolytic muscle fiber type (Beermann et al., 1987; Zeman et al., 1988; Polla et al., 2001).
In fact, we have shown that the amount of type IIB MyHC increases at the expense of those isoforms associated with slower-twitch, more oxidative fibers, suggesting that BAA stimulate muscle fibers to transition to a faster phenotype (Depreux et al., 2002). Taken together, these findings are consistent with the idea that improved efficiency in BAA-fed pigs may be a function of more favorable protein turnover rates in whiter muscle (Dadoune et al., 1978). Not only does skeletal muscle become more glycolytic after agonist administration (Vestergaard et al., 1994), but also BAA induce changes in energy metabolism (Rajab et al., 2000; Polla et al., 2001), suggesting that BAA stimulate muscle to change its functional and energetic capabilities.
The objective of the current study was to determine the effect of RAC on skeletal muscle fiber type-specific gene expression.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Two experiments were conducted to determine the role of RAC on muscle fiber type-specific gene expression. The first study was initially conducted to provide changes in muscle expression on a weekly basis. A subsequent study was conducted to define further the time, within hours, when these genes were altered, or if there were initial, or transient, changes that occurred within hours of treatment but subsided by 1 wk.
Animals
The Purdue Animal Care and Use Committee approved all procedures for the care and use of pigs.
Exp. 1.
Ractopamine-HCl (Elanco Animal Health, Greenfield, IN) was administered at 20 mg/kg daily in feed for 0, 1, 2, or 4 wk to 44 pigs (average BW 90 ± 10 kg) obtained from Purdue University Animal Sciences Research and Education Center. Diets were corn- and soybean meal-based and formulated to meet or exceed all nutrients requirements of these pigs (NRC, 1998). The diets were formulated to 1.15% Lys, 19.5% CP, and 5.0% supplemental fat, with or without RAC (as-fed basis). Control pigs (barrows and gilts) were slaughtered after a 4-wk period of growth, whereas pigs that were fed RAC for 1 or 2 wk received the treatment after 3- or 2-wk period of growth, respectively. This was an attempt to achieve the predetermined duration of feeding (4 wk for all pigs), and also to target an average slaughter BW of 115 kg. Pigs were processed according to normal industry procedures, and LM samples were taken immediately postexsanguination. Samples were frozen in liquid nitrogen and stored at –80°C.
Exp. 2.
In a second study, 48 pigs (barrows and gilts, average BW 98 ± 3 kg) obtained from Purdue University Animal Sciences Research and Education Center and used to establish, on an acute basis, the time needed to upregulate muscle fiber type-specific gene expression. When pigs reached an average BW of 90 kg, they were blocked by initial BW into 2 groups. Within these 2 groups, equal numbers of pens were randomly assigned to treatments of 0 or 20 mg/kg of RAC (Elanco Animal Health), for a total of 16 pens. Both diets were corn- and soybean meal-based and formulated to meet or exceed all nutrients requirements of these pigs (NRC, 1998). Diets were formulated to 1.15% Lys, 19.5% CP and 5.0% supplemental fat, with or without RAC (as-fed basis). After 12, 24, 48 or 96 h, 6 pigs were harvested for each level of RAC, for a total of 48 pigs. Pigs were processed according to normal industry procedures, and red semitendinosus (RST), white semitendinosus (WST), and LM samples were taken immediately postexsanguination. Samples were frozen in liquid nitrogen and stored at –80°C.
Total RNA Preparation
Total RNA was extracted from porcine skeletal muscle using the single-step method (Chomczynski and Sacchi, 1987), with modifications. Briefly, 100 mg of skeletal muscle that had been powdered in liquid nitrogen was placed into 3 mL of Solution D, composed of 4 M guanidium isothiocyanate (Sigma, St. Louis, MO), 25 mM sodium citrate (pH 7), 0.5% sarcosyl, and 0.1 M 2-mercaptoethanol, in a 50-mL conical tube held on ice. Tissue was homogenized using a Polytron Homogenizer (Brinkman Instruments, New York, NY) at speed 6, then transferred to a fresh 15-mL conical tube. Next, 300 µL of 2 M sodium acetate (pH 4) was added and vortexed. Protein and lipids in the preparation were extracted by sequentially adding 3 mL of phenol saturated with diethyl pyrocarbonate-treated water (pH 4.3), and 600 µL of chloroform:isoamylalcohol (40:1, vol/vol). After thorough vortexing, the tubes were placed on ice for 15 min. Samples were centrifuged at 10,000 x g at 4°C for 20 min to separate aqueous and organic phases. The aqueous phase (upper) was carefully transferred to a fresh, 15-mL conical tube. Attention was made to avoid disturbing the interphase. Precipitation of the RNA was facilitated by adding 3 mL of isopropanol and holding the samples at –20°C for 1 h. Precipitated RNA was then collected by centrifugation at 10,000 x g at 4°C for 30 min. Pellets were then redissolved in 0.9 mL solution D, 0.9 mL isopropanol was added, and RNA was allowed to precipitate at –20°C for 1 h. Precipitated RNA was then collected by centrifugation at 21,000 x g at 4°C for 30 min. Isopropanol was discarded, and the pellet was washed twice with 1 mL of 75% ethanol (vol/vol). The pellets were air-dried for 10 min and resuspended in 50 µL of TE-8, composed of 1 M tris-HCl (pH 8), 0.2 M EDTA (pH 8), and 0.1 M diethyl pyrocarbonate-treated water.
RNA Quantification
Total RNA isolated from skeletal muscle was measured using the Ribogreen quantification kit (Molecular Probes, Eugene, OR). The RNA was first treated with 4 U of recombinant DNAse (Ambion, Indianapolis, IN) in 2 µL of a 10x DNase buffer containing 100 mM Tris-HCl, pH 7.5; 25 mM MgCl2; 5 mM CaCl2; and 3 µL of nuclease-free water (Ambion) and digested for 30 min at 37°C. DNase Inactivation Reagent (5 µL; Ambion), a resin that binds DNase, was added to the reaction and mixed by flicking the tubes. The RNA was then loaded into 0.22 µm Spin-X Centrifuge Tube Filters (VWR International, West Chester, PA) and centrifuged at 10,000 x g for 1 min. A 1-µL sample of purified RNA was diluted with 249 µL of TE buffer containing 200 mM Tris-HCl and 20 mM EDTA (pH 7.5; Ambion). Next, 75 µL of TE buffer and 100 µL of Ribogreen reagent were combined in each well of a 96-well microplate. Prepared RNA samples (25 µL) were added to the reagents on the microplate. A GENios Pro fluorometer (Tecan, Durham, NC) was used to measure the fluorescent signal at 480 (excitation) and 520 (emission) nm.
cDNA Synthesis
The RNA was reverse transcribed to cDNA. A cDNA master mix was prepared from 5 U of Moloney murine leukemia virus reverse transcription (Invitrogen, Carlsbad, CA), 0.5 U of SUPERase-In (Ambion), 10 mM DTT (Invitrogen), and 5x First strand buffer (Invitrogen). Total RNA (0.5 µg) in 5 µL of nuclease-free water was added to 100 ng/µL of random hexamers (Invitrogen) and 100 µM of each dNTP (Eppendorf, Westbury, NY). Samples were denatured at 65°C for 5 min and then chilled to 4°C on ice. Four microliters of cDNA master mix was added to the denatured RNA and sequentially incubated at 25°C for 10 min, 37°C for 50 min, 70°C for 10 min, and chilled to 4°C on ice. Finally, cDNA samples were diluted with nuclease-free water and aliquoted so that gene quantification was based on 50 ng of total RNA per 5 µL.
Real-Time PCR
Exp. 1.
Relative transcript abundance was determined using quantitative, real-time PCR. Primer sequences for individual myosin heavy chain (MyHC) isoforms I, IIA, IIX, and IIB, total MyHC, glycogen synthase (GS), citrate synthase (CS), lactate dehydrogenase (LDH), peroxisome proliferator activated receptor (PPAR), ß1-adrenergic receptor (AR), ß2-AR, and ß-actin are shown in Table 1. Individual MyHC primers targeted the 5'-untranslated region of their respective cDNA (da Costa et al., 2002) to convey isoform specificity. Total MyHC primers targeted the S1 loop region, where homology among MyHC isoforms is essentially identical. Real-time PCR was carried out according to optimized gene specific protocols, shown in Table 2. The first cycle in the log-linear region of amplification at which a significant increase in fluorescence was detected above background was designated the threshold cycle (Ct). All expression data were normalized to ß-actin. Amplification of highly expressed genes, therefore, was detected at a lower cycle number than those genes expressed at a relatively low level, which require additional cycles; therefore, Ct and gene expression are inversely related.
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Statistical Analysis
Analysis of variance was generated by using the GLM procedure (SAS Inst. Inc., Cary, NC), with time and RAC as the main effects. Least square means and average SEM were calculated for all Ct (Exp. 1) or log starting abundance (Exp. 2) data, and the Student-Newman-Keuls procedure of SAS was used to separate means when a significant F-test (P < 0.05) was observed.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Real-time PCR was used to determine the effect of RAC on gene expression in porcine skeletal muscle following a period of 0, 1, 2 or 4 wk of feeding RAC. Beta-actin did not differ (P > 0.05) over the course of the experiment (data not shown). Expression of the type I MyHC gene was not altered by RAC during the entire length of the study (Figure 1, panel A). Type IIA MyHC gene expression decreased (P < 0.0001) by 1 wk of RAC administration, remained lower (P < 0.001) at 2 wk, but was not different from controls by 4 wk (Figure 1, panel B). Ractopamine did not influence MyHC type IIX gene expression by 1 wk but decreased (P < 0.001) by 2 and 4 wk (Figure 1, panel C). Expression of MyHC type IIB was increased (P < 0.0001) by 1 wk and remained elevated throughout the remainder of the study (Figure 1, panel D). Ractopamine increased (P < 0.001) total MyHC transcript abundance following 1 and 2 wk of administration, yet expression did not differ from controls at 4 wk (Figure 1, panel E).
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, panel A). Glycogen synthase (GS) mRNA abundance was greater (P < 0.05) by 1 wk of RAC administration, returned to the level of control at wk 2, and decreased (P < 0.01) by 4 wk (Figure 2, panel B). Lactate dehydrogenase gene expression was not affected by RAC (Figure 2, panel C). Expression of the PPAR gene decreased (P < 0.05) by 1 wk of RAC administration, but was not different from controls by 2 and 4 wk (Figure 2, panel D). Ractopamine had no effect on ß1-AR gene expression (Figure 2, panel E); however, ß2-AR gene expression decreased (P < 0.05) by 2 and 4 wk (Figure 2, panel F).
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ß-Actin gene expression, a control to test for experimental variation, was not affected by RAC (data not shown). Compared with LM muscles, MyHC type I gene expression was greater (P < 0.0001) in RST and lower (P < 0.0001) in WST (data not shown). Type IIA MyHC gene expression was also greatest (P < 0.0001) in RST muscles, but least (P < 0.0001) in LM muscles (data not shown). Ractopamine decreased (P < 0.0001) type IIA MyHC gene expression by 96 h compared with controls (Figure 3). No muscle x time interaction was observed. Ractopamine did not affect type IIX MyHC prior to 96 h, but gene expression was greater (P < 0.01) in WST than RST muscles, whereas LM muscles had least (P < 0.01) type IIX MyHC expression (data not shown). The RST muscles had lower (P < 0.0001) type IIB MyHC gene expression than WST and LM muscles (data not shown). However, RAC increased (P < 0.0001) type IIB MyHC gene expression in all muscles by 12 h and maintained this level of expression at all subsequent time points in the study (Figure 4, panel A). Glycogen synthase gene expression was greater (P < 0.05) in WST and LM muscles than RST muscles (data not shown). Furthermore, expression was increased (P < 0.05) by 12 h after RAC administration in all muscles studied (Figure 4, panel B). Gene expression of CS was not different among muscles and was unaffected by RAC in our study (data not shown). Gene expression of ß1-AR did not vary with treatment or muscle prior to 96 h (data not shown), whereas RST muscles had greater (P < 0.01) ß2-AR gene expression than WST and LM, but likewise did not change within 96 h of treatment (data not shown).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Koohmaraie et al., 1991; Grant et al., 1993); however, little data exist regarding the expression of multiple genes within a larger gene family, like myosin and its associated isoforms. Data from these studies show that RAC feeding differentially alters fiber type-specific gene expression rather than globally increasing the expression of all contractile proteins.
In our studies, type I MyHC gene expression was not affected by RAC feeding. These results are supported by earlier data indicating muscle hypertrophy and subsequent increases in muscle mass of those animals fed BAA generally lack increases in type I MyHC. Using histochemical approaches, Beermann et al. (1987) showed that type I fibers contributed little to cimaterol-induced hypertrophy in the biceps femoris, semitendinosus, and semimembranosus muscles of lambs. This was further verified in a separate lamb study by Kim et al. (1987). Furthermore, Aalhus et al. (1992) reported RAC did not significantly alter the cross sectional area of type I muscle fibers in pig muscle, whereas others have reported a reduction in these fibers in the biceps femoris of pigs fed salbutamol (Oksbjerg et al., 1994). Using a whole muscle enzyme linked ELISA-based assay, we showed that even feeding as much as 60 ppm of RAC for up to 42 d does not change the relative abundance of type I MyHC protein in the red and white semitendinosus muscle or LM of pigs (Depreux et al., 2002). The transient (2 wk) increase in total MyHC message observed in porcine muscle after BAA feeding suggests the proportion of type I MyHC may have increased slightly for 2 wk; however, this would be difficult to resolve given the aforementioned literature.
Type IIA MyHC gene expression first decreased between 48 and 96 h after RAC administration and continued to decrease to its lowest level by 1 wk, but returned to control levels by 4 wk. This pattern of gene expression represents a rapid downregulation of the type IIA gene followed by a gradual recovery to full, pretreatment levels. During the same time, however, type IIX MyHC gene expression decreased throughout the entire duration of the study, whereas increased type IIB MyHC gene expression was initially observed to increase by 12 h and remained elevated through the 4 wk feeding period. The majority of existing muscle fiber type data suggests that BAA-induced muscle hypertrophy results from increases in cross sectional area of type II fibers (Beermann et al., 1987; Zeman et al., 1988). Consistent with our results, several researchers have shown that the percentage of type IIA fibers decreases with BAA feeding, whereas type IIB increases (Aalhus et al., 1992; Vestergaard et al., 1994), suggesting the increased frequency of type IIB fibers in skeletal muscle may be responsible for BAA-improved growth (Wu et al., 1986; Beermann et al., 1987), albeit no cause and effect relationship has ever been established between the increase in muscle growth and the frequency, or size, of type IIB. Unfortunately, early studies were plagued by limitations in the specificity of fiber typing protocols (Lefaucheur et al., 2004). Muscle fibers histochemically identified as type IIB are composed of type IIX or IIB MyHC, and therefore, changes in fiber type composition may have been obscured by inclusion of different fiber types responding differently to treatments (Lefaucheur et al., 1998). In addition, means of reporting fiber type data vary from those reporting absolute frequencies to those reporting fiber type on a per cross-sectional area basis. Although both approaches are valid, it is difficult to collectively interpret these data. Data reported herein reflect whole muscle gene expression and essentially reflect data reported by Depreux et al. (2002) that RAC increases relative abundance of type IIB MyHC at the expense of type IIA and IIX using whole muscle MyHC preparations (Depreux et al., 2002). Even though variability in the antibodies used to quantify type IIA and IIX MyHC in that study have been identified (Lefaucheur et al., 2004), the fact that immunoreactivity to MyHC decreased while increases in IIB antigenicity were observed argues muscle becomes "faster" with BAA feeding at the expense of more oxidative, fast contracting fibers, namely IIA and IIX.
The enhanced protein synthesis observed in muscle cells cultured in the presence of BAA is blocked by compounds that bind specifically to the beta-adrenergic receptors (ß-AR; Anderson et al., 1990). To date, 3 ß-AR (ß1, ß2, and ß3) are known to exist in porcine skeletal muscle and represent 60, 39, and 0.7%, respectively, of the total ß-AR on muscle cells (McNeel and Mersmann, 1999). We were unable to detect any ß3-AR signal in the skeletal muscles studied using real-time PCR. The fact that we did not detect this species of ß-AR does not imply that it does not play an important role in modulating BAA effect in pigs. However, given our primers were specifically designed to porcine sequences, this simply indicates it is expressed at extremely low levels in skeletal muscle. The ß1-AR, however, was easily detected in porcine skeletal muscle, but significant changes were not noted. On the other hand, ß2-AR gene expression decreased significantly by 2 wk. Interpretation of these data is somewhat difficult given receptor binding studies were not conducted. However, if reflective of receptor numbers and binding, this suggests a portion of the loss of response to BAA feeding may be mediated through loss of ß2-AR. Ractopamine was originally thought to be selective for the ß1-AR subtype (Smith et al., 1990; Moody et al., 2000). However, Mills (2002) suggested that BAA-induced activation of the cAMP signaling cascade is more efficient through the ß2-AR even though RAC binds ß1- and ß2-AR with similar affinities (Spurlock et al., 1993; Liang et al., 2000; Mills, 2002). Kim et al. (1992) postulated the same after observing that ß-AR binding was reduced in the skeletal muscle of cimaterol-fed mice prior to the loss of growth-stimulation. Spurlock et al. (1994), however, showed no loss in receptor number in muscle of pigs fed RAC for up to 4 wk. These discrepancies may be related to a combination of posttranscriptional events and species differences. Curiously, the ß-AR are predominantly found in slow-twitch, oxidative muscles (Williams et al., 1984). Muscle fibers are extremely sensitive to hormonal or functional cues. In particular, muscle fiber type transitions occur along a well-characterized pathway (I
II-AIIXIIB). Even though our data are not based on individual fiber data and, therefore, cannot specifically address this pathway, per se, classical histochemical data firmly support the idea that BAA stimulation forces muscle to a faster-contracting phenotype. Therefore, it is possible that the loss of the ß2-AR may simply reflect the loss of slow fibers, which have greater ß-AR expression. However, our data does not support this mechanism because we failed to observe a muscle x time interaction for ß2-AR expression in Exp. 2, which would be expected given that greater expression is observed in the red portions of the semitendinosus. In addition, it is difficult to envision that changes in type IIA fibers would be enough to change whole muscle ß2-AR expression given IIA fibers represent only a small fraction (<10%; Lefaucheur et al., 2004) of the total fibers in the LM. Further studies will be required to understand fully how various ß-AR mediate this response, and more importantly, which fiber type (within the type II fibers), if any, respond directly to BAA feeding.
Muscles possessing different amounts and types of MyHC inherently contain different types of energy metabolisms to facilitate their function (Gauthier, 1969). A slow-to-fast transition of MyHC isoforms induced by clenbuterol is associated with decreases in the activity of oxidative enzymes and increases in activity of glycolytic enzymes in fast- and slow-twitch muscles (Polla et al., 2001). It is important to note, however, that enzyme activities are generally controlled by substrate availability or other types of posttranscriptional modifications and, therefore, need not correlate with mRNA levels. We did evaluate enzyme activity in our studies. Nonetheless, CS gene expression was decreased by RAC administration in our study by 2 and 4 wk. Our data are consistent with a BAA-induced decrease in aerobic enzyme activity in cattle and pigs indicating a shift toward the more glycolytic type metabolism that accompanies the transitioning fiber type profile in muscle of BAA treated animals (Zimmerli and Blum, 1990; Vestergaard et al., 1994). Curiously, GS gene expression increased by 12 h of RAC administration, returned to the level of controls by 2 wk, and further decreased by 4 wk where expression was significantly less than controls. Glycogen synthase is the rate-limiting enzyme for synthesis of glycogen, a major energy storage molecule in the liver and skeletal muscle. Highly selected pigs with a high rate of lean gain have intrinsically greater glycolytic metabolism, which can be enhanced by BAA administration (Oksbjerg et al., 1990). Of course, glycogenolysis is stimulated by acute administration of epinephrine (5 to 20 min; Richter et al., 1982). Epinephrine enhances glycogenolysis in skeletal muscle and glucose production from the liver as a result of direct stimulation of glycogenolysis and indirect stimulation of gluconeogenesis, respectively (Arinze and Kawai, 1983). Therefore, mobilization of energy stores from liver into the circulation by acute administration of BAA increases glucose availability to skeletal muscle (Mersmann et al., 1987; Assimacopoulos-Jeannet et al., 1991) and this cytosolic increase of glucose probably stimulates GS expression. Our data are consistent with these reports suggesting GS gene expression is increased by BAA acutely in an attempt to accommodate the increased glucose rapidly entering the cell. Afterwards, however, it is possible that chronic administration of RAC leads to depleted levels of glucose, most likely due to increased energy needs associated with increased muscle growth. As a result, less GS would be required, leading to downregulation of the gene. Although somewhat less reflective of fiber differences, these data provide insight into how BAA may enhance muscle growth.
Surprisingly, LDH gene expression was not significantly increased in our studies because BAA administration stimulates lactate production in sheep and pigs (Warriss et al., 1989, 1990). Clearly, LDH activity, an indicator of glycolytic energy metabolism, is increased by BAA feeding and corresponds with increased CSA of fast-twitch fibers (Pellegrino et al., 2004). Jungmann et al. (1983) showed that ß-AR-mediated signal transduction pathways transcriptionally regulate LDH expression. However, expression of LDH may be at an extremely high level in LM, demonstrating a further increase in gene expression may not be possible to detect.
Ractopamine decreased PPAR gene expression by 1 wk. Furthermore, PPAR, a ligand-activated nuclear hormone receptor first identified in mice, regulates lipid metabolism by stimulating expression of genes encoding enzymes required for peroxisomal ß-oxidation (Kliewer et al., 1994). The PPAR activates transcription by binding to peroxisome proliferator response elements in the DNA sequence and is abundantly expressed in tissues with high rates of ßoxidation including liver, kidney, heart, and skeletal muscle, promoting cellular uptake and oxidation of fatty acids (Kliewer et al., 1994; Braissant et al., 1996; Mukherjee et al., 2000). As mentioned above, RAC immediately mobilizes glucose, which likely reduces fatty acid oxidation and those associated enzymes. Of course, and consistent with the decreased need for GS, PPAR returns to normal by 2 wk and is maintained at this level at 4 wk, suggesting changes in ß-oxidation may be only indirectly associated with BAA feeding but clearly is consistent with reduced adipose tissue mass in BAA-fed animals (Crenshaw et al., 1987).
Data presented herein suggest RAC differentially induces expression of the type IIB MyHC gene at the expense of the other isforms. However, RAC was administered to pigs during the finishing phase where pigs normally exhibit muscle hypertrophy. Thus, it cannot be overstated that it remains highly possible that muscle assumes the aforementioned characteristics simply as a result of it growing faster or experiencing hypertrophy. Type II fibers are larger, more glycolytic, and exist in higher frequencies in muscles that experience the greatest amount of hypertrophy in a growing animal. Moreover, lines of pigs highly selected for augmented muscle growth contain more type II fibers and express greater amounts of type IIB MyHC (Oksbjerg et al., 1990). Therefore, muscle hypertrophy may stimulate transitions to the faster contracting, more glycolytic type II fiber phenotype, rather than vice versa. Additional studies will be necessary to document whether there is a true cause and effect relationship between muscle fiber type and muscle growth or simply reflects a close association between the whole animal and its underlying physiology.
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Footnotes |
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2 Corresponding author: dgerrard{at}purdue.edu
Received for publication August 4, 2006. Accepted for publication April 26, 2007.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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-Actin mRNA and insulin-like growth factor I mRNA in pigs during feeding and withdrawal of ractopamine. J. Anim. Sci. 71:3319–3326.[Abstract]This article has been cited by other articles:
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