Classical and atypical binding sites for beta-adrenoceptor ligands and activation of adenylyl cyclase in bovine skeletal muscle and adipose tissue membranes

Beta-Adrenergic Agonists, Dietary Protein, and Rumen Bacterial Community Interactions in Beef Cattle: A Review

Improving beef production efficiency, sustainability, and food security is crucial for meeting the growing global demand for beef while minimizing environmental impact, conserving resources, ensuring economic viability, and promoting animal welfare. Beta-adrenergic agonists and dietary protein have been critical factors in beef cattle production. Beta-agonists enhance growth, improve feed efficiency, and influence carcass composition, while dietary protein provides the necessary nutrients for muscle development and overall health. A balanced approach to their use and incorporation into cattle diets can lead to more efficient and sustainable beef production. However, microbiome technologies play an increasingly important role in beef cattle production, particularly by optimizing rumen fermentation, enhancing nutrient utilization, supporting gut health, and enhancing feed efficiency. Therefore, optimizing rumen fermentation, diet, and growth-promoting technologies has the potential to increase energy capture and improve performance. This review addresses the interactions among beta-adrenergic agonists, protein level and source, and the ruminal microbiome. By adopting innovative technologies, sustainable practices, and responsible management strategies, the beef industry can contribute to a more secure and sustainable food future. Continued research and development in this field can lead to innovative solutions that benefit both producers and the environment.

Short Communication: Beta-adrenergic agonists alter oxidative phosphorylation in primary myoblasts

Beta-adrenergic agonists (β-AAs) are widely used supplements in beef and pork production to improve feed efficiency and increase lean muscle mass, yet little is known about the molecular mechanism by which β-AAs achieve this outcome. Our objective was to identify the influence of ractopamine HCl and zilpaterol HCl on mitochondrial respiratory activity in muscle satellite cells isolated from crossbred beef steers (N = 5), crossbred barrows (N = 2), Yorkshire-cross gilts (N = 3), and commercial weather lambs (N = 5). Real-time measurements of oxygen consumption rates (OCRs) were recorded using extracellular flux analyses with a Seahorse XFe24 analyzer. After basal OCR measurements were recorded, zilpaterol HCl, ractopamine HCl, or no β-AA was injected into the assay plate in three technical replicates for each cell isolate. Then, oligomycin, carbonyl cyanide-p-trifluoromethoxyphenylhydrazone, and rotenone were injected into the assay plate sequentially, each inducing a different cellular state. This allowed for the measurement of OCR at these states and for the calculation of the following measures of mitochondrial function: basal respiration, non-mitochondrial respiration, maximal respiration, proton leak, adenosine triphosphate (ATP)-linked respiration, and spare respiratory capacity. Incubation of bovine cells with either zilpaterol HCl or ractopamine HCl increased maximal respiration (P = 0.046) and spare respiratory capacity (P = 0.035) compared with non-supplemented counterparts. No difference (P > 0.05) was observed between zilpaterol HCl and ractopamine HCl for maximal respiration and spare respiratory capacity in bovine cell isolates. No measures of mitochondrial function (basal respiration, non-mitochondrial respiration, maximal respiration, proton leak, ATP-linked respiration, and spare respiratory capacity) were altered by β-AA treatment in ovine or porcine cells. These findings indicate that β-AAs in cattle may improve the efficiency of oxidative metabolism in muscle satellite cells by modifying mitochondrial respiratory activity. The lack of response by ovine and porcine cells to β-AA incubation also demonstrates differing physiological responses to β-AA across species, which helps to explain the variation in its effectiveness as a growth supplement.

Lubabegron fumarate acts as a β-adrenergic receptor antagonist in cultured bovine intramuscular and subcutaneous adipocytes

We hypothesized that lubabegron fumarate (LUB) (Experior, Elanco Animal Health, Greenfield, IN) would act as an antagonist to β-adrenergic receptor (β-AR) subtypes in primary bovine subcutaneous (s.c.) and intramuscular (i.m.) adipocytes differentiated in culture. This study employed LUB, dobutamine (DOB, a selective β1-agonist), salbutamol (SAL, a selective β2-agonist), and propranolol (PRO, a non-selective β-AR antagonist). Preadipocytes were isolated by standard techniques from bovine longissimus muscle and overlying s.c. adipose tissue and differentiated to adipocytes for 14 d. The adipocyte source x stage of differentiation interaction was significant for β-adrenergic receptors-1 (ADRB1) (P = 0.001) and ADRB2 (P = 0.01) in that expression of ADRB1 and ADRB2 was greater in s.c. adipocytes than in s.c. preadipocytes; expression of the ADRB1-3 did not change after differentiation of i.m. adipocytes. CCATT/enhancer-binding protein alpha (CEBPA) expression increased upon differentiation in both s.c. and i.m. adipocytes (P = 0.006). The source x stage of differentiation interaction was significant for peroxisome proliferator-activated receptor gamma (PPARG) (P ≤ 0.001) and fatty acid binding protein-4 (FABP4) (P = 0.004). Expression of PPARG increased after differentiation of s.c. preadipocytes to adipocytes, but PPARG expression did not change with differentiation of i.m. preadipocytes to adipocytes. FABP4 expression increased after differentiation of both s.c. and i.m. adipocytes, but FABP4 expression increased to a greater extent in s.c. adipocytes. In s.c. adipocytes, DOB elevated cAMP and glycerol production and protein kinase A (PKA) activity, and SAL increased PKA activity; these effects were abolished by LUB and PRO (P < 0.001). Incubation of i.m. adipocytes with SAL increased cAMP production and PKA activity, which was attenuated by LUB and PRO (P ≤ 0.006). In s.c. adipocytes, SAL, LUB + SAL, and LUB + DOB upregulated hormone sensitive lipase (HSL) (P < 0.001) and perilipin (P = 0.002) gene expression. In i.m. adipocytes, DOB and LUB + DOB increased HSL gene expression (P = 0.001) and LUB + SAL depressed adipose triglyceride lipase expression below control levels (P = 0.001). These results demonstrate that LUB is a β-AR antagonist at the β1-AR and β2-AR subtypes in s.c. adipocytes, and that s.c. and i.m. exhibit different responses to β-AA and LUB.

Comparison of beta-ligands used in cattle production: structures, safety, and biological effects

Technologies that increase the efficiency and sustainability of food animal production to provide meat for a growing population are necessary and must be used in a manner consistent with good veterinary practices, approved labeled use, and environmental stewardship. Compounds that bind to beta-adrenergic receptors (β-AR), termed beta-adrenergic receptor ligands (β-ligands), are one such technology and have been in use globally for many years. Though all β-ligands share some similarities in structure and function, the significance of their structural and pharmacological differences is sometimes overlooked. Structural variations in these molecules can affect absorption, distribution, metabolism, and excretion as well as cause substantial differences in biological and metabolic effects. Several β-ligands are available for use specifically in cattle production. Ractopamine and zilpaterol are beta-adrenergic agonists approved to increase weight gain, feed efficiency, and carcass leanness in cattle. They both bind to and activate β1- and β2-AR. Lubabegron is a newly developed selective beta-adrenergic modulator with unique structural and functional features. Lubabegron displays antagonistic behavior at the β1- and β2-AR but agonistic behavior at the β3-AR. Lubabegron is approved for use in cattle to reduce ammonia emissions per unit of live or carcass weight. Additionally, lubabegron can withstand prolonged use as the β3-AR lacks structural features needed for desensitization. Due to these unique features of lubabegron, this new β-ligand provides an additional option in cattle production. The individual properties of each β-ligand should be considered when making risk management decisions, as unique properties result in varying human food safety profiles that can determine appropriate safe β-ligand use.

Characterization of β-adrenergic receptors in bovine intramuscular and subcutaneous adipose tissue: comparison of lubabegron fumarate with β-adrenergic receptor agonists and antagonists

Chinese hamster ovary cell constructs expressing either the β ₁-, β ₂- or β ₃-adrenergic receptor (AR) were used to determine whether a novel β-AR modulator, lubabegron fumarate (LUB; Experior, Elanco Animal Health) might exert greater potency for a specific β-AR subtype. EC₅₀ values calculated based on cAMP accumulation in dose response curves indicate that LUB is highly selective for the β ₃-AR subtype, with an EC₅₀ of 6 × 10^–9 M, with no detectible agonistic activity at the β ₂-AR. We hypothesized that the accumulation of lipolytic markers would reflect the agonist activity at each of the β-receptor subtypes of the specific ligand; additionally, there would be differences in receptor subtype expression in subcutaneous (s.c.) and intrmuscular (i.m.) adipose tissues. Total RNA was extracted from adipose tissue samples and relative mRNA levels for β ₁-, β₂-, and β ₃-AR were measured using real-time quantitative polymerase chain reaction. Fresh s.c. and i.m. adipose tissue explants were incubated with isoproterenol hydrochloride (ISO; β-AR pan-agonist), dobutamine hydrochloride (DOB; specific β ₁-AA), salbutamol sulfate (SAL; specific β ₂-AA), ractopamine hydrochloride (RAC), zilpaterol hydrochloride (ZIL), BRL-37344 (specific β ₃-agonist), or LUB for 30 min following preincubation with theophylline (inhibitor of phosphodiesterase). Relative mRNA amounts for β ₁-, β ₂-, and β ₃-AR were greater (P < 0.05) in s.c. than in i.m. adipose tissue. The most abundant β-AR mRNA in both adipose tissues was the β ₂-AR (P < 0.05), with the β ₁- and β ₃-AR subtypes being minimally expressed in i.m. adipose tissue. ISO, RH, and ZH stimulated the release of glycerol and nonesterified fatty acid (NEFA) from s.c. adipose tissue, but these β-AR ligands did not alter concentrations of these lipolytic markers in i.m. adipose tissue. LUB did not affect glycerol or NEFA concentrations in s.c. or i.m. adipose tissue, but attenuated (P < 0.05) the accumulation of cAMP mediated by the β ₁- and β ₂-AR ligands DOB and SAL in s.c. adipose tissue. Collectively, these data indicate that bovine i.m. adipose tissue is less responsive than s.c. adipose tissue to β-adrenergic ligands, especially those that are agonists at the β ₁- and β₃-receptor subtypes. The minimal mRNA expression of the β ₁- and β ₃ subtypes in i.m. adipose tissue likely limits the response potential to agonists for these β-AR subtypes.

Characterization of insulin and IGF-1 receptor binding in equine liver and lamellar tissue: implications for endocrinopathic laminitis

Although it is well established that equine laminitis can be triggered by extreme hyperinsulinemia, the mechanism of insulin action is not known. High concentrations of insulin lead to separation of the weight-bearing apparatus from the hoof wall and are associated with an increased cycle of cell death and proliferation in the lamellae. Gene expression and immunohistochemistry studies have indicated that the lamellae are sparsely populated with insulin receptors, whereas IGF-1 receptors (IGF-1R) are abundant, suggesting that the action of insulin may be mediated by insulin binding to the IGF-1R. To investigate this possibility, cell membrane fragments containing IGF-1R were extracted from the livers of 6 horses and the lamellae of >50 horses euthanized for nonresearch purposes at an abattoir. Radioligand-binding studies using ¹²⁵I-IGF-1 and ¹²⁵I-insulin confirmed an abundance of high-affinity IGF-1R in the liver (K_D 0.11 nM, B_max 223 fmol/mg protein) and lamellae (K_D 0.16 nM, B_max 243 fmol/mg protein). However, the affinity of insulin for binding to the lamellar IGF-1R (K_i 934 nM) was >5,800 fold less than that of IGF-1, suggesting that insulin is unlikely to bind to equine IGF-1R at physiological concentrations. Although insulin receptors could be detected in the liver (K_D 0.48 nM, B_max 123 fmol/mg protein), they were barely detectable in lamellae (estimated B_max 14 fmol/mg protein). There was no evidence to support the presence of insulin/IGF-1 hybrid receptors in either tissue. These findings suggest that insulin does not act directly through IGF-1 receptors and that an alternative theory is required to explain the mechanism of insulin action in laminitis.

Evaluation of the efficacy of Grofactor, a beta-adrenergic agonist based on zilpaterol hydrochloride, using feedlot finishing bulls

Beta-adrenergic agonists (β-AA) have been shown to positively impact finishing performance and some carcass traits of feedlot cattle. Our objective was to evaluate the efficacy of a β-AA on the basis of zilpaterol hydrochloride (Grofactor, Laboratorios Virbac México, Guadalajara, Mexico) on growth and DMI, carcass characteristics, and meat quality of finishing bulls. Forty-five bulls (75% 25% ) initially weighing 448.7 ± 2.58 kg were blocked by BW and randomly assigned to 1 of 3 diets, using pens of 3 animals, in a randomized complete block design: 1) daily feeding without β-AA in the basal diet (Control), 2) daily feeding with 0.15 mg/kg BW of Grofactor added to the basal diet (ZHG), or 3) daily feeding with 0.15 mg/kg BW of Zilmax (MSD Salud Animal México, Mexico City, Mexico) added to the basal diet (ZHZ). The duration of the feeding period was 30 d with a subsequent 4-d withdrawal period. Compared with Control bulls, the group fed ZHG had a 12% better ( < 0.025) G:F ratio, and their final BW ( 0.094) and ADG ( 0.084) tended to be enhanced. Feedlot performance of ZHG and ZHZ bulls was similar, although the DMI was ∼4% lower ( 0.05) in ZHG bulls vs. the ZHZ and Control groups. The HCW ( 0.001) and dressing percentage ( 0.015) were higher by 20 kg and 3%, respectively, in ZHG bulls vs. Control bulls. The KPH fat was lower ( 0.007) in bulls fed ZHG than in nonsupplemented bulls, but other carcass characteristics were not different in the ZHG and ZHZ bulls, and noncarcass components were not affected by ZHG or ZHZ supplementation. At 48 h postmortem, ZHG bulls had lower ( 0.007) water holding capacity and trended toward ( 0.06) increased chroma and reduced pH ( 0.09) compared to Control bulls. However, compared to ZHZ bulls, ZHG bulls had higher ( 0.02) chroma and a trend ( 0.08) toward increased hue angle. At 14 d postmortem, meat quality variables did not differ between the 3 groups of bulls. Supplementation of ZH Grofactor improved feedlot performance and some carcass characteristics of finishing bulls without affecting meat quality. The effects of Grofactor on feedlot performance, carcass traits, and meat quality were similar to those of Zilmax.

Historical Overview of the Effect of β-Adrenergic Agonists on Beef Cattle Production

Postnatal muscle hypertrophy of beef cattle is the result of enhanced myofibrillar protein synthesis and reduced protein turnover. Skeletal muscle hypertrophy has been studied in cattle fed β-adrenergic agonists (β-AA), which are receptor-mediated enhancers of protein synthesis and inhibitors of protein degradation. Feeding β-AA to beef cattle increases longissimus muscle cross-sectional area 6% to 40% compared to non-treated cattle. The β-AA have been reported to improve live animal performance, including average daily gain, feed efficiency, hot carcass weight, and dressing percentage. Treatment with β-AA increased mRNA concentration of the β₂ or β₁-adrenergic receptor and myosin heavy chain IIX in bovine skeletal muscle tissue. This review will examine the effects of skeletal muscle and adipose development with β-AA, and will interpret how the use of β-AA affects performance, body composition, and growth in beef cattle.

Effects of ovariectomization and ractopamine hydrochloride inclusion on heifer feedlot performance, meat yield, and tenderness of select muscles

Forty-eight British cross heifers were used to examine the effects of ovariectomization (OVX v. intact; INT) and ractopamine-HCl inclusion (0 or 0.41 mg/kg of BW day(-1); NORAC or RAC) for the final 31 days on performance, meat yield, and tenderness of select muscles. Ractopamine supplemented heifers had heavier BW (P≤0.05) and greater dressing percent than NORAC. Dressing percent was also greater (P<0.01) for INT v. OVX. LM area tended to be larger for RAC (P=0.07) and was larger for INT (P=0.05). Neither ractopamine inclusion nor sex class affected (P>0.08) organ weights or percent intramuscular fat. Ractopamine inclusion increased (P≤0.05) subprimal weight for the shoulder clod and bottom round. However, ractopamine inclusion and sex class had minimal effects on subprimal yields or slice shear force. Heifers receiving ractopamine had increased BW, DP, carcass weight and select subprimal weights without impacting meat quality. Ovariectomization did not influence the affects of ractopamine supplementation.

Effects of sequential feeding of β-adrenergic agonists on cull cow performance, carcass characteristics, and mRNA relative abundance

The objectives of this study were to determine the effects of supplementation with a single β-adrenergic agonist (β-AA) or a sequence of β-AA on cow performance, carcass characteristics, and mRNA relative abundance of cull cows implanted and fed a concentrate diet. Sixty cull cows were implanted with Revalor-200 (200 mg of trenbolone acetate and 20 mg of estradiol) and assigned to 1 of 4 treatments (n = 15/treatment): CON = fed a concentrate diet only; RH = supplemented with ractopamine-HCl for the last 25 d before slaughter; ZH = supplemented with zilpaterol-HCl for 20 d before a 3-d withdrawal before slaughter; RH + ZH = supplemented with RH for 25 d, followed by ZH for 20 d before a 3-d withdrawal before slaughter. Ractopamine-HCl was supplemented at a dose of 200 mg·animal(-1)·d(-1), and ZH was supplemented at 8.33 mg/kg (100% DM basis) of feed. All cows were fed a concentrate diet for 74 d. Each treatment had 5 cows per pen and 3 replicate pens. Body weights were collected on d 1, 24, 51, and 72. Muscle biopsies from the LM were collected on d 24, 51, and at slaughter from a subsample of 3 cows per pen. Carcass traits were evaluated postslaughter. The 2 ZH treatments averaged 15.3 kg more BW gain, 0.20 kg greater ADG, and 7.8 cm(2) larger LM area than CON and RH treatments, and 21 kg more HCW than CON, but these differences were not significant (P > 0.10), likely due to a sample size of n = 15/treatment. The sequence of RH followed by ZH tended to optimize the combination of HCW, LM area, percent intramuscular fat, and lean color and maturity compared with the ZH treatment. Abundance of β(2)-adrenergic receptor (AR) mRNA was not altered in the RH + ZH treatment during RH supplementation from d 24 to 51 of feeding. However, the abundance of β(2)-AR mRNA increased (P < 0.05) the last 23 d of feeding for the RH treatment and tended (P = 0.10) to increase in ZH cows during ZH supplementation. For all cows, abundance of type IIa myosin heavy chain (MHC-IIa) mRNA decreased (P < 0.05) after 24 d of feeding. Abundance of MHC-IIx mRNA increased (P < 0.05) for ZH and RH + ZH treatments the last 23 d of feeding during ZH supplementation. Although few significant differences were observed in performance or carcass traits, mRNA quantification indicated that β-AA supplementation elicited a cellular response in cull cows. Implanting and feeding cull cows for 74 d, regardless of β-AA supplementation, added economic value by transitioning cows from a cull cow to what is referred to in industry as a white cow market in which cows have white fat resulting from grain feeding.

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.

Effects of duration of zilpaterol hydrochloride and days on the finishing diet on carcass cutability, composition, tenderness, and skeletal muscle gene expression in feedlot steers

Preselected carcasses (n = 112) from feedlot steers fed zilpaterol hydrochloride (ZH; 8.33 mg/kg, DM basis) in a serial slaughter experiment were evaluated to determine the effects of ZH upon carcass cutability, composition, and tenderness. A 4 x 4 factorial arrangement of treatments in a completely random design was used with days on ZH (0, 20, 30, and 40 d before slaughter with a 3-d withdrawal) and days on the finishing diet (DOF; 136, 157, 177, and 198 d). No relevant ZH duration x slaughter group interactions were detected (P > 0.05) for carcass cutability, composition, or tenderness data. Exposure to ZH increased the lean yield of 22 of the 33 subprimals evaluated with every subprimal within the round showing increased cutability (P < or = 0.04). Carcass fat was decreased, whereas carcass protein and moisture were increased due to ZH (P < 0.01). Lengthening the ZH feeding period did not result in additive gains in subprimal yield or chemical composition (P > 0.05). Warner-Bratzler shear force analysis of the LM indicated that ZH caused a toughening effect (P < 0.01) regardless of the length of the aging period (7, 14, or 21 d). Extending the ZH dose duration caused a linear increase in Warner-Bratzler shear force at 7 (P = 0.06) and 21 d (P < 0.01) of aging. Within 10 min postmortem, samples (n = 48) were collected from the semimembranosus muscle for RNA isolation from 4 randomly selected steers from each treatment within the 157, 177, and 198 d slaughter groups. Feeding ZH did not alter beta1- or beta2-adrenergic receptor (AR), calpastatin (CAL), IGF-I, or myosin heavy chain (MHC) isoform I mRNA abundance (P > 0.10). There was a ZH duration x DOF interaction (P < 0.01) for the expression of MHC-IIa and -IIx. Expression of MHC-IIa was decreased in every ZH treatment within the 177 and 198 DOF groups (P < 0.02). Expression of MHC-IIx was increased in the 20-d ZH group in the 157 DOF group (P = 0.03), and the 40-d ZH group in the 177 (P = 0.10) and 198 (P = 0.03) DOF groups. There was a tendency for a linear decrease in CAL mRNA abundance as ZH duration increased (P = 0.07), and there was a linear increase in beta2-AR (P = 0.03) and CAL (P < 0.01) mRNA abundance as DOF increased. Collectively, the data indicate that ZH may influence net protein turnover by decreasing MHC-IIa mRNA transcription and possibly increasing MHC-IIx. Furthermore, a ZH feeding duration of 20 d appeared to be adequate for capturing lean yield benefits while limiting tenderness losses.

Dietary zilpaterol hydrochloride. I. Feedlot performance and carcass traits of steers and heifers

Experiments were conducted at 3 US locations (CA, ID, and TX) to determine the effects of dietary zilpaterol hydrochloride (Zilmax, Intervet Inc., Millsboro, DE) and duration of zilpaterol feeding on performance and carcass merit of finishing steers and heifers. At each site, 160 steers and 160 heifers were stratified within sex by initial BW (study d -1) and assigned randomly within BW strata to 1 of 4 treatments in a randomized complete block design (4 blocks/treatment for each sex). The 4 treatments were arranged in a 2 (no zilpaterol vs. zilpaterol) x 2 (20 or 40 d duration of zilpaterol feeding) factorial arrangement of treatments. When included in the diet, zilpaterol was supplemented at 8.3 mg/kg of DM. Each pen consisted of 10 animals. Each animal was individually weighed unshrunk on d 1, 21 or 41, and 66 of the experiment. Following d 66, cattle were slaughtered and carcass data collected. Feeding zilpaterol increased (P<0.01) final BW of steers and heifers by 11.6 and 6.7 kg, respectively. In addition, feeding zilpaterol hydrochloride increased (P <or= 0.001) ADG 36 and 18%, and increased (P<0.001) G:F 28 and 21% for steers and heifers, respectively. For heifers, DMI was decreased (P<0.001) 6.2% when zilpaterol was fed, whereas in steers DMI tended (P=0.09) to be decreased 2%. For steers and heifers, feeding zilpaterol increased (P<0.001) HCW 16.4 and 12.1 kg, dressing percentage 1.5 percentage units for each sex, and LM area 8.23 and 6.37 cm(2), respectively. Twelfth-rib fat (P >or= 0.12) and KPH (P >or= 0.70) were not affected by feeding zilpaterol to steers or heifers. Feeding zilpaterol decreased (i.e., improved; P=0.02) calculated yield grade of steer and heifer carcasses. Marbling score (P=0.002) and quality grade (P=0.002) were decreased when zilpaterol hydrochloride was fed to steers, and the decrease in marbling score and quality grade tended to be greater when zilpaterol was fed for 40 compared with 20 d (zilpaterol x duration interaction, P=0.07). For heifers, marbling score tended (P=0.07) to be decreased and quality grade was decreased (P=0.05) when zilpaterol hydrochloride was fed. In general, it appears from these data that zilpaterol hydrochloride fed for 20 to 40 d at the end of the finishing period enhances growth performance and carcass muscle deposition for steers and heifers.

Anabolic agents for improving muscle regeneration and function after injury

1. In the present review, we describe how muscles can be injured by external factors, internal factors or during the performance of some actions during sports. In addition, we describe the injury to a muscle that occurs when its blood supply is interrupted, an occurrence common in clinical settings. An overview of muscle regeneration is presented, as well as a discussion of some of the potential complications that can compromise successful muscle repair and lead to impaired function and permanent disability. 2. Improving muscle regeneration is important for hastening muscle repair and restoring muscle function and the present review describes ways in which this can be achieved. We describe recent advances in tissue engineering that offer considerable promise for treating muscle damage, but highlight the fact that these techniques require rigorous evaluation before they can become mainstream clinical treatments. 3. Growth-promoting agents are purported to increase the size of existing and newly regenerating muscle fibres and, therefore, could be used to improve muscle function if administered at appropriate times during the repair process. The present review provides an update on the efficacy of some growth-promoting agents, including anabolic steroids, insulin-like growth factor-I (IGF-I) and beta(2)-adrenoceptor agonists, to improve muscle function after injury. Although these approaches have clinical merit, a better understanding of the androgenic, IGF-I and b-adrenoceptor signalling pathways in skeletal muscle is important if we are to devise safe and effective therapies to enhance muscle regeneration and function after injury.

Characterization of porcine beta1- and beta2-adrenergic receptors in heart, skeletal muscle, and adipose tissue, and the identification of an atypical beta-adrenergic binding site

The objective of this study was to characterize porcine beta1- and beta2-adrenergic receptors (beta1-AR and beta2-AR) in heart, skeletal muscle, and adipose tissue by measuring the binding of a radioligand to cell membrane fragments. In skeletal muscle (LM), [3H]CGP12177 labeled a homogeneous population of beta2-AR as evidenced by the rank order of affinity of catecholamines [(-)isoproterenol > (-)epinephrine > (-)norepinephrine], a high affinity of the binding site for the beta2-AR-agonist clenbuterol (equilibrium dissociation constant, Kd = 16 nM), and a low affinity of the binding site for the beta1-AR-antagonist CGP20712A (Kd = 21 microM). The affinity of ICI118551, a ligand selective for beta2-AR in other species, was uncharacteristically low in porcine LM (Kd = 441 nM), but was consistent with a value reported for the cloned porcine beta2-AR. In heart ventricle, ligand binding revealed a predominant population of beta1-AR, judged by the rank order of affinity of catecholamines [(-)isoproterenol > (-)epinephrine > or = (-)norepinephrine] and high-affinity binding to CGP20712A (Kd = 40 nM). The Kd for ICI118551 (731 nM) was close to that observed at beta2-AR in LM, confirming that ICI118551 is not subtype-selective in the pig. Displacement studies using (-)propranolol, clenbuterol, and (-)isoproterenol revealed a second high-affinity binding site in the heart that was not a beta2-AR and could not be eliminated by guanosine 5'-triphosphate or guanylyli-midodiphosphate. In adipose tissue, an equal number of beta1- and beta2-AR was identified through the binding of clenbuterol and CGP20712A, whereas ICI118551 could not discriminate between these sites. In further experiments, we used 10 microM CGP20712A to eliminate beta1-AR binding and allow accurate Kd values to be determined at beta2-AR for nonselective ligands. Under these conditions, another binding site was observed that had a high affinity for (-)propranolol (Kd = 20 pM), which is inconsistent with beta3- or beta4-AR binding reported elsewhere. Our results indicate that porcine adipose tissue contains beta1-AR, beta2-AR, and an atypical binding site in the proportions 50, 34, and 16%, respectively, of the total binding sites labeled by [3H]CGP12177.

beta-Adrenoceptor signaling in regenerating skeletal muscle after beta-agonist administration

Stimulating the beta-adrenoceptor (beta-AR) signaling pathway can enhance the functional repair of skeletal muscle after injury, but long-term use of beta-AR agonists causes beta-AR downregulation, which may limit their therapeutic effectiveness. The aim was to examine beta-AR signaling during early regeneration in rat fast-twitch [extensor digitorum longus (EDL)] and slow-twitch (soleus) muscles after bupivacaine injury and test the hypothesis that, during regeneration, beta-agonist administration does not cause beta-AR desensitization. Rats received either the beta-AR agonist fenoterol (1.4 mgxkg(-1)xday(-1) ip) or saline for 7 days postinjury. Fenoterol reduced beta-AR density in regenerating soleus muscles by 42%. Regenerating EDL muscles showed a threefold increase in beta-AR density, and, again, these values were 43% lower with fenoterol treatment. An amplified adenylate cyclase (AC) response to isoproterenol was observed in cell membrane fragments from EDL and soleus muscles 7 days postinjury. Fenoterol attenuated this increase in regenerating EDL muscles but not soleus muscles. beta-AR signaling mechanisms were assessed using AC stimulants (NaF, forskolin, and Mn(2+)). Although beta-agonist treatment reduces beta-AR density in regenerating muscles, these muscles can produce large cAMP responses relative to healthy (uninjured) muscles. Desensitization of beta-AR signaling in regenerating muscles is prevented by altered rates of beta-AR synthesis and/or degradation, changes in G protein populations and coupling efficiency, and altered AC activity. These mechanisms have important therapeutic implications for modulating beta-AR signaling to enhance muscle repair after injury.

Response to ractopamine-hydrogen chloride is similar in yearling steers across days on feed

Yearling steers (n = 2,552; 314 kg of initial BW) were used to evaluate the effects of ractopamine-HCl (RAC) and days on feed on performance, carcass characteristics, and skeletal muscle gene expression in finishing steers. Treatment groups included serial slaughter dates of 150, 171, or 192 d on feed. Within each slaughter date, steers either received RAC (200 mg/steer) daily for the final 28 d or were not fed RAC. All steers were initially implanted with Revalor-IS and were reimplanted with Revalor-S after 75 d on feed. At slaughter, muscle samples from the semimembranosus were collected for mRNA analysis of the beta-adrenergic receptors (beta-AR). Ractopamine administration increased (P < 0.05) ADG, G:F, and HCW and increased (P = 0.08) LM area. Ractopamine did not affect the dressing percentage, USDA yield grade, or quality grade (P > 0.3). There was no change in overall feed intake across the entire feeding period; however, feed intake was increased during the 28-d period during which the steers were fed RAC (P < or = 0.05). Greater days on feed decreased (P < 0.05) ADG, G:F, DMI, and the number of yield grade 1 and 2 carcasses. Also, greater days on feed increased (P < 0.05) HCW, dressing percentage, and the number of prime and choice carcasses, as well as the number of yield grade 4 and 5 carcasses. Increasing days on feed decreased (P < 0.05) the abundance of beta(1)-AR and beta(3)-AR mRNA and increased (P < 0.05) the abundance of beta(2)-AR mRNA in skeletal muscle samples obtained at slaughter. Ractopamine had no effect (P > 0.10) on the abundance of beta(1)-AR or beta(3)-AR mRNA, but tended (P = 0.09) to increase beta(2)-AR mRNA. Additional time-course studies with primary muscle cell cultures revealed that advancing time in culture increased (P < 0.001) beta(2)-AR mRNA but had no effect (P > 0.10) on beta(1)-AR or beta(3)-AR mRNA. We conclude that days on feed and RAC are affecting beta-AR mRNA levels, which could, in turn, impact the biological response to RAC feeding in yearling steers.

Early effects of spinal cord transection on skeletal muscle properties

Modulation of β-adrenergic receptor (β-AR) activity affects muscle mass and could have a role in the reduction of muscle mass observed following spinal cord transection (Tx). The aims of this study were to examine the early acute effects of Tx on muscle mass, total and myofibrillar protein concentrations, cytochrome c oxidase activity, and β-AR density of skeletal muscle, to ascertain if any change in muscle properties could be related to β-AR signalling events. Female Sprague–Dawley rats (n = 33; ~255 g) were randomly assigned to 4 experimental groups: control 4 d, control 8 d, Tx 4 d, and Tx 8 d. A complete Tx was performed surgically at the T10 cord level. Compared with controls, muscle mass and muscle – body mass ratios decreased significantly following Tx, with no significant change observed in total and myofibrillar protein concentrations. Spinal cord Tx also resulted in a significant decrease in plantaris cytochrome c oxidase activity by 24% at Tx 4 d and 28% at Tx 8 d (p < 0.05). β-AR density of the lateral gastrocnemius was unchanged; however, the β-AR density of the forelimb triceps brachii m. was found to increase after Tx. Our results suggest that changes in muscle mass and cytochrome c oxidase activity rapidly occur after Tx and do not appear to be related to changes in β-AR density.

Beta 2-agonist administration reverses muscle wasting and improves muscle function in aged rats

The beta2-adrenoceptor agonist (beta2-agonist) fenoterol has potent anabolic effects on rat skeletal muscle. We conducted an extensive dose-response study to determine the most efficacious dose of fenoterol for increasing skeletal muscle mass in adult rats and used this dose in testing the hypothesis that fenoterol may have therapeutic potential for ameliorating age-related muscle wasting and weakness. We used adult (16-month-old) rats that had completed their growth and development, and old (28-month-old) rats that exhibited characteristic muscle wasting and weakness, and treated them daily with either fenoterol (1.4 mg kg(-1), i.p), or saline vehicle, for 4 weeks. Following treatment, functional characteristics of fast-twitch extensor digitorum longus (EDL) and predominantly slow-twitch soleus muscles of the hindlimb were assessed in vitro. Untreated old rats exhibited a loss of skeletal muscle mass and a decrease in force-producing capacity, in both fast and slow muscles, compared with adult rats (P < 0.05). However, there was no age-associated decrease in skeletal muscle beta-adrenoceptor density, nor was the muscle response to chronic beta-agonist stimulation reduced with age. Thus, muscle mass and force-producing capacity of EDL and soleus muscles from old rats treated with fenoterol was equivalent to, or greater than, untreated adult rats. The increase in mass and strength was attributed to a non-selective increase in the cross-sectional area of all muscle fibre types, in both the EDL and soleus. Fenoterol treatment caused a small increase in fatiguability due to a decrease in oxidative metabolism in both EDL and soleus muscles, with some cardiac hypertrophy. Further studies are needed to fully separate the desirable effects on skeletal muscle and the undesirable effects on the heart. Nevertheless, our results demonstrate that fenoterol is a powerful anabolic agent that can restore muscle mass and strength in old rats, and provide preliminary evidence of therapeutic potential for age-related muscle wasting and weakness.

The rainbow trout skeletal muscle beta-adrenergic system: characterization and signaling

The presence and functionality of beta-adrenoceptors (beta-ARs) were examined in red (RM) and white muscle (WM) membranes isolated from the rainbow trout Oncorhynchus mykiss. Specific binding assays revealed the presence of a single class of binding sites with similar affinities in both muscle types (K(d) in nM: 0.14 +/- 0.03 and 0.18 +/- 0.03 for RM and WM, respectively) but with a significantly higher number of binding sites in RM compared with WM (B(max) in fmol/mg protein: 3.22 +/- 0.11 and 2.60 +/- 0.13, respectively). Selective and nonselective beta-adrenergic agonists (beta-AAs) and antagonists indicated an atypical beta-AR pharmacology. This result may represent a nonmammalian beta-AR classification or, more likely, the presence of more than one beta-AR subtype in trout muscles with similar affinities that could not be kinetically resolved. Adenylyl cyclase (ACase) assays showed a dose-dependent increase in cAMP production as concentrations of beta(2)-AAs increased in both muscle membranes with significantly higher basal cAMP production in RM compared with WM (cAMP production in pmol cAMP. mg protein(-1). 10 min(-1): 24.67 +/- 3.06 and 9.64 +/- 3.45, respectively). The agonist-induced increase in cAMP production was blocked by the beta-adrenergic antagonist propranolol, while the ACase activator forskolin increased cAMP production by 7- to 14-fold above basal and approximately 3-fold above all beta-AAs tested. This study demonstrated the presence of atypical beta(2)-ARs on RM and WM membranes of trout, suggesting that beta(2)-AAs may be a tool to enhance protein accretion through this signaling pathway.

Manipulation of growth in pigs through treatment of the neonate with clenbuterol and somatotropin

Neonatal pigs were treated with lipolytic agents to determine whether this would cause a long-term decrease in their ability to deposit fat, with a consequent increase in muscle growth and feed efficiency. Groups of 25 female piglets were given clenbuterol (100 microg/kg BW), porcine somatotropin (pST; 100 microg/kg BW), pST plus clenbuterol, or saline injections from 3 d to 40 d of age. Five piglets from each group were then slaughtered to determine body composition. Clenbuterol and pST both increased ADG up to weaning when given separately (24%, P < 0.05; 20%, P < 0.1 respectively) but did not reduce fat deposition. In contrast, pigs given clenbuterol plus pST showed no increase in ADG and a 41% reduction in carcass fat (P < 0.05). Clenbuterol caused a marked decrease in beta2-adrenoceptor density in porcine adipose tissue (P < 0.001) and skeletal muscle (P < 0.01). This effect was attenuated by concurrent pST treatment, which helps to explain the synergistic effect of these drugs on fat deposition. Once the drugs were withdrawn at 40 d, the anabolic effect of pST gradually disappeared, so that the live weight of pST-treated and control pigs was identical at 168 d. Clenbuterol withdrawal caused the rapid loss of extra weight gained, plus an additional 4 to 5 kg live weight that was never recovered. During the 4-wk finishing period there was an increase in feed intake in pigs that had previously undergone treatment with pST (23%, P < 0.1), with no increase in ADG, and so feed efficiency was impaired (P < 0.05). Pigs that were treated with pST plus clenbuterol showed no marked increase in feed intake during this period. Carcasses from clenbuterol-treated pigs tended to be leaner at 168 d, but there was no long-term effect of pST or the combined treatment on carcass composition. Overall, the treatment of neonatal pigs with repartitioning agents was counter-productive, due to the withdrawal effects of the beta-adrenefgic agonist and the delayed long-term effect of pST on feed intake.

Skeletal muscle hypertrophy and anti-atrophy effects of clenbuterol are mediated by the beta2-adrenergic receptor

Analyses were performed to evaluate the roles of the beta1- and beta2-adrenergic receptors in the skeletal muscle hypertrophy and anti-atrophy response to the beta-adrenergic agonist, clenbuterol. Treatment of wild-type mice with clenbuterol resulted in statistically significant hypertrophy of the innervated tibialis anterior and medial gastrocnemius muscles and inhibition of denervation-induced atrophy of these muscles. Treatment of beta1-adrenergic receptor knockout mice with clenbuterol also resulted in statistically significant hypertrophy of the innervated tibialis anterior and medial gastrocnemius muscles and inhibition of denervation-induced atrophy of these muscles. In contrast, in beta2-adrenergic receptor knockout mice and in mice lacking both the beta1- and beta2-adrenergic receptors, clenbuterol treatment did not result in hypertrophy of the innervated tibialis anterior and medial gastrocnemius muscles, nor did it inhibit denervation-induced atrophy in these muscles. Together these data demonstrate that the beta2-adrenergic receptor is responsible for both the skeletal muscle hypertrophy and anti-atrophy effects of the beta-adrenergic agonist clenbuterol.

Biochemistry and physiology of anabolic hormones used for improvement of meat production

A number of hormones are involved in endocrine regulation of growth. In general, these hormones enhance body protein accretion and metabolise fat stores resulting in increased lean growth rates. Most practical importance was obtained by sex hormones (oestrogens and androgens), beta-agonists and growth hormone - whether legally or illegally. Efficiency of growth promotion ranges between 0% and +20% depending on the prerequisites such as species, breed, gender, age, reproductive status, body score or feeding of the animals. Oestrogens and androgens mediate their activity via intracellular receptors - directly in muscular tissue as well as indirectly via stimulation of growth hormone from the hypophysis and other growth factors from liver plus several further organs. In addition, mineral absorption in the gut is improved. The outstanding efficiency of trenbolone is based on its androgenic plus antiglucocorticoid activity. Melengestrolacetate is thought to act indirectly via stimulation of endogenous ovarian oestradiol in non-pregnant heifers. The necessary dosages and residue formations depend on the pharmacokinetic parameters of each substance and extrapolations between compounds are hardly possible. Growth hormone and beta2-agonists use independent pathways for growth promotion not related to steroid biochemistry.

Cyclic AMP accumulation in rat soleus muscle: stimulation by beta2- but not beta3-adrenoceptors

The beta-adrenoceptor subtypes involved in cyclic AMP accumulation in rat soleus muscle were studied using beta1- beta2- and beta3-adrenoceptor agonists and antagonists. Responses to (-)-isoprenaline were antagonised by (-)-propranolol (p KB = 8.32 at 0.1 microM) and by erythro-DL-1(7-methylindian-4-yloxy)-3-isopropylaminobuta n-2-ol (+/-)-ICI 118551) (pKB = 9.38 at 10 nM and 9.65 at 100 nM) but not by 2-hydroxy-5(2-((2-hydroxy-3-(4-((1-methyl-4-trifluoromethyl)1H-imidazole -2-yl)-phenoxy)propyl)amino)ethoxy)-benzamide monomethane sulfonate ((+/-)-CGP 20712A at 10 nM or 100 nM). The beta3-adrenoceptor agonist sodium-4-[-2[-2-hydroxy-2-(-3-chlorophenyl)ethylamino]propyl]phenoxya cetate (BRL 37344 at 10 pM or 10 microM) caused no significant change in basal cyclic AMP levels and had no effect on the level of cyclic AMP accumulation stimulated by (-)-isoprenaline, zinterol or forskolin. (-)-Isoprenaline pretreatment (400 microg kg(-1) h(-1), 14 days) abolished responses to (-)-isoprenaline (10 microM) and zinterol (1 microM) while BRL 37344 had no effect in either isoprenaline or vehicle-treated groups. These results show that beta3-adrenoceptor agonists do not stimulate cyclic AMP accumulation in rat soleus muscle and that (-)-isoprenaline induced increases in cyclic AMP levels are mediated predominantly by beta2-adrenoceptors. This suggests that the previously reported increase in glucose uptake by beta3-adrenoceptor agonists in skeletal muscle does not involve direct stimulation of adenylate cyclase.

Characterization of a melatonin binding site in Siberian hamster brown adipose tissue

Melatonin has been shown, in various rodent species, to mediate photoperiodic effects on body weight and, consequently, fat mass. Pharmacological investigations indicated that the brown adipose tissue of Siberian hamsters possesses a melatonin binding site with a dissociation constant of 570+/-300 pM and a density of 3.2+/-1.8 fmol/mg protein. This binding site can also be detected on mature brown adipocyte membranes. The rank order of potency of a variety of drugs to displace 2-[125I]iodomelatonin from binding sites on Siberian hamster brown adipose tissue was as follows: 2-iodomelatonin > melatonin = prazosin > GR135531 (5-methoxycarbonylamino-N-acetyltryptamine) > N-acetylserotonin > 6-chloromelatonin > S20304 (N-(2-(1-naphthyl)ethyl)cyclobutanecarboxamide) >> methoxamine, phenylephrine, serotonin. Mel(1a) mRNA was not detected by RT-PCR (reverse transcription-polymerase chain reaction) in brown adipose tissue. Melatonin had no effect on either basal or stimulated lipolysis. Moreover, melatonin did not modify intracellular cAMP accumulation or inositol phosphate content. Together, these results suggest that the melatonin binding site characterized in brown adipose tissue is clearly different from the Mel(1) cloned subtype and has some features different from those of the Mel2 subtype.

Biphasic effects of the beta-adrenoceptor agonist, BRL 37344, on glucose utilization in rat isolated skeletal muscle

1. The effects of the selective beta 3-adrenoceptor agonist, BRL 37344 (BRL) on glucose uptake and phosphorylation (i.e. glucose utilization; GU) and glycogen synthesis in rat isolated soleus and extensor digitorium longus (EDL) muscle preparations in vitro were investigated by use of 2-deoxy-[3H]-glucose (GU) and [U-14C]-glucose (glycogen synthesis). 2. Low concentrations of BRL (10(-11)-10(-9) M) significantly increased GU, with maximal increases of 30% in soleus and 24% in EDL at 10(-11) M. Neither the selective beta 1-adrenoceptor antagonist, atenolol (10(-8)-10(-6) M), nor the selective beta 2-adrenoceptor antagonist, ICI 118551 (10(-8)-10(-6) M) had any effect on the stimulation of GU induced by 10(-11) M BRL. 3. High concentrations of BRL (10(-6)-10(-5) M) caused significant inhibition (up to 30%) of GU in both soleus and EDL muscles. The inhibition of 10(-6) M BRL was blocked completely by 10(-6) and 10(-7) M ICI 118551 in soleus, and by 10(-6)-10(-8) M ICI 118551 in EDL; atenolol (10(-8)-10(6) M) had no effect. 4. Another selective beta 3-adrenoceptor agonist, CL 316,243, also caused a significant stimulation of muscle GU, with maximal increases of 43% at 10(-9) M in soleus and 45% at 10(-10) M in EDL. The stimulation of GU declined with further increases in the concentration of CL 316,243, but no inhibition of GU was seen, even at the highest concentration (10(-5) M) tested. 5. BRL at 10(-5) M inhibited completely insulin-stimulated glycogen synthesis in both soleus and EDL, but this inhibitory effect of BRL was abolished by 10(-6) M ICI 118551. BRL at 10(-11) M (with or without 10(-6) M ICI 118551) had no effect on insulin-stimulated glycogen synthesis. 6. It is concluded that: (i) low (< nM) concentrations of BRL stimulate GU via an atypical beta-adrenoceptor that is resistant to conventional beta 1-adrenoceptor and beta 2-adrenoceptor antagonists; (ii) the stimulation of GU is negated by the activation of beta 2-adrenoceptors that occurs at higher (> nM) concentrations of BRL; (iii) inhibition of GU via beta 2-adrenoceptor activation is associated with inhibition of glycogen synthesis, possibly due to activation of glycogenolysis; (iv) the opposing effects of beta 2-adrenoceptor and atypical beta-adrenoceptor activation on GU suggest that in skeletal muscle these adrenoceptors are linked to different post-receptor pathways.

Signal transduction pathway involved in beta 3-adrenoceptor-mediated relaxation in guinea pig taenia caecum

Experiments were carried out to examine the components of the intracellular second messenger system that is involved in beta 3-adrenoceptor (atypical beta-adrenoceptors)-mediated relaxation in the guinea pig taenia caecum. Propranolol and butoxamine caused competitive antagonism of the relaxant response to isoprenaline. However, propranolol or butoxamine did not significantly affect the relaxant responses to CGP 12177 (4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H- benzimidazol-2-one), a beta 3-adrenoceptor agonist. The concentration-response curves of the isoprenaline-induced increase in adenosine 3',5'-cyclic monophosphate (cyclic AMP) levels were shifted to the right in a parallel manner by propranolol and butoxamine. However, propranolol or butoxamine did not significantly affect the concentration-response curve for the CGP 12177-induced increase in cyclic AMP levels. MDL 12330 (cis-N-(2-phenylcyclopentyl)-azacyclotridec-1-en-2-amine) inhibited the isoprenaline- or CGP 12177-induced increase in cyclic AMP levels. These results suggest that the production of cyclic AMP contributes to the beta 3-adrenoceptor (or atypical beta-adrenoceptor)-mediated relaxation of the guinea pig taenia caecum.