Early-age heat exposure affects skeletal muscle satellite cell proliferation and differentiation in chicks

Meeting the meat: delineating the molecular machinery of muscle development

Muscle, studied mostly with respect to meat production, represents one of the largest protein reservoirs of the body. As gene expression profiling holds credibility to deal with the increasing demand of food from animal sources, excessive loss due to myopathies and other muscular dystrophies was found detrimental as it aggravates diseases that result in increased morbidity and mortality. Holding key point towards improving the developmental program of muscle in meat producing animals, elucidating the underlying mechanisms of the associated pathways in livestock animals is believed to open up new avenues towards enhancing the lean tissue deposition. To this end, identification of vital candidate genes having no known function in myogenesis, is believed to increase the current understanding of the physiological processes going on in the skeletal muscle tissue. Taking consequences of gene expression changes into account, knowledge of the pathways associated with their activation and as such up-regulation seems critical for the overall muscle homeostasis. Having important implications on livestock production, a thorough understanding of postnatal muscle development seems a timely step to fulfil the growing need of ever increasing populations of the world.

The effect of temperature on proliferation and differentiation of chicken skeletal muscle satellite cells isolated from different muscle types

Skeletal muscle satellite cells are a muscle stem cell population that mediate posthatch muscle growth and repair. Satellite cells respond differentially to environmental stimuli based upon their fiber-type of origin. The objective of this study was to determine how temperatures below and above the in vitro control of 38°C affected the proliferation and differentiation of satellite cells isolated from the chicken anaerobic pectoralis major (p. major) or mixed fiber biceps femoris (b.femoris) muscles. The satellite cells isolated from the p. major muscle were more sensitive to both cold and hot temperatures compared to the b.femoris satellite cells during both proliferation and differentiation. The expressions of myogenic regulatory transcription factors were also different between satellite cells from different fiber types. MyoD expression, which partially regulates proliferation, was generally expressed at higher levels in p. major satellite cells compared to the b.femoris satellite cells from 33 to 43°C during proliferation and differentiation. Similarly, myogenin expression, which is required for differentiation, was also expressed at higher levels in p. major satellite cells in response to both cold and hot temperatures during proliferation and differentiation than b. femoris satellite cells. These data demonstrate that satellite cells from the anaerobic p. major muscle are more sensitive than satellite cells from the aerobic b. femoris muscle to both hot and cold thermal stress during myogenic proliferation and differentiation.

Incubation temperature alters thermal preference and response to heat stress of broiler chickens along the rearing phase

The current study aimed to investigate whether embryonic temperature manipulation may alter thermal preference throughout the rearing phase of broiler chickens and how this manipulation may affect response to thermal challenge, metabolism, growth rate and feed intake rate. Eggs were exposed to a constant incubation temperature [machine temperatures: 36°C (Low), 37.5°C (Control), and 39°C (High); eggshell temperature of 37.4 ± 0.08°C, 37.8 ± 0.15°C, and 38.8 ± 0.33°C, respectively] from d 13 till hatching. Low treatment chickens showed lower plasma T3 and GH levels at d 1 of age and lower T3 level at d 42 of age compared to the Control treatment. Preferred ambient, rectal temperature, T4 level, growth rate, food intake rate, and response to thermal challenge were not altered in these chickens. On the other hand, High-treatment chickens exhibited high preferred ambient temperature and rectal temperature during the first 2 wk post-hatch, lower plasma T3 level at d 21 and 42 and a delayed increase in respiratory movement in response to thermal challenge compared to the Control treatment. However, chickens subjected to the Control and High treatments did not differ in T4 and GH level and performance. We conclude that exposure to high temperature during late embryonic development has long-lasting effects on the thermoregulatory system of broiler chickens by affecting the heat tolerance of these chickens. Moreover, the preferred ambient temperature of the chickens from heat-treated eggs correspond to those recommended for the strain under study, whereas for the cold-treated and control-chickens it was 1°C below, indicating that incubation temperature might have consequences on the ambient temperature chickens require during the rearing phase.

Temperature effect on proliferation and differentiation of satellite cells from turkeys with different growth rates

Poultry selected for growth have an inefficient thermoregulatory system and are more sensitive to temperature extremes. Satellite cells are precursors to skeletal muscle and mediate all posthatch muscle growth. Their physiological functions are affected by temperature. The objective of the current study was to determine how temperature affects satellite cells isolated from the pectoralis major (p. major) muscle (breast muscle) of turkeys selected for increased 16 wk body weight (F line) in comparison to a randombred control line (RBC2) from which the F line originated. Pectoralis major muscle satellite cells were thermally challenged by culturing between 33°C and 43°C to analyze the effects of cold and heat on proliferation and differentiation as compared to control temperature of 38°C. Expression levels of myogenic regulatory factors: myogenic differentiation factor 1 (MYOD1) and myogenin (MYOG) were quantified by quantitative polymerase chain reaction (qPCR). At all sampling times, proliferation increased at a linear rate across temperature in both the RBC2 and F lines. Differentiation also increased at a linear rate across temperature from 33 to 41°C at all sampling times in both the F and RBC2 lines. Satellite cells isolated from F line turkeys were more sensitive to both hot and cold temperatures as proliferation and differentiation increased to a greater extent across temperature (33 to 43°C) when compared with the RBC2 line. Expression of MYOD1 and MYOG increased as temperatures increased from 33 to 41°C at all sampling times in both the F and RBC2 lines. These results demonstrate that satellite cell function is sensitive to both cold and hot temperatures and p. major muscle satellite cells from F line turkeys are more sensitive to temperature extremes than RBC2 satellite cells.

Thermal manipulation during embryogenesis affects myoblast proliferation and skeletal muscle growth in meat-type chickens

Thermal manipulation (TM) of 39.5°C applied during mid-embryogenesis (embryonic d 7 to 16) has been proven to promote muscle development and enhance muscle growth and meat production in meat-type chickens. This study aimed to elucidate the cellular basis for this effect. Continuous TM or intermittent TM (for 12 h/d) increased myoblast proliferation manifested by higher (25 to 48%) myoblast number in the pectoral muscles during embryonic development but also during the first week posthatch. Proliferation ability of the pectoral-muscle-derived myoblasts in vitro was significantly higher in the TM treatments until embryonic d 15 (intermittent TM) or 13 (continuous TM) compared to that of controls, suggesting increased myogenic progeny reservoir in the muscle. However, the proliferation ability of myoblasts was lower in the TM treatments vs. control during the last days of incubation. This coincided with higher levels of myogenin expression in the muscle, indicating enhanced cell differentiation in the TM muscle. A similar pattern was observed posthatch: Myoblast proliferation was significantly higher in the TM chicks relative to controls during the peak of posthatch cell proliferation until d 6, followed by lower cell number 2 wk posthatch as myoblast number sharply decreases. Higher myogenin expression was observed in the TM chicks on d 6. This resulted in increased muscle growth, manifested by significantly higher relative weight of breast muscle in the embryo and posthatch. It can be concluded that temperature elevation during mid-term embryogenesis promotes myoblast proliferation, thus increasing myogenic progeny reservoir in the muscle, resulting in enhanced muscle growth in the embryo and posthatch.

Evaluation of capillary and myofiber density in the pectoralis major muscles of rapidly growing, high-yield broiler chickens during increased heat stress

Skeletal muscle development proceeds from early embryogenesis through marketing age in broiler chickens. While myofiber formation is essentially complete at hatching, myofiber hypertrophy can increase after hatch by assimilation of satellite cell nuclei into myofibers. As the diameter of the myofibers increases, capillary density peripheral to the myofiber is marginalized, limiting oxygen supply and subsequent diffusion into the myofiber, inducing microischemia. The superficial and deep pectoralis muscles constitute 25% of the total body weight in a market-age bird; thus compromise of those muscle groups can have profound economic impact on broiler production. We hypothesized that marginal capillary support relative to the hypertrophic myofibers increases the incidence of microischemia, especially in contemporary high-yield broilers under stressing conditions such as high environmental temperatures. We evaluated the following parameters in four different broiler strains at 39 and 53 days of age when reared under thermoneutral (20 to 25 C) versus hot (30 to 35 C) environmental conditions: capillary density, myofiber density and diameter, and degree of myodegeneration. Our data demonstrate that myofiber diameter significantly increased with age (P > or = 0.0001), while the absolute numbers of capillaries, blood vessels, and myofibers visible in five 400 x microscopic fields decreased (P > or = 0.0001). This is concomitant with marginalization of vascular support in rapidly growing myofibers. The myofiber diameter was significantly lower with hot environmental temperatures (P > or = 0.001); therefore, the absolute number of myofibers visible in five 400X microscopic fields was significantly higher. The incidence and subjective degree of myodegeneration characterized by loss of cross-striations, myocyte hyperrefractility, sarcoplasmic vacuolation, and nuclear pyknosis or loss also increased in hot conditions. Differences among strains were not observed.

DNA methylation and histone modification patterns during the late embryonic and early postnatal development of chickens

Early mammalian embryonic cells have been proven to be essential for embryonic development and the health of neonates. A series of epigenetic reprogramming events, including DNA methylation and histone modifications, occur during early embryonic development. However, epigenetic marks in late embryos and neonates are not well understood, especially in avian species. To investigate the epigenetic patterns of developing embryos and posthatched chicks, embryos at embryonic day 5 (E5), E8, E11, E14, E17, and E20 and newly hatched chicks on day of life 1 (D1), D7, D14, D21 were collected. The levels of global DNA methylation and histone H3 at lysine 9 residue (H3K9) modifications were measured in samples of liver, jejunum, and breast skeletal muscles by Western blotting and immunofluorescence staining. According to our data, decreased levels of proliferating cell nuclear antigen expression were found in the liver and a V-shaped pattern of proliferating cell nuclear antigen expression was found in the jejunum. The level of proliferating cell nuclear antigen in muscle was relatively stable. Caspase 3 expression gradually decreased over time in liver, was stable in the jejunum, and increased in muscle. Levels of DNA methylation and H3K9 acetylation decreased in liver over time, while the pattern was N-shaped in jejunal tissue and W-shaped in pectoral muscles, and these changes were accompanied by dynamic changes of DNA methyltransferases, histone acetyltransferases 1, and histone deacetylase 2. Moreover, dimethylation, trimethylation, and acetylation of H3K9 were expressed in a time- and tissue-dependent manner. After birth, epigenetic marks were relatively stable and found at lower levels. These results indicate that spatiotemporal specific epigenetic alterations could be critical for the late development of chick embryos and neonates.

Gestational heat stress alters postnatal offspring body composition indices and metabolic parameters in pigs

The study objectives were to test the hypothesis that heat stress (HS) during gestational development alters postnatal growth, body composition, and biological response to HS conditions in pigs. To investigate this, 14 first parity crossbred gilts were exposed to one of four environmental treatments (TNTN, TNHS, HSTN, or HSHS) during gestation. TNTN and HSHS dams were exposed to thermal neutral (TN, cyclical 18-22°C) or HS conditions (cyclical 28-34°C) during the entire gestation, respectively. Dams assigned to HSTN and TNHS treatments were heat-stressed for the first or second half of gestation, respectively. Postnatal offspring were exposed to one of two thermal environments for an acute (24 h) or chronic (five weeks) duration in either constant TN (21°C) or HS (35°C) environment. Exposure to chronic HS during their growth phase resulted in decreased longissimus dorsi cross-sectional area (LDA) in offspring from HSHS and HSTN treated dams whereas LDA was larger in offspring from dams in TNTN and TNHS conditions. Irrespective of HS during prepubertal postnatal growth, pigs from dams that experienced HS during the first half of gestation (HSHS and HSTN) had increased (13.9%) subcutaneous fat thickness compared to pigs from dams exposed to TN conditions during the first half of gestation. This metabolic repartitioning towards increased fat deposition in pigs from dams heat-stressed during the first half of gestation was accompanied by elevated blood insulin concentrations (33%; P = 0.01). Together, these results demonstrate HS during the first half of gestation altered metabolic and body composition parameters during future development and in biological responses to a subsequent HS challenge.

Thermal manipulation during embryogenesis has long-term effects on muscle and liver metabolism in fast-growing chickens

Fast-growing chickens have a limited ability to tolerate high temperatures. Thermal manipulation during embryogenesis (TM) has previously been shown to lower chicken body temperature (T_b) at hatching and to improve thermotolerance until market age, possibly resulting from changes in metabolic regulation. The aim of this study was to evaluate the long-term effects of TM (12 h/d, 39.5°C, 65% RH from d 7 to 16 of embryogenesis vs. 37.8°C, 56% RH continuously) and of a subsequent heat challenge (32°C for 5 h at 34 d) on the mRNA expression of metabolic genes and cell signaling in the Pectoralis major muscle and the liver. Gene expression was analyzed by RT-qPCR in 8 chickens per treatment, characterized by low T_b in the TM groups and high T_b in the control groups. Data were analyzed using the general linear model of SAS considering TM and heat challenge within TM as main effects. TM had significant long-term effects on thyroid hormone metabolism by decreasing the muscle mRNA expression of deiodinase DIO3. Under standard rearing conditions, the expression of several genes involved in the regulation of energy metabolism, such as transcription factor PGC-1α, was affected by TM in the muscle, whereas for other genes regulating mitochondrial function and muscle growth, TM seemed to mitigate the decrease induced by the heat challenge. TM increased DIO2 mRNA expression in the liver (only at 21°C) and reduced the citrate synthase activity involved in the Krebs cycle. The phosphorylation level of p38 Mitogen-activated-protein kinase regulating the cell stress response was higher in the muscle of TM groups compared to controls. In conclusion, markers of energy utilization and growth were either changed by TM in the Pectoralis major muscle and the liver by thermal manipulation during incubation as a possible long-term adaptation limiting energy metabolism, or mitigated during heat challenge.

Cyclic variations in incubation conditions induce adaptive responses to later heat exposure in chickens: a review

Selection programs have enabled broiler chickens to gain muscle mass without similar enlargement of the cardiovascular and respiratory systems that are essential for thermoregulatory efficiency. Meat-type chickens cope with high ambient temperature by reducing feed intake and growth during chronic and moderate heat exposure. In case of acute heat exposure, a dramatic increase in morbidity and mortality can occur. In order to alleviate heat stress in the long term, research has recently focused on early thermal manipulation. Aimed at stimulation of long-term thermotolerance, the thermal manipulation of embryos is a method based on fine tuning of incubation conditions, taking into account the level and duration of increases in temperature and relative humidity during a critical period of embryogenesis. The consequences of thermal manipulation on the performance and meat quality of broiler chickens have been explored to ensure the potential application of this strategy. The physiological basis of the method is the induction of epigenetic and metabolic mechanisms that control body temperature in the long term. Early thermal manipulation can enhance poultry resistance to environmental changes without much effect on growth performance. This review presents the main strategies of early heat exposure and the physiological concepts on which these methods were based. The cellular mechanisms potentially underlying the adaptive response are discussed as well as the potential interest of thermal manipulation of embryos for poultry production.

Exploring the molecular mechanism of acute heat stress exposure in broiler chickens using gene expression profiling

The process of heat regulation is complex and its exact molecular mechanism is not fully understood. In this study, to investigate the global gene regulation response to acute heat exposure, gene microarrays were exploited to analyze the effects of heat stress on three tissues (brain, liver, leg muscle) of the yellow broiler chicken (Gallus gallus). We detected 166 differentially expressed genes (DEGs) in the brain, 219 in the leg muscle and 317 in the liver. Six of these genes were differentially expressed in all three tissues and were validated by qRT-PCR, and included heat shock protein genes (HSPH1, HSP25), apoptosis-related genes (RB1CC1, BAG3), a cell proliferation and differentiation-related gene (ID1) and the hunger and energy metabolism related gene (PDK). All these genes might be important factors in chickens suffering from heat stress. We constructed gene co-expression networks using the DEGs of the brain, leg muscle and liver and two, four and two gene co-expression modules were identified in these tissues, respectively. Functional enrichment of these gene modules revealed that various functional clusters were related to the effects of heat stress, including those for cytoskeleton, extracellular space, ion binding and energy metabolism. We concluded that these genes and functional clusters might be important factors in chickens under acute heat stress. Further in-depth research on the newly discovered heat-related genes and functional clusters is required to fully understand their molecular functions in thermoregulation.

The effect of nutritional status and muscle fiber type on myogenic satellite cell fate and apoptosis

Satellite cells (SC) are multipotential stem cells that can be induced by nutrition to alter their cellular developmental fate, which may vary depending on their fiber type origin. The objective of the current study was to determine the effect of restricting protein synthesis on inducing adipogenic transdifferentiation and apoptosis of SC originating from fibers of the fast glycolytic pectoralis major (p. major) and fast oxidative and glycolytic biceps femoris (b. femoris) muscles of the chicken. The availability of the essential sulfur amino acids Met and Cys was restricted to regulate protein synthesis during SC proliferation and differentiation. The SC were cultured and treated with 1 of 6 Met/Cys concentrations: 60/192, 30/96 (control), 7.5/24, 3/9.6, 1/3.2, or 0/0 mg/L. Reductions in Met/Cys concentrations from the control level resulted in increased lipid staining and expression of the adipogenic marker genes peroxisome proliferator-activated receptor gamma and stearoyl-CoA desaturase during differentiation in the p. major SC. Although b. femoris SC had increased lipid staining at lower Met/Cys concentrations, there was no increase in expression of either adipogenic gene. For both muscle types, SC Met/Cys, concentration above the control increased the expression of peroxisome proliferator-activated receptor gamma and stearoyl-CoA desaturase during differentiation. As Met/Cys concentration was decreased during proliferation, a dose-dependent decline in all apoptotic cells occurred except for early apoptotic cells in the p. major, which had no treatment effect (P < 0.05). During differentiation, decreasing Met/Cys concentration caused an increase in early apoptotic cells in both fiber types and no effect on late apoptotic cells except for an increase in the p. major 7.5/24 mg/L of Met/Cys treatment. In general, the viability of the SC was unaffected by the Met/Cys concentration except during proliferation in the p. major 0/0 mg/L of Met/Cys treatment, which increased SC viability. These data demonstrate the effect of nutrition on SC transdifferentiation to an adipogenic lineage and apoptosis, and the effect of fiber type on this response in an in vitro context.

The effect of nutritional status and myogenic satellite cell age on turkey satellite cell proliferation, differentiation, and expression of myogenic transcriptional regulatory factors and heparan sulfate proteoglycans syndecan-4 and glypican-1

Posthatch satellite cell mitotic activity is a critical component of muscle development and growth. Satellite cells are myogenic stem cells that can be induced by nutrition to follow other cellular developmental pathways, and whose mitotic activity declines with age. The objective of the current study was to determine the effect of restricting protein synthesis on the proliferation and differentiation, expression of myogenic transcriptional regulatory factors myogenic determination factor 1, myogenin, and myogenic regulatory factor 4, and expression of the heparan sulfate proteoglycans syndecan-4 and glypican-1 in satellite cells isolated from 1-d-, 7-wk-, and 16-wk-old turkey pectoralis major muscle (1 d, 7 wk, and 16 wk cells, respectively) by using variable concentrations of Met and Cys. Four Met concentrations-30 (control), 7.5, 3, or 0 mg/L with 3.2 mg/L of Cys per 1 mg/L of Met-were used for culture of satellite cells to determine the effect of nutrition and age on satellite cell behavior during proliferation and differentiation. Proliferation was reduced by lower Met and Cys concentrations in all ages at 96 h of proliferation. Differentiation was increased in the 1 d Met-restricted cells, whereas the 7 wk cells treated with 3 mg/L of Met had decreased differentiation. Reduced Met and Cys levels from the control did not significantly affect the 16 wk cells at 72 h of differentiation. However, medium with no Met or Cys suppressed differentiation at all ages. The expression of myogenic determination factor 1, myogenin, myogenic regulatory factor 4, syndecan-4, and glypican-1 was differentially affected by age and Met or Cys treatment. These data demonstrate the age-specific manner in which turkey pectoralis major muscle satellite cells respond to nutritional availability and the importance of defining optimal nutrition to maximize satellite cell proliferation and differentiation for subsequent muscle mass accretion.

Developmental specificity in skeletal muscle of late-term avian embryos and its potential manipulation

Unlike the mammalian fetus, development of the avian embryo is independent of the maternal uterus and is potentially vulnerable to physiological and environmental stresses close to hatch. In contrast to the fetus of late gestation in mammals, skeletal muscle in avian embryos during final incubation shows differential developmental characteristics: 1) muscle mobilization (also called atrophy) is selectively enhanced in the type II fibers (pectoral muscle) but not in the type I fibers (biceps femoris and semimembranosus muscle), involving activation of ubiquitin-mediated protein degradation and suppression of S6K1-mediated protein translation; 2) the proliferative activity of satellite cells is decreased in the atrophied muscle of late-term embryos but enhanced at the day of hatch, probably preparing for the postnatal growth. The mobilization of muscle may represent an adaptive response of avian embryos to external (environmental) or internal (physiological) changes, considering there are developmental transitions both in hormones and requirements for glycolytic substrates from middle-term to late-term incubation. Although the exact mechanism triggering muscle fiber atrophy is still unknown, nutritional and endocrine changes may be of importance. The atrophied muscle fiber recovers as soon as feed and water are available to the hatchling. In ovo feeding of late-term embryos has been applied to improve the nutritional status and therein enhances muscle development. Similarly, in ovo exposure to higher temperature or green light during the critical period of muscle development are also demonstrated to be potential strategies to promote pre- and posthatch muscle growth.

The effect of nutritional status on myogenic satellite cell proliferation and differentiation

Early posthatch satellite cell (SC) mitotic activity is a critical component of muscle development and growth. Satellite cells are stem cells that can be induced by nutrition to follow other cellular developmental pathways. The objective of the current study was to determine the effect of restricting protein synthesis on the proliferation and differentiation of SC, using variable concentrations of Met and Cys to modulate protein synthesis. Broiler pectoralis major SC were cultured and treated with 1 of 6 different Met/Cys concentrations: 60/192, 30/96 (control), 7.5/24, 3/9.6, 1/3.2, or 0/0 mg/L. The effect of Met/Cys concentration on SC proliferation and differentiation was measured, and myonuclear accretion was measured by counting the number of nuclei per myotube during differentiation. The 30/96 mg/L Met/Cys treatment resulted in the highest rate of proliferation compared with all other treatments by 72 h of proliferation (P < 0.05). Differentiation was measured with Met/Cys treatments only during proliferation and the cultures receiving normal differentiation medium (R/N), normal proliferation medium and differentiation medium with variable Met/Cys (N/R), or both proliferation and differentiation receiving variable Met/Cys treatments (R/R). Differentiation responded in a dose-dependent manner to Met/Cys concentration under all 3 of these treatment regimens, with a degree of recovery in the R/N regimen cells following reinstatement of the control medium. Reductions in both proliferation and differentiation were more pronounced as Met/Cys concentrations were further reduced, whereas increased differentiation was observed under the increased Met/Cys concentration treatment when applied during differentiation in the N/R and R/R regimens. The number of nuclei per myotube was significantly decreased in the severely Met/Cys restricted treatments (P < 0.05). These data demonstrate the sensitivity of pectoralis major SC to nutritional availability and the importance of optimal nutrition during both proliferation and differentiation for maximizing SC activity, which will affect subsequent muscle mass accretion.

Thermal manipulation of the embryo modifies the physiology and body composition of broiler chickens reared in floor pens without affecting breast meat processing quality

Selection in broiler chickens has increased muscle mass without similar development of the cardiovascular and respiratory systems, resulting in limited ability to sustain high ambient temperatures. The aim of this study was to determine the long-lasting effects of heat manipulation of the embryo on the physiology, body temperature (Tb), growth rate and meat processing quality of broiler chickens reared in floor pens. Broiler chicken eggs were incubated in control conditions (37.8°C, 56% relative humidity; RH) or exposed to thermal manipulation (TM; 12 h/d, 39.5°C, 65% RH) from d 7 to 16 of embryogenesis. This study was planned in a pedigree design to identify possible heritable characters for further selection of broiler chickens to improve thermotolerance. Thermal manipulation did not affect hatchability but resulted in lower Tb at hatching and until d 28 post-hatch, with associated changes in plasma thyroid hormone concentrations. At d 34, chickens were exposed to a moderate heat challenge (5 h, 32°C). Greater O2 saturation and reduced CO2 partial pressure were observed (P < 0.05) in the venous blood of TM than in that of control chickens, suggesting long-term respiratory adaptation. At slaughter age, TM chickens were 1.4% lighter and exhibited 8% less relative abdominal fat pad than controls. Breast muscle yield was enhanced by TM, especially in females, but without significant change in breast meat characteristics (pH, color, drip loss). Plasma glucose/insulin balance was affected (P < 0.05) by thermal treatments. The heat challenge increased the heterophil/lymphocyte ratio in controls (P < 0.05) but not in TM birds, possibly reflecting a lower stress status in TM chickens. Interestingly, broiler chickens had moderate heritability estimates for the plasma triiodothyronine/thyroxine concentration ratio at d 28 and comb temperature during the heat challenge on d 34 (h(2) > 0.17). In conclusion, TM of the embryo modified the physiology of broilers in the long term as a possible adaptation for heat tolerance, without affecting breast meat quality. This study highlights the value of 2 new heritable characters involved in thermoregulation for further broiler selection.

Expression of myogenic genes in chickens stimulated in ovo with light and temperature

Since chicken myogenesis is tightly controlled by myogenic regulatory factors (MRFs), the external stimuli (e.g. light or temperature) affecting the proliferation and differentiation of the muscle cells have a primary effect on the gene expression of MRFs. The aim of this study was to analyze the expression of some of MRF genes (MyoD1, myogenin and Myf5) in response to the stimulation of chicken embryos with green light (AL group) or increased temperature (38.5°C; AT group) on day 18 of embryo development (18ED) as well as on days 4 (4PHD) and 8 (8PHD) post hatch. To achieve this goal a quantitative reverse transcription polymerase chain reaction was used. The most prominent differences in gene expression were observed before hatching. Relative expression of MyoD1 on 18ED was higher (p<0.05) in AL and control (AC) groups in comparison to the AT group. Myogenin expression on 18ED was lower (p<0.05) in control chickens than in both treated groups. Light stimulation in ovo decreased (p<0.05) the Myf5 expression on 18ED in comparison to the control group. Green-light illumination applied during in ovo development had more pronounced effects on mRNA level of MRFs genes measured during both the pre- and post hatch development. The elevated temperature applied during embryonic development affected only the 18ED time point. This suggests that the effect of green-light illumination on chicken myogenesis was more prolonged than that of elevated temperature.

Early-age feed restriction affects viability and gene expression of satellite cells isolated from the gastrocnemius muscle of broiler chicks

Background

Muscle growth depends on the fusion of proliferate satellite cells to existing myofibers. We reported previously that 0–14 day intermittent feeding led to persistent retardation in myofiber hypertrophy. However, how satellite cells respond to such nutritional insult has not been adequately elucidated.

Results

One-day-old broiler chicks were allocated to control (Con, ad libitum feeding), intermittent feeding (IF, feed provided on alternate days) and re-feeding (RF, 2 days ad libitum feeding after 12 days of intermittent feeding) groups. Chickens were killed on Day 15 and satellite cells were isolated. When cultured, satellite cells from the IF group demonstrated significant retardation in proliferation and differentiation potential, while RF partly restored the proliferation rate and differentiation potential of the satellite cells. Significant up-regulation of insulin like growth factor I receptor (IGF-IR) (P<0.05) and thyroid hormone receptor α (TRα) (P<0.05), and down-regulation of growth hormone receptor (GHR) (P<0.01) and IGF-I (P<0.01) mRNA expression was observed in freshly isolated IF satellite cells when compared with Con cells. In RF cells, the mRNA expression of IGF-I was higher (P<0.05) and of TRα was lower (P<0.01) than in IF cells, suggesting that RF restored the mRNA expression of TRα and IGF-I, but not of GHR and IGF-IR. The Bax/Bcl-2 ratio tended to increase in the IF group, which was reversed in the RF group (P<0.05), indicating that RF reduced the pro-apoptotic influence of IF. Moreover, no significant effect of T₃ was detected on cell survival in IF cells compared with Con (P<0.001) or RF (P<0.05) cells.

Conclusions

These data suggest that early-age feed restriction inhibits the proliferation and differentiation of satellite cells, induces changes in mRNA expression of the GH/IGF-I and thyroid hormone receptors in satellite cells, as well as blunted sensitivity of satellite cells to T₃, and that RF partially reverses these effects. Thus, a moderate nutritional strategy for feed restriction should be chosen in early chick rearing systems.

Acute heat stress prior to downhill running may enhance skeletal muscle remodeling

Heat shock proteins (HSPs) are chaperones that are known to have important roles in facilitating protein synthesis, protein assembly and cellular protection. While HSPs are known to be induced by damaging exercise, little is known about how HSPs actually mediate skeletal muscle adaption to exercise. The purpose of this study was to determine the effects of a heat shock pretreatment and the ensuing increase in HSP expression on early remodeling and signaling (2 and 48 h) events of the soleus (Sol) muscle following a bout of downhill running. Male Wistar rats (10 weeks old) were randomly assigned to control, eccentric exercise (EE; downhill running) or heat shock + eccentric exercise (HS; 41°C for 20 min, 48 h prior to exercise) groups. Markers of muscle damage, muscle regeneration and intracellular signaling were assessed. The phosphorylation (p) of HSP25, Akt, p70s6k, ERK1/2 and JNK proteins was also performed. As expected, following exercise the EE group had increased creatine kinase (CK; 2 h) and mononuclear cell infiltration (48 h) compared to controls. The EE group had an increase in p-HSP25, but there was no change in HSP72 expression, total protein concentration, or neonatal MHC content. Additionally, the EE group had increased p-p70s6k, p-ERK1/2, and p-JNK (2 h) compared to controls; however no changes in p-Akt were seen. In contrast, the HS group had reduced CK (2 h) and mononuclear cell infiltration (48 h) compared to EE. Moreover, the HS group had increased HSP72 content (2 and 48 h), total protein concentration (48 h), neonatal MHC content (2 and 48 h), p-HSP25 and p-p70s6k (2 h). Lastly, the HS group had reduced p-Akt (48 h) and p-ERK1/2 (2 h). These data suggest that heat shock pretreatment and/or the ensuing HSP72 response may protect against muscle damage, and enhance increases in total protein and neonatal MHC content following exercise. These changes appear to be independent of Akt and MAPK signaling pathways.

The effects of early-age thermal manipulation and daily short-term fasting on performance and body temperatures in broiler exposed to heat stress

This study was conducted to investigate the effects of thermal manipulation at 5 days of age and short-term fasting during the warmest part of the day on responses to prolonged heat stress of broilers. A total of 240-day-old Ross 308 female broiler chicks were divided into three groups: control, thermal manipulation (chicks were exposed to 36 °C for 24 h at 5 days of age) and short-term fasting during the warmest part of the day (10.00-17.00 h). Prolonged heat stress was induced daily from 28 to 42 days by heating until the ambient temperature reached 32-35 °C between 10.00 and 17.00 h. Both thermal manipulation and short-term fasting resulted in a decrease in rectal temperatures and haematocrit values at 35 and 41 days of age. Thermal manipulation improved body weight, feed consumption and feed conversion. However, short-term fasting caused a reduction in body weight and a deterioration in feed conversion. Short-term fasting lowered the percentages of carcass, whereas thermal manipulation highered breast yield. Both thermal manipulation and short-term fasting decreased heart mass and abdominal fat.

In ovo administration of rhIGF-1 to duck eggs affects the expression of myogenic transcription factors and muscle mass during late embryo development

In ovo administration of IGF-1 to poultry eggs has effective roles on post hatching muscle development. However, the secondary muscle development stages at the late embryo development stage are important for muscle fiber formation and differentiation. To investigate the roles of in ovo administration of IGF-1 on duck secondary muscle development, we injected rhIGF-1 into duck eggs in hatching at day 12. After administration on days 18, 21, 24, and 27 in hatching (E18d, E21d, E24d, and E27d, respectively), muscle samples were isolated, and the muscle tissue weight, muscle fiber parameters, and myoblast proliferation rate in leg and breast muscle were analyzed. Additionally, the expression levels of the transcription factors MyoG and MRF4 were detected using qPCR. Results show that embryo body weight and muscle fiber parameters, including muscle fiber diameter (MFD) and the number of myofibers per unit area, are upregulated in IGF-1-treated groups. Moreover, the transcription factors MyoG and MRF4 are expressed at higher levels in the experimental groups compared with the control groups. These results suggest that in ovo administration of IGF-1 to poultry eggs can mediate the expression of MyoG and MRF4, induce myoblast proliferation, and finally influence muscle development during the secondary muscle development stages.

Effect of in ovo feeding and its interaction with timing of first feed on glycogen reserves, muscle growth, and body weight

Chicks are commonly fasted for the first 36 to 72 h posthatch because of the logistics of commercial production. Fasting for 48 to 72 h posthatch results in retarded BW, delayed intestinal development, and lower pectoral muscle weight. This study is focused on the first 36 h of fasting and its interaction with feeding before hatch. Four treatment groups, differing in time of first feed, 6 h [early feeding (EF)] or 36 h [standard feeding procedure (SP)] posthatch, with or without in ovo feeding (IOF) with dextrin and β-hydroxy-β-methylbutyrate-calcium salt in a saline solution, were examined for glycogen status in the liver and pectoral muscle, myogenic cell proliferation, and myofiber diameter in embryos and chickens on various days posthatch. In addition, chicken BW, ADG, pectoral muscle weight, and pectoral muscle percentage of BW until 35 d of age were recorded. Results showed that delaying the first feed for 36 h posthatch (SP group) led to an irreversibly reduced growth rate compared with the EF group. However, IOF affected the growth of chickens in the SP group, whereas the control embryos had depleted glycogen reserves in the liver; IOF-treated embryos had elevated hepatic glycogen contents on embryonic day (E) 19, E20, and the day of hatch. In addition, on d 2 posthatch, although hatchlings in the SP group showed the predicted low levels of glycogen in their livers, birds in the EF group exhibited more than 30-fold and 3-fold increases in liver and muscle glycogen, respectively. In ovo-fed birds in the SP group also exhibited higher glycogen reserves, BW, pectoral muscle weight, and BW gain than control birds in the SP group. In ovo feeding had an immediate effect on promoting myoblast proliferation on E19, whereas on d 3 posthatch, the effect was pronounced only in the EF groups. On d 5, although myoblast proliferation in all groups declined, it remained higher in both IOF groups. These effects were expressed on d 3 and 35 by myofiber diameter. Together, IOF had a long-term supportive effect on BW and posthatch muscle growth when first feed was delayed by 36 h.

Effects of heat stress on proliferation, protein turnover, and abundance of heat shock protein messenger ribonucleic acid in cultured porcine muscle satellite cells

It is well established that heat stress (HS) negatively affects growth rate in swine. Although reduced feed intake undoubtedly plays a significant role in this reduction, studies in laboratory animals and other nonswine species indicate muscle growth also is affected by HS-related alterations in muscle physiology. Evidence is now emerging that heat shock proteins (Hsp), produced in response to HS and other types of cellular stress, may play an important role in regulating the rate and efficiency of muscle growth. Because muscle satellite cells play a crucial role in postnatal muscle growth, the effects of HS on rates of satellite cell proliferation, protein synthesis, and protein degradation play an important role in determining the rate and extent of muscle growth. Consequently, in the current study we have examined the effects of mild HS (40.5°C for 48 h) on the rates of proliferation, protein synthesis, and protein degradation and on quantities of Hsp90, Hsp70, and Hsp25/27 mRNA and protein in cultured porcine muscle satellite cells (PSC). Mild HS of PSC cultures resulted in 2.5-, 1.4-, and 6.5-fold increases (P < 0.05) in the abundance of Hsp90, Hsp70, and Hsp25/27 mRNA, respectively, relative to control cultures. Abundance of Hsp 90, 70, and 25/27 proteins was also increased in HS PSC cultures compared with those in control cultures. Proliferation rates in HS PSC cultures were 35% less (P < 0.05) than those in control cultures. Protein synthesis rates in HS-fused PSC cultures were 85% greater (P < 0.05) than those in control cultures, and protein degradation rates in HS-fused PSC were 23% less (P < 0.05) than those in control cultures. In light of the crucial role satellite cells play in postnatal muscle growth, the HS-induced changes we have observed in rates of proliferation, protein turnover, and abundance of Hsp mRNA and Hsp protein in PSC cultures indicate that mild HS affects the physiology of PSC in ways that could affect muscle growth in swine.

Expression analysis of global gene response to chronic heat exposure in broiler chickens (Gallus gallus) reveals new reactive genes

The process of heat regulation is complex and the exact molecular mechanism is not fully understood. To investigate the global gene response to chronic heat exposure, a breast muscle cDNA library and a liver tissue cDNA library from Silkie fowl were constructed and analyzed in bioinformatics. A total of 8,935 nonredundant EST were identified from and used for gene expression analysis. Microarray assay revealed that in breast muscle of broiler chickens (Gallus gallus), 110 genes changed expression levels after 3 wk of cycling heat stress. Ubiquitin B (UBB); ubiquitin C (UBC); tumor necrosis factor receptor-associated factor 3-interacting Jun amino-terminal kinase activating modulator (TRAF3IP3); eukaryotic translation initiation factor 3, subunit 6 (EIF3S6); poly(A) binding protein, cytoplasmic 1 (PABPC1); and F-box only protein 11 (FBXO11) were the only genes that have been reported to be involved in heat regulation; the majority of the other genes were shown to be related for the first time. The finding of new heat-reactive genes [mitogen-activated protein kinase activating protein PM20/PM21; suppressors of cytokine signaling (SOCS) box-containing protein 2 (ASB2); ubiquitin-specific proteinase 45 (USP45); and TRK-fused gene (TFG)] suggests that the mitogen-activated protein kinase pathways as well as the ubiquitin-proteasome pathways and the nuclear factor κB pathways play important roles in heat regulation. This study provides new information on the regulation of heat stress, though the mechanism is far from being understood. Further in-depth research on the newly discovered heat-reactive genes is required to fully understand their molecular functions in thermoregulation.

Influence of temperature stimulation during the last 4 days of incubation on secondary sex ratio and later performance in male and female broiler chicks

1. In 6 incubation trials a total of 9883 eggs (Ross 308) were incubated from d 1 to 17 under normal incubation conditions (37.2-37.4 degrees C) and then sorted into three hatch incubators (control: 37.2-37.4 degrees C; chronic warm incubation: 38.2-38.4 degrees C, 24 h daily; short-term warm stimulation: 38.2-38.4 degrees C, 2 h daily) in incubation trials 1 and 2 or two hatch incubators (control and short-term warm stimulation) in trials 3-6. 2. The one-day-old chicks were selected by sex and chick quality was analysed in random samples using the Pasgar score. A total of 120 male and 120 female one-day-old chickens from each incubator were used for a 35-d fattening period. 3. Neither chronic nor short-term increase in incubation temperature had a negative effect on hatchability and chick quality. Short-term warm stimulation improved hatchability by more than 1.5% and was associated with a significantly higher proportion of hatched male chicks. 4. In the subsequent broiler growth trial, the mean daily weight gain of the short-term warm stimulated male broiler chicks was significant higher than for the control group, which results in a body weight increase of 2.9%. 5. Feed conversion (feed:gain ratio) of the short-term warm stimulated male and female broilers was significantly lower than in the males and females of the control and chronic warm incubated groups. 6. In conclusion, an incubation temperature profile which includes short-term temperature variation can be important in improving poultry performance (European patent pending since March 2008).

Thermal manipulations in late-term chick embryos have immediate and longer term effects on myoblast proliferation and skeletal muscle hypertrophy

We investigated the cellular and molecular bases for the promotion of muscle development and growth by temperature manipulations (TMs) during late-term chick embryogenesis. We show that incubation at 39.5 degrees C (increase of 1.7 degrees C from normal conditions) from embryonic days 16 to 18 (E16 to E18) for 3 or 6 h daily increased diameter of myofibers as of day 13 of age and enhanced absolute muscle growth relative to controls, until day 35 of age. TMs had immediate (E17) and later (up to 2 wk posthatch) effects in elevating muscle cell proliferation relative to controls. This was indicated by higher DNA incorporation of thymidine and a higher number of cells expressing PCNA in intact muscle, accompanied by higher Pax7 levels, all reflecting a higher number of myogenic cells, and suggesting that the increased hypertrophy can be attributed to a higher reservoir of myogenic progeny cells produced in response to the TM. IGF-I levels were higher in the TM groups than in controls, implying a mechanism by which heat manipulations in chicks affect muscle development, with locally secreted IGF-I playing a major role. Whereas hypertrophy was similar in both TM groups, cell proliferation and Pax7 levels were more robust in the 6-h muscle, mainly posthatch, suggesting a differential effect of various TM periods on cell reservoir vs. hypertrophy and a high sensitivity of myoblasts to relatively small changes in heat duration with respect to these processes, which is manifested in the short and long term.

Tissue differentiation and bone regeneration in an osteotomized mandible: a computational analysis of the latency period

Mandibular symphyseal distraction osteogenesis is a common clinical procedure to modify the geometrical shape of the mandible for correcting problems of dental overcrowding and arch shrinkage. In spite of consolidated clinical use, questions remain concerning the optimal latency period and the influence of mastication loading on osteogenesis within the callus prior to the first distraction of the mandible. This work utilized a mechano-regulation model to assess bone regeneration within the callus of an osteotomized mandible. A 3D model of the mandible was reconstructed from CT scan data and meshed using poroelastic finite elements (FE). The stimulus regulating tissue differentiation within the callus was hypothesized to be a function of the strain and fluid flow computed by the FE model. This model was then used to analyse tissue differentiation during a 15-day latency period, defined as the time between the day of the osteotomy and the day when the first distraction is given to the device. The following predictions are made: (1) the mastication forces generated during the latency period support osteogenesis in certain regions of the callus, and that during the latency period the percentage of progenitor cells differentiating into osteoblasts increases; (2) reducing the mastication load by 70% during the latency period increases the number of progenitor cells differentiating into osteoblasts; (3) the stiffness of new tissue increases at a slower rate on the side of bone callus next to the occlusion of the mandibular ramus which could cause asymmetries in the bone tissue formation with respect to the middle sagittal plane. Although the model predicts that the mastication loading generates such asymmetries, their effects on the spatial distribution of callus mechanical properties are insignificant for typical latency periods used clinically. It is also predicted that a latency period of longer than a week will increase the risk of premature bone union across the callus.

Heat stress inhibits skeletal muscle hypertrophy

Heat shock proteins (Hsps) are molecular chaperones that aid in protein synthesis and trafficking and have been shown to protect cells/tissues from various protein damaging stressors. To determine the extent to which a single heat stress and the concurrent accumulation of Hsps influences the early events of skeletal muscle hypertrophy, Sprague-Dawley rats were heat stressed (42 degrees C, 15 minutes) 24 hours prior to overloading 1 plantaris muscle by surgical removal of the gastrocnemius muscle. The contralateral plantaris muscles served as controls. Heat-stressed and/or overloaded plantaris muscles were assessed for muscle mass, total muscle protein, muscle protein concentration, Type I myosin heavy chain (Type I MHC) content, as well as Hsp72 and Hsp25 content over the course of 7 days following removal of the gastrocnemius muscle. As expected, in non-heat-stressed animals, muscle mass, total muscle protein and MHC I content were significantly increased (P < 0.05) following overload. In addition, Hsp25 and Hsp72 increased significantly after 2 and 3 days of overload, respectively. A prior heat stress-elevated Hsp25 content to levels similar to those measured following overload alone, but heat stress-induced Hsp72 content was increased significantly greater than was elicited by overload alone. Moreover, overloaded muscles from animals that experienced a prior heat stress showed a lower muscle mass increase at 5 and 7 days; a reduced total muscle protein elevation at 3, 5, and 7 days; reduced protein concentration; and a diminished Type I MHC content accumulation at 3, 5, and 7 days relative to nonheat-stressed animals. These data suggest that a prior heat stress and/or the consequent accumulation of Hsps may inhibit increases in muscle mass, total muscle protein content, and Type I MHC in muscles undergoing hypertrophy.

In ovo exposure to monochromatic green light promotes skeletal muscle cell proliferation and affects myofiber growth in posthatch chicks

Our previous studies demonstrated that illumination of chicken embryos with monochromatic green light results in enhanced body and muscle weight at later posthatch stages. In the present study, we investigated the cellular and molecular basis of this phenomenon. First, we showed that on day 6 posthatch, myofibers were more uniform in the in ovo illuminated group than in the control group incubated in the dark, with respect to the number of myofibers displaying diameter values within the range of the mean value. Second, we tested the hypothesis that in ovo illumination causes an increase in the number of myoblasts; this in turn can promote posthatch muscle growth. Indeed, a significant increase in the number of skeletal muscle cells isolated from pectoralis muscle was observed in the in ovo illuminated group on days 1 and 3 posthatch relative to the control group. This increased cell number was accompanied by higher expression levels of Pax7 and myogenin proteins on posthatch days 1 and 3, respectively. A parallel analysis of proliferating cells in the intact muscle further demonstrated a significant increase in the number of cells positive for proliferating cell nuclear antigen in muscle from the in ovo illuminated group. Third, we demonstrated that the transition from fetal- to adult-type myoblasts, normally occurring in late stages of chicken embryogenesis, is initiated earlier in embryos subjected to in ovo green-light illumination. We suggest that the stimulatory effect of in ovo illumination on posthatch muscle growth is the result of enhanced proliferation and differentiation of adult myoblasts and myofiber synchronization.

Post-exercise leg and forearm flexor muscle cooling in humans attenuates endurance and resistance training effects on muscle performance and on circulatory adaptation

The influence of regular post-exercise cold application to exercised muscles trained by ergometer cycling (leg muscles) or handgrip exercise using a weight-loaded handgrip ergometer (forearm flexor muscles) was studied in human volunteers. Muscle loads were applied during exercise programs three to four times a week for 4-6 weeks. Besides measuring parameters characterizing muscle performance, femoral and brachial artery diameters were determined ultrasonographically. Training effects were identified by comparing pre- and post-training parameters in matched groups separately for the trained limbs cooled after exercise by cold-water immersion and the corresponding trained limbs kept at room temperature. Significant training effects were three times more frequent in the control than in the cold group, including increases in artery diameters in the control but not in the cold group. It is concluded that training-induced molecular and humoral adjustments, including muscle hyperthermia, are physiological, transient and essential for training effects (myofiber regeneration, muscle hypertrophy and improved blood supply). Cooling generally attenuates these temperature-dependent processes and, in particular, hyperthermia-induced HSP formation. This seems disadvantageous for training, in contrast to the beneficial combination of rest, ice, compression and elevation in the treatment of macroscopic musculo-tendinous damage.

Heat-Stress Enhances Proliferative Potential in Rat Soleus Muscle

The effects of heat-stress on proliferative potential in vivo were studied in rat skeletal muscle. Male Wistar rats (7-weeks-old) were divided into two groups: control (n=24) and heat-stressed (n=24). Rats in the experimental group were exposed to environmental heat-stress (41 degrees C for 60 min) in a heat chamber without anesthesia. The soleus muscles were dissected 1, 7, and 14 days after the heat exposure. The wet and dry weights of soleus muscle relative to body weight in the heat-stressed group were significantly higher than controls 7 days after the exposure (10.1% and 17.5%, respectively, p <0.05). The distribution of 5-bromo-2'-deoxyuridine and proliferating cell nuclear antigen-positive nuclei, that are the indicators for the cell proliferation, were increased by 2.2 and 5.1 times, respectively 1 day after heating (p <0.05). The expressions of heat shock protein 72 (58.0%) and phosphorylated p70S6 kinase (52.3%) were increased 1 day following heat exposure (p <0.05). These results suggest that heat-stress could promote the cell proliferation and induce muscular hypertrophy.

Effects of heat stress and mechanical stretch on protein expression in cultured skeletal muscle cells

Effects of heat stress, mechanical stretching or a combination of both on the expression of heat shock proteins (HSPs) and total protein level were studied in a culture system. Rat skeletal muscle cells (L6) were cultured on flexible-bottomed culture plates. They were subjected to one of the four following conditions: (1) 97 h incubation at 37 degrees C, (2) 1 h incubation at 41 degrees C followed by 96 h incubation at 37 degrees C, (3) 1 h incubation at 37 degrees C followed by 96 h cyclic stretching (18% of initial length, 2-s stretch and 4-s release) at 37 degrees C or (4) 1 h incubation at 41 degrees C followed by 96 h cyclic stretching at 37 degrees C. The expression of HSP72 and HSP90 and total protein was determined in the crude homogenates, supernatant and pellets. Cellular protein concentrations in the homogenates and pellets were increased by heat stress and/or mechanical stress (stretch). A cumulative effect of the combination of heating and stretch on the protein concentration in the homogenates and in the pellets was noted. The expressions of HSP72 and HSP90 in the pellets were also increased by heat stress and/or stretch. However, HSP90 in the supernatant did not change following heat stress and/or stretch. The regulation of HSP72 and HSP90 expression in skeletal muscle cells may be closely related to total protein, the abundance of which is also stimulated by mechanical and heat stresses. These observations suggest strongly that heating and passive stretch of muscle may be useful as a means of increasing muscle mass, not only in athletes but also in patients during rehabilitation.

Early age thermal conditioning immediately reduces body temperature of broiler chicks in a tropical environment

Early age thermal conditioning (TC) durably improves resistance of broilers to heat stress and reduces body temperature (Tb). Three experiments on broiler chicks were conducted to evaluate the effects of TC at 5 d of age on Tb variation measured by thermometer between 4 and 7 d of age, under a tropical environment. Because manipulation of chickens to measure Tb with a thermometer may increase Tb, a preliminary experiment on 13 3-to-4-wk-old male broilers compared Tb measured by telemetry to Tb measured in the terminal colon during three successive periods at 22, 33, and 22 degrees C. During heat exposure, Tb rapidly increased by 0.9 degrees C and plateaued over 24 h. During the last period, seven of the broilers rapidly reduced Tb to a plateau lower than the initial Tb, although six broilers exhibited more variable Tb. Measurement by thermometer underestimated on average core Tb by 0.28 degrees C at 22 degrees C and by 0.57 degrees C at 33 degrees C, whereas Tb recorded by telemetry was not affected by manipulation of the chickens. TC reduced Tb 24 h later in the three experiments. Compared to unexposed control chicks (N), 12 h of TC at 40 degrees C did not significantly reduce Tb at 7 d of age, although 24 h did. TC at 38 and 40 degrees C over 24 h significantly reduced Tb variation from 4 to 7 d of age compared to N chicks, whereas 36 degrees C did not. Withdrawing feed from the chicks for 2 h prior to measurement did not significantly reduce Tb at 4 and 7 d of age, but Tb reduction due to TC was greater in fed chicks (0.28 degrees C) than in chicks without feed (0.05 degrees C). Early age thermal conditioning at 38 to 40 degrees C at 5 d of age for 24 h reduced body temperature of 7-d-old male broilers.

Freshwater environment affects growth rate and muscle fibre recruitment in seawater stages of Atlantic salmon (Salmo salar L.)

The influence of freshwater environment on muscle growth in seawater was investigated in an inbred population of farmed Atlantic salmon (Salmo salar L.). The offspring from a minimum of 64 families per group were incubated at either ambient temperature (ambient treatment) or in heated water (heated treatment). Growth was investigated using a mixed-effect statistical model with repeated measures, which included terms for treatment effect and random fish effects for individual growth rate (alpha) and the instantaneous growth rate per unit change in temperature (gamma). Prior to seawater transfer, fish were heavier in the heated (61.6+/-1.0 g; N=298) than in the ambient (34.1+/-0.4 g; N=206) treatments, reflecting their greater growth opportunity: 4872 degree-days and 4281 degree-days, respectively. However, the subsequent growth rate of the heated group was lower, such that treatments had a similar body mass (3.7-3.9 kg) after approximately 450 days in seawater. The total cross-sectional area of fast muscle and the number (FN) and size distribution of the fibres was determined in a subset of the fish. We tested the hypothesis that freshwater temperature regime affected the rate of recruitment and hypertrophy of muscle fibres. There were differences in FN between treatments and a significant age x treatment interaction but no significant cage effect (ANOVA). Cessation of fibre recruitment was identified by the absence of fibres of <10 micro m diameter. The maximum fibre number was 22.4% more in the ambient (9.3 x 10(5)+/-2.0 x 10(4) than in the heated (7.6 x 10(5)+/-1.5 x 10(4)) treatments (N=44 and 40 fish, respectively; P<0.001). For fish that had completed fibre recruitment, there was a significant correlation between FN and individual growth rate, explaining 35% of the total variation. The density of myogenic progenitor cells was quantified using an antibody to c-met and was approximately 2-fold higher in the ambient than in the heated group, equivalent to 2-3% of the total muscle nuclei. The number of myonuclei in isolated fibre segments showed a linear relationship with fibre diameter. On average, there were 20.6% more myonuclei in 200-microm-diameter fibres isolated from the ambient (3734 myonuclei cm(-1)) than from the heated (3097 myonuclei cm(-1)) treatments. The maximum fibre diameter was greater in heated than in ambient groups, whereas the age x treatment interaction was not significantly different (ANCOVA). There were also no consistent differences in the rate of hypertrophy of muscle fibres between treatments. It was concluded that freshwater temperature regime affected fibre number and the nuclear content of fast muscle in seawater but not the rate of fibre hypertrophy. The mechanisms and life history consequences of developmental plasticity in fibre number are discussed.

IGF-I binding in primary culture of muscle cells of rainbow trout: changes during in vitro development

To characterize and study the variations of IGF-I binding during the development of trout muscle cells, in vitro experiments were conducted using myocyte cultures, and IGF-I binding assays were performed in three stages of cell development: mononuclear cells (day 1), small myotubes (day 4), and large myotubes (day 10). Binding experiments were done by incubating cells with IGF-I for 12 h at 4 degrees C. Specific IGF-I binding increased with the concentration of labeled IGF-I and reached a plateau at 32 pM. The displacement of cold human and trout IGF-I showed a very similar curve (EC(50) = 1.19 +/- 0.05 and 0.95 +/- 0.05 nM, respectively). IGF binding proteins did not interfere significantly because displacement of labeled IGF-I by either cold trout recombinant IGF-I or Des (1-3) IGF-I resulted in similar curves. Insulin did not displace labeled IGF-I even at very high concentrations (>1 microM), which indicates the specificity of IGF-I binding. The amount of receptor (R(0)) increased from 253 +/- 51 fmol/mg DNA on day 1 to 766 +/- 107 fmol/mg DNA on day 10. However, the affinity (K(d)) of IGF-I receptors did not change significantly during this development (from 1.29 +/- 0.19 to 0.79 +/- 0.13 nM). On the basis of our results, we conclude that rainbow trout muscle cells in culture express specific IGF-I receptors, which increase their number with development from mononuclear cells to large myotubes.