EFFECTS OF DIETARY SUPPLEMENTAL L-CARNITINE AND ASCORBIC ACID ON PERFORMANCE,CARCASS COMPOSITION AND PLASMA L-CARNITINE CONCENTRATION OF BROILER CHICKS REARED UNDER DIFFERENT TEMPERATURE

The present study was initiated to determine whether dietary supplemental L-carnitine and ascorbic acid affect growth performance, carcass yield and composition, abdominal fat and plasma L-carnitine concentration of broiler chicks reared under normal and high temperature. During the experiment, two temperature regimes were employed in two experimental rooms, which were identical but different in environmental temperature. The regimes were thermoneutral (20-22°C for 24 h) or recycling hot (34-36°C for 8 h and 20-22°C for 16 h). One-day-old broiler chicks (ROSS) were used in the experiment. A 2 x 2 x 2 factorial arrangement was employed with two levels (0 and 50 mg/kg) of supplemental L-carnitine and two levels (0 or 500 mg/kg) of supplemental ascorbic acid in drinking water under thermoneutral or high temperature regimes. Body weight gain was affected by high temperature. However, body weight gain was significantly improved in animals receiving supplemental L-carnitine, ascorbic acid or L-carnitine + ascorbic acid compared to animals receiving unsupplemented diet under high temperature. On the other hand, supplemental L-carnitine or L-carnitine + ascorbic acid reduced body weight gain under thermoneutral condition. Supplemental ascorbic acid significantly improved feed conversion efficiency, the improvement was relatively greater under high temperature. The L-carnitine content in the plasma was higher in the groups receiving supplemental L-carnitine and ascorbic acid under high temperature, while broilers fed supplemental L-carnitine and ascorbic acid had a decreased level of plasma L-carnitine concentration under normal temperature. It is concluded that dietary supplemental L-carnitine or L-carnitine + ascorbic acid may have positive effects on body weight gain, carcass weight under high temperature conditions.     

Effect of thermal stress on physiological parameters, feed intake and plasma thyroid hormones concentration in Alentejana, Mertolenga, Frisian and Limousine cattle breeds

The aim of the present study was to assess the heat tolerance of animals of two Portuguese (Alentejana and Mertolenga) and two exotic (Frisian and Limousine) cattle breeds, through the monitoring of physiological acclimatization reactions in different thermal situations characterized by alternate periods of thermoneutrality and heat stress simulated in climatic chambers. In the experiment, six heifers of the Alentejana, Frisian and Mertolenga breeds and four heifers of the Limousine breed were used. The increase in chamber temperatures had different consequences on the animals of each breed. When submitted to heat stress, the Frisian animals developed high thermal polypnea (more than 105 breath movements per minute), which did not prevent an increase in the rectal temperature (from 38.7°C to 40.0°C). However, only a slight depression in food intake and in blood thyroid hormone concentrations was observed under thermal stressful conditions. Under the thermal stressful conditions, Limousine animals decreased food intake by 11.4% and blood triiodothyronine (T3) hormone concentration decreased to 76% of the level observed in thermoneutral conditions. Alentejana animals had similar reactions. The Mertolenga cattle exhibited the highest capacity for maintaining homeothermy: under heat stressful conditions, the mean thermal polypnea increased twofold, but mean rectal temperature did not increase. Mean food intake decreased by only 2% and mean T3 blood concentration was lowered to 85,6% of the concentration observed under thermoneutral conditions. These results lead to the conclusion that the Frisian animals had more difficulty in tolerating high temperatures, the Limousine and Alentejana ones had an intermediate difficulty, and the Mertolenga animals were by far the most heat tolerant.     

Neue Ergebnisse über die Arbeitshyperthermie des Menschen

Nielsen (1938) demonstrated that hypothermia during exercise is independent of room temperature within a range from 5° to 32° C. Subsequently, other investigators confirmed this observation. From these results,Asmussen &Nielsen (1947) concluded that a resetting of the thermoregulatory centre brought about by impulses reaching the brain from the working muscles or from the motor centres takes place. In order to find out whether impulses from motor centres really affect the thermoregulatory centres, we tried, by administration of curare in human beings, to increase the frequency of impulses necessary to bring about a certain amount of mechanical work. However, in 2 series of experiments at room temperatures of 23° and 34° C respectively and at a constant work output of 3 mkp/sec, no significant differences in body temperature (measured in the lower esophagus) before and during curare infusion could be detected. In the resting conditions, curare exerts no influence on body temperature. In normal subjects performing low work (3 mkp/sec) the body temperature decreases at a room temperature of 23° C but increases at a room temperature of 32° C. In conclusion, the hyperthermia during exercise cannot be due to a resetting of thermoregulatory centres. It works rather like a proportional closed loop control system. The relative independence of the hyperthermia from the environmental temperature suggests a considerable influence of the latter parameter via the thermoreceptors of the skin on the thermoregulation during work.     

Cutaneous vascular responses to isometric handgrip exercise during local heating and hyperthermia.

The dramatic increase in skin blood flow and sweating observed during heat stress is mediated by poorly understood sympathetic cholinergic mechanisms. One theory suggests that a single sympathetic cholinergic nerve mediates cutaneous active vasodilation (AVD) and sweating via cotransmission of separate neurotransmitters, because AVD and sweating track temporally and directionally when activated during passive whole body heat stress. It has also been suggested that these responses are regulated independently, because cutaneous vascular conductance (CVC) has been shown to decrease, whereas sweat rate increases, during combined hyperthermia and isometric handgrip exercise. We tested the hypothesis that CVC decreases during isometric handgrip exercise if skin blood flow is elevated using local heating to levels similar to that induced by pronounced hyperthermia but that this does not occur at lower levels of skin blood flow. Subjects performed isometric handgrip exercise as CVC was elevated at selected sites to varying levels by local heating (which is independent of AVD) in thermoneutral and hyperthermic conditions. During thermoneutral isometric handgrip exercise, CVC decreased at sites in which blood flow was significantly elevated before exercise (-6.5 +/- 1.8% of maximal CVC at 41 degrees C and -10.5 +/- 2.0% of maximal CVC at 43 degrees C; P < 0.05 vs. preexercise). During isometric handgrip exercise in the hyperthermic condition, an observed decrease in CVC was associated with the level of CVC before exercise. Taken together, these findings argue against withdrawal of AVD to explain the decrease in CVC observed during isometric handgrip exercise in hyperthermic conditions.     

Sex difference in cerebral blood flow velocity during exercise hyperthermia

IntroductionCerebral blood flow and thermal perception during physical exercise under hyperthermia conditions in females are poorly understood. Because sex differences exist for blood pressure control, resting middle cerebral artery velocity (MCA_Vmean), and pain, we tested the hypothesis that females would have greater reductions in MCAv_mean and increased thermal perceptual strain during exercise hyperthermia compared to males.MethodsTwenty-two healthy active males and females completed 60 min of matched exercise metabolic heat production in a 1) control cool (24.0 ± 0.0 °C; 14.4 ± 3.4% Rh) and 2) hot (42.3 ± 0.3 °C; 28.4 ± 5.2% Rh) conditions in random order, separated by at least 3 days while MCAv_mean, thermal comfort, and preference was obtained during the exercise.ResultsCompared to 36 °C mean body temperature (M^bt), as hyperthermia increased to 39 °C Mbt, females had a greater reduction in absolute (MCAv_mean), and relative change (%Δ MCAv_mean) and conductance (%Δ MCAv_mean conductance) in MCA_Vmean compared to males (Interaction: Temperature x Sex, P ≤ 0.002). During exercise in cool conditions, absolute and conductance MCAv_mean was maintained from rest through exercise; however, females had greater MCA_Vmean compared to males (Main effect: Sex, P < 0.0008). We also found disparities in females' perceptual thermal comfort and thermal preference. These differences may be associated with a greater reduction in partial pressure of end-tidal CO₂, and different cardiovascular and blood pressure control to exercise under hyperthermia.ConclusionsIn summary, females exercise cerebral blood flow velocity is reduced to a greater extent (25% vs 15%) and the initial reduction occurs at lower hyperthermia mean body temperatures (~38 °C vs ~39 °C) and are under greater thermal perceptual strain compared to males.     

Effects of heat exposure in the absence of hyperthermia on power output during repeated cycling sprints

The aim of this study was to investigate the effects of heat exposure in the absence of hyperthermia on power output during repeated cycling sprints. Seven males performed four 10-s cycling sprints interspersed by 30 s of active recovery on a cycle ergometer in hot-dry and thermoneutral environments. Changes in rectal temperature were similar under the two ambient conditions. The mean 2-s power output over the 1st–4th sprints was significantly lower under the hot-dry condition than under the thermoneutral condition. The amplitude of the electromyogram was lower under the hot-dry condition than under the thermoneutral condition during the early phase (0–3 s) of each cycling sprint. No significant difference was observed for blood lactate concentration between the two ambient conditions. Power output at the onset of a cycling sprint during repeated cycling sprints is decreased due to heat exposure in the absence of hyperthermia.     

THERMOREGULATORY EFFECT IN HUMANS OF SUPPRESSED ENDOGENOUS MELATONIN BY PRE-SLEEP BRIGHT-LIGHT EXPOSURE IN A COLD ENVIRONMENT

This study investigated the physiological function of suppressed melatonin through thermoregulation in a cold environment. Interactions between thermoregulation directly affected by exposure to a cold environment and indirectly affected by endogenous melatonin suppression by bright-light exposure were examined. Ten male subjects were exposed to two different illumination intensities (30 and 5000 lux) for 4.5?h, and two different ambient temperatures (15 and 27°C) for 2?h before sleep under dark and thermoneutral conditions. Salivary melatonin level was suppressed by bright light (p?<?0.001), although the ambient temperature condition had no significant effect on melatonin. During sleep, significant effects of pre-sleep exposure to a cold ambient temperature (p?<?0.001) and bright light (p?<?0.01) on rectal temperature (T_re) were observed. Pre-sleep, bright-light exposure led to an attenuated fall in T_re during sleep. Moreover, T_re dropped more precipitously after cold exposure than thermoneutral conditions (cold: ?0.54?±?0.07°C/h; thermoneutral: ?0.16?±?0.03°C/h; p?<?0.001). Pre-sleep, bright-light exposure delayed the nadir time of T_re under thermoneutral conditions (p?<?0.05), while cold exposure masked the circadian rhythm with a precipitous decrease in T_re. A significant correlation between the T_re nadir and melatonin level (r?=??0.774, p?<?0.05) indicated that inter-individual differences with higher melatonin levels lead to a reduction in T_re after cold exposure. These results suggest that suppressed endogenous melatonin inhibits the downregulation of the body temperature set-point during sleep. (Author correspondence: ishibasi@design.kyushu-u.ac.jp)     

Central administration of peptides alters thermoregulation in the rabbit

Sixteen peptides were injected intracerebroventricularly to test their effects on rectal temperature of rabbits in a thermoneutral environment. In initial tests 5 μg α-MSH, ACTH(1–24), oxytocin, vasopressin and glucagon altered body temperature while ACTH(1–10), cholecystokinin, contraceptive tetrapeptide, gastrin, insulin, interferon, leupeptin, LHRH, panhibin (somatostatin), and proctolin did not. Bombesin also altered body temperature but in no consistent direction. In further tests on the effective peptides 1.25–5.0 μg α-MSH and ACTH(1–24) produced dose-related decreases in rectal temperature as great as 1.0°C. The same doses of oxytocin and glucagon produced small, prolonged hyperthermias which did not exceed 0.4°C. Vasopressin caused rapid development of small increases in rectal temperature; temperature returned to normal in 2–3 hr. The results suggest that five of the peptides tested may have roles in central mediation of normal body temperature, hypothermia, hyperthermia and fever.     

Nocturnal loss of body reserves reveals high survival risk for subordinate great tits wintering at extremely low ambient temperatures

Winter acclimatization in birds is a complex of several strategies based on metabolic adjustment accompanied by long-term management of resources such as fattening. However, wintering birds often maintain fat reserves below their physiological capacity, suggesting a cost involved with excessive levels of reserves. We studied body reserves of roosting great tits in relation to their dominance status under two contrasting temperature regimes to see whether individuals are capable of optimizing their survival strategies under extreme environmental conditions. We predicted less pronounced loss of body mass and body condition and lower rates of overnight mortality in dominant great tits at both mild and extremely low ambient temperatures, when ambient temperature dropped down to ?43 °C. The results showed that dominant great tits consistently maintained lower reserve levels than subordinates regardless of ambient temperature. However, dominants responded to the rising risk of starvation under low temperatures by increasing their body reserves, whereas subdominant birds decreased reserve levels in harsh conditions. Yet, their losses of body mass and body reserves were always lower than in subordinate birds. None of the dominant great tits were found dead, while five young females and one adult female were found dead in nest boxes during cold spells when ambient temperatures dropped down to ?43 °C. The dead great tits lost up to 23.83 % of their evening body mass during cold nights while surviving individuals lost on average 12.78 % of their evening body mass. Our results show that fattening strategies of great tits reflect an adaptive role of winter fattening which is sensitive to changes in ambient temperatures and differs among individuals of different social ranks.     

Prediction of human core body temperature using non-invasive measurement methods

The measurement of core body temperature is an efficient method for monitoring heat stress amongst workers in hot conditions. However, invasive measurement of core body temperature (e.g. rectal, intestinal, oesophageal temperature) is impractical for such applications. Therefore, the aim of this study was to define relevant non-invasive measures to predict core body temperature under various conditions. We conducted two human subject studies with different experimental protocols, different environmental temperatures (10 °C, 30 °C) and different subjects. In both studies the same non-invasive measurement methods (skin temperature, skin heat flux, heart rate) were applied. A principle component analysis was conducted to extract independent factors, which were then used in a linear regression model. We identified six parameters (three skin temperatures, two skin heat fluxes and heart rate), which were included for the calculation of two factors. The predictive value of these factors for core body temperature was evaluated by a multiple regression analysis. The calculated root mean square deviation (rmsd) was in the range from 0.28 °C to 0.34 °C for all environmental conditions. These errors are similar to previous models using non-invasive measures to predict core body temperature. The results from this study illustrate that multiple physiological parameters (e.g. skin temperature and skin heat fluxes) are needed to predict core body temperature. In addition, the physiological measurements chosen in this study and the algorithm defined in this work are potentially applicable as real-time core body temperature monitoring to assess health risk in broad range of working conditions.     

Time course of photosynthetic temperature acclimation in Carex eleocharis Bailey

Abstract Photosynthetic temperature acclimation in Carex eleocharis has been demonstrated in a previous study in which warm grown (35/15°C) plants were shown to have photosynthetic temperature optima approximately 14°C higher than cool grown (20/15°C) plants (Monson, Littlejohn & Williams, 1983). The current study examined the time course of this acclimation by determining photo-synthetic temperature optima as a function of time, of cool grown plants moved to warm growing conditions. Leaves which had developed under cool conditions were capable of an upward adjustment of 6–8°C of their optimum photosynthetic temperature within a time span of 6–14 d. For greatest photosynthetic temperature acclimation it was necessary for leaves to form and develop entirely under warm conditions. These leaves exhibited a 14–15°C upward adjustment of their optimum temperature for photosynthesis within 20–31 d since moving plants from cool to warm growing conditions. Thus, the time course of this acclimation is of short enough duration to be significant during the growing season and presumably contributes toward the ability of this species to maintain active growth during the cool and warm portions of the growing season. It is also noted that the plant with its capacity to form new leaves, has a much wider acclimation capacity than any single leaf.     

Torpor is not the only option: seasonal variations of the thermoneutral zone in a small primate

The reddish-gray mouse lemur (Microcebus griseorufus) is one of only a few small mammals inhabiting the spiny forest of southwestern Madagascar. In this study we investigated the physiological adjustments which allow these small primates to persist under the challenging climatic conditions of their habitat. To this end we measured energy expenditure (metabolic rate) and body temperature of 24 naturally acclimatized mouse lemurs, kept in outdoor enclosures, during different seasons (summer, winter, and the transition period between the two seasons). Mouse lemurs displayed two main physiological strategies to compensate seasonal and diurnal fluctuations of ambient temperature. On the one hand, individuals entered hypometabolism with decreasing ambient temperature (T _a) during the transition period and winter, enabling them to save up to 21 % energy per day (92 % per hour) compared with the normal resting metabolic rate at comparable T _a. On the other hand, euthermic mouse lemurs also showed physiological adjustments to seasonality when resting: the lower critical temperature of the thermoneutral zone decreased from summer to winter by 7.5 °C, which allowed mouse lemurs to keep energy demands constant despite colder T _as during winter. In addition, the basal metabolic rate was substantially lowered prior to the winter period, which facilitated accumulation of fat reserves. The combination of physiological modifications during euthermia in addition to hypometabolism, which can be individually adjusted according to external parameters and respective body condition, is important as it allows M. griseorufus to cope with the environmental variability of an energetically challenging habitat.     

Brain temperature and limits on transcranial cooling in humans: quantitative modeling results

Selective brain cooling (SBC) of varying strengths has been demonstrated in a number of mammals and appears to play a role in systemic thermoregulation. Although primates lack obvious specialization for SBC, the possibility of brain cooling in humans has been debated for many years. This paper reports on the use of mathematical modeling to explore whether surface cooling can control effectively the temperature of the human cerebrum. The brain was modeled as a hemisphere with a volume of 1.33 1 and overlying layers of cerebrospinal fluid, skull, and scalp. Each component was assigned appropriate dimensions, physical properties and physiological characteristics that were determined from the literature. The effects of blood flow and of thermal conduction were modeled using the steady-state form of the bio-heat equation. Input parameters included core (arterial) temperature: normal (37°C) or hyperthermic (40°C), air temperature: warm (30°C) or hot (40°C), and sweat evaporation rate: 0, 0.25, or 0.50 l · m⁻² · h⁻¹. The resulting skin temperatures of the model ranged from 31.8°C to 40.2°C, values which are consistent with data obtained from the literature. Cerebral temperatures were generally insensitive to surface conditions (air temperature and evaporation rate), which affected only the most superficial level of the cerebrum (≤1.5 mm) The remaining parenchymal temperatures were 0.2–0.3°C above arterial temperatures, regardless of surface conditions. This held true even for the worst-case conditions combining core hyperthermia in a hot environment with zero evaporative cooling. Modeling showed that the low surface-to-volume ratio, low tissue conductivity, and high rate of cerebral perfusion combine to minimize the potential impact of surface cooling, whether by transcranial venous flow or by conduction through intervening layers to the skin or mucosal surfaces. The dense capillary network in the brain assures that its temperature closely follows arterial temperature and is controlled through systemic thermoregulation independent of head surface temperature. A review of the literature reveals several independent lines of evidence which support these findings and indicate the absence of functionally significant transcranial venous flow in either direction. Given the fact that humans sometimes work under conditions which produce face and scalp temperatures that are above core temperature, a transcranial thermal link would not necessarily protect the brain, but might instead increase its vulnerability to environmentally induced thermal injury. Accepted: 11 March 1998     

Predicting military working dog core temperature during exertional heat strain: Validation of a Canine Thermal Model

Military working dogs (MWDs) operate under a wide range of conditions, including hot environments. Predicting how long a MWD can safely work without overheating is important for both health and performance. A Canine Thermal Model (CTM) was developed to predict core temperature (Tc) of MWDs. The CTM calculates heat storage from the balance of heat production from metabolism and heat exchange with the environment. Inputs to the CTM are: meteorological conditions (ambient temperature, relative humidity, solar radiation and wind speed), physical characteristics of the dog (mass, length), and metabolic activity (MET level, estimated from accelerometer data). The CTM was validated against Tc measured in 23 MWDs during training sessions (11.6 ± 5.0 min (mean ± standard deviation), range 4–26 min) in October (24 °C, 52% RH), March (14 °C, 74% RH), or August (28 °C, 64% RH), and 24 kennel MWDs during a standard exercise walk (11.4 ± 3.3 min, range 5.6–18 min) in July (26 °C, 77% RH). The CTM was considered acceptable if predicted Tc was within ±0.5 °C of measured Tc at the end of exercise. Compared to Tc at the end of training sessions (39.8 ± 0.6 °C, range 38.4–41.1 °C) and exercise walks (40.0 ± 0.7 °C, range 38.9–41.4 °C), the CTM-predicted Tc was within ±0.5 °C for 71 of 84 cases (85%) and 19 of 24 cases (79%), respectively. The mean difference between CTM-predicted and measured final Tc during training was -0.04 ± 0.43 °C, with 80 of 84 cases (95%) within the range of ±2 SD (Bland Altman comparison). During exercise walks the mean difference was -0.15 °C ± 0.57, with 23 of 24 cases (96%) within ±2 SD. These results support the use of the CTM to predict Tc of MWDs for the types of physical activities described above.     

Physical exercise-induced changes in the core body temperature of mice depend more on ambient temperature than on exercise protocol or intensity

The mechanisms underlying physical exercise-induced hyperthermia may be species specific. Therefore, the present study aimed to investigate the effects of exercise intensity and ambient temperature on the core body temperature (T _core) of running mice, which provide an important experimental model for advancing the understanding of thermal physiology. We evaluated the influence of different protocols (constant- or incremental-speed exercises), treadmill speeds and ambient temperatures (T _a) on the magnitude of exercise-induced hyperthermia. To measure T _core, a telemetric sensor was implanted in the abdominal cavity of male adult Swiss mice under anesthesia. After recovering from the surgery, the animals were familiarized to running on a treadmill and then subjected to the different running protocols and speeds at two T _a: 24 °C or 34 °C. All of the experimental trials resulted in marked increases in T _core. As expected, the higher-temperature environment increased the magnitude of running-induced hyperthermia. For example, during incremental exercise at 34 °C, the maximal T _core achieved was increased by 1.2 °C relative to the value reached at 24 °C. However, at the same T _a, neither treadmill speed nor exercise protocol altered the magnitude of exercise-induced hyperthermia. We conclude that T _core of running mice is influenced greatly by T _a, but not by the exercise protocols or intensities examined in the present report. These findings suggest that the magnitude of hyperthermia in running mice may be regulated centrally, independently of exercise intensity.     

Response of mice to continuous 5-day passive hyperthermia resembles human heat acclimation

Chronic repeated exposure to hyperthermia in humans results in heat acclimation (HA), an adaptive process that is attained in humans by repeated exposure to hyperthermia and is characterized by improved heat elimination and increased exercise capacity, and acquired thermal tolerance (ATT), a cellular response characterized by increased baseline heat shock protein (HSP) expression and blunting of the acute increase in HSP expression stimulated by re-exposure to thermal stress. Epidemiologic studies in military personnel operating in hot environments and elite athletes suggest that repeated exposure to hyperthermia may also exert long-term health effects. Animal models demonstrate that coincident exposure to mild hyperthermia or prior exposure to severe hyperthermia can profoundly affect the course of experimental infection and injury, but these models do not represent HA. In this study, we demonstrate that CD-1 mice continuously exposed to mild hyperthermia (ambient temperature ~37°C causing ~2°C increase in core temperature) for 5 days and then exposed to a thermal stress (42°C ambient temperature for 40 min) exhibited some of the salient features of human HA, including (1) slower warming during thermal stress and more rapid cooling during recovery and (2) increased activity during thermal stress, as well as some of the features of ATT, including (1) increased baseline expression of HSP72 and HSP90 in lung, heart, spleen, liver, and brain; and (2) blunted incremental increase in HSP72 expression following acute thermal stress. This study suggests that continuous 5-day exposure of CD-1 mice to mild hyperthermia induces a state that resembles the physiologic and cellular responses of human HA. This model may be useful for analyzing the molecular mechanisms of HA and its consequences on host responsiveness to subsequent stresses.     

Non-thermal signals govern selective brain cooling in pigs

We used implanted miniature data loggers and fine thermistors to measure arterial blood and brain temperatures in four female pigs, to a resolution of 0.04 °C, every 5 min, for 4 weeks. Within that period, pigs were exposed on different days, and in random order, to a cold (5 °C) or hot (38 °C) environment. In the thermoneutral environment of the pigs' home pens, brain temperature was usually lower than blood temperature. Such selective brain cooling was absent for 2 days after surgery, during handling and transport stress, and on waking. The magnitude of selective brain cooling was greatest when pigs were sleeping and body temperatures were low, and was smallest, or even absent, during hyperthermia and natural fever. Our results showed that selective brain cooling was present in pigs, but there was no clear relationship between blood temperature and the magnitude of selective brain cooling. Instead, the degree of selective brain cooling in pigs was governed by non-thermal factors, especially those associated with high sympathetic nervous system activity. Our results further support the concept that selective brain cooling does not serve to protect the brain from thermal damage during heat stress. Accepted: 14 September 1999     

Internal Control of Rate of Leaf Production in Sngar Beet

The theory that the rate at which leaves appear is little affected by the current environment has been tested with sugar-beet plants. Seedlings were raised vinder 4 different conditions, viz., 8 hour photoperiod 20°C, 16 hour photoperiod 20°, continuous light or in the glasshouse. After 2 weeks all were put in the glasshouse under the same growing conditions. The treatments wbich the seedlings received continued to affect the rate at which leaves were produced during the next 20 weeks.     

Effects of different levels of selenium on growth performance and immunocompetence of broilers under heat stress

An experiment was undertaken to evaluate the effect of dietary selenium (Se) levels on growth performance and immune competence of broilers under heat stress. Birds were raised in either a thermoneutral (TN, 23.9°C constant) or heat stress conditions (HS, 23.9°C to 38°C cycling) and were fed a corn-soybean meal basal diet supplemented with Se at 0, 0.2 and 0.4 mg/kg. A total of 240 one-day-old male broiler chicks were randomly assigned to six groups; each group had four replicates of 10 birds. Body weight and feed intake were not influenced by dietary Se, while feed conversion was significantly improved by a Se-supplementation of 0.2 mg/kg. HS significantly reduced body weight, feed intake and feed conversion. Numbers of abdominal exudate cells (AEC), percentage of macrophages in AEC, phagocytic macrophages, internalized opsonised and unopsonised sheep red blood cells (SRBC) were significantly increased by dietary Se. Both primary and secondary antibody responses were characterised by increasing titres of antibody to SRBC by dietary Se when birds were exposed to HS (p < 0.05). Lymphoid organ weights, antibody responses, incidence of macrophages in AEC, and phagocytic ability of macrophages were also significantly reduced under HS. These results indicated that HS severely reduced growth performance and immunocompetence of broilers, whereas the immune response of broilers improved by dietary Se supplementation under HS.     

Effects of passive hyperthermia versus exercise-induced hyperthermia on immune responses: hormonal implications

Different stress hormones are released during prolonged exercise and passive hyperthermia. We hypothesized that these different hormonal responses could contribute to the different changes in the immune response during these two challenges. Methods: Eight subjects completed three trials in a randomized order. In the control trial (C), the subjects remained in a sitting posture for three hours in thermoneutral conditions. In the exercise hyperthermia trial (E), they exercised for two hours on a treadmill at 65% max in thermoneutral conditions, followed by 1-h recovery in thermoneutral conditions; in the passive hyperthermia trial (PH), the subjects remained in a semi-recumbent position in a climatic chamber for two hours in hot conditions, followed by 1-h recovery in thermoneutral conditions. During the E and PH trials, wind speed and thermal conditions were modulated to reach a rectal temperature (Tre) of 38.5 degrees C at 60 min and 39 degrees C at 120 min. The subjects did not drink during the experiments. Blood samples (10 mL) were taken at 0, 60, 120 and 180 min of each trial. The total white cell count and its subsets were measured; plasma catecholamines, cortisol and prolactin were assayed. In a whole blood assay, blood leukocytes were stimulated by lipopolysaccharide (LPS) or phytohemagglutinin (PHA) for 24 and 48 hours, respectively. Cytokines, such as TNF-alpha, IL-10 and INF-gamma were measured in the culture supernatant. RESULTS: The plasma levels of catecholamines were increased only during E, prolactin was increased only during PH, and cortisol was increased in both E and PH. Only the exercise caused a mobilization of blood leukocytes and leukocyte subsets. The INF-gamma and TNF-alpha production by PHA- and LPS-stimulated blood, respectively, were inhibited in a substantial way in both E and PH compared to control when Tre reached 39 degrees C. Only LPS-induced IL-10 production was enhanced during the exercise. The effects of the challenges were increased with 39 degrees C compared to 38.5 degrees C. CONCLUSIONS: Catecholamines play a major role in the mobilization of immunocompetent cells and the production of IL-10 during exercise. Prolactin and catecholamines have adverse role on the immune response, whereas cortisol exerts similar effects during both trials. The consequence could be a protection against inflammatory overshooting.