Days Of Heat Exposure Research Articles

Declining semen quality and steadying seminal plasma ions in heat-stressed boar model.

There are increasing concerns about infertility of male exposure to high environmental temperatures. Nevertheless, the relationship between heat and accessory sex gland secretion underlying the high ambient temperature-induced poor semen quality has not yet been addressed. In the present study, five boars were used as an animal model to evaluate semen quality and the secretory function of accessory sex glands. After the boars received 3 days of heat exposure, semen collection was standardized to 18 continual times with a 3-day interval to determine the semen variables of semen volume, semen concentration, abnormal spermatozoa, seminal plasma composition, and testosterone level in the seminal plasma and serum. The total sperm count was lowest by the end of week 2. The higher abnormal spermatozoa percentage were observed by the end of week 2 and persisted until week 6 after heat exposure. Additionally, there was no significant change in semen volume, testosterone level, and concentrations of ions and total protein in the seminal plasma before and after heat exposure. A single 3-day heat exposure caused poor semen quality, but did not disturb accessory sex gland secretion in boars. Declining semen quality might be mainly due to the damaged germ cells, which were sensitive to elevated temperature in hot summer months.

Effect of acute heat stress on adrenocorticotropic hormone, cortisol, interleukin-2, interleukin-12 and apoptosis gene expression in rats.

The aim of the present study was to investigate the effect of acute heat stress on the neuroendocrine and immunological function in rats. Male Sprague-Dawley rats were randomly divided into two groups and respectively exposed to heat (32°C) or to room temperature (24°C). After 7 days of heat exposure, the heat-stress rat model was established. The organ coefficients of the pituitary and adrenal glands were determined. The body temperature was measured by telemetry. The average contents of adrenocorticotropic hormone (ACTH), cortisol (Cor), interleukin-2 (IL-2) and IL-12 in serum were detected. The expression of apoptotic genes in the spleen was measured. The results showed that acute heat stress did not evidently affect the body temperature and body weight (P>0.05), but the exposure increased the organ coefficients of the pituitary and adrenal glands (P<0.05). Heat exposure significantly elevated the level of ACTH, Cor, IL-2 and IL-12 (P<0.05). The expression of caspase-3 and Bax were not changed significantly (P>0.05), while Bcl2 was reduced (P<0.05).

Topical curcumin-loaded hydrogels obtained using galactomannan from Schizolobium parahybae and xanthan.

The curcumin (CUR)-loaded binary hydrogel was formulated using xanthan and galactomannan from Schizolobium parahybae (guapuruvu). The binary hydrogels presented gel characteristics, stable pH values and mechanical stress resistance even after 45 days of heat exposure (45 °C). The CUR-loaded hydrogel content was 98.6% for XGMC (xanthan and galactomannan with CUR-microemulsion) after the stability test. The in vitro cytotoxicity analysis suggested non-cutaneous membrane irritation, and the in vitro skin permeation analysis indicated 2.15 to 2.50 μg mL(-1) CUR at the stratum corneum, epidermal and dermal levels. The XGEC (xanthan and galactomannan with CUR solubilized in ethanol) and XGMC hydrogels presented 76.8 and 63.2% inhibition of topical inflammation, respectively. Chemical stability and non-cytotoxicity analysis confirm the safety of prolonged exposure of the skin during the topical treatment, offering long-lasting XGEC and XGMC action.

Effects of heat exposure during fixed time on thoracic aorta contractility in rats

Objective To investigate the effect of heat exposure during fixed time on the thoracic aorta contractile properties of rats and the related mechanism.Methods Male Wistar rats were randomly divided into control group,random-time heat exposure group and fixed-time heat exposure group(ambient temperature to 24℃;the two heat exposure groups were exposed to heat \ 6 h daily). After 7 days of heat exposure,the body core temperature of rats was measured by telemetry technology.The response of isolated thoracic aorta to noradrenaline(NA) was determined by organ bath system.Expression of aorta eNOS mRNA and plasma NOx(NO2-and NO3-) content were also examined.Results Heat exposure showed no significant effects on body core temperature of rats(P0.05).Compared with control group,the fixed-time heat exposure group had significantly lowered thoracic aortic vascular reactivity to NA(P0.01).Compared with control group and random-time heat exposed group,the fixed-time heat exposure group had significantly increased eNOS mRNA expression and plasma NOx content(P0.05). ConclusionFixed-time exposure to heat can change the reactivity of the thoracic aorta in rats,and vascular endothelial cell-derived NOS pathway may be involved in the vasodilatory effects.

Climate change threats to population health and well-being: the imperative of protective solutions that will last

BackgroundThe observational evidence of the impacts of climate conditions on human health is accumulating. A variety of direct, indirect, and systemically mediated health effects have been identified. Excessive daily heat exposures create direct effects, such as heat stroke (and possibly death), reduce work productivity, and interfere with daily household activities. Extreme weather events, including storms, floods, and droughts, create direct injury risks and follow-on outbreaks of infectious diseases, lack of nutrition, and mental stress. Climate change will increase these direct health effects. Indirect effects include malnutrition and under-nutrition due to failing local agriculture, spread of vector-borne diseases and other infectious diseases, and mental health and other problems caused by forced migration from affected homes and workplaces. Examples of systemically mediated impacts on population health include famine, conflicts, and the consequences of large-scale adverse economic effects due to reduced human and environmental productivity. This article highlights links between climate change and non-communicable health problems, a major concern for global health beyond 2015.DiscussionDetailed regional analysis of climate conditions clearly shows increasing temperatures in many parts of the world. Climate modelling indicates that by the year 2100 the global average temperature may have increased by 3-4°C unless fundamental reductions in current global trends for greenhouse gas emissions are achieved. Given other unforeseeable environmental, social, demographic, and geopolitical changes that may occur in a plus-4-degree world, that scenario may comprise a largely uninhabitable world for millions of people and great social and military tensions.ConclusionIt is imperative that we identify actions and strategies that are effective in reducing these increasingly likely threats to health and well-being. The fundamental preventive strategy is, of course, climate change mitigation by significantly reducing global greenhouse gas emissions, especially long-acting carbon dioxide (CO2), and by increasing the uptake of CO2 at the earth's surface. This involves urgent shifts in energy production from fossil fuels to renewable energy sources, energy conservation in building design and urban planning, and reduced waste of energy for transport, building heating/cooling, and agriculture. It would also involve shifts in agricultural production and food systems to reduce energy and water use particularly in meat production. There is also potential for prevention via mitigation, adaptation, or resilience building actions, but for the large populations in tropical countries, mitigation of climate change is required to achieve health protection solutions that will last.

Heat acclimation-induced changes in heart glycogen/glucose metabolism in rats

Based on the observation that heat acclimation is a slowly developing response, evoked by continuous exposure to moderate heat, we investigated the time-dependent acclimatory changes of heart glycogen metabolism. Cardiac levels of key carbohydrate-related enzymes and substrates were studied in the function of the duration of short-term (STHA; 6, 12, 24 and 48 h) and long-term heat acclimation (LTHA; 7, 14, 21 and 30 days) to high environmental temperature (35 ± 1°C). The changes in heart glycogen metabolism during STHA could be separated in two phases: up to 12 h exposure, where significant decrease of the heart glycogen (Glk), glucose-6-phosphate (G6P), hexokinase (HK) activity as well as increase of heart glucose was observed; and from 24 to 48 h exposure, manifested with elevation of Glk, Glu, glycogen phosphorylase a (GPa), phosphofructokinase (PFK) and HK activities. The metabolic changes in the period of LTHA could also be seen as separate phases: in a period of 7-14 days of heat exposure there was an increase of heart Glk, Glu, G6P, HK, as well as a decrease of GPa and PFK, while in the period of 21-28 days there was more intensive rebound of Glk and G6P, increase of GPa activity and non-significant changes of Glu, HK and PFK. The results obtained have showed that acclimation to moderate hyperthermic environment has caused significant changes in examined parameters which differ depending on duration to the exposure: intensive stress-induced glycogenolytic and glycolytic processes in the period of STHA and intensive energy sparing, manifested by Glk deposition in the period of LTHA.

Effect of an increase in environmental temperature on testicular androgenesis and spermatogenesis in toad (Bufo melanostictus) during hibernating season

Activities of key testicular androgenic enzymes [Δ(5), 3β-hydroxysteroid dehydrogenase (Δ(5), 3β-HSD) and 17β-hydroxysteroid dehydrogenase (17β-HSD)], plasma levels of testosterone, and testicular gametogenic activities were studied in heat-exposed adult male toads during hibernating season for two consecutive years. Exposure of toads to an elevated environmental temperature for 14 and 21 days resulted in significant elevation of testicular Δ(5), 3β-HSD and 17β-HSD activities, along with plasma levels of testosterone. Testicular gametogenic activity, by means of quantity of all stages of spermatogenic cycle, were elevated significantly at the same experimental schedule, but 7 days of heat exposure resulted in significant elevation only in stage IV. The results indicated that environmental temperature is an important modulator of breeding activities of male toads. It also demonstrated that testicular activities in seasonally breeding toads are probably not linked to hibernating cycle.

Time course of ROS production in skeletal muscle mitochondria from chronic heat-exposed broiler chicken

This study was designed to elucidate physiological changes of skeletal muscle mitochondria from broiler chickens ( Gallus gallus) during chronic heat exposure. Chickens (19-day-old) were exposed to either constant heat stress (34 °C) or kept at control temperature (24 °C) for 14 days. Mitochondrial ROS production for control group showed little changes during the experimental periods, whereas that for the heat-stressed group was increased after 3, 5 and 9 days of heat exposure and returned to original levels at day 14. Mitochondrial membrane potential in state 4 for heat-stressed birds was higher than those of control birds after 3 and 5 days, but was not at day 14. Mitochondrial oxygen consumption rate in state 3 was increased after 3 and 5 days, and also returned to original levels by day 14. These results suggest that chronic heat stress induces increased ROS production in skeletal muscle mitochondria, probably via elevation of the membrane potential in state 4, resulting from enhanced oxygen consumption in the initial stage of heat exposure. These physiological changes were no longer observed at day 14, possibly because the animals had acclimatized to environmental heat stress.

Influence of lesions in the limbic-hypothalamic system on metabolic responses of pyruvate to daily repeated heat exposures in rabbits.

The effects of lesions of the limbic-hypothalamic structures on the pyruvate metabolic responses to daily repeated heat exposures in the liver of rabbits have been investigated. The experimental results were as follows: 1. The pyruvate metabolic responses to the 1st heat exposure (heat exposure on the 1st day) were altered by lesions of periventricular arcuate nucleus (ARC), ventromedial hypothalamus (VMH), stria terminalis (ST) and dorsal fornix (FX). 2. The pyruvate metabolic responses to heat exposure were completely abolished by the ten times repetition of daily heat exposures in rabbits with lesions of ARC, VMH and ST, as same as in each sham-operated group. 3. In rabbits with FX lesions, the pyruvate metabolic responses to heat exposure remained even after the ten times repetition of daily heat exposures. 4. From these results, it might be suggested that ARC, VMH, amygdala (AMYG)-ST system and dorsal hippocampus (HPC)-FX system played a certain role in the mechanisms of pyruvate metabolic responses to the 1st heat exposure, but only the HPC-FX system participated in the process of pyruvate metabolic adaptation to daily repeated heat exposures.

Proliferation of neuronal progenitor cells and neuronal differentiation in the hypothalamus are enhanced in heat-acclimated rats

Male Wistar rats, initially maintained at an ambient temperature (T (a)) of 24 degrees C, were subjected to a constant high T (a) of 32 degrees C (HE) or were constantly kept at 24 degrees C (controls, CN). Bromodeoxyuridine (BrdU) was intraperitoneally injected daily for five consecutive days after commencing heat exposure. On the 6th, 13th, 23rd, 33rd, 43rd, and 53rd day of heat exposure, rats' brains were removed. Immunohistochemical analysis showed that the numbers of BrdU-positive cells in the hypothalamus of HE were significantly and consistently greater than those of CN. In HE, the number of BrdU-positive cells double-stained by a mature neuron marker increased abruptly after 33 days of heat exposure by about seven times. This was not the case in CN. The results suggest that heat exposure facilitates proliferation of neuronal progenitor cells in the hypothalamus and promotes differentiation to neurons, which might have certain relation to establishing long-term heat acclimation in rats.

The impact of excess heat events in Maricopa County, Arizona: 2000–2005

Exposure to excess heat is preventable yet it is the primary weather-related cause of mortality in the United States. In the Southwest United States, high temperatures are common and indoor environments often have cooling devices. In summer 2005, Maricopa County, Arizona experienced a 182% increase in reported heat-related deaths in comparison to 2000--2004. We examined at-risk populations and excess mortality. We characterized heat-related deaths using descriptive and multivariate time-series analyses of county vital record data from June-September 2000--2005. Dose-response relationships for heat-related mortality and heat index were evaluated using linear and quadratic splines. From June-September, 2000--2005, 136 heat-related deaths (0.68 per 100,000) were reported; 49 (36%) occurred in 2005. In July 2005, a 14-day heat wave resulted in 28 (57%) reported deaths--a 102% increase in comparison to the same time period in 2000--2004. Decedent demographics in 2005 did not differ from previous years. The mean age of all 136 deaths was 56 years (range: 7-92 years). Of those with discernable reported injury locations, 62 (66%) were identified outdoors. Forty-eight (77%) decedents identified outdoors were < 5 years; conversely, 26 (82%) decedents who were found indoors were > or = 65 years. A 6% (95% CI: 1.00-1.13) increase in mortality risk was observed for each degree (F) increase in heat index. Excess heat impacted a younger population in Maricopa County and many deaths occurred outdoors. Consecutive days of heat exposure--even among a heat-acclimated population--can increase mortality risk. Public health response activities guided by locally obtained data will better target those at risk.

Effects of Plantain (Plantago lanceolata L.) Herb and Heat Exposure on Plasma Glucose Metabolism in Sheep

ABSTRACT : An experiment was conducted using a [6, 6- 2 H]glucose isotope dilution method to determine the effects of plantain ( Plantago lanceolata L.) on plasma glucose metabolism in sheep taken from a thermoneutral environment and exposed to a hot environment. The sheep were fed either mixed hay (MH) of orchardgrass ( Dactylis glomerata L.) and reed canarygrass ( Phalaris arundinacea L.) at a 60:40 ratio or MH and plantain (PL) at a 9:1 ratio in a crossover design for each 23-day period. In both dietary treatments the metabolizable energy (ME) and crude protein intake were designed to be isoenergetic and isoproteinous at around maintenance level. The sheep were taken from a thermoneutral environment (20°C, 70% RH) and exposed to a hot environment (28-30°C, 70% RH) for 5 days. The isotope dilution method using a single injection of [6, 6- 2 H]glucose was performed on the 18 th day of the thermoneutral environment and on the 5 th day of heat exposure. Plasma glucose pool size was numerically lower (p = 0.26) during heat exposure on both dietary treatments, and numerically higher (p = 0.13) on the MH diet irrespective of environmental temperature. Plasma NEFA concentration (p = 0.01) and glucose turnover rate (p = 0.03) were decreased during heat exposure, but remained similar between diets. It could be concluded that, although no positive impact of plantain on glucose metabolism was found under the present experimental conditions (plantain constituted only 10% of basal diet), plantain herb is an alternative to MH for rearing sheep in both thermoneutral and hot environments. (

Responses of Thyroid Hormones of Market-Size Broilers to Thermoneutral Constant and Warm Cyclic Temperatures

This study characterizes the responses, particularly diurnal variations, of thyroid hormones (TH) [3,5,3′-triiodothyronine (T3), thyroxine (T4), and T3:T4 ratio (T3/T4)] of Arbor Acres broilers to constant thermoneutral (TN) or warm cyclic (WC) temperatures. There existed distinct circadian variations—2 peaks in TH under the TN and WC conditions. The 2 peaks of T3, T4, and T3/T4 of the broilers occurred, respectively, at 0 and 16 h, 8 and 16 h, and 0 and 12 h under the TN condition; but at 0 and 12 h, 0 and 8 h, and 4 and 12 h under the WC conditions. During a 5-d heat exposure (HE) to the WC regimens, T3 and T4 showed continual decrease. The daily mean of T3 declined significantly (P < 0.05) on the first day of HE, whereas significant decrease of T4 (P < 0.05) occurred on the second day of HE. There was no significant change in daily mean of T3/T4 during the same HE period. Hence, results of the study indicate that T3 provides a better heat stress indicator than T4. The nature of circadian variations in TH makes it necessary to collect blood samples more than once a day, e.g., 6 times at 0, 4, 8, 12, 16, and 20 h, to ensure full evaluation of hormonal responses of market-size broilers undergoing thermal challenges.

RESPONSE OF COMMERCIAL LAYERS TO HOUSING AT DIFFERENT CAGE DENSITIES AND HEAT STRESS CONDITIONS. 1- PHYSIOLOGICAL INDICATORS AND IMMUNE RESPONSE

An experiment was conducted to determine the effects of heat stress and different cage densities on blood indices and Immune response. The hypothesis that, commercial layers, caged at lower densities, react less intensely, to heat stress, than would the hens caged at higher densities was also tested. A total of 936, Lohman Selected Leghorn (LSL) hens, fifty eight weeks old, were used in this experiment. These hens were originally housed, at eighteen weeks of age, as follows: 1- The first group (468 hens) was housed in a conventional open-sided house (Heat Stressed). 2The second group (468 hens) was housed in a conventional open-sided house with desert cooling systems (Control). Pullets were placed at 4, 5, 6, 7, 8 and 9 birds per cage in wire cages (60 cm wide by 50 cm deep and 40 cm height). At 58 weeks of age, the first group was exposed to high environmental temperatures, ranging from 4042 O C and 65-70% relative humidity (five hours daily for seven consecutive days). After each daily heat exposure, the hens were exposed to the normal ambient temperature of the season (20-26 O C and 60-65% relative humidity). The control group was exposed to the ambient temperatures of the season, ranging from 20-26 O C and 60-65% relative humidity. Data were collected during the period of heat stress exposure . Results indicated that rectal temperature and Heterophil: Lymphocyte ratios (H:L ratios) of heat-stressed birds, increased significantly as compared to the control group, at the first and last days of heat stress. However, red blood cell count, packed cell volume, hemoglobin concentration, plasma total protein, globulin and serum potassium levels, of heat stressed birds, decreased significantly as compared to the control ones, when tested at both the first and last day of heat stress. Plasma albumin and serum calcium levels, of heat stressed birds, decreased significantly as compared to the control group, only at the first day of heat stress. Also, heat exposure reduced antibody titers against sheep red blood cell's (SRBC's) on 3, 5 and 7 days post immunization. No significant detrimental effects, due to heat stress, on serum calcium and potassium levels, were observed, when hens were housed at the lowest cage density (4 hens/cage), at the first day of heat stress. At the last day of heat stress, plasma proteins, serum calcium and potassium levels, and H:L ratio, were also not affected, when hens were housed at the lowest cage density. Antibody titers against SRBC's, were significantly reduced due to heat stress, when hens were housed at 9 per cage,

Effect Of A 14 Day High Intensity Exercise Protocol On Heat Acclimation Responses

0569 Heat acclimation (HA) reduces physiological strain, optimizes performance and lessens thermal injury risk during occupational or athletic activities. Few HA protocols have focused on relative exercise intensities > 60% VO2max. Little is known about time course or magnitude of high intensity-based adaptive responses beyond 9 days of heat exposure. This is pertinent, for higher intensity exercise HA protocols are more applicable than low intensity protocols to many work, exercise or athletic settings. It is unknown how individuals of different ages will respond differentially to a high intensity protocol. This has practical implications for many older adults who regularly exercise, work, or compete in athletic events in the heat. PURPOSE: To assess: 1) effects of a 14 day high intensity running protocol on time course and magnitude of HA responses and 2) age effect on adaptive responses among highly fit males. METHODS: Nine younger (20.4 ± 0.8 years) and eight older (45.8 ± 1.9 years) aerobically fit males ran daily for 30 minutes at 75% VO2max in a hot, humid (35°C, 35% RH) environment. Core temperature (T c) and heart rate (HR) were monitored continuously; perceived exertion (RPE), thermal sensation (TS) and sweat volume were measured during each trial. Physiological strain (PSI) was measured as per Moran during each trial. Blood sampling and sweat electrolyte measures were conducted across protocol. RESULTS: VO2max did not differ significantly across protocol or between groups (55.2 ± 2.4 older vs. 57.1 ± 1.3 ml.kg−1.min−1 younger pre-acclimation, 56.1 ± 2.3 older vs. 58.5 ± 1.2 ml.kg−1.min−1 younger post-acclimation). T c, HR, PSI, RPE and TS significantly decreased across protocol for both groups (P < 0.05). HR and PSI were significantly less for older when compared with younger runners. HR decreased similarly for younger (7.4%) and older (10.3%) participants despite the agerelated HR difference. Participants experienced a 2.9% resting plasma volume increase across protocol with no significant difference between groups. Participants experienced significant decreases in sweat sodium concentration for forehead and back regions with no significant difference between groups. Sweat volume did not significantly differ across protocol or between groups. Despite age-related differences in HR and PSI, younger and older participants underwent similar changes in cardiovascular and thermal measures across protocol. CONCLUSIONS: Most adaptations to high intensity HA protocols occur by day 8. Highly fit older and younger males have similar heat tolerance capabilities following high intensity HA protocols. Supported by Gatorade Sports Science Institute.

A SPME–GC procedure for monitoring peppermint flavor in tablets

A method was developed using solid-phase microextraction (SPME) and gas chromatography to monitor the peppermint flavor loss in a taste-masked tablet formulation. This was accomplished by headspace sampling of two major components of peppermint: menthone and menthol. It was found that the excipients from the tablet produced an important matrix effect and that standard addition analysis was necessary for improved accuracy of the determination. The method was shown to be specific and precise. Furthermore, the method produced acceptable results with adequate quantitation limits to determine peppermint flavors in taste-masked tablets. The optimized extraction procedure was successfully used to monitor the stability of peppermint flavor in an oral solid formulation. The accelerated stability studies of the tablet showed that the menthone and menthol was lost in an exponential manner and levels off after several days of heat exposure.

Time memory for heat exposure limited to a fixed daily time in heat-acclimated rats and humans

After daily heat exposure for 4–5 h limited to a fixed time for more than 5 consecutive days, body core temperature of rats and humans drops before and during the period they were previously exposed to heat. The fall in body temperature persists for a few days after terminating the timed daily heat exposure. Additionally, various thermoregulatory changes to resist heat are brought about especially during the specific period. It is, therefore, hypothesized that the thermoregulatory system can memorize the time for heat exposure and according to the memory, thermoregulatory responses for neutralizing heat stress are induced around the period corresponding to that of the previous heat exposure time even without actual heat exposure.

Anticipatory fall in core temperature in rats acclimated to heat given for various hours at a fixed daily time.

Rats were subjected to daily heat exposure limited to a fixed time for 10 consecutive days. An ambient temperature (T(a)) inside an animal room initially set at 24.0 degrees C was raised to 32.0 degrees C, starting at the middle of the dark phase. The high T(a) was maintained for a subsequent 1.5, 3, or 5 h and was then returned to 24.0 degrees C. After the heat exposure schedule, their core temperature significantly fell for about 30 min before and during the period when they had previously been exposed to heat without thermal stimuli. In rats, a time memory for heat exposure could be formed even when the duration of daily heat exposure was short.

Fluid and electrolyte supplementation for exercise heat stress

During exercise in the heat, sweat output often exceeds water intake, resulting in a body water deficit (hypohydration) and electrolyte losses. Because daily water losses can be substantial, persons need to emphasize drinking during exercise as well as at meals. For persons consuming a normal diet, electrolyte supplementation is not warranted except perhaps during the first few days of heat exposure. Aerobic exercise is likely to be adversely affected by heat stress and hypohydration; the warmer the climate the greater the potential for performance decrements. Hypohydration increases heat storage and reduces a person's ability to tolerate heat strain. The increased heat storage is mediated by a lower sweating rate (evaporative heat loss) and reduced skin blood flow (dry heat loss) for a given core temperature. Heat-acclimated persons need to pay particular attention to fluid replacement because heat acclimation increases sweat losses, and hypohydration negates the thermoregulatory advantages conferred by acclimation. It has been suggested that hyperhydration (increased total body water) may reduce physiologic strain during exercise heat stress, but data supporting that notion are not robust. Research is recommended for 3 populations with fluid and electrolyte balance problems: older adults, cystic fibrosis patients, and persons with spinal cord injuries.

Altered arterial concentrations of placental hormones during maximal placental growth in a model of placental insufficiency.

Pregnant ewes were exposed chronically to thermoneutral (TN; 20+/-2 degrees C, 30% relative humidity; n=8) or hyperthermic (HT; 40+/-2 degrees C 12 h/day, 35+/-2 degrees C 12 h/day, 30% relative humidity, n=6) environments between days 37 and 93 of pregnancy. Ewes were killed following 56 days of exposure to either environment (days in treatment (dit)), corresponding to 93+/-1 day post coitus (dpc). Maternal core body temperatures (CBT) in HT ewes were significantly elevated above the TN ewes (HT; 39.86+/-0.1 degrees C vs TN; 39.20+/-0.1 degrees C; P<0.001). Both groups of animals displayed circadian CBT, though HT ewes had elevated amplitudes (HT; 0.181+/-0.002 degrees C vs TN; 0.091+/-0.002 degrees C; P<0.001) and increased phase shift constants (HT; 2100 h vs TN; 1800 h; P<0.001). Ewes exposed to chronic heat stress had significantly reduced progesterone and ovine placental lactogen (oPL) concentrations from 72 and 62 dpc respectively (P<0.05), corresponding to approximately 30 dit. However, when compared with the TN ewes, HT cotyledonary tissue oPL mRNA and protein concentrations were not significantly different (P>0.1). Prolactin concentrations rose immediately upon entry into the HT environment, reaching concentrations approximately four times that of TN ewes, a level maintained throughout the study (HT; 216.31+/-32.82 vs TN; 54. 40+/-10.0; P<0.0001). Despite similar feed intakes and euglycemia in both groups of ewes, HT fetal body weights were significantly reduced when compared with TN fetuses (HT; 514.6+/-48.7 vs TN; 703. 4+/-44.8; P<0.05), while placental weights (HT; 363.6+/-63.3 vs TN; 571.2+/-95.9) were not significantly affected by 56 days of heat exposure. Furthermore, the relationship between body weight and fetal length, the ponderal index, was significantly reduced in HT fetuses (HT; 3.01+/-0.13 vs TN; 3.57+/-0.18; P<0.05). HT fetal liver weights were also significantly reduced (HT; 27.31+/-4.73 vs TN; 45.16+/-6.16; P<0.05) and as a result, the brain/liver weight ratio was increased. This study demonstrates that chronic heat exposure lowers circulating placental hormone concentrations. The observation that PL mRNA and protein contents are similar across the two treatments, suggests that reduced hormone concentrations are the result of impaired trophoblast cell development, specifically trophoblast migration. Furthermore, the impact of heat exposure during maximal placental growth is great enough to restrict early fetal development, even before the fetal maximal growth phase (100 dpc-term). These data highlight that intrauterine growth retardation (IUGR) may result primarily from placental trophoblast cell dysfunction, and secondarily from later reduced placental size.