Heat Loss Pathways Research Articles

Evaluation of thermal loss of Chinese solar greenhouse cover under all operating conditions

The solar greenhouses in China are composed of gables on the east and west sides, north walls, and north and south roofs. The south roof is the main heat dissipation surface of the greenhouse. To quantitatively and comprehensively analyse the heat transfer characteristics and heat loss pathways of the south roof of a solar greenhouse, based on the actual usage of the greenhouse, the heat transfer of the south roof is classified into two working conditions: heat transfer of the transparent covering material (TCM) and heat transfer of the composite thermal insulation coating (CTIC). A theoretical heat transfer model of the covering layer of the solar greenhouse under all working conditions is constructed by applying the heat transfer theory. The model was tested using field test data from two areas different regions in China: Beijing and Lhasa. The results show good agreement between the simulated and measured values, the relative errors between simulated and measured values were 18.54% and 8.17%, respectively, with root mean square errors of 1.69 and 1.92. The model can comprehensively and accurately reflect the comprehensive heat transfer coefficient and heat loss changes of the south roof of the solar greenhouse. It can be used to analyze the proportion of heat loss caused by radiation, convection, and water vapor condensation. The analysis shows that condensation heat loss cannot be ignored during the winter night period. Measures such as covering the roof with plastic film and controlled irrigation can help reduce condensation heat loss.

Experimental and theoretical analysis of sub-ambient cooling with longwave radiative coating

With radiative cooling coating it is possible to keep the temperature of a surface around or even below air temperature under solar radiation exposure. This study conducted experimental and theoretical analysis of its effectiveness. Setups with different surface coatings and boundary conditions were placed on a roof of a building in Chongqing, China. The 24-h measurements on a representative summer day showed that different boundary conditions at the bottom of the coating led to varied cooling effects, but the surface temperature of the radiative cooling coating stayed below air temperature most of the day when the incident solar radiation was below 800 W m−2. Theoretical models were also established to investigate the heat transfer process. Analysis showed that the longwave radiation absorption amounted to 89.0%–91.9% of the total daily heat gain of the cooling coating. The longwave radiative emission was the sole and significant heat loss pathway for the cooling coating, making the net longwave radiative heat transfer a pathway for heat dissipation. Sensitivity analysis further demonstrated that on a typical summer day in Chongqing, shortwave reflectivity and incident solar radiation were the dominant factors impacting the diurnal cooling effect, while wind velocity was the most important parameter for nocturnal cooling.

Avian thermoregulation in the heat: evaporative cooling in five Australian passerines reveals within-order biogeographic variation in heat tolerance.

Evaporative heat loss pathways vary among avian orders, but the extent to which evaporative cooling capacity and heat tolerance vary within orders remains unclear. We quantified the upper limits to thermoregulation under extremely hot conditions in five Australian passerines: yellow-plumed honeyeater (Lichenostomus ornatus; ∼17 g), spiny-cheeked honeyeater (Acanthagenys rufogularis; ∼42 g), chestnut-crowned babbler (Pomatostomus ruficeps; ∼52 g), grey butcherbird (Cracticus torquatus; ∼86 g) and apostlebird (Struthidea cinerea; ∼118 g). At air temperatures (Ta) exceeding body temperature (Tb), all five species showed increases in Tb to maximum values around 44-45°C, accompanied by rapid increases in resting metabolic rate above clearly defined upper critical limits of thermoneutrality and increases in evaporative water loss (EWL) to levels equivalent to 670-860% of baseline rates at thermoneutral Ta Maximum cooling capacity, quantified as the fraction of metabolic heat production dissipated evaporatively, ranged from 1.20 to 2.17, consistent with the known range for passerines, and well below the corresponding ranges for columbids and caprimulgids. Heat tolerance limit (HTL, the maximum Ta tolerated) scaled positively with body mass, varying from 46°C in yellow-plumed honeyeaters to 52°C in a single apostlebird, but was lower than that of three southern African ploceid passerines investigated previously. We argue this difference is functionally linked to a smaller scope for increases in EWL above baseline levels. Our data reiterate the reliance of passerines in general on respiratory evaporative heat loss via panting, but also reveal substantial within-order variation in heat tolerance and evaporative cooling capacity.

The Effects of Heat Adaptation on Physiology, Perception and Exercise Performance in the Heat: A Meta-Analysis.

Exercise performance and capacity are impaired in hot, compared to temperate, conditions. Heat adaptation (HA) is one intervention commonly adopted to reduce this impairment because it may induce beneficial exercise performance and physiological and perceptual adaptations. A number of investigations have been conducted on HA but, due to large methodological differences, the effectiveness of different HA regimens remain unclear. (1) To quantify the effect of different HA regimens on exercise performance and the physiological and perceptual responses to subsequent heat exposure. (2) To offer practical HA recommendations and suggestions for future HA research based upon a systematic and quantitative synthesis of the literature. PubMed was searched for original research articles published up to, and including, 16 February 2016 using appropriate first- and second-order search terms. English-language, peer-reviewed, full-text original articles using human participants were reviewed using the four-stage process identified in the PRISMA statement. Data for the following variables were obtained from the manuscripts by at least two of the authors: participant sex, maximal oxygen consumption and age; HA duration, frequency, modality, temperature and humidity; exercise performance and capacity; core and skin temperature; heart rate, stroke volume, cardiac output, skin blood flow, sweat onset temperature, body mass loss, sweat rate, perception of thirst, volitional fluid consumption, plasma volume changes; sweat concentrations of sodium, chloride and potassium; aldosterone, arginine vasopressin, heat shock proteins (Hsp), ratings of perceived exertion (RPE) and thermal sensation. Data were divided into three groups based upon the frequency of the HA regimen. Performance and capacity data were also divided into groups based upon the type of HA used. Hedges' g effect sizes and 95% confidence intervals were calculated. Correlations were run where appropriate. Ninety-six articles were reviewed. The most common duration was 7-14days and the most common method of HA was the controlled work-rate approach. HA had a moderately beneficial effect on exercise capacity and performance in the heat irrespective of regimen; however, longer regimens were more effective than shorter approaches. HA had a moderate-to-large beneficial effect on lowering core body temperature before and during exercise, maintaining cardiovascular stability, and improving heat-loss pathways. Data are limited but HA may reduce oxygen consumption during subsequent exercise, improve glycogen sparing, increase the power output at lactate threshold, reduce lactate concentrations during exercise, have a trivial effect on increasing extracellular concentrations of Hsp, and improve perceived ratings of exertion and thermal sensation. HA regimens lasting <14days induce many beneficial physiological and perceptual adaptations to high ambient temperatures, and improve subsequent exercise performance and capacity in the heat; however, the extent of the adaptations is greatest when HA regimens lasting longer than 14days are adopted. Large methodological differences in the HA literature mean that there is still uncertainty regarding the magnitude and time course of potential adaptation for a number of physiological and perceptual variables.

Evaporative heat loss in Bos taurus: Do different cattle breeds cope with heat stress in the same way?

The aim of this study was to compare two Portuguese (Alentejana and Mertolenga) and two exotic (Frisian and Limousine) cattle breeds in terms of the relationship between the increase in ambient temperature and the responses of the evaporative heat loss pathways and the effects on homeothermy. In the experiment, six heifers of the Alentejana, Frisian, and Mertolenga breeds and four heifers of the Limousine breed were used. The animals were placed in four temperature levels, the first one under thermoneutral conditions and the other ones with increase levels of thermal stress. When submitted to severe heat stress, the Frisian developed high thermal tachypnea (125mov/min) and moderate sweating rates (117gm−2h−1), which did not prevent an increase in the rectal temperature (from 38.4°C to 40.0°C). Moderate increases in rectal temperature were observed in the Alentejana (from 38.8°C to 39.4°C) and Limousine (from 38.6°C to 39.4°C), especially in the period of highest heat stress. The Limousine showed moderate levels of tachypnea (101mov/min) while showing the lowest sweating rates. The Alentejana showed significant increases in sweating rate (156gm−2h−1) that played a major role in homeothermy. The Mertolenga showed a superior stability of body temperature, even in the period of highest heat stress (from 38.5°C to 39.1°C). Uncommonly, the maintenance of homeothermy during moderate heat stress was achieved primarily by intense tachypnea (122mov/min). The sweating rate remained abnormally low under conditions of moderate heat stress, rising significantly (110gm−2h−1) without evidence of stabilization, only when tendency for heat storage occurred. This unusual response of the evaporative heat loss pathways infers a different thermoregulatory strategy, suggesting a different adaptation to semi-arid environment and strong association with water metabolism.

Change in Heat Loss after Light-Short Exercise and Food Intake

Changes in heat loss after light-short exercise and unwarmed food intake were investigated using partitional calorimetry to test the hypothesis that heat loss following these treatments is due to sweat evaporation (Esk). Four Japanese male subjects exercised at 30% of their maximum workload for 10 min and rested for 60 min. They ate 200 g almonds, drank 100 g green tea and rested for 120 min on different occasion. Rectal temperature, oxygen consumption, body weight, skin temperatures and local heat flow rates at seven sites on the skin surface were measured at 2-min intervals. The results demonstrated that the heat stored during exercise was dissipated by Esk with a 16 min delay whereas the heat produced by diet induced thermogenesis was dissipated by Esk without delay. Therefore, it was demonstrated that the supplemental heat loss pathway after both light-short exercise and unwarmed food intake was Esk.

The effect of menthol application to the skin on sweating rate response during exercise in swimmers and controls

We tested the hypothesis that menthol application would reduce the magnitude and initiation of sweating via excitation of cold-sensitive afferent pathways and concurrently via a cross-inhibition of heat loss pathways in acclimatized (swimmers, SW) and non acclimatized (control, CON) subjects in cool water. It was expected this effect to be exaggerated in SW subjects. Eight SW and eight CON subjects cycled at 60% of their VO(2)max, as long as to reach 38 degrees C in rectal temperature (Tre), without or with (4.6 g per 100 ml of water) all-body application of menthol sediment. Heart rate (HR), Tre, sweating rate (SwR), the proximal-distal skin temperature gradient (TSk(f-f)), and oxygen consumption (VO(2)) were measured continuously. VO(2) and HR were similar between groups and conditions. Menthol increased TSk(f-f), Tre threshold for SwR [+0.32 (0.01) degrees C] and Tre gain, while menthol reduced exercise time by 8.1 (4.1) min. SW group showed higher changes in Tre threshold for SwR [+0.50 (0.01) degrees C for SW vs. +0.13 (0.03) degrees C for CON], higher Tre gain, lower time for Tre increase and shorter exercise time [-10.7 (7) min for SW vs. -4.9 (4) min for CON] in menthol condition. Upon exercise initiation, previously applied menthol on the skin seems to induce vasoconstriction, results in a delayed sweating, which in turn affects the rectal temperature. Acclimatized subjects showed higher delay in SwR and earlier rise in Tre, which most probably is due to the inter-group differences in cold receptors activity.

Understanding thermal integration issues and heat loss pathways in a planar microscale fuel processor: Demonstration of an integrated silicon microreactor-based methanol steam reformer

Methanol reforming offers an attractive source of hydrogen for proton exchange membrane fuel cell (PEMFC) systems for portable power. At present, the greatest obstacle to realization of these systems is the extraction of hydrogen from the hydrocarbon fuel, i.e., the fuel processing part. Because of size and portability, microchemical technology has shown assuring results in the field of fuel processing. However, several roadblocks persist in the development of an integrated micro-fuel processor and fuel cell system. Thermal management in miniature systems is perhaps the most crucial of these challenges. In this study, a silicon microreactor-based catalytic methanol steam reforming reactor was demonstrated in the context of complete thermal integration to understand this critical issue. Detailed experiments were carried out to quantify heat losses through various pathways from the planar microreactor structure. Based on these experiments, an empirical correlation is developed to predict natural convection heat transfer coefficient from meso to microscale devices. The result provides fundamental insight of critical thermal issues such as transfer of heat between reactor components, control of temperature, insulation, and heat losses. Based on this understanding, suggestions are made for scale up of reactor components and a packaging scheme for reduction of convective and radiative losses.

Risperidone-Induced Neuroleptic Malignant Syndrome in Neurodegenerative Disease

Risperidone-Induced Neuroleptic Malignant Syndrome in Neurodegenerative Disease

Evidence for heat losses via party wall cavities in masonry construction

This paper presents empirical evidence and analysis that supports the existence of a significant heat loss mechanism resulting from air movement through cavities in party walls in masonry construction. A range of heat loss experiments were undertaken as part of the Stamford Brook housing field trial in Altrincham in the United Kingdom. Co-heating tests showed a large discrepancy between the predicted and measured whole house heat loss coefficients. Analysis of the co-heating results, along with internal temperature data, thermal imaging and a theoretical analysis indicated that the most likely explanation for the discrepancy was bypassing of the thermal insulation via the uninsulated party wall cavities. The data show that such a bypass mechanism is potentially the largest single contributor to heat loss in terraced dwellings built to the 2006 revision of the Building Regulations. A comparable convective heat bypass associated with masonry party walls was identified in the late 1970s during the course of the Twin Rivers Project in the United States, albeit in a somewhat different construction from that used at Stamford Brook. A similar effect was also reported in the United Kingdom in the mid 1990s. However, it appears that no action was taken at that time either to confirm the results, to develop any technical solutions, or to amend standards for calculating heat losses from buildings. Current conventions for heat loss calculations in the United Kingdom do not take account of heat losses associated with party walls and it is suggested by the authors that such conventions may need to be updated to take account of the effect described in this paper. In the final part of the paper, the authors propose straightforward solutions to prevent bypassing of roof insulation via party walls by for example filling the cavity of the party wall with mineral fibre insulation, or by inserting a cavity closer across the cavity in the plane of the roof insulation. Practical application: The heat bypass mechanism described in this paper is believed by the authors to contribute to a significant proportion of heat loss from buildings in the UK constructed with clear cavities such as those found in separating walls between cavity masonry dwellings. It is proposed that relatively simple design changes could be undertaken to eliminate such heat loss pathways from new buildings. In addition, simple and cost effective measures are envisaged that could be used to minimise or eliminate the bypass from existing buildings. Such an approach could give rise to a significant reduction in carbon emissions from UK housing. a The Department for Communities and Local Government, previously the Office of the Deputy Prime Minister, ODPM. References produced before the change of name are listed under ODPM. b Difficulties in the recruitment of households have delayed the long term monitoring programme to 2007 but, fortuitously, this has provided the capacity for a more detailed investigation of the party wall heat loss issues discussed in this paper with further co-heating tests (incorporating a more detailed measurements) planned for the winter of 2006/07. c A co-heating test involves electrically heating the inside of a house to a constant temperature over a period of several weeks. Correlation of the measured electrical heat input with external temperature and solar insolation then allows an estimation of the total dwelling heat loss coefficient. d House A is an end-of-terrace house with one adjacent house. House E is a mid-terrace house with two adjacent houses, D and F. e In the UK, the regulations consist of a broadly framed, performance based, legal requirement supported by detailed technical guidance on compliance. Such guidance is provided by a system of approved documents issued by the Secretary of State. The current methodology for dwelling heat loss calculations is contained in the Standard Assessment Procedure 200512 which is invoked by regulation 17A and Approved Document L1A.2 f The Building Regulations do limit the leakage of air from within each dwelling into the cavity through an overall limit on dwelling permeability. g The higher-than-normal inside-outside temperature difference during the co-heating tests implies that background ventilation rates and ventilation heat loss coefficients during these tests are likely to have exceeded those in the table. Calculations based on a semi-analytical model of air flow14 suggest that the increase is of the order of 3 WK—1 (10%) for the two-storey house A and 15 WK—1 (27%) for the three-storey house E. h Note that the correction is only used to improve the graphical presentation of the data. The heat loss coefficient is calculated directly from multiple regression of heating power against ΔT and S.

Perioperative shivering: physiology and pharmacology.

IN homeothermic species, a thermoregulatory system coordinates defenses against cold and heat to maintain internal body temperature within a narrow range, thus optimizing normal physiologic and metabolic function. The combination of anesthetic-induced thermoregulatory impairment and exposure to a cool environment makes most unwarmed surgical patients hypothermic. Although shivering is but one consequence of perioperative hypothermia, and rarely the most serious, it occurs frequently (i.e., 40–60% after volatile anesthetics), and it remains poorly understood. While coldinduced thermoregulatory shivering remains an obvious etiology, the phenomenon has also been attributed to numerous other causes. Our first goal is to review the organization of the thermoregulatory system, and particularly the physiology of postanesthetic shivering. We then discuss the pharmacology of thermoregulation and review the putative mechanisms and sites of action of various antishivering drugs.

Effect of central and peripheral cholinergic antagonists on chlorpyrifos-induced changes in body temperature in the rat

Exposure to the organophosphate (OP)-based pesticide chlorpyrifos (CHP) in the rat results in an initial period of hypothermia lasting <24 h, followed by a fever lasting 48–72 h. The purpose of this study was to determine how cholinergic pathways participate in the mediation of the thermoregulatory effects of CHP. The corn oil (CO) vehicle or CHP (25 mg/kg; p.o.) was administered to female rats while core temperature ( T c) and motor activity (MA) were monitored by radiotelemetry. The peripheral muscarinic antagonist, methyl scopolamine (MS) and central antagonist, scopolamine (S) were administered during the period of CHP-induced hypothermia and fever. The hypothermia was attenuated by scopolamine but not by methyl scopolamine. The delayed fever was augmented by scopolamine but blocked by methyl scopolamine. The results indicate that CHP-induced hypothermia is mediated by cholinergic stimulation of heat loss pathways in CNS thermoregulatory centers. Peripheral cholinergic pathways appear to have a minimal role in mediating chlorpyrifos-induced hypothermia. On the other hand, the chlorpyrifos-induced fever appears to be mediated by a peripheral pathway that is blocked by methyl scopolamine. The data provides a possible explanation for the persistent fever in humans exposed to OP pesticides and treated with atropine. Methyl atropine or methyl scopolamine may be a more effective therapy in the treatment of the fever.

Sympathoadrenal hyperactivity and the etiology of neuroleptic malignant syndrome.

The author's goal was to develop a pathophysiological model for neuroleptic malignant syndrome with greater explanatory power than the alternative hypotheses of hypothalamic dopamine antagonism (elevated set point) and direct myotoxicity (malignant hyperthermia variant). Published clinical findings on neuroleptic malignant syndrome were integrated with data from human and animal studies of muscle physiology, thermoregulation, and autonomic nervous system function. The data show that the sympathetic nervous system's latent capacity for autonomous activity is expressed when tonic inhibitory inputs from higher central nervous system centers are disrupted. These tonic inhibitory inputs are relayed to preganglionic sympathetic neurons by way of dopaminergic hypothalamospinal tracts. The sympathetic nervous system mediates hypothalamic coordination of thermoregulatory activity and is a primary regulator of muscle tone and thermogenesis, augmenting both of these when stimulated. In addition, the sympathetic nervous system modulates all of the other end-organs that function abnormally in neuroleptic malignant syndrome. There is substantial evidence to support the hypothesis that dysregulated sympathetic nervous system hyperactivity is responsible for most, if not all, features of neuroleptic malignant syndrome. A predisposition to more extreme sympathetic nervous system activation and/or dysfunction in response to emotional or psychological stress may constitute a trait vulnerability for neuroleptic malignant syndrome, which, when coupled with state variables such as acute psychic distress or dopamine receptor antagonism, produces the clinical syndrome of neuroleptic malignant syndrome. This hypothesis provides a more comprehensive explanation for existing clinical data than do the current alternatives.

The possible central role of GABA in thermoregulation in the sheep

Injections of GABA and its agonist muscimol into a lateral cerebral ventricle of sheep depressed both evaporative heat loss (40 and 58% respectively) at 41°C ambient temperature ( T a) and metabolic heat production (27 and 32% respectively) at 3°C T a. The intracerebroventricular (ICV) injection of the GABA antagonist, bicuculline (Bi), prior to an ICV injection of either serotonin (5-HT) or carbamyl choline (CCh) prevented the inhibitory effects of an ICV injection of 5-HT at 3°C or CCh at 41°C on heat production and heat loss respectively [Bligh et al., J. Physiol., Lond. 212, 377–392 (1971)]. Simultaneously the inhibitory effects of hot and cold ambient temperatures on heat production and heat loss, respectively, were blocked and both the heat production and heat loss pathways were activated concurrently. These results suggest that GABA may be a terminal transmitter of reciprocal inhibition between the two major thermosensor to thermoregulatory effector pathways in the sheep.

Alterations in α-Adrenergic and Muscarinic Cholinergic Receptor Binding in Rat Brain Following Nonionizing Radiation

Microwave radiation produces hyperthermia. The mammalian thermoregulatory system defends against changes in temperature by mobilizing diverse control mechanisms. Neurotransmitters play a major role in eliciting thermoregulatory responses. The involvement of adrenergic and muscarinic cholinergic receptors was investigated in radiation-induced hyperthermia. Rats were subjected to radiation at 700 MHz frequency and 15 mW/cm2 power density and the body temperature was raised by 2.5 degrees C. Of six brain regions investigated only the hypothalamus showed significant changes in receptor states, confirming its pivotal role in thermoregulation. Adrenergic receptors, studied by [3H]clonidine binding, showed a 36% decrease in binding following radiation after a 2.5 degrees C increase in body temperature, suggesting a mechanism to facilitate norepinephrine release. Norepinephrine may be speculated to maintain thermal homeostasis by activating heat dissipation. Muscarinic cholinergic receptors, studied by [3H]quinuclidinyl benzilate binding, showed a 65% increase in binding at the onset of radiation. This may be attributed to the release of acetylcholine in the hypothalamus in response to heat cumulation. The continued elevated binding during the period of cooling after radiation was shut off may suggest the existence of an extra-hypothalamic heat-loss pathway.

Neuroanatomical mapping of hypothalamic regions mediating verapamil hyper- and hypothermia in the cat

Guide cannulae for microinjection and push-pull perfusion in the unrestrained cat were implanted bilaterally in the anterior hypothalamic, prepotic area (AH/POA) and posterior hypothalamus (PH). Postoperatively, the region was first identified in AH/POA which was reactive to norepinephrine or in PH to excess Ca ++ ions; in both cases a hypothermie response was produced. Then either an artificial CSF control vehicle or the Ca ++ ion channel blocking agent, verapamil, was perfused for 30 min by means of push-pull cannulae at a rate of 25.0 μl/min. Verapamil (0.4, 2.0 and 4.0 μg/μl) induced a concentrationdependent hypothermia when perfused within AH/POA sites but hyperthermia when perfused in the caudal hypothalamus. An anatomical analysis of the sites of perfusion revealed that verapamil's thermolytic effect was localized within the classical thermosensitive region of the cat's diencephalon, a region ventral to the anterior commissure and dorsal to the optic chiasm. On the other hand, the loci in which verapamil evoked thermogenesis were localized to a region dorsal to the mammillary bodies and caudal to the descending columns of the fornix. It is suggested that verapamil interferes with Ca ++ ion channels in the PH to shift the cat's “set-point” temperature. Conversely, however, verapamil apparently could act on catecholaminergic terminals in AH/POA to enhance the presynaptic release of norepinephrine which, in turn, stimulates the heat loss pathway to yield hypothermia.

5-HT receptors and hyper- or hypothermia: Elucidation by catecholamine antagonists injected into the cat hypothalamus

The receptor mechanisms in the hypothalamus and preoptic area which mediate thermoregulatory changes produced by serotonin (5-HT) were examined in the unrestrained cat. Stainless steel guide tubes were implanted just above the rostral diencephalon or preoptic area in each of twelve cats. 5-HT in a dose of 1.5 to 10.5 μg was micro-injected in a volume of 0.75 to 1.5 μl into a total of 256 test sites in the hypothalamus. An individual site at which 5-HT evoked a rise or a fall in core temperature of ⩾ 0.5°C within 30 min was considered to be reactive to the indoleamine. Within such a region of maximum sensitivity, i.e., the cat's anterior hypothalamic, preoptic area (AH/POA), either norepinephrine (NE) or dopamine (DA) micro-injected in a dose of 2.33 to 14.0 μg in similar volumes evoked only a dose-dependent decline in the body temperature of the cat. 5-HT was found to cause either hyper- or hypothermia. The direction of temperature change caused by 5-HT was dependent principally upon the neuroanatomical locus of injection. Each of the 5-HT reactive sites was characterized pharmacologically by their pre-treatment with phentolamine (PHT), d-butaclamol (BUTAC) or methysergide (METHY) in doses of 1.0 to 10.0 μg. At the most rostral sites in POA, the catecholamine receptor antagonists selectively delayed the onset of the 5-HT hypothermia and limited the magnitude of the response. Within sites located in the AH in which 5-HT induced only a rise in body temperature, prior treatment of the site with the catecholamine receptor antagonists PHT or BUTAC failed to modify the response. These results indicate that 5-HT in producing hypothermia not only may saturate the 5-HT receptor sites but also may be taken up by catecholamine receptors which mediate the diencephalic heat loss pathway.

The function of dopamine receptors in the central thermoregulatory pathways of the rat

At neutral (25°C) and low (17°C) ambient temperatures apomorphine exerted a hypothermic effect in the rat, apparently through an action on dopamine receptors in the hypothalamic thermoregulatory pathways. This fall in core temperature was brought about by a co-ordinated response of thermoregulatory effectors including; vasodilation, reduced heat production, as indicated by decreased oxygen consumption, and an increase in evaporative water loss. At high ambient temperatures (35 C), when vasodilation was already maximal, apomorphine increased core temperature apparently through an increase in metabolic heat production. At this elevated ambient temperature the rat attempted to compensate for the apomorphine-induced hyperthermia by an increase in evaporative water loss. These results support the hypothesis that dopamine receptors play an important physiological role in the heat loss pathways, but provide no evidence for such a role in heat gain pathways.

Effects of apomorphine and pimozide on temperature regulation during exercise in the rat.

The purpose of this study was to determine the effects of a specific dopamine receptor agonist (apomorphine) and antagonist (pimozide) on thermoregulation when the heat loss pathway was activated by the stress of exercise. Apomorphine or its control vehicle (0.9% wt/vol saline) was injected systemically (0.5, 1.0, 1.5 mg/kg ip) or within the preoptic-anterior hypothalamus (5, 10, 20 micrograms) immediately before the start of treadmill exercise at 21.5 m/min. Colonic, tailskin, and ambient temperatures were recorded each minute. Oxygen consumption was calculated from on-line measurements of percent O2 and CO2. Pimozide injected systemically (0.5 mg/kg ip) had no effect on resting colonic temperature, but caused a significant (P less than 0.05) hyperthermia during treadmill exercise compared to saline controls. Central and systemic injections of apomorphine caused a dose-dependent hypothermia that was blocked by pretreatment with pimozide. Oxygen uptake values during exercise following the central injection of apomorphine were virtually identical to those following the injection of saline, but colonic temperature was significantly (P less than 0.05) lower than saline controls, indicating that the hypothermia observed was not due to a reduction in metabolic rate. These data indicate that dopamine receptors in the preoptic-anterior hypothalamus of the rat participate in the mediation of heat dissipation when the animal is challenged with a heat stress.

The influence of 4-methylhistamine, an agonist of histamine H 2 receptors on body temperature in rats

4-Methylhistamine, an agonist of histamine H 2 receptors, injected directly into the lateral ventricle of the rat brain, produced a dose-dependent hypothermia, which was antagonized by cimetidine, an antagonist of histamine H 2 receptors. The action of 4-methyhistamine was abolished at an ambient temperature of 32°C. The thermoregulatory behaviour of rats after injection of 4-methylhistamine tended to compensate for the fall of body temperature. It is concluded that the hypothermic action of 4-methylhistamine is due to stimulation of heat loss pathways.