Heat transfer in a microvascular network: the effect of heart rate on heating and cooling in reptiles (Pogona barbata and Varanus varius)

Thermally-induced changes in heart rate and blood flow in reptiles are believed to be of selective advantage by allowing animal to exert some control over rates of heating and cooling. This notion has become one of the principal paradigms in reptilian thermal physiology. However, the functional significance of changes in heart rate is unclear, because the effect of heart rate and blood flow on total animal heat transfer is not known. I used heat transfer theory to determine the importance of heat transfer by blood flow relative to conduction. I validated theoretical predictions by comparing them with field data from two species of lizard, bearded dragons (Pogona barbata) and lace monitors (Varanus varius). Heart rates measured in free-ranging lizards in the field were significantly higher during heating than during cooling, and heart rates decreased with body mass. Convective heat transfer by blood flow increased with heart rate. Rates of heat transfer by both blood flow and conduction decreased with mass, but the mass scaling exponents were different. Hence, rate of conductive heat transfer decreased more rapidly with increasing mass than did heat transfer by blood flow, so that the relative importance of blood flow in total animal heat transfer increased with mass. The functional significance of changes in heart rate and, hence, rates of heat transfer, in response to heating and cooling in lizards was quantified. For example, by increasing heart rate when entering a heating environment in the morning, and decreasing heart rate when the environment cools in the evening a Pogona can spend up to 44 min longer per day with body temperature within its preferred range. It was concluded that changes in heart rate in response to heating and cooling confer a selective advantage at least on reptiles of mass similar to that of the study animals (0. 21-5.6 kg).     

Coloration affects heating and cooling in three color morphs of the Australian bluetongue lizard,Tiliqua scincoides

The color-mediated thermoregulation hypothesis predicts that dark body color (low reflectance) allows organisms to gain heat more efficiently than does pale coloration (high reflectance). This prediction is intuitive and widely assumed to be true, but has poor empirical support. We used rare, captive-bred, mutant melanistic, albino and wild-type Australian bluetongue lizards, Tiliqua scincoides to measure the effects of skin reflectance on the heating and cooling rates. We measured heating under an artificial radiant heat source and cooling rates in an ice-cooled box using live lizards in a room with still air. The effect of skin reflectance on heat transfer was clear, despite the substantial influence of body size. Melanistic T. scincoides showed low reflectance and gained heat faster than highly reflective albinos. Melanistic lizards also lost heat faster than albinos. Wild-type lizards were intermediate in reflectance, gained heat at rates indistinguishable from melanistic lizards, and lost heat at rates indistinguishable from albino lizards. This study system allowed us to control for variables that were confounded in other studies and may explain the inconsistent support for the color-mediated thermoregulation hypothesis. Our results provide clear evidence that skin reflectance influences the rate of heating and cooling in ectotherms.     

PCR amplification using electrolytic resistance for heating and temperature monitoring

An alternative method of rapid-cycle PCR for DNA amplification is demonstrated using electrolyte resistance for heating and temperature monitoring. The PCR amplification solution is electrically conductive and can be heated by passing an alternating current through the sample. The temperature of the solution is evaluated by monitoring its electrical resistance. Cooling is accomplished by forced air convection at ambient temperature. Heating and cooling rates of up to 20 degrees C/s were achieved. The 35 cycles of PCR were completed in less than 12 min with product yields equivalent to conventional temperature cycling. Electrolyte resistance provides a method for both direct heating and monitoring the temperature of PCR samples.     

Skin and core temperature response to partial- and whole-body heating and cooling

1. Human subjects were exposed to partial- and whole-body heating and cooling in a controlled environmental chamber to quantify physiological and subjective responses to thermal asymmetries and transients.

2. Skin temperatures, core temperature, thermal sensation, and comfort responses were collected for 19 local body parts and for the whole body.

3. Core temperature increased in response to skin cooling and decreased in response to skin heating.

4. Hand and finger temperatures fluctuated significantly when the body was near a neutral thermal state.

5. When using a computer mouse in a cool environment, the skin temperature of the hand using the mouse was observed to be 2–3 °C lower than the unencumbered hand.     

Physiological and ecological implications of a simple model of heating and cooling in reptiles

A model of heat exchange in reptiles is used to investigate the role of blood flow in controlling rates of heating/cooling in animals in complex thermal environments. The model suggests an allometry of heating and cooling time constants and of the effects of blood flow on those time constants that accords with published data. The model suggests a simple physical reason for the increased effect of blood flow on time constants in large animals. Two tools (the model and an impulse response method) are presented to allow projection of body temperatures in complex thermal habitats. Application of the model to ecologically important situations suggest that mass, blood flow, and shuttling schedules affect the rate of heating and cooling and the effect of blood flow on the range of body temperatures experienced.     

The effect of rate of heating or cooling prior to heating on tumor and normal tissue microcirculatory blood flow

Single vessel responses to hyperthermia were studied in tumor and normal tissues using a transparent access window chamber. Rates of heating less than or equal to .68 degrees C/minute preserved relatively better vascular function in normal than tumor tissue. A rate of heating of 1.0 degrees C/minute lowered normal tissue statis temperatures so they were no different from tumor. Cooling to 30 degrees C prior to heating slowed normal arteriolar flows to less than 5% of 38 degrees C controls. Heating resulted in increased flow in those vessels, but maximum flows never exceeded 5% of flows achieved in similar vessels which were not cooled first. The implications of this work are that rate of heating and cooling prior to heating can alter normal tissue vascular response to heat in a way that could prove deleterious to maintaining efficient vascular function in that tissue relative to tumor.     

Influence of heat transmission mode on heating rates and on the selection of patches for heating in a mediterranean lizard

Heliothermy (heat gain by radiation) has been given a prominent role in basking lizards. However, thigmothermy (heat gain by conduction) could be relevant for heating in small lizards. To ascertain the importance of the different heat transmission modes to the thermoregulatory processes, we conducted an experimental study where we analyzed the role of heat transmission modes on heating rates and on the selection of sites for heating in the Mediterranean lizard Acanthodactylus erythrurus (Lacertidae). The study was conducted under laboratory conditions, where two situations of different operative temperatures (38 degrees and 50 degrees C) were simulated in a terrarium. In a first experiment, individuals were allowed to heat up during 2 min at both temperatures and under both heat transmission modes. In a second experiment, individuals were allowed to select between patches differing in the main transmission mode, at both temperatures, to heat up. Experiences were conducted with live, nontethered lizards with a starting body temperature of 27 degrees C. Temperature had a significant effect on the heating rate, with heat gain per unit of time being faster at the higher operative temperature (50 degrees C). The effect of the mode of heat transmission on the heating rate was also significant: at 50 degrees C, heating rate was greater when the main heat transmission mode was conduction from the substrate (thigmothermy) than when heating was mainly due to heat gain by radiation (heliothermy); at 38 degrees C, heating rates did not significantly differ between transmission modes. At 38 degrees C, selection of the site for heating was not significantly different from that expected by chance. However, at 50 degrees C, the heating site offering the slowest heating rate (heliothermic patch) was selected. These results show that heating rates vary not only with environmental temperature but also with different predominant heat transmission modes. Lizards are able to identify and exploit this heterogeneity, selecting the source of heat gain (radiation) that minimizes the risk of overheating when temperature is high.     

Prostaglandins are important in thermoregulation of a reptile (Pogona vitticeps)

The effectiveness of behavioural thermoregulation in reptiles is amplified by cardiovascular responses, particularly by differential rates of heart beat in response to heating and cooling (heart-rate hysteresis). Heart-rate hysteresis is ecologically important in most lineages of ectothermic reptile, and we demonstrate that heart-rate hysteresis in the lizard Pogona vitticeps is mediated by prostaglandins. In a control treatment (administration of saline), heart rates during heating were significantly faster than during cooling at any given body temperature. When cyclooxygenase 1 and 2 enzymes were inhibited, heart rates during heating were not significantly different from those during cooling. Administration of agonists showed that thromboxane B(2) did not have a significant effect on heart rate, but prostacyclin and prostaglandin F(2alpha) caused a significant increase (3.5 and 13.6 beats min(-1), respectively) in heart rate compared with control treatments. We speculate that heart-rate hysteresis evolved as a thermoregulatory mechanism that may ultimately be controlled by neurally induced stimulation of nitric oxide production, or maybe via photolytically induced production of vitamin D.     

Integration of autonomic and local mechanisms in regulating cardiovascular responses to heating and cooling in a reptile (<Emphasis Type="Italic">Crocodylus porosus</Emphasis>)

Reptiles change heart rate and blood flow patterns in response to heating and cooling, thereby decreasing the behavioural cost of thermoregulation. We tested the hypothesis that locally produced vasoactive substances, nitric oxide and prostaglandins, mediate the cardiovascular response of reptiles to heat. Heart rate and blood pressure were measured in eight crocodiles (Crocodylus porosus) during heating and cooling and while sequentially inhibiting nitric-oxide synthase and cyclooxygenase enzymes. Heart rate and blood pressure were significantly higher during heating than during cooling in all treatments. Power spectral density of heart rate and blood pressure increased significantly during heating and cooling compared to the preceding period of thermal equilibrium. Spectral density of heart rate in the high frequency band (0.19–0.70 Hz) was significantly greater during cooling in the saline treatment compared to when nitric-oxide synthase and cyclooxygenase enzymes were inhibited. Cross spectral analysis showed that changes in blood pressure preceded heart rate changes at low frequencies (<0.1 Hz) only. We conclude that the autonomic nervous system controls heart rate independently from blood pressure at higher frequencies while blood pressure changes determine heart rate at lower frequencies. Nitric oxide and prostaglandins do not control the characteristic heart rate hysteresis response to heat in C. porosus, although nitric oxide was important in buffering blood pressure against changes in heart rate during cooling, and inhibition caused a compensatory decrease in parasympathetic stimulation of the heart.     

Redistribution of blood within the body is important for thermoregulation in an ectothermic vertebrate (<Emphasis Type="Italic">Crocodylus porosus</Emphasis>)

Changes in blood flow are a principal mechanism of thermoregulation in vertebrates. Changes in heart rate will alter blood flow, although multiple demands for limited cardiac output may compromise effective thermoregulation. We tested the hypothesis that regional differences in blood flow during heating and cooling can occur independently from changes in heart rate. We measured heart rate and blood pressure concurrently with blood flow in the crocodile, Crocodylus porosus. We measured changes in blood flow by laser Doppler flowmetry, and by injecting coloured microspheres. All measurements were made under different heat loads, with and without blocking cholinergic and β-adrenergic receptors (autonomic blockade). Heart rates were significantly faster during heating than cooling in the control animals, but not when autonomic receptors were blocked. There were no significant differences in blood flow distribution between the control and autonomic blockade treatments. In both treatments, blood flow was directed to the dorsal skin and muscle and away from the tail and duodenum during heating. When the heat source was switched off, there was a redistribution of blood from the dorsal surface to the duodenum. Blood flow to the leg skin and muscle, and to the liver did not change significantly with thermal state. Blood pressure was significantly higher during the autonomic blockade than during the control. Thermal time constants of heating and cooling were unaffected by the blockade of autonomic receptors. We concluded that animals partially compensated for a lack of differential heart rates during heating and cooling by redistributing blood within the body, and by increasing blood pressure to increase flow. Hence, measures of heart rate alone are insufficient to assess physiological thermoregulation in reptiles.     

Effects of heating and cooling on nerve terminal impulses recorded from cold-sensitive receptors in the guinea-pig cornea

An in vitro preparation of the guinea-pig cornea was used to study the effects of changing temperature on nerve terminal impulses recorded extracellularly from cold-sensitive receptors. At a stable holding temperature (31-32.5 degrees C), cold receptors had an ongoing periodic discharge of nerve terminal impulses. This activity decreased or ceased with heating and increased with cooling. Reducing the rate of temperature change reduced the respective effects of heating and cooling on nerve terminal impulse frequency. In addition to changes in the frequency of activity, nerve terminal impulse shape also changed with heating and cooling. At the same ambient temperature, nerve terminal impulses were larger in amplitude and faster in time course during heating than those recorded during cooling. The magnitude of these effects of heating and cooling on nerve terminal impulse shape was reduced if the rate of temperature change was slowed. At 29, 31.5, and 35 degrees C, a train of 50 electrical stimuli delivered to the ciliary nerves at 10-40 Hz produced a progressive increase in the amplitude of successive nerve terminal impulses evoked during the train. Therefore, it is unlikely that the reduction in nerve terminal impulse amplitude observed during cooling is due to the activity-dependent changes in the nerve terminal produced by the concomitant increase in impulse frequency. Instead, the differences in nerve terminal impulse shape observed at the same ambient temperature during heating and cooling may reflect changes in the membrane potential of the nerve terminal associated with thermal transduction.     

Effect of cooling and heating on the regional distribution of blood flow in fetal sheep

This is a study on the effect of cooling and heating amniotic fluid on blood flow to fetal tissues and organs. In 8 unanaesthetized, chronically-catheterised fetal sheep (129-137 days gestation) cold or warm water was passed through tubing encircling the fetus in utero and blood flow was measured using the radionuclide-labelled 15 mu spheres. Following cooling for 30 min, amniotic fluid temperature fell 9.6 degrees C to 29.9 +/- 2.1 degrees C (SEM) fetal arterial temperature fell 2.37 degrees C to 37.30 +/- 0.36, and maternal arterial temperature fell 0.53 degrees C to 38.58 +/- 0.16. Blood flow through the fetal skin fell 60% (P less than 0.01) to 13.6 ml/min per 100 g tissue. Blood flow to the brown fat increased 186% (P less than 0.05) to 99.6 ml/min per 100 g. Following warming for 20 min, fetal temperature rose to 40.43 +/- 0.19 degrees C, and skin blood flow did not change significantly relative to initial control period but rose 200% above that during cooling (P less than 0.01). During both cooling and heating, blood flow to the adrenals rose significantly (P less than 0.05) whereas flow to the carcass, brain, kidneys, and placenta was not altered detectably. Continuous sampling of blood from the inferior vena cava during microsphere injection failed to detect any evidence of arterio-venous shunting through the skin at any temperature studied. Overall, the blood flow responses are consistent with a thermoregulatory role for the skin and brown fat in the near-term fetal sheep.     

Modulation of arterial baroreflex control of heart rate by skin cooling and heating in humans.

The purpose of this study was to examine the effects of skin cooling and heating on the heart rate (HR) control by the arterial baroreflex in humans. The subjects were 15 healthy men who underwent whole body thermal stress (esophageal temperatures, approximately 36.8 and approximately 37.0 degrees C; mean skin temperatures, approximately 26.4 and approximately 37.7 degrees C, in skin cooling and heating, respectively) produced by a cool or hot water-perfused suit during supine rest. The overall arterial baroreflex sensitivity in the HR control was calculated from spontaneous changes in beat-to-beat arterial pressure and HR during normothermic control and thermal stress periods. The carotid baroreflex sensitivity was evaluated from the maximum slope of the HR response to changes in carotid distending pressure, calculated as mean arterial pressure minus neck pressure. The overall arterial baroreflex sensitivity at existing arterial pressure increased during cooling (-1.32 +/- 0.25 vs. -2.13 +/- 0.20 beats. min(-1). mmHg(-1) in the control and cooling periods, respectively, P < 0.05), whereas it did not change significantly during heating (-1.39 +/- 0. 23 vs. -1.40 +/- 0.15 beats. min(-1). mmHg(-1) in the control and heating periods, respectively). Neither the cool nor heat loadings altered the carotid baroreflex sensitivity in the HR control. These results suggest that the sensitivity of HR control by the extracarotid (presumably aortic) baroreflex was augmented by whole body skin cooling, whereas the sensitivities of HR control by arterial baroreflex remain unchanged during mild whole body heating in humans.     

Effects of emissions from different type of residential heating upon cyclic guanosine monophosphate (cGMP) in blood platelets of residents

We hypothesized that different types of residential heating would be associated with different levels of indoor carbon monoxide (CO) and further that this might result in a differential in the concentration of cyclic 3′:5′ guanosine monophosphate (cGMP) in blood platelets in exposed residents. Individuals, who were recruited from homes using different fuel for heating, donated a venous blood sample in the winter and in the summer. In the winter the median blood platelet cGMP value for the group using liquid propane gas (LPG) was 65% higher than for the group using piped natural gas for heating (p <0.001). Also in the group using LPG, the median concentration of cGMP in the winter was 39% higher than the summer median (p?<?0.003). The mean indoor concentrations of CO were measured over a period of 1 week during the winter and were <1 ppm. We conclude that observed differences were associated with emissions from different types of heating but that CO exposure alone is too low to explain these.     

Hysteresis on heating and cooling of E. coli alkaline phosphatase

Measurements of [theta](222) of E. coli phosphatase on heating from 20 degrees to 90 degrees and subsequent cooling to 20 degrees shows a gradual increase in [theta](222) on heating, while cooling shows a symmetric transition centered at 45 degrees . Reheating and cooling shows the same phenomenon. Enzyme heated and cooled once is fully active. The activity of the enzyme depends on its storage conditions (buffer and pH for example), but such changes are least to some extent reversible, especially by heating in different solvents. We conclude the enzyme exists in several forms which are in slow equilibrium with each other, so that the enzyme responds slowly when heated and hence is not at equilibrium during heating/cooling experiments.     

Effect of peripheral heating and cooling on spinal reflex activity

The effects of heating and cooling the skin of the thigh and leg on the character of changes in monosynaptic and polysynaptic reflexes and of the first negative wave (N_`) of the dorsal cord potential was studied in acute experiments on lightly anesthetized cats. During cooling of the skin the amplitude of the monosynaptic reflex and N₁-wave was increased while the polysynaptic reflex was inhibited; during heating the amplitude of the monosynaptic reflex and N₁-wave was reduced while the polysynaptic reflex was increased. After de-efferentation the cooling effect was reduced, and after division of the spinal cord responses to cooling of the skin were considerably inhibited. The mechanism of the selective action of cooling and heating of the skin on monosynaptic and polysynaptic reflexes are discussed.Institute of Physiology, Academy of Sciences of the Kazakh SSR, Alma-Ata. Translated from Neirofiziologiya, Vol. 5, No. 2, pp. 181–185, March–April, 1973.     

The effect of sub-floor heating on house-dust-mite populations on floors and in furniture

It is well known that dehydrating conditions for house dust mites can be created by simply raising the temperature, causing loss of body water and eventually death. Thus, it can be expected that conditions for dust mites are less favourable on floors supplied with sub-floor heating. This was examined in a study of 16 houses with sub-floor heating and 21 without. The pattern of changes in air humidity and temperature on the floors was investigated and compared to known data of the tolerance of dust mites. Also the resident mite populations were compared. Floors with sub-floor heating had, on average, fewer mites, but the difference with unheated floors was small. It was remarkable that mite numbers were also lower in upholstered furniture. Another important observation was that some houses with sub-floor heating had high mite numbers, indicating that this type of heating is compatible with a thriving mite population. Temperature and humidity conditions of heated floors may allow mites not only to survive, but also to remain active in winter. A moderate increase in temperature, a moderate decrease inf (absolute) air humidity, or a combination of both, will suffice to keep the humidity all winter below the Critical Equilibrium Humidity, the level of air humidity that is critical for mite growth and reproduction, hence for allergen production. However, it is argued that measures to suppress allergen production by house dust mites are likely to be far more effective if taken in summer rather than in winter. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physiological and behavioral control of heating and cooling rates in rubber boas, Charina bottae

1. We examined the ability of rubber boas (Charina bottae) to physiologically and behaviorally control their heating and cooling rates between 5 and 35 °C with constrained and unconstrained postural treatments.     

Changes in heart rate are important for thermoregulation in the varanid lizard Varanus varius

Laboratory studies and a single field study have shown that heart rate in some reptiles is faster during heating than during cooling at any given body temperature. This phenomenon, which has been shown to reflect changes in peripheral blood flow, is shown here to occur in the lizard Varanus varius (lace monitor) in the wild. On a typical clear day, lizards emerged from their shelters in the morning to warm in the sun. Following this, animals were active, moving until they again entered a shelter in the evening. During their period of activity, body temperature was 34-36 degrees C in all six study animals (4.0-5.6 kg), but the animals rarely shuttled between sun and shade exposure. Heart rate during the morning heating period was significantly faster than during the evening cooling period. However, the ratio of heating to cooling heart rate decreased with increasing body temperature, being close to 2 at body temperatures of 22-24 degrees C and decreasing to 1.2-1.3 at body temperatures of 34-36 degrees C. There was a significant decrease in thermal time constants with increasing heart rate during heating and cooling confirming that changes in heart rate are linked to rates of heat exchange.