Influence of gender and menstrual cycle on a cold air tolerance test and its relationship to thermosensitivity

This investigation evaluated the influence of gender and phase of menstrual cycle [follicular (FOL): Days 2-6) and luteal (LUT: Days 19-24)] on a cold air tolerance test (CATT: 90-min of exposure to 5 degrees C air) in 8 females (22.7 +/- 3.0 yr) and 15 males (22.3 +/- 2.9 yr). In addition, central thermosensitivity (beta; W x kg(-1) x degrees C(-1) [i.e., the slope of the relationship between the decrease in esophageal temperature (Tes) and the increase in heat production (HP)], gathered during a separate water trial in 20 degrees C water, was correlated to the change (delta) in Tes and HP across the 90 min of resting exposure during the CATT. Analysis of variance revealed no significant differences between phase of menstrual cycle or gender for HP, mean skin temperature (Tsk), and insulation; however, a main effect for time for these parameters was demonstrated. Despite these similarities, Tes differed (P < 0.05) between males and females. Additionally, no relationship was found between beta and deltaHP and deltaTes in the males and females. Also, there was no relationship between beta and thermoregulation during the CATT in these subjects. These data suggest that menstrual cycle phase did not cause a differential response in Tes, Tsk, and HP during a CATT. Furthermore, women maintained a higher Tes than men during the CATT despite similarities in HP and Tsk.

The effect of timing of initial bath on newborn's temperature

OBJECTIVE

To determine the effect of the early timing of an initial bath on newborn temperature.

DESIGN

Random assignment to group, comparative study.

SETTING

Newborn nursery of a 20-bed obstetric unit in a regional hospital in the Midwest.

PATIENTS

80 healthy, full-term newborns.

INTERVENTIONS

40 neonates were bathed at 1 hour of age and 40 were bathed at 2 hours of age.

MAIN OUTCOME MEASURES

Axillary temperatures were measured before the admission bath and at 10, 20, and 60 minutes after the bath with disposable thermometers; the maximum drop in temperature also was determined.

RESULTS

No significant differences were noted in axillary temperatures between the two groups before the bath or at 10, 20, or 60 minutes after the bath. The groups did not differ significantly by gender, birth weight, length of bath in minutes, room temperature during bath, Apgar score at 5 minutes, or gestational age.

CONCLUSION

Healthy full-term newborns with an axillary temperature > or = 36.8 degrees C (98.2 degrees F) can be bathed after 1 hour of age when appropriate care is taken to support thermal stability.

Thermal regulation in Macaca mulatta during space flight

The results of studies of body temperature and thermal regulation in Macaca mulatta flown on biosatellites Bion 6-11 are presented. The effect of microgravity on deep body temperature as compared to skin temperature was investigated. In most animals, deep body temperature declined moderately and then tended to return to normal. Brain temperature/ankle temperature correlation changed. The system of thermal regulation was found to function adequately in space.

Effects of changes in packed cell volume on the specific heat capacity of blood: implications for studies measuring heat exchange in extracorporeal circuits

Extracorporeal circuits such as cardiopulmonary bypass (CPB) and renal dialysis machines cause active and/or passive loss of body heat. Attempts to quantify this heat loss are generally based on the Fick principle which requires knowledge of the specific heat capacity (SHC) of blood. As changes in packed cell volume are common, we investigated the effect of these changes on the SHC of blood over a range of packed cell volumes (PCV) from whole blood at 43.1% (3594 J kg-1 degrees C-1) to pure Hartmann's solution (4153 J kg-1 degrees C-1). The SHC of other fluids used during CPB was also measured and found to be 4139 J kg-1 degrees C-1 and 4082 J kg-1 degrees C-1 for normal saline and Gelofusine, respectively. The maximum variability in SHC over the range of PCV values encountered during CPB was calculated to be small (5.5%). We conclude that use of a constant value of SHC for calculation of thermal energy transfer is currently justified.

Involvement of sympathetic activation and brown adipose tissue in calcitonin gene-related peptide-induced heat production in the rat

We previously reported that microinjection of calcitonin gene-related peptide (CGRP; 1.6-8.0 pmol, 0.2-1.0 microliter) into the ventromedial hypothalamus (VMH) increased oxygen consumption (VO(2)), heart rate (HR), colonic temperature (T(co)), and temperature of interscapular brown adipose tissue (T(IBAT)). In the present study, we investigated whether the autonomic nervous system is involved in the CGRP-induced heat production in urethane-anesthetized rats. Intraperitoneal administration of the ganglion blocker hexamethonium (20 mg/kg) or the beta-adrenergic antagonist propranolol (5 mg/kg) suppressed the CGRP-induced increases in VO(2), HR, T(co), and T(IBAT). Pretreatment with the alpha-adrenergic antagonist phentolamine (5 mg/kg) partly attenuated the heat production response but did not affect the tachycardiac response. Bilateral sectioning of the nerves supplying the IBAT attenuated the CGRP-induced increase in T(IBAT) but not significantly that in VO(2) or T(co). In rats with adrenal demedullation, the effects of CGRP were similar to those in intact rats. These results suggest that the CGRP-induced heat production is mediated by the sympathetic nervous system and, at least in part, by the BAT through the alpha- and beta-adrenoceptors.

The effects of different materials of protective gloves on thermoregulatory responses

The effects of two kinds of protecting gloves for pesticide spraying made of different materials on thermoregulatory responses during exercise were studied at ambient temperature of 28 degrees C and relative humidity of 60% in six healthy females, aged 19. One kind of gloves was made of polyurethane (A) and the other of Goretex (B) with cotton lining in each glove. Both kinds of gloves had almost the same volume. Main results of the experiment were summarised as follows: (1) during the exercise an increase of rectal temperature was inhibited more effectively in B than in A; (2) skin temperature of hand was significantly lower in B than in A; (3) absolute humidity and temperature inside the gloves were significantly lower during the period from the gripping bar exercise to the end of the experiment; (4) the number of contractions by the handgrip exercise performed immediately after the second turning of the screw was significantly smaller in A than in B. The findings presented suggest that the gloves made of Goretex material could reduce thermal strain during intermittent work in warm environmental conditions.

Analysis of heart rate in developing bird embryos: effects of developmental mode and mass

Bird embryos may be regarded as developing in their thermo-neutral zone, at rest, and stay in the egg for a fixed period of time until hatching. It is therefore interesting to investigate if they follow the same 'rule' set for adult homeotherms, which states that, within a taxonomically or functionally defined category such as mammals or birds, the number of heart beats throughout the life span (sL) is more or less constant. This rule stems from the allometric relationships between heart rate (fH) and body mass (mB) and between sL and mB. As a step towards understanding the general allometric nature of avian embryonic physiology we analyzed the fH values of avian embryos in relation to their incubation span (sI). Data from 30 species were selected from the scientific literature for the analyses. Values obtained from invasive methods which were judged to grossly alter natural incubation conditions, or from undefined or unmatched temperature conditions were not used. These include most values obtained below the first 30% of the incubation. Also, data obtained after internal pipping were discarded since hatching activity influences them. Values for sI and egg mass (mE) as representatives of embryonic mass were also collected. Embryonic fH was normalized to 70.1-80% sI. At 20.1-30% sI it was only 85% of the value at 70.1-80% sI and increased to a plateau at about 50.1-60% sI. It was almost constant among species between 50.1 and 60% sI and pre-internal pipping (PIP) time and thus, the mean fH value between 50.1 and 60% sI and between 90.1 and 100% excluding pipped eggs (fH) was taken as a representative value for each given species. The fH (min-1) and the corresponding sI (days) values for the 30 species, scaled with mE (g) as follows: fH = 371.1.mE-0.112 and: sI = 12.29.mE+0.209. Both powers were significantly different from 0. The product of fH and sI (fH.sI), representing the total number of heartbeats throughout the incubation, scaled with mE for the entire data set as follows: fH.sI = 6.565 x 10(+6).mE+0.096, where the +0.096 power is significantly different from 0. Values for fH.sI from embryos of altricial birds tended to concentrate at the low mE end of the plot while those of the precocial ones tended towards the high end. Separate analyses showed that the mE power for the combined altricial and semi-altricial species (ASA), and the combined precocial and semi precocial species (PSP), of log fH.sI against log mE regressions, were both insignificantly different from 0. Thus, means of fH.sI for ASA and PSP were calculated. The mean ASA value of 7.27 x 10(+6) heartbeats for fH.sI, was significantly different from the mean PSP value of 10.93 x 10(+6). The difference of 3.66 x 10(-6) (33.5%) heartbeats can be attributed to either the more advanced stage of the PSP hatchlings at hatch, to the larger mE values of these hatchlings, to the difference in water fraction of the hatchlings or all. The result of a linear regression of fH.sI against the rate of sI completion (the inverse of incubation span, fI; day-1) was: fH.10(-6) = 0.205 + 3.940.sI-1. Thus, the faster is the average rate of development accomplished per day (shorter incubation) the higher is daily heart rate. Data tended to cluster such that large eggs, mostly of the PSP type with relatively low fH, complete 2-4% of their incubation per day, while small, ASA type eggs with relatively high fH, complete 6-8% of their incubation time per day. We conclude that, at this stage of knowledge, the data is insufficient to resolve whether the different modes of hatch stage alone can explain differences in the total number of heartbeats throughout embryonic life among all bird species, or egg mass and water content differences contribute variability. This should be investigated on a larger sample of species in more depth.

Hypothalamic neuronal loss and altered circadian rhythm of temperature in a rat model of mesial temporal lobe epilepsy

PURPOSE

Numerous dysfunctions in endogenous hypothalamic function have been associated with mesial temporal lobe epilepsy (MTLE). One endogenous activity is the circadian rhythm of temperature (CRT). In this study we examined whether hypothalamically mediated function is altered in the electrically induced, self-sustained, limbic status epilepticus model of MTLE. We then wished to determine whether there was a structural basis for regulatory alterations.

METHODS

We measured CRT with peritoneal temperature telemetry obtained in light-entrained (LD) and in free-running, constant-dark (DD) conditions. CRT from epileptic and controls of normal animals and kindled animals were quantized by fast Fourier transform-nonlinear least squares analysis to determine rhythmic complexity.

RESULTS

The circadian component of CRT was preserved in all animals. In DD, CRTs of epileptic animals were more complex than those of normal animals. CRT of kindled animals showed no increased complexity after electrically induced seizures. Neuronal density was decreased in regions of the anterior and posterior hypothalamus but not in the suprachiasmatic nuclei from the epileptic rats.

CONCLUSIONS

Alterations in CRT due to the epileptic state were independent of isolated seizures. Altered circadian thermoregulation in epileptic rats corresponded to regional hypothalamic neuronal loss. Structural changes of the hypothalamus may explain alterations in endogenous rhythms in MTLE.

Spectral analysis of heart rate and blood pressure variability

The cardiovascular system contains negative feedback and time-delay mechanisms resulting in oscillations in heart rate and blood pressure, which are apparent when these signals are measured over time. Specifically, spectral analysis and nonlinear analysis techniques allow quantification of heart rate and blood pressure variability. Alterations in heart rate and blood pressure variability as evidenced from the spectral characteristics of these physiological signals allow the clinicians to have indicators of aberrations of neural control in various conditions or when drugs such as beta blockers, adrenergic agents, methylxanthines, etc., are being administered.

Brain uncoupling protein 2: uncoupled neuronal mitochondria predict thermal synapses in homeostatic centers

Distinct brain peptidergic circuits govern peripheral energy homeostasis and related behavior. Here we report that mitochondrial uncoupling protein 2 (UCP2) is expressed discretely in neurons involved in homeostatic regulation. UCP2 protein was associated with the mitochondria of neurons, predominantly in axons and axon terminals. UCP2-producing neurons were found to be the targets of peripheral hormones, including leptin and gonadal steroids, and the presence of UCP2 protein in axonal processes predicted increased local brain mitochondrial uncoupling activity and heat production. In the hypothalamus, perikarya producing corticotropin-releasing factor, vasopressin, oxytocin, and neuropeptide Y also expressed UCP2. Furthermore, axon terminals containing UCP2 innervated diverse hypothalamic neuronal populations. These cells included those producing orexin, melanin-concentrating hormone, and luteinizing hormone-releasing hormone. When c-fos-expressing cells were analyzed in the basal brain after either fasting or cold exposure, it was found that all activated neurons received a robust UCP2 input on their perikarya and proximal dendrites. Thus, our data suggest the novel concept that heat produced by axonal UCP2 modulates neurotransmission in homeostatic centers, thereby coordinating the activity of those brain circuits that regulate daily energy balance and related autonomic and endocrine processes.

The effect of submaximal exercise on recovery hemodynamics and thermoregulation in men and women

Five women and 5 men were studied to examine the effects of submaximal exercise on thermoregulatory and hemodynamic variables during recovery in two environments: (a) control (C), 22 degrees C, 33% rh; and (b) hot humid (H), 32 degrees C. The participants exercised on a cycle ergometer at 60% of peak oxygen consumption for 35 min prior to 90 min of seated recovery. Sessions were identical, except for environment. Variables evaluated (p < .05) were: core temperature (TR), mean skin temperature (Ts), sweat rate (SR), heart rate (HR), stroke index (SI), cardiac index (CI), forearm blood flow (FBF), systolic blood pressure (SBP), and diastolic blood pressure (DBP). Men and women exhibited similar patterns of TR, Ts, and SR in both environments. Ts and SR (collapsed means for gender) were higher in the H than in the C. DBP was higher in men than in women throughout recovery in both environments. With combined means for gender, HR was higher in the H than in the C. CI, SI, FBF, and SBP were similar in both environments and returned to baseline within 15 min into recovery. These data suggest that heat dissipation during extended recovery was accomplished with similar contributions of cutaneous vasodilation and sweating in M and F. Furthermore, the moderate exercise level did not influence hemodynamics beyond 15 min of recovery in either environment.

Energy content of the evening meal alters nocturnal body temperature but not sleep

Meals of varying energy content and episodes of sleep influence body temperature. We compared the effect of an evening meal, varying from high-energy (11.91 +/- 0.86 MJ) to average (5.74 +/- 0.88 MJ) and a 10-h fast (no evening meal), on nocturnal body temperature and sleep. Seven healthy men (20-24 years, mean body mass index of 23.4 +/- 2.6 kg/m2) reported to the laboratory for an evening meal at 2000 h having consumed similar amounts of food before 1300 h. After completing the meal, subjective hunger ratings were assessed, and a venous blood sample taken. The subjects spent 4 nonconsecutive nights (an adaptation night, followed by either of the two meal conditions or the fast in random order) in the sleep laboratory when polysomnographic recordings were made from 2300 to 0700 h. Meal energy content and serum concentrations of insulin, triglyceride, and low-density lipoproteins (LDL) varied significantly. Lower rectal temperatures were measured during the fast than following the meals. Over the 8-h recording period, thermal response indices (TRI) varied with higher body temperatures following the higher energy meal. Similar rectal temperatures were attained by the end of the sleep periods. There were no significant differences in any of the subjective or objective sleep measures. The physiological responses associated with the transient dietary changes of an evening meal or a 10-h fast altered nocturnal body temperature but did not significantly affect sleep of good sleepers when sleep was initiated 2 to 3 h after finishing the meal.

Dopamine and the origins of human intelligence

A general theory is proposed that attributes the origins of human intelligence to an expansion of dopaminergic systems in human cognition. Dopamine is postulated to be the key neurotransmitter regulating six predominantly left-hemispheric cognitive skills critical to human language and thought: motor planning, working memory, cognitive flexibility, abstract reasoning, temporal analysis/sequencing, and generativity. A dopaminergic expansion during early hominid evolution could have enabled successful chase-hunting in the savannas of sub-Saharan Africa, given the critical role of dopamine in counteracting hyperthermia during endurance activity. In turn, changes in physical activity and diet may have further increased cortical dopamine levels by augmenting tyrosine and its conversion to dopamine in the central nervous system (CNS). By means of the regulatory action of dopamine and other substances, the physiological and dietary changes may have contributed to the vertical elongation of the body, increased brain size, and increased cortical convolutedness that occurred during human evolution. Finally, emphasizing the role of dopamine in human intelligence may offer a new perspective on the advanced cognitive reasoning skills in nonprimate lineages such as cetaceans and avians, whose cortical anatomy differs radically from that of primates.

Spontaneous activity, sleep, and body temperature in rats lacking the CCK-A receptor

Because of a genetic mutation, the Otsuka-Long-Evans-Tokushima Fatty (OLETF) rat, a model for human non-insulin-dependent diabetes mellitus (NIDDM), shows no expression of the CCK-A receptor gene. We investigated the spontaneous physical activity, sleep, and body temperature in young OLETF rats that had not yet developed diabetes mellitus, and compared these data with age-matched control LETO (non-diabetic strain, Long-Evans-Tokushima-Otsuka) rats. The amount of large movements during the dark phase for the OLETF rats was significantly less than that of control rats. Thus, the amounts of total daily large movement and the ratio of dark-to-light phase movement in the OLETF rats were less than those of control rats, although the amount of small movement was similar for both groups. The diurnal rhythm of body temperature was similar for both groups. In addition, the amount of and circadian rhythm for each vigilance state and slow-wave activity were similar for the two groups. This study demonstrates that the CCK-A receptor might play a role in affecting the level of motor activity, adding hyperphagia, and the circadian rhythm of large movement in these rats prior to the manifestation of NIDDM. In contrast, a CCK-A receptor deficiency does not appear to affect sleep or body temperature in these rats.

Biological rhythms in birds--development, insights and perspectives

The aim of this review is to show that probably the internal clock of precocial birds is imprinted in the prenatal period by exogenous factors (zeitgeber). The activity of organ functions occurs early during embryonic development, before this function is ultimately necessary to ensure the survival of the embryo. Prenatal activation of some functional systems may have a training effect on the postnatal efficiency. The development of physiological control systems is influenced by endogenous and exogenous factors during the late prenatal and early postnatal period: epigenetic adaptation processes play an important role in the development of animals; they have acquired characteristics which are innated but not genetically fixed. As a rule, the actual value during the determination period has a very strong influence on the set-point of the system. This will be explained using the example of thermoregulation. It is shown in detail that it seems to be possible to imprint the prenatal development of circadian rhythms by periodic changes of the light-dark cycle but not by rhythmic influence of acoustic signals. Altogether, there are more questions open than solved concerning the perinatal genesis of circadian rhythms in birds. Topics are given for the future research.

Cardiopulmonary baroreceptor control of muscle sympathetic nerve activity in heat-stressed humans

Whole body heating decreases central venous pressure (CVP) while increasing muscle sympathetic nerve activity (MSNA). In normothermia, similar decreases in CVP elevate MSNA, presumably via cardiopulmonary baroreceptor unloading. The purpose of this project was to identify whether increases in MSNA during whole body heating could be attributed to cardiopulmonary baroreceptor unloading coincident with the thermal challenge. Seven subjects were exposed to whole body heating while sublingual temperature, skin blood flow, heart rate, arterial blood pressure, and MSNA were monitored. During the heat stress, 15 ml/kg warmed saline was infused intravenously over 7-10 min to increase CVP and load the cardiopulmonary baroreceptors. We reported previously that this amount of saline was sufficient to return CVP to pre-heat stress levels. Whole body heating increased MSNA from 25 +/- 3 to 39 +/- 3 bursts/min (P < 0. 05). Central blood volume expansion via rapid saline infusion did not significantly decrease MSNA (44 +/- 4 bursts/min, P > 0.05 relative to heat stress period) and did not alter mean arterial blood pressure (MAP) or pulse pressure. To identify whether arterial baroreceptor loading decreases MSNA during heat stress, in a separate protocol MAP was elevated via steady-state infusion of phenylephrine during whole body heating. Increasing MAP from 82 +/- 3 to 93 +/- 4 mmHg (P < 0.05) caused MSNA to decrease from 36 +/- 3 to 15 +/- 4 bursts/min (P < 0.05). These data suggest that cardiopulmonary baroreceptor unloading during passive heating is not the primary mechanism resulting in elevations in MSNA. Moreover, arterial baroreceptors remain capable of modulating MSNA during heat stress.

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 degree C, every 5 min, for 4 weeks. Within that period, pigs were exposed on different days, and in random order, to a cold (5 degrees C) or hot (38 degrees 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.

Thermoregulatory model of sleep control: losing the heat memory

Thermoregulatory mechanisms were hypothesized to provide primary control of non-rapid-eye-movement sleep (NREM). On the basis of this hypothesis, we incorporated the thermoregulatory feedback loops mediated by the "heat memory," heat load, and loss processes associated with sleep-wake cycles, which were modulated by two circadian oscillators. In addition, hypnogenic warm-sensitive neurons (HWSNs) were assumed to integrate thermoregulation and NREM control. The heat memory described above could be mediated by some sleep-promoting substances. In this paper, considering the possible carrier of the heat memory, its losing process is newly included in the model. The newly developed model can generate the appropriate features of human sleep-wake patterns. One of the special features of the model is to generate the bimodal distribution of the sleepiness. This bimodality becomes distinct, as the losing rate of the heat memory decreases or the amplitude of the Y oscillator increases. The theoretical analysis shows the losing rate of the heat memory control's rapidity of model response to a thermal perturbation, which is confirmed by simulating the responses with various losing rates to transient heat loads ("heat load pulse"). The sleepiness exhibits large responses to the heat load pulses applied in the early and late phases of wake period, while the response is significantly reduced to the pulse applied in the supposed wake-maintenance zone. This bimodality of the response appears to reflect the sensitivity of the HWSNs. In addition, the early pulse raises the immediate sleepiness rather than the nocturnal sleepiness, while the heat load pulse applied in the later phase of waking period significantly raises the sleepiness during a nocturnal sleep. In simulations of sleep deprivation, the discontinuous relationship between recovery sleep length and deprivation time is reproduced, where the critical sleep deprivation time at which the recovery sleep length jumps is extended as the losing rate increases. This is possibly due to the dissipation of the heat memory accumulated by the sleep deprivation. The simulation results here qualitatively reproduce the experimental observations or predict the intriguing phenomena of human circadian rhythms. Therefore, our model could provide a novel framework for investigating the relationship between thermoregulation and sleep control processes.

A comparison of Hsp70 expression and thermotolerance in adults and larvae of three Drosophila species

Heat shock proteins (Hsps) and other molecular chaperones perform diverse physiological roles. One is to facilitate, in part, organismal thermotolerance, of which the functional consequences depend on Hsp70 concentration and developmental stage in Drosophila melanogaster. To test whether an Hsp70-thermotolerance relationship is a general phenomenon within Drosophila, I assayed Hsp70 concentration at a range of temperatures in intact larvae and adults of three species, D. melanogaster, D. simulans, and D. mojavensis, and compared those results to the increase in survival to heat shock that occurs after an Hsp70 inducing pretreatment. Larvae of D. melanogaster and D. simulans responded similarly to heat; they expressed Hsp70 maximally at 36-37 degrees C, and their tolerance of 1 h heat shocks increased by 1.5-2 degrees C. By contrast, D. mojavensis, which tolerates higher temperatures than do D. melanogaster and D. simulans, expressed Hsp70 only at higher temperatures, although the 36 degrees C pretreatment still increased thermotolerance. Critically, the temperature that maximally induced Hsp70 was a poor inducer of thermotolerance in D. mojavensis and may have harmed larvae. Results for Drosophila adults, which tolerated heat poorly compared to larvae, likewise suggest that a close link between peak Hsp70 expression and maximal induction of thermotolerance is a feature of D. melanogaster, and not of the other species. Neither D. simulans nor D. mojavensis adults increased tolerance after exposure to the temperatures that maximally induced Hsp70.

Effect of precooling on high intensity cycling performance

OBJECTIVE

To examine the effects of precooling skin and core temperature on a 70 second cycling power test performed in a warm and humid environment (29 degrees C, 80% relative humidity).

METHODS

Thirteen male national and international level representative cyclists (mean (SD) age 24.1 (4.1) years; height 181.5 (6.2) cm; weight 75.5 (6.4) kg; maximal oxygen uptake (VO2peak) 66.1 (7.0) ml/kg/min) were tested in random order after either 30 minutes of precooling using cold water immersion or under control conditions (no precooling). Tests were separated by a minimum of two days. The protocol consisted of a 10 minute warm up at 60% of VO2peak followed by three minutes of stretching. This was immediately followed by the 70 second power test which was performed on a standard road bicycle equipped with 172.5 mm powermeter cranks and mounted on a stationary ergometer.

RESULTS

Mean power output for the 70 second performance test after precooling was significantly (p<0.005) increased by 3.3 (2.7)% from 581 (57) W to 603 (60) W. Precooling also significantly (p<0.05) decreased core, mean body, and upper and lower body skin temperature; however, by the start of the performance test, lower body skin temperature was no different from control. After precooling, heart rate was also significantly lower than control throughout the warm up (p<0.05). Ratings of perceived exertion were significantly higher than the control condition at the start of the warm up after precooling, but lower than the control condition by the end of the warm up (p<0.05). No differences in blood lactate concentration were detected between conditions.

CONCLUSIONS

Precooling improves short term cycling performance, possibly by initiating skin vasoconstriction which may increase blood availability to the working muscles. Future research is required to determine the physiological basis for the ergogenic effects of precooling on high intensity exercise.

Whole-body pre-cooling and heat storage during self-paced cycling performance in warm humid conditions

The aim of this study was to establish the effect that pre-cooling the skin without a concomitant reduction in core temperature has on subsequent self-paced cycling performance under warm humid (31 degrees C and 60% relative humidity) conditions. Seven moderately trained males performed a 30 min self-paced cycling trial on two separate occasions. The conditions were counterbalanced as control or whole-body pre-cooling by water immersion so that resting skin temperature was reduced by approximately 5-6 degrees C. After pre-cooling, mean skin temperature was lower throughout exercise and rectal temperature was lower (P < 0.05) between 15 and 25 min of exercise. Consequently, heat storage increased (P < 0.003) from 84.0+/-8.8 W x m(-2) to 153+/-13.1 W x m(-2) (mean +/- s(mean)) after pre-cooling, while total body sweat fell from 1.7+/-0.1 l x h(-1) to 1.2+/-0.1 l h(-1) (P < 0.05). The distance cycled increased from 14.9+/-0.8 to 15.8+/-0.7 km (P < 0.05) after pre-cooling. The results indicate that skin pre-cooling in the absence of a reduced rectal temperature is effective in reducing thermal strain and increasing the distance cycled in 30 min under warm humid conditions.

Water and sodium balances and metabolic physiology of house mice (Mus domesticus) and short-tailed mice (Leggadina lakedownensis) under laboratory conditions

A laboratory study investigated the metabolic physiology, and response to variable periods of water and sodium supply, of two arid-zone rodents, the house mouse (Mus domesticus) and the Lakeland Downs short-tailed mouse (Leggadina lakedownensis) under controlled conditions. Fractional water fluxes for M. domesticus (24 +/- 0.8%) were significantly higher than those of L. lakedownensis (17 +/- 0.7%) when provided with food ad libitum. In addition, the amount of water produced by M. domesticus and by L. lakedownensis from metabolic processes (1.3 +/- 0.4 ml.day-1 and 1.2 +/- 0.4 ml.day-1, respectively) was insufficient to provide them with their minimum water requirement (1.4 +/- 0.2 ml.day-1 and 2.0 +/- 0.3 ml.day-1, respectively). For both species of rodent, evaporative water loss was lowest at 25 degrees C, but remained significantly higher in M. domesticus (1.1 +/- 0.1 mg H2O.g-0.122.h-1) than in L. lakedownensis (0.6 +/- 0.1 mg H2O.g-0.122.h-1). When deprived of drinking water, mice of both species initially lost body mass, but regained it within 18 days following an increase in the amount of seed consumed. Both species were capable of drinking water of variable saline concentrations up to 1 mol.l-1, and compensated for the increased sodium in the water by excreting more urine to remove the sodium. Basal metabolic rate was significantly higher in M. domesticus (3.3 +/- 0.2 mg O2.g-0.75.h-1) than in L. lakedownensis (2.5 +/- 0.1 mg O2.g-0.75.h-1). The study provides good evidence that water flux differences between M. domesticus and L. lakedownensis in the field are due to a requirement for more water in M. domesticus to mect their physiological and metabolic demands. Sodium fluxes were lower than those observed in free-ranging mice, whose relatively high sodium fluxes may reflect sodium associated with available food.

[Ca(2+) level in the animal blood and their cold resistance]

Administration of small doses of the EDTA decreased by 15-20% the Ca2+ contentn in the blood plasma of rabbits and rats. The decrease coincided with an abrupt stimulation of the thermoregulation system of cooled animals. Restoration of the Ca2+ content in circulating blood coincided in time with repeated suppression of the system's functions. The findings corroborate the theory of a key role of the Ca2+ in sensitivity of the homoiothermal organism to cold and substantiates the method of restoring physiological functions in deep hypothermia without rewarming the body.

Do changing patterns of heat and humidity influence thermoregulation and endurance performance?

The purpose of this project was to determine whether changing patterns of temperature and humidity, as expected in the morning versus afternoon, had a differential effect on thermoregulation and endurance performance. Eight male distance runners each participated in two heat pattern tests consisting of two hours treadmill running at 70%-maximum oxygen consumption. The mean heat load for each test was identical (22.2 degrees C wet bulb temperature) but either dry bulb temperature increased (24 to 27.5 degrees C) or decreased (27.5 to 24 degrees C) over the course of the two hour heat stress test. Whole body sweat rate was 10.7% higher (p<0.05) and there was greater plasma volume loss (2.7 versus 1.6%, p<0.05) in the cooling versus warming pattern test. Mean skin and body temperature changed in a significantly different (p<0.05) manner between the two patterns and closely followed ambient dry bulb temperature change. The thermoregulatory variables of heart rate and rectal temperature were not affected and performance did not differ between pattern tests. Ratings of perceived exertion (RPE) and oxygen consumption were also not significantly different between cooling and warming test. In summary, although some minor differences were noted, thermal homeostasis was maintained equally well during either warming or cooling for wet bulb temperatures between 24 and 27 degrees C. The mean heat load is therefore more important than changing patterns of temperature and humidity in determining an individual's physiological response to exercise in a warm environment.

Amino acid-induced thermogenesis reduces hypothermia during anesthesia and shortens hospital stay

Amino acid infusion during general anesthesia induces thermogenesis and prevents postoperative hypothermia and shivering. We propose that amino acid prevention of hypothermia during anesthesia shortens the hospital stay. Core temperatures and pulmonary oxygen uptake were measured in 45 patients, receiving an IV amino acid mixture, 126 mL/h, before and/or during isoflurane anesthesia and 30 control patients receiving acetated Ringer's solution. At awakening, mean core temperature was 36.5 degrees+/-0.1 degrees C in the amino acid group and 35.7 degrees+/-0.1 degrees C (P < 0.001) in the controls. Energy expenditure increased by 54%+/-9% from baseline in amino acid patients in whom shivering was uncommon, but only by 5%+/-4% (P < 0.001) in control patients, of whom the majority developed postoperative shivering. The estimated difference in hospital stay between the two groups was 2.7 days (CI 95%: 1.3-4.0). Multiple regression analysis showed that the variables best predicting hospitalization were duration of surgery, amino acid treatment, and awakening temperatures. Duration of surgery was similar in the two groups and core temperatures at awakening were a result of amino acid infusion, which indicates that amino acid infusion during anesthesia and surgery was the most important factor for the shorter hospitalization.

IMPLICATIONS

Amino acid infusion during general anesthesia induces thermogenesis and prevents postoperative hypothermia and shivering. Multiple regression analysis indicated that this resulted in a shorter hospital stay.