The median preoptic nucleus is involved in the facilitation of heat-escape/cold-seeking behavior during systemic salt loading in rats

Systemic salt loading has been reported to facilitate operant heat-escape/cold-seeking behavior. In the present study, we hypothesized that the median preoptic nucleus (MnPO) would be involved in this mechanism. Rats were divided into two groups (n = 6 each): one group had the MnPO lesion with ibotenic acid (4.0 mug) and the other was the vehicle control. After subcutaneous injection (10 ml/kg) of either isotonic- (154 mM) or hypertonic-saline (2,500 mM), each rat was placed in a behavior box, where the ambient temperature was changed to 26 degrees C, 35 degrees C, and 40 degrees C every 1 h. The position of a rat in the box and the body core temperature (T(core)) were monitored. A rat could trigger 0 degrees C air for 45 s in the 35 degrees C and 40 degrees C heat when moved in a specific area in the box (operant behavior). In the control group, counts of the operant behavior were greater (P < 0.05) in the hypertonic- than in the isotonic-saline injection (17 +/- 2 and 10 +/- 2 at 35 degrees C, 24 +/- 2 and 18 +/- 1 at 40 degrees C). T(core) remained unchanged throughout the exposure, although the level was lower (P < 0.05) in the hypertonic- than in the isotonic-saline trial (36.6 +/- 0.2 degrees C and 37.4 +/- 0.1 degrees C at 26 degrees C and 36.9 +/- 0.2 degrees C and 37.4 +/- 0.1 degrees C at 40 degrees C, respectively). However, in the MnPO-lesion group, counts of the behavior were similar between the hypertonic- and isotonic-saline injection trials (10 +/- 2 and 8 +/- 1 at 35 degrees C, and 17 +/- 1 and 16 +/- 1 at 40 degrees C, respectively). T(core) increased (P < 0.05) in the heat in both trials (36.8 +/- 0.1 degrees C and 37.4 +/- 0.1 degrees C at 26 degrees C and 37.4 +/- 0.2 degrees C and 37.8 +/- 0.2 degrees C at 40 degrees C in the hypertonic- and isotonic-saline injection trials, respectively). These results may suggest that, at least in part, the MnPO is involved in the facilitation of heat-escape/cold-seeking behavior during osmotic stimulation.     

Effects of estrogen on thermoregulatory tail vasomotion and heat-escape behavior in freely moving female rats

Effects of estrogen on thermoregulatory vasomotion and heat-escape behavior were investigated in ovariectomized female rats supplemented with estrogen (replaced estrogen rats) or control saline (low estrogen rats). First, we measured tail temperature of freely moving rats at ambient temperatures (T(a)) between 13 and 31 degrees C. Tail temperature of the low estrogen rats was higher than that of the replaced estrogen rats at T(a) between 19 and 25 degrees C, indicating that the low estrogen rats exhibit more skin vasodilation than the replaced estrogen rats. There was no significant difference in oxygen consumption and core temperature between the two groups. Second, we analyzed heat-escape behaviors in a hot chamber where rats could obtain cold air by moving in and out of a reward area. The low estrogen rats kept T(a) at a lower level than did the replaced estrogen rats. These results imply that the lack of estrogen facilitates heat dissipation both by skin vasodilation and by heat-escape behavior. Ovariectomized rats may mimic climacteric hot flushes not only for autonomic skin vasomotor activity but also for thermoregulatory behavior.     

Gastrointestinal osmoreceptors and renal sodium excretion in humans

The hypothesis that natriuresis can be induced by stimulation of gastrointestinal osmoreceptors was tested in eight supine subjects on constant sodium intake (150 mmol NaCl/day). A sodium load equivalent to the amount contained in 10% of measured extracellular volume was administered by a nasogastric tube as isotonic or hypertonic saline (850 mM). In additional experiments, salt loading was replaced by oral water loading (3.5% of total body water). Plasma sodium concentration increased after hypertonic saline (+3.1 +/- 0.7 mM), decreased after water loading (-3.8 +/- 0.8 mM), and remained unchanged after isotonic saline. Oncotic pressure decreased by 9.4 +/- 1.2, 3.7 +/- 1.2, and 10.7 +/- 1.3%, respectively. Isotonic saline induced an increase in renal sodium excretion (104 +/- 15 to 406 +/- 39 micromol/min) that was larger than seen with hypertonic saline (85 +/- 15 to 325 +/- 39 micromol/min) and water loading (88 +/- 11 to 304 +/- 28 micromol/min). Plasma ANG II decreased to 22 +/- 6, 35 +/- 6, and 47 +/- 5% of baseline after isotonic saline, hypertonic saline, and water loading, respectively. Plasma atrial natriuretic peptide (ANP) concentrations and urinary excretion rates of endothelin-1 were unchanged. In conclusion, stimulation of osmoreceptors by intragastric infusion of hypertonic saline is not an important natriuretic stimulus in sodium-replete subjects. The natriuresis after intragastric salt loading was independent of ANP but can be explained by inhibition of the renin-angiotensin system.     

Effect of central hypertonic stimulation on plasma atrial natriuretic peptide and oxytocin in conscious rats

The effect of the intracerebroventricular (i.c.v.) injection of hypertonic sodium chloride on plasma atrial natriuretic peptide (ANP) and oxytocin (OT) was evaluated in conscious freely moving rats. A hypertonic or isotonic NaCl solution was injected into the third ventricle. Blood pressure and heart rate were monitored and blood samples were collected. I.c.v. injection of the hypertonic solution resulted in a significant increase in mean arterial pressure (105.3 +/- 2.9 mmHg at time 0 to 124.2 +/- 4.4 mmHg at 5 min, P less than 0.01) and heart rate (350.0 +/- 25.0 bpm at time 0 to 420.8 +/- 13.6 bpm at 20 min, P less than 0.01). Plasma OT increased 4-fold over the basal values 5 min after the injection (4.5 +/- 1.1 to 20.1 +/- 3.2 pg/ml, P less than 0.01), while there was no significant change in plasma ANP (37.3 +/- 9.1 to 46.6 +/- 12.6 pg/ml, n.s.). The control injection produced no significant changes in any parameters. These results show that hemodynamic changes are not necessarily associated with alterations in plasma ANP. Furthermore they suggest that central osmoreceptors are not involved in the control of ANP secretion.     

Effect of saline infusion on body temperature and endurance during heavy exercise

We tested the hypothesis that volume infusion during strenuous exercise, by expanding blood volume, would allow better skin blood flow and better temperature homeostasis and thereby improve endurance time. Nine males exercised to exhaustion at 84.0 +/- 3.14% (SE) of maximum O2 consumption on a cycle ergometer in a double-blind randomized protocol with either no infusion (control) or an infusion of 0.9% NaCl (mean vol 1,280.3 +/- 107.3 ml). Blood samples and expired gases (breath-by-breath), as well as core and skin temperatures, were analyzed. Plasma volume decreased less during exercise with the infusion at 15 min (-13.7 +/- 1.4% control vs. -5.3 +/- 1.7% infusion, P less than 0.05) and at exhaustion (-13.6 +/- 1.2% vs. -1.3 +/- 2.2%, P less than 0.01). The improved fluid homeostasis was associated with a lower core temperature during exercise (39.0 +/- 0.2 degrees C for control and 38.5 +/- 0.2 degrees C for infusion at exhaustion, P less than 0.01) and lower heart rate (194.1 +/- 3.9 beats/min for control and 186.0 +/- 5.1 beats/min for infusion at exhaustion, P less than 0.05). However, endurance time did not differ between control and infusion (21.96 +/- 3.56 and 20.82 +/- 2.63 min, respectively), and neither did [H+], peak O2 uptake, and CO2 production, end-tidal partial pressure of CO2, blood lactate, or blood pressure. In conclusion, saline infusion increases heat dissipation and lowers core temperature during strenuous exercise but does not influence endurance time.     

Thermodynamics of ribonuclease T1 denaturation.

Differential scanning calorimetry has been used to investigate the thermodynamics of denaturation of ribonuclease T1 as a function of pH over the pH range 2-10, and as a function of NaCl and MgCl2 concentration. At pH 7 in 30 mM PIPES buffer, the thermodynamic parameters are as follows: melting temperature, T1/2 = 48.9 +/- 0.1 degrees C; enthalpy change, delta H = 95.5 +/- 0.9 kcal mol-1; heat capacity change, delta Cp = 1.59 kcal mol-1 K-1; free energy change at 25 degrees C, delta G degrees (25 degrees C) = 5.6 kcal mol-1. Both T1/2 = 56.5 degrees C and delta H = 106.1 kcal mol-1 are maximal near pH 5. The conformational stability of ribonuclease T1 is increased by 3.0 kcal/mol in the presence of 0.6 M NaCl or 0.3 M MgCl2. This stabilization results mainly from the preferential binding of cations to the folded conformation of the protein. The estimates of the conformational stability of ribonuclease T1 from differential scanning calorimetry are shown to be in remarkably good agreement with estimates derived from an analysis of urea denaturation curves.     

Heat stress induces a biphasic thermoregulatory response in mice

Previous animal models of heat stress have been compromised by methodologies, such as restraint and anesthesia, that have confounded our understanding of the core temperature (T(c)) responses elicited by heat stress. Using biotelemetry, we developed a heat stress model to examine T(c) responses in conscious, unrestrained C57BL/6J male mice. Before heat stress, mice were acclimated for >4 wk to an ambient temperature (T(a)) of 25 degrees C. Mice were exposed to T(a) of 39.5 +/- 0.2 degrees C, in the absence of food and water, until they reached maximum T(c) of 42.4 (n = 11), 42.7 (n = 12), or 43.0 degrees C (n = 11), defined as mild, moderate, and extreme heat stress, respectively. Heat stress induced an approximately 13% body weight loss that did not differ by final group T(c); however, survival rate was affected by final T(c) (100% at 42.4 degrees C, 92% at 42.7 degrees C, and 46% at 43 degrees C). Hypothermia (T(c) < 34.5 degrees C) developed after heat stress, with the depth and duration of hypothermia significantly enhanced in the moderate and extreme compared with the mild group. Regardless of heat stress severity, every mouse that transitioned out of hypothermia (survivors only) developed a virtually identical elevation in T(c) the next day, but not night, compared with nonheated controls. To test the effect of the recovery T(a), a group of mice (n = 5) were acclimated for >4 wk and recovered at T(a) of 30 degrees C after moderate heat stress. Recovery at 30 degrees C resulted in 0% survival within approximately 2 h after cessation of heat stress. Using biotelemetry to monitor T(c) in the unrestrained mouse, we show that recovery from acute heat stress is associated with prolonged hypothermia followed by an elevation in daytime T(c) that is dependent on T(a). These thermoregulatory responses to heat stress are key biomarkers that may provide insight into heat stroke pathophysiology.     

Effects of active and passive hyperthermia on heat shock protein 70 (HSP70)

Summary. The purpose of this study was to delineate the effects of hyperthermia and physical exercise on the heat shock protein 70 (HSP70) response in circulating peripheral blood mononuclear cells (PBMCs). Six healthy, young (age: 24 ± 3 yrs), moderately trained males (VO_2max: 48.9 ± 2.7 ml · kg · min⁻¹) undertook two experimental trials in a randomised fashion in which the core temperature (T _c) was increased and then maintained at 39 °C during a 90 min bout by either active (AH) or passive (PH) means. AH involved subjects cycling at 90% of their lactate threshold in attire designed to impede heat loss mechanisms. In the PH trial, subjects were immersed up to the neck in a hot bath (40.2 ± 0.4 °C), once the critical T _c was achieved, intermittent cycling and water immersions were prescribed for the AH and PH conditions, respectively, to maintain the T _c at 39 °C. HSP70 was measured intracellularly pre, post and 4 h after trials, from circulating PBMCs using an ELISA technique. T _c reached 39 °C quicker in PH than during AH trials (PH: 21 ± 4 min vs. AH: 39 ± 6 min; P < 0.01), thereafter T _c was maintained around 39 °C (PH: 39.1 ± 0.2 °C; AH: 38.8 ± 0.3 °C; P > 0.05). AH induced a marked leukocytosis in all sub-sets (P < 0.05). PH generated significant monocytosis and granulocytosis (P < 0.05), without changes in lymphocyte counts (P > 0.05). There were no significant increases in intracellular HSP70 at 0 h (AH: Δ − 21.1 ± 44.8; PH: Δ + 12.5 ± 32.4 ng/mg TP/10³/μl PBMCs; P > 0.05) and 4 h (AH: Δ − 30.0 ± 40.1; PH: Δ + 36.3 ± 70.4 ng/mg TP/10³/μl PBMCs; P > 0.05) post active and passive heating. Peak HSP70 expressed as a fold-change from rest was also not increased by AH (1.1 ± 0.9; P > 0.05) or PH (3.2 ± 4.8; P > 0.05). There were no significant differences between the AH and PH trials at any time-point, and the HSP70 response appeared to be individual specific. These results did not allow us to delineate the effects of hyperthermia and other exercise associated stressors on the heat shock response and therefore further work is warranted. Authors’ address: Ric Lovell, Department of Sport, Health and Exercise Science, University of Hull, Hull HU6 7RX, U.K.     

Cardiac-specific Nkx2.5 homeodomain: conformational stability and specific DNA binding of Nkx2.5(C56S)

The cardiac-specific Nkx2.5 homeodomain has been expressed as a 79-residue protein with the oxidizable Cys(56) replaced with Ser. The Nkx2.5 or Nkx2.5(C56S) homeodomain is 73% identical in sequence to and has the same NMR structure as the vnd (ventral nervous system defective)/NK-2 homeodomain of Drosophila when bound to the same specific DNA. The thermal unfolding of Nkx2.5(C56S) at pH 6.0 or 7.4 is a reversible, two-state process with unit cooperativity, as measured by differential scanning calorimetry (DSC) and far-UV circular dichroism. Adding 100 mM NaCl to Nkx2.5(C56S) at pH 7.4 increases T(m) from 44 to 54 +/- 0.2 degrees C and DeltaH from 34 to 45 +/- 2 kcal/mol (giving a DeltaC(p) of approximately 1.2 kcal K(-)(1) mol(-)(1) for homeodomain unfolding). DSC profiles of Nkx2.5 indicate fluctuating nativelike structures at <37 degrees C. Titrations of specific 18 bp DNA with Nkx2.5(C56S) in buffer at pH 7.4 with 100 mM NaCl yield binding constants of 2-6 x 10(8) M(-)(1) from 10 to 37 degrees C and a stoichiometry of 1:1 for homeodomain binding DNA, using isothermal titration calorimetry. The DNA binding reaction of Nkx2.5 is enthalpically controlled, and the temperature dependence of DeltaH gives a DeltaC(p) of -0.18 +/- 0.01 kcal K(-)(1) mol(-)(1). This corresponds to 648 +/- 36 A(2) of buried apolar surface upon Nkx2.5(C56S) binding duplex B-DNA. Thermodynamic parameters differ for Nkx2.5 and vnd/NK-2 homeodomains binding specific DNA. Unbound NK-2 is more flexible than Nkx2.5.     

Stability of recombinant Lys25-ribonuclease T1

The conformational stability of recombinant Lys25-ribonuclease T1 has been determined by differential scanning microcalorimetry (DSC), UV-monitored thermal denaturation measurements, and isothermal Gdn.HCl unfolding studies. Although rather different extrapolation procedures are involved in calculating the Gibbs free energy of stabilization, there is fair agreement between the delta G degrees values derived from the three different experimental techniques at pH 5, theta = 25 degrees C: DSC, 46.6 +/- 2.1 kJ/mol; UV melting curves, 48.7 +/- 5 kJ/mol; Gdn.HCl transition curves, 40.8 +/- 1.5 kJ/mol. Thermal unfolding of the enzyme is a reversible process, and the ratio of the van't Hoff and calorimetric enthalpy, delta HvH/delta Hcal, is 0.97 +/- 0.06. This result strongly suggests that the unfolding equilibrium of Lys25-ribonuclease T1 is adequately described by a simple two-state model. Upon unfolding the heat capacity increases by delta Cp degrees = 5.1 +/- 0.5 kJ/(mol.K). Similar values have been found for the unfolding of other small proteins. Surprisingly, this denaturational heat capacity change practically vanishes in the presence of moderate NaCl concentrations. The molecular origin of this effect is not clear; it is not observed to the same extent in the unfolding of bovine pancreatic ribonuclease A, which was employed in control experiments. NaCl stabilizes Lys25-ribonuclease T1. The transition temperature varies with NaCl activity in a manner that suggests two limiting binding equilibria to be operative. Below approximately 0.2 M NaCl activity unfolding is associated with dissociation of about one ion, whereas above that concentration about four ions are released in the unfolding reaction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heterogeneity in radiosensitization by heat of cloned cell lines derived from a single human melanoma xenograft

Heterogeneity in radiosensitization by heat was studied using one uncloned and five cloned cell lines isolated from a single tumour of a human melanoma xenograft. Cells from passages 7-12 in vitro were given heat treatments of 42.5 degrees C (45 min), 43.5 degrees C (45 min) or 44.5 degrees C (45 min) immediately after exposure to graded doses of radiation. The survival curves after irradiation alone had similar D0 values but differed in the size of the shoulder. The heterogeneity in heat radiosensitization was reflected in differences in decrease of the D0 values. The thermal enhancement ratios, calculated from the D0 values, were in the ranges 1.2 +/- 0.2-1.7 +/- 0.2 (42.5 degrees C), 1.4 +/- 0.3-2.4 +/- 0.4 (43.5 degrees C) and 2.3 +/- 0.4-3.4 +/- 0.4 (44.5 degrees C). Moreover, at 43.5 degrees C the heterogeneity was also reflected in different modifications of the shape of the survival curves. Two lines showed survival curves with a significant shoulder and a relatively low D0 value whereas two other lines had lost the shoulder almost completely but showed relatively high D0 values. All lines showed survival curves with a broad shoulder after heating at 42.5 degrees C, whereas none of the lines showed survival curves with a significant shoulder after heating at 44.5 degrees C.     

Ca-dependent slow action potentials in human skeletal muscle

Slow Ca-action potentials (CaAP) were studied in normal human skeletal muscle fibers obtained during surgery (fibers with both ends cut). Control studies also were carried out with intact as well as cut rat skeletal muscle fibers. Experiments were performed in hypertonic Cl-free saline with 10 or 84 mM Ca and K-channel blockers; muscles were preincubated in a saline containing Cs and tetraethylammonium. A current-clamp technique with two intracellular microelectrodes was used. In human muscle, 14.5% of the fibers showed fully developed CaAPs, 21% displayed nonregenerative Ca responses, and 64.5% showed only passive responses; CaAPs were never observed in 10 mM Ca. In rat muscle, nearly 90% of the fibers showed CaAPs, which were not affected by the cut-end condition. Human and rat muscle fibers had similar membrane potential and conductance in the resting state. In human muscle (22-32 degrees C, 84 mM Ca), the threshold and peak potential during a CaAP were +26 +/- 6 mV and +70 +/- 3 mV, respectively, and the duration measured at threshold level was 1.7 +/- 0.5 sec. In rat muscle, the duration was four times longer. During a CaAP, membrane conductance was assumed to be a leak conductance in parallel with a Ca and a K conductance. In human muscle (22-32 degrees C, 84 mM Ca, 40 micron fiber diameter), values were 0.4 +/- 0.1 microS, 1.1 +/- 0.7 microS, and 0.9 +/- 0.4 microS, respectively. Rat muscle (22-24 degrees C, 84 mM Ca) showed leak and K conductances similar to those found in human fibers. Ca-conductance in rat muscle was double the values obtained in human muscle fibers.     

Possible biphasic sweating response during short-term heat acclimation protocol for tropical natives

The aim of the present study was to evaluate the sweat loss response during short-term heat acclimation in tropical natives. Six healthy young male subjects, inhabitants of a tropical region, were heat acclimated by means of nine days of one-hour heat-exercise treatments (40+/-0 degrees C and 32+/-1% relative humidity; 50% (.)VO(2peak) on a cycle ergometer). On days 1 to 9 of heat acclimation whole-body sweat loss was calculated by body weight variation corrected for body surface area. On days 1 and 9 rectal temperature (T(re)) and heart rate (HR) were measured continuously, and rating of perceived exertion (RPE) every 4 minutes. Heat acclimation was confirmed by reduced HR (day 1 rest: 77+/-5 b.min(-1); day 9 rest: 68+/-3 b.min(-1); day 1 final exercise: 161+/-15 b.min(-1); day 9 final exercise: 145+/-11 b.min(-1), p<0.05), RPE (13 vs. 11, p<0.05) and T(re) (day 1 rest: 37.2+/-0.2 degrees C; day 9 rest: 37.0+/-0.2 degrees C; day 1 final exercise: 38.2+/-0.2 degrees C; day 9 final exercise: 37.9+/-0.1 degrees C, p<0.05). The main finding was that whole-body sweat loss increased in days 5 and 7 (9.49+/-1.84 and 9.56+/-1.86 g.m(-2).min(-1), respectively) compared to day 1 (8.31+/-1.31 g.m(-2).min(-1), p<0.05) and was not different in day 9 (8.48+/-1.02 g.m(-2).min(-1)) compared to day 1 (p>0.05) of the protocol. These findings are consistent with the heat acclimation induced adaptations and suggest a biphasic sweat response (an increase in the sweat rate in the middle of the protocol followed by return to initial values by the end of it) during short-term heat acclimation in tropical natives.     

Effects of fasting on thermoregulatory processes and the daily oscillations in rats

To investigate the mechanism involved in the reduction of body core temperature (T(core)) during fasting in rats, which is selective in the light phase, we measured T(core), surface temperature, and oxygen consumption rate in fed control animals and in fasted animals on day 3 of fasting and day 4 of recovery at an ambient temperature (T(a)) of 23 degrees C by biotelemetry, infrared thermography, and indirect calorimetry, respectively. On the fasting day, 1) T(core) in the light phase decreased (P < 0.05) from the control; however, T(core) in the dark phase was unchanged, 2) tail temperature fell from the control (P < 0.05, from 30.7 +/- 0.1 to 23.9 +/- 0.1 degrees C in the dark phase and from 29.4 +/- 0.1 to 25.2 +/- 0.2 degrees C in the light phase), 3) oxygen consumption rate decreased from the control (P < 0.05, from 24.37 +/- 1.06 to 16.24 +/- 0.69 ml. min(-1). kg body wt(-0.75) in the dark phase and from 18.91 +/- 0.64 to 14.00 +/- 0.41 ml. min(-1). kg body wt(-0.75) in the light phase). All these values returned to the control levels on the recovery day. The results suggest that, in the fasting condition, T(core) in the dark phase was maintained by suppression of the heat loss mechanism, despite the reduction of metabolic heat production. In contrast, the response was weakened in the light phase, decreasing T(core) greatly. Moreover, the change in the regulation of tail blood flow was a likely mechanism to suppress heat loss.     

Hypertonic cryohemolysis: Ionophore and pH effects

Human erythrocytes suspended at 37 degrees C in hypertonic solution of either electrolytes or nonelectrolytes undergo hemolysis when the temperature is lowered toward 0 degrees C (Green, F.A., Jung, C.Y. 1977 J. Membrane Biol. 33:249). In the present studies this hypertonic cryohemolysis was profoundly affected by the pH of incubation, and was completely abolished at ph 5. In hypertonic NaCl, there was an apparent pH optimum at 6--6.5. In hypertonic sucrose, on the other hand, hemolysis increased progressively with increasing pH between 6 and 9. Amphotericin B inhibited hypertonic cryohemolysis in NaCl or KCl solution. No inhibiting effect of amphotericin B was observed when hypertonicity was due to sodium sulfate or sucrose. Valinomycin also inhibited hypertonic cryohemolysis in KCl, but did not affect the process in NaCl or sucrose solution. SITS (4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate) and phloretin interfered with this valinomycin effect, whereas phlorizin did not. These results indicate that dissipation of an osmotic gradient across membranes may be responsible for the inhibition of the hemolysis by these inophores. Iso-osmotic cell shrinkage induced by valinomycin in 150 mM NaCl solution did not result in cryohemolysis.     

Low doses of melatonin and diurnal effects on thermoregulation and tolerance to uncompensable heat stress.

This study examined whether the reported hypothermic effect of melatonin ingestion increased tolerance to exercise at 40 degrees C, for trials conducted either in the morning or afternoon, while subjects were wearing protective clothing. Nine men performed four randomly ordered trials; two each in the morning (0930) and afternoon (1330) after the double-blind ingestion of either two placebo capsules or two 1-mg capsules of melatonin. Despite significant elevations in plasma melatonin to over 1,000 ng/ml 1 h after the ingestion of the first 1-mg dose, rectal temperature (T(re)) was unchanged before or during the heat-stress exposure. Also, all other indexes of temperature regulation and the heart rate response during the uncompensable heat stress were unaffected by the ingestion of melatonin. Initial T(re) was increased during the afternoon (37.1 +/- 0.2 degrees C), compared with the morning (36.8 +/- 0.2 degrees C) exposures, and these differences remained throughout the uncompensable heat stress, such that final T(re) was also increased for the afternoon (39.2 +/- 0.2 degrees C) vs. the morning (39.0 +/- 0.3 degrees C) trials. Tolerance times and heat storage were not different among the exposures at approximately 110 min and 16 kJ/kg, respectively. It was concluded that this low dose of melatonin had no impact on tolerance to uncompensable heat stress and that trials conducted in the early afternoon were associated with an increased T(re) tolerated at exhaustion that offset the circadian influence on resting T(re) and thus maintained tolerance times similar to those of trials conducted in the morning.     

Febrile response to infection in the American alligator (Alligator mississippiensis).

Temperature probes were inserted into the stomachs of juvenile American alligators (Alligator mississippiensis) maintained outdoors at ambient fluctuating temperatures. Internal body temperatures (T(b)) were measured every 15 min for two days, and then the alligators were injected with bacterial lipopolysaccharide (LPS), pyrogen-free saline, or left untreated. Alligators injected intraperitoneally with LPS exhibited maximum T(b)s 2.6+/-1.1 degrees C and 3.5+/-1.2 degrees C higher than untreated control animals on days one and two after treatment, respectively. T(b)s for these animals fell to within control ranges by day three postinjection. Similarly, mean preferred body temperatures (MPBTs) were significantly higher for LPS-injected alligators on days one (4.2+/-1.8 degrees C) and two (3.5+/-1.6 degrees C) after treatment. Intraperitoneal injection of heat-killed Aeromonas hydrophila, a gram-negative bacterium known to infect crocodilians, resulted in a fever while injection of Staphylococcus aureus (gram positive) did not elicit a febrile response. Injection of LPS in alligators maintained indoors in a constant temperature environment resulted in no increase in internal T(b). These results indicate that alligators did not exhibit a febrile response in the absence of a thermal gradient, and suggest that febrile responses observed are probably behavioral in nature.     

Thermal effects of whole head submersion in cold water on nonshivering humans.

This study isolated the effect of whole head submersion in cold water, on surface heat loss and body core cooling, when the confounding effect of shivering heat production was pharmacologically eliminated. Eight healthy male subjects were studied in 17 degrees C water under four conditions: the body was either insulated or uninsulated, with the head either above the water or completely submersed in each body-insulation subcondition. Shivering was abolished with buspirone (30 mg) and meperidine (2.5 mg/kg), and subjects breathed compressed air throughout all trials. Over the first 30 min of immersion, exposure of the head increased core cooling both in the body-insulated conditions (head out: 0.47 +/- 0.2 degrees C, head in: 0.77 +/- 0.2 degrees C; P < 0.05) and the body-exposed conditions (head out: 0.84 +/- 0.2 degrees C and head in: 1.17 +/- 0.5 degrees C; P < 0.02). Submersion of the head (7% of the body surface area) in the body-exposed conditions increased total heat loss by only 10%. In both body-exposed and body-insulated conditions, head submersion increased core cooling rate much more (average of 42%) than it increased total heat loss. This may be explained by a redistribution of blood flow in response to stimulation of thermosensitive and/or trigeminal receptors in the scalp, neck and face, where a given amount of heat loss would have a greater cooling effect on a smaller perfused body mass. In 17 degrees C water, the head does not contribute relatively more than the rest of the body to surface heat loss; however, a cold-induced reduction of perfused body mass may allow this small increase in heat loss to cause a relatively larger cooling of the body core.     

Time course of cytokine, corticosterone, and tissue injury responses in mice during heat strain recovery.

Elevated circulating cytokines are observed in heatstroke patients, suggesting a role for these substances in the pathophysiological responses of this syndrome. Typically, cytokines are determined at end-stage heatstroke such that changes throughout progression of the syndrome are poorly understood. We hypothesized that the cytokine milieu changes during heatstroke progression, correlating with thermoregulatory, hemodynamic, and tissue injury responses to heat exposure in the mouse. We determined plasma IL-1alpha, IL-1beta, IL-2, IL-4, IL-6, IL-10, IL-12p40, IL-12p70, IFN-gamma, macrophage inflammatory protein-1alpha, TNF-alpha, corticosterone, glucose, hematocrit, and tissue injury during 24 h of recovery. Mice were exposed to ambient temperature of 39.5 +/- 0.2 degrees C, without food and water, until maximum core temperature (T(c,Max)) of 42.7 degrees C was attained. During recovery, mice displayed hypothermia (29.3 +/- 0.4 degrees C) and a feverlike elevation at 24 h (control = 36.2 +/- 0.3 degrees C vs. heat stressed = 37.8 +/- 0.3 degrees C). Dehydration ( approximately 10%) and hypoglycemia ( approximately 65-75% reduction) occurred from T(c,Max) to hypothermia. IL-1alpha, IL-2, IL-4, IL-12p70, IFN-gamma, TNF-alpha, and macrophage inflammatory protein-1alpha were undetectable. IL-12p40 was elevated at T(c,Max), whereas IL-1beta, IL-6, and IL-10 inversely correlated with core temperature, showing maximum production at hypothermia. IL-6 was elevated, whereas IL-12p40 levels were decreased below baseline at 24 h. Corticosterone positively correlated with IL-6, increasing from T(c,Max) to hypothermia, with recovery to baseline by 24 h. Tissue lesions were observed in duodenum, spleen, and kidney at T(c,Max), hypothermia, and 24 h, respectively. These data suggest that the cytokine milieu changes during heat strain recovery with similarities between findings in mice and those described for human heatstroke, supporting the application of our model to the study of cytokine responses in vivo.