Metabolic Reactions to Changes in Core and Skin Temperature in Man

The metabolic cold response, i.e. the increase in oxygen consumption above that for the given activity in a neutral environment, was measured in 7 subjects during cooling, resting or swimming in cold water (14, 16, 18, 20°C) and during rewarming in air (T_a 20, 30, 40°C), bicycling or resting. Esophageal temperatures varied between 38 and 34°C. Mean skin temperature was considered as equal to water temperature during cooling, and ranged between 25–35°C during rewarming in the different environments. Both central and peripheral cold stimulation induced metabolic cold responses. The skin temperature was the dominating factor in determining the response, especially in transient states. During rewarming a rising skin temperature suppressed the effects of even very low core temperatures.     

The contribution of blood flow to the skin temperature responses during a cold sensitivity test

Purpose

The presumption in a cold sensitivity test (CST) used for cold injuries is that the skin temperature (T _sk) observed reflects the return of blood flow to the extremity following a local cold challenge. We questioned this assumption.

Methods

Six non-cold injured participants undertook two CSTs in 30 °C air. The control (CON) CST involved 12 min gentle exercise prior to immersing the foot into 15 °C water for 2 min followed by 15 min of spontaneous rewarming. The occlusion (OCC) CST was the same except that blood flow to the foot was occluded during the rewarming period. These results were compared to CSTs from six individuals with non-freezing cold injury and moderate–severe cold sensitivity (CS) and a non-perfused human digit model (NPDM).

Results

Before immersion, great toe skin blood flow (SkBF) was similar in CON and OCC conditions [255 (107) laser Doppler units (LDU)] and was higher than CS [59 (52) LDU]. During rewarming, SkBF in CON returned to 104 % of the pre-immersion value and was higher than both OCC and CS. Great toe T _sk before immersion was lower in CS [28.5 (2.1) °C] compared to CON [34.7 (0.4) °C], OCC [34.6 (0.9) °C] and NPDM [35.0 (0.4) °C]. During rewarming skin/surface temperature in OCC, CS and NPDM were similar and all lower than CON.

Conclusions

SkBF does contribute to the skin rewarming profile during a CST as a faster rate of rewarming was observed in CON compared to either OCC or NPDM. The lower T _sk in CS may be due to a reduced basal SkBF.     

Diabetes affects blood pressure and heart rate responses during acute hypothermia

Many diabetics are cold-intolerant and experience dramatic changes in normal systemic function during hypothermia. Little is known of the cardiovascular adjustments in diabetics exposed to an acute cold stress. In an effort to identify the alterations in mean arterial blood pressure (MAP) and heart rate (HR) in the diabetic during environmental cooling (10 ± 2 °C), we compared the in vivo MAP and HR responses of urethane-anaesthetized (1.5 g kg^?1), streptozotocin-diabetic (STZ, 65 mg kg^?1, n = 12) and control (CON, n = 10) rats during acute hypothermia. MAP was measured directly via an indwelling carotid artery cannula and HR was calculated from the peak systolic pressure waves. Overall, the STZ rats were more cold-intolerant than CON as evidenced by the greater rate of decline in colonic temperature (T_c) from 36 to 28 °C (STZ, 0.16 °C min^?1 vs. CON, 0.06 °C min^?1; P < 0.05). Prior to cooling, HR was significantly lower (P < 0.05) in STZ (282 ± 9 beats min^?1) than in CON rats (399 ± 24 beats min^?1); however, during the acute hypothermic period, HR displayed a similar rate of decline in both groups. With respect to MAP, both groups demonstrated similar pre-experimental pressor responses (CON, 81.7 ± 5.4 vs. STZ, 83.2 ± 5.1 mmHg, P > 0.05). During progressive hypothermia, MAP gradually increased (P < 0.05) in the CON group from baseline (T_c = 36 °C) and reached peak values (118.4 ± 2.5 mmHg) at T_c = 30 °C, while the STZ group failed to exhibit any cold pressor response. At the conclusion of the experiment (T_c = 28 °C), the STZ group pressor response to hypothermia was not different from baseline (T_c = 36 °C, 83.2 ± 5.1 vs. T_c = 28 °C, 77.4 ± 3.4 mmHg; P > 0.05). The absence of any pressor response in the diabetic group during progressive hypothermia reflects the poor overall vasoconstrictive capacity to cooling and could partially explain the rapid decline of core temperature in this group.     

Effect of the adrenocorticostatic agent,metopirone, on thermoregulatory heat production in the European hedgehog

1. Non-shivering thermogenesis (NST) was studied in 8 hedgehogs before and after a single injection of metopirone ditartrate (150–450 mg/kg, i.p.) at a thermoneutral chamber temperature (T_a) of 28°C and during cold exposure (T_a=8°C for 20 min).
2. An average metabolic increase of 149% of the standard metabolic rate (SMR) was observed 12–26 min after an injection of metopirone ditartrate at thermoneutral chamber temperature. Average temperatures simultaneously increased by 2.1°C in the brown adipose tissue (BAT) and 2.0°C in the deep colon. This initial effect lasted for 40±8 min.
3. In a period of 3–48 h after injection of metopirone ditartrate, cold-induced NST was reduced by 89% of SMR (av.). Concomitant exposure to cold caused average temperatures to decrease by 1.0°C in the BAT and 1.1°C in the colon relative to control experiments.
4. Our results suggest the participation of corticosteroids in the control of NST in the hedgehog. As metopirone blocks enzymatic 11 β-hydroxylation in the steroid ring, there is a pronounced increase of endogenous 11-deoxycorticosteroids, such as deoxycorticosterone (DOC). An injection of DOC (3 mg/kg, i.m.) increases NST at thermoneutrality similar to the initial metabolic effect elicited by metopirone ditartrate. The reduced response to cold exposure after several hours may be explained by competitive inhibition of glucocorticoid receptors since there is also an increased production of other 11-deoxycorticosteroids.

     

Serum levels of thyroid and adrenal hormones,testosterone, TSH,LH, GH and prolactin in men after a 2-h stay in a cold room

Twenty healthy male volunteers, dressed in shorts, stayed for 30 min in a room with an ambient temperature of 28°C followed by a stay in a room with a temperature of 10°C for 120 min. The mean skin temperature fell rapidly during the first minutes in the cold but the rectal temperature began to fall as late as at 60 min (o. 1°C) and was 0.4°C lower at the end of the cold exposure than before it. The metabolic rate, and the systolic and diastolic blood pressures, increased, and the pulse rate fell, in the cold. Serum samples were taken before moving to the cold (10°C) room and after the 2-h stay and assayed for 11 hormones. There were no significant changes in the serum concentration of adrenalin, T₃, T₄, testosterone, TSH or LH. The serum level of noradrenaline increased from 4.5 to 6.3 nmol L¹ (P < 0.01) and those of Cortisol, GH and prolactin fell by 20, 87 and 48% (all P < 0.01). The total serum proteins increased by 11% and free fatty acids by 28%(P < 0.01). Our results show that the short-term exposure of adult man to low ambient temperature does not have any effect on the pituitary-thyroid and pituitary-testis axes and adrenal medulla. The increase of noradrenaline is probably due to general activation of the sympathetic nerves at low temperatures. The decreases in the serum levels of GH and prolactin reflect a true decrease in their secretions and may be mediated by inhibitory hypothalamic mechanisms.     

Skin cooling aids cerebrovascular function more effectively under severe than moderate heat stress

Skin surface cooling has been shown to improve orthostatic tolerance; however, the influence of severe heat stress on cardiovascular and cerebrovascular responses to skin cooling remains unknown. Nine healthy males, resting supine in a water-perfusion suit, were heated to +1.0 and +2.0°C elevation in body core temperature (T _c). Blood flow velocity in the middle cerebral artery (transcranial Doppler ultrasound), mean arterial pressure (MAP; photoplethysmography), stroke volume (SV; Modelflow), total peripheral resistance (TPR; Modelflow), heart rate (HR; ECG) and the partial pressure of end-tidal carbon dioxide (P_ETCO₂) were measured continuously during 1-min baseline and 3-min lower body negative pressure (LBNP, −15 mm Hg) when heated without and again with skin surface cooling. Nine participants tolerated +1°C and six participants reached +2°C. Skin cooling elevated (P = 0.004) MAP ~4% during baseline and LBNP at +1°C T _c. During LBNP, skin cooling increased SV (9%; P = 0.010) and TPR (0.9 mm Hg L⁻¹ min, P = 0.013) and lowered HR (13 b min⁻¹, P = 0.012) at +1°C T _c and +2°C T _c collectively. At +2°C T _c, skin cooling elevated P_ETCO₂ ~4.3 mm Hg (P = 0.011) and therefore reduced cerebral vascular resistance ~0.1 mm Hg cm⁻¹ s at baseline and LBNP (P = 0.012). In conclusion, skin cooling under severe heating and mild orthostatic stress maintained cerebral blood flow more effectively than it did under moderate heating, in conjunction with elevated carbon dioxide pressure, SV and arterial resistance.     

Thermoresponsive Properties of Sugar Sensitive Copolymer of N‐Isopropylacrylamide and 3‐(Acrylamido)phenylboronic Acid

Summary: The copolymer of N‐isopropylacrylamide and 3‐(acrylamido)phenylboronic acid (82:18, = 47 000 g · mol^?1) was prepared by free radical polymerization. The copolymer showed typical thermal precipitation behavior in aqueous solutions, its precipitation temperature (T_P) being increased from 23 to 32 °C by increasing the pH from 6.5 to 9.7, because of ionization of the phenylboronate units. The pK_a was evaluated as 8.9 ± 0.1 from the effect of pH on T_P. At pH > 9, i.e., in the anionic form of the copolymer, T_P was affected by a very low concentration of glucose (5.6 μM , ΔT_P = 1–1.5 °C), because of complex formation with a high binding constant. At a higher concentration of polyols (560 μM , pH > 8) the increase of T_P was maximal for the copolymer complexes with fructose (7–10 °C) and decreased in the order: fructose > glucose ≈ mannitol > pentaerythritol > galactose > Tris >glycerol. Di‐ and oligosaccharides (lactose, sucrose, and dextran) caused a slight increase of T_P at pH 7.5–8.7 while no effect was observed at pH > 9. Isothermal dissolution of the copolymer suspension in water (27 °C, pH 8.5) was possible in the presence of fructose or mannitol but required higher concentrations (1.4–3.6 × 10³ μM ) as compared to those which enabled the shift of T_P in the soluble copolymer. The dissolution rate increased with fructose concentrations.

Effect of pH on T_P of poly(NIPAAM‐co‐AAPBA) in the presence of various monosaccharides.     

Passive temperature lability in the elderly

Thermoregulatory responses of nine healthy elderly [seven men and two women; mean age (SD) 73.9 (4.8) years] were compared to those of nine young adult men [26.6 (5.2) years]. They exercised on a cycle ergometer for 20 min at an intensity inducing a heart rate equivalent to 65% of their predicted maximum, and were thereafter immersed in 28°C water. The exercise was conducted to elevate tympanic temperature (T _ty) and initiate a steady rate of sweating. The post-exercise immersion period induced gradual cooling ofT _ty, and changes inT _ty relative to resting levels (ΔT _ty) at which sweating abated and shivering commenced were defined as the ΔT _ty thresholds for the cessation of sweating (T _sw) and onset of shivering (T _sh), respectively. In addition toT _ty, oxygen uptake ( ; 1 · min⁻¹), sweating rate (g · m⁻² · min⁻¹), and forehead skin blood perfusion were also measured during the trials. The mean (SD)T _sw occurred at a significantly (P <0.005) higher ΔT _ty [0.48 (0.18)°C] in the elderly than in the young adults [0.21(0.06)°C], while the Tsh occurred at significantly (P < 0.005) lower ΔT _ty in the elderly [ −0.64 (0.34)°C] than in young adults [−0.22 (0.10)°C]. Decreases in ΔT _ty below the shivering threshold were met with a significantly (P <0.01) reduced . The range of temperature lability between Ts, andT _sh, defined as the null-zone, was significantly greater in the elderly [1.12 (0.39)°C] than in the young adults [0.43 (0.12)°C], and the slope of the vasoconstrictor response in the null-zone was significantly (P <0.001) lower in the elderly subjects. The present study demonstrates a greater passive core temperature lability in older individuals, since the effector responses of sweating and shivering were initiated at higher and lower levels ofT _ty, respectively. The magnitudes of the effector responses beyond the thresholds were also significantly reduced, suggesting that the elderly may be more susceptible to hypo-/hyperthermia during periods of endogenous and/or exogenous thermal stress.     

The effect of exercise intensity on the post-exercise esophageal temperature response

On 2 separate days, nine volunteers aged 23.8 (2.0) years performed 15-min bouts of treadmill running in a temperature-controlled chamber at 29°C at a power output that elicited either 70% (moderate) or 93% (intense) of maximum oxygen consumption. Exercise was followed by a 45-min recovery period. End-exercise esophageal temperature (T _es) was elevated by 0.97°C and 2.17°C above baseline for the moderate and intense exercise trials, respectively. Post-exercise T _es achieved a sustained elevated value of 0.38°C and 0.79°C within 15 min of exercise cessation. Systolic blood pressure (SBP) for both exercise trials became hypotensive for the full recovery period, with the magnitude of the reduction being greater for the intense exercise (P<0.05). Diastolic blood pressure (DBP) was unaffected by exercise intensity and values were lower than baseline between 15 min and 30 min post-exercise (P<0.05). Mean arterial pressure (MAP) was reduced from baseline for both exercise trials, with intense exercise showing a greater decrement (P<0.05). It was shown that the increase in the post-exercise hypotensive response, induced by exercise of increasing intensity, was paralleled by an increase in the magnitude of the post-exercise elevation in T _es (i.e., a difference of 0.41°C between conditions). Electronic Publication     

Shivering endurance and fatigue during cold water immersion in humans

An important component of survival time during cold exposure is shivering endurance. Nine male and three female healthy and fit subjects [mean (SD) age 24.8 (6.3) years, body mass 71.7 (13.2) kg, height 1.75 (0.10) m, body fat 22.7 (7.4)%] were immersed to the upper chest level in cold water for periods ranging from 105 to 388 min on two occasions to test a prediction of shivering endurance. The water was cooled from 20 to 8°C during the first 15 min of immersion and subsequently rewarmed (<20°C) to elicit a near constant submaximal shivering response. The data were divided according to moderate (M) and high (H) levels of shivering intensity. Respective mean total immersion times were 250 (75) and 199 (80) min (P=0.086) at different average shivering intensities of 61 (10) and 69 (8)% relative to maximal shivering (P<0.001). Blood plasma glucose concentration increased during the immersion [from 3.44 (0.54) pre- to 3.94 (0.60) mmol·l^–1 post-immersion (P=0.037)] and levels were higher during M (P=0.012). When compared to a model prediction of shivering endurance, shivering activity continued well beyond the predicted endurance times in 18 out of the 24 trials. The average rates of oxygen consumption over the entire immersion period were lower (P=0.002) during M [0.93 (0.20) l·min^–1] compared to H [1.05 (0.21) l·min^–1), and while these rates did not change during the last 90 min of immersion, there was an increase in fat oxidation. There were no trial differences in the average esophageal (T _es) and mean skin temperatures during the entire immersion period (36.0 and 18.0°C, respectively), yet T _es decreased (P=0.003) approximately 0.4°C during the last 90 min of immersion. When the shivering intensity was normalized to account for this decrease, a significant downward trend of approximately 17%·h^–1 in the normalized shivering intensity was found after the predicted end of shivering endurance. These results suggest that shivering drive, and not shivering intensity per se, decreased during the latter stages of the immersion. Underlying mechanisms such as fatigue and habituation for this diminishing cold sensitivity are discussed. Electronic Publication     

Thermal status of wet-suited divers using closed circuit O2 apparatus in sea water of 17–18.5°C

A wet suit may not provide adequate thermal protection when diving in moderately cold water (17–18°C), and any resultant mild hypothermia may impair performance during prolonged diving. We studied heat exchange during a dive to a depth of 5?m in sea water (17–18.5°C) in divers wearing a full wet suit and using closed-circuit oxygen breathing apparatus. Eight fin swimmers dived for 3.1?h and six underwater scooter (UWS) divers propelled themselves through the water for 3.7?h. The measurements taken throughout the dive were the oxygen pressure in the cylinder and skin and rectal temperatures (T _re). Each subject also completed a cold score questionnaire. The T _re decreased continuously in all subjects. Oxygen consumption in the fin divers (1.40?l?·?min^?1) was higher than that of the UWS divers (1.05?l?·?min^?1). The mean total insulation was 0.087°C?·?m²?·?W^?1 in both groups. Mean body insulation was 37% of the total insulation (suit insulation was 63%). The reduction in T _re over the 1st hour was related to subcutaneous fat thickness. There was a correlation between cold score and T _re at the end of 1?h, but not after that. A full wet suit does not appear to provide adequate thermal protection when diving in moderately cold water.     

The effect of ethnicity on the vascular responses to cold exposure of the extremities

Purpose

Cold injuries are more prevalent in individuals of African descent (AFD). Therefore, we investigated the effect of extremity cooling on skin blood flow (SkBF) and temperature (T _sk) between ethnic groups.

Methods

Thirty males [10 Caucasian (CAU), 10 Asian (ASN), 10 AFD] undertook three tests in 30 °C air whilst digit T _sk and SkBF were measured: (i) vasomotor threshold (VT) test—arm immersed in 35 °C water progressively cooled to 10 °C and rewarmed to 35 °C to identify vasoconstriction and vasodilatation; (ii) cold-induced vasodilatation (CIVD) test—hand immersed in 8 °C water for 30 min followed by spontaneous warming; (iii) cold sensitivity (CS) test—foot immersed in 15 °C water for 2 min followed by spontaneous warming. Cold sensory thresholds of the forearm and finger were also assessed.

Results

In the VT test, vasoconstriction and vasodilatation occurred at a warmer finger T _sk in AFD during cooling [21.2 (4.4) vs. 17.0 (3.1) °C, P = 0.034] and warming [22.0 (7.9) vs. 12.1 (4.1) °C, P = 0.002] compared with CAU. In the CIVD test, average SkBF during immersion was greater in CAU [42 (24) %] than ASN [25 (8) %, P = 0.036] and AFD [24 (13) %, P = 0.023]. Following immersion, SkBF was higher and rewarming faster in CAU [3.2 (0.4) °C min^?1] compared with AFD [2.5 (0.7) °C min^?1, P = 0.037], but neither group differed from ASN [3.0 (0.6) °C min^?1]. Responses to the CS test and cold sensory thresholds were similar between groups.

Conclusion

AFD experienced a more intense protracted finger vasoconstriction than CAU during hand immersion, whilst ASN experienced an intermediate response. This greater sensitivity to cold may explain why AFD are more susceptible to cold injuries.     

Using skin temperature gradients or skin heat flux measurements to determine thresholds of vasoconstriction and vasodilatation

Forearm–fingertip skin temperature differentials (T _sk-diff) are used to indicate vasomotor tone, vasoconstriction defined as having occurred when T _sk-diff≥4°C (Sessler et al. 1987, 1988a, b). This study was conducted to determine whether T _sk-diff or finger pad heat flux (HF) can be used to predict when vasoconstriction and vasodilatation occur. Seven subjects (one female) sat in water at [mean (SD)] 40.7 (0.8)°C until their core temperature (T _c) increased by 1°C, ensuring vasodilatation. The water was then cooled [at a rate of 0.6 (0.1)°C.min^–1] until T _c fell to 0.5°C below pretesting values, causing vasoconstriction. Subjects were then rewarmed in water [41.2 (1.0)°C]. Skin blood flow (SkBF) was measured using laser Doppler flowmetry (LDF) on the left second finger pad [immersed in water at 10.4 (1.4)°C as part of another experiment], and infrared plethysmography on the third finger pad of both hands. T _sk-diff and HF were measured on the right upper limb, which remained in air. When vasodilated, the subjects had a stable T _sk-diff and HF. During cooling, rapid-onset vasoconstriction occurred coincidental with large gradient changes in HF and T _sk-diff (inflection points). In two subjects the original vasoconstriction definition (T _sk-diff≥4°C) was not attained, in the other five this was achieved 31–51 min after vasoconstriction. During rewarming, the T _sk-diff and HF inflection points less accurately reflected the onset of vasodilatation, although with one exception they were within 5 min of the LDF changes. We conclude that T _sk-diff and HF inflection points predict vasoconstriction accurately, and better than T _sk-diff≥4°C. Electronic Publication     

The effects of two nights of sleep deprivation with or without energy restriction on immune indices at rest and in response to cold exposure

The purpose of the study was to determine the effects of two nights of sleep deprivation with or without energy restriction on immune indices at rest and in response to cold exposure. On three randomised occasions ten males slept normally [mean (SD): 436 (21) min night⁻¹; CON], were totally sleep-deprived (SDEP), or were totally sleep-deprived and 90% energy-restricted (SDEP + ER) for 53 h. After 53 h (1200 h) participants performed a seated cold air test (CAT) at 0.0°C until T _re decreased to 36.0°C. Circulating leucocyte counts, neutrophil degranulation, stress hormones and saliva secretory IgA (S-IgA) were determined at 0 h, 24 h, 48 h, pre-CAT, post-CAT, 1-h and 2-h post-CAT. One night on SDEP increased bacterially stimulated neutrophil degranulation (21%, P < 0.05), and two nights on SDEP and SDEP + ER increased S-IgA concentration (40 and 44%; P < 0.01). No other significant effects were observed for immuno-endocrine measures prior to CAT. CAT duration was not different between trials [mean (SD): 133 (53) min] and T _re decreased to 35.9 (0.3)°C. Modest whole-body cooling decreased circulating lymphocyte counts (25%; P < 0.01), S-IgA concentration (36%; P < 0.01) and secretion rate (24%; P < 0.05). A neutrophilia occurred post-CAT on CON and SDEP and 2-h post-CAT on SDEP + ER (P < 0.01). Modest whole-body cooling also decreased neutrophil degranulation on CON (22%) and SDEP (18%; P < 0.05). Plasma cortisol and norepinephrine increased post-CAT (31 and 346%, P < 0.05), but modest whole-body cooling did not alter plasma epinephrine. In conclusion, two nights of SDEP or SDEP + ER did not compromise resting immune indices. However, modest whole-body cooling (T _re 35.9°C) decreased circulating lymphocytes, neutrophil degranulation and S-IgA, but responses were not amplified by prior SDEP or SDEP + ER.     

Effect of photoperiod and acclimation temperature on nonshivering thermogenesis and GDP-binding of brown fat mitochondria in the Djungarian hamsterPhodopus s. sungorus

Acclimation to short photoperiod at 23° C constantT _a causedP. sungorus to improve their NST capacity from 752 to 1,082 mW. Chronic cold exposure in short photoperiod further enhanced the NST capacity, reaching a maximum level of 1,573 mW at –5° C acclimation temperature. Improvements in NST capacity were always accompanied by an increase in brown fat mitochondrial mass and GDP-binding of brown fat mitochondria, in proportion with the cold load applied during temperature acclimation (23°, 15°, 5°, –5° C). Brown fat mitochondrial protein increased from 7.41 mg (23° CT _a, long photoperiod) through 21.6 mg (23° CT _a, short photoperiod) and 81.6 mg (–5° CT _a, short photoperiod). This 10-fold increase was accompanied by a 35-fold increase in GDP-binding (2.0, 7.3 and 71.6 nmol GDP bound, respectively), demonstrating that the increase in capacity for uncoupled respiration in brown fat is of primary significance for thermogenic acclimation to cold as well as to short photoperiod.Abbreviations Cox cytochrome c oxidase - GDP guanosine-5-diphosphate - NST nonshivering thermogenesis - K-Tes potassium salt of N-tris-(hydroxymethyl)-methyl-2-amino-ethane-sulfonic-acid - T _a ambient temperatures - U units of enzyme activity (mol min^–1 at 25° C     

Appropriate and inappropriate hypothalamic cold thermosensitivity in Willow Ptarmigan

Hypothalamic thermosensitivity has been investigated in conscious Willow Ptarmigan (Lagopus lagopus lagopus) provided with chronically implanted hypothalamic perfusion thermodes. The birds were exposed to either cold (T_a-10°C) or warm (T_a+ 25°C) ambient conditions while hypothalamic temperature (T_hy) was clamped for periods of 20 min at different set levels between 28°C and 43°C. The responses of the animals to hypothalamic thermal stimulation were classified by comparing them with those normally found in mammals. At T_a– 10°C hypothalamic heating inhibited ongoing shivering, causing a fall in body-core temperature (T_c) (appropriate mammalian-like response). Strong levels of hypothalamic cooling (T_hy < 34.0°C) also caused a fall in T_c due to inhibition of shivering (inappropriate mammalian-like response). However, weaker levels of hypothalamic cooling (T_hy 34–36°C), facilitated ongoing shivering, resulting in small increases in T_c (appropriate mammalian-like response). At T_a+25°C hypothalamic heating facilitated ongoing panting while weak (T_hy 38°C) levels of hypothalamic cooling inhibited ongoing panting (both mammalian-like responses). The observation of a weak mammalian-like cold hypothalamic thermosensitivity in Willow Ptarmigan indicates that these birds possess some specific cold thermosensors in the hypothalamic region. This finding suggests that hypothalamic temperature dependence in birds and mammals is fundamentally similar.     

Effect of a short period of abstinence from smoking on rewarming patterns of the hands following local cooling

The purpose of the study was to examine the effect of a 12 h period of abstinence from smoking in young and old habitual smokers, on skin rewarming patterns of a hand following local cooling. This was done by comparing changes in peripheral circulation, measured indirectly by monitoring changes in skin surface temperatures of the hand with both infrared (IR) thermography and thermocouples before, during and after immersing the right hand for 2 min in water at 10°C. Included in the study were young male non-smokers (n = 14) and smokers (n = 13), and elderly non-smokers (n = 12) and smokers (n = 14). The results showed no statistically significant difference between young non-smokers and smokers when comparing their response to the local cold challenge. The elderly smokers had a significantly higher hand skin temperature prior to cooling (34.0 ± 0.2°C) and after 80% rewarming (32.1 ± 0.2°C) (i.e. when the skin temperature in the “cooled” hand has regained 80% of the cold induced drop in temperature), compared to elderly non-smokers (33.3 ± 0.2 and 31.3 ± 0.2°C, respectively). The elderly smoking subjects also had a faster recovery after cooling (9.7 ± 0.8 min) than the elderly non-smoking subjects (16.7 ± 2.6 min). A follow-up study with seven elderly smokers, who had smoked as usual until 2 h before the experiment, showed responses lying between the non-smokers and smokers who had had a longer period of abstinence (12 h) from smoking. In conclusion, we have demonstrated using IR-thermal imaging that elderly subjects who have smoked for many years have slightly warmer hand skin temperature when they abstain from smoking. Even a period of abstinence from smoking of a few hours can affect the way in which elderly subjects respond to a local cold challenge, recovering more rapidly then their non-smoking counterparts.     

Down syndrome, autoimmunity and T regulatory cells

Autoimmune diseases are more represented in Down syndrome (DS) individuals compared to chromosomally normal people. Natural T regulatory cells (nT_reg) have been considered to be primary in the role of controlling the intensity and targets of the immune response. We have investigated the phenotypical and functional alteration of nT_reg in a group of DS people. The phenotypical characteristic of T_reg cells of 29 DS was analysed and compared with an age‐matched healthy control group. The inhibitory potential of CD4⁺CD25^highCD127^low T regulatory cells was evaluated on autologous CD4⁺CD25^‐ T cell proliferation in response to activation with a mytogenic pan‐stimulus (anti‐CD2, anti‐CD3 and anti‐CD28 antibodies). The CD4⁺CD25^high cells in the DS and control groups were 2·692 ± 0·3808%, n = 29 and 1·246 ± 0·119, n = 29%, respectively (P = 0.0007), with a percentage of forkhead box protein 3 (FoxP3)‐expressing cells of 79·21 ± 3·376%, n = 29 and 59·75 ± 4·496%, respectively (P = 0.0015). CD4⁺CD25⁺FoxP3⁺ cells were increased in peripheral blood from DS subjects (DS mean 5·231 ± 0·6065% n = 29, control mean 3·076 ± 0·3140% n = 29). The majority of CD4⁺CD25^high were CD127^low and expressed a high percentage of FoxP3 (natural T_reg phenotype). While the proliferative capacity of DS T cells was not altered significantly compared to normal individuals, a reduced inhibitory potential of T_reg compared to healthy controls was clearly observed (mean healthy control inhibition in T_eff : T_reg 1:1 co‐culture: 58·9% ± 4·157%, n = 10 versus mean DS inhibition in T_eff : T_reg 1:1 co‐culture: 39·8 ± 4·788%, n = 10, P = 0.0075; mean healthy control inhibition in T_eff : T_reg 1:0·5 co‐culture: 45·10 ± 5·858%, n = 10 versus DS inhibition in T_eff : T_reg 1:0·5 co‐culture: 24·10 ± 5·517%, n = 10, P = 0.0177). DS people present an over‐expressed peripheral nT_reg population with a defective inhibitory activity that may partially explain the increased frequency of autoimmune disease.     

Thermoregulation, pacing and fluid balance during mass participation distance running in a warm and humid environment

Deep body temperature (T _c), pacing strategy and fluid balance were investigated during a 21-km road race in a warm and humid environment. Thirty-one males (age 25.3 ± 3.2 years; maximal oxygen uptake 59.1 ± 4.2 ml kg⁻¹ min⁻¹) volunteered for this study. Continuous T _c responses were obtained in 25 runners. Research stations at approximately 3-km intervals permitted accurate assessment of split times and fluid intake. Environmental conditions averaged 26.4°C dry bulb temperature and 81% relative humidity. Peak T _c was 39.8 ± 0.5 (38.5–40.7) °C with 24 runners achieving T _c > 39.0°C, 17 runners ≥39.5°C, and 10 runners ≥40.0°C. In 12 runners attaining peak T _c ≥ 39.8°C, running speed did not differ significantly when T _c was below or above this threshold (208 ± 15 cf. 205 ± 24 m min⁻¹; P = 0.532). Running velocity was the main significant predictor variable of ∆T _c at 21 km (R ² = 0.42, P < 0.001) and was the main discriminating variable between hyperthermic (T _c ≥ 39.8°C) and normothermic runners (T _c < 39.8°C) up to 11.8 km. A reverse J-shaped pacing profile characterised by a marked reduction in running speed after 6.9 km and evidence of an end-spurt in 16 runners was observed. Variables relating to fluid balance were not associated with any T _c parameters or pacing. We conclude that hyperthermia, defined by a deep body temperature greater than 39.5°C, is common in trained individuals undertaking outdoor distance running in environmental heat, without evidence of fatigue or heat illness.     

Critical power of knee extension exercises does not depend upon maximal strength

A possible dependence of critical power (CP) and the Y-intercept of the work/exhaustion time relationship (Y _intercept) on maximal muscular strength of the same muscle group has been studied in nine endurance-trained subjects, seven gymnasts, and seven weight-lifters. CP was calculated as being equal to the slope of the linear relationship between exhaustion time and the work performed at exhaustion on a knee extension ergometer. Y _intercept was equal to the intercept between this relationship and the work axis. The muscular strength of the knee was evaluated by measuring the torques exerted on a Biodex knee isokinetic dynamometer at four angular velocities: 0° · s⁻¹ (T₀), 90° · s⁻¹ (T₉₀), 180° · s⁻¹ (T₁₈₀) and 240° · s⁻¹ (T₂₄₀). The results of the present study do not support the hypothesis that CP depends upon maximal strength. Indeed, CP was not correlated with T₀, T₉₀, T₁₈₀ or T₂₄₀ (|r| < 0.01). Y _intercept was significantly and positively correlated only with T₉₀. Accepted: 1 November 1999