Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress

The purpose ofthe present study was to determine the separate and combined effects ofaerobic fitness, short-term heat acclimation, and hypohydration ontolerance during light exercise while wearing nuclear, biological, andchemical protective clothing in the heat (40°C, 30% relativehumidity). Men who were moderately fit [(MF); <50ml · kg¹ · min¹maximal O₂ consumption;n = 7] and highly fit[(HF); >55ml · kg¹ · min¹maximal O₂ consumption;n = 8] were tested while theywere euhydrated or hypohydrated by ~2.5% of body mass throughexercise and fluid restriction the day preceding the trials. Tests wereconducted before and after 2 wk of daily heat acclimation (1-htreadmill exercise at 40°C, 30% relative humidity, while wearingthe nuclear, biological, and chemical protective clothing). Heatacclimation increased sweat rate and decreased skin temperature andrectal temperature (T_re) in HF subjects but had no effecton tolerance time (TT). MF subjects increased sweat rate but did notalter heart rate, Tre, or TT. In both MF and HF groups, hypohydration significantly increased T_re and heart rate and decreasedthe respiratory exchange ratio and the TT regardless of acclimationstate. Overall, the rate of rise of skin temperature was less, whileT_re, the rate of rise of T_re, and the TTwere greater in HF than in MF subjects. It was concluded thatexercise-heat tolerance in this uncompensable heat-stress environmentis not influenced by short-term heat acclimation but is significantlyimproved by long-term aerobic fitness.

     

Posthemorrhagic antipyresis: what stage of fever genesis is affected?

Romanovsky, Andrej A., and Yelena K. Karman.Posthemorrhagic antipyresis: what stage of fever genesis isaffected? J. Appl. Physiol. 83(2):359-365, 1997.It has been shown that hemorrhage leads to adecreased thermal responsiveness to lipopolysaccharide (LPS). The aimof this study was to clarify what stage of fever genesis[production of endogenous pyrogens such as interleukin-1 (IL-1),increase of the prostaglandin E₂(PGE₂) concentration in braintissue, activation of cold-defense effectors] is deficient inposthemorrhagic antipyresis. In adult rabbits, we evaluated the effectof acute hemorrhage (15 ml/kg) on the rectal temperature (T_re) responses to LPS fromSalmonella typhi (200 ng/kg iv),ethanol-purified preparation of homologous IL-1 (1 ml from 3.5 × 10⁷ cells, 1.5 ml/kg iv), andPGE₂ (1 µg,intracisternal injection). The effect of hemorrhage onT_re was also studied in afebrilerabbits, both at thermoneutrality (23°C) and during ramp cooling(to 7°C). The hemorrhage strongly attenuated the biphasicLPS-induced fever (a T_re rise of0.4 ± 0.1 instead of 1.2 ± 0.2°C at the time of the secondpeak), the monophasic T_re responseto IL-1 (by ~0.5°C for over 1-5 h postinjection), and thePGE₂-induced hyperthermia (0.4 ± 0.1 vs. 0.9 ± 0.1°C, maxima). In afebrileanimals, the hemorrhage neither affectedT_re at thermoneutrality norchanged the T_re response to coldexposure. The data suggest that neither insufficiency of cold-defenseeffectors nor lack of endogenous mediators of fever (IL-1,PGE₂) can be the only or eventhe major cause of posthemorrhagic antipyresis. Wespeculate that fever genesis is altered at a stage occurring after theintrabrain PGE₂ level is increasedbut before thermoeffectors are activated.

     

Dehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise

González-Alonso, José, RicardoMora-Rodríguez, Paul R. Below, and Edward F. Coyle.Dehydration markedly impairs cardiovascular function inhyperthermic endurance athletes during exercise. J. Appl. Physiol. 82(4): 1229-1236, 1997.Weidentified the cardiovascular stress encountered by superimposingdehydration on hyperthermia during exercise in the heat and themechanisms contributing to the dehydration-mediated stroke volume (SV)reduction. Fifteen endurance-trained cyclists [maximalO₂ consumption(O_{2 max}) = 4.5 l/min] exercised in the heat for 100-120 min and either became dehydrated by 4% body weight or remained euhydrated by drinkingfluids. Measurements were made after they continued exercise at 71%O_{2 max} for 30 minwhile 1) euhydrated with anesophageal temperature (T_es) of38.1-38.3°C (control); 2)euhydrated and hyperthermic (39.3°C);3) dehydrated and hyperthermic withskin temperature (T_sk) of34°C; 4) dehydrated withT_es of 38.1°C and T_sk of 21°C; and5) condition4 followed by restored blood volume. Compared withcontrol, hyperthermia (1°C T_esincrease) and dehydration (4% body weight loss) each separatelylowered SV 7-8% (11 ± 3 ml/beat;P < 0.05) and increased heart ratesufficiently to prevent significant declines in cardiac output.However, when dehydration was superimposed on hyperthermia, thereductions in SV were significantly (P < 0.05) greater (26 ± 3 ml/beat), and cardiac output declined 13% (2.8 ± 0.3 l/min). Furthermore, mean arterialpressure declined 5 ± 2%, and systemic vascular resistanceincreased 10 ± 3% (both P < 0.05). When hyperthermia wasprevented, all of the decline in SV with dehydration was due to reducedblood volume (~200 ml). These results demonstrate that thesuperimposition of dehydration on hyperthermia during exercise in theheat causes an inability to maintain cardiac output and blood pressurethat makes the dehydrated athlete less able to cope with hyperthermia.

     

Thermal and metabolic responses to cold-water immersion at knee, hip, and shoulder levels

Lee, Dae T., Michael M. Toner, William D. McArdle, IoannisS. Vrabas, and Kent B. Pandolf. Thermal and metabolic responses tocold-water immersion at knee, hip, and shoulder levels.J. Appl. Physiol. 82(5):1523-1530, 1997.To examine the effect of cold-water immersion atdifferent depths on thermal and metabolic responses, eight men (25 yrold, 16% body fat) attempted 12 tests: immersed to the knee (K), hip(H), and shoulder (Sh) in 15 and 25°C water during both rest (R) orleg cycling [35% peak oxygen uptake; (E)] for up to 135 min. At 15°C, rectal (T_re)and esophageal temperatures(T_es) between R and E were notdifferent in Sh and H groups (P > 0.05), whereas both in K group were higher during E than R(P < 0.05). At 25°C,T_re was higher(P < 0.05) during E than R at alldepths, whereas T_es during E washigher than during R in H and K groups.T_re remained at control levels inK-E at 15°C, K-E at 25°C, and in H-E groups at 25°C,whereas T_es remained unchanged inK-E at 15°C, in K-R at 15°C, and in all 25°C conditions (P > 0.05). During R and E, themagnitude of T_re change wasgreater (P < 0.05) than themagnitude of T_es change in Sh andH groups, whereas it was not different in the K group(P > 0.05). Total heat flow wasprogressive with water depth. During R at 15 and 25°C, heatproduction was not increased in K and H groups from control level(P > 0.05) but it did increase in Shgroup (P < 0.05). The increase inheat production during E compared with R was smaller(P < 0.05) in Sh (121 ± 7 W/m² at 15°C and 97 ± 6 W/m² at 25°C) than in H (156 ± 6 and 126 ± 5 W/m²,respectively) and K groups (155 ± 4 and 165 ± 6 W/m², respectively). These datasuggest that T_re andT_es respond differently duringpartial cold-water immersion. In addition, water levels above knee in15°C and above hip in 25°C cause depression of internal temperatures mainly due to insufficient heat production offsetting heatloss even during light exercise.

     

A relationship between heat loss and sleepiness: effects of postural change and melatonin administration

Kräuchi, Kurt, Christian Cajochen, and AnnaWirz-Justice. A relationship between heat loss and sleepiness:effects of postural change and melatonin administration.J. Appl. Physiol. 83(1): 134-139, 1997.Both the pineal hormone melatonin (Mel) and postural changeshave thermoregulatory sequelae. The purpose of the study was toevaluate their relationship to subjective sleepiness. Eight healthyyoung men were investigated under the unmasking conditions of aconstant routine protocol. Heart rate, rectal temperature(T_re), skin temperatures (foot,T_fo; and stomach), and subjectivesleepiness ratings were continuously recorded from 1000 to 1700. Mel (5 mg po) was administered at 1300, a time when Mel should not phaseshift the circadian system. Both the postural change at1000 from upright to a supine position (lying down in bed) and Meladministration at 1300 reduced T_reand increased T_fo in parallel withincreased sleepiness. These findings suggest that under comfortableambient temperature conditions, heat loss via the distal skin regions(e.g., feet) is a key mechanism for induction of sleepiness as corebody temperature declines.

     

Chronic hormone replacement therapy alters thermoregulatory and vasomotor function in postmenopausal women

Brooks, E. M., A. L. Morgan, J. M. Pierzga, S. L. Wladkowski, J. T. O'Gorman, J. A. Derr, and W. L. Kenney. Chronic hormone replacement therapy alters thermoregulatory and vasomotor function in postmenopausal women. J. Appl.Physiol. 83(2): 477-484, 1997.This investigationexamined effects of chronic (2 yr) hormone replacement therapy (HRT),both estrogen replacement therapy (ERT) and estrogen plus progesteronetherapy (E+P), on core temperature and skin blood flow responses ofpostmenopausal women. Twenty-five postmenopausal women [9 not onHRT (NO), 8 on ERT, 8 on E+P] exercised on a cycle ergometer for1 h at an ambient temperature of 36°C. Cutaneous vascularconductance (CVC) was monitored by laser-Doppler flowmetry, and forearmvascular conductance (FVC) was measured by using venous occlusionplethysmography. Iontophoresis of bretylium tosylate was performedbefore exercise to block local vasoconstrictor (VC) activity at oneskin site on the forearm. Rectal temperature (T_re) was ~0.5°C lower forthe ERT group (P < 0.01) comparedwith E+P and NO groups at rest and throughout exercise. FVC: mean body temperature (T_b) and CVC:T_b curves were shifted~0.5°C leftward for the ERT group(P < 0.0001). Baseline CVC wassignificantly higher in the ERT group(P < 0.05), but there was nointeraction between bretylium treatment and groups once exercise wasinitiated. These results suggest that1) chronic ERT likely acts centrally to decrease T_re,2) ERT lowers theT_re at which heat-loss effector mechanisms are initiated, primarily by actions on active cutaneous vasodilation, and 3) addition ofexogenous progestins in HRT effectively blocks these effects.

     

Influence of pregnancy on the febrile response to ICV administration of PGE1 in rats studied in a thermocline

Eliason, Heather L., and James E. Fewell. Influence ofpregnancy on the febrile response to ICV administration ofPGE₁ in rats studied in athermocline. J. Appl. Physiol. 82(5):1453-1458, 1997.Rats near term of pregnancy have an attenuatedfebrile response to intracerebroventricular (ICV) injection ofprostaglandin E₁ (PGE₁) when they are studied atan ambient temperature below their thermoneutral zone. Given thatnonshivering thermogenesis in brown adipose tissue is impaired inrodents near term of pregnancy, it is possible that the attenuatedfebrile response is forced by impairment of this component of theautonomic thermoregulatory response. If this were the case, thennear-term pregnant rats should develop a "normal" fever afterPGE₁ administration if they werestudied in a thermocline where they could utilize behavioral as well asautonomic thermoregulatory effectors to increase their body coretemperature (T_bc). Experimentswere, therefore, carried out on 13 nonpregnant and 14 pregnantchronically instrumented rats in a thermocline (temperature gradient10-40°C) to investigate theirT_bc responses to ICV injection ofPGE₁. ICV injection of 0.2 µgPGE₁ produced significantincreases in T_bc and fever index in both nonpregnant and pregnant animals (day19 of gestation); the increases, however, weresignificantly attenuated in the pregnant compared with the nonpregnantrats. Behavioral (e.g., selected ambient temperature) and autonomic(e.g., oxygen consumption) thermoregulatory effectors were activated toincrease T_bc after ICVPGE₁ in both groups of animals,but the duration of activation was shortened in pregnant compared withnonpregnant rats. The abbreviated thermoregulatory effector responsesand the resulting attenuated febrile response toPGE₁ in the pregnant rats may have resulted from a pregnancy-related activation of an endogenous antipyretic system.

     

HSP70 expression in the CNS in response to exercise and heat stress in rats

We havepreviously documented the regional distribution of 70-kDa heat shockprotein (HSP70) in brains of rats made hyperthermic by brief exposureto high-powered microwaves (HPM; 2.06 GHz). We now compare HSP70expression induced by HPM exposure to that induced by exertionaland/or environmental heat stress. Rats were chronicallyimplanted with a temperature probe guide in the hypothalamic region ofthe brain (T_br). After recovery,the following treatment groups were examined: HPM; sham exposed;treadmill exercise at room temperature (24°C; Ex-1); treadmillexercise in a warm environment (34°C; Ex-2); and sedentary groups(Sed-1 and Sed-2), in which ambient temperature was adjusted so thatthe T_br mimicked the T_br in the corresponding exercisegroups. Significant HSP70 expression occurred only in the hyperthermic(Ex-2, Sed-2, and HPM) groups. The pattern of HSP70 expression wassimilar among Ex-2 and Sed-2 rats but differed from that in HPM rats.We conclude that 1) the pattern ofHSP70 expression differs between HPM and nonmicrowave heating, and2) exercise alone was not sufficientto induce central HSP70 expression.

     

Heat stroke: opioid-mediated mechanisms

Romanovsky, Andrej A., and Clark M. Blatteis. Heatstroke: opioid-mediated mechanisms. J. Appl.Physiol. 81(6): 2565-2570, 1996.In our previousstudy in guinea pigs, intensive and prolonged intraperitoneal heating(IPH) caused heat stroke characterized by high mortality andaccompanied by two paradoxical phenomena: ear skin vasoconstriction ata high body temperature (T_b)(hyperthermia-induced vasoconstriction) and a post-IPHT_b fall at an ambient temperature (T_a) below thermoneutrality(hyperthermia-induced hypothermia). In this study, we tested thehypothesis that the mechanisms of the two phenomena involve endogenousopioid agonists. Experiments were conducted in 24 unanesthetized,lightly restrained guinea pigs, each chronically implanted with anintraperitoneal thermode and intrahypothalamic thermocouple. Thethermoregulatory effects of a wide-spectrum opioid-receptor antagonist,naltrexone (NTX; 50 or 0 µmol/kg sc), were studied in IPH-inducedheat stroke and under normal conditions. IPH was accomplished byperfusing (50 ml/min; 80 min) water (45°C) through the thermode.T_a was maintained at ~24°C.Skin vasodilation occurred at the onset of IPH but later changed tovasoconstriction despite high T_band continuing IPH. IPH-induced hyperthermia (1.8 ± 0.1°C) was followed by a post-IPH T_b fall (5.1 ± 0.7°C; calculated for the survivors only). The 48-h mortality ratewas 50%. NTX prevented the hyperthermia-induced vasoconstriction andattenuated the hyperthermia-induced hypothermia (1.8 ± 0.4°C). None of the NTX-treated animals died. The effects of NTX onT_b regulation under normalconditions were minor. These results indicate that the phenomena ofboth hyperthermia-induced vasoconstriction and hyperthermia-inducedhypothermia are opioid dependent. The latter is speculated to reflectopioid-mediated inhibition of metabolism; the former is thought toresult from opioid-induced hemodynamic alterations. Because bothphenomena did not occur in the NTX-treated survivors, the skinvasoconstriction at high T_b andthe posthyperthermia T_b fall maybe viewed as markers of the severity of heat stroke. It is suggestedthat opioid antagonists may have therapeutic potential in heat-induceddisorders.

     

Age, fitness, and regional blood flow during exercise in the heat

Ho, C. W., J. L. Beard, P. A. Farrell, C. T. Minson, and W. L. Kenney. Age, fitness, and regional blood flow during exercisein the heat. J. Appl. Physiol. 82(4):1126-1135, 1997.During dynamic exercise in warm environments,the requisite increase in skin blood flow (SkBF) is supported by anincrease in cardiac output (c) and decreases insplanchnic (SBF) and renal blood flows (RBF). To examine interactionsbetween age and fitness in determining this integrated response, 24 men, i.e., 6 younger fit (YF), 6 younger sedentary (YS), 6 older fit (OF), and 6 older sedentary (OS) rested for 50 min, thenexercised at 35 and 60% maximalO₂ consumption(O_{2 max}) at36°C ambient temperature. YF had a significantly higherc and SkBF than any other group during exercise,but fitness level had no significant effect on any measured variable inthe older men. At 60%O_{2 max}, younger subjects had significantly greater decreases in SBF and RBF than theolder men, regardless of fitness level. Total flow redirected fromthese two vascular beds (SBF + RBF) followed YF >> YS > OF > OS. A rigorous 4-wk endurance training programincreased exercise SkBF in OS, but SBF and RBF were unchanged.Under these conditions, older men distribute cdifferently to regional circulations, i.e., smaller increases in SkBFand smaller decreases in SBF and RBF. In younger subjects, the higherSkBF associated with a higher fitness level is a function of both ahigher c and a greater redistribution of flow fromsplanchnic and renal circulations, but the attenuated splanchnic andrenal vasoconstriction in older men does not appear to change withenhanced aerobic fitness.

     

Recovery from mild hypothermia can be accelerated by mechanically distending blood vessels in the hand

Peripheral vasoconstriction decreases thermalconductance of hypothermic individuals, making it difficult to transferexternally applied heat to the body core. We hypothesizedthat increasing blood flow to the skin of a hypothermic individualwould enhance the transfer of exogenous heat to the body core, therebyincreasing the rate of rewarming. External auditory meatus temperature(T_EAM) was monitored inhypothermic subjects during recovery from general anesthesia. In 10 subjects, heat (45-46°C, water-perfused blanket) was appliedto a single forearm and hand that had been placed in a subatmosphericpressure environment (30 to 40 mmHg) to distend the bloodvessels. Heat alone was applied to control subjects (n = 6). The application ofsubatmospheric pressure resulted in a 10-fold increase in rewarmingrates as determined by changes inT_EAM [13.6 ± 2.1 (SE)°C/h in the experimental group vs. 1.4 ± 0.1°C/h in thecontrol group; P < 0.001]. Inthe experimental subjects, the rate of change ofT_EAM decreased sharply asT_EAM neared the normothermic range.

     

Thermoregulatory responses to cold transients: effects of menstrual cycle in resting women

Effects of themenstrual cycle on heat loss and heat production(M) and core and skin temperatureresponses to cold were studied in six unacclimatized female nonsmokers(18-29 yr of age). Each woman, resting supine, was exposed to acold transient (ambient temperature = mean radiant temperature = 20 to5°C at 0.32°C/min, relative humidity = 50 ± 2%, wind speed = 1 m/s) in the follicular (F) phase(days 2-6) and midluteal (L)phase (days 19-23) of her menstrual cycle. Clothed in each of two ensembles with different thermal resistances, women performed multiple experiments in the F andL phases. Thermal resistance was 0.2 and 0.4 m² · K · W¹for ensembles A andB, respectively. Esophagealtemperature (T_es), mean weightedskin temperature(_sk),finger temperature (T_fing), andarea-weighted heat flux were recorded continuously. Rate of heat debt(S) and integrated mean bodytemperature(_b,i)were calculated by partitional calorimetry throughout the cold ramp. Extensive peripheral vasoconstriction in the F phase during early periods of the ramp elevated T_esabove thermoneutral levels. Shivering thermogenesis(M = M  M_basal,W /m²) was highly correlated withdeclines in_sk andT_fing(P <0.0001). There was a reducedslope in M as a function of_b,i inthe L phase with ensembles A(P < 0.02) andB (P < 0.01). Heat flux was higher andS was less in the L phases withensemble A(P < 0.05). An analytic modelrevealed that_sk andT_es contribute as additive inputsand T_fing has a multiplicativeeffect on the total control of Mduring cold transients(R² = 0.9).Endogenous hormonal levels at each menstrual cycle phase, coretemperature and_skinputs, vascular responses, and variations in body heat balance must beconsidered in quantifying thermoregulatory responses in women duringcold stress.

     

Blood-gas measurements adjusted for temperature at three sites during incremental exercise in the horse

Rectal temperature(T_re) is often used to adjustmeasurements of blood gases, but these adjusted measurements may notapproximate temperatures during intense exercise at main sites of gasexchange: muscle and lung. To evaluate differences in blood gasesbetween sites, temperatures (T) were measured with thermocouples in the rectum (re), in mixed venous blood (), ingluteal muscle (mu), and on the skin (sk) in seven Arabian horses asthey underwent an incremental exercise test on a treadmill. Bloodsamples were drawn from the carotid artery and pulmonary artery (mixedvenous) 30 s before each increase in speed and during recovery. Blood gases and pH were measured at 37°C, and all variables were adjusted to T_re,, andT_mu. Adjusted variables duringexercise and recovery were significantly different from each other atthe three sites. Linear and polynomial equations described the timecourse of venous temperature and fromT_re andT_sk during exercise and fromT_sk during recovery.Interpretation of changes in muscle metabolism and gas exchanges basedon blood-gas measurements is improved if they are adjustedappropriately to T_mu or, which may be predicted fromT_sk in addition toT_re during strenuous exercise andfrom T_sk during recovery.

     

Expanded blood volumes contribute to the increased cardiovascular performance of endurance-trained older men

To determine whether expanded intravascular volumes contributeto the older athlete's higher exercise stroke volume and maximal oxygen consumption(O_{2 max}),we measured peak upright cycle ergometry cardiac volumes(^99mTc ventriculography) andplasma (¹²⁵I-labeled albumin) andred cell (NaCr⁵¹) volumes in 7 endurance-trained and 12 age-matched lean sedentary men. The athleteshad ~40% higherO_{2 max} values thandid the sedentary men and larger relative plasma (46 vs. 38 ml/kg), red cell (30 vs. 26 ml/kg), and total blood volumes (76 vs. 64 ml/kg) (allP < 0.05). Athletes hadlarger peak cycle ergometer exercise stroke volume indexes (75 vs. 57 ml/m²,P < 0.05) and 17% largerend-diastolic volume indexes. In the total group,O_{2 max}correlated with plasma, red cell, and total blood volumes(r = 0.61-0.70,P < 0.01). Peakexercise stroke volume was correlated directly with the blood volumevariables (r = 0.59-0.67,P < 0.01). Multiple regressionanalyses showed that fat-free mass and plasma or total blood volume,but not red cell volume, were independent determinants ofO_{2 max} andpeak exercise stroke volume. Plasma and total blood volumes correlated with the stroke volume and end-diastolic volume changes from rest topeak exercise. This suggests that expanded intravascular volumes, particularly plasma and total blood volumes, contribute to the higherpeak exercise left ventricular end-diastolic volume, stroke volume, andcardiac output and hence the higherO_{2 max} in master athletes by eliciting both chronic volume overload and increased utilization of the Frank-Starling effect during exercise.

     

Small bowel tonometry is more accurate than gastric tonometry in detecting gut ischemia

Gastric tonometerPCO₂ measurement may help identifygut ischemia in critically ill patients but is frequentlyassociated with large measurement errors. We tested the hypothesis thatsmall bowel tonometer PCO₂measurement yields more accurate information. In 10 anesthetized,mechanically ventilated pigs subject to progressive hemorrhage, wemeasured gut oxygen delivery and consumption. We also measuredtonometer PCO₂ minus arterialPCO₂(PCO₂) and calculated the corresponding intracellular pH from tonometers placed in the stomach and jejunum. We found that the correlation coefficient(r²) forbiphasic gut oxygen delivery-PCO₂relationships was 0.29 ± 0.52 for the gastric tonometer vs. 0.76 ± 0.25 for the small bowel tonometer(P < 0.05). In addition, thecritical gastric tonometer PCO₂was excessively high and variable (62.9 ± 39.6) compared with thecritical small bowel tonometerPCO₂ (17.0 ± 15.0, P < 0.01). Small bowel tonometerPCO₂ was closely correlated withsuperior mesenteric vein PCO₂(r² = 0.81, P < 0.001), whereas gastrictonometer PCO₂ was not(r² = 0.13, P = not significant). Weconclude that measurement of gastric tonometerPCO₂ yields excessively noisy andinaccurate data on the onset of gut anaerobic metabolism in hemorrhagicshock. Small bowel tonometer PCO₂ isless noisy and, as a result, is superior in detecting gut hypoperfusionand the onset of anaerobic metabolism.

     

Appropriate thermal manipulations eliminate tremors in rats recovering from halothane anesthesia

Grahn, D. A., M. C. Heller, J. E. Larkin, and H. C. Heller.Appropriate thermal manipulations eliminate tremors in ratsrecovering from halothane anesthesia. J. Appl.Physiol. 81(6): 2547-2554, 1996.Tremors arecommon in mammals emerging from anesthesia. To determine whetherappropriate thermal manipulations immediately before emergence fromanesthesia are sufficient to eliminate these tremors,electroencephalographic (EEG) and electromyographic (EMG) activities,hypothalamic temperature (T_hy),and O₂ consumption were monitoredin 12 rats recovering from halothane anesthesia under three thermalregimes. EEG and EMG activities were recorded throughout anesthesia andserved as feedback signals for controlling anesthetic depth. Duringanesthesia, T_hy was either1) allowed to fall to32-34°C, 2) maintained at37-39°C, or 3) allowed to fall to 32-34°C and then raised to 37-39°C. Whenhypothermic on emergence from anesthesia, all of the animals exhibitedpostanesthetic tremors that persisted untilT_hy values returned tonormothermia. None of the animals expressed postanesthetic tremors whennormothermic on emergence from anesthesia. In addition, the timebetween emergence from anesthesia (as determined by EEG/EMG parameters)and the initiation of coordinated motor activities was significantlydecreased in the normothermic animals.

     

Mechanisms of Thermoregulation in Flying Bees

SYNOPSIS. Thermoregulation of elevated thorax temperatures isnecessary for bees to achieve the high rates of power productionrequired for flight, and is a key factor allowing them to occupywidely varying thermal environments. However, the mechanismsby which bees thermoregulate during flight are poorly understood.Thermoregulation is accomplished by balancing heat gain andheat loss via the following routes: convection, evaporation,and metabolic heat production. There appears to be a diversityof thermoregulatory mechanisms employed during flight amongbee species. Some species, particularly Bombus spp., activelyincrease the distribution of thoracic heat to the abdomen duringflight as air temperature (T_a) rises, and apparently thermoregulateby varying convective heat loss. However, thermal variationin convection has not been directly measured for any free-flyingbee. Above 33°C, flying Apis mellifera greatly increaseevaporative heat loss with T_a, and many other species "tongue-lash"during flight at high T_as or when artificially heated. Thus,evaporation seems to be important for preventing overheatingduring flight at very high T_as. Flying A. mellifera and Centrispallida strongly decrease metabolic rate as T_a increases, suggestingthat they are varying metabolic heat production for thermoregulationand not aerodynamic requirements. Variation in metabolic heatproduction appears to be mediated by changes in wingbeat kinematics,since wingbeat frequency decreases with T_a for A. melliferaand Centris spp. It is unknown if the decrease in flight metabolicrate at higher T_as occurs secondarily as a consequence of greaterefficiency or if it is truly an active response.     

Effects of hyperthermia on enzymes and electrolytes in blood and cerebrospinal fluid in dogs

The effects of exposure to various degrees of heat stress on serum glutamate—oxaloacetate transaminase (SGOT), serum glutamate-pyruvate transaminase (SGPT), alkaline phosphatase (ALK-P-ase), calcium and chlorides have been studied on 75 dogs. Rectal temperature (T_re) was recorded before and after exposure to heat stress. These dogs were divided into 5 groups, according to the T_re level attained after exposure to heat stress. Rectal temperature was raised from normal to 39.45±0.47C in the first group, to 40.93±0.17C in the second group, to 41.87±0.22C in the third group, to 42.90 ± 0.21C in the fourth group and to 43.93±0.19C in the fifth group. The concentration of enzymes SGOT, SGPT and ALK-P-ase in blood and cerebrospinal fluid (CSF) increased significantly with hyperthermia. Calcium and chlorides concentrations in blood and in CSF tended to increase in hyperthermia. The integrity of the blood brain barrier for these enzymes and calcium is maintained under mild hyperthermia but it breaks down partially under influence of more severe hyperthermia. Core temperature above 41C results in damage to tissues and consequential rise of plasma enzymes. This degree of hyperthermia also seems to mark the beginning of injury to blood brain barrier. Critical core temperature tolerated by 50% of animals was 44C.     

Differential Effects of Day and Night Temperature on Development to Flowering in Rice

There are conflicting reports with regard to difference in effectsof day temperature (T_D) and night temperatures (T_N) on plantdevelopment. The objective of this study is to determine whetherthere are different effects ofT_DandT_Non development from sowingto flowering in rice (Oryza sativaL.). Plants of 24 rice cultivars were grown in naturally-lightedgrowth chambers at five diurnally constant (22, 24, 26, 28 and32 °C) and four diurnally fluctuating temperatures (26 /22,30 /22, 22 /26 and 22 /30 °C forT_D/T_Nwith 12hd^-1each) witha constant photoperiod of 12hd^-1. The treatments were selectedto enable the separation of effects ofT_DandT_Non developmentrate (DR). The response of DR to constant temperatures was typically nonlinear.This nonlinearity could not explain the difference in floweringdates between fluctuating temperatures with the same mean dailyvalue but oppositeT_D/T_Ndifferences. Differential effects ofT_DandT_NonDR to flowering were detected in all but one cultivar. In mostcases,T_Dexerted a greater influence thanT_N, in contrast withmany previous reports based on the assumption of a linearitybetween DR and temperature. The data were further analysed bya nonlinear model which separated effects ofT_DandT_N. The estimatedvalue for the optimumT_Nwas generally 25 –29 °C, about2 –4 °C lower than the estimated optimumT_Din mostcultivars. The effects ofT_DandT_Non DR were found to be interactivein some cultivars. These results form a new basis for modellingflowering dates in rice. Oryza sativa; rice; flowering; development; day and night temperature; thermoperiodicity     

Transverse Relaxation Time of Leaf Water Protons and Membrane Injury in Wheat (Triticum aestivum L.) in Response to High Temperature

The relationship between high temperature stress injury andtemperature dependence of the transverse relaxation time (T₂)of leaf water was examined using NMR in four cultivars of wheatdiffering in their sensitivity to high temperature stress. TheT₂declined with increasing temperature between 25 and 35 °C.A comparison of relative injury based on electrolyte leakageand T₂, between 40 and 50 °C, indicated that while membranepermeability increased with increasing temperature there wasan increase in T₂until 44 and 48 °C in susceptible and tolerantcultivars respectively, followed by a sharp decline. This patternof change in T₂with increasing temperature was consistent whetherthe same or different samples were used for each treatment temperature.Loss of temperature dependence of T₂after heat killing indicatedirreversible changes in T₂, probably due to the loss of membraneintegrity. Heat tolerant varieties, which suffered less membraneinjury, had a higher T₂compared to susceptible varieties. Tolerantvarieties also maintained the T₂of leaf water protons to highertemperatures than did sensitive varieties. This NMR-based, non-invasive,rapid technique could be used to efficiently detect heat injuryin leaf tissues. Copyright 1999 Annals of Botany Company Membrane integrity, transverse relaxation time, high temperature stress, Triticum aestivum L.