Heat-acclimated Rats Research Articles

Salivary Immunoglobulin A Secretion and Polymeric Ig Receptor Expression in the Submandibular Glands Are Enhanced in Heat-Acclimated Rats.

Salivary immunoglobulin A (IgA) plays a critical role in mucosal immunity. Chronic exposure to moderate heat induces heat acclimation, which modifies salivary functions. However, the changes in salivary IgA secretion in heat-acclimated rats are unclear. In this study, we investigated salivary IgA secretion and the expression of polymeric Ig receptor (pIgR), a key mediator of mucosal IgA secretion, in the submandibular glands (SMGs) of heat-acclimated rats. Following maintenance at an ambient temperature (Ta) of 24 ± 0.1 °C for 10 days, male Wistar rats were subjected to Ta of 32 ± 0.2 °C for 5 days (HE group) for heat acclimation or maintained at Ta of 24 ± 0.1°C (CN group). The rats were then anesthetized, pilocarpine (0.5 mg/kg) was intraperitoneally injected, and saliva was collected. Afterward, the SMGs and plasma were sampled. The salivary IgA concentration and IgA flow rate were significantly higher in the HE group than in the CN group. Similarly, SMG pIgR expression was significantly higher in HE rats. The levels of plasma cytokines, including interleukin (IL)-5, IL-6, and interferon-γ, were significantly greater in HE rats than in CN rats. Heat acclimation may enhance oral immunity through salivary IgA secretion and pIgR upregulation in the SMGs.

Circular RNA Expression Profiling and Selection of Key Circular RNAs in the Hypothalamus of Heat-Acclimated Rats.

Circular RNAs (circRNAs) have vital roles in great variety of biological processes. However, expression levels and functions of circRNAs related to heat acclimation (HA) are poorly understood. This study is the first time an in-depth circRNA expression profiling were used to investigate circRNA–miRNA interactions in HA rats in order to further comprehend the mechanisms underlying HA. CircRNA expression profile was performed in rats’ hypothalamus of HA and control group with microarray assays and their functions were predicted by using Bioinformatics analysis. Differential circRNAs and their regulated downstream miRNAs and mRNAs were quantitatively validated by means of quantitative polymerase chain reaction in real-time (RT-qPCR). Enzyme-linked immunosorbent assay (ELISA) was then applied to predict the expression of proteins. In total, 53 circRNAs were expressed distinctively between the HA and Control; up- and down-regulation of circRNAs were 28 and 25, respectively, in HA (fold change > 1.5, P < 0.05). Three circRNAs and two miRNAs and three predicted mRNAs were obviously regulated after validated by RT-qPCR in HA rats. Two proteins expression were proportional to their mRNA changes. Further analysis demonstrates that circRNAs closest to HA can be categorized into three signal pathways: including rno_circRNA_014301-vs-rno-miR-3575-vs-Hif-1α, rno_circRNA_014301-vs-rno-miR-3575-vs-Lppr4, and rno_circRNA_010393-vs-rno-miR-20b-3p-vs-Mfap4 in hypoxia response pathways, substance/energy metabolism, and inflammatory response pathways. Our findings implicate that many circRNAs regulate expressions of genes that interact with each other to exert their functions during HA.

Direct exposure to mild heat promotes proliferation and neuronal differentiation of neural stem/progenitor cells in vitro.

Heat acclimation in rats is associated with enhanced neurogenesis in thermoregulatory centers of the hypothalamus. To elucidate the mechanisms for heat acclimation, we investigated the effects of direct mild heat exposure on the proliferation and differentiation of neural stem/progenitor cells (NSCs/NPCs). The NSCs/NPCs isolated from forebrain cortices of 14.5-day-old rat fetuses were propagated as neurospheres at either 37.0°C (control) or 38.5°C (mild heat exposure) for four days, and the effects on proliferation were investigated by MTS cell viability assay, measurement of neurosphere diameter, and counting the total number of cells. The mRNA expressions of heat shock proteins (HSPs) and brain-derived neurotrophic factor (BDNF), cAMP response element-binding (CREB) protein and Akt phosphorylation levels, and intracellular reactive oxygen species (ROS) levels were analyzed using real time PCR, Western blotting and CM-H2DCFDA assay respectively. Heat exposure under proliferation condition increased NSC/NPC viability, neurosphere diameter, and cell count. BDNF mRNA expression, CREB phosphorylation, and ROS level were also increased by heat exposure. Heat exposure increased HSP27 mRNA expression concomitant with enhanced p-Akt level. Moreover, treatment with LY294002 (a PI3K inhibitor) abolished the effects of heat exposure on NSC/NPC proliferation. Furthermore, heat exposure under differentiation conditions increased the proportion of cells positive for Tuj1 (a neuronal marker). These findings suggest that mild heat exposure increases NSC/NPC proliferation, possibly through activation of the Akt pathway, and also enhances neuronal differentiation. Direct effects of temperature on NSCs/NPCs may be one of the mechanisms involved in hypothalamic neurogenesis in heat-acclimated rats. Such heat-induced neurogenesis could also be an effective therapeutic strategy for neurodegenerative diseases.

Neural progenitor cell proliferation in the hypothalamus is involved in acquired heat tolerance in long-term heat-acclimated rats.

Constant exposure to moderate heat facilitates progenitor cell proliferation and neuronal differentiation in the hypothalamus of heat-acclimated (HA) rats. In this study, we investigated neural phenotype and responsiveness to heat in HA rats’ hypothalamic newborn cells. Additionally, the effect of hypothalamic neurogenesis on heat acclimation in rats was evaluated. Male Wistar rats (5 weeks old) were housed at an ambient temperature (Ta) of 32°C for 6 days (STHA) or 40 days (LTHA), while control (CN) rats were kept at a Ta of 24°C for 6 days (STCN) or 40 days (LTCN). Bromodeoxyuridine (BrdU) was intraperitoneally injected daily for five consecutive days (50 mg/kg/day) after commencing heat exposure. The number of hypothalamic BrdU-immunopositive (BrdU+) cells in STHA and LTHA rats was determined immunohistochemically in brain samples and found to be significantly greater than those in respective CN groups. In LTHA rats, approximately 32.6% of BrdU+ cells in the preoptic area (POA) of the anterior hypothalamus were stained by GAD67, a GABAergic neuron marker, and 15.2% of BrdU+ cells were stained by the glutamate transporter, a glutamatergic neuron marker. In addition, 63.2% of BrdU+ cells in the POA were immunolabeled with c-Fos. Intracerebral administration of the mitosis inhibitor, cytosine arabinoside (AraC), interfered with the proliferation of neural progenitor cells and acquired heat tolerance in LTHA rats, whereas the selected ambient temperature was not changed. These results demonstrate that heat exposure generates heat responsive neurons in the POA, suggesting a pivotal role in autonomic thermoregulation in long-term heat-acclimated rats.

Heat acclimation and thirst in rats.

The effects of heat acclimation on water intake and urine output responses to thermal dehydration and other thirst stimuli were studied in male Sprague-Dawley rats. Rats were heat acclimated by continuous exposure to a 34°C environment for at least 6weeks. Thermal dehydration-induced thirst was brought about by exposing the heat-acclimated rats and control rats housed at 24°C to a 37.5°C environment for 4h without access to food or water. Heat acclimation reduced evaporative and urinary water losses and the increases in plasma sodium and osmolality during thermal dehydration, which led to a reduction in thermal dehydration-induced thirst. Heat acclimation reduced the rate of rehydration following thermal dehydration but did not alter the final rehydration level, indicating that heat acclimation does not alter the primary control of thermal dehydration-induced thirst. Heat acclimation did not alter water intake or urine output following administration of hypertonic saline, which selectively stimulates intracellular thirst, but led to greater water intake following administration of angiotensin II, which plays an important role in extracellular/volemic thirst, and following water deprivation, which activates both thirst pathways. Cardiovascular responses to angiotensin II were not altered by heat acclimation. Heat acclimation thus reduces water loss during heat exposure in rats, but does not have major effects on thermal dehydration-induced or extracellular thirst but does appear to alter volemic thirst.

Aging attenuates acquired heat tolerance and hypothalamic neurogenesis in rats.

This study investigated age-dependent changes in heat exposure-induced hypothalamic neurogenesis and acquired heat tolerance in rats. We previously reported that neuronal progenitor cell proliferation and neural differentiation are enhanced in the hypothalamus of long-term heat-acclimated (HA) rats. Male Wistar rats, 5 weeks (Young), 10-11 months (Adult), or 22-25 months (Old) old, were subjected to an ambient temperature of 32°C for 40-50 days (HA rats). Rats underwent a heat tolerance test. In HA rats, increases in abdominal temperature (Tab ) in the the Young, Adult, and Old groups were significantly smaller than those in their respective controls. However, the increase in Tab of HA rats became greater with advancing age. The number of hypothalamic bromodeoxyuridine (BrdU)-immunopositive cells double stained with a mature neuron marker, neuronal nuclei (NeuN), of HA rats was significantly higher in the Young group than that in the control group. In Young HA, BrdU/NeuN-immunopositive cells of the preoptic area/anterior hypothalamus appeared to be the highest among regions examined. Large numbers of newborn neurons were also located in the ventromedial and dorsomedial nuclei, as well as the posterior hypothalamic area, whereas heat exposure did not increase such numbers in the Adult and Old groups. Aging may interfere with heat exposure-induced hypothalamic neurogenesis and acquired heat tolerance in rats.

Effect of neonatal or adult heat acclimation on testicular and epididymal morphometry and sperm production in rats

The accessory gland weight, testicular and epididymal morphometry and sperm production were analyzed in four groups of rats housed at 20 or 34°C: (1) control rats (CR) kept at 20°C from birth to day 90; (2) adult heat-acclimated rats (AHA) kept at 20°C from birth to day 45 followed by 34°C to day 90; (3) neonatal heat-acclimated rats (NHA) kept at 34°C from birth to day 90 and (4) de-acclimated rats (DA) kept at 34°C from birth to day 45 followed by 20°C to day 90. In NHA and DA rats, accessory gland weight was higher than in controls. Despite the lack of differences in testicular and epididymal morphometry, curvilinear velocity of spermatozoa was lower in the NHA group compared to controls. Areas of seminiferous tubules were lower in the DA than in CR and NHA groups, however, sperm concentration and motility were not affected by the treatment in this group. In AHA rats, epithelium of approximately 20% of seminiferous tubules was degenerated and Sertoli cell number was lower in the remaining tubules. In contrast to sperm motility, epididymal duct area, area of the duct occupied by spermatozoa and cauda epididymis sperm concentration were lower in AHA rats than in the other groups. In conclusion, neonatal heat acclimation did not affect the testicular morphometry and epididymal sperm concentration, suggesting adjustment to high ambient temperature. On the contrary, adult heat acclimation of rats affected the examined parameters, leading to decreased sperm concentration.

Distinct Testicular Steroidogenic Response Mechanisms Between Neonatal and Adult Heat-Acclimated Male Rats

Background: In comparison to short-term gonad heat exposure, little is known about the molecular mechanisms that regulate testicular steroidogenesis during long-term whole body heat acclimation. Material and Methods: Testicular slices from neonatal (NHA) and adult (AHA) heat-acclimated Wistar rats were analysed in vitro to assess the mRNA expression and enzymatic activity of steroidogenic enzymes under basal and luteinising hormone (LH) or prolactin (PRL) stimulated conditions compared with control rats (CR). Furthermore, a de-acclimated group (DA) was created by transferring adult NHA rats to control conditions. Results: Heat acclimation significantly increased plasma LH levels in the AHA group and LH and PRL in the NHA group compared with the CR group; however, after heat acclimation, the T and E2 levels did not differ from the control levels. All heat-acclimated groups showed high basal intra-testicular steroid production in vitro. Moreover, basal Cyp11a1 and Hsd3b1 levels were upregulated in vitro in the NHA and DA groups versus the CR group. LH in vitro stimulation upregulated Cyp11a1 expression in the NHA and AHA groups and PRL stimulation upregulated Cyp17a1 levels in the NHA and DA groups compared with the basal expression levels. In the AHA group, decreased basal Star and CYP11A activities but increased HSD3B1 and CYP17A1 activities were found. Conclusion: Our data revealed that despite the similar steroid levels in plasma and secreted in vitro by neonatal and adult heat-acclimated rat testicular slices, the molecular mechanisms underlying the steroidogenic response to heat acclimation during these different developmental stages were distinct.

Upregulation of aquaporin expression in the salivary glands of heat-acclimated rats

It is known that aquaporin (AQP) 5 expression in the apical membrane of acinar cells in salivary glands is important for the secretion of saliva in rodents and humans. Although heat acclimation enhances saliva secretion in rodents, the molecular mechanism of how heat induces saliva secretion has not been determined. Here, we found that heat acclimation enhanced the expression of AQP5 and AQP1 in rat submandibular glands concomitant with the promotion of the HIF-1α pathway, leading to VEGF induction and CD31-positive angiogenesis. The apical membrane distribution of AQP5 in serous acinar cells enhanced after heat acclimation, while AQP1 expression was restricted to the endothelial cells in the submandibular glands. A network of AQPs may be involved in heat-acclimated regulation in saliva secretion. Because AQPs probably plays a crucial role in saliva secretion in humans, these findings may lead to a novel strategy for treating saliva hyposecretion.

Proliferation of neuronal progenitor cells and neuronal differentiation in the hypothalamus are enhanced in heat-acclimated rats

Male Wistar rats, initially maintained at an ambient temperature (T (a)) of 24 degrees C, were subjected to a constant high T (a) of 32 degrees C (HE) or were constantly kept at 24 degrees C (controls, CN). Bromodeoxyuridine (BrdU) was intraperitoneally injected daily for five consecutive days after commencing heat exposure. On the 6th, 13th, 23rd, 33rd, 43rd, and 53rd day of heat exposure, rats' brains were removed. Immunohistochemical analysis showed that the numbers of BrdU-positive cells in the hypothalamus of HE were significantly and consistently greater than those of CN. In HE, the number of BrdU-positive cells double-stained by a mature neuron marker increased abruptly after 33 days of heat exposure by about seven times. This was not the case in CN. The results suggest that heat exposure facilitates proliferation of neuronal progenitor cells in the hypothalamus and promotes differentiation to neurons, which might have certain relation to establishing long-term heat acclimation in rats.

Changes in the hypothalamo-pituitary-adrenocortical and hypothalamo-pituitary-thyroid axes in diabetic rats acclimated to moderate hyperthermic environment

We examined the effect of acclimation to moderate hyperthermic environment on the ACTH, TSH, T 3, T 4 and corticosterone level, as well as the relative weight of hypophysis, thyroid and adrenal glands in streptozotocin-diabetic rats. Increased activity of the hypothalamo-pituitary-adrenocortical (HPA) axis has been demonstrated in diabetic animals, whereas insulin treatment restores the changes. Heat acclimation reduces the level of ACTH and corticosterone in control animals and moderates the hormonal disturbances caused by diabetes. Simultaneously, our study revealed impairment in the activity of the hypothalamo-pituitary-thyroid (HPT) axis. Acclimation to 35±1 °C resulted in significantly lower T 3 and T 4 levels in control, diabetic and insulin-treated animals. Relative weight of the hypophysis, thyroid and adrenal glands is decreased in heat-acclimated rats. Our assumption is that there might be a cross tolerance between diabetes and heat acclimation on a hormonal level.

Changes of noradrenaline-induced contractility and gene expression in aorta of rats acclimated to heat in two different modes

We investigated changes in responsiveness to noradrenaline (NA) and gene expression in aorta, and plasma nitrite/nitrate (NOx) levels in heat-acclimated rats. Male rats were exposed for 10 consecutive days to either 32 degrees C in the second half of the dark phase (HI) or to 32 degrees C constantly (HC). Controls were kept at 24 degrees C. After heat acclimation, thoracic aorta was excised and blood was taken in the first (Dark1) and second (Dark2) halves of the dark phase, and in the light phase. Contractile response to NA in HI was significantly depressed in Dark1 and Dark2, whereas in HC the contraction response to NA was significantly attenuated in all phases. Endothelial NO synthase (eNOS) mRNA expression was significantly enhanced in HI in Dark2. Inducible NOS mRNA was undetectable. mRNA expression of period2 was not modified by heat acclimation. Plasma NOx concentrations in HI were significantly elevated in Dark1 and Dark2, while in HC, the level was significantly raised in all phases. Our results suggest that in rats, heat acclimation improves distensibility of thoracic aorta due to attenuated responsiveness to NA, which might be attributable in part to an upregulated eNOS expression and an increase in circulating NO. The phases of the day when such changes occurred appear to be closely related to the periods when the animals were previously subjected to heat.

The effect of fasting on the glycogen metabolism in heat-acclimated rats

We investigated the influence of successive fasting for 24,48,72, and 96 h on some key enzymes and substrates of liver, kidney, and muscle in control and heat-acclimated (30days at 35 ? 1?C)rats. Short-term fasting (for 24 and 48 h)resulted in decrease of liver glycogen content, blood glucose level, and concentration of glucose-6-phosphate, as well as increase of glucose-6-phosphatase activity, regardless of the previous temperature of acclimation. During a period of prolonged fasting (for 72 and 96 h),there was a rebound of liver glycogen content only in animals kept at room temperature. Fasting induced increase of renal glycogen content in animals kept at room temperature and increase of renal glucose-6-phosphatase activity in both experimental groups. As for muscle metabolism, endogenous nutrition resulted in decrease of muscle glycogen content in heat-acclimated animals. Activity of muscle glycogen phosphorylase (a+b)was decreased in the control and increased in heat-acclimated animals. The obtained results indicate that the examined carbohydrate-related parameters show time-dependent changes during 4 days of fasting. Twenty-four- and 48-h fasting intensifies glycogenolytic processes, while 72- and 96-h fasting intensifies gluconeogenic processes, doing so to a lesser extent in heat-acclimated animals. The changes caused by the fasting were modified by acclimation to moderate heat, primarily in the liver and to a lesser extent in the kidney and muscle.

Changes in Vascular Functions in Heat-Acclimated Rats

Changes in Vascular Functions in Heat-Acclimated Rats

Altered Ca2+handling and myofilament desensitization underlie cardiomyocyte performance in normothermic and hyperthermic heat-acclimated rat hearts

Heat acclimation (AC) improves cardiac mechanical and metabolic performance. Using cardiomyocytes and isolated hearts from 30-day and 2-day acclimated rats (AC and AC-2d, 34 degrees C), we characterized cellular contractile mechanisms under normothermic (37 degrees C) and hyperthermic (39-42 degrees C) conditions. To determine contractile responses, Ca2+ transients (Ca2+ T), sarcoplasmic reticulum (SR) Ca2+ pool size (fura-2/indo-1 fluorescence), force generation [amplitude systolic motion (ASM)], L-type Ca2+ channels [dihydropyridine receptor (DHPR)], ryanodine receptors (RyRs), and total (PLBt) and phosphorylated phospholamban [serine phosphorylated (PLBs) and theonine phosphorylated (PLBtr)] proteins and transcripts were measured (Western blot, RT-PCR). Cardiac mechanical performance was measured using a Langendorff system. We demonstrated that AC and AC-2d increased Ca2+ T amplitude (148% and 147%, respectively) and twitch force (180% and 130%, respectively) and desensitized myofilaments, as indicated by a rightward shift in the ASM-Ca2+ relationships, despite no change in SR Ca2+ pool size. Hence, generation of higher Ca2+ T underlies greater force development in AC and AC-2d myocytes. In isolated hearts, ryanodine administration eliminated differences between AC and control (C) hearts, implying an important role for RyRs in that acclimation phase. Increased expression of DHPR and RyRs, and decreased PLBs/PLBt in AC hearts only, suggest that different pathways increase force generation in the AC-2d vs. AC myocytes. At basal beating rates, hyperthermia (39-41 degrees C) enhanced pressure generation in AC hearts. C hearts failed to restitute pressure beyond 39 degrees C. Increased beating frequency produced negative inotropic response. In C cardiomyocytes, hyperthermia elevated basal cytosolic Ca2+ and tension, Ca2+ T, and ASM. AC myocytes enhanced Ca2+ T but showed myofilament desensitization, suggesting its involvement in cardiac protection against hyperthermia. Collectively, both Ca2+ turnover and myofilament responsiveness are important adaptive acclimatory targets during normothermic and hyperthermic conditions.

Contribution of the Suprachiasmatic Nucleus to the Formation of a Time Memory for Heat Exposure in Rats

We have reported that after rats were acclimated to heat for about 5 h daily at a fixed time, the pattern of day-night variations of core temperature (T(cor)) altered, i.e., their T(cor) fell, especially during the period when they had previously been exposed to heat. The suprachiasmatic nucleus (SCN) of the hypothalamus is known to be indispensable for the genesis of circadian rhythms of T(cor). We therefore investigated the involvement of the SCN in the characteristic fall in T(cor) in heat-acclimated rats. The rats were exposed to an ambient temperature of 33 degrees C only in the last half of the dark phase for 10 consecutive days. After the heat exposure schedule, the nocturnal pattern of T(cor) variations and Fos expression in the dorsomedial SCN altered so that the T(cor) and the number of Fos immunoreactive cells decreased in the last half of the dark phase. The bilateral lesions of the SCN of rats were made electrically, and the electrical lesions of the SCNs abolished the daily cycle of T(cor). In the SCN-lesioned rats, theT(cor) levels were significantly lowered after the 10-day heat exposure schedule. However, their T(cor) did not specifically drop during the period when they had previously been exposed to heat. These findings suggest that the SCN is crucial for establishing a time memory for heat stress, and it plays a minimal role in heat acclimation-induced changes in T(cor) in rats.

WARM HEARTS PREPARE RATS FOR BEING SHORT OF BREATH

Organisms that have acclimated to a particular stressor often find that they can cope better with another stressor that they were not exposed to during their acclimation, a phenomenon known as cross-tolerance. For example,when rats acclimate to high temperatures, their hearts step up their rate of ATP production through anaerobic glycolytic pathways, which suggests that the warm-acclimated heart is also able to cope better with stress caused by oxygen depletion. Since oxygen depletion is a leading cause of heart injury, it would be intriguing to find a molecular switch that confers cross-tolerance to reduced oxygen levels in the heat-acclimated mammalian heart. To find one such potential switch, Maloyan and colleagues from The Hebrew University in Jerusalem examined heat-acclimated rat hearts, focusing on the activation of hypoxia-inducible factor 1 (HIF-1), a transcription factor of genes that are expressed during oxygen depletion.HIF-1 consists of two subunits, α and β, which together form the active transcription factor that binds to DNA. Unlike HIF-1β, which is continually present, HIF-1α levels increase in response to low oxygen levels. To evaluate the chronic response of HIF-1α to heat, Maloyan and colleagues acclimated 3-week-old rats to either 24°C or 34°C for 30 days. They used immunoblot assays to determine that rats acclimated to 34°C had higher HIF-1α levels in their hearts than those acclimated to 24°C. To measure the acute response of HIF-1α to heat, they exposed acclimated animals to 41°C for 2 h. They found that HIF-1αlevels increased in response to an acute heat shock in non-acclimated but not in heat-acclimated rats. But they didn't see an increase in rats' HIF-1αmRNA levels following heat acclimation, suggesting that changes in translation or degradation are responsible for the higher HIF-1α levels.HIF-1α is only active if it associates with the β-subunit to create the HIF-1 dimer, so the authors set out to prove that heat stress results in the formation of HIF-1 dimers. They used an anti-HIF-1βantibody that binds to the β-subunit of the protein; when they analysed the resulting antibody-protein complex, they found that the α-subunits were also bound to the complex, which meant that they had associated with theβ-subunits. Using this technique, the team showed that chronic as well as acute heat stress increases the dimerization of the protein. They further showed that these HIF-1 dimers are active when present at high levels and bind to a HIF-1 DNA-binding element, which results in the activation of many of the HIF-1 target genes. For example, the team presented evidence that these higher HIF-1 levels stimulate the expression of erythropoietin, a protein that activates the production of red blood cells, which helps the animals cope with lower oxygen levels.Does this increase in HIF-1 due to heat acclimation translate into greater protection of the heart from the damaging effects of low oxygen levels? The authors evaluated the rats' cross-tolerance to reduced oxygen levels by lowering the perfusion rate of isolated rat hearts by 75%. Sure enough, they found evidence for cross-tolerance; they saw that heat-acclimated rat hearts had smaller patches of injured heart tissue than non-acclimated hearts. They also showed that, like heat acclimation, acute oxygen depletion activates an increase in HIF-1 levels, with similar consequences. This may be the best evidence yet that a warm heart helps animals survive shortness of breath.

Heat acclimation prolongs the time to central nervous system oxygen toxicity in the rat: Possible involvement of HSP72

Oxygen toxicity of the central nervous system (CNS-OT) can occur during diving with oxygen-enriched gas mixtures, or during hyperbaric medical treatment. CNS-OT is characterised by convulsions and sudden loss of consciousness, which may be fatal in diving. Heat acclimation is known to provide cross-tolerance to various forms of stress in different organs, including the brain. We hypothesised that heat acclimation may delay the onset of CNS-OT in the rat. Male Sprague–Dawley rats were acclimated to an ambient temperature of 32 °C for 4 weeks. Rats in the control group were kept at 24 °C. Both groups were exposed to oxygen at 608 kPa. EEG was recorded continuously until the appearance of the first electrical discharge preceding clinical convulsions. CO 2 production was measured simultaneously with the EEG. Latency to CNS-OT was measured and brain samples were taken for evaluation of heat shock protein 72 (HSP72) levels by Western blot analysis at the end of the acclimation period and during 4 weeks of deacclimation. Latency to CNS-OT was twice as long in the heat-acclimated rat, with insignificant changes in CO 2 production. This prolongation continued for 2 weeks during deacclimation. There was a significant increase in the level of HSP72 following heat acclimation, with a subsequent decrease during deacclimation. We conclude that heat acclimation prolongs latency to CNS-OT in a way that does not involve changes in metabolic rate. During deacclimation there was a linear relationship between latency to CNS oxygen toxicity and the level of HSP72. A possible beneficial effect of HSP72 is discussed.

Heat acclimation-induced elevated glycogen, glycolysis, and low thyroxine improve heart ischemic tolerance.

Based on our observations of energy sparing in heat-acclimated (AC) rat hearts, we investigated whether changes in preischemic glycogen level, glycolytic rate, and plasma thyroxine level mediate cardioprotection induced in these hearts during ischemia-reperfusion insults. Control (C) (24 degrees C), AC (34 degrees C, 30 days), acclimated-euthyroid (34 degrees C + 3 ng/ml l-thyroxine), and control hypothyroid (24 degrees C + 0.02% 6-n-propyl-2-thiouracil) groups were studied. Preischemic glycogen was higher in AC than in C hearts [39.0 +/- 8.5 vs. 19.2 +/- 4.2 (SE) micromol glucose/g wet wt; P < 0.0006], and the lactate produced vs. glycogen level during total ischemia ((13)C-NMR spectroscopy) was markedly slower (AC: -0.82x, r = 0.98 vs. C: -4.7x, r = 0.9). Time to onset of ischemic contracture was lengthened, and the fraction of hearts experiencing ischemic contracture was lowered. Pulse pressure recovery was improved in AC compared with C animals before, but not after, absolute sodium iodoacetate-induced glycolysis inhibition. Acclimated-euthyroid hearts exhibited decreased ischemic tolerance, whereas induced hypothyroidism in C improved cardiotolerance. Thus higher preischemic glycogen and slowed glycolysis are associated with hypothyroidism and are likely important mediators of the improved ischemic tolerance exhibited by AC hearts.

Heat acclimation in rats: modulation via lipid polyunsaturation.

Heat acclimation of rats has been shown to enhance endurance of rat hearts to ischemic insult and acute heat stress. Common protective features have been shown to be operative during both these stress-inducing conditions. To explore the role of membrane lipid composition in the adaptive response, we analyzed two major parameters that impact membrane dynamics and order, the nonesterified cholesterol levels and the acyl chain composition of phospholipids, in rat heart and salivary glands, both major thermoregulatory organs, in short- and long-term heat-acclimated rats. Before exposure to heat, control salivary gland tissue has a higher cholesterol-to-phospholipid mole ratio (0.32 +/- 0.02) than heart (0.14 +/- 0.01), and the acyl chains of its phospholipids are 50% more saturated. The remodeling strategies of the tissues after exposure to heat differed. Heart cholesterol levels increased after short-term heat acclimation (approximately 50%), whereas salivary gland cholesterol levels decreased in acute heat stress and long-term heat acclimation (approximately 32%). Remodeling of phospholipid acyl chains, particularly an increase in docosahexaenoic acid, was a protective strategy in both tissues (57% in heart and >100% in salivary glands). Modifying membrane lipid composition by treating rats with liposomes composed of egg phosphatidylcholine (PC) before exposure to heat resulted in a 38% increase in endurance to thermal stress. The density and affinity of muscarinic receptors of submaxillary salivary glands, involved in the acclimation response, were measured in control and PC liposome-treated rats, and then both groups were subjected to short-term heat acclimation. After PC treatment the well-established compensatory upregulation of the muscarinic receptors and concomitant decrease in their affinity was blunted. The substantial increase in the thermal endurance of heat-challenged intact rats after treatment with PC liposomes (600 vs. 200 min) suggests that membrane lipid composition plays a role in the ability of these tissues to respond to heat stress.