Redistribution of cardiac output in anesthetized thermally dehydrated rats

Cardiac output (CO) and its distribution were studied in dehydrated (37 degrees C) anesthetized (Na thiopentone) rats prior to and following heat acclimation (at 34 degrees C), using 57Co 15 micron microspheres. In non-acclimated dehydrated rats, CO decreased while heart rate (HR) increased significantly. Following acclimation CO increased without any change in HR; during dehydration CO remained elevated together with a significant increase in HR. In non-acclimated rats at low dehydration blood perfusion to peripheral thermoregulatory areas increased while perfusion of splanchnic area decreased; at high dehydration level peripheral blood flow decreased whereas splanchnic blood flow was augmented. In acclimated dehydrated rats, CO distribution to thermoregulatory areas did not change while perfusion of the splanchnic area decreased. It is suggested that following acclimation, the increased CO contributes to maintenance of thermoregulatory peripheral blood flow; in non-acclimated rats severe dehydration leads to augmented blood flow in the permeable splanchnic vascular bed, increasing efflux of plasma protein and failure of plasma volume conservation.     

Heat stress and thermal dehydration: lactacidemia and plasma volume regulation.

This investigation was undertaken to study heat stress and dehydration effects on 1) plasma lactic acid (LA) concentration and 2) plasma LA effect on plasma volume conservation during thermal dehydration. Experiments were performed on conscious nonacclimated and heat-acclimated laboratory rats subjected to various levels of heat stress and/or dehydration (37-42 degrees C with and without drinking water). During the exposures, rectal temperature (Tre), plasma LA pyruvic acids, and hematocrit were measured. From these data, excess LA, indicative of anaerobic metabolism, was calculated. In separate experiments, transvascular protein efflux (half time of Evans blue-labeled albumin) was measured before and after plasma LA elevation, either by LA infusion or thermal dehydration. The results show that elevation of plasma LA was associated with a rise in Tre, with accelerated elevation within a Tre range of 41-42 degrees C. LA concentrations were similar for the same Tre in all experimental groups. In nonacclimated rats, this rise was accompanied by a significant rise in excess LA. In acclimated rats, only a minor rise in excess LA was observed. A positive correlation was found between plasma LA elevation and the increase in plasma protein efflux. It is concluded that there is a temperature threshold for the rise in plasma LA. In nonacclimated rats, local hypoxia may contribute to this rise. The data also suggest that, in nonacclimated rats, lactacidemia accelerates plasma protein and fluid loss, leading to circulatory failure during acute thermal dehydration.     

The role of the liver in lipid metabolism during cold acclimation in non-hibernator rodents

Cold exposure increases the demand for energy substrates. Cold acclimation of rats led to a 3-fold increase in fatty acid (FA) beta-oxidation (P<0.01) for ex vivo livers perfused at 37 degrees C. This increase was preserved following perfusion at 25 degrees C (P<0.001). In vitro measurement of absolute rates of hepatic beta-oxidation revealed no significant difference following cold acclimation, implying changes in fatty acid flux through beta-oxidation rather than increased oxidation capacity. Total FA uptake was increased one-third following perfusion at 25 degrees C (P<0.001) and cold acclimation (P<0.05) and cold acclimation led to diversion of tissue FA from storage to beta-oxidation (P<0.01). In separate experiments, in vivo hepatic lipogenesis rates for saponifiable lipids doubled (P<0.01) and cholesterol synthesis increased one-third (P<0.001). Taken together these data suggest the oxidation and synthesis of lipids occur simultaneously in hepatic tissue possibly to increase prevailing tissue FA concentrations and to generate heat through increased metabolic flux rates.     

Heat loss responses in rats acclimated to heat loaded intermittently

The present study examined the heat loss response of heat-acclimated rats to direct body heating with an intraperitoneal heater or to indirect warming by elevating the ambient temperature (Ta). The heat acclimation of the rats was attained through exposure to Ta of 33 or 36 degrees C for 5 h daily during 15 consecutive days. Control rats were kept at Ta of 24 degrees C for the same acclimation period. Heat acclimation lowered the body core temperature at Ta of 24 degrees C, and the core temperature level was lowered as acclimation temperature increased. When heat was applied by direct body heating, the threshold hypothalamic temperature (Thy) for the tail skin vasodilation was also lower in heat-acclimated rats than in the control rats. However, the amount of increase in Thy from the resting level to the threshold was the same in all three groups. When heat was applied by indirect warming, threshold Thy was slightly higher in heat-acclimated than in control rats. The amount of increase in Thy from the resting level to the threshold was significantly greater in heat-acclimated rats. In addition, Ta and the skin temperature at the onset of skin vasodilation were significantly higher in heat-acclimated rats. The results indicate that heat-acclimated rats were less sensitive to the increase in skin temperature in terms of threshold Thy. The gain constant of nonevaporative heat loss response was assessed by plotting total thermal conductance against Thy.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of acclimation temperature on thermal tolerance and membrane phospholipid composition in the fruit fly Drosophila melanogaster

Adaptative responses of ectothermic organisms to thermal variation typically involve the reorganization of membrane glycerophospholipids (GPLs) to maintain membrane function. We investigated how acclimation at 15, 20 and 25 degrees C during preimaginal development influences the thermal tolerance and the composition of membrane GPLs in adult Drosophila melanogaster. Long-term cold survival was significantly improved by low acclimation temperature. After 60 h at 0 degrees C, more than 80% of the 15 degrees C-acclimated flies survived while none of the 25 degrees C-acclimated flies survived. Cold shock tolerance (1h at subzero temperatures) was also slightly better in the cold acclimated flies. LT50 shifted down by ca 1.5 degrees C in 15 degrees C-acclimated flies in comparison to those acclimated at 25 degrees C. In contrast, heat tolerance was not influenced by acclimation temperature. Low temperature acclimation was associated with the increase in proportion of ethanolamine (from 52.7% to 58.5% in 25 degrees C-acclimated versus 15 degrees C-acclimated flies, respectively) at the expense of choline in GPLs. Relatively small, but statistically significant changes in lipid molecular composition were observed with decreasing acclimation temperature. In particular, the proportions of glycerophosphoethanolamines with linoleic acid (18:2) at the sn-2 position increased. No overall change in the degree of fatty acid unsaturation was observed. Thus, cold tolerance but not heat tolerance was influenced by preimaginal acclimation temperature and correlated with the changes in GPL composition in membranes of adult D. melanogaster.     

Triggering of cryoprotectant synthesis in the woolly bear caterpillar (Pyrrharctia isabella Lepidoptera: Arctiidae)

Isabella tiger moths (Pyrrharctia isabella) overwinter as caterpillars (i.e., woolly bears) that can survive freezing at moderate subzero temperatures. We observed an increase in hemolymph osmolality for field-collected woolly bears during October (325 +/- 47 to 445 +/- 27 mOsmol/liter) and tested the influence of temperature and moisture levels on cryoprotectant production. Laboratory acclimation was done at 5 degrees C in moist conditions and at 25 degrees C acclimation in both dry and moist conditions. Body water contents were diminished by dehydration at 25 degrees C for 4 days (57 +/- 4%). Caterpillars collected in early October did not alter their hemolymph osmolality during cold acclimation, but caterpillars increased by 45% (to 647 +/- 90 mOsmol/liter) after 4 days at 5 degrees C following their collection in late October. Hemolymph composition was markedly changed in caterpillars experiencing dehydration at 25 degrees C (1042 +/- 200 mOsmol/liter; 507 +/- 225 mmol glycerol/liter), whereas caterpillars showed no change in their hemolymph composition when kept moist at 25 degrees C. Our experiments reveal that both dehydration and cold acclimation rapidly induce cryoprotectant synthesis in P. isabella caterpillars. J. Exp. Zool. 286:367-371, 2000.     

Acclimation of entomopathogenic nematodes to novel temperatures: trehalose accumulation and the acquisition of thermotolerance

The effect of thermal acclimation on trehalose accumulation and the acquisition of thermotolerance was studied in three species of entomopathogenic nematodes adapted to either cold or warm temperatures. All three Steinernema species accumulated trehalose when acclimated at either 5 or 35 degrees C, but the amount of trehalose accumulation differed by species and temperature. The trehalose content of the cold adapted Steinernema feltiae increased by 350 and 182%, of intermediate Steinernema carpocapsae by 146 and 122% and of warm adapted Steinernema riobrave by 30 and 87% over the initial level (18.25, 27.24 and 23.97 microg trehalose/mg dry weight, respectively) during acclimation at 5 and 35 degrees C, respectively. Warm and cold acclimation enhanced heat (40 degrees C for 8h) and freezing (-20 degrees C for 4h) tolerance of S. carpocapsae and the enhanced tolerance was positively correlated with the increased trehalose levels. Warm and cold acclimation also enhanced heat but not freezing tolerance of S. feltiae and the enhanced heat tolerance was positively correlated with the increased trehalose levels. In contrast, warm and cold acclimation enhanced the freezing but not heat tolerance of S. riobrave, and increased freezing tolerance of only warm acclimated S. riobrave was positively correlated with the increased trehalose levels. The effect of acclimation on maintenance of original virulence by either heat or freeze stressed nematodes against the wax moth Galleria mellonella larvae was temperature dependent and differed among species. During freezing stress, both cold and warm acclimated S. carpocapsae (84%) and during heat stress, only warm acclimated S. carpocapsae (95%) maintained significantly higher original virulence than the non-acclimated (36 and 47%, respectively) nematodes. Both cold and warm acclimated S. feltiae maintained significantly higher original virulence (69%) than the non-acclimated S. feltiae (0%) during heat but not freezing stress. In contrast, both warm and cold acclimated S. riobrave maintained significantly higher virulence (41%) than the non-acclimated (14%) nematodes during freezing, but not during heat stress. Our data indicate that trehalose accumulation is not only a cold associated phenomenon but is a general response of nematodes to thermal stress. However, the extent of enhanced thermal stress tolerance conferred by the accumulated trehalose differs with nematode species.     

Effect of thermal history on the rat's response to varying environmental temperature

After acclimating individually housed male rats to temperatures of either 24.5 +/- 0.1 or 29.2 +/- 0.1 degrees C for 14 days, randomly paired animals from each group were acutely exposed (3 h) in series to experimental temperatures between 18.0 and 34.5 degrees C in a controlled environment room. Relative humidity of 50 +/- 0.3% and a 12-h light-dark photoperiod (light from 0900 to 2100 h) were maintained. Metabolic rate (MR) and evaporative water loss (EWL) were-measured using an open-flow system; thermistors were used to measure the rectal (Tre) and tail skin (Tts) temperatures. MR was relatively constant over a temperature range of 22.2 to 27.0 degrees C for rats acclimated to 24.5 degrees C and 20.0 to 29.2 degrees C for rats acclimated to 29.2 degrees C. Above and below these ranges, MR for both groups was significantly (P less than 0.05) elevated. At their respective acclimation temperatures, the absolute Tre and Tts of 29.2 degrees C rats were maintained at an elevated level compared with 24.5 degrees C rats. Although EWL for both groups was relatively constant between 18.0 and 27.0 degrees C, 24.5 degrees C rats displayed higher EWL changes at most environmental temperatures above 27.0 degrees C. At 34.5 degrees C, 29.2 degrees C rats dissipated 26% more metabolic heat by evaporation compared with 24.5 degrees C rats. These data suggest that acclimation temperatures of rats affected the thermoneutral zone and alter the set-point temperature around which thermal responses are regulated.     

The effects of acclimation temperature on pituitary and plasma beta-endorphin in rats at 32.5 degrees C

Endocrine and thermoregulatory responses were studied in male rats exposed to heat (32.5 +/- 0.1 degrees C) from acclimation temperatures of either 24.5 +/- 0.1 degrees C or 29.2 +/- 0.1 degrees C. After 1 hr in the heat, evaporative water loss and tail skin temperature changes in the 24.5 degrees C acclimated rats were greater than in the 29.2 degrees C acclimated rats; both groups displayed similar changes in metabolic rate and rectal temperature. At the respective acclimation temperatures, 29.2 degrees C rats displayed lowered plasma thyroid hormones, elevated beta-endorphin-like immunoreactivity (beta-END-LI) in the plasma, neurointermediate and anterior lobes of the pituitary gland, and no change in plasma corticosterone levels compared to 24.5 degrees C rats. After exposure to 32.5 degrees C for 1 hr, both groups of rats maintained similar plasma corticosterone levels; however, only the 24.5 degrees C group increased plasma thyroxine and beta-END-LI. These data suggest that beta-endorphin may be involved in body temperature regulation during acclimation to elevated environmental temperatures.     

Changes in thyroid hormone and insulin status of the brown adipose tissue of cold acclimated rats on short term exposure to heat

Acclimation of rats to cold caused 45% increase in the concentration of triidothyronine (T3) and 35% increase in the concentration of thyroxine (T4) in serum. Exposure of cold-acclimated rats to heat (12 hr, 37 degrees C) failed to decrease the concentrations of thyroid hormones in circulation. The concentration of T3 in brown adipose tissue (BAT) increased almost 10-fold on cold acclimation. Iodothyronine deiodinase activity also registered 3-fold increase. Exposure of cold-acclimated animals to heat caused decrease in the concentration of T3 in BAT without appreciably affecting T4 concentration. In liver tissue, the changes in hormone concentrations were quite small compared to those in BAT. On thyroidectomy or when fed with propyl thiouracil, rats could not survive exposure to the cold. The concentration of insulin in circulation showed small increase, while that in the tissues showed significant decrease on acclimation of rats to the cold. The concentration of the hormone in BAT registered significant increase on exposure of cold-acclimated animals to heat (12 hr, 37 degrees C). The increase in liver was marginal. The temperature-dependent response of T3 indicates an important role for this hormone in rapid physiological response in BAT.     

Acclimation of the photosynthetic machinery to high temperature in Chlamydomonas reinhardtii requires synthesis de novo of proteins encoded by the nuclear and chloroplast genomes

The mechanism responsible for the enhancement of the thermal stability of the oxygen-evolving machinery of photosystem II during acclimation of Chlamydomonas reinhardtii to high temperatures such as 35 degrees C remains unknown. When cells that had been grown at 20 degrees C were transferred to 35 degrees C, the thermal stability of the oxygen-evolving machinery increased and within 8 h it was equivalent to that in cells grown initially at 35 degrees C. Such enhancement of thermal stability was prevented by cycloheximide and by lincomycin, suggesting that the synthesis de novo of proteins encoded by both the nuclear and the chloroplast genome was required for this process. No increase in thermal stability was observed when cells that had been grown at 35 degrees C were exposed to heat shock at 41 degrees C, optimum conditions for the induction of the synthesis of homologs of three heat shock proteins (Hsps), namely, Hsp60, Hsp70, and Hsp22. Moreover, no synthesis of these homologs of Hsps was induced at 35 degrees C. Thus it appears likely that Hsps are not involved in the enhancement of the thermal stability of the oxygen-evolving machinery.     

滨海电厂冷却水余热和余氯对中华哲水蚤的影响

依据滨海电厂冷却系统的实际运作情况,模拟研究了滨海电厂冷却水余热和余氯对中华哲水蚤的影响.结果表明：中华哲水蚤的热忍受能力随暴露时间延长而降低,随驯化温度升高而升高.驯化温度为16 ℃～27 ℃的中华哲水蚤在持续受到15、30、45 min热冲击及持续升温暴露24和48 h的致死温度分别为29.9 ℃～31.7 ℃、29.4 ℃～31.0 ℃、28.9 ℃～30.3 ℃和26.9 ℃～28.5 ℃、26.4 ℃～28.0 ℃.当驯化温度升高到一定程度后,其热忍受能力不再上升.电厂冷却水中余氯对中华哲水蚤的毒性随驯化温度升高、升温幅度增大及暴露时间延长而增强.     

Interrelationships between heat acclimation and salivary cooling mechanism in conscious rats

1. Adaptation of salivary cooling mechanism during acclimation to heat (34 degrees C) and its role in thermoregulation of the rats was studied on conscious rats with either one submaxillary gland chronically cannulated or both submaxillaries ligated. 2. During heat stress (40 degrees C) acclimated rats showed a decrease both in rectal temperature threshold for salivation (Tre-TS), in salivary flow rate and in Tre (hyperthermic plateau). Animals survived for extended periods and rats with ligated glands survived 40% less than non-ligated rats. 3. For both cannulated and ligated rats short term acclimation (5 days) was the most effective. 4. It is suggested that earlier activation of salivation mechanism is associated with the decreased hyperthermic plateau and that the decreased salivary flow rate allows better control of water balance of the animals. Consequently, survival period during heat stress is extended.     

Heat acclimation and hypohydration in aged rats: The involvement of adrenergic pathways in thermal-induced vasomotor responses in the portal circulation

1. 1. The beneficial effects of heat acclimation on thermal induced vasomotor responses of hypohydrated aged rats were assessed by measuring the isometric tension of aortic and portal rings of old and young rats under heat acclimation and hypohydration in response to -adrenergic (-AR) and β-adrenergic (β-AR) stimulation (phenylephrine 10⁻⁹–10⁻² mM and isoprenaline 10⁻⁹–10⁻⁴ respectively). In parallel, portal blood flow (PBF), which drains the splanchnic vasculature, was measured in conscious rats, before and during heat stress (42°C).

2. 2. In the aorta, heat acclimation augmented phenylephrine (-AR) induced tension, to a great extent in the older rats. Hypohydration increased -AR sensitivity in all experimental groups. Acclimation and aging brought about decreased responsiveness in isoprenaline induced relaxation (β-AR) in both the aorta and the portal vein. Hypohydration increased β-AR responsiveness in the portal vein of OR, acclimated and acclimated-hypohydrated rats.

3. 3. Normothermic euhydrated resting PBF was similar for young and old rats. Hypohydration decreased resting PBF. Upon heat stress, thermal induced vasoconstriction in hypohydrated YR and OR occurred earlier than in the euhydrated groups and was more pronounced. The latter responses were attenuated in the old rats.

4. 4. Taken together, these results imply that chronic environmental stressors such as heat acclimation and hypohydration produce selective alterations in AR responsiveness of the vasculature in both young and old rats. Consequently, thermoregulatory vasomotor mediated mechanisms, as exhibited in this study in PBF, may differ in their responsiveness in these two age groups.

     

Desiccation tolerance and drought acclimation in the Antarctic collembolan Cryptopygus antarcticus

The availability of water is recognized as the most important determinant of the distribution and activity of terrestrial organisms within the maritime Antarctic. Within this environment, arthropods may be challenged by drought stress during both the austral summer, due to increased temperature, wind, insolation, and extended periods of reduced precipitation, and the winter, as a result of vapor pressure gradients between the surrounding icy environment and the body fluids. The purpose of the present study was to assess the desiccation tolerance of the Antarctic springtail, Cryptopygus antarcticus, under ecologically-relevant conditions characteristic of both summer and winter along the Antarctic Peninsula. In addition, this study examined the physiological changes and effects of mild drought acclimation on the subsequent desiccation tolerance of C. antarcticus. The collembolans possessed little resistance to water loss under dry air, as the rate of water loss was >20% h(-1) at 0% relative humidity (RH) and 4 degrees C. Even under ecologically-relevant desiccating conditions, the springtails lost water at all relative humidities below saturation (100% RH). However, slow dehydration at high RH dramatically increased the desiccation tolerance of C. antarcticus, as the springtails tolerated a greater loss of body water. Relative to animals maintained at 100% RH, a mild drought acclimation at 98.2% RH significantly increased subsequent desiccation tolerance. Drought acclimation was accompanied by the synthesis and accumulation of several sugars and polyols that could function to stabilize membranes and proteins during dehydration. Drought acclimation may permit C. antarcticus to maintain activity and thereby allow sufficient time to utilize behavioral strategies to reduce water loss during periods of reduced moisture availability. The springtails were also susceptible to desiccation at subzero temperatures in equilibrium with the vapor pressure of ice; they lost approximately 40% of their total body water over 28 d when cooled to -3.0 degrees C. The concentration of solutes in the remaining body fluids as a result of dehydration, together with the synthesis of several osmolytes, dramatically increased the body fluid osmotic pressure. This increase corresponded to a depression of the melting point to approximately -2.2 degrees C, and may therefore allow C. antarcticus to survive much of the Antarctic winter in a cryoprotectively dehydrated state.     

Thermoregulatory responses to acute body heating in rats acclimated to continuous heat exposure

Thermoregulatory responses to an acute heat load with intraperitoneal heating (IH) or indirect external warming (EW) by increasing ambient temperature (Ta) were investigated with direct and indirect calorimetry in rats acclimated to environments of 24.0 degrees C (Cn), 29.4 degrees C (H1), and 32.8 degrees C (H2) for greater than 15 days. The rats were placed in a direct calorimeter where the air temperature was maintained at 24 degrees C for the initial 3 h. IH was then made for 30 min through an electric heater implanted chronically (6.5 W.kg-1) in the peritoneal cavity, and EW was performed by raising the jacket water temperature surrounding the calorimeter from 24 to 39 degrees C (0.19 degrees C.min-1). Hypothalamic (Thy) and colonic temperature immediately before the start of the heat load tended to be higher as the acclimation temperature increased. During IH, the threshold Thy for the tail skin vasodilation (Tth) was significantly higher in H2 than in Cn rats. During EW, however, there was no difference in Tth between the groups. Metabolic heat production (M) was slightly suppressed during IH and significantly depressed only in H2 rats. During EW, M was suppressed in all the groups. The magnitude and duration of suppression were greater in H2 rats than in the other two groups. The responses in nonevaporative heat loss and thermal conductance (C) to the rise in Thy did not differ among the three groups during IH. According to the rise in Thy, however, there was a greater C increase in H2 than in Cn and H1 rats during EW.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in body temperature of rats acclimated to heat with different acclimation schedules

Male Wistar rats, initially maintained at an ambient temperature (Ta) of 23.8 degrees C, were subjected to one of seven different heat acclimation schedules under a 12:12-h light-dark cycle (lights on at 0600 h). Two groups of rats were exposed to Ta of 32.4 degrees C all day for 5 (HC5) or 10 (HC10) days. The other four groups were exposed to Ta of 32.8 degrees C for 5 h/day during the last half of the dark phase for 5 (NI5) or 10 (NI10) consecutive days or during the last half of the light phase for 5 (DI5) or 10 (DI10) consecutive days. Control rats (C) were kept at 23.8 degrees C throughout the experiment. Hypothalamic temperature (Thy) was measured every 5 min with a chronically implanted thermocouple from 1 day before the beginning to 2 days after the end of the heat acclimation periods. During the heat acclimation periods, daily mean Thy rose significantly in HC5 and HC10 rats but decreased significantly in NI5 and NI10 rats. Daily mean Thy did not change in C, DI5, and DI10 rats. Thy in HC10 rats sharply decreased at the end of the heat acclimation periods and remained at low levels for approximately 3 h. On the 2nd postacclimation day, however, mean Thy returned and remained at a significantly higher level. In NI10 rats, the mean Thy in the postacclimation period was significantly lower than the preacclimation values. No such changes in mean Thy were observed in DI10 rats. Five-days of heat exposure had little effect on the postacclimation Thy.(ABSTRACT TRUNCATED AT 250 WORDS)     

The physiological strain index applied to heat-stressed rats.

A physiological strain index (PSI) based on heart rate (HR) and rectal temperature (Tre) was recently suggested to evaluate exercise-heat stress in humans. The purpose of this study was to adjust PSI for rats and to evaluate this index at different levels of heat acclimation and training. The corrections of HR and Tre to modify the index for rats are as follows: PSI = 5 (Tre t - Tre 0). (41.5 - Tre 0)-1 + 5 (HRt - HR0). (550 - HR0)-1, where HRt and Tre t are simultaneous measurements taken at any time during the exposure and HR0 and Tre 0 are the initial measurements. The adjusted PSI was applied to five groups (n = 11-14 per group) of acclimated rats (control and 2, 5, 10, and 30 days) exposed for 70 min to a hot climate [40 degrees C, 20% relative humidity (RH)]. A separate database representing two groups of acclimated or trained rats was also used and involved 20 min of low-intensity exercise (O2 consumption approximately 50 ml. min-1. kg-1) at three different climates: normothermic (24 degrees C, 40% RH), hot-wet (35 degrees C, 70% RH), and hot-dry (40 degrees C, 20% RH). In normothermia, rats also performed moderate exercise (O2 consumption approximately 60 ml. min-1. kg-1). The adjusted PSI differentiated among acclimation levels and significantly discriminated among all exposures during low-intensity exercise (P < 0.05). Furthermore, this index was able to assess the individual roles played by heat acclimation and exercise training.     

Heat shock protein induction in the freshwater prawn Macrobrachium malcolmsonii: acclimation-influenced variations in the induction temperatures for Hsp70

The intracellular build-up of thermally damaged proteins following exposure to heat stress results in the synthesis of heat shock proteins (Hsps). In the present study, the upper thermal tolerance and expression of heat shock protein 70 (Hsp70) were examined in juveniles of the freshwater prawn Macrobrachium malcolmsonii that had been acclimated at two different temperatures, i.e. 20 degrees C (group A) and 30 degrees C (group B), in the laboratory for 30 days. Upper thermal tolerance was determined by a standard method. For heat-shock experiments, prawns in groups A and B were exposed to various elevated temperatures for 3 h each, followed by 1 h recovery at the acclimation temperature. Endogenous levels of Hsp70 were determined in the gill, heart, hepatopancreas and skeletal muscle tissues by Western blotting analysis of one dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The critical thermal maximum (CT max) for prawns in groups A and B was 37.7+/-0.27 degrees C and 41.41+/-0.16 degrees C, respectively. In general, Western blotting analysis for Hsp70 revealed one band at the 70 kDa region, containing both constitutive (Hsc70) and inducible (Hsp70) isoforms, in the gill and heart tissues; these were not detected in the hepatopancreas and skeletal muscle tissues. The onset temperature for Hsp70 induction in both gill and heart tissues was 30 degrees C for prawns in group A and 34 degrees C for those in group B. The optimum induction temperatures (at which Hsp70 induction was maximum) were found to be 34 degrees C and 32 degrees C, respectively, in the gill and heart tissues of group A prawns, and 38 degrees C and 36 degrees C, respectively, for group B prawns. These results suggest that the temperature at which acclimation occurs influences both upper thermal tolerance and Hsp70 induction in M. malcolmsonii.     

Response of superoxide dismutase isoenzymes in tomato plants (Lycopersicon esculentum) during thermo-acclimation of the photosynthetic apparatus

Seedlings of Lycopersicon esculentum Mill. var. Amalia were grown in a growth chamber under a photoperiod of 16 h light at 25 degrees C and 8 h dark at 20 degrees C. Five different treatments were applied to 30-day-old plants: Control treatment (plants maintained in the normal growth conditions throughout the experimental time), heat acclimation (plants exposed to 35 degrees C for 4 h in dark for 3 days), dark treatment (plants exposed to 25 degrees C for 4 h in dark for 3 days), heat acclimation plus heat shock (plants that previously received the heat acclimation treatment were exposed to 45 degrees C air temperature for 3 h in the light) and dark treatment plus heat shock (plants that previously received the dark treatment were exposed to 45 degrees C air temperature for 3 h in the light). Only the heat acclimation treatment increased the thermotolerance of the photosynthesis apparatus when the heat shock (45 degrees C) was imposed. In these plants, the CO(2) assimilation rate was not affected by heat shock and there was a slight and non-significant reduction in maximum carboxylation velocity of Rubisco (V(cmax)) and maximum electron transport rate contributing to Rubisco regeneration (J(max)). However, the plants exposed to dark treatment plus heat shock showed a significant reduction in the CO(2) assimilation rate and also in the values of V(cmax) and J(max). Chlorophyll fluorescence measurements showed increased thermotolerance in heat-acclimated plants. The values of maximum chlorophyll fluorescence (F(m)) were not modified by heat shock in these plants, while in the dark-treated plants that received the heat shock, the F(m) values were reduced, which provoked a significant reduction in the efficiency of photosystem II. A slight rise in the total superoxide dismutase (SOD) activity was found in the plants that had been subjected to both heat acclimation and heat shock, and this SOD activity was significantly higher than that found in the plants subjected to dark treatment plus heat shock. The activity of Fe-SOD isoenzymes was most enhanced in heat-acclimated plants but was unaltered in the plants that received the dark treatment. Total CuZn-SOD activity was reduced in all treatments. Darkness had an inhibitory effect on the Mn-SOD isoenzyme activity, which was compensated by the effect of a rise in air temperature to 35 degrees C. These results show that the heat tolerance of tomatoplants may be increased by the previous imposition of a moderately high temperature and could be related with the thermal stability in the photochemical reactions and a readjustment of V(cmax) and J(max). Some isoenzymes, such as the Fe-SODs, may also play a role in the development of heat-shock tolerance through heat acclimation. In fact, the pattern found for these isoenzymes in heat-acclimated Amalia plants was similar to that previously described in other heat-tolerant tomato genotypes.