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PubMed1998

Sensory nerves affect the recruitment and differentiation of rat periovarian brown adipocytes during cold acclimation.

Rat periovarian adipose tissue contains a low number of uncoupling protein-expressing brown adipocytes scattered into lobules of white fat. Their increase following cold acclimation is matched by a major increase in noradrenergic and neuropeptide Y-, substance P- and calcitonin gene-related peptide-containing nerves. To ascertain whether periovarian fat is provided with sensory nerves, and whether any relationship exists between such nerves (in particular the calcitonin gene-related peptide-containing fibers found in cold-acclimated rats in close association with brown adipocytes) and brown fat recruitment, the effects of capsaicin desensitization on neuropeptide-containing nerves and brown adipocyte density were studied in the periovarian tissue of rats kept at 20 degrees C and on a group acclimated to 4 degrees C for 14 days. In both groups, systemic capsaicin administration considerably reduced the expression of substance P and calcitonin gene-related peptide in vascular-nerve bundles and parenchyma. In cold-acclimated rats, the increase in brown adipocyte density was significantly checked by capsaicin administration (21.11 versus 7.96 brown adipocytes/mm2, P<0.05). Finally, ultrastructural investigation showed the occurrence of brown adipocyte precursors filled with aggregates of glycogen and poorly differentiated multilocular adipocytes in capsaicin-treated cold-acclimated rats. These data suggest that periovarian adipose tissue is indeed provided with sensory neuropeptide-containing nerves and that they play a role in the recruitment and differentiation of brown adipocytes.

Acclimatization↗

PubMed2003

Plasma membrane rafts of rainbow trout are subject to thermal acclimation.

Rafts are cholesterol- and sphingolipid-enriched microdomains of the plasma membrane (PM) that organize many signal transduction pathways. Interactions between cholesterol and saturated lipids lead to patches of liquid-ordered membrane (rafts) phase-separating from the remaining PM. Phase behavior is temperature sensitive, and acute changes in temperature experienced by poikilotherms would be expected to perturb raft structure, necessitating an acclimatory response. Therefore, with thermal acclimation, we would expect compositional changes in the raft directed to offset this perturbation. Using differential and density gradient centrifugation, we separated PM from the livers of rainbow trout acclimated to 5 degrees C and 20 degrees C into raft-enriched (raft) and raft-depleted PM (RDPM). Compared with RDPM, the raft fractions were enriched in cholesterol, the beta(2)-adrenergic receptor and adenylyl cyclase, which are commonly used markers for this microdomain. Furthermore, cholesterol was enriched in all fractions from warm-compared with cold-acclimated animals, but this increase was 3.4 times greater in raft than in PM. We developed a novel approach for measuring membrane molecular interaction strength (and thus the tendency to stabilize raft structure) based on the susceptibility of membranes to detergent. Specifically, studies with model vesicles demonstrated that the capacity of a membrane to accommodate detergent prior to solubilization (saturation point) was a good index of this property. The saturation point of the isolated membrane preparations was temperature sensitive and was significantly different in 5 degrees C- and 20 degrees C-acclimated RDPM when assayed at 5 degrees C and 20 degrees C, respectively. By contrast, this comparison in rafts was not significantly different, suggesting compensation of this property. These data suggest that compositional changes made in the PM during thermal acclimation act to offset thermal perturbation of the raft but not the RDPM structural integrity.

Acclimatization↗

PubMed2004

Seawater acclimation causes independent alterations in Na+/K+- and H+-ATPase activity in isolated mitochondria-rich cell subtypes of the rainbow trout gill.

Mitochondria-rich cells (MR cells) of the gills of rainbow trout undergo changes in relative distribution and biochemical function during acclimation to partial-strength (10 per thousand ) and full-strength (30 per thousand ) seawater. In isolated total gill cells, Na(+)/K(+)-ATPase activity increased fivefold and H(+)-ATPase activity decreased fourfold when trout were acclimated to either 10 per thousand or 30 per thousand seawater. When total MR gill cells were separated based on differential binding to peanut lectin agglutinin (PNA), the PNA subtypes underwent a change in relative distribution in seawater-acclimated fish. In freshwater, the ratio of PNA(-):PNA(+) was 65:35 while in seawater the distribution changed to 20:80 PNA(-):PNA(+). Additionally, differential changes in Na(+)/K(+)-ATPase and H(+)-ATPase activity in each of the independent cell types occurred during seawater acclimation; Na(+)/K(+)-ATPase activity in the PNA(-) cells increased by 197% while in PNA(+) cells Na(+)/K(+)-ATPase decreased by 57%. However, H(+)-ATPase activity was decreased in both PNA(-) (84%) and PNA(+) (72%) subtypes during acclimation to seawater.

Acclimatization↗

PubMed2005

Effects of hypoxia acclimation on morpho-physiological traits over three generations of Daphnia magna.

The mechanisms, dynamics and effects of hypoxia acclimation were studied in the water flea Daphnia magna over three successive generations (parental, first and second filial generation: P, F1 and F2). The P generation was raised under normoxic conditions at 20 degrees C and became exposed to environmental hypoxia (10-19% air saturation) at maturity. Their progenies (F1 and F2) experienced hypoxia from birth onwards. Controls were kept under normoxic conditions. Individuals were successively sampled in a 3-day interval from each acclimation group to determine morpho-physiological parameters relevant in oxygen transport and regulation. Hypoxia acclimation induced adjustments at the haemoglobin (Hb) and metabolic level (within 3 days) but none at the systemic level. The convective performance and oxygen-sensitive control of the ventilatory and circulatory systems were the same in both acclimation groups. The Hb concentration and oxygen affinity increased by 266% and 32%, respectively. The 22% decrease in mass-specific oxygen consumption rate reduced the energy allocation to somatic growth without greatly affecting reproduction. The onset and duration of hypoxic exposure during ontogenesis have had a significant influence on Hb oxygen affinity and body size. Transgenerational effects of hypoxia acclimation could not be observed. The adjustments at the Hb and metabolic levels in combination with the smaller body size, which is advantageous to diffusive oxygen transport, reduced the critical ambient oxygen tension by approximately 50%.

Acclimatization↗

PubMed2006

Effects of acclimation temperature and cadmium exposure on cellular energy budgets in the marine mollusk Crassostrea virginica: linking cellular and mitochondrial responses.

In order to understand the role of metabolic regulation in environmental stress tolerance, a comprehensive analysis of demand-side effects (i.e. changes in energy demands for basal maintenance) and supply-side effects (i.e. metabolic capacity to provide ATP to cover the energy demand) of environmental stressors is required. We have studied the effects of temperature (12, 20 and 28 degrees C) and exposure to a trace metal, cadmium (50 microg l(-1)), on the cellular energy budget of a model marine poikilotherm, Crassostrea virginica (eastern oysters), using oxygen demand for ATP turnover, protein synthesis, mitochondrial proton leak and non-mitochondrial respiration in isolated gill and hepatopancreas cells as demand-side endpoints and mitochondrial oxidation capacity, abundance and fractional volume as supply-side endpoints. Cadmium exposure and high acclimation temperatures resulted in a strong increase of oxygen demand in gill and hepatopancreas cells of oysters. Cd-induced increases in cellular energy demand were significant at 12 and 20 degrees C but not at 28 degrees C, possibly indicating a metabolic capacity limitation at the highest temperature. Elevated cellular demand in cells from Cd-exposed oysters was associated with a 2-6-fold increase in protein synthesis and, at cold acclimation temperatures, with a 1.5-fold elevated mitochondrial proton leak. Cellular aerobic capacity, as indicated by mitochondrial oxidation capacity, abundance and volume, did not increase in parallel to compensate for the elevated energy demand. Mitochondrial oxidation capacity was reduced in 28 degrees C-acclimated oysters, and mitochondrial abundance decreased in Cd-exposed oysters, with a stronger decrease (by 20-24%) in warm-acclimated oysters compared with cold-acclimated ones (by 8-13%). These data provide a mechanistic basis for synergism between temperature and cadmium stress on metabolism of marine poikilotherms. Exposure to combined temperature and cadmium stress may result in a strong energy deficiency due to the elevated energy demand on one hand and a reduced mitochondrial capacity to cover this demand on the other hand, which may have important implications for surviving seasonally and/or globally elevated temperatures in polluted estuaries.

Acclimatization↗

PubMed2006

Reciprocal expression of gill Na+/K+-ATPase alpha-subunit isoforms alpha1a and alpha1b during seawater acclimation of three salmonid fishes that vary in their salinity tolerance.

The upregulation of gill Na+/K+-ATPase activity is considered critical for the successful acclimation of salmonid fishes to seawater. The present study examines the mRNA expression of two recently discovered alpha-subunit isoforms of Na+/K+-ATPase (alpha1a and alpha1b) in gill during the seawater acclimation of three species of anadromous salmonids, which vary in their salinity tolerance. Levels of these Na+/K+-ATPase isoforms were compared with Na+/K+-ATPase activity and protein abundance and related to the seawater tolerance of each species. Atlantic salmon (Salmo salar) quickly regulated plasma Na+, Cl- and osmolality levels within 10 days of seawater exposure, whereas rainbow trout (Oncorhynchus mykiss) and Arctic char (Salvelinus alpinus) struggled to ionoregulate, and experienced greater perturbations in plasma ion levels for a longer period of time. In all three species, mRNA levels for the alpha1a isoform quickly decreased following seawater exposure whereas alpha1b levels increased significantly. All three species displayed similar increases in gill Na+/K+-ATPase activity during seawater acclimation, with levels rising after 10 and 30 days. Freshwater Atlantic salmon gill Na+/K+-ATPase activity and protein content was threefold higher than those of Arctic char and rainbow trout, which may explain their superior seawater tolerance. The role of the alpha1b isoform may be of particular importance during seawater acclimation of salmonid fishes. The reciprocal expression of Na+/K+-ATPase isoforms alpha1a and alpha1b during seawater acclimation suggests they may have different roles in the gills of freshwater and marine fishes; ion uptake in freshwater fish and ion secretion in marine fishes.

Acclimatization↗

PubMed2006

Acclimation to different thermal conditions in a northerly wintering shorebird is driven by body mass-related changes in organ size.

Seasonal acclimatization and experimental acclimation to cold in birds typically results from increased shivering endurance and elevated thermogenic capacity leading to improved resistance to cold. A wide array of physiological adjustments, ranging from biochemical transformations to organ mass variations, are involved in this process. Several studies have shown that improved cold endurance is accompanied by increases in summit metabolic rate (M(sum)), a measure of maximal heat production and an indicator of the level of sustainable thermogenic capacity. However, improved endurance to cold can also be achieved without significant changes in M(sum). The same is true for basal metabolic rate (BMR), which is known to increase in association with cold acclimatization or acclimation in some species but not in others. We investigated cold acclimation in a migrant shorebird known for extreme physiological flexibility, the red knot (Calidris canutus, the northerly wintering subspecies islandica). We measured BMR and M(sum) over two months in birds caught in the wild and transferred to experimentally controlled conditions representative of aspects of their seasonal thermal environment (two groups at constant 25 degrees C, one group at constant 4 degrees C and two groups experiencing variable outdoor temperatures). Birds maintained in both cold and variable ambient temperatures showed a 14-15% higher body mass, 33-45% higher food intake, and 26% and 13% elevations in BMR and M(sum), respectively, compared with birds kept at thermoneutrality. These results, together with data on alimentary tract size and pectoral muscle thickness measured by ultrasonography, suggest that red knots acclimate to cold primarily through modulation of (lean) body mass components. Heavier individuals have larger muscles, which allow higher maximal heat production and better thermal compensation. Cold acclimation effects on BMR are most probably due to changes in the size of visceral organs, although not the alimentary tract in this specific case. The liver, known for its thermogenic capacity, is a probable candidate. Overall, our results indicate that relatively small changes in body mass and muscle size allow enough reserve capacity in terms of heat production to cope with typical wintering ambient temperature variations as measured on the red knot's wintering grounds.

Acclimatization↗

PubMed1987

Thermal dependence of maximum Ca2+-activated force in skinned muscle fibres of the toad Bufo marinus acclimated at different temperatures.

Mechanically skinned muscle fibres from the twitch region of the iliofibularis muscle of cool- (16 +/- 1 degree C) and warm- (32 +/- 1 degree C) acclimated cane toads (Bufo marinus) were activated maximally by Ca2+ in solutions of different pH and at different temperatures (approx. 1-35 degrees C). Acclimation of up to 12 weeks at 16 degrees C and up to 8 weeks at 32 degrees C did not modify the marked thermal dependence of isometric force in the skeletal muscle fibres of the cane toad. The prominent decline of maximum Ca2+-activated force at lower temperatures, a property which is not characteristic of muscles from other anurans, was associated with an obvious decline in fibre stiffness at temperatures below about 20 degrees C, regardless of the temperatures at which the toads were kept prior to experimentation. The results suggest that the decline of isometric force at lower temperatures is due both to a reduction in the number of cross-bridges and to a decrease in the force output per cross-bridge. The maximum Ca2+-activated force response increased when fibres were activated in solutions of increasing pH at all temperatures investigated. This trend is expected to have a compensatory effect on the thermal dependence of the maximum Ca2+-activated force under physiological conditions, because of the elevation of intracellular pH as temperature declines. The isometric force did not depend on the concentration of the zwitterionic species of the pH buffer in solutions. The skinned fibre preparation developed a Ca2+-insensitive residual force following maximal activation. The increment in residual force followed a linear relationship with the duration of activation at a given temperature and a power relationship of activation temperature for a given duration of activation. Fibres from warm-acclimated animals developed less residual force following activations at 15 degrees C than did fibres from cool-acclimated animals, suggesting that thermal acclimation may substantially reduce the magnitude of this phenomenon at temperatures below 20 degrees C.

Acclimatization↗

PubMed1995

The effects of softwater acclimation on respiratory gas transfer in the rainbow trout Oncorhynchus mykiss

Gill O2 uptake, CO2 excretion, ventilation and blood respiratory/acidbase variables were evaluated in control and softwater-acclimated trout (Oncorhynchus mykiss) to test the hypothesis that gill chloride cell (CC) proliferation, elicited by 2 weeks of softwater exposure, impairs the diffusion of respiratory gases across the gill. The proliferation of CCs in softwater fish was verified using light microscopy, and its impact on respiratory gas transfer was assessed in vivo by continuous monitoring of arterial blood PO2 (PaO2), PCO2 (PaCO2) and pH (pHa) using an extracorporeal blood circulation under conditions of normoxia and graded hypoxia [water PO2 (PwO2) was lowered from 20.0 kPa to 5.3 kPa within 20 min]. During normoxia, ventilation frequency was significantly higher in the softwater trout (78±4 versus 57±4 breaths min-1; mean ± s.e.m.), while ventilation amplitude was similar in both groups (1.01.1 cm opercular displacement). PaCO2 and plasma HCO3- concentration were significantly lower in the softwater fish and the blood acidbase status was characterized by a mixed respiratory alkalosis and metabolic acidosis such that blood pH was not statistically different between the two groups. CO2 excretion (2.52.8 mmol kg-1 h-1) and O2 uptake rates (2.35.1 mmol kg-1 h-1), as measured during normoxia, were unaffected by acclimation to soft water. During hypoxia, ventilation frequency and amplitude increased in the control trout, whereas only ventilation amplitude increased in the softwater-acclimated fish. The rate of PaO2 reduction during hypoxia was significantly greater in the softwater fish (0.84±0.06 versus 0.65±0.06 kPa PaO2 kPa-1 PwO2) and, at the most severe level of hypoxia (PwO2=5.3 kPa), PaO2 was significantly lower in the softwater fish. The rate of PaCO2 reduction (caused by hyperventilation) was significantly lower in the softwater-acclimated fish (0.002±0.001 versus 0.005±0.001 kPa PaCO2 kPa-1 PwO2; mean ± s.e.m.; P<0.06) and, indeed, was not statistically different from zero. Blood pH did not change significantly during hypoxia in either group but, through much of the hypoxic period (715 kPa PwO2), pHa was statistically lower in the softwater-acclimated fish. These results demonstrate that exposure of trout to soft water for 2 weeks is associated with proliferation of lamellar CCs and impaired branchial gas transfer. Hyperventilation was identified as a compensatory physiological adjustment.

Journal Article↗

PubMed1998

Effects of temperature on cuticular lipids and water balance in a desert Drosophila: is thermal acclimation beneficial?

The desert fruit fly Drosophila mojavensis experiences environmental conditions of high temperature and low humidity. To understand the physiological mechanisms allowing these small insects to survive in such stressful conditions, we studied the effects of thermal acclimation on cuticular lipids and rates of water loss of adult D. mojavensis. Mean hydrocarbon chain length increased at higher temperatures, but cuticular lipid melting temperature (Tm) did not. Lipid quantity doubled in the first 14 days of adult life, but was unaffected by acclimation temperature. Despite these changes in cuticular properties, organismal rates of water loss were unaffected by either acclimation temperature or age. Owing to the smaller body size of warm-acclimated flies, D. mojavensis reared for 14 days at 33 degrees C lost water more rapidly on a mass-specific basis than flies acclimated to 25 degrees C or 17 degrees C. Thus, apparently adaptive changes in cuticular lipids do not necessarily result in reduced rates of water loss. Avoidance of high temperatures and desiccating conditions is more likely to contribute to survival in nature than changes in water balance mediated by surface lipids.

Acclimatization↗

PubMed1999

Lipid restructuring does not contribute to elevated activities of Na(+)/K(+)-ATPase in basolateral membranes from the gill of seawater-acclimated eel (Anguilla rostrata)

In teleost fishes, increases in gill Na(+)/K(+)-ATPase activity accompanying the transition from fresh water to sea water may be attributed to changes in either the numbers of enzyme molecules present or to turnover number (k(cat)). The sensitivity of Na(+)/K(+)-ATPase to its chemical/physical environment in the membrane makes it plausible that modulation of enzyme activity may be driven, in part, by changes in membrane properties. In the current study, I test the hypothesis that lipid compositional changes (restructuring) contribute to the modulation of gill Na(+)/K(+)-ATPase activity. An enriched preparation of basolateral membranes was prepared from the gills of freshwater- and seawater-acclimated American eel (Anguilla rostrata). Phospholipid class distribution, fatty acyl chain compositions and cholesterol contents were determined. Phosphatidylcholine, the most abundant phospholipid present in gill basolateral membranes, makes up more than 60 % of the total phospholipid content in both freshwater- and seawater-acclimated animals. The contents of other phospholipids and major fatty acyl chains are also similar for the two acclimation groups. Cholesterol/phospholipid molar ratios are 0.28 for freshwater and 0.29 for seawater animals. The similarity between lipid compositions in membranes from freshwater- and seawater-acclimated eels indicates that lipid restructuring is not a mechanism for modulation of gill Na(+)/K(+)-ATPase activity in Anguilla rostrata, at least during the acclimation time course used in the present study.

Journal Article↗

PubMed1999

Contractile properties of atrial and ventricular myocardium of the heart of rainbow trout oncorhynchus mykiss: effects of thermal acclimation

Atrial and ventricular myocardium perform different tasks in the pumping work of the vertebrate heart, which are reflected in their contractile properties. Although atrial contraction is assumed to have an important role in the function of fish heart, the contractile properties of atrial and ventricular myocardium have not been directly compared in any fish species. The objective of this study was to clarify any contractile differences in the heart of teleost fish and, in particular, to elucidate the contribution of myofibrillar ATPase and intracellular Ca(2+) stores to the characteristics of atrial and ventricular contraction. Experiments were conducted on thermally acclimated rainbow trout Oncorhynchus mykiss to determine whether the effects of temperature adaptation are the same in atrial and ventricular tissue. It was shown that the rate of isometric contraction is much faster in atrial than in ventricular tissue of the fish heart and that acclimation to cold increases the rate of contraction in both cardiac compartments. The rapid contraction kinetics of the atrial tissue were associated with higher myofibrillar ATPase activity and faster Ca(2+) uptake rate of the sarcoplasmic reticulum (SR) compared with ventricular tissue. Similarly, the faster kinetics of contraction following cold acclimation could be attributed to enhancement of the myofibrillar and/or SR function. The atrio-ventricular and temperature-induced differences were also expressed in the recovery of force from inactivation, i.e. in the mechanical restitution. The refractory period and the rate constant of force restitution were shorter in atrial than in ventricular muscle tissue. Similar differences also existed between the tissues of cold-acclimated (CA, 4 degrees C) and warm-acclimated (WA, 17 degrees C) fish. The fast recovery of force from inactivation in the heart of the CA trout was, at least in part, due to more active SR. Furthermore, it was shown that the force of atrial contraction in the CA trout is sensitive to ryanodine (10 (&mgr;)mol l(-)(1)), a Ca(2+)-release channel blocker of SR, at physiological body temperature (4 degrees C) and at a physiological pacing rate (0.6 Hz). This finding indicates that the Ca(2+) stores of SR contribute to activation of cardiac contraction in the fish heart, and that the SR of fish heart is able to retain its Ca(2+) load at low body temperatures, i.e. the Ca(2+ )release channels of SR are not leaky in the cold. The present data show that in the atrial tissue of CA trout, the SR directly contributes to the cytosolic Ca(2+) and that in the atrium and ventricle of CA trout, the SR significantly accelerates the recovery of contractility from inactivation. The fast recovery from inactivation allows relatively high heart rates and therefore adequate cardiac outputs at low environmental temperatures for the cold-active rainbow trout.

Journal Article↗

PubMed2000

Chloride conductance across toad skin: effects of ionic acclimations and cyclic AMP and relationship to mitochondria-rich cell density.

The anionic conductance across toad (Bufo viridis) skin was studied using the voltage-clamp technique following long-term (more than 10 days) acclimation to NaCl and KCl solutions. The non-specific baseline conductance was approximately 0.6 mS cm(-)(2) and was similar in skins from all acclimation conditions. The voltage-activated Cl(-) conductance (G(Cl)) was maximal in skins from distilled-water- and KCl-acclimated toads (>3 mS cm(-)(2)) and was greatly reduced following acclimation to NaCl solutions. Cyclic AMP (EC(50)=13 micromol l(-)(1)) and isobutylmethyl xanthine (IBMX) (EC(50)=69 micromol l(-)(1)) exerted different effects on the activated conductance. IBMX only sensitized the activated conductance, whereas cyclic AMP (CPTcAMP) at high concentrations induced an increase in anionic conductance that was insensitive to electrical potential. Furthermore, external Cl(-) was not required for the stimulatory effect of cyclic AMP, and the conductive pathway had low selectivity. The effects of the two agonists were reversible and depended on the acclimation conditions. Following electrical measurements, the skin of the toads was removed and stained with silver to measure mitochondria-rich cell density (D(mrc)). There was no correlation between D(mrc) and Cl(-) conductance in the present study.

1-Methyl-3-isobutylxanthine↗

PubMed2002

Thermal acclimation changes DNA-binding activity of heat shock factor 1 (HSF1) in the goby Gillichthys mirabilis: implications for plasticity in the heat-shock response in natural populations.

The intracellular build-up of thermally damaged proteins following exposure to heat stress results in the synthesis of a family of evolutionarily conserved proteins called heat shock proteins (Hsps) that act as molecular chaperones, protecting the cell against the aggregation of denatured proteins. The transcriptional regulation of heat shock genes by heat shock factor 1 (HSF1) has been extensively studied in model systems, but little research has focused on the role HSF1 plays in Hsp gene expression in eurythermal organisms from broadly fluctuating thermal environments. The threshold temperature for Hsp induction in these organisms shifts with the recent thermal history of the individual but the mechanism by which this plasticity in Hsp induction temperature is achieved is unknown. We examined the effect of thermal acclimation on the heat-activation of HSF1 in the eurythermal teleost Gillichthys mirabilis. After a 5-week acclimation period (at 13, 21 or 28 degrees C) the temperature of HSF1 activation was positively correlated with acclimation temperature. HSF1 activation peaked at 27 degrees C in fish acclimated to 13 degrees C, at 33 degrees C in the 21 degrees C group, and at 36 degrees C in the 28 degrees C group. Concentrations of both HSF1 and Hsp70 in the 28 degrees C group were significantly higher than in the colder acclimated fish. Plasticity in HSF1 activation may be important to the adjustable nature of the heat shock response in eurythermal organisms and the environmental control of Hsp gene expression.

Acclimatization↗

PubMed2002

Interspecific- and acclimation-induced variation in levels of heat-shock proteins 70 (hsp70) and 90 (hsp90) and heat-shock transcription factor-1 (HSF1) in congeneric marine snails (genus Tegula): implications for regulation of hsp gene expression.

In our previous studies of heat-shock protein (hsp) expression in congeneric marine gastropods of the genus Tegula, we observed interspecific and acclimation-induced variation in the temperatures at which heat-shock gene expression is induced (T(on)). To investigate the factors responsible for these inter- and intraspecific differences in T(on), we tested the predictions of the 'cellular thermometer' model for the transcriptional regulation of hsp expression. According to this model, hsps not active in chaperoning unfolded proteins bind to a transcription factor, heat-shock factor-1 (HSF1), thereby reducing the levels of free HSF1 that are available to bind to the heat-shock element, a regulatory element upstream of hsp genes. Under stress, hsps bind to denatured proteins, releasing HSF1, which can now activate hsp gene transcription. Thus, elevated levels of heat-shock proteins of the 40, 70 and 90 kDa families (hsp 40, hsp70 and hsp90, respectively) would be predicted to elevate T(on). Conversely, elevated levels of HSF1 would be predicted to decrease T(on). Following laboratory acclimation to 13, 18 and 23 degrees C, we used solid-phase immunochemistry (western analysis) to quantify endogenous levels of two hsp70 isoforms (hsp74 and hsp72), hsp90 and HSF1 in the low- to mid-intertidal species Tegula funebralis and in two subtidal to low-intertidal congeners, T. brunnea and T. montereyi. We found higher endogenous levels of hsp72 (a strongly heat-induced isoform) at 13 and 18 degrees C in T. funebralis in comparison with T. brunnea and T. montereyi. However, T. funebralis also had higher levels of HSF1 than its congeners. The higher levels of HSF1 in T. funebralis cannot, within the framework of the cellular thermometer model, account for the higher T(on) observed for this species, although they may explain why T. funebralis is able to induce the heat-shock response more rapidly than T. brunnea. However, the cellular thermometer model does appear to explain the cause of the increases in T(on) that occurred during warm acclimation of the two subtidal species, in which warm acclimation was accompanied by increased levels of hsp72, hsp74 and hsp90, whereas levels of HSF1 remained stable. T. funebralis, which experiences greater heat stress than its subtidal congeners, consistently had higher ratios of hsp72 to hsp74 than its congeners, although the sum of levels of the two isoforms was similar for all three species except at the highest acclimation temperature (23 degrees C). The ratio of hsp72 to hsp74 may provide a more accurate estimate of environmental heat stress than the total concentrations of both hsp70 isoforms.

Acclimatization↗

PubMed1982

Thermal acclimation in a crustacean neuromuscular system.

1. Effects of temperature on the muscle fibre membrane and synapses of stretcher muscle preparations made from autotomized limbs of the Pacific shore crab (Pachygrapsus crassipes) were investigated. 2. Acclimation of the crabs to different temperatures modified properties of both muscle fibre membrane and synapses. 3. Increased temperature produced an increase in membrane potential of the muscle fibres. A semi-log plot of these data revealed two linear phases of the membrane potential-temperature relationship, with a change in slope near the acclimation temperature. 4. Maximum values for excitatory junction potential (EJP) amplitude and time constant of EJP decay, and minimum values for facilitation were obtained at temperatures close to the acclimation temperature. It is suggested that the decline in EJP amplitude and time constant of decay produced by deviations in temperature from the acclimation temperature is compensated for by an increase in the amount of facilitation. In this way, maximum tension can be produced by the stretcher muscle in a range of at least 8 degrees C around the acclimation temperature.

Adaptation, Physiological↗

PubMed2005

Differential and coordinated expression of Cbf and Cor/Lea genes during long-term cold acclimation in two wheat cultivars showing distinct levels of freezing tolerance.

The cold acclimation process in plants is primarily regulated through the signal transduction pathways that lead to the induction and enhancement of expression of different sets of Cor/Lea genes. Winter wheat 'Mironovskaya 808' (M808) exhibited a much higher level of freezing tolerance than spring wheat 'Chinese Spring' (CS), and the difference became clearer after the long-term cold acclimation. To understand the molecular basis of this cultivar difference, we isolated two CBF/DREB1 homologs, Wcbf2, which are the candidate gene for a transcription factor of the Cor/Lea genes. Expression of the Wcbf2 gene was induced rapidly by low temperature (LT) and drought but not by abscisic acid (ABA). The gene expression was temporal and at least twice up-regulated by LT. The first up-regulation occurred within 1-4 h, which might correspond to the rapid response to LT, while the second up-regulation occurred during 2-3 weeks of cold acclimation. After the second up-regulation, the amount of Wcbf2 transcript greatly decreased in CS, while it increased again in M808 after 4 weeks until 9 weeks (end of the test period). The maintenance of this high level of the Wcbf2 transcript might represent the long-term effect of cold acclimation. The activation of Cor/Lea genes followed the accumulation of Wcbf2 transcript suggested direct involvement of the Wcbf2 gene in the induction and enhancement of the Cor/Lea gene expression. The cultivar difference in freezing tolerance developed during different stages of cold acclimation can be at least partly explained by the differential and coordinated regulation of the predicted Cor/Lea gene signal transduction pathway that is mediated by the CBF/DREB1 transcription factors in common wheat.

Acclimatization↗

PubMedDate not supplied

Mechanism of heat acclimation induced bradycardia in the sand rat.

The effect of heat acclimation on chronotropic response of the heart under normothermic and hyperthermic conditions was studied in the sand rat, Psammomys obesus, a diurnal desert species. All animals were acclimated at 34 degrees C for 0, 5, 14, 30 or 60 days; heat stress was achieved by exposure at 38 degrees C. Continuous measurements of heart rate (HR) were carried out on conscious animals, using chronic subcutaneous electrodes. Atropine (0.1 mg/100 g) and propranolol (1 mg/100 g) were administered to evaluate the para-sympathetic (V) and sympathetic (S) influences on HR. Intrinsic HR (HRi) was measured following administration of both drugs simultaneously. P. obesus developed bradycardia from day 5 of the acclimation. This bradycardia was induced solely by decreased HRi, overriding partial vagal withdrawal. During hyperthermia apparent thermal insensitivity of HR was observed. This was attained by partial sympathetic withdrawal compensating for the increase in HRi due to body temperature rise. It can be concluded that in P. obesus, heat acclimation induced bradycardia is attained by intrinsic changes in the pacing cells. It also emerges that the hyperthermic response is independent of and is not affected by heat acclimation.

Acclimatization↗