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PubMed1984

Isoproterenol-induced blood flow in rats acclimated to room temperature and cold.

Blood flow was measured with labeled microspheres in cold (6 degrees C)- and room-temperature-acclimated rats at rest and during infusion of the beta-agonist, isoproterenol. Isoproterenol elicited decreased mass-specific blood flows (ml . g-1 . min-1) to liver (55% of control), kidney (48%), and white fat (64%) in room-temperature-acclimated (RT) rats. Blood flow was similarly decreased in liver (51%), kidney (37%), and white fat (70%) in cold-acclimated (CA) rats. In contrast, isoproterenol increased blood flows to brown fat and cardiac and skeletal muscles. The blood flows to cardiac muscle during isoproterenol infusion were comparable in both RT (12.7 ml . g-1 . min-1) and CA (11.7 ml . g-1 . min-1) animals, representing increases of 2.9- and 2.6-fold above control values, respectively. Blood flow to skeletal muscle was also similarly elevated in RT and CA animals, representing increases 5.9 and 5.6 times those of their respective control values. In contrast, although isoproterenol greatly stimulated (ca. 8-fold) blood flow to brown fat (interscapular plus cervical depots) in RT animals, it had a greater effect on these two depots in the CA rats (18-fold increase). These data emphasize the importance of brown fat as a major effector of nonshivering thermogenesis as well as the importance of beta-adrenergic receptors in mediating the metabolic response of nonshivering thermogenesis.

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PubMed1988

Membrane lipid composition and overwintering strategy in thermally acclimated crayfish.

The membrane lipid composition of hepatopancreas tissue was analyzed from two phylogenetically related species of crayfish after thermal acclimation to either 5 or 20 degrees C. One species overwinters in a quiescent state (Orconectes propinquus) and the other remains active throughout the winter (Cambarus bartoni). Both species significantly decreased the level of saturates in the major phosphatides, phosphatidylcholine (PC) and phosphatidylethanolamine (PE; difference not significant in C. bartoni), and both winter-active and winter-quiescent animals tended to be enriched in n-3 acids in PC. Orconectes animals tended to be enriched in n - 3 acids in PC. Orconectes accumulated n - 6 acids in PE at low temperature, whereas Cambarus lost n - 6 acids in this lipid, a change that was offset by the accumulation of monoenes. Cardiolipin became more saturated with cold acclimation in both species. The fatty acid content of the triacylglycerol fraction showed little change with acclimation history, although O. propinquus did accumulate n - 3 acids in triglycerides at low temperature. In O. propinquus, head-group composition was not significantly altered by temperature, but in C. bartoni, cold temperature increased the relative amounts of PE, sphingomyelin, and phosphatidylinositol at the expense of PC. Adaptations to temperature in membrane composition of poikilotherms are related to the overwintering strategy of the animal.

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PubMed1990

Increase in cytochrome oxidase capacity of BAT and other tissues in cold-acclimated gerbils.

To determine the effects of cold acclimation on the oxidative capacity of different tissues and their possible role in nonshivering thermogenesis (NST) in a desert rodent, the gerbil (Gerbillus campestris), measurements of cytochrome oxidase activity (COX) were performed on homogenates of liver, brown adipose tissue (BAT), and subsarcolemmal (SS) and intermyofibrillar (IMF) fractions of skeletal muscle. Total organ mass was also measured. Gerbils were maintained either at thermoneutrality (TN) or cold [4 degrees C, ambient temperature (Ta)] for 4 (CA4) or 8 (CA8) wk. A comparative study was made with mice (Mus musculus). Total and relative masses of BAT increased significantly in both gerbils and mice during cold acclimation, whereas muscle mass decreased in CA4 gerbils. Specific and total COX (TCOX) increased in the three tissues. A considerable increase (+170%, P less than 0.05) of TCOX and protein content in liver of CA4 gerbils compared with controls was observed, whereas no significant changes occurred in liver of CA4 mice. In muscle, an increase of TCOX in SS and a reduction in IMF cell compartments were noted. The increase in BAT oxidative capacity in CA4 gerbils represented only a small portion of that in liver (36%) and in SS fraction of muscle (41%) at the same stage of acclimation. The ability of the three tissues to contribute to components of thermogenesis in vivo was evaluated. The sum of oxidative capacity of the three tissues was largely above the peak metabolic rate (PMR), whereas that of muscle exceeded maximum shivering thermogenesis, which developed both in TN and CA4 gerbils.(ABSTRACT TRUNCATED AT 250 WORDS)

Acclimatization↗

PubMed1990

Thermal acclimation and muscle contractile properties in cyprinid fish.

After several weeks of cold acclimation, the swimming performance of some fish is increased at low temperatures and decreased at high temperatures. The temperature compensation of locomotory activity involves changes in central patterns of muscle fiber recruitment and in the properties of the peripheral nervous system and muscle tissues. In some freshwater fish belonging to the family Cyprinidae, including the goldfish (Carassius auratus), the common carp (Cyprinus carpio), and the roach (Rutilus rutilus), the intrinsic contractile properties of muscles are modified by thermal acclimation. Parameters that can be altered by temperature acclimation in both fast and slow muscle fibers include isometric twitch contraction time, maximum force production, and unloaded shortening speed. The molecular mechanisms responsible for these changes in contractility are discussed.

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PubMed1990

Mechanisms of temperature acclimation in the carp: a molecular biology approach.

Most fish are unable to regulate their body temperature. Therefore their muscle power output would be expected to decline markedly with decreased environmental temperature. However, some species including carp show a remarkable ability to acclimate. The nature and significance of changes in enzymatic myosin ATPase activity and the recruitment of different muscle fiber types in relation to acclimation and environmental temperature in carp are reviewed. These changes presumably involve qualitative and quantitative changes in gene expression. To approach the problem, we have constructed a carp genomic library and isolated 28 nonoverlapping clones containing sequences of different myosin heavy chain (MHC) isoforms along with a clone containing carp actin sequences. Plasmids containing probes specific for a carp fast muscle MHC and actin RNA were constructed. Hybridization results suggest that the expression of this MHC isoform RNA is increased in warm- relative to cold-acclimated carp. These data indicate that there are different MHC isoform genes that are expressed at warm and cold environmental temperatures.

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PubMed1999

Heat stress induces ultrastructural changes in cutaneous capillary wall of heat-acclimated rock pigeon.

In heat-acclimated rock pigeons, cutaneous water evaporation is the major cooling mechanism when exposed at rest to an extremely hot environment of 50-60 degrees C. This evaporative pathway is also activated in room temperature by a beta-adrenergic antagonist (propranolol) or an alpha-adrenergic agonist (clonidine) and inhibited by a beta-adrenergic agonist (isoproterenol). In contrast, neither heat exposure nor drug administration activates cutaneous evaporation in cold-acclimated pigeons. To elucidate the mechanisms underlying this phenomenon, we studied the role of the ultrastructure and permeability of the cutaneous vasculature. During both heat stress and the administration of propranolol and clonidine, we observed increased capillary fenestration and endothelial gaps. Similarly, propranolol increased the extravasation of Evans blue-labeled albumin in the skin tissue. We concluded that heat acclimation reinforces a mechanism by which the activation of adrenergic signal transduction pathways alters microvessel permeability during heat stress. Consequently the flux of plasma proteins and water into the interstitial space is accelerated, providing an interstitial source of water for sustained cutaneous evaporative cooling.

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PubMed1975

Orthostatism and heat acclimation.

Three groups of subjects (6 subj in each group) underwent the following precedures: group A was given a 20-min head-up tilt at 21 degrees C followed by 4 h of exercise at 33.9 degrees C DB, 32.2 degrees C WB, and a repetition of tilting after exercise in heat; group B underwent the same procedure at 21 degrees C; group C was tilted at 21 degrees C, rested in heat for 4 h and was retilted in heat. The above procedures were repeated for 8 days, and on the last day groups B and C underwent the same treatment as group A. Group A showed the usual decreases in heart rate and rectal temperature and an increase in sweat rate on acclimation. This corresponded to marked improvements in heat-orthostatism. While five subjects in group A fainted during post-exposure tilting on the first exposure, none fainted on the last day. Resting in heat (group C) did not cause any acclimation to work in heat. This corresponded to poor heat-orthostatism after the work-heat procedure when five subjects fainted. Mild training at 21 degrees C (group B) resulted in minor improvements to work in heat as evident by some improvements in heart rate responses after the 1st and 2nd h of exposure. This corresponded to better heat-orthostatism and fewer men fainting than in group C. The results indicated that heat-orthostatism improves on acclimation to the same extent as exercise heart rate and rectal temperature.

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PubMed1977

Control of liver size in heat-acclimated hamsters.

In the hamster, heat acclimation reduces liver weight more than it does body weight. Therefore, liver weight constitutes a lower percentage of body weight during exposure to high ambient temperature. This change is not a result of dehydration since water content of the whole body and of the liver is not altered during heat acclimation. However, changes in lactic dehydrogenase isozyme proportions indicate a higher rate of liver degradation during the first 2 wk of heat exposure. These changes are accompanied by enhancement of DNA synthesis which is found to be elevated during the early period of heat exposure and later to fall to the control levels. The enhanced DNA synthesis might be a result of a high rate of tissue regeneration which probably takes place in the organ following the commencement of the degradative processes as was suggested in partial hepatectomy. Since the activity of DNA synthesis is negatively correlated with cyclic AMP levels, it is suggested that cyclic AMP plays some role in controlling hepatic DNA synthesis during heat acclimation.

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PubMed1981

Proposed mechanism for increased thyroxine deiodination in cold-acclimated rats.

Groups of male rats were administered isoproterenol (ISO), thyroxine (T4), or both (ISO + T4) daily for 20 days in an attempt to mimic the effect of cold acclimation on the rate of outer-ring deiodination of T4 to triiodothyronine (T3) by 2,000-g supernatants of homogenates of liver and kidney. The rates of hepatic and renal deiodination of T4 to T3 in an additional group of rats exposed to 4 +/- 1 degree C for 20 days were 53 and 71% higher, respectively, than control. Administration of ISO (100 micrograms . kg-1 . day-1) did not affect the rate of deiodination of T4 to T3 by either hepatic or renal tissue. Administration of T4 (50 micrograms . kg-1 . day-1) resulted in rates of hepatic and renal deiodination of T4 that were 297 and 222% higher, respectively, than control. Administration of ISO + T4 resulted in a rate of conversion of T4 to T3 not significantly different from that observed when T4 was administered alone. Serum concentration of T4 was elevated after administration of both T4 and ISO + T4, whereas serum concentration of T3 was elevated significantly above that of control only in the cold-acclimated group. These results suggest that the increased rate of 5'-monodeiodination of T4 by hepatic and renal homogenates from cold-acclimated rats is not a result of increased beta-adrenergic activity but can be accounted for by the increase in thyroid activity observed in these animals.

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PubMed1982

Protein kinase activity in liver of heat-acclimated hamsters.

Protein kinase activity in incubated liver slices from 35 degrees C heat-acclimated (HA) hamsters was 70% higher than in similar slices from 23 degrees C control (C) hamsters. Adding glucagon to the incubation medium increased protein kinase activity by 65% in slices from C animals, but by only 30% in slices from HA animals. Binding of [3H]cAMP to proteins of a low-speed supernatant fraction of incubated and homogenized slices was 30% lower for HA than for C hamsters. For each acclimation group this binding was reduced 30% by incubation of the slices with glucagon. The activities of phosphorylase kinase, phosphorylase phosphatase, and phosphorylase alpha in slices incubated with or without glucagon did not differ between groups. Addition of glucagon increased phosphorylase kinase by 30% and phosphorylase alpha by 40% but caused no change in phosphorylase phosphatase activity. These results suggest that heat acclimation of the hamster increases the amount of a species of liver protein kinase that is different from the one that mediates the effect of glucagon on glycogenolysis.

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PubMed1987

Substrate utilization in leg muscle of men after heat acclimation.

Eight men were heat acclimated (39.6 degrees C and 29.2% rh) for 8 days to examine changes in substrate utilization. A heat exercise test (HET), (cycling for 60 min; 50% maximal O2 consumption) was performed before (UN-HET) and after (ACC-HET) the acclimation period. Muscle glycogen utilization (67.0 vs. 37.6 mmol/kg wet wt), respiratory exchange ratio (0.85 +/- 0.002 vs. 0.83 +/- 0.001), and calculated rate of carbohydrate oxidation (75.15 +/- 1.38 vs. 64.80 +/- 1.52 g/h) were significantly reduced (P less than 0.05) during the ACC-HET. Significantly lower (P less than 0.05) femoral venous glucose (15, 30, and 45 min) and lactate (15 min) levels were observed during the ACC-HET. No differences were observed in plasma free fatty acid (FFA) and glycerol concentrations or glucose, lactate and glycerol arteriovenous uptake/release between tests. A small but significant increase (P less than 0.05) above resting levels in FFA uptake was observed during the ACC-HET. Leg blood flow was slightly greater (P greater than 0.05) during the ACC-HET (4.64 +/- 0.13 vs. 4.80 +/- 0.13 l/min). These findings indicate a reduced use of muscle glycogen following heat acclimation. However, the decrease is not completely explained by a shift toward greater lipid oxidation or increased blood flow.

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PubMed1987

Differential effects of hypoxia on sleep of warm- and cold-acclimated rats.

We studied the effect of different levels of hypoxia (10, 12 or 13, 15, and 18% O2) on the sleep-waking pattern (SWP) and the maximum-minimum core temperature of warm-acclimated (WA) and cold-acclimated (CA) rats at their neutral temperature, 29 degrees C. Whereas the SWP of WA rats showed a trend toward increasing disruption as the degree of hypoxia increased, CA rats exhibited no such trend. The effect was chiefly on the frequency of state changes and less on epoch durations. The SWP of WA rats was more vulnerable to hypoxia than that of CA rats. Maximum and minimum body temperatures of WA and CA rats were not significantly affected by O2 lack down to 10% inspired O2. We conclude that in the rat 1) hypoxia primarily affects the neural mechanism that governs the frequency of changes in sleep-waking states; 2) the extent of alterations in SWP's depends on the ambient temperature to which the rats are acclimated; and 3) hypoxia does not significantly affect deep body temperature at the animal's neutral temperature.

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PubMed1991

Effects of hypoxia and cold acclimation on thermoregulation in the rat.

The effects of hypoxia (inspired O2 fraction = 0.12) on thermoregulation and on the different sources of thermogenesis were studied in rats before and after periods of 1-4 wk of cold acclimation. Measurements of metabolic rate (VO2) and body temperature (Tb) were made at 5-min intervals, and shivering activity was recorded continuously in groups of rats subjected to three protocols. In protocol 1, rats were exposed to normoxia to an ambient temperature (Ta) of 5 degrees C for 2 h. In protocol 2, at Ta of 5 degrees C, rats were exposed for 30 min to normoxia, then for 45 min to hypoxia, and finally for 30 min to normoxia. In protocol 3, in the non-cold-acclimated (NCA) rats, Ta was decreased from 30 to 5 degrees C in steps of 5 degrees C and of 30-min duration while in cold-acclimated (CA) rats at 5 degrees C for 4-wk, Ta was increased from 5 to 30 degrees C in steps of 5 degrees C and of 30-min duration. Recordings were made in normoxia and in hypoxia on different days in the same animals. The results showed that 1) in NCA rats, cold exposure in normoxia induced increases in VO2 and shivering that were proportional to the decrease in Ta; 2) in CA rats in normoxia, for a given Ta, VO2 and Tb were higher than in NCA rats, whereas shivering was generally lower; and 3) in both NCA and CA rats, hypoxia induced a transient decrease in shivering and a sustained decrease in nonshivering thermogenesis associated with a marked decrease in Tb that was about the same in NCA and CA rats. We speculate that hypoxia acts on Tb control to produce a general inhibition of thermogenesis. Nonshivering thermogenesis is markedly sensitive to hypoxia, especially demonstrable in CA rats; a recovery or even an increase in shivering can compensate for the decrease in nonshivering thermogenesis.

Acclimatization↗

PubMed2002

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.

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PubMed1999

Is the growth of brown adipocytes necessary for cold acclimation in mice?

Chronic exposure to cold increases the growth of brown adipose tissue and the resistance to more severe cold, thus improving thermogenesis. The present study examined the possibility that dietary compounds can modify cold acclimation. Adenosine (ADO) or adenine (ADE) were administered in drinking water (0.05%, w/v) for 5 weeks to male ddY strain mice from 4 weeks of age. At 5 weeks of age, the mice were exposed to 4 degrees C for 4 weeks. After termination of this period, the interscapular brown adipose tissue (IBAT) and rectal temperature on acute exposure to severe cold (-20 degrees C) for 60 min were measured. Chronic exposure to cold increased the weight of IBAT and made mice resistant to a fall in rectal temperature on exposure to severe cold. The growth of IBAT and improvement in thermogenesis can be used as a cold acclimation profile in ddY mice. The growth of IBAT was selectively prevented by ingestion of ADO. The improvement in thermogenesis was reduced, but only a little, by the ingestion of either ADO or ADE. Thus, growth of brown adipocytes might not be necessary for cold acclimation in mice.

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PubMed2003

On-line venous oxygen tensions in rainbow trout during graded exercise at two acclimation temperatures.

For most teleost fish, the majority of the myocardial oxygen supply is provided by the oxygen reserve remaining in venous blood after other tissues have extracted oxygen. We examined the effect of graded exercise and water temperature on this venous blood oxygen supply to the heart (termed the cardiac circulation) by performing novel on-line measurements of venous partial pressure of oxygen (Pv(O(2))) using a fibreoptic micro-optode implanted in the ductus Cuvier of rainbow trout (Oncorhynchus mykiss). As expected, Pv(O(2)) decreased progressively and significantly as swimming velocity approached the critical swimming speed (U(crit)). Unsteady swimming behaviours during the graded exercise, however, caused abrupt and generally short-lived decreases in Pv(O(2)). For the cold-acclimated (6-10 degrees C) fish, Pv(O(2)) reached a minimum plateau value of 15.3+/-3.7 torr (1 torr=133.3 Pa) before U(crit) was reached, and so increased swimming effort near to U(crit) did not reduce Pv(O(2)) further. Warm-acclimated fish had a significantly higher Pv(O(2)) (28.9+/-3.5 torr) at U(crit). Despite this difference in the Pv(O(2)) at U(crit), we estimated that there was little difference between warm- and cold-acclimated fish in terms of oxygen supply in the cardiac circulation because of a right-shift in the haemoglobin-oxygen dissociation curve at warm temperatures. Furthermore, although Pv(O(2)) decreased significantly at U(crit), our estimates suggest that the expected increase in cardiac output would easily maintain the oxygen supply in venous blood at a level similar to that found in resting fish. Although unsteady swimming behaviours decreased Pv(O(2)), unsteady swimming rarely decreased the minimum Pv(O(2)) value observed at U(crit) by more than 10%. The findings are discussed in terms of a threshold Pv(O(2)) required to maintain adequate rates of oxygen diffusion from the cardiac circulation to the myocardial tissues.

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PubMed2005

The effects of acclimation to reversed seasonal temperatures on the swimming performance of adult brown trout Salmo trutta.

Adult brown trout (Salmo trutta) were acclimatised to and maintained at seasonal temperatures (5 degrees C in winter; 15 degrees C in summer) and acclimated to reversed seasonal temperatures (15 degrees C in winter; 5 degrees C in summer) while exposed to the natural (i.e. seasonally variable) photoperiod. The mean critical swimming speeds (U(crit)) of animals acclimatised to the seasonal temperatures were similar, but more than 30% greater than those for fish acclimated to the reversed seasonal temperatures. The lower values of U(crit) that accompanied acclimation to reversed seasonal temperatures appeared largely to result from the inability of white muscle to function maximally, since the concentrations of lactate and ammonia in white muscle of fish swum to U(crit) at reversed seasonal temperatures were significantly lower than those in fish swum at seasonal temperatures. These observations, together with biochemical and morphometric attributes of muscle tissue, suggest that swimming ability is influenced, at least in part, by seasonal factors other than temperature. These data have important implications for the design of experiments using fish that experience predictable, usually seasonal, changes in their natural environment (temperature, dissolved oxygen, changes in water levels, etc.).

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PubMed2005

Low temperature acclimated populations of the grain aphid Sitobion avenae retain ability to rapidly cold harden with enhanced fitness.

In contrast to previous studies of rapid cold-hardening (RCH), which have investigated the responses of insects maintained under 'summer conditions' (20 degrees to 25 degrees C), this study focuses on the ability of low-temperature acclimated insects to undergo RCH. When the grain aphid Sitobion avenae Fabricus was low-temperature acclimated by rearing for three generations at 10 degrees C, the discriminating temperatures (temperature that results in approximately 20% survival after direct transfer from the rearing temperature to a sub-zero temperature for a period of 3 h), of first instar nymphs and adult aphids were -11.5 degrees and -12 degrees C, respectively. Maximum rapid cold-hardening was induced by cooling aphids at 0 degrees C for 2 h (nymphs) or 30 min (adults), resulting in survival at the respective discriminating temperatures increasing from 26% to 96% (nymphs) and 22% to 70% (adults). Cooling from 10 degrees to 0 degrees C at 1 degree, 0.1 degrees and 0.05 degrees C min-1 significantly increased survival of nymphs at the discriminating temperature, but not of adults. There were no ;ecological costs' associated with rapid cold-hardening at 0 degrees C, or with exposure of rapidly cold-hardened aphids to the discriminating temperatures; fecundity and longevity, in both nymphs and adults were either similar to control aphids or significantly increased. The study demonstrates that rapid cold-hardening ability is retained in aphids that have already undergone cold-acclimation, as would be the case in overwintering aphids. Both rapid cold-hardening and subsequent exposure at previously lethal temperatures can enhance fitness in surviving individuals.

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