Thermal acclimation,mitochondrial capacities and organ metabolic profiles in a reptile (<Emphasis Type="Italic">Alligator mississippiensis</Emphasis>)

Reptiles thermoregulate behaviourally, but change their preferred temperature and the optimal temperature for performance seasonally. We evaluated whether the digestive and locomotor systems of the alligator show parallel metabolic adjustments during thermal acclimation. To this end, we allowed juvenile alligators to grow under thermal conditions typical of winter and summer, providing them with seasonally appropriate basking opportunities. Although mean body temperatures of alligators in these groups differed by approximately 10°C, their growth and final anatomic status was equivalent. While hepatic mitochondria isolated from cold-acclimated alligators had higher oxidative capacities at 30°C than those from warm-acclimated alligators, the capacities did not differ at 20°C. Cold acclimation decreased maximal oxidative capacities of muscle mitochondria. For mitochondria from both organs and acclimation groups, palmitate increased oligomycin-inhibited respiration. GDP addition reduced palmitate-uncoupled rates more in liver mitochondria from warm- than cold-acclimated alligators. In muscle mitochondria, carboxyatractyloside significantly reduced palmitate-uncoupled rates. This effect was not changed by thermal acclimation. The aerobic capacity of liver, skeletal muscle and duodenum, as estimated by activities of cytochrome c oxidase (COX), increased with cold acclimation. At acclimation temperatures, the activities of COX and citrate synthase (CS) in these organs were equivalent. By measuring COX and CS in isolated mitochondria and tissue extracts, we estimated that cold acclimation did not change the mitochondrial content in liver, but increased that of muscle. The thermal compensation of growth rates and of the aerobic capacity of the locomotor and digestive systems suggests that alligators optimised metabolic processes for the seasonally altered, preferred body temperature. The precision of this compensatory response exceeds that typically shown by aquatic ectotherms whose body temperatures are at the mercy of their habitat.     

Temperature induced variation in the distribution of different types of muscle fibre in the goldfish (Carassius auratus)

Summary Goldfish (Carassius auratus L.) were acclimated to environmental temperatures of 3 °C, 18 °C and 31 °C for a period of three months. Cytochemical techniques were used to study the metabolism and myofibrillar ATPase activities of individual muscle fibres. Fish muscle is composed of three basic fibre types each with distinct contractile and metabolic characteristics. Cold acclimation resulted in a shift to a more aerobic type of metabolism, particularly in the red and pink fibres. In addition, environmental temperature was found to affect the size and relative distribution of the different fibre types in the myotome. The total number of pink and red fibres increased significantly with cold acclimation. Mechanisms of environmentally-induced adaptation of muscle fibre phenotype are discussed.In addition to changes in the metabolism and distribution of muscle-fibre types, biochemical studies have provided evidence for different kinetic forms of Mg²⁺Ca²⁺ myofibrillar ATPase at different environmental temperatures. Activities of myofibrillar ATPase assayed at 31 °C were 2–3 times higher in fish acclimated to the higher environmental temperature. Activation enthalpy (H ) of the ATPase was also signficantly reduced in the cold adapted enzyme. Reduction of H in the cold acclimated ATPase is thought to reduce the temperature sensitivity of the activation process thus partly compensating for the reduced cell temperature.     

Effects of temperature acclimation on cardiorespiratory performance of the Antarctic notothenioid Trematomus bernacchii

Notothenioid fishes of the Southern Ocean have evolved under cold and stable temperatures for millions of years. Due to rising temperatures in the Southern Ocean, investigating thermal limits and the capacities for inducing a temperature acclimation response in notothenioids has become of increasing interest. Here, we investigated effects of temperature acclimation on cardiorespiratory responses and cardiac and skeletal muscle energy metabolism in a benthic Antarctic notothenioid, Trematomus bernacchii. We acclimated specimens to ?1, 2 and 4.5 °C for 14 days and quantified heart rates and ventilation rates during an acute increase in temperature. Ventilation rates showed an effect of acclimation both at initial steady-state acclimation conditions and during an acute temperature increase, suggesting a partial thermal compensatory response. However, acclimation did not affect heart rates at steady-state acclimation conditions and the temperatures at which onset of cardiac arrhythmia occurred, suggesting lack of inducible thermal tolerance in cardiac performance. Citrate synthase (CS), lactate dehydrogenase (LDH) and 3-hydroxyacyl dehydrogenase activities in skeletal muscle tissues suggested acclimation-induced shifts in metabolic fuel preferences, and a marked increase in LDH activity with acclimation to 4.5 °C showed an increase in anaerobic metabolism. In heart tissue, CS and LDH activities decreased with acclimation to 4.5 °C, suggesting reduced cardiac ATP production. Overall, the data suggest a partial acclimatory response to temperature by T. bernacchii and support the hypothesis that reduced cardiac acclimatory capacity may play a role in limiting the thermal plasticity of T. bernacchii.     

Can phenotypic plasticity facilitate the geographic expansion of the tilapia Oreochromis mossambicus?

Climate influences the distribution of organisms because of the thermal sensitivity of biochemical processes. Animals may compensate for the effects of variable temperatures, and plastic responses may facilitate radiation into different climates. The tropical fish Oreochromis mossambicus has radiated into climates that were thought to be thermally unsuitable. Here, we test the hypothesis that thermal acclimation will extend the locomotory and metabolic performance range of O. mossambicus. Juvenile fish were acclimated to 14 degrees, 17 degrees, and 22 degrees C. We measured responses to acclimation at three levels of organization: whole-animal performance (sustained swimming and resting and recovery rates of oxygen consumption), mitochondrial oxygen consumption in caudal muscle, and metabolic enzyme activities in muscle and liver at 12 degrees, 14 degrees, 17 degrees, 22 degrees, and 26 degrees C. Thermal optima of sustained swimming performance (U(crit)) changed significantly with acclimation, but acclimation had no effect on either resting or recovery oxygen consumption. Fish compensated for cold temperatures by upregulating state 3 mitochondrial oxygen consumption and increasing activity of lactate dehydrogenase in the liver. The capacity for phenotypic plasticity in O. mossambicus means that the fish would not be limited by its locomotor performance or metabolic physiology to expand its range into cooler thermal environments from its current distribution.     

Thermal acclimation effects differ between voluntary, maximum, and critical swimming velocities in two cyprinid fishes

Temperature acclimation may be a critical component of the locomotor physiology and ecology of ectothermic animals, particularly those living in eurythermal environments. Several studies of fish report striking acclimation of biochemical and kinetic properties in isolated muscle. However, the relatively few studies of whole-animal performance report variable acclimation responses. We test the hypothesis that different types of whole-animal locomotion will respond differently to temperature acclimation, probably due to divergent physiological bases of locomotion. We studied two cyprinid fishes, tinfoil barbs (Puntius schwanenfeldii) and river barbels (Barbus barbus). Study fish were acclimated to either cold or warm temperatures for at least 6 wk and then assayed at four test temperatures for three types of swimming performance. We measured voluntary swimming velocity to estimate routine locomotor behavior, maximum fast start velocity to estimate anaerobic capacity, and critical swimming velocity to estimate primarily aerobic capacity. All three performance measures showed some acute thermal dependence, generally a positive correlation between swimming speed and test temperature. However, each performance measure responded quite differently to acclimation. Critical speeds acclimated strongly, maximum speeds not at all, and voluntary speeds uniquely in each species. Thus we conclude that long-term temperature exposure can have very different consequences for different types of locomotion, consistent with our hypothesis. The data also address previous hypotheses that predict that polyploid and eurythermal fish will have greater acclimation abilities than other fish, due to increased genetic flexibility and ecological selection, respectively. Our results conflict with these predictions. River barbels are eurythermal polyploids and tinfoil barbs stenothermal diploids, yet voluntary swimming acclimated strongly in tinfoil barbs and minimally in river barbels, and acclimation was otherwise comparable.     

Phenotypic flexibility in the metabolic response of the limpet Cellana tramoserica to thermally different microhabitats

Temperature determines all physiological responses by limiting cellular reaction rates. Daily temperature variation differs between microhabitats, which means that subpopulations of the same species may respond differently to temperature. The aim of this study is to determine how physiological responses to temperature of the limpet Cellana tramoserica differ between limpets from variable and from stable thermal environments. Oxygen consumption and anaerobic and aerobic metabolic capacities were measured over a range of temperatures in limpets from thermally stable and variable field sites in summer and winter, and in laboratory acclimation treatments. Limpets from both variable and stable sites, showed acclimatisation of anaerobic and aerobic potentials. Limpets from stable environments, but not from variable environments, showed increased oxygen consumption in winter. Comparison of field and laboratory data showed that temperature was the signal for acclimatisation. The physiological response of C. tramoserica to temperature depends on season and microhabitat. Care must therefore be taken when conducting interspecies comparisons of response to temperature to address the confounding effects of phenotypic plasticity. Differences in physiological response to temperature in phenotypically flexible species like C. tramoserica may simply reflect individual reactions to immediate environmental conditions.     

The ontogeny of contractile performance and metabolic capacity in a high-frequency muscle

High-performance muscles such as the shaker muscles in the tails of western diamond-backed rattlesnakes (Crotalus atrox) are excellent systems for studying the relationship between contractile performance and metabolic capacity. We observed that shaker muscle contraction frequency increases dramatically with growth in small individuals but then declines gradually in large individuals. We tested whether metabolic capacity changed with performance, using shaker muscle contraction frequency as an indicator of performance and maximal activities of citrate synthase and lactate dehydrogenase as indicators of aerobic and anaerobic capacities, respectively. Contraction frequency increased 20-fold in 20-100-g individuals but then declined by approximately 30% in individuals approaching 1,000 g. Mass-independent aerobic capacity was positively correlated with contractile performance, whereas mass-independent anaerobic capacity was slightly but negatively correlated with performance; body mass was not correlated with performance. Rattle mass increased faster than the ability to generate force. Early in ontogeny, shaker muscle performance appears to be limited by aerobic capacity, but later performance becomes limited equally by aerobic capacity and the mechanical constraint of moving a larger mass without proportionally thicker muscles. This high-performance muscle appears to shift during ontogeny from a metabolic constraint to combined metabolic and mechanical constraints.     

Effects of hibernation on mitochondrial regulation and metabolic capacities in myocardium of painted turtle (Chrysemys picta)

Painted turtles hibernating during winter may endure long-term exposure to low temperature and anoxia. These two conditions may affect the aerobic capacity of a tissue and might be of particular importance to the cardiac muscle normally highly reliant on aerobic energy production. The present study addressed how hibernation affects respiratory characteristics of mitochondria in situ and the metabolic pattern of turtle myocardium. Painted turtles were acclimated to control (25 degrees C), cold (5 degrees C) normoxic and cold anoxic conditions. In saponin-skinned myocardial fibres, cold acclimation increased mitochondrial respiratory capacity and decreased apparent ADP-affinity. Concomitant anoxia did not affect this. Creatine increased the apparent ADP-affinity to similar values in the three acclimation groups, suggesting a functional coupling of creatine kinase to mitochondrial respiration. As to the metabolic pattern, cold acclimation decreased glycolytic capacity in terms of pyruvate kinase activity and increased lactate dehydrogenase (LHD) activity. Concomitant anoxia counteracted the cold-induced decrease in pyruvate kinase activity and increased creatine kinase activity. In conclusion, cold acclimation seems to increase aerobic and decrease anaerobic energy production capacity in painted turtle myocardium. Importantly, anoxia does not affect the mitochondrial functional integrity but seems to increase the capacity for anaerobic energy production and energy buffering.     

Thermogenic side effects to migratory predisposition in shorebirds

In the calidrine sandpiper red knot (Calidris canutus), the weeks preceding takeoff for long-distance migration are characterized by a rapid increase in body mass, largely made up of fat but also including a significant proportion of lean tissue. Before takeoff, the pectoral muscles are known to hypertrophy in preparation for endurance flight without any specific training. Because birds facing cold environments counterbalance heat loss through shivering thermogenesis, and since pectoral muscles represent a large proportion of avian body mass, we asked the question whether muscle hypertrophy in preparation for long-distance endurance flight would induce improvements in thermogenic capacity. We acclimated red knots to different controlled thermal environments: 26 degrees C, 5 degrees C, and variable conditions tracking outdoor temperatures. We then studied within-individual variations in body mass, pectoral muscle size (measured by ultrasound), and metabolic parameters [basal metabolic rate (BMR) and summit metabolic rate (M(sum))] throughout a 3-mo period enclosing the migratory gain and loss of mass. The gain in body mass during the fattening period was associated with increases in pectoral muscle thickness and thermogenic capacity independent of thermal acclimation. Regardless of their thermal treatment, birds showing the largest increases in body mass also exhibited the largest increases in M(sum). We conclude that migratory fattening is accompanied by thermoregulatory side effects. The gain of body mass and muscle hypertrophy improve thermogenic capacity independent of thermal acclimation in this species. Whether this represents an ecological advantage depends on the ambient temperature at the time of fattening.     

Thermal variation near the thermal optimum does not affect the growth,metabolism or swimming performance in wild Atlantic salmon Salmo salar

Typically, laboratory studies on the physiological effects of temperature are conducted using stable acclimation temperatures. Nonetheless, information extrapolated from these studies may not accurately represent wild populations living in thermally variable environments. The aim of this study was to compare the growth rate, metabolism and swimming performance of wild Atlantic salmon exposed to cycling temperatures, 16–21°C, and stable acclimation temperatures, 16, 18.5, 21°C. Growth rate, metabolic rate, swimming performance and anaerobic metabolites did not change among acclimation groups, suggesting that within Atlantic salmon's thermal optimum range, temperature variation has no effect on these physiological properties.     

Thermal acclimation is not necessary to maintain a wide thermal breadth of aerobic scope in the common killifish (Fundulus heteroclitus)

Loss of aerobic scope at high and low temperatures is a physiological mechanism proposed to limit the thermal performance and tolerance of organisms, a theory known as oxygen- and capacity-limited thermal tolerance (OCLTT). Eurythermal organisms maintain aerobic scope over wide ranges of temperatures, but it is unknown whether acclimation is necessary to maintain this breadth. The objective of this study was to examine changes in aerobic scope in Fundulus heteroclitus, a eurythermal fish, after acclimation and acute exposure to temperatures from 5° to 33°C. The range of temperatures over which aerobic scope was nonzero was similar in acclimated and acutely exposed fish, suggesting that acclimation has modest effects on the thermal breadth of aerobic scope. However, in acclimated fish, there was a clear optimum temperature range for aerobic scope between 25° and 30°C, whereas aerobic scope was relatively constant across the entire temperature range with acute temperature exposure. Therefore, the primary effect of acclimation was to increase aerobic scope between 25° and 30°C, which paradoxically resulted in a narrower temperature range of optimal performance in acclimated fish compared to acutely exposed fish. There was only weak evidence for correlations between the thermal optimum of aerobic scope and the thermal optimum of measures of performance (specific growth rate and gonadosomatic index), and indicators of anaerobic metabolism (lactate accumulation and lactate dehydrogenase activity) only increased at high temperatures. Together these data fit many, but not all, of the predictions made by OCLTT.     

Adapt,move or die – how will tropical coral reef fishes cope with ocean warming?

Previous studies hailed thermal tolerance and the capacity for organisms to acclimate and adapt as the primary pathways for species survival under climate change. Here we challenge this theory. Over the past decade, more than 365 tropical stenothermal fish species have been documented moving poleward, away from ocean warming hotspots where temperatures 2–3 °C above long‐term annual means can compromise critical physiological processes. We examined the capacity of a model species – a thermally sensitive coral reef fish, Chromis viridis (Pomacentridae) – to use preference behaviour to regulate its body temperature. Movement could potentially circumvent the physiological stress response associated with elevated temperatures and may be a strategy relied upon before genetic adaptation can be effectuated. Individuals were maintained at one of six temperatures (23, 25, 27, 29, 31 and 33 °C) for at least 6 weeks. We compared the relative importance of acclimation temperature to changes in upper critical thermal limits, aerobic metabolic scope and thermal preference. While acclimation temperature positively affected the upper critical thermal limit, neither aerobic metabolic scope nor thermal preference exhibited such plasticity. Importantly, when given the choice to stay in a habitat reflecting their acclimation temperatures or relocate, fish acclimated to end‐of‐century predicted temperatures (i.e. 31 or 33 °C) preferentially sought out cooler temperatures, those equivalent to long‐term summer averages in their natural habitats (~29 °C). This was also the temperature providing the greatest aerobic metabolic scope and body condition across all treatments. Consequently, acclimation can confer plasticity in some performance traits, but may be an unreliable indicator of the ultimate survival and distribution of mobile stenothermal species under global warming. Conversely, thermal preference can arise long before, and remain long after, the harmful effects of elevated ocean temperatures take hold and may be the primary driver of the escalating poleward migration of species.     

Developmental and adult acclimation effects of ambient temperature on temperature regulation of mice selected for high and low levels of nest-building

Summary Genetic and environmental components of adaptation to cold inMus musculus were assessed in a study of the effects of selective breeding for behavioral temperature regulation (indexed by high and low levels of nest-building), rearing mice from birth in the cold, and cold acclimation of adult animals, on thermoregulatory traits. Mice from the eleventh selected generation of a high-nesting line maintained higher resting metabolic rates and body temperatures, while at the same time consuming less food when compared with mice from the low-nesting line (Table 1). High-nesting mice were also more discriminating in their temperature preference when placed on a thermal gradient. Thus, common genetic loci must influence a variety of energy conservation measures important for survival in the cold, including insulative nest-building, metabolic efficiency, and optimum microhabitat selection.Rearing mice at 5°C from birth until 70 days of age resulted in permanent increases in nonshivering thermogenesis, weight of interscapular brown adipose tissue, and core body temperature when compared to mice raised at 22°C (Table 1). These greater heat production capacities were accompanied by consumption of more food. Cold acclimation of adults at 5°C for 3 weeks similarly increased measures of thermogenic capacity (nonshivering thermogenesis and interscapular brown adipose tissue) as well as food consumption, when compared to the effects of warm acclimation, but differed from the effects of cold-rearing in that while resting metabolic rates were elevated, no significant differences in body temperature were found (Table 1).Sex differences were also noted for most of the thermoregulatory measures, with the lighter females scoring higher on thermal preference, resting metabolic rate, nonshivering thermogenesis, brown fat, and food consumption.In general, these results suggest that a more precise partitioning of the genetic and environmental factors which influence thermoregulatory traits in mammals could eventually result in a better understanding of the differences which exist between acclimated and acclimatized animals.     

Thermal sensitivity of metabolic rates and swimming performance in two latitudinally separated populations of cod, <Emphasis Type="Italic">Gadus morhua</Emphasis> L.

Atlantic cod populations live in a wide thermal range and can differ genetically and physiologically. Thermal sensitivity of metabolic capacity and swimming performance may vary along a latitudinal gradient, to facilitate performance in distinct thermal environments. To evaluate this hypothesis, we compared the thermal sensitivity of performance in two cod stocks from the Northwest Atlantic that differ in their thermal experience: Gulf of St Lawrence (GSL) and Bay of Fundy (BF). We first compared the metabolic, physiological and swimming performance after short-term thermal change to that at the acclimation temperature (7°C) for one stock (GSL), before comparing the performance of the two stocks after short-term thermal change. For cod from GSL, standard metabolism (SMR) increased with temperature, while active metabolism (AMR, measured in the critical swimming tests), EMR (metabolic rate after an exhaustive chase protocol), aerobic scope (AS) and critical swimming speeds (U _crit and U _b–c) were lower at 3°C than 7 or 11°C. In contrast, anaerobic swimming (sprint and burst-coasts in U _crit test) was lower at 11 than 7 or 3°C. Factorial AS (AMR SMR⁻¹) decreased as temperature rose. Time to exhaustion (chase protocol) was not influenced by temperature. The two stocks differed little in the thermal sensitivities of metabolism or swimming. GSL cod had a higher SMR than BF cod despite similar AMR and AS. This led factorial AS to be significantly higher for the southern stock. Despite these metabolic differences, cod from the two stocks did not differ in their U _crit speeds. BF cod were better sprinters at both temperatures. Cod from GSL had a lower aerobic cost of swimming at intermediate speeds than those from BF, particularly at low temperature. Only the activity of cytochrome C oxidase (CCO) in white muscle differed between stocks. No enzymatic correlates were found for swimming capacities, but oxygen consumption was best correlated with CCO activity in the ventricle for both stocks. Overall, the stocks differed in their cost of maintenance, cost of transport and sprint capacity, while maintaining comparable thermal sensitivities.     

Cold acclimation induces proliferation of sarcoplasmic reticulum without increase in Ca2+-ATPase activity in white axial muscle of striped bass (Morone saxatilis)

The effects of acclimation of striped bass to cold (5 degrees C) and warm (25 degrees C) temperatures upon ultrastructural features of white axial skeletal muscle are quantified. Surface density of sarcoplasmic reticulum (SR) increased by almost 30%, and SR volume density increased by about 20% during cold acclimation. Proliferation of SR suggests an increase in available SR surface for re-sequestration of Ca2+ and a decrease in diffusion path length for Ca2+ during cold acclimation. Average cross-sectional areas and cross-sectional perimeters of myofibrils situated in the center of muscle fibers decreased during cold acclimation by approximately 20% and 11%, respectively. Additionally, average major and minor axes of ellipses fit to central myofibrillar cross-sections decreased by approximately 12% and 8%, respectively, during cold acclimation. These measurements define a decrease in average myofibrillar diameter and suggest a decrease in diffusion path length for Ca2+ to and from myofibrillar activation sites. Measurements of peripheral myofibrils that had elongated profiles in cross-sections indicate that maximum profile length of these myofibrils decreases by about 17%. Peripheral myofibrils may break up into smaller myofibrils with more rounded cross-sectional profiles during cold acclimation. SR Ca2+-ATPase of white axial muscle was also measured in unfractionated homogenates and in crude SR-enriched subcellular fractions from cold- and warm-acclimated striped bass. No difference in SR Ca2+-ATPase activity per g wet weight was observed between cold- and warm-acclimated animals. Lack of increase in SR Ca2+-ATPase per g wet weight, despite a significant proliferation of SR, probably results in a decrease in average Ca2+-ATPase pump density within the SR membrane during cold acclimation. Thus, compensation for decreased diffusion coefficient of Ca2+ during cold acclimation appears due to the combined effects of proliferation of SR surface density and a decrease in average myofibrillar diameter.     

Biochemical responses to temperature in the contractile protein complex of striped bass Morone saxatilis

The effects of acute and long-term changes in temperature upon catalytic and calcium regulatory function of red (slow oxidative) and white (fast glycolytic) muscle from striped bass (Morone saxatilis) were determined. Acclimation to 5 degrees C or 25 degrees C had no significant effect on catalytic function (ATPase activity) or regulatory sensitivity (Ca++-activation) of myofibrils from either muscle type. Substantial differences between red and white muscle were found in the intrinsic thermal sensitivity of maximally-activated Mg++-Ca++ myofibrillar ATPase. Arrhenius plots of myofibrillar ATPase from white muscle show one significant breakpoint at 29 degrees C, with activation energies (Ea) of 2.3 and 23.4 kcal mole-1 at temperatures above and below this transition, respectively. Arrhenius plots of myofibrillar ATPase from red muscle show two transitions occurring at 22 and 9 degrees C, with Ea of 7.6 kcal mole-1 above 22 degrees C and 18.3 kcal mole-1 between 9 and 22 degrees C. Activation energies for myofibrils from red muscle increase substantially to approximately 107.3 kcal mole-1 below the 9 degrees C breakpoint. Differences in the intrinsic thermal sensitivity of red and white muscle catalytic function are apparently due to interaction of actomyosins and calcium regulatory proteins which are specific to each muscle type. The results suggest that capacity for sustained swimming in striped bass, which is powered exclusively by red muscle, will be severely impaired at cold temperature unless compensations occur above the level of contractile proteins.     

Divergence in aerobic scope and thermal tolerance is related to local thermal regime in two populations of introduced Nile perch (Lates niloticus)

We tested whether thermal tolerance and aerobic performance differed between two populations of Nile perch (Lates niloticus) originating from the same source population six decades after their introduction into two lakes in the Lake Victoria basin in East Africa. We used short-term acclimation of juvenile fish to a range of temperatures from ambient to +6°C, and performed critical thermal maximum (CT_max) and respirometry tests to measure upper thermal tolerance, resting and maximum metabolic rates, and aerobic scope (AS). Across acclimation temperatures, Nile perch from the cooler lake (Lake Nabugabo, Uganda) tended to have lower thermal tolerance (i.e., CT_max) and lower aerobic performance (i.e., AS) than Nile perch from the warmer waters of Lake Victoria (Bugonga region, Uganda). Effects of temperature acclimation were more pronounced in the Lake Victoria population, with the Lake Nabugabo fish showing less thermal plasticity in most metabolic traits. Our results suggest phenotypic divergence in thermal tolerance between these two introduced populations in a direction consistent with an adaptive response to local thermal regimes.     

Biochemical transformation of canine skeletal muscle for use in cardiac-assist devices

Skeletal muscle has an inherent biochemical phenotypic plasticity that provides the possibility for it to be remodeled into a "heart-like" muscle for use in cardiac-assist devices. The purpose of this study was to chronically stimulate skeletal muscle electrically to transform the biochemical capacities of the three major subcellular systems (i.e., metabolic, calcium regulating, and contractile) to resemble those of heart muscle. The latissimus dorsi muscle (LDM) of mongrel dogs weighing 22-27 kg was stimulated via the thoracodorsal nerve at 2 Hz for 6-8 wk. This stimulation protocol reduced the phosphorylase (glycogenolytic) and phosphofructokinase (glycolytic) activities by 70%. The aerobic (citrate synthase activity) and fatty acid oxidative (3-hydroxyacyl-CoA dehydrogenase activity) capacities were not significantly increased by chronic stimulation and remained at about one-fourth those in the canine heart. The calcium-dependent sarcoplasmic reticulum adenosinetriphosphatase (ATPase) activity in the microsomal fraction, which was sixfold greater in the nonstimulated LDM than in the heart, was reduced by electrical stimulation to a level similar to that of the dog heart. The contractile capacity was evaluated by determining the percentage of types I and II fibers, the myofibrillar ATPase activity, and the proportion of myosin isoforms. The transformed muscle was comprised of 93 +/- 2% type I fibers, a myofibrillar ATPase activity similar to that in heart with primarily a slow-twitch muscle myosin isoform. In conclusion, electrical stimulation of canine LDM at 2 Hz for 6-8 wk resulted in two of the three biochemical systems, which confer physiological expression and fatigue resistance to muscle being transformed to resemble those of the myocardium.     

Disentangling environmental drivers of metabolic flexibility in birds: the importance of temperature extremes versus temperature variability

Examining physiological traits across large spatial scales can shed light on the environmental factors driving physiological variation. For endotherms, flexibility in aerobic metabolism is especially important for coping with thermally challenging environments and recent research has shown that aerobic metabolic scope [the difference between maximum thermogenic capacity (M_sum) and basal metabolic rate (BMR)] increases with latitude in mammals. One explanation for this pattern is the climatic variability hypothesis, which predicts that flexibility in aerobic metabolism should increase as a function of local temperature variability. An alternative explanation is the cold adaptation hypothesis, which predicts that cold temperature extremes may also be an important driver of variation in metabolic scope. To determine the thermal drivers of aerobic metabolic flexibility in birds, we combined data on metabolic scope from 40 bird species sampled across a range of environments with several indices of local ambient temperature. Using phylogenetically‐informed analyses, we found that minimum winter temperature was the best predictor of variation in avian metabolic scope, outperforming all other thermal variables. Additionally, M_sum was a better predictor of latitudinal patterns of metabolic scope than BMR, with species inhabiting colder environments exhibiting increased M_sum over their counterparts in warmer environments. Taken together, these results suggest that cold temperature extremes drive latitudinal patterns of metabolic scope via selection for enhanced thermogenic performance in cold environments, supporting the cold adaptation hypothesis. Temperature extremes may therefore be an important selective pressure driving macrophysiological trends of aerobic performance in endotherms.     

Metabolic responses to low temperature in fish muscle

For most fish, body temperature is very close to that of the habitat. The diversity of thermal habitats exploited by fish as well as their capacity to adapt to thermal change makes them excellent organisms in which to examine the evolutionary and phenotypic responses to temperature. An extensive literature links cold temperatures with enhanced oxidative capacities in fish tissues, particularly skeletal muscle. Closer examination of inter-species comparisons (i.e. the evolutionary perspective) indicates that the proportion of muscle fibres occupied by mitochondria increases at low temperatures, most clearly in moderately active demersal species. Isolated muscle mitochondria show no compensation of protein-specific rates of substrate oxidation during evolutionary adaptation to cold temperatures. During phenotypic cold acclimation, mitochondrial volume density increases in oxidative muscle of some species (striped bass Morone saxatilis, crucian carp Carassius carassius), but remains stable in others (rainbow trout Oncorhynchus mykiss). A role for the mitochondrial reticulum in distributing oxygen through the complex architecture of skeletal muscle fibres may explain mitochondrial proliferation. In rainbow trout, compensatory increases in the protein-specific rates of mitochondrial substrate oxidation maintain constant capacities except at winter extremes. Changes in mitochondrial properties (membrane phospholipids, enzymatic complement and cristae densities) can enhance the oxidative capacity of muscle in the absence of changes in mitochondrial volume density. Changes in the unsaturation of membrane phospholipids are a direct response to temperature and occur in isolated cells. This fundamental response maintains the dynamic phase behaviour of the membrane and adjusts the rates of membrane processes. However, these adjustments may have deleterious consequences. For fish living at low temperatures, the increased polyunsaturation of mitochondrial membranes should raise rates of mitochondrial respiration which would in turn enhance the formation of reactive oxygen species (ROS), increase proton leak and favour peroxidation of these membranes. Minimisation of mitochondrial oxidative capacities in organisms living at low temperatures would reduce such damage.