Cold resistance of the brain during hibernation: I. K+ transport in cerebral cortex slices

The brains of the hibernating hamsters and 13-lined ground squirrels maintain Na⁺ and K⁺ at the same concentrations as in the awake state. The ability of slices of the cerebral cortex when incubated in vitro to accumulate or retain K⁺ is similar in the awake hamster and rat at both 38 and 5 ° C. On the other hand, slices of cerebral cortex from the hibernating hamster retained slightly more K⁺ at 5 °C than did those of awake hamster or rat. It was concluded that the cerebral cortex of the awake hamster is probably not cold resistant with respect to the maintenance of cation balance. Further, the cold resistance that exists in the cerebral cortex of the hibernating hamster is largely destroyed when the brain is disrupted by slicing.     

Comparative temperature dependence of (Na+ + K+)-ATPase.

The temperature dependence of (Na+ + K+)-ATPase was measured, utilizing preparations of enzyme from heat and kidney of rats, hamsters, guinea pigs, ground squirrels, turtles, chickens, and ducks. The two hibernating species, hamsters and ground squirrels, were studied awake at normothermia and hibernating at 4 degrees C. The results for every species except the turtles showed the same temperature dependence established for (Na++K+)-ATPase from rabbit kidney with a quasi-linear dependence above 15 degrees C and little or no activity below 15 degrees C. Turtle enzymes showed a broad activity versus temperature curve with a fall-off at high and low temperatures. The data in all cases, including the turtle data, may be fitted by a previously described thermodynamic kinetic model. Further, the model will fith the turnover or decrease in enzyme activity at higher temperatures observed in a number of cases. These results do not support the widely imputed ion pumping role for (Na++K+)-ATPase.     

Resistance of Erythrocytes of Hibernating Mammals to Loss of Potassium during Hibernation and during Cold Storage

In two species of hibernators, hamsters and ground squirrels, erythrocytes were collected by heart puncture and the K content of the cells of hibernating individuals was compared with that of awake individuals. The K concentration of hamsters did not decline significantly during each bout of hibernation (maximum period of 5 days) but in long-term bouts in ground squirrels (i.e. more than 5 days) the K concentration of cells dropped significantly. When ground squirrels were allowed to rewarm the K content of cells rose toward normal values within a few hours. Erythrocytes of both hamsters and ground squirrels lose K more slowly than those of guinea pigs (nonhibernators) when stored in vitro for up to 10 days at 5°C. In ground squirrels the rate of loss of K during storage is the same as in vivo during hibernation, and stored cells taken from hibernating ground squirrels also lose K at the same rate. The rate of loss of K from guinea pig cells corresponded with that predicted from passive diffusion unopposed by transport. The actual rate of loss of K from ground squirrel cells was slower than such a predicted rate but corresponded with it when glucose was omitted from the storage medium or ouabain was added to it. Despite the slight loss of K that may occur in hibernation, therefore, the cells of hibernators are more cold adapted than those of a nonhibernating mammal, and this adaptation depends in part upon active transport.     

Cultured cells from renal cortex of hibernators and nonhibernators. Regulation of cell K+ at low temperature

Cells were grown as primary monolayer cultures from kidney cortex of guinea pigs (nonhibernators), hamsters and ground squirrels (both hibernating species). When plates of cells were placed at 5 °C, cells of guinea pigs lost 37% of their K⁺ in 2 h and those of the hibernator lost about 10%.Uptake of ⁴²K into the cells exhibited a simple, single exponential time course at both temperatures. Unidirectional efflux of K⁺ was equal to K⁺ influx in all cultures at 37 °C and, within limits of error, in hibernator cells at 5 °C. Efflux was 3- to 5-fold greater than influx in guinea pig cells at 5 °C.After 2 h in the cold the ouabain-sensitive K⁺ influx remaining (7–15% of that at 37 °C) was about the same in the cells of the 3 species. Cells from active hamsters and from hibernating ground squirrels, however, exhibited significantly greater pump activity after 45 min in the cold (19 and 14%, respectively). The stimulation of K⁺ influx by increasing [K⁺]_o did not show an increase in K_m⁺ at 5 °C in cells of guinea pigs and ground squirrels. Lowering [K⁺]_c and/or raising [Na⁺]_c by treatment in low- and high-K⁺ media caused only slight stimulation of K⁺ influx, except in cells of ground squirrels at 5 °C in which the stimulation was at least 11-times greater than at 37 °C or in cells of guinea pigs at either temperature.This altered kinetic response of K⁺ transport to cytoplasmic ion stimulation with cooling accounted for about one-third of the improved regulation of K⁺ at 5 °C in ground squirrel cells; the other two-thirds was attributable to a greater decrease in K⁺ leak with cooling. The inhibition of active transport by cold in all 3 species was much less severe than that previously seen in any (Na⁺ + K⁺)-ATPase of mammalian cells.     

Regulation of glucokinase activity in liver of hibernating ground squirrel Spermophilus undulatus

The kinetic properties of glucokinase (GLK) from the liver of active and hibernating ground squirrels Spermophilus undulatus have been studied. Entrance of ground squirrels into hibernation from their active state is accompanied by a sharp decrease in blood glucose (Glc) level (from 14 to 2.9 mM) and with a significant (7-fold) decrease of GLK activity in the liver cytoplasm. Preparations of native GLK practically devoid of other molecular forms of hexokinase were obtained from the liver of active and hibernating ground squirrels. The dependence of GLK activity upon Glc concentration for the enzyme from active ground squirrel liver showed a pronounced sigmoid character (Hill coefficient, h = 1.70 and S _0.5 = 6.23 mM; the experiments were conducted at 25°C in the presence of enzyme stabilizers, K⁺ and DTT). The same dependence of enzyme activity on Glc concentration was found for GLK from rat liver. However, on decreasing the temperature to 2°C (simulation of hibernation conditions), this dependency became almost hyperbolic (h = 1.16) and GLK affinity for substrate was reduced (S _0.5 = 23 mM). These parameters for hibernating ground squirrels (body temperature 5°C) at 25°C were found to be practically equal to the corresponding values obtained for GLK from the liver of active animals (h = 1.60, S _0.5 = 9.0 mM, respectively); at 2°C sigmoid character was less expressed and affinity for Glc was drastically decreased (h = 1.20, S _0.5 = 45 mM). The calculations of GLK activity in the liver of hibernating ground squirrels based on enzyme kinetic characteristics and seasonal changes in blood Glc concentrations have shown that GLK activity in the liver of hibernating ground squirrels is decreased about 5500-fold.     

Regulation of the α-ketoglutarate dehydrogenasecomplex during hibernation in a small mammal,the Richardson's ground squirrel (Urocitellus richardsonii)

The citric acid cycle (CAC) is a central metabolic pathway that links carbohydrate, lipid, and amino acid metabolism in the mitochondria and, hence, is a crucial target for metabolic regulation. The α-ketoglutarate dehydrogenase complex (KGDC) is the rate-limiting step of the CAC, the three enzymes of the complex catalyzing the transformation of α-ketoglutarate to succinyl-CoA with the release of CO₂ and reduction of NAD to NADH. During hibernation, the metabolic rate of small mammals is suppressed, in part due to reduced body temperature but also active controls that suppress aerobic metabolism. The present study examined KGDC regulation during hibernation in skeletal muscle of the Richardson's ground squirrel (Urocitellus richardsonii). The KGDC was partially purified from skeletal muscle of euthermic and hibernating ground squirrels and kinetic properties were evaluated at 5°, 22°, and 37 °C. KGDC from hibernator muscle at all temperatures compared with euthermic controls exhibited a decreased affinity for CoA as well as reduced activation by Ca²⁺ ions at 5 °C from both euthermic and hibernating conditions. Co-immunoprecipitation was employed to isolate the E1, E2 and E3 enzymes of the complex (OGDH, DLST, DLD) to allow immunoblot analysis of post-translational modifications (PTMs) of each enzyme. The results showed elevated phospho-tyrosine content on all three enzymes during hibernation as well as increased ADP-ribosylation and succinylation of hibernator OGDH. Taken together these results show that the KGDC is regulated by posttranslational modifications and temperature effects to reorganize enzyme activity and mitochondrial function to aid suppression of mitochondrial activity during hibernation.     

Relation of Na-K-ATPase to acute changes in renal tubular sodium and potassium transport

Renal Na-K-ATPase activity changes adaptively in response to chronic alterations in sodium reabsorption or potassium secretion, but the role of this enzyme in rapid adjustments of renal tubular Na and K transport is not known. To evaluate this question, microsomal Na-K-ATPase specific activity and kinetics were determined in the rat and guinea pig kidney after massive but short-term (3 h) sodium or potassium loading. In other experiments renal sodium handling was evaluated in hydropenic and saline-loaded rats in which enzyme synthesis was prevented by the concurrent administration of actinomycin D or cycloheximide. Saline loading increased net sodium reabsorption in both rats and guinea pigs, but microsomal Na-K-ATPase from the outer medulla (where the reabsorptive increment is greatest) did not change significantly in either species. In vitro [3H]ouabain bidint to guinea pig microsomes and apparent Km for sodium of rat microsomal Na-K-ATPase, both from outer medulla, were also unaltered. Actinomycin D and cycloheximide failed to increase sodium excretion and microsomal Na-K-ATPase remained unchanged. KCL loading resulted in a 10-fold increase in K excretion but again Na-K-ATPase specific activity (in cortex, outer medulla, and papilla), and its apparent Km for potassium were not affected. Taken together these results suggest that rapid adjustments in remal tubular Na or K transport are mediated by mechanisms that do not involve the Na-K-ATPase enzyme system.     

Characteristics of ion transport in kidney cortex of mammalian hibernators

Slices of kidney cortex of two species of hibernating mammals (hamsters and ground squirrels) have been leached of K, and their subsequent ability to reaccumulate K in vitro has been determined at temperatures between 38° and 0°C. At 5°C (body temperature of a hibernating mammal) uptake is appreciable in kidney cortex of both species. In the kidney cortex of hamsters, for example, the tissue K of slices incubated at 5°C reaches the same steady-state concentration after 2 hours that is observed in slices at 38°C after 20 minutes. At 0°C there is also a measurable uptake. This K transport is blocked by metabolic inhibitors and, in ground squirrel kidneys, by ouabain. In kidney cortex slices from guinea pigs net K accumulation is slight at 5°C and absent at 0°C. The initial rapid uptake of K at 38°C occurs at the same rate in kidney cortex slices of hamsters as in those of rabbits. Lowering the temperature of incubation decreases this initial rate of uptake in hamster kidney slices with a Q ₁₀ of 1.8 between 38° and 15° and of 5.7 between 15° and 0°C. In hamsters this uptake of K has been shown to require the outward extrusion of Na. Conversely, about half of the outward extrusion of Na requires K in the medium, while the remainder appears to be independent of K. The conclusions warranted are that kidney cells of hibernators possess an unusual ability to transport ions at low temperature, that this ability does not depend upon a more rapid rate at higher temperatures, and that the characteristics of transport at low temperature are qualitatively similar to those at 38°C in cells of nonhibernators.     

高温胁迫下两种藓类植物过氧化物酶活性的变化

在不同的高温胁迫条件下 ,对湿地匍灯藓 (Plagiomnium acutum)和大羽藓 (Thuidium cymbifolium)过氧化物酶 (POD)活性及其与处理时间和处理温度的关系进行了初步研究。结果表明 ,在一定的温度范围内 ,随着温度的升高 ,POD活性增加 ,二者成线形关系。在一定温度条件下 ,一般随着处理时间的延长 ,POD活性增加。但是当超过一定的温度 (4 5～ 5 0°C)以及一定的处理时间 (4～ 6h) ,POD活性有所下降。结果还表明 ,湿地匍灯藓的POD活性显著高于大羽藓。而且在高温胁迫下 ,湿地匍灯藓 POD活性变化比大羽藓活跃 ,其变化幅度也比大羽藓大。     

Enhancement of the thermostability of a recombinant β-agarase, AgaB, from Zobellia galactanivorans by random mutagenesis

Random mutagenesis was performed on β-agarase, AgaB, from Zobellia galactanivorans to improve its catalytic activity and thermostability. The activities of three mutants E99K, T307I and E99K–T307I were approx. 140, 190 and 200%, respectively, of wild type β-agarase (661 U/mg) at 40°C. All three mutant enzymes were stable up to 50°C and E99K–T307I had the highest thermostability. The melting temperature (T _m) of E99K–T307I, determined by CD spectra, was increased by 5.2°C over that of the wild-type enzyme (54.6°C). Activities of both the wild-type and E99K–T307I enzymes, as well as their overall thermostabilities, increased in 1 mM CaCl₂. The E99K–T307I enzyme was stable at 55°C with 1 mM CaCl₂, reaching 260% of the activity the wild-type enzyme held at 40°C without CaCl₂.     

The Infestiveness of Heat and Gold Exposed Diphyllobothrium Latum Plerocercoids on Golden Hamster

The infestiveness of heat (56°C/5 min. and 50°C/5 min.), and cold (−10°C and −6°C) exposed Diphyllobothrium latum plerocercoids was studied by administration to golden hamsters (Mesocricetus auratus). The cold exposed larvae were subjected to an exposure, analogous the freezing of fish and which is cut off when the temperature has declined either to −6°C or −10°C. The plerocercoids were administered to the hamsters under inhalation anaesthesia, 5 larvae per hamster. Out of 150 normal control plerocercoids administered on 30 hamsters, 74 adult worms developed, indicating 49 % infestiveness. Tests carried out with 105 56°C/5 min. exposed larvae on 21 hamsters, 45 50°C/5 min. exposed larvae on 9 hamsters, and 100 larvae exposed to −10°C on 20 hamsters all gave negative results, while out of 50 −6°C exposed larvae on 10 hamsters 3 developed to adult worms. The results show that the 56°C/5 min. and −10°C exposed larvae, which in previous studies have been considered to be inactivated on the basis of the resultant permanent immobilization, were not able to develop further in the host. Thus permanent immobility seems to be a reliable criterion of larval death.     

cAMP-dependent protein kinase from brown adipose tissue: temperature effects on kinetic properties and enzyme role in hibernating ground squirrels

Arousal from hibernation requires thermogenesis in brown adipose tissue, a process that is stimulated by β-adrenergic signals, leading to a rise in intracellular 3′,5′-cyclic adenosine monophosphate AMP (cAMP) and activating cAMP-dependent protein kinase A (PKA) to phosphorylate a suite of target proteins and activate lipolysis and uncoupled respiration. To determine whether specific adaptations (perhaps temperature-dependent) facilitate PKA kinetic properties or protein-phosphorylating ability, the catalytic subunit of PKA (PKAc) from interscapular brown adipose of the ground squirrel Spermophilus richardsonii, was purified (final specific activity = 279 nmol phosphate transferred per min per mg protein) and characterized. Physical properties of PKAc included a molecular weight of 41 kDa and an isoelectric point of 7.8 ± 0.08. A change in assay temperature from a euthermic value (37 °C) to one typical of hibernating body temperature (5 °C) had numerous significant effects on ground squirrel PKAc including: (a) pH optimum rose from 6.8 at 37 °C to 8.7 at 5 °C, (b) K_m values at 37 °C for Mg.ATP (49.2±3.4 M) and for two phosphate acceptors, Kemptide (50.0±5.5 M) and Histone IIA (0.41 ± 0.05 mg/ml) decreased by 53%, 80% and 51%, respectively, at 5 °C, and (c) inhibition by KCl, NaCl and NH₄Cl was reduced. However, temperature change had little or no effect on K_m values of rabbit PKAc, suggesting a specific positive thermal modulation of the hibernator enzyme. Arrhenius plots also differed for the two enzymes; ground squirrel PKAc showed a break in the Arrhenius relationship at 9 °C and activation energies that were 29.1 ± 1.0 kJ/mol for temperatures >9 °C and 2.3-fold higher at 68.1 ± 2.1 kJ/mol for temperatures <9 °C, whereas the rabbit enzyme showed a breakpoint at 17 °C with a 13-fold higher activation energy over the lower temperature range. However, fluorescence analysis of PKAc in the absence of substrates, showed a linear change in fluorescence intensity and wavelength of maximal fluorescence over the entire temperature range; this suggested that the protein conformational change indicated by the break in the Arrhenius plot was substrate-related. Temperature change also affected the Hill coefficient for cAMP dissociation of the ground squirrel PKA holoenzyme which rose from 1.12 ± 0.18 at 37 °C to 2.19 ± 0.07 at 5 °C, making the release of catalytic subunits at low temperature much more responsive to small changes in cAMP levels. Analysis of PKAc function via in vitro incubations of extracts of ground squirrel brown adipose with ³²P-ATP + cAMP in the presence versus absence of a PKA inhibitor, also revealed major differences in the patterns of phosphoproteins, both between euthermic and hibernating animals as well as between 37 and 5 °C incubation temperatures; this suggests that there are both different targets of PKAc phosphorylation in the hibernating animal and that temperature affects the capacity of PKAc to phosphorylate different targets. Both of these observations, plus the species-specific and temperature-dependent changes in ground squirrel PKAc kinetic properties, suggest differential control of the enzyme in vivo at euthermic versus hibernating body temperatures in a manner that would facilitate a rapid and large activation of the enzyme during arousal from torpor. Accepted: 10 July 1998     

Temperature effects on evoked potentials of hippocampal slices from noncold-acclimated, cold-acclimated and hibernating hamsters

1. 1.|Neural activity was recorded in hippocampal slices from noncold-acclimated, cold-acclimated and hibernating hamsters.

2. 2.|Action potentials from a population of hippocampal pyramidal neurons were evoked by stimulating an afferent fiber tract, the Schaffer collaterals. The temperature of the artificial cerebrospinal fluid bathing the slice was varied by controlling the temperature of a water chamber jacketing the recording chamber.

3. 3.|The temperature just below that at which a population spike could be evoked, T_t, was 15.8 ± 0.9°C (mean ± SEM) for noncold-acclimated hamsters, 13.9 ± 0.3°C for cold-acclimated hamsters and 12.3 ± 0.3°C for hibernating hamsters.

4. 4.|These thresholds for evoked activity were significantly different in noncold-acclimated, cold-acclimated and hibernating hamsters, and may reflect acclimation of hippocampal neurons to cold.

The effects of day length, hibernation, and ambient temperature on incisor dentin in the Turkish hamster (Mesocricetus brandti)

Dentin is deposited on a circadian basis, and daily layers manifest as bands on the medial surfaces of rodent incisors. Hibernation alters dentin deposition, and a distinct hibernation mark has been described on incisor surfaces of several rodent species; the factors that influence the morphology of this mark are poorly understood. We tested the effects of day length, torpor expression, and ambient temperature on incisor surface morphology in Turkish hamsters housed in one of four conditions: long days (LDs) at 22 °C, short days (SDs) at 22 °C, SDs at 5 °C, and SDs at 13 °C. Body temperature was monitored continuously with implanted radio transmitters, and teeth examined postmortem. Teeth of SD hamsters had narrower, less distinct circadian increments than those of LD hamsters, but the width of ultradian increments was similar in both photoperiods. Hibernation at both 5 and 13 °C was associated in most specimens with very narrow, sharply defined dentin increments and increased tooth heterogeneity. Hamsters in SDs at 5 °C that did not hibernate lacked characteristic hibernation increments. At 5 °C, but not 13 °C, the number and cumulative width of hibernation increments were related to number and cumulative duration of periodic arousals. Our results suggest that incremental deposition of dentin in rodent incisors may be a useful trait for characterizing hibernation behavior in both evolutionary and historical contexts.     

Effect of osmolality on renal medullary Na-K-ATPase activity in the postobstructive kidney.

The aim of this study was to determine the effect of changes in osmolality on the reduced renal medullary Na-K-ATPase (EC 3.6.1.3) activity of the postobstructive kidney. The effect of osmolality on renal medullary Na-K-ATPase activity was studied by incubating tissue slices from sham-operated and bilaterally obstructed rats in media with osmolality varied before enzyme isolation using sodium chloride, choline chloride, or sucrose. Both sham-operated and bilaterally obstructed rat renal medullary enzyme showed a similar increase in activity with increased osmolality due to sodium chloride. Medullary Na-K-ATPase from the postobstructive kidney also showed increased activity with osmotic changes induced by choline chloride or sucrose. It is proposed that the decrease of Na-K-ATPase activity observed after bilateral ureteral obstruction is due, at least in part, to the loss of the solute concentration gradient in the kidney.     

T3 increases Na-K-ATPase activity via a MAPK/ERK1/2-dependent pathway in rat adult alveolar epithelial cells

Thyroid hormone (T3) increases Na-K-ATPase activity in rat adult alveolar type II cells via a PI3K-dependent pathway. In these cells, dopamine and beta-adrenergic agonists can stimulate Na-K-ATPase activity through either PI3K or MAPK pathways. We assessed the role of the MAPK pathway in the stimulation of Na-K-ATPase by T3. In the adult rat alveolar type II-like cell line MP48, T3 enhanced MAPK/ERK1/2 activity in a dose-dependent manner. Increased ERK1/2 phosphorylation was observed within 5 min, peaked at 20 min, and then decreased. Two MEK1/2 inhibitors, U0126 and PD-98059, each abolished the T3-induced increase in the quantity of Na-K-ATPase alpha(1)-subunit plasma membrane protein and Na-K-ATPase activity. T3 also increased the phosphorylation of MAPK/p38; however, SB-203580, a specific inhibitor of MAPK/p38 activity, did not prevent the T3-induced Na-K-ATPase activity. SP-600125, a specific inhibitor of the MAPK/JNK pathway, also did not block the T3-induced Na-K-ATPase activity. Phorbol 12-myristate 13-acetate (PMA) significantly increased ERK1/2 phosphorylation and Na-K-ATPase activity. The PMA-induced Na-K-ATPase activity was inhibited by U0126. These data indicate that activation of MAPK-ERK1/2 was required for the T3-induced increase in Na-K-ATPase activity in addition to the requirement for the PI3K pathway.     

New isolate of Streptomyces sp. with novel thermoalkalotolerant cellulases

A Streptomyces sp. was isolated that produced novel thermoalkalotolerant cellulase activity after growth on crystalline cellulose at 50°C. Three major components of the cellulases (CMCase, Avicelase and cellobiase) were produced with maximal activities (11.8, 7.8 and 3.9 IU/ml) and maximum specific activities 357, 276 and 118 IU/mg protein, respectively, after 120 h growth. Maximum CMCase activity was between 50 and 60°C measured over 3 h. The enzyme also retained 88% of its maximum activity at 70°C and pH 5, and 80% of the activity at pH 10 and 50°C when assayed after 1 h. After incubation at 40°C for 1 h with commercial detergent (Tide) at pH 11, 95% activity was retained. The enzyme mixture produced glucose from crystalline cellulose.     

Abnormal response of glycolipid synthesis to temperature in transformed BHK cells thermosensitive for growth control.

In DMN4B cells, a line of chemically mutagenized BHK hamster cells which exhibit transformed behavior at 38.5°C but not at 32°C, [¹⁴C]-palmitate incorporation into mono-, di-, and trihexosylceramides was unimpaired at 32°C when compared with incorporation rates in untransformed BHK cells. At 38.5°C, labeling of these glycolipids increased greatly in the BHK cells, but failed to increase comparably in the DMN4B cells. Assay of cell-free preparations of the galactosyltransferase which catalyzes trihexosylceramide synthesis revealed a stimulatory effect of increased temperature on activity of the BHK enzyme, but not the DMN4B enzyme. The results suggest that transformation can result from a mutation affecting glycolipid synthesis.     

Different mechanisms for the induction of acetylcholinesterase in neuroblastoma cells.

Neuroblastoma cells, incliding clones selected for resistance to dibutyryl-cAMP or for their ability to survive and multiply at 40°C, were used to study differences in the induction of acetylcholinesterase activity by dibutyryl-cAMP and 5-bromodeoxyuridine. In nonselected neuroblastoma cells both of these compounds induced this enzyme activity. Actinomycin D inhibited induction by 5-bromodeoxyuridine but did not inhibit inducion by dibutyryl-cAMP. Enzyme activity in dibutyryl-cAMP-resistant cells was induced by 5-bromodeoxyuridine and not by dibutyryl-cAMP. In the temperature-resistant cells, induction by 5-bromodeoxyuridine was lower at 40 than at 37°C and induction by dibutyryl-cAMP was higher at 40 than at 37°C. This difference in induction at the two temperatures was associated with a higher inhibition of cell multiplication at 40°C by both compounds. The results indicate that 5-bromodeoxyuridine and dibutyryl-cAMP induce acetylcholinesterase activity in neuroblastoma cells by different mechanisms.     

Hibernation patterns of Turkish hamsters: influence of sex and ambient temperature

Turkish hamsters (Mesocricetus brandti) are a model organism for studies of hibernation, yet a detailed account of their torpor characteristics has not been undertaken. This study employed continuous telemetric monitoring of body temperature (T _b) in hibernating male and female Turkish hamsters at ambient temperatures (T _as) of 5 and 13 °C to precisely characterize torpor bout depth, duration, and frequency, as well as rates of entry into and arousal from torpor. Hamsters generated brief intervals of short (<12 h), shallow test bouts (T _b > 20 °C), followed by deep torpor bouts lasting 4–6 days at T _a = 5 °C and 2–3 days at T _a = 13 °C. Females at T _a = 5 °C had longer bouts than males, but maintained higher torpor T _b; there were no sex differences at T _a = 13 °C. Neither body mass loss nor food intake differed between the two T _as. Hamsters entered torpor primarily during the scotophase (subjective night), but timing of arousals was highly variable. Hamsters at both T _as generated short, shallow torpor bouts between deep bouts, suggesting that this species may be capable of both hibernation and daily torpor.