Analysis of changes in heart rate and temperature of the ground squirrel Citellus undulatus in various physiological states

Heart rate (HR) of ground squirrel C. undulatus was studied in dependence of season, level of activity, physiological state and air temperature (T). In summer HR varies from 110-130 beat/min in sleep up to 420 beat/min at flight from danger. During winter hibernation HP was minimal (3-5 beat/min) at T 1-4 degrees C, the increase in T induced the growth of HP in correspondence to the Arrenius van't Hoff law. The temperature of the body in hibernation exceeded T on 1.5-3 degrees C. The time of getting off the hibernation increased with the decrease in T (6-7 hours at -1 degree C and 2.5-3 hours at +18 degrees C). At phase of increased thermogenesis during arousal heart temperature exceeded rectal one on 10-12 degrees C and heart rate run up to 360-420 beat/min i.e. 2-3 time higher than in active state. The decrease in T stimulated the increase in HP up to 3.8 in winter and 5.3 beat/min/degree C in summer. The highest values of Q10 for HP were revealed at the beginning of hibernation (15-20) and at the beginning of arousal (6-7), in other periods Q10 was similar to the normal biological values (2-2.5). Thus, at the beginning of transitional periods changes of HP were determined mainly by endogenic mechanisms that inhibited myocardium at the beginning of hibernation and activated in arousal. Some mechanisms of coordination between activities of heart and other systems of organism are considered. The features of hit exchange providing the hibernation in wide range of T are discussed.     

Comparison of surface temperature in 13-lined ground squirrel (Spermophilus tridecimlineatus) and yellow-bellied marmot (Marmota flaviventris) during arousal from hibernation

Surface temperatures (Ts) of eight 13-lined ground squirrels and seven yellow-bellied marmots were measured during arousal from hibernation using infrared thermography (IRT) and recorded on videotape. Animals aroused normally in 5 degrees C cold rooms. Body temperatures were recorded during arousal using both cheek pouch and interscapular temperature probes. Warming rate in arousal was exponential. Mean mass specific warming rates show the squirrels warm faster (69.76 degrees C/h/kg) than the marmots (4.49 degrees C/h/kg). Surface temperatures (Ts) for 11 regions were measured every few minutes during arousal. The smaller ground squirrel shows the ability to perfuse distal regions without compromising rise in deep body temperature (Tb). All squirrel Ts's remained low as Tb rose to 18 degrees C, at which point, eyes opened, squirrels became more active and all Ts's rose parallel to Tb. Marmot Ts remained low as Tb rose initially. Each marmot showed a plateau phase where Tb remained constant (mean Tb 20.3+/-1.0 degrees C, duration 9.4+/-4.1 min) during which time all Ts's rose, and then remained relatively constant as Tb again began to rise. An anterior to posterior Ts gradient was evident in the ground squirrel, both body and feet. This gradient was only evident in the feet of the marmots.     

The hibernating 13-lined ground squirrel as a model organism for potential cold storage of platelets

Hibernating mammals have developed many physiological adaptations to extreme environments. During hibernation, 13-lined ground squirrels (Ictidomys tridecemlineatus) must suppress hemostasis to survive prolonged body temperatures of 4-8°C and 3-5 heartbeats per minute without forming lethal clots. Upon arousal in the spring, these ground squirrels must be able to quickly restore normal clotting activity to avoid bleeding. Here we show that ground squirrel platelets stored in vivo at 4-8°C were released back into the blood within 2 h of arousal in the spring with a body temperature of 37°C but were not rapidly cleared from circulation. These released platelets were capable of forming stable clots and remained in circulation for at least 2 days before newly synthesized platelets were detected. Transfusion of autologous platelets stored at 4°C or 37°C showed the same clearance rates in ground squirrels, whereas rat platelets stored in the cold had a 140-fold increase in clearance rate. Our results demonstrate that ground squirrel platelets appear to be resistant to the platelet cold storage lesions observed in other mammals, allowing prolonged storage in cold stasis and preventing rapid clearance upon spring arousal. Elucidating these adaptations could lead to the development of methods to store human platelets in the cold, extending their shelf life.     

Analysis of the brain bioelectrical activity of the suslik Citellus parryi during hibernation

Computer analysis has been made of the diurnal rhythm of the brain activity in the ground squirrel Citellus parryi during hibernation (body temperature 7-8 degrees C). The data obtained are presented in the form of graphical plots with two-dimensional density of probability of distribution of intermittent bursts of spindles and isoelectric EEG.     

Periodic arousal from hibernation is necessary for initiation of immune responses in ground squirrels

Golden-mantled ground squirrels (Spermophilus lateralis) undergo seasonal hibernation during which core body temperature (T(b)) values are maintained 1-2 degrees C above ambient temperature. Hibernation is not continuous. Squirrels arouse at approximately 7-day intervals, during which T(b) increases to 37 degrees C for approximately 16 h; thereafter, they return to hibernation and sustain low T(b)s until the next arousal. Over the course of the hibernation season, arousals consume 60-80% of a squirrel's winter energy budget, but their functional significance is unknown and disputed. Host-defense mechanisms appear to be downregulated during the hibernation season and preclude normal immune responses. These experiments assessed immune function during hibernation and subsequent periodic arousals. The acute-phase response to bacterial lipopolysaccharide (LPS) was arrested during hibernation and fully restored on arousal to normothermia. LPS injection (ip) resulted in a 1-1.5 degrees C fever in normothermic animals that was sustained for > 8 h. LPS was without effect in hibernating squirrels, neither inducing fever nor provoking arousal, but a fever did develop several days later, when squirrels next aroused from hibernation; the duration of this arousal was increased sixfold above baseline values. Intracerebroventricular infusions of prostaglandin E(2) provoked arousal from hibernation and induced fever, suggesting that neural signaling pathways that mediate febrile responses are functional during hibernation. Periodic arousals may activate a dormant immune system, which can then combat pathogens that may have been introduced immediately before or during hibernation.     

Effects of changing ambient temperature on metabolic, heart, and ventilation rates during steady state hibernation in golden-mantled ground squirrels (Spermophilus lateralis)

To determine whether metabolic rate is suppressed in a temperature-independent fashion in the golden-mantled ground squirrel during steady state hibernation, we measured body temperature and metabolic rate in ground squirrels during hibernation at different T(a)'s. In addition, we attempted to determine whether heart rate, ventilation rate, and breathing patterns changed as a function of body temperature or metabolic rate. We found that metabolic rate changed with T(a) as it was raised from 5 degrees to 14 degrees C, which supports the theory that different species sustain falls in metabolic rate during hibernation in different ways. Heart rate and breathing pattern also changed with changing T(a), while breathing frequency did not. That the total breathing frequency did not correlate closely with oxygen consumption or body temperature, while the breathing pattern did, raises important questions regarding the mechanisms controlling ventilation during hibernation.     

The role of succinate dehydrogenase and oxaloacetate in metabolic suppression during hibernation and arousal

Hibernation elicits a major reduction in whole-animal O₂ consumption that corresponds with active suppression of liver mitochondrial electron transport capacity at, or downstream of, succinate dehydrogenase (SDH). During arousal from the torpor phase of hibernation this suppression is reversed and metabolic rates rise dramatically. In this study, we used the 13-lined ground squirrel (Ictidomys tridecemlineatus) to assess isolated liver mitochondrial respiration during the torpor phase of hibernation and various stages of arousal to elucidate a potential role of SDH in metabolic suppression. State 3 and state 4 respiration rates were seven- and threefold lower in torpor compared with the summer-active and interbout euthermic states. Respiration rates increased during arousal so that when body temperature reached 30°C in late arousal, state 3 and state 4 respiration were 3.3- and 1.8-fold greater than during torpor, respectively. SDH activity was 72% higher in interbout euthermia than in torpor. Pre-incubating with isocitrate [to alleviate oxaloacetate (OAA) inhibition] increased state 3 respiration rate during torpor by 91%, but this rate was still fourfold lower than that measured in interbout euthermia. Isocitrate pre-incubation also eliminated differences in SDH activity among hibernation bout stages. OAA concentration correlated negatively with both respiration rates and SDH activity. These data suggest that OAA reversibly inhibits SDH in torpor, but cannot fully account for the drastic metabolic suppression observed during this hibernation phase.     

The functional brain state of hibernators and nonhibernators at different animals temperatures

Literature and our own data on structural and functional state of neocortex and hippocampus during both entrance in hibernation of ground squirrel (Spermophilus undulates) and Wistar rats in hypothermia were generalized. During hibernation when body temperature is about 2-4 degrees C the suppression of both bioelectrical and protein-synthesizing activity, the decrease of neuronal cell bodies and the branching of dendrites, retraction of dendritic spines, and a decrease of postsynaptic active zones of synapses were observed. Similar changes in those parameters were triggered for rats during hypoxia-hypercapnia at body temperature 17-19 degrees C. Hypoxia-hypercapnia facilitates the entrance in torpid state for hole animals. Nonhibernating animals during cooling and hypoxia-hypercapnia trigger functioning some mechanisms similar hibernators during entrance in hibernation. Similar morphological and functional changes for both hibernators and nonhibernators at low temperature state show similarity of mechanisms which induce a low level of brain activity of different animals.     

The Dynamics of Adaptive Changes in the Spleen of the Hibernating Ground Squirrel <Emphasis Type="Italic">Spermophilus undulatus</Emphasis>

In hibernation season during torpor bouts, the spleen weight and the hemoglobin level, as well as the total and extracted protein contents in the spleen of the ground squirrel Spermophilus undulatus are increased when animals enter torpor and reach maximum values when the body temperature drops below 25°C. All these parameters return to the characteristic values of the euthermic animals during arousal, before the body temperature increases to 20°C. There were no significant differences in the numbers of splenocytes between ground squirrels in interbout euthermia and torpor. The minimum number of splenocytes was observed in animals that entered torpor when the core body temperature was approximately 18°C. The activity of ornithine decarboxylase, a key enzyme in polyamine synthesis, which is correlated with the functional and proliferative status of lymphoid tissue, was the same for the euthermic and summer ground squirrels and decreased monotonically during torpor. Upon arousal of the animals when body temperature was below 29°C, no resumption of the spleen ornithine decarboxylase activity was observed.     

Comparisons of the effects of temperature on the liver fatty acid binding proteins from hibernator and nonhibernator mammals.

Hibernating mammals rely heavily on lipid metabolism to supply energy during hibernation. We wondered if the fatty acid binding protein from a hibernator responded to temperature differently than that from a nonhibernator. We found that the Kd for oleate of the liver fatty acid binding protein (1.5 microM) isolated from ground squirrel (Spermophilus richardsonii) was temperature insensitive over 5-37 degrees C, while the rat liver fatty acid binding protein was affected with the Kd at 37 degrees C being about half (0.8 microM) that found at lower temperatures. This same trend was observed when comparing the specificity of various fatty acids of differing chain length and degree of unsaturation for the two proteins at 5 and 37 degrees C. At the lower temperature, ground squirrel protein bound long-chain unsaturated fatty acids, particularly linoleate and linolenate, at least as well as at the higher temperature and matched requirements for these fatty acids in the diet. The most common long-chain fatty acid, palmitate, was a more effective ligand for ground squirrel liver fatty acid binding protein at 5 degrees C than at 37 degrees C, with the opposite occurring in the eutherm. Rat protein was clearly not adapted to function optimally at temperatures lower than the animal's body temperature.     

Changes of characteristics of preoptic neurons and NA metabolism in hypothalamus of ground squirrel (Citelleus Dautieus) in different seasons and hibernating phases

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The dependence of the rate of heart contractions in the long-tailed Yakutsk suslik Citellus undulatus on environmental temperature]

Decrease of ambient temperature (Ta) leads to the increase of the heart rate (HR) in active ground squirrels C. undulatus by 5.3/min/1 degree C in summer and by 3.8/min/1 degree C in winter. In a hibernation state, the dependence of the HR on Ta was in a good agreement with equation HT = 2.53.exp.(0.1.Ta). On entering into hibernation and on arousal, the HR change outruns the corresponding body temperature (Tb) change by 1.5-2 hours. A maximum HR level (up to 400/min and more) was registered on arousal when Tb reached 17-20 degrees C. A minimal HR level (4-5/min) was observed during hibernation at Ta 2-5 degrees C. The maximum Ta, at witch C. undulatus was hibernating, reached 23-24 degrees C, the HR being 23-25/min.     

Effect of insulin on the myocardium of the active, hibernating and wakening ground squirrel Citellus undulatus

The effect of insulin (0.1-100 nM) on isometric force of contraction in isolated ground squirrel papillary muscle was investigated. In summer, autumn and winter active animals, insulin had a negative inotropic effect on papillary muscles, decreasing the amplitude of contraction by about 30% of the control value. In some cases, predominantly in the summer group of animals, insulin produced different effects on contractility: low doses (0.1-0.5 nM) caused a transient activation of isometric contraction by about 10-15% of control, whereas high doses produced a negative inotropic effect by about 30% of the control level. During deep hibernation (at 5-6 degrees C of heart temperature) and during arousal from hibernation (from 3 to 20 degrees C), insulin had no significant effect on contractility. Opposite inotropic effects of insulin at concentrations of 0.1-50 nM were found during arousal: from 26 to 31 degrees C of heart temperature--a positive inotropic effect by about 20-25% of control, and from 32 to 36 degrees C--a negative one by about 30-40% of the control value.     

Electrophysiological characteristics of cardiac function and the intensity of protein synthesis in cardiomyocytes during the arousal of susliks from hibernation

It has been demonstrated that during winter hibernation (body temperature 2-4 degrees C), the heart rate in ground squirrels is equal to 100 10-12 beats/min. At the initial stage of the arousal, while body temperature remains still low (9-10 degrees C), the heart rate may increase up to 160-200 beats/min. At this stage, practically all electrophysiological parameters of the heart correspond to those in active animals. These results may indicate the ability of "cold" heart in arousing ground squirrels to operate as a normothermic organ and reveal certain role of the heart in body warming. Significant increase of the intensity of protein synthesis in cardiomyocytes together with periodic changes in protein composition of their membranes were found during arousal which may account for regulation of the level of metabolism in cells and for adaptation of the latter to different temperatures.     

Changes of characteristics of preoptic neurons and NA metabolism in hypothalamus of ground squirrel (Citelleus Dautieus) in different seasons and hibernating phases

The unit firing activities of neurons in the preoptic area (POA) of ground squirrel hypothalamic tissue slices were recorded and the metabolism of NA in hypothalamus was measured with high performance liquid chromatography (HPLC). Thermosensitivity, proportions, the critical temperature (Tc) and the lowest temperature (TL) of firing activity of the above-mentioned neurons, and NA metabolism in hypothalamus were compared in different seasons and hibernating phases. In comparison with that in summer euthermar, it was shown that (i) the percentage and thermosensitivity of the POA neurons varied respectively in the hibernating phases; (ii) TL and Tc of the POA neurons in winter, both euthermar and hibernation, were markedly decreased; (iii) the POA neurons in hibernation became much more sensitive to NA, and the response of cold-sensitive neurons to NA changed from inhibiting pattern in summer to exciting one in hibernation; (iv) the contents and metabolism of NA in hypothalamus decreased significantly in the entering phase and deep hibernation phase, while the metabolism of NA increased remarkably in the arousal phase. These changes might explain the regulatory mechanism how ground squirrel actively decreases body temperature (Tb) in entering into hibernation and quickly recovers body temperature in arousal phase. Project supported by the National Natural Science Foundation of China (Grant Nos. 39230060 and 39570100)     

Translational initiation is uncoupled from elongation at 18 degrees C during mammalian hibernation

Cellular and organismal homeostasis must be maintained across a body temperature (Tb) range of 0 to 37 degrees C during mammalian hibernation. Hibernators depress biosynthetic activities including protein synthesis, concordant with limited energy availability and temperature effects on reaction rates. We used polysome analysis to show that initiation of protein synthesis ceases during entrance into torpor in golden-mantled ground squirrels (Spermophilus lateralis) when Tb reaches 18 degrees C. Elongation of preinitiated polypeptides continues slowly throughout the torpor bout. As Tb begins to rise, initiation resumes even at temperatures below 18 degrees C, although the euthermic polysome pattern is not reestablished. At precisely 18 degrees C, there is a large increase in initiation events and a complete restoration of euthermic polysome distribution patterns. These data indicate a role for both passive and active depression of translation during torpor and are consistent with a requirement for new protein biosynthesis during each interbout arousal.     

Increased heat loss affects hibernation in golden-mantled ground squirrels

During hibernation at ambient temperatures (T(a)) above 0 degrees C, rodents typically maintain body temperature (T(b)) approximately 1 degrees C above T(a), reduce metabolic rate, and suspend or substantially reduce many physiological functions. We tested the extent to which the presence of an insulative pelage affects hibernation. T(b) was recorded telemetrically in golden-mantled ground squirrels (Spermophilus lateralis) housed at a T(a) of 5 degrees C; food intake and body mass were measured at regular intervals throughout the hibernation season and after the terminal arousal. Animals were subjected to complete removal of the dorsal fur or a control procedure after they had been in hibernation for 3-4 wk. Shaved squirrels continued to hibernate with little or no change in minimum T(b), bout duration, duration of periodic normothermic bouts, and food intake during normothermia. Rates of rewarming from torpor were, however, significantly slower in shaved squirrels, and rates of body mass loss were significantly higher, indicating increased depletion of white adipose energy stores. An insulative pelage evidently conserves energy over the course of the hibernation season by decreasing body heat loss and reducing energy expenditure during periodic arousals from torpor and subsequent intervals of normothermia. This prolongs the hibernation season by several weeks, thereby eliminating the debilitating consequences associated with premature emergence from hibernation.     

Possible mechanism of the reorganization of the cardiomyocyte plasma membrane in the suslik in the hibernation-arousal-wakefulness cycle

Studies have been made on the ultrastructure of cardiomyocytes during hibernation and arousal of the ground squirrel C. undulatus. It was found that the number of elements of the rough endoplasmic reticulum, Golgi complex, vesicles and ribosomes increases in the perinuclear areas of cardiomyocytes during arousal of animals. These areas are saturated with lipid inclusions and mitochondria. Numerous vesicles and fringed bubbles were found near the plasma membrane which has many caveolae. These findings may indicate the intense metabolism of the membrane material between plasmalemma and cytoplasmic vesicles. Possible mode of rapid reorganization of the sarcolemma and changes in its functional properties during hibernation-arousal stages are suggested. It is concluded that apart from structural and functional properties which are acquired by cells during preparation of animals to hibernation and which exhibit only small changes during the whole period of hibernation, cyclic changes in plasmalemma structure and function occur during arousal of the ground squirrels.     

The effect of insulin on the heart rate and temperature of the ground squirrel <Emphasis Type="Italic">Spermofilus undulatus</Emphasis> during arousal from hibernation

The effect of insulin on the heart rate and body temperature, measured per rectum, of ground squirrels (Spermophilus undulatus) during triggered arousal from winter hibernation was studied. We found that the outcomes of insulin injection to hibernating ground squirrels varied in the course of arousal. During the first stage, while body temperatures were less than 10°C, the heart rates and rectal temperatures in both control and insulin-treated groups changed in the same manner. During the next stage of arousal, when the body temperature rose above 12°C, elevation of the heart rate and rectal temperature in the insulin-treated animals was significantly retarded and lasted 110 min compared to 80 min in the control group. Conversely, in the final stage of arousal at body temperatures above 20°C, the heart rate and body temperature increased more rapidly in the insulin-treated animals that reached normal body temperature within 40 min compared to 60 min in the control group. Suggested mechanisms of bidirectional effects of insulin on the heart rates and body temperatures in ground squirrels at the particular stages of arousal, with regard to the progression of endogenous insulin and glucose levels in the blood serum, are discussed.