Shivering in the cold: from mechanisms of fuel selection to survival

Fatty acid binding protein 4 and 5 play a crucial role in thermogenesis under the conditions of fasting and cold stress

Hypothermia is rapidly induced during cold exposure when thermoregulatory mechanisms, including fatty acid (FA) utilization, are disturbed. FA binding protein 4 (FABP4) and FABP5, which are abundantly expressed in adipose tissues and macrophages, have been identified as key molecules in the pathogenesis of overnutrition-related diseases, such as insulin resistance and atherosclerosis. We have recently shown that FABP4/5 are prominently expressed in capillary endothelial cells in the heart and skeletal muscle and play a crucial role in FA utilization in these tissues. However, the role of FABP4/5 in thermogenesis remains to be determined. In this study, we showed that thermogenesis is severely impaired in mice lacking both FABP4 and FABP5 (DKO mice), as manifested shortly after cold exposure during fasting. In DKO mice, the storage of both triacylglycerol in brown adipose tissue (BAT) and glycogen in skeletal muscle (SkM) was nearly depleted after fasting, and a biodistribution analysis using 125I-BMIPP revealed that non-esterified FAs (NEFAs) are not efficiently taken up by BAT despite the robustly elevated levels of serum NEFAs. In addition to the severe hypoglycemia observed in DKO mice during fasting, cold exposure did not induce the uptake of glucose analogue 18F-FDG by BAT. These findings strongly suggest that DKO mice exhibit pronounced hypothermia after fasting due to the depletion of energy storage in BAT and SkM and the reduced supply of energy substrates to these tissues. In conclusion, FABP4/5 play an indispensable role in thermogenesis in BAT and SkM. Our study underscores the importance of FABP4/5 for overcoming life-threatening environments, such as cold and starvation.

Mice lacking G0S2 are lean and cold-tolerant

G 0/G 1 switch gene 2 (G0S2) is a protein that was first identified in a search for lymphocyte G 0/G 1 switch genes. A direct role for G0S2 in cell cycle regulation has proven elusive. Yet, there is prior evidence for G0S2 functioning in tumor suppression, immune regulation and lipolysis. To explore definitively G0S2 functions, mice lacking G0S2 were generated and characterized. G0S2(-/-) mice were born at a Mendelian ratio and were phenotypically normal, with the exception of a possible lactation defect. G0S2(-/-) female mice carried viable pups to term, but could not typically sustain them beyond 48 h. G0S2 is shown here to be most highly expressed in adipose tissue. It is also expressed in liver, skeletal muscle, lung, ventricles of the heart, and components of the kidney. G0S2 loss significantly decreased relative body weight gain as compared with wild-type (WT) (G0S2(+/+)) mice, with a significant decrease in gonadal fat pad weight and a significant increase in serum glycerol levels. This decreased relative body weight gain is not associated with a significant decrease in food intake or increase in activity of G0S2(-/-) mice. In fact, G0S2(-/-) mice were significantly less active at night than G0S2(+/+) mice. When fed with a high fat diet (45% fat diet), G0S2 loss did not prevent diet-induced obesity in mice. Intriguingly, G0S2 loss improved acute cold tolerance, augmenting expression of genes involved in thermogenesis. In summary, in vivo roles for G0S2 were found in lactation, energy balance, and thermogenesis. This study provides a basis for tumor suppressive effects of G0S2 by regulating lipolysis.

Fuel selection during short-term submaximal treadmill exercise in the cold is not affected by pre-exercise low-intensity shivering

Exercise and shivering rely on different metabolic pathways and consequently, fuel selection. The present study examined the effects of a pre-exercise low-intensity shivering protocol on fuel selection during submaximal exercise in a cold environment. Nine male subjects exercised 4 times for 60 min at 50% (LOW) or 70% (MOD) of their peak oxygen consumption on a motorized treadmill in a climatic chamber set at 0 °C with (SHIV) and without (CON) a pre-exercise cooling protocol, inducing low-intensity shivering. Thermal, cardiorespiratory and metabolic responses were measured every 15 min whereas blood samples were collected every 30 min to assess serum nonesterified fatty acids (NEFA), glycerol, glucose, β-hydroxybutyrate (BHB) and plasma catecholamine concentrations. Rectal and skin temperatures were lower in the SHIV condition, within LOW and MOD conditions, during the first 45 min of exercise. Norepinephrine (NE) concentration was greater in SHIV vs. CON within LOW (1.39 ± 0.17 vs. 0.98 ± 0.17 ng·mL(-1)) and MOD (1.50 ± 0.20 vs. 1.01 ± 0.09 ng·mL(-1)), whereas NEFA, glycerol and BHB were greater in SHIV vs. CON (1060 ± 49 vs. 898 ± 78 μmol·L(-1); 0.27 ± 0.02 vs. 0.22 ± 0.03 mmol·L(-1); 0.39 ± 0.06 vs. 0.27 ± 0.04 mmol·L(-1), respectively) within MOD only. No changes were observed in fat or carbohydrate oxidation between SHIV and CON during exercise. Despite increases in NE, NEFA, glycerol and BHB from pre-exercise low-intensity shivering, fuel selection during short-term submaximal exercise in the cold was unaltered.

Effect of ambient temperature during acute aerobic exercise on short-term appetite, energy intake, and plasma acylated ghrelin in recreationally active males

Ambient temperature during exercise may affect energy intake regulation. Compared with a temperate (20 °C) environment, 1 h of running followed by 6 h of rest tended to decrease energy intake from 2 ad libitum meals in a hot (30 °C) environment but increase energy intake in a cool (10 °C) environment (p = 0.08). Core temperature changes did not appear to mediate this trend; whether acylated ghrelin is involved is unclear. Further research is warranted to clarify these findings.

Whole body oxygen uptake and evoked knee torque in response to low frequency electrical stimulation of the quadriceps muscles: V•O2 frequency response to NMES

Background

There is emerging evidence that isometric Neuromuscular Electrical Stimulation (NMES) may offer a way to elicit therapeutically significant increases in whole-body oxygen uptake in order to deliver aerobic exercise to patients unable to exercise volitionally, with consequent gains in cardiovascular health. The optimal stimulation frequency to elicit a significant and sustained pulmonary oxygen uptake has not been determined. The aim of this study was to examine the frequency response of the oxygen uptake and evoked torque due to NMES of the quadriceps muscles across a range of low frequencies spanning the twitch to tetanus transition.

Methods

Ten healthy male subjects underwent bilateral NMES of the quadriceps muscles comprising eight 4 minute bouts of intermittent stimulation at selected frequencies in the range 1 to 12 Hz, interspersed with 4 minutes rest periods. Respiratory gases and knee extensor torque were simultaneously monitored throughout. Multiple linear regression was used to fit the resulting data to an energetic model which expressed the energy rate in terms of the pulse frequency, the torque time integral and a factor representing the accumulated force developed per unit time.

Results

Additional oxygen uptake increased over the frequency range to a maximum of 564 (SD 114) ml min^-1 at 12 Hz, and the respiratory exchange ratio was close to unity from 4 to 12 Hz. While the highest induced torque occurred at 12 Hz, the peak of the force development factor occurred at 6 Hz. The regression model accounted for 88% of the variability in the observed energetic response.

Conclusions

Taking into account the requirement to avoid prolonged tetanic contractions and to minimize evoked torque, the results suggest that the ideal frequency for sustainable aerobic exercise is 4 to 5 Hz, which coincided in this study with the frequency above which significant twitch force summation occurred.

Effects of ambient temperature on mechanomyography of resting quadriceps muscle

It has been speculated that resting muscle mechanical activity, also known as minor tremor, microvibration, and thermoregulatory tonus, has evolved to maintain core temperature in homeotherms, and may play a role in nonshivering thermogenesis. This experiment was done to determine whether resting muscle mechanical activity increases with decreasing ambient temperature. We cooled 20 healthy, human, resting, supine subjects from an ambient temperature of 40° to 12 °C over 65 min. Core temperature, midquadriceps mechanomyography, surface electromyography, and oxygen consumption ([Formula: see text]O2) were recorded. Resting muscle mechanical and electrical activity in the absence of shivering increased significantly at temperatures below 21.5 °C. Women defended core temperature more effectively than men, and showed increased resting muscle activity earlier than men. Metabolism measured by [Formula: see text]O2 correlated with resting muscle mechanical activity (R = 0.65; p = 0.01). Resting muscle mechanical activity may have evolved, in part, to maintain core temperature in the face of mild cooling.

Effects of green tea extracts on non-shivering thermogenesis during mild cold exposure in young men

The effects of epigallocatechin-3-gallate (EGCG) and caffeine on non-shivering thermogenesis (NST) during cold exposure is unknown. The purpose of the present study was to quantify the effects of co-ingesting EGCG and caffeine on the thermogenic responses of a 3 h cold exposure. A total of eight healthy males were exposed to mild cold, using a liquid-conditioned suit perfused with 158C water, on two occasions and consumed a placebo or an extract of 1600 mg of EGCG and 600 mg of caffeine (Green tea). Thermic, metabolic and electromyographic measurements were monitored at baseline and during the cold exposure. Results showed that the AUC of shivering intensity over the cold exposure period was reduced by approximately 20% in the Green tea (266 (SEM 6)% maximal voluntary contraction (MVC) x min) compared with the Placebo (332 (SEM 69)%MVC x min) (P=0·01) treatments. In contrast, the total AUC for energy expenditure (EE) was approximately 10% higher in the Green tea (23·5 (SEM 1·4) kJ/kg x 180 min) compared with the Placebo (327 (SEM 74) kJ/kg 180 min) (P=0·007) treatments. The decrease in shivering activity combined with an increase in EE, following the ingestion of EGCG and caffeine during the cold exposure, indicates that NST pathways can be significantly stimulated in adult human subjects. The present study provides an experimental approach for human investigations into the potential role of diet and bioactive food ingredients in modulating NST during cold exposure. Stimulating NST pathways in such a manner may also provide important targets in the search of targets for the management of obesity and diabetes.

Translational barriers and opportunities for emergency preservation and resuscitation in severe injuries

BACKGROUND

Hypothermia is commonly used for organ and tissue preservation in multiple clinical settings, but its role in the management of injured patients remains controversial. There is no doubt that temperature modulation is a powerful tool, and hypothermia has been shown to protect cells during ischaemia and reperfusion, decrease organ damage and improve survival. Yet hypothermia is a double-edged sword: unless carefully managed, its induction can be associated with a number of complications.

METHODS

A literature review was performed to include important papers that address the impact of hypothermia on key biological processes, and explore the potential therapeutic role of hypothermia in trauma/haemorrhage models.

RESULTS

No clinical studies have been conducted to test the therapeutic benefits of hypothermia in injured patients. However, numerous well designed animal studies support this concept. Despite excellent preclinical data, there are several potential barriers to translating hypothermia into clinical practice.

CONCLUSION

Therapeutic hypothermia is a promising life-saving strategy. Appropriate patient selection requires a thorough understanding of how temperature modulation affects various biological mechanisms.

Type-2 iodothyronine 5'deiodinase in skeletal muscle of C57BL/6 mice. I. Identity, subcellular localization, and characterization

RT-PCR shows that mouse skeletal muscle contains type-2 iodothyronine deiodinase (D2) mRNA. However, the D2 activity has been hard to measure. Except for newborn mice, muscle homogenates have no detectable activity. However, we have reported D2 activity in mouse muscle microsomes. As the mRNA, activity is higher in slow- than in fast-twitch muscle. We addressed here the major problems in measuring D2 activity in muscle by: homogenizing muscle in high salt to improve yield of membranous structures; separating postmitochondrial supernatant between 38 and 50% sucrose, to eliminate lighter membranes lacking D2; washing these with 0.1 M Na(2)CO(3) to eliminate additional contaminating proteins; pretreating all buffers with Chelex, to eliminate catalytic metals; and eliminating the EDTA from the assay, as this can bind iron that enhances dithiothreitol oxidation and promotes peroxidation reactions. Maximum velocity of T(3) generation by postgradient microsomes from red muscles was approximately 1100 fmol/(h · mg) protein with a Michaelis-Menten constant for T(4) of 1.5 nM. D2-specific activity of Na(2)CO(3)-washed microsomes was 6-10 times higher. The enrichment in D2 activity increased in parallel with the capacity of microsomes to load (sarco/endoplasmic reticulum Ca(2+)-ATPase) and bind Ca(2+) (calsequestrin), indicating that D2 resides in the inner sarcoplasmic reticulum, close to the nuclei. The presence of D3 in the sarcolemma suggests that the most of D2-generated T(3) acts locally. Estimates from maximum velocity, Michaelis-Menten constant, and muscle T(4) content suggest that mouse red, type-1, aerobic mouse muscle fibers can generate physiologically relevant amounts of T(3) and, further, that muscle D2 plays an important role in thyroid hormone-dependent muscle thermogenesis.

Effects of the menstrual cycle on muscle recruitment and oxidative fuel selection during cold exposure

Differences in core temperature and body heat content, generally observed between the luteal and follicular phase of the menstrual cycle, have been reported to modulate the thermogenic activity of cold-exposed women. However, it is unclear how this change in whole body shivering activity will influence fuel selection. The goal of this study was to quantify the effects of the menstrual cycle on muscle recruitment and oxidative fuel selection during low-intensity shivering. Electromyographic activity of eight large muscles was monitored while carbohydrate, lipid, and protein utilization was simultaneously quantified in the follicular and luteal phases of the menstrual cycle in nonacclimatized women shivering at a low intensity. The onset (∼25 min), intensity (∼15% of maximal voluntary contraction), and pattern (∼6 shivering bursts/min) of the shivering response did not differ between menstrual cycle phases, regardless of differences in core temperature and hormone levels. This resulted in lipids remaining the predominant substrate, contributing 75% of total heat production, independent of menstrual phase. We conclude that hormone fluctuations inherent in the menstrual cycle do not affect mechanisms of substrate utilization in the cold. Whether the large contribution of lipids to total heat production in fuel selection confers a survival advantage remains to be established.

Central efferent pathways for cold-defensive and febrile shivering

Shivering is a remarkable somatomotor thermogenic response that is controlled by brain mechanisms. We recorded EMGs in anaesthetized rats to elucidate the central neural circuitry for shivering and identified several brain regions whose thermoregulatory neurons comprise the efferent pathway driving shivering responses to skin cooling and pyrogenic stimulation. We simultaneously monitored parameters from sympathetic effectors: brown adipose tissue (BAT) temperature for non-shivering thermogenesis and arterial pressure and heart rate for cardiovascular responses. Acute skin cooling consistently increased EMG, BAT temperature and heart rate and these responses were eliminated by inhibition of neurons in the median preoptic nucleus (MnPO) with nanoinjection of muscimol. Stimulation of the MnPO evoked shivering, BAT thermogenesis and tachycardia, which were all reversed by antagonizing GABA(A) receptors in the medial preoptic area (MPO). Inhibition of neurons in the dorsomedial hypothalamus (DMH) or rostral raphe pallidus nucleus (rRPa) with muscimol or activation of 5-HT1A receptors in the rRPa with 8-OH-DPAT eliminated the shivering, BAT thermogenic, tachycardic and pressor responses evoked by skin cooling or by nanoinjection of prostaglandin (PG) E2, a pyrogenic mediator, into the MPO. These data are summarized with a schematic model in which the shivering as well as the sympathetic responses for cold defence and fever are driven by descending excitatory signalling through the DMH and the rRPa, which is under a tonic inhibitory control from a local circuit in the preoptic area. These results provide the interesting notion that, under the demand for increasing levels of heat production, parallel central efferent pathways control the somatic and sympathetic motor systems to drive thermogenesis.

Adipose acyl-CoA synthetase-1 directs fatty acids toward beta-oxidation and is required for cold thermogenesis

Long-chain acyl-CoA synthetase-1 (ACSL1) contributes 80% of total ACSL activity in adipose tissue and was believed to be essential for the synthesis of triacylglycerol. We predicted that an adipose-specific knockout of ACSL1 (Acsl1(A-/-)) would be lipodystrophic, but compared to controls, Acsl1(A-/-) mice had 30% greater fat mass when fed a low-fat diet and gained weight normally when fed a high-fat diet. Acsl1(A-/-) adipocytes incorporated [(14)C]oleate into glycerolipids normally, but fatty acid (FA) oxidation rates were 50%-90% lower than in control adipocytes and mitochondria. Acsl1(A-/-) mice were markedly cold intolerant, and beta(3)-adrenergic agonists did not increase oxygen consumption, despite normal adrenergic signaling in brown adipose tissue. The reduced adipose FA oxidation and marked cold intolerance of Acsl1(A-/-) mice indicate that normal activation of FA for oxidation in adipose tissue in vivo requires ACSL1. Thus, ACSL1 has a specific function in directing the metabolic partitioning of FAs toward beta-oxidation in adipocytes.

Hypothermia in bleeding trauma: a friend or a foe?

The induction of hypothermia for cellular protection is well established in several clinical settings. Its role in trauma patients, however, is controversial. This review discusses the benefits and complications of induced hypothermia--emphasizing the current state of knowledge and potential applications in bleeding patients. Extensive pre-clinical data suggest that in advanced stages of shock, rapid cooling can protect cells during ischemia and reperfusion, decrease organ damage, and improve survival. Yet hypothermia is a double edged sword; unless carefully managed, its induction can be associated with a number of complications. Appropriate patient selection requires a thorough understanding of the pre-clinical literature. Clinicians must also appreciate the enormous influence that temperature modulation exerts on various cellular mechanisms. This manuscript aims to provide a balanced view of the published literature on this topic. While many of the advantageous molecular and physiological effects of induced hypothermia have been outlined in animal models, rigorous clinical investigations are needed to translate these promising findings into clinical practice.

Effects of two glucose ingestion rates on substrate utilization during moderate-intensity shivering

Although the importance of food consumption to survive in the cold is well established, most shivering studies have focused on fuel selection in fasting subjects. Therefore, the aim of the present study was to provide the first estimates of exogenous glucose as well as liver and muscle glycogen oxidation rates of non-cold acclimatized men (n = 6) ingesting glucose in trace amounts (Control; C), and at rates of 400 mg min(-1) (Low Glucose; LG), and 800 mg min(-1) (High Glucose; HG) during moderate-intensity shivering (~3 times resting metabolic rate or ~20% VO(2max)) using indirect calorimetry and stable isotope methodologies. Exogenous glucose oxidation peaked at ~200 mg min(-1) at the lowest glucose ingestion rate (~400 mg min(-1)). In addition, glucose ingestion increased the contribution of plasma glucose to total heat production by ~50% but did not change the role played by muscle glycogen (~27% of heat production for control condition and ~23-28% for LG and HG). Instead, the contribution of liver-derived glucose to total heat production was reduced by 40-60% in LG and HG, respectively. In conclusion, glucose ingestion even at low rates contributes a significant proportion of total heat production during moderate intensity shivering and reduces the utilization of liver-derived glucose but not muscle glycogen.

Cold exposure increases adiponectin levels in men

Sympathetic nerve activation is recognized at the adipose tissue level during cold exposure. Adiponectin is a key protein produced by adipose tissue, but its acute modulation remains unknown in humans exposed to cold. The aim of this study were (1) to examine the acute effects of cold exposure on circulating adiponectin and (2) to determine whether the changes are modulated by (a) an acute glucose ingestion as well as (b) a short-term modulation in carbohydrate (CHO) availability. Using a random crossover design, 6 healthy men were exposed to cold for 120 minutes with ingestion of beverages containing low (Control, 0.04 g/min) or high (High, 0.8 g/min) amounts of glucose during the course of the experiment (study 1). In study 2, 6 healthy men were exposed twice to cold for 120 minutes after equicaloric low-CHO diet and exercise and high-CHO diet without exercise. Plasma adiponectin concentrations were quantified before and during cold exposure. In study 1, adiponectin levels did not change during High, whereas a 20% rise was observed during Control (condition x time interaction, P = .06). In study 2, adiponectin levels increased by approximately 70% during cold exposure after both low- and high-CHO diets (effect of time, P < .05). A 120-minute period of cold exposure is accompanied by a significant increase in adiponectin levels in young healthy men. The rise in adiponectin levels observed during shivering is inhibited with glucose ingestion but not after diets varying in CHO content.

Central control of thermogenesis in mammals

Thermogenesis, the production of heat energy, is an essential component of the homeostatic repertoire to maintain body temperature in mammals and birds during the challenge of low environmental temperature and plays a key role in elevating body temperature during the febrile response to infection. The primary sources of neurally regulated metabolic heat production are mitochondrial oxidation in brown adipose tissue, increases in heart rate and shivering in skeletal muscle. Thermogenesis is regulated in each of these tissues by parallel networks in the central nervous system, which respond to feedforward afferent signals from cutaneous and core body thermoreceptors and to feedback signals from brain thermosensitive neurons to activate the appropriate sympathetic and somatic efferents. This review summarizes the research leading to a model of the feedforward reflex pathway through which environmental cold stimulates thermogenesis and discusses the influence on this thermoregulatory network of the pyrogenic mediator, prostaglandin E(2), to increase body temperature. The cold thermal afferent circuit from cutaneous thermal receptors ascends via second-order thermosensory neurons in the dorsal horn of the spinal cord to activate neurons in the lateral parabrachial nucleus, which drive GABAergic interneurons in the preoptic area to inhibit warm-sensitive, inhibitory output neurons of the preoptic area. The resulting disinhibition of thermogenesis-promoting neurons in the dorsomedial hypothalamus and possibly of sympathetic and somatic premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, activates excitatory inputs to spinal sympathetic and somatic motor circuits to drive thermogenesis.

Increased thermoregulation in cold-exposed transgenic mice overexpressing lipoprotein lipase in skeletal muscle: an avian phenotype?

LPL is an enzyme involved in the breakdown and uptake of lipoprotein triglycerides. In the present study, we examined how the transgenic (Tg) overexpression of human LPL in mouse skeletal muscle affected tolerance to cold temperatures, cold-induced thermogenesis, and fuel utilization during this response. Tg mice and their nontransgenic controls were placed in an environmental chamber and housed in metabolic chambers that monitored oxygen consumption and carbon dioxide production with calorimetry. When exposed to 4 degrees C, an attenuation in the decline in body temperature in Tg mice was accompanied by an increased metabolic rate (15%; P < 0.001) and a reduction in respiratory quotient (P < 0.05). Activity levels, the expression of uncoupling proteins in brown fat and muscle, and lean mass failed to explain the enhanced cold tolerance and thermogenesis in Tg mice. The more oxidative type IIa fibers were favored over the more glycolytic type IIb fibers (P < 0.001) in the gastrocnemius and quadriceps muscles of Tg mice. These data suggest that Tg overexpression of LPL in skeletal muscle increases cold tolerance by enhancing the capacity for fat oxidation, producing an avian-like phenotype in which skeletal muscle contributes significantly to the thermogenic response to cold temperatures.

American College of Sports Medicine position stand: prevention of cold injuries during exercise

It is the position of the American College of Sports Medicine that exercise can be performed safely in most cold-weather environments without incurring cold-weather injuries. The key to prevention is use of a comprehensive risk management strategy that: a) identifies/assesses the cold hazard; b) identifies/assesses contributing factors for cold-weather injuries; c) develops controls to mitigate cold stress/strain; d) implements controls into formal plans; and e) utilizes administrative oversight to ensure controls are enforced or modified. The American College of Sports Medicine recommends that: 1) coaches/athletes/medical personnel know the signs/symptoms and risk factors for hypothermia, frostbite, and non-freezing cold injuries, identify individuals susceptible to cold injuries, and have the latest up-to-date information about current and future weather conditions before conducting training sessions or competitions; 2) cold-weather clothing be chosen based on each individual's requirements and that standardized clothing ensembles not be mandated for entire groups; 3) the wind-chill temperature index be used to estimate the relative risk of frostbite and that heightened surveillance of exercisers be used at wind-chill temperatures below -27 degrees C (-18 degrees F); and 4) individuals with asthma and cardiovascular disease can exercise in cold environments, but should be monitored closely.