Ascorbate distribution during hibernation is independent of ascorbate redox state

Erythrocytes of Little Ground Squirrels Undergo Reversible Oxidative Stress During Arousal From Hibernation

The hibernation of small mammals is characterized by long torpor bouts alternating with short periods of arousal. During arousal, due to a significant increase in oxygen consumption, tissue perfusion, and the launch of thermogenesis in cells, a large amount of reactive oxygen species (ROS) and nitrogen (RNS) can be formed, which can trigger oxidative stress in cells. To estimate this possibility, we studied the intensity of free-radical processes in the red blood cells (RBCs) of little ground squirrels (LGS; Spermophilus pygmaeus) in the dynamics of arousal from hibernation. We found that in the torpid state, the degree of generation of ROS and RNS (8.3%, p>0.09; 20.7%, p<0.001, respectively), the degree of oxidative modification of membrane lipids and RBC proteins is at a low level (47%, p<0.001; 82.7%, p<0.001, respectively) compared to the summer control. At the same time, the activity of superoxide dismutase (SOD) and catalase (CAT) in RBC is significantly reduced (32.8%, p<0.001; 22.2%, p<0.001, respectively), but not the level of glutathione (GSH). In the torpid state, SOD is activated by exogenous GSH in concentration-dependent manner, which indicates reversible enzyme inhibition. During the arousal of ground squirrels, when the body temperature reaches 25°C, RBCs are exposed oxidative stress. This is confirmed by the maximum increase in the level of uric acid (25.4%, p<0.001) in plasma, a marker of oxidative modification of lipids [thiobarbituric acid reactive substances (TBARS); 82%, p < 0.001] and proteins (carbonyl groups; 499%, p < 0.001) in RBC membranes, as well as the decrease in the level of GSH (19.7%, p < 0.001) in erythrocytes relative to the torpid state and activity of SOD and CAT in erythrocytes to values at the Tb 20°C. After full recovery of body temperature, the level of GSH increases, the ratio of SOD/CAT is restored, which significantly reduces the degree of oxidative damage of lipids and proteins of RBC membranes. Thus, the oxidative stress detected at Tb 25°C was transient and physiologically regulated.

Hepatic resistance to cold ferroptosis in a mammalian hibernator Syrian hamster depends on effective storage of diet-derived α-tocopherol

Mammalian hibernators endure severe and prolonged hypothermia that is lethal to non-hibernators, including humans and mice. The mechanisms responsible for the cold resistance remain poorly understood. Here, we found that hepatocytes from a mammalian hibernator, the Syrian hamster, exhibited remarkable resistance to prolonged cold culture, whereas murine hepatocytes underwent cold-induced cell death that fulfills the hallmarks of ferroptosis such as necrotic morphology, lipid peroxidation and prevention by an iron chelator. Unexpectedly, hepatocytes from Syrian hamsters exerted resistance to cold- and drug-induced ferroptosis in a diet-dependent manner, with the aid of their superior ability to retain dietary α-tocopherol (αT), a vitamin E analog, in the liver and blood compared with those of mice. The liver phospholipid composition is less susceptible to peroxidation in Syrian hamsters than in mice. Altogether, the cold resistance of the hibernator’s liver is established by the ability to utilize αT effectively to prevent lipid peroxidation and ferroptosis.

Daisuke Anegawa et al. investigated the mechanisms responsible for cold resistance in the Syrian hamster’s hepatocytes, which exhibited remarkable resistance to prolonged cold culture. Their results suggest that hepatocytes exhibit diet-dependent resistance to cold, which is linked to the retention of α-tocopherol in the liver.

Mitochondrial metabolic suppression and reactive oxygen species production in liver and skeletal muscle of hibernating thirteen-lined ground squirrels

During hibernation, animals cycle between periods of torpor, during which body temperature (Tb) and metabolic rate (MR) are suppressed for days, and interbout euthermia (IBE), during which Tb and MR return to resting levels for several hours. In this study, we measured respiration rates, membrane potentials, and reactive oxygen species (ROS) production of liver and skeletal muscle mitochondria isolated from ground squirrels ( Ictidomys tridecemlineatus ) during torpor and IBE to determine how mitochondrial metabolism is suppressed during torpor and how this suppression affects oxidative stress. In liver and skeletal muscle, state 3 respiration measured at 37°C with succinate was 70% and 30% lower, respectively, during torpor. In liver, this suppression was achieved largely via inhibition of substrate oxidation, likely at succinate dehydrogenase. In both tissues, respiration by torpid mitochondria further declined up to 88% when mitochondria were cooled to 10°C, close to torpid Tb. In liver, this passive thermal effect on respiration rate reflected reduced activity of all components of oxidative phosphorylation (substrate oxidation, phosphorylation, and proton leak). With glutamate + malate and succinate, mitochondrial free radical leak (FRL; proportion of electrons leading to ROS production) was higher in torpor than IBE, but only in liver. With succinate, higher FRL likely resulted from increased reduction state of complex III during torpor. With glutamate + malate, higher FRL resulted from active suppression of complex I ROS production during IBE, which may limit ROS production during arousal. In both tissues, ROS production and FRL declined with temperature, suggesting ROS production is also reduced during torpor by passive thermal effects.

Physiological oxidative stress after arousal from hibernation in Arctic ground squirrel

Hibernation in Arctic ground squirrels (AGS), Spermophilus parryii, is characterized by a profound decrease in oxygen consumption and metabolic demand during torpor that is punctuated by periodic rewarming episodes, during which oxygen consumption increases dramatically. The extreme physiology of torpor or the surge in oxygen consumption during arousal may increase production of reactive oxygen species, making hibernation an injurious process for AGS. To determine if AGS tissues experience cellular stress during rewarming, we measured carbonyl proteins, lipid peroxide end products and percent oxidized glutathione in brown adipose tissue (BAT) and liver of torpid, hibernating (hAGS), late arousal (laAGS), and cold-adapted, euthermic AGS (eAGS). In BAT carbonyl proteins and lipid peroxide end products were higher in eAGS and laAGS than in hAGS. By contrast, in liver, no significant difference in carbonyl proteins was observed. In another group of animals, comparison of carbonyl proteins and percent oxidized glutathione in frontal cortex, liver, and BAT of eAGS and hAGS showed no evidence of oxidative stress associated with torpor. These results indicate that increased thermogenesis associated with arousal AGS results in tissue specific oxidative stress in BAT but not in liver. Moreover, torpor per se is largely devoid of oxidative stress, likely due to suppression of oxidative metabolism.

Lessons from comparative physiology: could uric acid represent a physiologic alarm signal gone awry in western society?

Uric acid has historically been viewed as a purine metabolic waste product excreted by the kidney and gut that is relatively unimportant other than its penchant to crystallize in joints to cause the disease gout. In recent years, however, there has been the realization that uric acid is not biologically inert but may have a wide range of actions, including being both a pro- and anti-oxidant, a neurostimulant, and an inducer of inflammation and activator of the innate immune response. In this paper, we present the hypothesis that uric acid has a key role in the foraging response associated with starvation and fasting. We further suggest that there is a complex interplay between fructose, uric acid and vitamin C, with fructose and uric acid stimulating the foraging response and vitamin C countering this response. Finally, we suggest that the mutations in ascorbate synthesis and uricase that characterized early primate evolution were likely in response to the need to stimulate the foraging "survival" response and might have inadvertently had a role in accelerating the development of bipedal locomotion and intellectual development. Unfortunately, due to marked changes in the diet, resulting in dramatic increases in fructose- and purine-rich foods, these identical genotypic changes may be largely responsible for the epidemic of obesity, diabetes and cardiovascular disease in today's society.

Brain ECF antioxidant interactions in hamsters during arousal from hibernation

Warming from hibernation to cenothermia involves intense metabolic activity and large fluxes in regional blood flow and volume. During this transition, levels of the antioxidants, ascorbate (AA), urate and glutathione (GSH) in brain tissue, extracellular fluid (ECF) and plasma change substantially. Striatal ECF was sampled and manipulated using very slow perfusion microdialysis to examine the mechanisms that influence the changing profile of striatal ECF AA, urate and GSH levels during arousal from hibernation to cenothermia in Syrian hamsters (Mesocricetus auratus). Omission of glucose from the perfusate had no effect upon the respective decrease, increase and transient increase in striatal ECF levels of AA, GSH and urate observed during arousal from hibernation to cenothermia. In contrast, inhibition of xanthine dehydrogenase/oxidase (XOR) activity by reverse dialysis with oxypurinol, itself a free radical scavenger, decreased ECF urate and preserved ECF AA levels. This suggests that some ECF AA is oxidized by free radical products of XOR flux and/or by other free radical producing processes activated during the transition from hibernation to cenothermia. Local supplementation of ECF AA, GSH and cystiene had no effect upon the profile of transient increase of ECF urate observed during arousal from hibernation. The production of free radicals by XOR and the disappearance of AA from the ECF continues for at least 2h immediately after the hamster has attained cenothermia. The hamster, immediately after arousal from hibernation, can be utilized as a natural model to study free radical production and effective scavenging at cenothermia.

Brain antioxidant levels in hamsters during hibernation, arousal and cenothermia

Warming from hibernation to cenothermia involves intense metabolic activity coincident with large fluxes in blood flow and is considered to be a period of oxidative stress during which utilization of endogenous antioxidants prevents pathology. Very slow flow brain microdialysis enabled temperature independent sampling of the brain striatal extracellular fluid (ECF) during hibernation, arousal and cenothermia in Syrian hamsters (Mesocricetus auratus). Brain tissue and dialysates were analyzed to provide the first profile of changes in ECF levels of ascorbate (AA), glutathione (GSH) and urate during hibernation and the transition to cenothermia. Brain tissue content of AA and GSH was unchanged between hibernation and cenothermia; however, arousal was associated with substantial oxidation of AA from the brain ECF and plasma compartments. ECF GSH increased during arousal. Brain tissue urate content was decreased 50% during hibernation. ECF urate levels were unchanged in hibernation and cenothermia but transiently increased 100% during arousal. These experiments demonstrate that arousal from hibernation is a suitable experimental model for examination of the mechanisms by which non-pathological tissue integrity is maintained in the face of the generation of free radicals during increasing metabolism, temperature and cerebral reperfusion.

Absence of cellular stress in brain after hypoxia induced by arousal from hibernation in Arctic ground squirrels

Although hypoxia tolerance in heterothermic mammals is well established, it is unclear whether the adaptive significance stems from hypoxia or other cellular challenge associated with euthermy, hibernation, or arousal. In the present study, blood gases, hemoglobin O2 saturation (S(O2), and indexes of cellular and physiological stress were measured during hibernation and euthermy and after arousal thermogenesis. Results show that arterial O2 tension (Pa(O2)) and S(O2) are severely diminished during arousal and that hypoxia-inducible factor (HIF)-1alpha accumulates in brain. Despite evidence of hypoxia, neither cellular nor oxidative stress, as indicated by inducible nitric oxide synthase (iNOS) levels and oxidative modification of biomolecules, was observed during late arousal from hibernation. Compared with rats, hibernating Arctic ground squirrels (Spermophilus parryii) are well oxygenated with no evidence of cellular stress, inflammatory response, neuronal pathology, or oxidative modification following the period of high metabolic demand necessary for arousal. In contrast, euthermic Arctic ground squirrels experience mild, chronic hypoxia with low S(O2) and accumulation of HIF-1alpha and iNOS and demonstrate the greatest degree of cellular stress in brain. These results suggest that Arctic ground squirrels experience and tolerate endogenous hypoxia during euthermy and arousal.

Hypoxia tolerance in mammalian heterotherms

Heterothermic mammals tolerate severe hypoxia, as well as a variety of central nervous system insults, better than homeothermic mammals. Tolerance to hypoxia may stem from adaptations associated with the ability to survive hibernation and periodic arousal thermogenesis. Here, we review evidence and mechanisms of hypoxia tolerance during hibernation, euthermy and arousal in heterothermic mammals and consider potential mechanisms for regenerative-like processes, such as synaptogenesis, observed within hours of hypoxic stress associated with arousal thermogenesis.