Lori K. Bogren

Resistance to systemic inflammation and multi organ damage after global ischemia/reperfusion in the arctic ground squirrel.

Introduction Cardiac arrest (CA) and hemorrhagic shock (HS) are two clinically relevant situations where the body undergoes global ischemia as blood pressure drops below the threshold necessary for adequate organ perfusion. Resistance to ischemia/reperfusion (I/R) injury is a characteristic of hibernating mammals. The present study sought to determine if arctic ground squirrels (AGS) are protected from systemic inflammation and multi organ damage after CA- or HS-induced global I/R and if, for HS, this protection is dependent upon their hibernation season. Methods For CA, rats and summer euthermic AGS (AGS-EU) were asphyxiated for 8 min, inducing CA. For HS, rats, AGS-EU, and winter interbout arousal AGS (AGS-IBA) were subject to HS by withdrawing blood to a mean arterial pressure of 35 mmHg and maintaining that pressure for 20 min before reperfusion with Ringers. For both I/R models, body temperature (Tb) was kept at 36.5–37.5°C. After reperfusion, animals were monitored for seven days (CA) or 3 hrs (HS) then tissues and blood were collected for histopathology, clinical chemistries, and cytokine level analysis (HS only). For the HS studies, additional groups of rats and AGS were monitored for three days after HS to access survival and physiological impairment. Results Rats had increased serum markers of liver damage one hour after CA while AGS did not. For HS, AGS survived 72 hours after I/R whereas rats did not survive overnight. Additionally, only rats displayed an inflammatory response after HS. AGS maintained a positive base excess, whereas the base excess in rats was negative during and after hemorrhage. Conclusions Regardless of season, AGS are resistant to organ damage, systemic inflammation, and multi organ damage after systemic I/R and this resistance is not dependent on their ability to become decrease Tb during insult but may stem from an altered acid/base and metabolic response during I/R.

1H–NMR Metabolomic Biomarkers of Poor Outcome after Hemorrhagic Shock are Absent in Hibernators

Background Hemorrhagic shock (HS) following trauma is a leading cause of death among persons under the age of 40. During HS the body undergoes systemic warm ischemia followed by reperfusion during medical intervention. Ischemia/reperfusion (I/R) results in a disruption of cellular metabolic processes that ultimately lead to tissue and organ dysfunction or failure. Resistance to I/R injury is a characteristic of hibernating mammals. The present study sought to identify circulating metabolites in the rat as biomarkers for metabolic alterations associated with poor outcome after HS. Arctic ground squirrels (AGS), a hibernating species that resists I/R injury independent of decreased body temperature (warm I/R), was used as a negative control. Methodology/principal findings Male Sprague-Dawley rats and AGS were subject to HS by withdrawing blood to a mean arterial pressure (MAP) of 35 mmHg and maintaining the low MAP for 20 min before reperfusing with Ringers. The animals’ temperature was maintained at 37±0.5°C for the duration of the experiment. Plasma samples were taken immediately before hemorrhage and three hours after reperfusion. Hydrophilic and lipid metabolites from plasma were then analyzed via 1H–NMR from unprocessed plasma and lipid extracts, respectively. Rats, susceptible to I/R injury, had a qualitative shift in their hydrophilic metabolic fingerprint including differential activation of glucose and anaerobic metabolism and had alterations in several metabolites during I/R indicative of metabolic adjustments and organ damage. In contrast, I/R injury resistant AGS, regardless of season or body temperature, maintained a stable metabolic homeostasis revealed by a qualitative 1H–NMR metabolic profile with few changes in quantified metabolites during HS-induced global I/R. Conclusions/significance An increase in circulating metabolites indicative of anaerobic metabolism and activation of glycolytic pathways is associated with poor prognosis after HS in rats. These same biomarkers are absent in AGS after HS with warm I/R.

Hibernation: A Natural Model of Tolerance to Cerebral Ischemia/Reperfusion

Hibernation, a means of systemic energy conservation, defies the need for most life-sustaining processes. Hibernation is recognized by a state of prolonged torpor where whole body metabolic rate, core body temperature (T b), heart rate, and blood flow decrease to 1–10 % of values observed during sleep. These bouts of torpor are interrupted at regular intervals by brief episodes of heterogeneous rewarming and reperfusion of vital organs, including the brain. Despite the reduction of cerebral blood flow during torpor or the return of cerebral blood flow during interbout arousals, hibernation produces no evidence of neuropathology. Multiple adaptations, at the whole animal and tissue levels, during torpor reveal a combination therapy–like scenario that likely contributes to ischemic tolerance. Nonetheless, some hibernating species tolerate ischemic-like cerebral blood flow even when not hibernating outside of the hibernation season and when T b is maintained at 37 °C. The arctic ground squirrel (Urocitellus parryii), a species studied extensively as a model of cerebral ischemia tolerance, resists neuronal pathology following cardiac arrest in vivo and following various paradigms designed to mimic cerebral ischemia in brain slices and cultured neurons. Here we review evidence that supports and refutes hypothesized mechanisms of ischemia tolerance in arctic ground squirrels with the caveat that much remains to be learned about mechanisms of ischemia tolerance in arctic ground squirrels and in other mammalian hibernators. Although hibernating mammals resist injury following cerebral ischemia/reperfusion even when not hibernating, torpor nonetheless is a phenotype with obvious neuroprotective advantages including cold tissue temperatures, a decrease in metabolic demand, and suppressed immune responsiveness. Thus we also review recent breakthroughs in the understanding of how the central nervous system regulates the onset of hibernation and discuss prospects for inducing hibernation in humans.

The effects of hibernation and forced disuse (neurectomy) on bone properties in arctic ground squirrels

Bone loss is a well‐known medical consequence of disuse such as in long‐term space flight. Immobilization in many animals mimics the effects of space flight on bone mineral density. Decreases in metabolism are also thought to contribute to a loss of skeletal mass. Hibernating mammals provide a natural model of disuse and metabolic suppression. Hibernating ground squirrels have been shown to maintain bone strength despite long periods of disuse and decreased metabolism during torpor. This study examined if the lack of bone loss during torpor was a result of the decrease in metabolic rate during torpor or an evolutionary change in these animals affording protection against disuse. We delineated changes in bone density during natural disuse (torpor) and forced disuse (sciatic neurectomy) in the hind limbs of the arctic ground squirrel (AGS) over an entire year. We hypothesized that the animals would be resistant to bone loss due to immobilization and disuse during the winter hibernation season when metabolism is depressed but not the summer active season. This hypothesis was not supported. The animals maintained bone density (dual‐energy X‐ray absorptiometry) and most bone structural and mechanical properties in both seasons. This was observed in both natural and forced disuse, regardless of the known metabolic rate increase during the summer. However, trabecular bone volume fraction (microcomputed tomography) in the distal femur was lower in neurectomized AGS at the study endpoint. These results demonstrate a need to better understand the relationship between skeletal load (use) and bone density that may lead to therapeutics or strategies to maintain bone density in disuse conditions.

Ischemia/reperfusion injury resistance in hibernators is more than an effect of reduced body temperature or winter season

Hibernating mammals are resistant to injury following cardiac arrest. The basis of this protection has been proposed to be due to their ability to lower body temperature or metabolic rate in a seasonally-dependent manner. However, recent studies have shown that neither reduced body temperature nor hibernation season are components this protection.

Potential Mechanisms of Metabolic SuppressionDownstream of Central A1AR Activation During Onset of Torpor

Hibernating animals demonstrate a nadir in metabolic demand and body temperature (T b) during torpor that is fundamental to adaptation to seasonal periods of reduced resource availability. A recent study shows how the brain regulates metabolic suppression during onset of torpor suggesting that central A1 adenosine receptor signaling is both necessary and sufficient to trigger decreases in metabolic rate and T b. This leads to an interesting question of how central signals are transduced to the periphery to elicit global suppression of metabolism and this chapter discusses relevant hypotheses.

Optimization of Thermolytic Response to A 1 Adenosine Receptor Agonists in Rats

Cardiac arrest is a leading cause of death in the United States, and, currently, therapeutic hypothermia, now called targeted temperature management (TTM), is the only recent treatment modality proven to increase survival rates and reduce morbidity for this condition. Shivering and subsequent metabolic stress, however, limit application and benefit of TTM. Stimulating central nervous system A1 adenosine receptors (A1AR) inhibits shivering and nonshivering thermogenesis in rats and induces a hibernation-like response in hibernating species. In this study, we investigated the pharmacodynamics of two A1AR agonists in development as antishivering agents. To optimize body temperature (Tb) control, we evaluated the influence of every-other-day feeding, dose, drug, and ambient temperature (Ta) on the Tb-lowering effects of N6-cyclohexyladenosine (CHA) and the partial A1AR agonist capadenoson in rats. The highest dose of CHA (1.0 mg/kg, i.p.) caused all ad libitum–fed animals tested to reach our target Tb of 32°C, but responses varied and some rats overcooled to a Tb as low as 21°C at 17.0°C Ta. Dietary restriction normalized the response to CHA. The partial agonist capadenoson (1.0 or 2.0 mg/kg, i.p.) produced a more consistent response, but the highest dose decreased Tb by only 1.6°C. To prevent overcooling after CHA, we studied continuous i.v. administration in combination with dynamic surface temperature control. Results show that after CHA administration control of surface temperature maintains desired target Tb better than dose or ambient temperature.