Comfortably Numb and Back: Plasma Metabolomics Reveals Biochemical Adaptations in the Hibernating 13-Lined Ground Squirrel

Hibernation is an evolutionary adaptation that affords some mammals the ability to exploit the cold to achieve extreme metabolic depression (torpor) while avoiding ischemia/reperfusion or hemorrhagic shock injuries. Hibernators cycle periodically out of torpor, restoring high metabolic activity. If understood at the molecular level, the adaptations underlying torpor-arousal cycles may be leveraged for translational applications in critical fields such as intensive care medicine. Here, we monitored 266 metabolites to investigate the metabolic adaptations to hibernation in plasma from 13-lined ground squirrels (57 animals, 9 time points). Results indicate that the periodic arousals foster the removal of potentially toxic oxidative stress-related metabolites, which accumulate in plasma during torpor while replenishing reservoirs of circulating catabolic substrates (free fatty acids and amino acids). Specifically, we identified metabolic fluctuations of basic amino acids lysine and arginine, one-carbon metabolism intermediates, and sulfur-containing metabolites methionine, cysteine, and cystathionine. Conversely, reperfusion injury markers such as succinate/fumarate remained relatively stable across cycles. Considering the cycles of these metabolites with the hibernator’s cycling metabolic activity together with their well-established role as substrates for the production of hydrogen sulfide (H₂S), we hypothesize that these metabolic fluctuations function as a biological clock regulating torpor to arousal transitions and resistance to reperfusion during arousal

Physiological parameters change for all groups during hemorrhage and resuscitation.

<p>AGS had higher pH, BE, P_O2, and HCO₃⁻ than rats. Plasma glucose levels were higher for AGS-IBA than rat at the end of reperfusion and three hours after reperfusion and AGS-EU were lower than rat at the start of hemorrhage (different letters indicate <i>p</i><0.05, Tukey between groups at that timepoint).</p><p>** n = 5 for AGS-IBA P_O2.</p

AGS maintain positive whole blood base excess values before, during, and after hemorrhagic shock.

<p>All animals initially had positive BE values with the AGS (both seasons) having a starting BE higher than the rats. Data presented as mean±SEM; * Tukey, <i>p</i><0.05 between AGS-EU Sham HS (SHS) and HS, ** Tukey, <i>p</i><0.05 between rat SHS and HS.</p

Serum markers of organ damage after hemorrhagic shock.

<p>Serum markers indicate that the AGS do not sustain kidney damage as indicated by increased serumcreatinine. Dark areas indicate baseline levels obtained at the start of hemorrhage. Light areas are the levels three hours after end of resuscitation. Raw data is shown and expressed as mean ± SEM Statistical analysis with an ANOVA was performed on the difference between three hours after resuscitation and baseline. *Tukey, <i>p</i><0.05 between SHS and HS; n = 4–8.</p

Rats have a higher average heart rate before, during and after hemorrhagic shock than AGS.

<p>Shaded region indicates period of hemorrhage. Rats differed from both AGS groups (a, Tukey, p<0.05), just the AGS-IBA (b, Tukey, p<0.05), or AGS-EU (c, Tukey p<0.05). AGS-EU and –IBA did not differ from each other at any timepoint. Data shown as mean±SEM, n = 6–7 for all treatment groups.</p

Blood serum markers for liver damage after cardiac arrest.

<p>AGS did not show an increase in blood serum markers for liver damage one hour after CA. Rats had an increase in both ALT (top, * Tukey, <i>p</i><0.05) and AST (bottom, * Tukey, <i>p</i><0.05) one hour after CA compared to all other groups. Dark bars indicate baseline values. Light bars are values one hour after CA. Raw data is shown as mean ± SEM. Statistical analysis was done on the difference between baseline and one hour after CA. For all groups n = 5–6.</p

Fold changes in circulating cytokine levels after hemorrhagic shock.

<p>AGS do not have a significant fold change in plasma cytokine levels immediately prior to hemorrhage and three hours after resuscitation. Cytokines levels assessed for: IL-1 alpha (A), IL-1 beta (B), IL-6 (C), IL-10 (D), TNF-alpha (E), and INF-gamma (F); *<i>p</i><0.05, Tukey versus corresponding sham; n = 6–8 for each group.</p

The mean arterial blood pressure and average heart rate decrease during cardiac arrest.

<p>Both mean arterial pressure (MAP; top) and heart rate (HR; bottom) decrease similarly for both AGS and rat and return to near normal after CA. Darkened area represents period of asphyxia. MAP and HR were recorded immediately before asphyxiation (Before CA) and one hour after CA (After CA). Data shown as mean ± SEM, * Tukey, <i>p</i><0.05 between rat and AGS. For both groups n = 5.</p

Resistance to Systemic Inflammation and Multi Organ Damage after Global Ischemia/Reperfusion in the Arctic Ground Squirrel

<div><p>Introduction</p><p>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.</p><p>Methods</p><p>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.</p><p>Results</p><p>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.</p><p>Conclusions</p><p>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.</p></div

Percent survival after hemorrhagic shock.

<p>Extended survival assessed up to 72(MAP ∼35 mmHg for 20 min) in euthermic AGS, interbout arousal AGS, and rats (control; n = 4 for all groups; A). Survival up to three hours after 55–60% blood loss in euthermic- (n = 4) and interbout arousal (n = 4) AGS (B).</p