Prolonged stable hypothermia: effect on blood gases and pH in rats and ground squirrels

Hypothermia can reverse hepatic oxidative stress damage induced by hypoxia in rats

Our previous findings demonstrated that hypothermia enhances the reduction potential in the liver and helps to maintain the plasmatic antioxidant pool. Here, we aimed to elucidate if hypothermia protects against hypoxia-induced oxidative stress damage in rat liver. Several hepatic markers of oxidative stress were compared in three groups of animals (n = 8 in each group): control normothermic group ventilated with room air and two groups under extreme hypoxia (breathing 10 % O₂), one kept at normothermia (HN) (37 °C) and the other under deep hypothermia (HH) (central body temperature of 21-22 °C). Hypoxia in normothermia significantly increased the levels of hepatic nitric oxide, inducible nitric oxide synthase expression, protein oxidation, Carbonilated proteins, advanced oxidation protein products, 4-hydroxynonenal (HNE) protein adducts, and lipid peroxidation when compared to the control group (p < 0.05). However, when hypoxia was induced under hypothermia, results from the oxidative stress biomarker analyses did not differ significantly from those found in the control group. Indeed, 4-HNE protein adduct amounts were significantly lower in the HH versus HN group (p < 0.05). Therefore, hypothermia can mitigate hypoxia-induced oxidative stress damage in rat liver. These effects could help clarify the mechanisms of action of therapeutic hypothermia.

Acid-base balance in the developing marsupial: from ectotherm to endotherm

Marsupial joeys are born ectothermic and develop endothermy within their mother's thermally stable pouch. We hypothesized that Tammar wallaby joeys would switch from α-stat to pH-stat regulation during the transition from ectothermy to endothermy. To address this, we compared ventilation (V̇e), metabolic rate (V̇o2), and variables relevant to blood gas and acid-base regulation and oxygen transport including the ventilatory requirements (V̇e/V̇o2 and V̇e/V̇co2), partial pressures of oxygen (PaO2), carbon dioxide (PaCO2), pHa, and oxygen content (CaO2) during progressive hypothermia in ecto- and endothermic Tammar wallabies. We also measured the same variables in the well-studied endotherm, the Sprague-Dawley rat. Hypothermia was induced in unrestrained, unanesthetized joeys and rats by progressively dropping the ambient temperature (Ta). Rats were additionally exposed to helox (80% helium, 20% oxygen) to facilitate heat loss. Respiratory, metabolic, and blood-gas variables were measured over a large body temperature (Tb) range (∼15–16°C in both species). Ectothermic joeys displayed limited thermogenic ability during cooling: after an initial plateau, V̇o2 decreased with the progressive drop in Tb. The Tb of endothermic joeys and rats fell despite V̇o2 nearly doubling with the initiation of cold stress. In all three groups the changes in V̇o2 were met by changes in V̇e, resulting in constant V̇e/V̇o2 and V̇e/V̇co2, blood gases, and pHa. Thus, although thermogenic capability was nearly absent in ectothermic joeys, blood acid-base regulation was similar to endothermic joeys and rats. This suggests that unlike some reptiles, unanesthetized mammals protect arterial blood pH with changing Tb, irrespective of their thermogenic ability and/or stage of development.

Oxidative stress and antioxidant activity in hypothermia and rewarming: can RONS modulate the beneficial effects of therapeutic hypothermia?

Hypothermia is a condition in which core temperature drops below the level necessary to maintain bodily functions. The decrease in temperature may disrupt some physiological systems of the body, including alterations in microcirculation and reduction of oxygen supply to tissues. The lack of oxygen can induce the generation of reactive oxygen and nitrogen free radicals (RONS), followed by oxidative stress, and finally, apoptosis and/or necrosis. Furthermore, since the hypothermia is inevitably followed by a rewarming process, we should also consider its effects. Despite hypothermia and rewarming inducing injury, many benefits of hypothermia have been demonstrated when used to preserve brain, cardiac, hepatic, and intestinal function against ischemic injury. This review gives an overview of the effects of hypothermia and rewarming on the oxidant/antioxidant balance and provides hypothesis for the role of reactive oxygen species in therapeutic hypothermia.

Effects of hetastarch and mannitol on prolonging survival in stable hypothermia in rats

In rats, prolonged stable hypothermia ( approximately 24 h at body temperature of 19 degrees C) is characterized by a time-dependent increase in hematocrit, plasma osmolality, and red blood cell fragility and a decrease in plasma volume. These changes impede tissue microcirculation and could limit survival. As a countermeasure, we used plasma volume expanders of both long (hetastarch)- and short-lasting (mannitol) characteristics to improve microcirculation and hopefully hypothermia survival. Infusion of 6% hetastarch at hour 3 in hypothermia significantly (P < 0.05) enhanced survival over saline control (33.5 vs. 23.8 h); a significant delay in the increases of hematocrit and cell fragility was also observed compared with those in saline controls. Treating the animal with 6% hetastarch at hour 20 during hypothermia caused a similar but less-effective improvement in survival. In contrast, treating the rats with 6% mannitol at hour 3 or 20 during hypothermia failed to enhance survival over saline control, although transient improvement in plasma volume was observed. Our results indicate that by using a long-lasting volume expander, which tends to better maintain plasma volume and rheological parameters governing microcirculation than does saline or a short-lasting volume expander, hypothermia survival can be significantly improved.

Liver metabolism in cold hypoxia: a comparison of energy metabolism and glycolysis in cold-sensitive and cold-resistant mammals

The effects of cold hypoxia were examined during a time-course at 2 degrees C on levels of glycolytic metabolites: glycogen, glucose, glucose-1-phosphate, glucose-6-phosphate, fructose-6-phosphate, fructose-1,6-bisphosphate, phosphoenolpyruvate, pyruvate, lactate and energetics (ATP, ADP, AMP) of livers from rats and columbian ground squirrels. Responses of adenylate pools reflected the energy imbalance created during cold hypoxia in both rat and ground squirrel liver within minutes of organ isolation. In rat, ATP levels and energy charge values for freshly isolated livers were 2.54 mumol.g-1 and 0.70, respectively. Within 5 min of cold hypoxia, ATP levels had dropped well below control values and by 8 h storage, ATP, AMP, and energy charge values were 0.21 mumol.g-1, 2.01 mumol.g-1, and 0.17, respectively. In columbian ground squirrels the patterns of rapid ATP depletion and AMP accumulation were similar to those found in rat. In rat liver, enzymatic regulatory control of glycolysis appeared to be extremely sensitive to the decline in cellular energy levels. After 8 h cold hypoxia levels of fructose-6-phosphate decreased and fructose-1,6-bisphosphate increased, thus reflecting an activation of glycolysis at the regulatory step catalysed by phosphofructokinase fructose-1,6-bisphosphatase. Despite an initial increase in flux through glycolysis over the first 2 min (lactate levels increased 3.7 mumol.g-1), further flux through the pathway was not permitted even though glycolysis was activated at the phosphofructokinase/fructose-1,6-bisphosphatase locus at 8 h, since supplies of phosphorylated substrate glucose-1-phosphate or glucose-6-phosphate remained low throughout the duration of the 24-h period.(ABSTRACT TRUNCATED AT 250 WORDS)

Erythrocyte osmotic resistance during acute hypothermia in awake unrestrained rats

Erythrocyte osmotic fragility and plasma ionic composition were studied in rats subjected to acute hypothermia. A decrease in osmotic fragility and a significant increase in plasma magnesium and total phosphorus were observed in blood from hypothermic rats in relation to control. A decrease in erythrocyte osmotic fragility from hypothermic animals was observed when the test was performed at 37 degrees C, whereas osmotic fragility was unaltered if the test was carried out at body temperature. This could be interpreted as an adaptative response to counteract the opposite effect on erythrocyte osmotic fragility observed at low temperature 'in vitro'.