Intracellular pH during daily torpor in Peromyscus maniculatus

Evolutionary constraint and innovation across hundreds of placental mammals

Zoonomia is the largest comparative genomics resource for mammals produced to date. By aligning genomes for 240 species, we identify bases that, when mutated, are likely to affect fitness and alter disease risk. At least 332 million bases (~10.7%) in the human genome are unusually conserved across species (evolutionarily constrained) relative to neutrally evolving repeats, and 4552 ultraconserved elements are nearly perfectly conserved. Of 101 million significantly constrained single bases, 80% are outside protein-coding exons and half have no functional annotations in the Encyclopedia of DNA Elements (ENCODE) resource. Changes in genes and regulatory elements are associated with exceptional mammalian traits, such as hibernation, that could inform therapeutic development. Earth's vast and imperiled biodiversity offers distinctive power for identifying genetic variants that affect genome function and organismal phenotypes.

Embryonic common snapping turtles (Chelydra serpentina) preferentially regulate intracellular tissue pH during acid-base challenges

The nests of embryonic turtles naturally experience elevated CO2 (hypercarbia), which leads to increased blood PCO2 and a respiratory acidosis resulting in reduced blood pH [extracellular pH (pHe)]. Some fishes preferentially regulate tissue pH [intracellular pH (pHi)] against changes in pHe; this has been proposed to be associated with exceptional CO2 tolerance and has never been identified in amniotes. As embryonic turtles may be CO2 tolerant based on nesting strategy, we hypothesized that they preferentially regulate pHi, conferring tolerance to severe acute acid-base challenges. This hypothesis was tested by investigating pH regulation in common snapping turtles (Chelydra serpentina) reared in normoxia then exposed to hypercarbia (13kPa PCO2) for 1h at three developmental ages, 70 and 90% of incubation, and in yearlings. Hypercarbia reduced pHe but not pHi, at all developmental ages. At 70% of incubation, pHe was depressed by 0.324 pH units while pHi of brain, white muscle, and lung increased; heart, liver, and kidney pHi remained unchanged. At 90% of incubation, pHe was depressed by 0.352 pH units but heart pHi increased with no change in pHi of other tissues. Yearling exhibited a pHe reduction of 0.235 pH units but had no changes in pHi of any tissues. The results indicate common snapping turtles preferentially regulate pHi during development, but the degree of the response is reduced throughout development. This is the first time preferential pHi regulation has been identified in an amniote. These findings may provide insight into the evolution of acid-base homeostasis during development of amniotes, and vertebrates in general.

Spontaneous daily torpor and fasting-induced torpor in Djungarian hamsters are characterized by distinct patterns of metabolic rate

The Djungarian hamster is a rodent species that expresses both spontaneous daily torpor (SDT) when acclimated to winter conditions as well as fasting-induced torpor (FIT) during summer. In an earlier report we argued that these two thermoregulatory phenomena differ in several parameters. In the present study, we further complete this comparison by showing that metabolic rate patterns differ between both SDT and FIT. SDT bouts were significantly longer and deeper compared to FIT bouts. Additionally, respiratory quotient measures support the view that SDT is entered from a state of energetic balance while FIT appears to be an emergency shutdown of energy demanding thermogenesis due to a shortage of energy sources. In a second experiment, we also confirm that brief periods of food restriction during the hamsters' torpor season increase the frequency of SDT, but do not affect its depth or duration. Although winter-acclimated animals could flexibly alter torpor frequency in order to stay in energetic balance, we also found evidence for torpor expression patterns that resembled FIT, rather than SDT. Consequently, if energetic challenges cannot be compensated with increased SDT expression any longer, the hamsters seem to be driven in a negative energy balance resulting in FIT as a last resort.

Metabolic adjustments during daily torpor in the Djungarian hamster

Djungarian hamsters (Phodopus sungorus) acclimated to a short photoperiod (8:16-h light-dark cycle) display spontaneous daily torpor with ad libitum food availability. The time course of body temperature (Tb), metabolic rate, respiratory quotient (RQ), and substrate and enzyme changes was measured during entrance into torpor and in deep torpor. RQ, blood glucose, and serum lipids are high during the first hours of torpor but then gradually decline, suggesting that glucose is the primary fuel during the first hours of torpor, with a gradual change to lipid utilization. No major changes in enzyme activities were observed during torpor except for inactivation of the pyruvate dehydrogenase (PDH) complex in liver, brown adipose tissue, and heart muscle. PDH inactivation closely correlates with the reduction of total metabolic rate, whereas in brain, kidney, diaphragm, and skeletal muscle, PDH activity was maintained at the initial level. These findings suggest inhibition of carbohydrate oxidation in heart, brown adipose tissue, and liver during entrance into daily torpor.

Glycolytic enzyme binding during entrance to daily torpor in deer mice (Peromyscus maniculatus)

Associations of glycolytic enzymes with the subcellular particulate fraction of skeletal muscle and heart were examined during entrance to daily torpor in deer mice (Peromyscus maniculatus). In skeletal muscle a significant decrease in enzyme binding occurred during torpor entrance for phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate kinase, with an additional significant decrease for phosphofructokinase and pyruvate kinase during the deepest state of torpor. Reductions in enzyme binding during torpor entrance also occurred in heart; significant changes were observed in hexokinase, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate kinase binding. Contrary to the finding of additional reductions in enzyme binding seen in skeletal muscle, significant increases in enzyme binding during the deepest torpor state were observed for hexokinase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase, pyruvate kinase, and lactate dehydrogenase in heart. These results suggest that a decrease in the binding of glycolytic enzymes to subcellular structures in skeletal muscle and heart may be at least partially responsible for initiating the reduction in metabolic rate during torpor entrance. This decreased binding may continue to mediate the metabolic reduction in skeletal muscle throughout torpor; heart, however, may require the use of different molecular mechanisms. The increased binding in heart during the deepest state of torpor may represent an anticipatory response in preparation for increased activity during arousal.

Effectors of metabolic depression in an estivating pulmonate snail (Helix aspersa): whole animal and in vitro tissue studies

We have examined metabolic depression in the land snail (Helix aspersa) during estivation, and have developed a tissue model of metabolic depression using an in vitro mantle preparation. The metabolic rate of H. aspersa is depressed by 84% in vivo within 4 weeks of onset of estivation, and this metabolic depression is accompanied by a decrease in haemolymph PO2 and pH, and an increase in haemolymph PCO2. The in vitro mantle preparation has a stable O2 consumption and energy charge, and an energy charge similar to that of mantle in vivo. The in vitro mantle is an O2-conforming tissue, with VO2 varying curvilinearly with PO2. Consequently, we have developed a mathematical method of calculating tissue VO2 at any PO2. These calculations show that under appropriate incubation conditions of pH and PO2, the mantle from estivating animals shows a stable in vitro metabolic depression of 48% compared to mantle from control snails. The extrinsic effects of PO2 and pH account for 70% of the total in vitro metabolic depression of mantle tissue; intrinsic effectors contribute a further 30%.

Effects of moderate hypercapnia on hypothermia induced by cold He-O2 in rats

1. Effects of moderate hypercapnia (10% CO2) on rectal temperature, oxygen consumption and body weight loss were examined during and after acute hypothermia induced by cold and helium-oxygen. 2. Hypothermia induction time was reduced significantly by hypercapnia. Rewarming tended to be faster in hypercapnic animals than in normocapnic animals. 3. Hypercapnia significantly reduced body weight loss when measured during hypothermia and during normothermia after rewarming. 4. Oxygen consumption during cooling was decreased by hypercapnia. 5. Exposure to 10% CO2 during cooling may spare energy substrate and favor survival in hypothermia.

Tissue-specific metabolism during normothermy and daily torpor in deer mice (Peromyscus maniculatus)

Previous work with deer mice (Peromyscus maniculatus) has demonstrated that a significant acidosis occurs during daily torpor. In addition, carbohydrate levels are significantly lower, whereas fatty acid and ketone levels are significantly higher during torpor. The present study examined the effects of these in vivo acid-base and metabolite adjustments on in vitro 14C-glucose metabolism in tissues taken from normothermic and torpid deer mice. Glucose oxidation in liver and vastus lateralis taken from normothermic animals was reduced by a change in incubation temperature from 37 degrees C to 25 degrees C (liver, 0.44 to 0.23 mumoles/g.h; vastus, 0.66 to 0.25 mumoles/g.h), whereas heart from normothermic mice exhibited an increase from 0.57 to 0.99 mumoles/g.h at the lower temperature. Altering acid-base conditions or metabolite levels had no effect on glucose metabolism in the heart or liver. However, both of these factors significantly influenced metabolism in vastus. Vastus taken from normothermic mice had an increased glucose oxidation rate under the more acidic torpor conditions (0.25 to 0.33 mumoles/g.h), whereas a reduction in oxidation occurred when incubated with torpor substrate concentrations (0.33 to 0.22 mumoles/g.h). In vastus taken from torpid mice, changing acid-base state and metabolites to torpor levels resulted in a significant reduction in glucose oxidation (0.26 to 0.12 mumoles/g.h). Under most incubation conditions, glucose oxidation was significantly lower in tissues taken from torpid mice than in these tissues from normothermic mice, suggesting that adjustments in addition to changes in acid-base and substrate parameters may be necessary to account for metabolic modifications during daily torpor.