Cold acclimation in Peromyscus: individual variation and sex effects in maximum and daily metabolism, organ mass and body composition

Aqp5-/- mice exhibit reduced maximal body O2 consumption under cold exposure, normal pulmonary gas exchange, and impaired formation of brown adipose tissue

The fundamental body functions that determine maximal O₂ uptake (V̇o_2max) have not been studied in Aqp5^-/- mice (aquaporin 5, AQP5). We measured V̇o_2max to globally assess these functions and then investigated why it was found altered in Aqp5^-/- mice. V̇o_2max was measured by the Helox technique, which elicits maximal metabolic rate by intense cold exposure of the animals. We found V̇o_2max reduced in Aqp5^-/- mice by 20%-30% compared with wild-type (WT) mice. As AQP5 has been implicated to act as a membrane channel for respiratory gases, we studied whether this is caused by the known lack of AQP5 in the alveolar epithelial membranes of Aqp5^-/- mice. Lung function parameters as well as arterial O₂ saturation were normal and identical between Aqp5^-/- and WT mice, indicating that AQP5 does not contribute to pulmonary O₂ exchange. The cause for the decreased V̇o_2max thus might be found in decreased O₂ consumption of an intensely O₂-consuming peripheral organ such as activated brown adipose tissue (BAT). We found indeed that absence of AQP5 greatly reduces the amount of interscapular BAT formed in response to 4 wk of cold exposure, from 63% in WT to 25% in Aqp5^-/- animals. We conclude that lack of AQP5 does not affect pulmonary O₂ exchange, but greatly inhibits transformation of white to brown adipose tissue. As under cold exposure, BAT is a major source of the animals' heat production, reduction of BAT likely causes the decrease in V̇o_2max under this condition.

Phenotypic plasticity to chronic cold exposure in two species of Peromyscus from different environments

Effective thermoregulation is important for mammals, particularly those that remain winter-active. Adjustments in thermoregulatory capacity in response to chronic cold can improve capacities for metabolic heat production (cold-induced maximal oxygen consumption, [Formula: see text]), minimize rates of heat loss (thermal conductance), or both. This can be challenging for animals living in chronically colder habitats where necessary resources (i.e., food, O₂) for metabolic heat production are limited. Here we used lowland native white-footed mice (Peromyscus leucopus) and highland deer mice (P. maniculatus) native to 4300 m, to test the hypothesis that small winter-active mammals have evolved distinct cold acclimation responses to tailor their thermal physiology based on the energetic demands of their environment. We found that both species increased their [Formula: see text] after cold acclimation, associated with increases in brown adipose tissue mass and expression of uncoupling protein 1. They also broadened their thermoneutral zone to include lower ambient temperatures. This was accompanied by an increase in basal metabolic rate but only in white-footed mice, and neither species adjusted thermal conductance. Unique to highland deer mice was a mild hypothermia as ambient temperatures decreased, which reduced the gradient for heat loss, possibly to save energy in the chronically cold high alpine. These results highlight that thermal acclimation involves coordinated plasticity of numerous traits and suggest that small, winter-active mammals may adjust different aspects of their physiology in response to changing temperatures to best suit their energetic and thermoregulatory needs.

Effects of short- and long-term cold acclimation on morphology, physiology, and exercise performance of California mice (Peromyscus californicus): potential modulation by fatherhood

California mice (Peromyscus californicus) differ from most other mammals in that they are biparental, genetically monogamous, and (compared with other Peromyscus) relatively large. We evaluated effects of cold acclimation on metabolic rate, exercise performance, and morphology of pair-housed male California mice, as well as modulation of these effects by fatherhood. In Experiment 1, virgin males housed at 5° or 10 °C for approximately 25 days were compared with virgins housed at standard vivarium temperature of 22 °C. Measures included resting metabolic rate (RMR), maximal oxygen consumption ([Formula: see text]max), grip strength, and sprint speed. In Experiment 2, virgin males housed at 22 °C were compared with three groups of males housed at 10 °C: virgins, breeding males (housed with a female and their pups), and non-breeding males (housed with an ovariectomized, estrogen- and progesterone-treated female) after long-term acclimation (mean 243 days). Measures in this experiment included basal metabolic rate (BMR), [Formula: see text]max, maximal thermogenic capacity ([Formula: see text]sum), and morphological traits. In Experiment 1, virgin males housed at 5° and 10 °C had higher RMR and [Formula: see text]max than those at 22 °C. In Experiment 2, 10 °C-acclimated groups had shorter bodies; increased body, fat, and lean masses; higher BMR and [Formula: see text]sum, and generally greater morphometric measures and organ masses than virgin males at 22 °C. Among the groups housed at 10 °C, breeding males had higher BMR and lower [Formula: see text]max than non-breeding and/or virgin males. Overall, we found that effects of fatherhood during cold acclimation were inconsistent, and that several aspects of cold acclimation differ substantially between California mice and other small mammals.

Diet composition modulates intestinal hydrolytic enzymes in white-footed mice (Peromyscus leucopus)

We tested whether white-footed mice (Peromyscus leucopus) modulate the activity of three key intestinal digestive enzymes (maltase, sucrase, and aminopeptidase-N [APN]) based on diet composition. To test the adaptive modulation hypothesis (AMH), we fed mice either of three kinds of synthetic diet, high starch (HS, 50% carbohydrate), high protein (HP, 60% protein), and high lipid (HL, 25% lipid), and determined their digestive responses. First, there was no effect of either diet itself, or time eating the diet, on body mass, or mass and length of small intestine. Second, the activity of both disaccharidases summed over the entire small intestine was highest on the HS diet, which was higher than on the HP diet by about 45% and higher than on the HL diet by 400%. This was consistent with our prediction that starch induces disaccharidase activity, and demonstrated induction of disaccharidase activities by high dietary carbohydrate in a wild mammal. Third, both summed and mass-specific activity of maltase and sucrase of HL mice were lower than those of HP mice, even though their diets had the same content of starch, which suggests that lipid in the HL diet inhibited disaccharidase activity. Finally, the summed activity of APN was highest on the HP diet, which was higher than on the HS diet or HL diet by ~100%, consistent with our prediction that high protein content induces peptidase activity. Taken together, our results support the AMH, though they also illustrate that high lipid content in the diet can confound some predicted patterns. Flexibility of digestive enzyme activity is likely important in allowing white-footed mice to cope with fluctuations in the environmental availability of different food types.

Probamos si el ratón de patas blancas (Peromyscus leucopus) modula las actividades de tres enzimas digestivas intestinales claves – maltasa, sacarasa y N-aminopeptidasa- al modificarse la composición de la dieta. Para someter a prueba la hipótesis de la modulación adaptativa, se alimentaron paralelamente ratones con tres tipos de dietas semi-sintéticas, una alta en almidón (HS, 50% carbohidratos), otra alta en proteína (HP, 60% proteínas), y una alta en lípidos (HL, 25% lípidos), y se determinaron sus respuestas digestivas. No se observó un efecto de la dieta o del tiempo que la consumieron sobre la masa corporal o la masa y el largo del intestino delgado (SI). La sumatoria de las actividades de cada una de las disacaridasas a lo largo de todo el intestino delgado fue más alta con la dieta HS que con las dietas HP y HL, un 45% y un 400% mayor, respectivamente. Esto fue consistente con nuestra predicción acerca de que el almidón induce la actividad disacaridásica, constituyendo el primer estudio que demuestra inequívocamente en un animal silvestre, que la inducción de las actividades de las disacaridasas intestinales es mediada por un incremento de los carbohidratos en la dieta. Las actividades hidrolíticas totales y masa-específicas de la maltasa y sacarasa de los ratones HL fueron más bajas que las de los alimentados con dieta HP, aun cuando sus dietas tenían el mismo contenido de almidón, lo que sugiere que los lípidos en la dieta HL inhiben la actividad de las disacaridasas. La actividad hidrolítica total de la N-aminopeptidasa fue mayor con la dieta HP, ~100% más alta que para las dietas HS y HL, de manera consistente con la predicción que propone que la presencia de mayor cantidad de proteína en la dieta induce la actividad peptidásica. En conjunto nuestros resultados dan soporte a la hipótesis de la modulación adaptativa, además de ilustrar que los lípidos en las dietas pueden confundir la predicción de patrones de procesamiento de alimentos. La flexibilidad de la actividad de las enzimas digestivas es probablemente importante para los ratones de patas blancas, ya que les permite adecuarse a las fluctuaciones ambientales de disponibilidad de diferentes tipos de recursos.

More than just the numbers-contrasting response of snake erythrocytes to thermal acclimation

Acclimation to lower temperatures decreases energy expenditure in ectotherms but increases oxygen consumption in most endotherms, when dropped below thermoneutrality. Such differences should be met by adjustments in oxygen transport through blood. Changes in hematological variables in correspondence to that in metabolic rates are, however, not fully understood, particularly in non-avian reptiles. We investigated the effect of thermal acclimation on a snake model, the grass snakes (Natrix natrix). After 6 months of acclimation to either 18 °C or 32 °C hematocrit, hemoglobin concentration, erythrocyte number, and size were assessed. All variables revealed significantly lower values under warm compared to cold ambient temperature. Our data suggest that non-avian reptiles, similarly as birds, reduce erythrocyte fraction under energy-demanding temperatures. Due to low deformability of nucleated erythrocytes in sauropsids, such reduced fraction may be important in decreasing blood viscosity to optimize blood flow. Novel findings on flexible erythrocyte size provide an important contribution to this optimization process.

Acclimatization of low altitude-bred deer mice ( Peromyscus maniculatus) to high altitude

A colony of deer mice subspecies ( Peromyscus maniculatus sonoriensis) native to high altitude (HA) has been maintained at sea level for 18-20 generations and remains genetically unchanged. To determine if these animals retain responsiveness to hypoxia, one group (9-11 wk old) was acclimated to HA (3,800 m) for 8 wk. Age-matched control animals were acclimated to a lower altitude (LA; 252 m). Maximal O₂ uptake (V̇o_2max) was measured at the respective altitudes. On a separate day, lung volume, diffusing capacity for carbon monoxide (DL_CO), and pulmonary blood flow were measured under anesthesia using a rebreathing technique at two inspired O₂ tensions. The HA-acclimated deer mice maintained a normal V̇o_2max relative to LA baseline. Compared with LA control mice, antemortem lung volume was larger in HA mice in a manner dependent on alveolar O₂ tension. Systemic hematocrit, pulmonary blood flow, and standardized DL_CO did not differ significantly between groups. HA mice showed a higher postmortem alveolar-capillary hematocrit, larger alveolar ducts, and smaller distal conducting structures. In HA mice, absolute volumes of alveolar type I epithelia and endothelia were higher whereas that of interstitia was lower than in LA mice. These structural changes occurred without a net increase in whole-lung septal tissue-capillary volumes or surface areas. Thus, deer mice bred and raised to adulthood at LA retain phenotypic plasticity and adapt to HA without a decrement in V̇o_2max via structural (enlarged airspaces, alveolar septal remodeling) and nonstructural (lung expansion under hypoxia) mechanisms and without an increase in systemic hematocrit or compensatory lung growth. NEW & NOTEWORTHY Deer mice ( Peromyscus maniculatus) are robust and very active mammals that are found across the North American continent. They are also highly adaptable to extreme environments. When introduced to high altitude they retain remarkable adaptive ability to the low-oxygen environment via lung expansion and remodeling of existing lung structure, thereby maintaining normal aerobic capacity without generating more red blood cells or additional lung tissue.

Solar radiation during rewarming from torpor in elephant shrews: supplementation or substitution of endogenous heat production?

Many small mammals bask in the sun during rewarming from heterothermy, but the implications of this behaviour for their energy balance remain little understood. Specifically, it remains unclear whether solar radiation supplements endogenous metabolic thermogenesis (i.e., rewarming occurs through the additive effects of internally-produced and external heat), or whether solar radiation reduces the energy required to rewarm by substituting (i.e, replacing) metabolic heat production. To address this question, we examined patterns of torpor and rewarming rates in eastern rock elephant shrews (Elephantulus myurus) housed in outdoor cages with access to either natural levels of solar radiation or levels that were experimentally reduced by means of shade cloth. We also tested whether acclimation to solar radiation availability was manifested via phenotypic flexibility in basal metabolic rate (BMR), non-shivering thermogenesis (NST) capacity and/or summit metabolism (M_sum). Rewarming rates varied significantly among treatments, with elephant shrews experiencing natural solar radiation levels rewarming faster than conspecifics experiencing solar radiation levels equivalent to approximately 20% or 40% of natural levels. BMR differed significantly between individuals experiencing natural levels of solar radiation and conspecifics experiencing approximately 20% of natural levels, but no between-treatment difference was evident for NST capacity or M_sum. The positive relationship between solar radiation availability and rewarming rate, together with the absence of acclimation in maximum non-shivering and total heat production capacities, suggests that under the conditions of this study solar radiation supplemented rather than substituted metabolic thermogenesis as a source of heat during rewarming from heterothermy.

The relationship between cardiopulmonary size and aerobic performance in adult deer mice at high altitude

Deer mice (Peromyscus maniculatus sonoriensis) populations in the White Mountains of Eastern California are found across a substantial range of partial pressures of oxygen (PO₂). Reduction in PO₂ at high altitude can have a negative impact on aerobic performance. We studied plastic changes in organ mass and volume involved in aerobic respiration in response to acclimation to high altitude, and how those changes are matched with aerobic performance measured by VO₂,max. Adult deer mice born and raised at 340 m were acclimated at either 340 or 3800 m for a period of 9 weeks. Lung volume increased by 9% in mice acclimated to high altitude. VO₂,max was also significantly higher under hypoxic conditions after high altitude acclimation compared with controls. Body mass-corrected residuals of VO₂,max were significantly correlated with an index of cardiopulmonary size (summed standardized residuals of lung volume and heart mass) under both hypoxic and normoxic conditions. These data show that phenotypic plasticity in lung volume and heart mass plays an important role in maintaining aerobic performance under hypoxic conditions, and accounts for up to 55% of the variance in aerobic performance.

Phenotypic plasticity in blood-oxygen transport in highland and lowland deer mice

In vertebrates living at high altitude, arterial hypoxemia may be ameliorated by reversible changes in the oxygen-carrying capacity of the blood (regulated by erythropoiesis) and/or changes in blood-oxygen affinity (regulated by allosteric effectors of hemoglobin function). These hematological traits often differ between taxa that are native to different elevational zones, but it is often unknown whether the observed physiological differences reflect fixed, genetically based differences or environmentally induced acclimatization responses (phenotypic plasticity). Here, we report measurements of hematological traits related to blood-O2 transport in populations of deer mice (Peromyscus maniculatus) that are native to high- and low-altitude environments. We conducted a common-garden breeding experiment to assess whether altitude-related physiological differences were attributable to developmental plasticity and/or physiological plasticity during adulthood. Under conditions prevailing in their native habitats, high-altitude deer mice from the Rocky Mountains exhibited a number of pronounced hematological differences relative to low-altitude conspecifics from the Great Plains: higher hemoglobin concentrations, higher hematocrits, higher erythrocytic concentrations of 2,3-diphosphoglycerate (an allosteric regulator of hemoglobin-oxygen affinity), lower mean corpuscular hemoglobin concentrations and smaller red blood cells. However, these differences disappeared after 6 weeks of acclimation to normoxia at low altitude. The measured traits were also indistinguishable between the F1 progeny of highland and lowland mice, indicating that there were no persistent differences in phenotype that could be attributed to developmental plasticity. These results indicate that the naturally occurring hematological differences between highland and lowland mice are environmentally induced and are largely attributable to physiological plasticity during adulthood.

Contributions of phenotypic plasticity to differences in thermogenic performance between highland and lowland deer mice

Small mammals face especially severe thermoregulatory challenges at high altitude because the reduced O2 availability constrains the capacity for aerobic thermogenesis. Adaptive enhancement of thermogenic performance under hypoxic conditions may be achieved via physiological adjustments that occur within the lifetime of individuals (phenotypic plasticity) and/or genetically based changes that occur across generations, but their relative contributions to performance differences between highland and lowland natives are unclear. Here, we examined potentially evolved differences in thermogenic performance between populations of deer mice (Peromyscus maniculatus) that are native to different altitudes. The purpose of the study was to assess the contribution of phenotypic plasticity to population differences in thermogenic performance under hypoxia. We used a common-garden deacclimation experiment to demonstrate that highland deer mice have enhanced thermogenic capacities under hypoxia, and that performance differences between highland and lowland mice persist when individuals are born and reared under common-garden conditions, suggesting that differences in thermogenic capacity have a genetic basis. Conversely, population differences in thermogenic endurance appear to be entirely attributable to physiological plasticity during adulthood. These combined results reveal distinct sources of phenotypic plasticity for different aspects of thermogenic performance, and suggest that thermogenic capacity and endurance may have different mechanistic underpinnings.