Effect of cold acclimation on oxidative capacity and respiratory properties of liver and muscle mitochondria in ducklings, Cairina moschata

Ontogeny of muscle bioenergetics in Adélie penguin chicks (Pygoscelis adeliae)

The ontogeny of pectoralis muscle bioenergetics was studied in growing Adélie penguin chicks during the first month after hatching and compared with adults using permeabilized fibers and isolated mitochondria. With pyruvate-malate-succinate or palmitoyl-carnitine as substrates, permeabilized fiber respiration markedly increased during chick growth (3-fold) and further rose in adults (1.4-fold). Several markers of muscle fiber oxidative activity (cytochrome oxidase, citrate synthase, hydroxyl-acyl-CoA dehydrogenase) increased 6- to 19-fold with age together with large rises in intermyofibrillar (IMF) and subsarcolemmal (SS) mitochondrial content (3- to 5-fold) and oxidative activities (1.5- to 2.4-fold). The proportion of IMF relative to SS mitochondria increased with chick age but markedly dropped in adults. Differences in oxidative activity between mitochondrial fractions were reduced in adults compared with hatched chicks. Extrapolation of mitochondrial to muscle respirations revealed similar figures with isolated mitochondria and permeabilized fibers with carbohydrate-derived but not with lipid-derived substrates, suggesting diffusion limitations of lipid substrates with permeabilized fibers. Two immunoreactive fusion proteins, mitofusin 2 (Mfn2) and optic atrophy 1 (OPA1), were detected by Western blots on mitochondrial extracts and their relative abundance increased with age. Muscle fiber respiration was positively related with Mfn2 and OPA1 relative abundance. Present data showed by two complementary techniques large ontogenic increases in muscle oxidative activity that may enable birds to face thermal emancipation and growth in childhood and marine life in adulthood. The concomitant rise in mitochondrial fusion protein abundance suggests a role of mitochondrial networks in the skeletal muscle processes of bioenergetics that enable penguins to overcome harsh environmental constraints.

Differential effects of thyroid status on regional H₂O₂ production in slow- and fast-twitch muscle of ducklings

Birds seem to employ powerful physiological strategies to curb the harmful effects of reactive oxygen species (ROS) because they generally live longer than predicted by the free radical theory of aging. However, little is known about the physiological mechanisms that confer protection to birds against excessive ROS generation. Hence, we investigated the ability of birds to control mitochondrial ROS generation during physiologically stressful periods. In our study, we analyzed the relationship between the thyroid status and the function of intermyofibrillar and subsarcolemmal mitochondria located in glycolytic and oxidative muscles of ducklings. We found that the intermyofibrillar mitochondria of both glycolytic and oxidative muscles down regulate ROS production when plasma T₃ levels rise. The intermyofibrillar mitochondria of the gastrocnemius muscle (an oxidative muscle) produced less ROS and were more sensitive than the pectoralis muscle (a glycolytic muscle) to changes in plasma T₃. Such differences in the ROS production by glycolytic and oxidative muscles were associated with differences in the membrane proton permeability and in the rate of free radical leakage within the respiratory chain. This is the first evidence which shows that in birds, the amount of ROS that the mitochondria release is dependent on: (1) their location within the muscle; (2) the type of muscle (glycolytic or oxidative) and (3) on the thyroid status. Reducing muscle mitochondrial ROS generation might be an important mechanism in birds to limit oxidative damage during periods of physiological stress.

Acclimation to different thermal conditions in a northerly wintering shorebird is driven by body mass-related changes in organ size

Seasonal acclimatization and experimental acclimation to cold in birds typically results from increased shivering endurance and elevated thermogenic capacity leading to improved resistance to cold. A wide array of physiological adjustments, ranging from biochemical transformations to organ mass variations, are involved in this process. Several studies have shown that improved cold endurance is accompanied by increases in summit metabolic rate(Msum), a measure of maximal heat production and an indicator of the level of sustainable thermogenic capacity. However, improved endurance to cold can also be achieved without significant changes in Msum. The same is true for basal metabolic rate (BMR),which is known to increase in association with cold acclimatization or acclimation in some species but not in others. We investigated cold acclimation in a migrant shorebird known for extreme physiological flexibility, the red knot (Calidris canutus, the northerly wintering subspecies islandica). We measured BMR and Msumover two months in birds caught in the wild and transferred to experimentally controlled conditions representative of aspects of their seasonal thermal environment (two groups at constant 25°C, one group at constant 4°C and two groups experiencing variable outdoor temperatures). Birds maintained in both cold and variable ambient temperatures showed a 14-15% higher body mass, 33-45% higher food intake, and 26% and 13% elevations in BMR and Msum, respectively, compared with birds kept at thermoneutrality. These results, together with data on alimentary tract size and pectoral muscle thickness measured by ultrasonography, suggest that red knots acclimate to cold primarily through modulation of (lean) body mass components. Heavier individuals have larger muscles, which allow higher maximal heat production and better thermal compensation. Cold acclimation effects on BMR are most probably due to changes in the size of visceral organs, although not the alimentary tract in this specific case. The liver,known for its thermogenic capacity, is a probable candidate. Overall, our results indicate that relatively small changes in body mass and muscle size allow enough reserve capacity in terms of heat production to cope with typical wintering ambient temperature variations as measured on the red knot's wintering grounds.

Chronic cold exposure increases liver oxidative capacity in the marsupial Monodelphis domestica

Marsupials lack brown adipose tissue, and therefore rely exclusively on other tissues for thermogenesis. To determine the magnitude of phenotypic plasticity of the liver in response to changing metabolic demand, gray short-tailed opossums (M. domestica) were exposed to thermoneutral (28 degrees C) or cold (9-12 degrees C) conditions continuously for 6 weeks. Half of each group was also endurance trained with a treadmill program during their respective temperature exposure. Mass specific summit metabolism (VO(2)summit) increased 11% following cold acclimation, though there was no significant main effect by training on VO(2)summit. To estimate the contribution of the liver to whole animal oxidative activity, we determined liver mass, mitochondrial volume density, and total mitochondrial volume. Relative liver mass was 48% greater in cold-acclimated animals, whereas training had no effect on liver mass. The stereological analysis of hepatocyte ultrastructure suggests the percentage of intracellular volumes remained unchanged in response to either aerobic challenge. Thus, following cold-acclimation, there is a 20% increase in the total mitochondrial volume of the liver. This increase could account for nearly half (44%) of the observed increase in whole animal VO(2)summit following cold exposure.