Role of glucagon in the control of heat production in ducklings.

Glucagon is known to be a central modulator of neural activity and a peripheral thermogenic effect. The purpose of this study was to better understand the role of glucagon in the control of heat production, shivering and particularly as a mediator of nonshivering thermogenesis (NST) in ducklings. In order to study the mechanism of NST, an intracerebroventricular (i.c.v.) injection of glucagon (10(-7) M) in to thermoneutral (TN), chronically glucagon treated (GT) and cold acclimatized (CA) ducklings exposed to acute cold (4 degrees C) or a thermoneutrality (25 degrees C), was performed. At 25 degrees C ambient temperature (Ta), the metabolic rate (MR) remained unchanged after glucagon injection. At 4 degrees C Ta i.c.v. glucagon injection, no significant change in MR was observed in GT and CA ducklings during 160 min of cold exposure, whereas there was 63% decrease in MR in (TN) ducklings (5.02 +/- 0.1 2 vs 7.91 +/- 0.1 4 W/kg(-1) p < 0.05). Shivering activity was completely suppressed in TN and GT ducklings after glucagon administration. The NST was estimated to be 3.26 W/kg. This findings suggest that glucagon administered into the brain has no thermogenic effect but could be involved in the central control of somatic motricity, and here we demonstrated for the first time, of our knowledge, that central glucagon have a role in the development of nonshivering thermogenesis during prolonged cold via an inhibition of shivering in birds.

Role of catecholamines in glucagon-induced thermogenesis.

The present work was undertaken in order to investigate whether the observed thermogenesis following glucagon injection requires the participation of catecholamines. Our experiments aim at studying the effects of intraperitoneal injection of glucagon on metabolic rates, plasma catecholamine and fuel metabolites in guanethidine-treated ducklings reared at thermoneutrality (25 degrees C). The chronic guanethidine treatment induced a marked decrease in catecholamines levels in peripheral tissues (heart, muscle and intestine) but not in adrenals. At thermoneutrality, intraperitoneal injection of glucagon had lower thermogenic effects in guanethidine-treated compared to control ducklings. Glucagon injection elicited a concomitant increase of plasma norepinephrine, metabolic rate and energy metabolites in control ducklings, whereas in guanethidine-treated ducklings, the plasma catecholamines and metabolic rate did not undergo any consistent change. The thermogenic action of glucagon in birds involves at least the mobilization of lipids and catecholaminergic system stimulation.

Use of 137Cs measurements to estimate changes in soil erosion rates associated with changes in soil management practices on cultivated land.

Intensification of agricultural production in south-central Chile since the 1970s has caused problems of increased soil erosion and associated soil degradation. These problems have prompted a shift from conventional tillage to no-till management practices. Faced with the need to establish the impact of this shift in soil management on rates of soil loss, the use of caesium-137 (137Cs) measurements has been explored. A novel procedure for using measurements of the 137Cs depth distribution to estimate rates of soil loss at a sampling point under the original conventional tillage and after the shift to no-till management has been developed. This procedure has been successfully applied to a study site at Buenos Aires farm near Carahue in the 9th region of Chile. The results obtained indicate that the shift from conventional tillage to no-till management has caused net rates of soil loss to decrease to about 40% of those existing under conventional tillage. This assessment of the impact of introducing no-till management at the study site must, however, be seen as provisional, since only a limited number of sampling points were used. A simplified procedure aimed at documenting the reduction in erosion rates at additional sampling points, based solely on measurements of the 137Cs inventory of bulk cores and the 137Cs activity in the upper part of the soil has been developed and successfully tested at the study site. Previous application of 137Cs measurements to estimate erosion rates has been limited to estimation of medium-term erosion rates during the period extending from the beginning of fallout receipt to the time of sampling. The procedures described in this paper, which permits estimation of the change in erosion rates associated with a shift in land management practices, must be seen as representing a novel application of 137Cs measurements in soil erosion investigations.

Effect of voluntary exercise on H2O2 release by subsarcolemmal and intermyofibrillar mitochondria.

Previous data have demonstrated that, to handle the oxidative stress encountered with training at high intensity, skeletal muscle relies on an increase in mitochondrial biogenesis, a reduced H(2)O(2) production, and an enhancement of antioxidant enzymes. In the present study, we evaluated the influence of voluntary running on mitochondrial O(2) consumption and H(2)O(2) production by intermyofibrillar mitochondria (IFM) and subsarcolemmal mitochondria (SSM) isolated from oxidative muscles in conjunction with the determination of antioxidant capacities. When mitochondria are incubated with succinate as substrate, both maximal (state 3) and resting (state 4) O(2) consumption were significantly lower in SSM than in IFM populations. Mitochondrial H(2)O(2) release per unit of O(2) consumed was 2-fold higher in SSM than in IFM. Inhibition of H(2)O(2) formation by rotenone suggests that complex I of the electron transport chain is likely the major physiological H(2)O(2)-generating system. In Lou/C rats (an inbred strain of rats of Wistar origin), neither O(2) consumption nor H(2)O(2) release by IFM and SSM were affected by long-term, voluntary wheel training. In contrast, glutathione peroxidase and catalase activity were significantly increased despite no change in oxidative capacities with long-term, voluntary exercise. Furthermore, chronic exercise enhanced heat shock protein 72 accumulation within skeletal muscle. It is concluded that the antioxidant status of muscle can be significantly improved by prolonged wheel exercise without necessitating an increase in mitochondrial oxidative capacities.

The heart rate/oxygen consumption relationship during cold exposure of the king penguin: a comparison with that during exercise.

This study investigated whether exposure to low ambient temperature could be used as an alternative to exercise for calibrating heart rate (fH) against rate of oxygen consumption ((O(2))) for subsequent use of fH to estimate (O(2)) in free-ranging animals. Using the relationship between the oxygen pulse (OP, the amount of oxygen used per heart beat) and an index of body condition (or nutritional index, NI), a relationship between fH and (O(2)) was established for resting king penguins exposed to a variety of environmental temperatures. Although there was a small but significant increase in the OP above and below the lower critical temperature (-4.9 degrees C), there was no difference in the relationship obtained between the OP and body condition (NI) obtained above or below the lower critical temperature. These results were then compared with those obtained in a previous study in which the relationship between fH and (O(2)) had been established for king penguins during steady-state exercise. The relationship between OP and NI in the present study was not significantly different from the relationship between resting OP and NI in the previous study. However, the relationship was different from that between active OP and NI. We conclude that, at least for king penguins, although thermoregulation does not affect the relationship between resting OP and NI, temperature cannot be used as an alternative to exercise for calibrating fH against (O(2)) for subsequent use of fH to estimate (O(2)) in free-ranging animals.

Responsiveness of plasma catecholamines to intracerebroventricular injection of glucagon in Muscovy ducklings.

Recent investigations have demonstrated a modulatory action of glucagon on shivering via the central nervous system in ducklings. The aim of this study was to investigate the effects of intracerebroventricular injection (i.c.v.) of glucagon on metabolic rates (MR) and plasma catecholamines in control ducklings (TN) and in ducklings exhibiting nonshivering thermogenesis after chronic glucagon treatment: (GT). At thermoneutrality (25 degrees C, Ta), i.c.v. injection of glucagon had no thermogenic effects in TN and GT ducklings. At cold (+4 degrees C, Ta), i.c.v. glucagon injection elicited a concomitant decrease of plasma norepinephrine (NE) and MR in TN ducklings, whereas in GT ducklings, the plasma catecholamines and the MR remained unchanged. These results indicate that glucagon treatment rendered the catecholaminergic system of GT ducklings insensitive to cold or i.c.v. glucagon injection.

Cold acclimation or grapeseed oil feeding affects phospholipid composition and mitochondrial function in duckling skeletal muscle.

The phospholipid fatty acid (FA) composition and functional properties of skeletal muscle and liver mitochondria were examined in cold-acclimated (CA, 4 degrees C) ducklings. Phospholipid FA of isolated muscle mitochondria from CA birds were longer and more unsaturated than those from thermoneutral (TN, 25 degrees C) reared ducklings. The rise in long-chain and polyunsaturated FA (PUFA, mainly 20:4n-6) was associated with a higher State 4 respiration rate and a lower respiratory control ratio (RCR). Hepatic mitochondria, by contrast, were much less affected by cold acclimation. The cold-induced changes in phospholipid FA profile and functional properties of muscle mitochondria were reproduced by giving TN ducklings a diet enriched in grapeseed oil (GO, rich in n-6 FA), suggesting a causal relationship between the membrane structure and mitochondrial functional parameters. However, hepatic mitochondria from ducklings fed the GO diet also showed an enrichment in long-chain PUFA but opposite changes in their biochemical characteristics (lower State 4, higher RCR). It is suggested that the differential modulation of mitochondrial functional properties by membrane lipid composition between skeletal muscle and liver may depend on muscle-specific factors possibly interacting with long-chain PUFA and affecting the proton leakiness of mitochondrial membranes.

Differential effects of endurance training and creatine depletion on regional mitochondrial adaptations in rat skeletal muscle.

To examine the combined effects of 2-week endurance training and 3-week feeding with beta-guanidinopropionic acid (GPA) on regional adaptability of skeletal muscle mitochondria, intermyofibrillar mitochondria (IFM) and subsarcolemmal mitochondria (SSM) were isolated from quadriceps muscles of sedentary control, trained control, sedentary GPA-fed and trained GPA-fed rats. Mitochondrial oxidative phosphorylation was assessed polarographically by using pyruvate plus malate, succinate (plus rotenone), and ascorbate plus N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) (plus antimycin) as respiratory substrates. Assays of cytochrome c oxidase and F(1)-ATPase activities were also performed. In sedentary control rats, IFM exhibited a higher oxidative capacity than SSM, whereas F(1)-ATPase activities were similar. Training increased the oxidative phosphorylation capacity of mitochondria with both pyruvate plus malate and ascorbate plus TMPD as substrates, with no differences between IFM and SSM. In contrast, the GPA diet mainly improved the overall SSM oxidative phosphorylation capacity, irrespective of the substrate used. Finally, the superimposition of training to feeding with GPA strongly increased both oxidase and enzymic activities in SSM, whereas no cumulative effects were found in IFM mitochondria. It therefore seems that endurance training and feeding with GPA, which are both known to alter the energetic status of the muscle cell, might mediate distinct biochemical adaptations in regional skeletal muscle mitochondria.

Effects of intracerebroventricular injections of des-His1 (Glu9) glucagon amide on the regulatory thermogenesis in muscovy ducklings.

Recent investigations have demonstrated a modulatory action of glucagon on shivering via the central nervous system in ducklings. Such an action could be mediated by glucagon receptors that have been recently detected in several brain areas involved in the central control of the involuntary motricity in this avian species. The present study using des-His1 (Glu9) glucagon amide, was performed to investigate the central mechanisms of glucagon on shivering. This glucagon analog was found to be an antagonist of glucagon devoid of adenylate cyclase activity (GR2) by triggering the breakdown of inositol phosphate (GR1) in mammals hepatocytes. The intracerebroventricular administration of des-His1 (Glu9) glucagon amide or glucagon induced a marked inhibition of shivering in ducklings exposed to cold. It seems likely that GR1 receptors contribute to decreased shivering in ducklings exposed to cold. Central glucagon or des-His1 (Glu9) glucagon amide were devoid of thermogenic effect at thermoneutrality.

Involvement of the catecholaminergic system in glucagon-induced thermogenesis in Muscovy ducklings (Cairina moschata).

Physiological studies have shown that glucagon is a potential mediator of nonshivering thermogenesis (NST) in birds. The present work was undertaken in order to investigate whether the observed thermogenesis results from a direct action of glucagon on avian thermoregulatory mechanisms or in fact requires the participation of other agents such as catecholamines. Our experiments were performed using cold-acclimated (CA) ducklings which developed muscle NST. A comparison was made with thermoneutral (TN) ducklings of the same age. Our principal results showed that: (1) at ambient temperature (25 degrees C), circulating norepinephrine (NE) was markedly decreased in CA ducklings (-42%), while circulating epinephrine (E) did not undergo any consistent change; (2) in CA and TN ducklings, an intraperitoneal injection of glucagon (360 microg x kg(-1)) was followed after 10 min by prominent lipolysis and a large increase in circulating NE (4- to 6-fold) and E (14- to 17-fold), which was sustained for at least 1 h. The elevation of circulating NE was less pronounced in CA ducklings. The thermogenic action of glucagon in birds is probably indirect and involves at least the mobilization of lipids and sympatho-adrenal stimulation. The changes in peripheral noradrenergic activity during cold acclimation could be associated with adaptive changes leading to NST.

Effects of cold acclimation and palmitate on energy coupling in duckling skeletal muscle mitochondria.

Gastrocnemius subsarcolemmal and intermyofibrillar mitochondria were isolated from 5-week-old cold-acclimated and thermoneutral control ducklings. In vitro respiration (polarography) and ATP synthesis (bioluminescence) were determined at 25 degrees C. Subsarcolemmal mitochondria showed a higher state 4 respiration and lower respiratory control and ADP/O ratio in cold-acclimated than in thermoneutral ducklings. Palmitate decreased the rate of ATP synthesis in both mitochondrial populations to about 30% of maximal but failed to abolish this process even at high concentrations. It is suggested that both expensive ATP synthesis and increased ATP hydrolysis could contribute synergistically to muscle non-shivering thermogenesis in cold-acclimated ducklings.

Increased in vitro fatty acid supply and cellular transport capacities in cold-acclimated ducklings (Cairina moschata).

In cold-acclimated (CA) birds, lipids play a crucial role in regulatory thermogenesis by acting both as substrates for and activators of thermogenic processes. The capacity to supply lipids to thermogenic tissues, which could limit cold thermogenesis, was assessed in CA ducklings (5 wk old, 4 degrees C) and compared with thermoneutral controls (TN, 25 degrees C). In CA ducklings, basal lipolytic activity of adipose tissue fragments was higher (202 +/- 9 vs. 130 +/- 14 nmol glycerol released . 100 mg tissue-1 . h-1, +55%) than in TN controls, while glucagon had a much higher stimulatory effect (+140 to +500% depending on dose). This was consistent with increased plasma levels of nonesterified fatty acids (FA, +57%) and glycerol (+31%) in vivo. In vitro endothelial lipase activity per organ was higher in CA than in TN ducklings in red gastrocnemius muscle (6.3 +/- 0.6 vs. 3.5 +/- 0.3 microeq nonesterified FA released per hour, +80%) and liver (+55%). The intracellular FA-binding capacity of (12-18 kDa) proteins was higher in gastrocnemius muscle (+43%) and liver (+74%) from CA ducklings. In gastrocnemius, it was linked to a higher content (21 +/- 2 vs. 15 +/- 2 microg/mg protein, +37%) of an intracellular 15.4-kDa FA-binding protein. These in vitro results indicate that coordinated increases in FA supply from adipose tissue, cellular uptake of lipoprotein-derived FA, and intracellular FA transport capacity occur in CA ducklings endowed with higher thermogenic capacity and cold endurance.

Inhibition of shivering thermogenesis by centrally applied glucagon in muscovy ducklings.

Glucagon has marked thermogenic and lipolytic effects in birds but could also be involved in the central modulation of neural activity on the basis of the recently discovered glucagon receptors in several areas of the brain in ducklings. The aim of this work was to investigate the possible role of these receptors in the modulation of thermogenic processes. Glucagon was infused into the lateral ventricle of the brain in ducklings after an acute cold exposure (4 degrees C, 2 h) or at thermoneutrality (25 degrees C). Electromyographic (EMG) data were simultaneously recorded with electrodes implanted in the gastrocnemius muscle. Glucagon (10(-4) M) was infused at a rate of 8 microliters/min. When acutely exposed to cold, ducklings increased their metabolic rate by shivering thermogenesis. A significant decrease in shivering activity was elicited after 5 min of glucagon infusion. After 16 +/- 2 min of glucagon infusion, shivering was completely inhibited, corresponding to a total dose of 36 +/- 4 micrograms/kg. The suppression of shivering was accompanied by a diminution of metabolic rate (5.3 +/- 0.3 vs. 8.5 +/- 0.2 W/kg, P < 0.05). The values of metabolic rate obtained at 4 degrees C after glucagon infusion were not significantly different from those measured at 25 degrees C before glucagon infusion (6.4 +/- 0.3 W/kg, P > 0.05). The infusion of the same dose of glucagon did not induce any change in EMG activity and resting metabolic rate at 25 degrees C. These findings suggest that glucagon infused into the brain has no thermogenic effect but could be involved in the central control of somatic motricity. Although the origin and the mechanisms of action of the endogenous peptide still remain unknown, glucagon might have a role in the development of non shivering thermogenesis during prolonged cold exposure via an inhibition of shivering in birds.

Dual core and shell temperature regulation during sea acclimatization in Gentoo penguins (Pygoscelis papua).

Penguins are able to maintain a high and constant body temperature despite a thermally constraining environment. Evidence for progressive adaptation to cold and marine life was sought by comparing body and peripheral skin temperatures, metabolic rate, and thermal insulation in juvenile and adult Gentoo penguins exposed to various ambient temperatures in air (from -30 to +30 degrees C) and water (3-35 degrees C). Juvenile penguins in air showed metabolic and insulative capacities comparable with those displayed by adults. Both had a lower critical temperature (LCT) close to 0 degree C. In both adults and juveniles, the intercept of the metabolic curve with the abscissa at zero metabolic rate was far below body temperature. This was accompanied by a decrease in thermal insulation below LCT, allowing the preservation of a threshold temperature in the shell. However, this shell temperature maintenance was progressively abandoned in immersed penguins as adaptation to marine life developed, probably because of its prohibitive energy cost in water. Thus adaptation to cold air and to cold water does not rely on the same kind of reactions. Both of these strategies fail to follow the classical sequence linking metabolic and insulative reactions in the cold.

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

1. The effect of cold acclimation on the oxidative capacity of the skeletal muscle (gastrocnemius and pectoralis) and the liver of ducklings was investigated. 2. In cold-acclimated (CA) ducklings, the oxidative capacity of the liver was higher (+40%) than in ducklings reared at thermoneutraility (TN). In these animals an increase in state 4 respiration and a decrease in the respiratory control index (RCI) was also found. 3. The oxidative capacity of both pectoralis and gastrocnemius muscles also increased in CA animals. 4. In these muscles the oxidatibe capacity of the subsarcolemmal mitochrondrial fraction of CA ducklings was higher (+96% in the gastrocnemius and +58% in the pectoralis) than the intermyofibrillar one (+51% in the gastrocnemius and +33% in the pectoralis). No variations were observed in either the RCI or the ADP/O ratios. 5. These findings indicate that the energy expenditure needed for non-shivering thermogenesis (NST) in cold-acclimated ducklings can be met by the increase in the oxidative capacity of the skeletal muscle and the liver, each by different mechanisms; the gastrocnemius muscle would seem to play a prominent role.

Histochemical arguments for muscular non-shivering thermogenesis in muscovy ducklings.

1. The histochemical characteristics of gastrocnemius muscle were investigated in 6-week-old cold-acclimated (5 weeks, 4 degrees C) and glucagon-treated (5 weeks, 25 degrees C, 103 nmol/kg I.P. twice daily) muscovy ducklings, two groups able to develop non-shivering thermogenesis in vivo. A comparison was made with thermoneutral controls (25 degrees C) of the same age. All animals were fed ad libitum. Fibre type, fibre area and capillary supply have been studied. Further, a quantitative histochemical method for mitochondrial Mg(2+)-ATPase activity was developed to characterize the mitochondrial coupling state in situ. 2. White gastrocnemius was composed of fast glycolytic (FG) and fast oxidative glycolytic (FOG) fibres, while red gastrocnemius contained FOG and slow oxidative (SO) fibres. In white gastrocnemius, the proportion of FG fibres was higher in glucagon-treated than in control or cold-acclimated ducklings. In red gastrocnemius, the proportion of SO fibres was higher in both cold-acclimated and glucagon-treated ducklings than in controls. The area of all fibres was generally lower in glucagon-treated than in other ducklings. 3. The capillary density was higher in both red and white components of the gastrocnemius muscle in cold-acclimated and glucagon-treated than in control ducklings, as a result of an increased number of capillaries around each fibre. 4. In all fibres, except the FG type in cold-acclimated ducklings, the staining intensity of the Mg(2+)-ATPase reaction was higher in cold-acclimated and glucagon-treated than in control ducklings whereas the staining intensity with maximal decoupling of oxidative phosphorylation by dinitrophenol was unchanged. This indicated a more loose-coupled state of mitochondria in situ in all fibres of cold-acclimated ducklings, and in FOG fibres of white gastrocnemius and SO fibres of red gastrocnemius in glucagon-treated ducklings. 5. These results indicated a higher oxidative metabolism of skeletal muscle in both cold-acclimated and glucagon-treated than in control ducklings, and for most of the parameters studied, a similarity between cold acclimation and glucagon treatment. Because of the higher loose-coupled state of muscle mitochondria in cold-acclimated and glucagon-treated than in control ducklings, the higher oxidative capacity of skeletal muscle in these ducklings could be used for heat production rather than ATP synthesis and account for muscular non-shivering thermogenesis.

Nonshivering thermogenesis in king penguin chicks. I. Role of skeletal muscle.

In cold-acclimatized (CA) king penguin chicks exhibiting nonshivering thermogenesis (NST), protein content and cytochrome oxidase (CO) activity of tissue homogenates were measured together with protein content, CO, and respiration rates of isolated mitochondria from skeletal muscle (gastrocnemius and pectoralis) and liver. The comparison was made with chicks reared at thermoneutrality (TN) for at least 3 wk. In CA chicks showing a NST despite the lack of brown adipose tissue, an increase in thermogenic capacity was observed in skeletal muscle in which the oxidative capacity rose (+28% and +50% in gastrocnemius and pectoralis muscles, respectively), whereas no change occurred in the liver. Oxidative capacity of skeletal muscle increased together with the development of mitochondrial inner membrane plus cristae in muscles of CA chicks contrary to their TN littermates (+30 to +50%). Subsarcolemmal mitochondria of CA chicks had a higher protein content (+65% in gastrocnemius muscle) and higher oxidative capacities than in controls. The lower respiratory control ratio of these mitochondria might result from a low ADP phosphorylation rate. No change occurred in the intermyofibrillar fraction nor in liver mitochondria. These findings together with earlier results obtained in cold-acclimated ducklings indicate the marked and suited adaptation of skeletal muscle and in particular of subsarcolemmal mitochondria allowing them to play a role in NST.

Nonshivering thermogenesis in king penguin chicks. II. Effect of fasting.

The effect of fasting on the energy metabolism of skeletal muscle and liver was investigated in cold-acclimatized short-term fasting (STF) (3 wk) and naturally long-term fasting (LTF) (4-5 mo) king penguin chicks, both groups exhibiting nonshivering thermogenesis (NST). A comparison was made with nourished cold-acclimatized controls. In these chicks, no brown adipose tissue deposits could be found on electron-microscopic observations of fat deposits. Protein content and cytochrome oxidase (CO) activity of tissue homogenates were measured in liver and pectoralis and gastrocnemius muscles, as were protein content, CO activity, and respiration rates of mitochondria isolated from these organs. Fasting-induced protein loss affected the pectoralis more than the gastrocnemius muscle, thus preserving locomotor function. In STF chicks, specific mitochondrial protein content and specific tissue CO activity were preserved but total organ CO capacity was reduced by half in pectoralis and liver following the fall in organ mass. In LTF chicks, both specific and total CO activity were drastically reduced in muscles, whereas specific CO activity was preserved in liver. In these LTF chicks, muscle mitochondria showed an energized configuration associated with an increased area of inner membrane in gastrocnemius. A reduction of respiratory control ratio (RCR) was observed in subsarcolemmal muscle mitochondria of STF chicks, whereas intermyofibrillar and liver mitochondria kept high RCR values.(ABSTRACT TRUNCATED AT 250 WORDS)

Increase in cytochrome oxidase capacity of BAT and other tissues in cold-acclimated gerbils.

To determine the effects of cold acclimation on the oxidative capacity of different tissues and their possible role in nonshivering thermogenesis (NST) in a desert rodent, the gerbil (Gerbillus campestris), measurements of cytochrome oxidase activity (COX) were performed on homogenates of liver, brown adipose tissue (BAT), and subsarcolemmal (SS) and intermyofibrillar (IMF) fractions of skeletal muscle. Total organ mass was also measured. Gerbils were maintained either at thermoneutrality (TN) or cold [4 degrees C, ambient temperature (Ta)] for 4 (CA4) or 8 (CA8) wk. A comparative study was made with mice (Mus musculus). Total and relative masses of BAT increased significantly in both gerbils and mice during cold acclimation, whereas muscle mass decreased in CA4 gerbils. Specific and total COX (TCOX) increased in the three tissues. A considerable increase (+170%, P less than 0.05) of TCOX and protein content in liver of CA4 gerbils compared with controls was observed, whereas no significant changes occurred in liver of CA4 mice. In muscle, an increase of TCOX in SS and a reduction in IMF cell compartments were noted. The increase in BAT oxidative capacity in CA4 gerbils represented only a small portion of that in liver (36%) and in SS fraction of muscle (41%) at the same stage of acclimation. The ability of the three tissues to contribute to components of thermogenesis in vivo was evaluated. The sum of oxidative capacity of the three tissues was largely above the peak metabolic rate (PMR), whereas that of muscle exceeded maximum shivering thermogenesis, which developed both in TN and CA4 gerbils.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonshivering thermogenesis and adaptation to fasting in king penguin chicks.

The ability to develop nonshivering thermogenesis (NST) and the effect of fasting on thermogenic response to cold were studied in winter-acclimatized king penguin chicks. Metabolic rate (MR) and integrated electrical muscle activity were measured at different ambient temperatures. In cold-acclimatized (5 degrees C) fed chicks, shivering threshold temperature (STT) was 9.4 degrees C lower than lower critical temperature (LCT), indicating that NST (0.7 W/kg) occurs at moderate cold, whereas in control chicks fed and reared at 25 degrees C for 3 wk, LCT and STT were similar. Chicks reared in the cold and fasting for 3 wk or 4-5 mo (natural winter fast) developed an NST of 0.8 and 2.4 W/kg, respectively, despite the fast. In fasting chicks, the intercept of the metabolic curve with the abscissa at zero MR was far below body temperature, contrasting with the classic model for heat loss. Their low LCT indicates the capacity of a large reduction in convective conductance characteristic of diving animals and allows energy sparing in moderate cold. Below LCT, conductance reincreases progressively, leading to a steeper than expected slope of the metabolic curve and allowing preservation of a threshold temperature in the shell. These results show for the first time in a wild young bird the development of NST after cold acclimatization. Further, at the temperature of cold acclimatization, an energy-sparing mechanism is shown in response to long-term fast adaptation.