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The objective of this paper is to evaluate adaptations in hepatic mitochondrial protein mass, function and efficiency in a rat model of high-fat diet-induced obesity and insulin resistance that displays several correlates to human obesity. Adult male rats were fed a high-fat diet for 7 weeks. Mitochondrial state 3 and state 4 respiratory capacities were measured in liver homogenate and isolated mitochondria by using nicotinamide adenine dinucleotide, flavin adenine dinucleotide and lipid substrates. Mitochondrial efficiency was evaluated by measuring proton leak kinetics. Mitochondrial mass was assessed by ultrastructural observations and citrate synthase (CS) activity measurements. Mitochondrial oxidative damage and antioxidant defence were also considered by measuring lipid peroxidation, aconitase and superoxide dismutase (SOD) specific activity. Whole body metabolic characteristics were obtained by measuring 24-h oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory quotient (RQ) and nonprotein respiratory quotient (NPRQ), using indirect calorimetry with urinary nitrogen analysis. Whole body glucose homeostasis was assessed by measuring plasma insulin and glucose levels after a glucose load. Adult rats fed a high-fat diet for 7 weeks, exhibit not only obesity, insulin resistance and hepatic steatosis, but also reduced respiratory capacity and increased oxidative stress in liver mitochondria. Our present results indicate that alterations in the mitochondrial compartment induced by a high-fat diet are associated with the development of insulin resistance and ectopic fat storage in the liver. Our results thus fit in with the emerging idea that mitochondrial dysfunction can led to the development of metabolic diseases, such as obesity, type 2 diabetes mellitus and nonalcoholic steatohepatitis.

The increase in the use of refined food, which is rich in fructose, is of particular concern in children and adolescents, since the total caloric intake and the prevalence of metabolic syndrome are increasing continuously in these populations. Nevertheless, the effects of high fructose diet have been mostly investigated in adults, by focusing on the effect of a long-term fructose intake. Notably, some reports evidenced that even short-term fructose intake exerts detrimental effects on metabolism. Therefore, the aim of this study was to compare the metabolic changes induced by the fructose-rich diet in rats of different age, i.e., young (30 days old) and adult (90 days old) rats. The fructose-rich diet increased whole body lipid content in adult, but not in young rats. The analysis of liver markers of inflammation suggests that different mechanisms depending on the age might be activated after the fructose-rich diet. In fact, a pro-inflammatory gene-expression analysis showed just a minor activation of macrophages in young rats compared to adult rats, while other markers of low-grade metabolic inflammation (TNF-alpha, myeloperoxidase, lipocalin, haptoglobin) significantly increased. Inflammation was associated with oxidative damage to hepatic lipids in young and adult rats, while increased levels of hepatic nitrotyrosine and ceramides were detected only in young rats. Interestingly, fructose-induced hepatic insulin resistance was evident in young but not in adult rats, while whole body insulin sensitivity decreased both in fructose-fed young and adult rats. Taken together, the present data indicate that young rats do not increase their body lipids but are exposed to metabolic perturbations, such as hepatic insulin resistance and hepatic oxidative stress, in line with the finding that increased fructose intake may be an important predictor of metabolic risk in young people, independently of weight status. These results indicate the need of corrective nutritional interventions for young people and adults as well for the prevention of fructose-induced metabolic alterations.

The drastic increase in the consumption of fructose encouraged the research to focus on its effects on brain physio-pathology. Although young and adults differ largely by their metabolic and physiological profiles, most of the previous studies investigated brain disturbances induced by long-term fructose feeding in adults. Therefore, we investigated whether a short-term consumption of fructose (2 weeks) produces early increase in specific markers of inflammation and oxidative stress in the hippocampus of young and adult rats. After the high-fructose diet, plasma lipopolysaccharide and tumour necrosis factor (TNF)-alpha were found significantly increased in parallel with hippocampus inflammation, evidenced by a significant rise in TNF-alpha and glial fibrillar acidic protein concentrations in both the young and adult groups. The fructose-induced inflammatory condition was associated with brain oxidative stress, as increased levels of lipid peroxidation and nitro-tyrosine were detected in the hippocampus. The degree of activation of the protein kinase B, extracellular signal-regulated kinase 1/2, and insulin receptor substrate 1 pathways found in the hippocampus after fructose feeding indicates that the detrimental effects of the fructose-rich diet might largely depend on age. Mitochondrial function in the hippocampus, together with peroxisome proliferator-activated receptor gamma coactivator 1-alpha content, was found significantly decreased in fructose-treated adult rats. In vitro studies with BV-2 microglial cells confirmed that fructose treatment induces TNF-alpha production as well as oxidative stress. In conclusion, these results suggest that unbalanced diet, rich in fructose, may be highly deleterious in young people as in adults and must be strongly discouraged for the prevention of diet-associated neuroinflammation and neurological diseases.

The present study investigated the effect of 2 weeks of energy restriction on whole body, liver and skeletal muscle energy handling. We measured whole-body oxygen consumption, as well as mitochondrial protein mass, respiratory capacity and energetic coupling in liver and skeletal muscle from food-restricted (FR) rats, age- and weight-matched controls. We also assessed markers of oxidative damage and antioxidant defences. The present results show that, in response to energy restriction, an adaptive decrease in whole-body energy expenditure is coupled with structural and functional changes in mitochondrial compartment, both in liver and skeletal muscle. In fact, liver mitochondrial mass per g of liver significantly increased, whereas total hepatic mitochondrial oxidative capacity was lower in FR than in control rats, because of a significant decrease in liver contribution to total body weight. In skeletal muscle, sub-sarcolemmal (SS) mitochondrial respiratory capacity, as well as SS and inter-myofibrillar (IMF) mitochondrial protein mass per g of tissue, was significantly lower in FR rats, compared to controls. Finally, a decrease in oxidative damage was found in liver but not in skeletal muscle mitochondria from FR rats, whereas an increase in antioxidant defence was found in both tissues. From the present results, it appears that skeletal muscle is involved in the decrease in energy expenditure induced by energy restriction. Energy sparing is achieved through changes in the activity (SS), mass (SS and IMF) and efficiency (IMF) of mitochondrial compartment.

Evidence indicates that many forms of fructose‐induced metabolic disturbance are associated with oxidative stress and mitochondrial dysfunction. Mitochondria are prominent targets of oxidative damage; however, it is not clear whether mitochondrial DNA (mtDNA) damage and/or its lack of repair are events involved in metabolic disease resulting from a fructose‐rich diet. In the present study, we evaluated the degree of oxidative damage to liver mtDNA and its repair, in addition to the state of oxidative stress and antioxidant defense in the liver of rats fed a high‐fructose diet. We used male rats feeding on a high‐fructose or control diet for eight weeks. Our results showed an increase in mtDNA damage in the liver of rats fed a high‐fructose diet and this damage, as evaluated by the expression of DNA polymerase γ, was not repaired; in addition, the mtDNA copy number was found to be significantly reduced. A reduction in the mtDNA copy number is indicative of impaired mitochondrial biogenesis, as is the finding of a reduction in the expression of genes involved in mitochondrial biogenesis. In conclusion, a fructose‐rich diet leads to mitochondrial and mtDNA damage, which consequently may have a role in liver dysfunction and metabolic diseases.

Objectives We assessed the alterations in mitochondrial function in skeletal muscle that were elicited by short-term high-fat feeding in sedentary rats. Methods Two groups of rats were pair-fed for 1 wk and received a low-fat or high-fat diet. Body composition, energy balance, and glucose homeostasis were measured. Mitochondrial mass, oxidative capacity, and energetic efficiency as well as parameters of oxidative stress and antioxidant defense were evaluated in subsarcolemmal and intermyofibrillar mitochondria from the skeletal muscle. Results Body energy, lipid content, and metabolic efficiency were significantly higher and energy expenditure was significantly decreased among rats that were fed a high-fat diet, as compared with controls. Skeletal muscle mitochondrial energetic efficiency, oxidative capacity for lipid substrates, and antioxidant defense were significantly increased in rats that were fed a high-fat diet as compared with controls. Conclusions Acute isocaloric high-fat feeding is able to induce increased phosphorylation efficiency in skeletal muscle subsarcolemmal and intermyofibrillar mitochondria. This modification implies a reduced oxidation of energy substrates that may contribute to the early onset of obesity.

The purpose of the present study was to assess the impact of previous period of caloric restriction on energy balance and skeletal muscle mitochondrial energetics in response to high-fat (HF) diet. To this end, 1 group of rats was subjected to 2 weeks of caloric restriction with nonpurified diet and then fed HF diet (430 kJ metabolizable energy/day) for 1 week, while the second group was fed ad libitum with nonpurified diet for 2 weeks and then fed HF diet (430 kJ metabolizable energy/day) for 1 week. Body composition, energy balance, and glucose homeostasis were measured. Mitochondrial mass, oxidative capacity and efficiency, parameters of oxidative stress, and antioxidant defense were evaluated in subsarcolemmal and intermyofibrillar mitochondria from skeletal muscle. Body energy and lipid content, plasma insulin, and metabolic efficiency were significantly higher, while energy expenditure significantly decreased, in food-restricted rats fed HF diet compared to controls. Mitochondrial efficiency and oxidative damage in skeletal muscle were significantly increased, while antioxidant defence was significantly lower in food-restricted rats fed HF diet, compared with controls. Finally, food-restricted rats fed HF diet exhibited significant reduction in subsarcolemmal mitochondrial mass. In conclusion, caloric restriction elicits higher mitochondrial efficiency and predisposes skeletal muscle to high fat-induced oxidative damage, which in turn could lead to impaired glucose homeostasis in food-restricted rats fed HF diet.

The recovery of body weight after a period of caloric restriction is accompanied by an enhanced efficiency of fat deposition and hyperinsulinemia—which are exacerbated by isocaloric refeeding on a high fat diet rich in saturated and monounsaturated fatty acids (SFA-MUFA), and poor in polyunsaturated fatty acids (PUFA), and associated with a blunting of de novo lipogenesis in adipose tissue and liver. As high fat diets rich in PUFA have been shown to limit the excess fat deposition and improve glucose homeostasis, we investigated here the extent to which de novo lipogenesis in liver and adipose tissues (white and brown), as well as hepatic oxidative stress, are influenced by refeeding on diets rich in PUFA.Design: In rats calorically restricted for 14 days and refed for 14 days on isocaloric amounts of a high fat diet rich in lard (i.e., high SFA- MUFA) or in safflower and linseed oils (rich in PUFA), we investigated energy balance, body composition, glycemic profile, and the regulation of fatty acid synthase (rate- limiting enzyme of de novo lipogenesis) in liver, white and brown adipose tissue. We also evaluated oxidative stress in liver and skeletal muscle and markers of hepatic inflammation.Results: Rats refed the PUFA diet gained less lipids and more proteins compared to rats refed SFA-MUFA diet and showed lower amount of visceral and epididymal white adipose tissue, but increased depots of interscapular brown adipose tissue, with higher expression of the uncoupling protein 1. A significant increase in non- protein respiratory quotient and carbohydrate utilization was found in rats refed PUFA diet. Rats refed PUFA diet showed improved glucose homeostasis, as well as lower triglycerides and cholesterol levels. Fatty acid synthase activity was significantly higher in liver, white and brown adipose tissue, while lipid peroxidation and the degree of inflammation in the liver were significantly lower, in rats refed PUFA diet.Conclusions: When considering the composition of high fat diets for nutritional rehabilitation, the inclusion of PUFA could be useful for improving protein deposition and maintaining glucose homeostasis, while limiting lipid storage in adipose tissue and oxidative stress and inflammation in the liver.

We have investigated whether altered hepatic mitochondrial energetics could explain the differential effects of high-fat diets with low or high ω6 polyunsaturated fatty acid content (lard vs. safflower oil) on the efficiency of body fat recovery (catch-up fat) during refeeding after caloric restriction. After 2 weeks of caloric restriction, rats were isocalorically refed with a low-fat diet (LF) or high-fat diets made from either lard or safflower oil for 1 week, and energy balance and body composition changes were assessed. Hepatic mitochondrial energetics were determined from measurements of liver mitochondrial mass, respiratory capacities, and proton leak. Compared to rats refed the LF, the groups refed high-fat diets showed lower energy expenditure and increased efficiency of fat gain; these differences were less marked with high-safflower oil than with high-lard diet. The increase in efficiency of catch-up fat by the high-fat diets could not be attributed to differences in liver mitochondrial activity. By contrast, the lower fat gain with high-safflower oil than with high-lard diet is accompanied by higher mitochondrial proton leak and increased proportion of arachidonic acid in mitochondrial membranes. In conclusion, the higher efficiency for catch-up fat on high-lard diet than on LF cannot be explained by altered hepatic mitochondrial energetics. By contrast, the ability of the high-safflower oil diet to produce a less pronounced increase in the efficiency of catch-up fat may partly reside in increased incorporation of arachidonic acid in hepatic mitochondrial membranes, leading to enhanced proton leak and mitochondrial uncoupling.

Mitochondria are the main cellular sites devoted to ATP production and lipid oxidation. Therefore, the mitochondrial dysfunction could be an important determinant of cellular fate of circulating lipids, that accumulate in the cytoplasm, if they are not oxidized. The ectopic fat accumulation is associated with the development of insulin resistance, and a link between mitochondrial dysfunction and insulin resistance has been proposed.Recent data on the possible link existing between mitochondrial dysfunction in the liver and diet induced obesity will be summarized, focusing on the three factors that affect the mitochondrial oxidation of metabolic fuels, i.e. organelle number, organelle activity, and energetic efficiency of the mitochondrial machinery in synthesizing ATP. Search in PubMed relevant articles from 2003 to 2014 was conducted, by using query “liver mitochondria and obesity” “hepatic mitochondria and obesity” “liver mitochondria and high fat diet” and “hepatic mitochondria and high fat diet” and including related articles by the same groups.Several works, by using different physiological approaches, have dealt with alteration in mitochondrial function in obesity and diabetes. Most results show that hepatic mitochondrial function is impaired in models of obesity and insulin resistance induced by high-fat or highfructose feeding.Since mitochondria are the main producers of both cellular energy and free radicals, dysfunctional mitochondria could play an important role in the development of insulin resistance and ectopic fat storage in the liver, thus supporting the emerging idea that mitochondrial dysfunction is closely related to the development of obesity, type 2 diabetes mellitus and non-alcoholic steatohepatitis.

An accelerated rate of fat recovery (catch-up fat) and insulin resistance are characteristic features of weight recovery after caloric restriction, with implications for the pathophysiology of catch-up growth and weight fluctuations. Using a previously described rat model of weight recovery in which catch-up fat and skeletal muscle insulin resistance have been linked to suppressed thermogenesis per se, we investigated alterations in mitochondrial energetics and oxidative stress in subsarcolemmal (SS) and intermyofibrillar (IMF) skeletal muscle mitochondria. After 2 weeks of semistarvation followed by 1 week of refeeding, the refed rats show persistent and selective reductions in SS mitochondrial mass (assessed from citrate synthase activity in tissue homogenate and isolated mitochondria) and oxidative capacity. Furthermore, the refed rats show, in both SS and IMF muscle mitochondria, a lower aconitase activity (whose inactivation is an index of increased reactive oxygen species [ROS]), associated with higher superoxide dismutase activity and increased proton leak. Taken together, these studies suggest that diminished skeletal muscle mitochondrial mass and function, specifically in the SS mitochondrial compartment, contribute to the high metabolic efficiency for catch-up fat after caloric restriction and underscore a potential link between diminished skeletal muscle SS mitochondrial energetics, increased ROS concentration, and insulin resistance during catch-up fat.

Insulin resistance, "a relative impairment in the ability of insulin to exert its effects on glucose, protein and lipid metabolism in target tissues," has many detrimental effects on metabolism and is strongly correlated to deposition of lipids in non-adipose tissues. Mitochondria are the main cellular sites devoted to ATP production and fatty acid oxidation. Therefore, a role for mitochondrial dysfunction in the onset of skeletal muscle insulin resistance has been proposed and many studies have dealt with possible alteration in mitochondrial function in obesity and diabetes, both in humans and animal models. Data reporting evidence of mitochondrial dysfunction in type two diabetes mellitus are numerous, even though the issue that this reduced mitochondrial function is causal in the development of the disease is not yet solved, also because a variety of parameters have been used in the studies carried out on this subject. By assessing the alterations in mitochondrial efficiency as well as the impact of this parameter on metabolic homeostasis of skeletal muscle cells, we have obtained results that allow us to suggest that an increase in mitochondrial efficiency precedes and therefore can contribute to the development of high-fat-induced insulin resistance in skeletal muscle.

To study the effect of isoenergetic administration to adult rats of high-fat or high-fat--high-fructose diet for 2 weeks on skeletal muscle mitochondrial energetic.Body and skeletal muscle composition, energy balance, plasma lipid profile and glucose tolerance were measured, together with mitochondrial functionality, oxidative stress and antioxidant defense.Rats fed high-fat--high-fructose diet exhibited significantly higher plasma triglycerides and non-esterified fatty acids, together with significantly higher plasma glucose and insulin response to glucose load. Skeletal muscle triglycerides and ceramide were significantly higher in rats fed high-fat--high-fructose diet. Skeletal muscle mitochondrial energetic efficiency and uncoupling protein 3 content were significantly higher, while adenine nucleotide translocase content was significantly lower, in rats fed high-fat or high-fat--high-fructose diet.The results suggest that a high-fat--high-fructose diet even without hyperphagia is able to increase lipid flow to skeletal muscle and mitochondrial energetic efficiency, with two detrimental effects: (a) energy sparing that contributes to the early onset of obesity and (b) reduced oxidation of fatty acids and lipid accumulation in skeletal muscle, which could generate insulin resistance.

OBJECTIVE: To study the influence of thyroid hormones on the relationship between serum leptin and fat mass, as well as on energy and macronutrient balance. DESIGN: Rats with different thyroid states were obtained by 7 and 15 days of treatment with the antithyroid drug propylthiouracil or with triiodothyronine (T3). MEASUREMENTS: Energy balance, macronutrient balance and serum leptin concentrations. RESULTS: In hypothyroid rats we found a decrease in metabolizable energy (ME) intake and energy expenditure together with an increase in lipid gain/lipid intake ratio and a decrease in protein gain/protein intake ratio. Consequently, body lipid percentage significantly increased compared to euthyroid rats. Hyperthyroid rats first increased energy expenditure and later ME intake, so that increased metabolism was balanced by increased intake, and energy gain was similar to that found in euthyroid rats. CONCLUSION: These results indicate that T3 plays a major role in the maintenance of energy and lipid balance. Our results also indicate that an inverse relationship exists between T3 and leptin serum concentrations, and that this relationship is not only the result of changes in body fat stores induced by changed T3 concentrations.

The effect of treatment with acetyl-L-carnitine on hepatic mitochondrial respiration and biosynthetic function in perfused liver from young (90 days) and old (22-24 months) rats was studied. Rats were given a 1.5% (w/v) solution of acetyl-L-carnitine in their drinking water for 1 month and oxygen consumption together with the rate of gluconeogenesis, urea synthesis, and ketogenesis with and without added substrates were measured in perfused liver. Mitochondrial oxygen consumption was also assessed in liver homogenate and isolated mitochondria to determine the maximal capacity for oxidative phosphorylation. Acetyl-L-carnitine treatment almost completely restored the age-dependent decline in oxygen consumption, gluconeogenesis, urea synthesis, and ketogenesis found in perfused liver of old rats to the levels found in young rats. In addition, acetyl-L-carnitine treatment increased oxygen consumption and biosynthetic function in perfused liver from young rats. After acetyl-L-carnitine treatment, we found detectable 3-oxoacyl-CoA-transferase activity associated with a consumption of ketone bodies in young and old rats. Finally, oxygen consumption measured in homogenate and isolated mitochondria did not change with age and acetyl-L-carnitine treatment. Our results show that in perfused liver, acetyl-L-carnitine treatment slows the age-associated decline in mitochondrial respiration and biosynthetic function. In addition, treatment of young rats with acetyl-L-carnitine has a stimulating effect on liver metabolism, probably through an increase in ATP production.

Variations in energy balance, body composition, and nutrient partitioning induced by high-fat feeding, cold exposure or by concomitant high-fat feeding and cold exposure were studied in young Wistar rats. Changes in hepatic metabolism as well as in serum free triiodothyronine and leptin levels were also evaluated. Rats were exposed to either 24 or 4 degrees C and fed either a low- or high-fat diet (10 % or 50 % energy respectively) for 2 weeks. Relative to low-fat feeding at 24 degrees C, both energy intake and expenditure were increased by high-fat feeding or by cold exposure, and these changes were accompanied by increased serum triiodothyronine levels. In response to concomitant high-fat feeding and cold exposure, serum triiodothyronine tended to be further elevated, but no further increases in energy intake or energy expenditure were observed. Independently of diet, the increased energy expenditure in cold-exposed rats was not completely balanced by adaptive hyperphagia, with consequential reductions in protein and fat gain, accompanied by marked decreases in serum leptin. Furthermore, unlike high-fat feeding at 24 degrees C, cold exposure enhanced hepatic mitochondrial oxidative capacity both in the low-fat- and high-fat-fed groups. It is concluded that in this strain of young Wistar rats, despite similarly marked stimulation of energy expenditure by high-fat feeding at 24 degrees C, by cold exposure and by concomitant high-fat feeding and cold exposure, an increased hepatic oxidative capacity occurred only in the presence of the cold stimulus.

In the present study we investigated the changes in the hepatic mitochondrial respiratory system in the transition from weaning to adulthood in the rat. We conceptually divided the system into blocks of reactions that produced or consumed mitochondrial membrane potential and then measured the kinetic responses of these blocks of reactions to changes in this potential in isolated liver mitochondria from 25- and 60-day-old rats using succinate as substrate. Moreover, we considered the mitochondrial membrane potential producers to be divided into blocks of reactions that reduced or oxidized ubiquinone-2 (Q-2) and then measured the kinetic responses of these two blocks to changes in Q-2 redox state as well as the flux control coefficients and the cytochrome content. We found that adult rats exhibited significantly higher state 3 respiratory rates with increased kinetic response of the substrate oxidation pathway to the mitochondrial membrane potential, slightly decreased activity of the phosphorylating system, increased kinetic responses of both Q-2 reducers and oxidizers to Q-2 redox state, and increased cytochrome content. Our results indicate that important changes in the hepatic mitochondrial respiratory system occur in the transition from weaning to adulthood in rats.

In the present study we investigated the changes in hepatic mitochondrial function in the transition from weaning to adulthood in the rat. We measured mitochondrial respiration using FAD- and NAD-linked substrates in 25 and 60 day old rats. The results show that adult rats exhibited significantly higher respiratory rates with all the substrates used except pyruvate. Our results indicate that the transition from weaning to adulthood induces important changes in hepatic mitochondrial function.

Background: We considered of interest to evaluate how aging affects mitochondrial function in skeletal muscle. Methods: We measured mitochondrial oxidative capacity and proton leak, together with lipid oxidative damage, superoxide dismutase specific activity and uncoupling protein 3 content, in subsarcolemmal and intermyofibrillar mitochondria from adult (six months) and old (two years) rats. Body composition, resting metabolic rate and plasma non esterified fatty acid levels were also assessed. Results: Old rats displayed significantly higher body energy and lipids, while body proteins were significantly lower, compared to adult rats. In addition, plasma non esterified fatty acid levels were significantly higher, while resting metabolic rates were found to be significantly lower, in old rats compared to adult ones. Significantly lower oxidative capacities in whole tissue homogenates and in intermyofibrillar and subsarcolemmal mitochondria were found in old rats compared to adult ones. Subsarcolemmal and intermyofibrillar mitochondria from old rats exhibited a significantly lower proton leak rate, while oxidative damage was found to be significantly higher only in subsarcolemmal mitochondria. Mitochondrial superoxide dismutase specific activity was not significantly affected in old rats, while significantly higher content of uncoupling protein 3 was found in both mitochondrial populations from old rats compared to adult ones, although the magnitude of the increase was lower in subsarcolemmal than in intermyofibrillar mitochondria. Conclusions: The decrease in oxidative capacity and proton leak in intermyofibrillar and subsarcolemmal mitochondria could induce a decline in energy expenditure and thus contribute to the reduced resting metabolic rate found in old rats, while oxidative damage is present only in subsarcolemmal mitochondria.

To explore the effect of a fructose-rich diet on morphological and functional changes in white adipose tissue (WAT) that could contribute to the development of insulin resistance. Adult sedentary rats were fed a fructose-rich diet for 8 weeks. Glucose tolerance test was carried out together with measurement of plasma triglycerides, non-esterified fatty acids and lipid peroxidation. In subcutaneous abdominal and intra-abdominal WAT, number and size of adipocytes together with cellular insulin sensitivity and lipolytic activity were assessed. Rats fed a fructose-rich diet exhibited a significant increase in plasma insulin, triglycerides, non-esterified fatty acids and lipid peroxidation, together with significantly increased body lipids and epididymal and mesenteric WAT, compared to controls. Mean adipocyte volume in subcutaneous abdominal WAT was significantly lower, while mean adipocyte volume in intra-abdominal WAT was significantly higher, in rats fed a fructose-rich diet compared to controls. A significant increase in larger adipocytes and a significant decrease in smaller adipocytes were found in intra-abdominal WAT in rats fed a fructose-rich diet compared to controls. Insulin’s ability to inhibit lipolysis was blunted in subcutaneous abdominal and intra-abdominal adipocytes from fructose-fed rats. Accordingly, lower p-Akt/Akt ratio was found in WAT in rats fed a fructose-rich diet compared to controls. Long-term consumption of high levels of fructose elicits remarkable morphological and functional modifications, particularly in intra-abdominal WAT, that are highly predictive of obesity and insulin resistance and that contribute to the worsening of metabolic alterations peculiar in a fructose-rich, hypolipidic diet.

We have carried out measurements of energy balance in hypothyroid rats fed a low-fat or a high-fat diet for eighteen days. We have also measured cephalic and processing thermic effect of food (TEF) after a low-fat or a high-fat meal. Body lipid gain, carcass lipid content and gross efficiency were significantly (P < 0·05) higher in hypothyroid rats fed a high-fat diet compared with hypothyroid rats fed a low-fat diet, while metabolizable energy intake and energy expenditure remained unchanged. Cephalic TEF after a low-fat meal was significantly (P < 005) lower in hypothyroid rats fed a high-fat diet compared with hypothyroid rats fed a low-fat diet, while it was significantly (P Our results indicate that hypothyroid rats are unable to develop increased energy expenditure and increased TEF in response to a high-fat diet. European Journal of Endocrinology 136 309–315

To assess hepatic de novo lipogenesis and mitochondrial energetics as well as whole-body energy homeostasis in sedentary rats fed a fructose-rich diet. Male rats of 90 days of age were fed a high-fructose or control diet for 8 weeks. Body composition, energy balance, oxygen consumption, carbon dioxide production, non-protein respiratory quotient, de novo lipogenesis and insulin resistance were measured. Determination of specific activity of hepatic enzymes of de novo lipogenesis, mitochondrial mass, oxidative capacity and degree of coupling, together with parameters of oxidative stress and antioxidant defence, was also carried out. Body energy and lipid content as well as plasma insulin and non-esterified fatty acids were significantly higher in fructose-fed than in control rats. Significantly higher rates of net de novo lipogenesis and activities of hepatic lipogenic enzymes fatty acid synthase and stearoyl CoA desaturase-1 were found in fructose-fed rats compared to controls. Mitochondrial protein mass and degree of coupling were significantly higher in fructose-fed rats compared to controls. Hepatic mitochondria showed oxidative damage, both in the lipid and in the protein component, together with decreased activity of antioxidant defence. Liver mitochondrial compartment is highly affected by fructose feeding. The increased mitochondrial efficiency allows liver cells to burn less substrates to produce ATP for de novo lipogenesis and gluconeogenesis. In addition, increased lipogenesis gives rise to whole body and ectopic lipid deposition, and higher mitochondrial coupling causes mitochondrial oxidative stress.

In the present study, the effect of long-term fructose feeding on skeletal muscle mitochondrial energetics was investigated. Measurements in isolated tissue were coupled with the determination of whole-body energy expenditure and insulin sensitivity. A significant increase in plasma NEFA, as well as in skeletal muscle TAG and ceramide, was found in fructose-fed rats compared with the controls, together with a significantly higher plasma insulin response to a glucose load, while no significant variation in plasma glucose levels was found. Significantly lower RMR values were found in fructose-fed rats starting from week 4 of the dietary treatment. Skeletal muscle mitochondrial mass and degree of coupling were found to be significantly higher in fructose-fed rats compared with the controls. Significantly higher lipid peroxidation was found in fructose-fed rats, together with a significant decrease in superoxide dismutase activity. Phosphorylated Akt levels normalised to plasma insulin levels were significantly lower in fructose-fed rats compared with the controls. In conclusion, a fructose-rich diet has a deep impact on a metabolically relevant tissue such as skeletal muscle. In this tissue, the consequences of high fructose feeding are altered glucose tolerance, elevated mitochondrial biogenesis and increased mitochondrial coupling. This latter modification could have a detrimental metabolic effect by causing oxidative stress and energy sparing that contribute to the high metabolic efficiency of fructose-fed rats.

1. 1. We have studied fatty acid oxidation and ketone bodies production in liver mitochondria from rats exposed to cold for 10 days and in rats adapted to cold for 40 days. We have examined State 3 respiration and ketone bodies production with palmitoylcarnitine as substrate in isolated mitochondria, together with mitochondrial protein mass and hepatocyte oxygen consumption. 2. 2. Mitochondrial protein mass increases both in cold exposed and in cold adapted rats. 3. 3. State 3 respiration and hepatocyte respiration increase only in cold exposed rats. 4. 4. Ketone bodies production increases in cold adapted rats. 5. 5. Taken together, our results suggest that the increased hepatic mitochondrial protein mass and the increased fatty acid oxidation subserve two different roles: an increase in ATP production in cold exposed rats and an increase in ketone bodies production in cold adapted rats.

We measured mitochondrial protein mass as well as State 4 and 3 respiratory rates using different substrates in isolated liver mitochondria from 30-day cold-exposed rats. In addition, we measured the respiration under different conditions of stimulation in isolated hepatocytes from long-term cold-exposed rats. The results show that long-term cold exposure elicits a significant increase in hepatic mass and mitochondrial protein mass. No variation was found in oxygen consumption of isolated mitochondria and hepatocytes. On the whole, the results indicate that long-term exposure elicits an increase in hepatic mitochondrial protein mass but not in hepatic oxygen consumption.

The objective of the study was to investigate whether changes in liver mitochondrial energetics could underlie the enhanced energetic efficiency that drives accelerated body fat recovery (catch-up fat) during refeeding after caloric restriction. Rats were subjected to caloric restriction (50% of ad libitum intake) for 15 days and then refed for 1 or 2 weeks on an amount of chow equal to that of controls matched for weight at the onset of refeeding. Whole-body metabolism was characterized by energy balance and body composition determinations as well as by indirect calorimetric measurements of 24-hour energy expenditure, substrate oxidation, and whole-body de novo lipogenesis estimated from nonprotein respiratory quotient. Hepatic mitochondrial energetics were determined from measurements of liver mitochondrial mass, respiratory capacities, and proton leak (both basal and fatty acid stimulated), whereas hepatic oxidative status was assessed from measurements of hepatic mitochondrial lipid peroxidation, aconitase, and superoxide dismutase activity. Furthermore, hepatic lipogenic capacity was determined from assays of fatty acid synthase activity. Compared with controls, isocalorically refed rats showed an elevated energetic efficiency and body fat gain over both week 1 and week 2 of refeeding, as well as a lower 24-hour energy expenditure and higher rates of whole-body de novo lipogenesis at the end of both week 1 and week 2 of refeeding. Analysis of the liver revealed that after 1 week (but not after 2 weeks) of refeeding, the mitochondrial mass (but not mitochondrial density) was lower in refed rats than in controls, associated with higher state 3 mitochondrial respiratory capacity, increased superoxide dismutase activity, as well as higher fatty acid synthase activity. These results suggest that, although at the whole-body level elevations in energy efficiency and de novo lipogenesis are coordinated toward catch-up fat, the overall hepatic mitochondrial energetic status during refeeding is more consistent with a contributory role of the liver in the enhanced de novo lipogenic machinery during catch-up fat rather than in the energy-conservation mechanisms (elevated energetic efficiency) that spare energy for catch-up fat.

The effects of thyroid state on liver mitochondrial protein mass was investigated in rats at 24 and 4 °C, as was oxidative phosphorylation using substrates which represent the final catabolic products of the metabolic fuels. In rats at 24 °C, a significant increase in mitochondrial protein mass (about +40%) was observed only in hyperthyroid animals, while a significant increase due to cold exposure was found in hypothyroid (+ 45%) and euthyroid (+ 35%) rats. In rats at 24 °C, hypothyroidism significantly decreased the oxidation of glutamate and palmitoyl carnitine but not of pyruvate, while hyperthyroidism only increased the oxidation of palmitoyl carnitine. On the other hand, exposure to cold significantly increased the oxidation of glutamate and pyruvate only in the presence of tri-iodothyronine. Our results underline not only the fact that a simple and single hypothesis for thyroid effects cannot be adopted, but also that any study concerning oxidative metabolism should be carried out using different substrates and involving different pathways of oxidation. Journal of Endocrinology (1991) 131, 67–73

Objective: To investigate whether changes in body energy balance induced by long-term high-fat feeding in adult rats could be associated with modifications in energetic behaviour and oxidative stress of skeletal muscle subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondrial populations.Design: Adult rats were fed low-fat or high-fat diet for 7 weeks.Measurements: Body energy balance and composition analysis together with plasma insulin and glucose level determination in the whole animal. Oxidative capacity, basal and induced proton leaks as well as aconitase and superoxide dismutase activities in SS and IMF mitochondria from skeletal muscle.Results: High-fat fed rats exhibit increased body lipid content, as well as hyperinsulinemia, hyperglycaemia and higher plasma non-esterified fatty acids. In addition, SS mitochondria display lower respiratory capacity and a different behaviour of SS and IMF mitochondria is found in the prevention from oxidative damage.Conclusions: A deleterious consequence of decreased oxidative capacity in SS mitochondria from rats fed high-fat diet would be a reduced utilization of energy substrates, especially fatty acids, which may lead to intracellular triglyceride accumulation, lipotoxicity and insulin resistance development. Our results thus reveal a possible role for SS mitochondria in the impairment of glucose homeostasis induced by high-fat diet.

This study was designed to examine energetic behaviour of skeletal muscle subsarcolemmal and intermyofibrillar mitochondrial populations. The data show that subsarcolemmal mitochondria exhibited a lower degree of coupling and efficiency than intermyofibrillar ones, and can therefore be considered less efficient at producing ATP. In addition, subsarcolemmal mitochondria showed an increased sensitivity to palmitate-induced uncoupling, in line with high adenine nucleotide translocator content and decreased oxidative damage. We then determined the effect of 24 h fasting on energetic characteristics of skeletal muscle mitochondrial populations. We found that fasting enhanced proton leak and decreased the degree of coupling and efficiency, both in the absence and in the presence of palmitate only in subsarcolemmal mitochondria. Moreover, this mitochondrial population showed lower oxidative damage, probably due to a counter-regulatory mechanism mediated by uncoupling protein 3. Subsarcolemmal and intermyofibrillar mitochondria appear to exhibit different energetic characteristics and can be differently affected by physiological stimuli.

This study deals with mitochondrial energy efficiency in liver and skeletal muscle mitochondria in 15 days cold exposed rats. Cold exposure strongly increases the sensitivity to uncoupling by added palmitate of skeletal muscle but not liver mitochondria, while mitochondrial energy coupling in the absence of fatty acids is only slightly affected by cold in liver and skeletal muscle. In addition, uncoupling protein 3 content does not follow changes in skeletal muscle mitochondrial coupling. It is therefore concluded that skeletal muscle could play a direct thermogenic role based on fatty acid-induced mild uncoupling of mitochondrial oxidative phosphorylation.

This study was designed to examine the effect of youth-adulthood transition on hepatic mitochondrial energy efficiency. The changes in basal and palmitate-induced proton leak, which contribute to mitochondrial efficiency, were evaluated in mitochondria isolated from the liver of young and adult rats. Alterations in mitochondrial cytochrome oxidase and aconitase specific activities, and in adenine nucleotide translocator content were also assessed. There was no difference in basal proton leak or thermodynamic coupling and efficiency of oxidative phosphorylation in liver mitochondria between the two rat groups. On the other hand, palmitate-induced proton leak increased significantly in adult rats. The function of this uncoupling could be avoidance of elevated formation of reactive oxygen species, which are known to accelerate ageing.

The transition from young to adult age is associated with decreased insulin sensitivity. To investigate whether changes in skeletal muscle mitochondrial function could be involved in the development of insulin resistance, we measured the oxidative capacity and energetic efficiency of subsarcolemmal and intermyofibrillar mitochondria isolated from the skeletal muscle of 60- and 180-day-old rats. Mitochondrial efficiency was tested by measuring the degree of thermodynamic coupling and optimal thermodynamic efficiency, as well as mitochondrial proton leak, which was determined in both the absence (basal) and the presence (fatty acid induced) of palmitate. Serum glucose, insulin, and HOMA index were also measured. The results show that in adult rats, concomitant with increased HOMA index, skeletal muscle mitochondria display higher respiratory capacity and energy efficiency. In fact, thermodynamic coupling and optimal thermodynamic efficiency significantly increased and fatty acid-induced proton leak was significantly lower in the skeletal muscle mitochondria from adult than in younger rats. A deleterious consequence of increased mitochondrial efficiency would be a reduced utilization of energy substrates, especially fatty acids, leading to intracellular triglyceride accumulation and lipotoxicity, thus contributing to the onset of skeletal muscle insulin resistance.

The changes in metabolic efficiency, body composition, and nutrient partitioning induced by high-fat feeding were evaluated in adult rats (90d of age). The alterations in serum free triiodothyronine, insulin, and leptin levels, as well as in hepatic and skeletal muscle metabolism, were also assessed. Rats were fed either a low- or a high-fat diet for 2 weeks. Relative to the low-fat feeding, energy intake and expenditure, as well as body-energy gain, lipid gain, and energetic efficiency, were increased by the high-fat feeding. Increased serum leptin levels accompanied these variations. A positive correlation between serum leptin levels and percentage of body fat was found in the rats fed the low- or high-fat diet, with a significant divergence between the slope of the regression lines. Furthermore, a negative correlation between serum leptin level and energy intake was found in the rats fed the low-fat diet, while a positive correlation was found in the rats fed the high-fat diet. Finally, the high-fat feeding decreased the hepatic and skeletal muscle mitochondrial oxidative capacity. It is concluded that, in adult rats, a nutritional factor such as a high level of fat in the diet induces obesity, leptin resistance, and impairment of mitochondrial capacity, all phenomena typical of unrestrained aged rats.

We have studied changes in hepatic mitochondrial efficiency induced by 24-h fasting or acclimation at 29 degrees C, two conditions of reduced thermogenesis. Basal and palmitate-induced proton leak, which contribute to mitochondrial efficiency, are not affected after 24-h fasting, when serum free triiodothyronine decreases significantly and serum free fatty acids increase significantly. In rats at 29 degrees C, in which serum free triiodothyronine and fatty acids decrease significantly, basal proton leak increases significantly, while no variation is found in palmitate-induced proton leak. The present results indicate that mitochondrial efficiency in the liver is not related to a physiological decrease in whole body thermogenesis.

We investigated, in skeletal muscle mitochondria isolated from semistarved and refed rats, the relation between the protein expression of uncoupling protein 3 (UCP3) and mitochondrial oxidative capacity, assessed as state 4 and state 3 respiration rates in presence of substrates that are either non-lipids (glutamate, succinate) or lipids (palmitoyl CoA, palmitoylcarnitine). During semistarvation, when whole-body thermogenesis is diminished, state 3 respiration was lower than in fed controls by about 30% independently of substrate types, while state 4 respiration was lower by 20% only during succinate oxidation, but UCP3 was unaltered. After 5 days of refeeding, when thermogenesis is still diminished, neither state 4, state 3 nor UCP3 were lower than in controls. Refeeding on a high-fat diet, which exacerbates the suppression of thermogenesis, resulted in a two-fold elevation in UCP3 but no change in state 4 or state 3 respiration. These results during semistarvation and refeeding, in line with those previously reported for fasting, are not in support of the hypothesis that UCP3 is a mediator of adaptive thermogenesis pertaining to weight regulation, and underscore the need for caution in interpreting parallel changes in UCP3 and mitochondrial oxidative capacity as the reflection of mitochondrial uncoupling by UCP3.

To establish whether changes in skeletal muscle mitochondrial efficiency contribute to increased energy expenditure and decreased metabolic efficiency of overeating rats with increased thermogenesis, we measured basal proton leak, fatty acid-induced uncoupling and uncoupling protein 3 (UCP3) content in subsarcolemmal and intermyofibrillar skeletal muscle mitochondria. Intermyofibrillar, but not subsarcolemmal, mitochondria from rats with increased thermogenesis exhibited a lower proton leak compared with controls. In both mitochondrial populations from rats with increased thermogenesis, fatty acid-induced uncoupling was increased significantly and a small recoupling effect of GDP was detected. In addition, intermyofibrillar and subsarcolemmal mitochondria from rats with increased thermogenesis showed higher UCP3 contents than controls. These results point out that metabolic efficiency in subsarcolemmal and intermyofibrillar mitochondria from rats with increased thermogenesis is differently regulated. In fact, in intermyofibrillar mitochondria both basal proton leak and fatty acid-induced uncoupling are altered, while in subsarcolemmal mitochondria only fatty acid-induced uncoupling increases. Both mitochondrial populations in skeletal muscle cells from rats with increased thermogenesis display an increased fatty acid-induced uncoupling and UCP3 content, which could contribute to avoiding obesity.

Variations in energy balance, body composition, and nutrient partitioning induced by acetyl-L-carnitine (ALCAR) supplementation were studied in young (2 mo) and old (24 mo) Wistar rats. Changes in skeletal muscle metabolism as well as in serum free triiodothyronine and leptin levels were also evaluated. Rats were administered 0 (control) or 15 g/L ALCAR in their drinking water for 1 mo. ALCAR treatment significantly decreased body lipid percentage in young rats and significantly increased body protein percentage in old rats. The percentage of metabolizable energy (ME) intake stored as lipid was lower in ALCAR-treated young rats, whereas the percentage of ME intake stored as protein was greater in ALCAR-treated old rats compared with their age-matched controls. In addition, ALCAR supplementation significantly decreased serum leptin levels in old rats. Elevated skeletal muscle respiration was found in old rats treated with ALCAR, due to an increase in mitochondrial protein mass. In conclusion, ALCAR supplementation decreases efficiency of lipid deposition in young rats and increases efficiency of protein deposition in old rats. In addition, ALCAR supplementation partly reduces the leptin resistance that occurs in old rats, and improves ATP production in skeletal muscle mitochondria through an increase in mitochondrial protein content.

OBJECTIVE: To investigate whether young rats respond to high-fat feeding through changes in energy efficiency and fuel partitioning at the level of skeletal muscle, to avoid obesity development. In addition, to establish whether the two mitochondrial subpopulations, which exist in skeletal muscle, ie subsarcolemmal and intermyofibrillar, are differently affected by high-fat feeding. DESIGN: Weaning rats were fed a low-fat or a high-fat diet for 15 days. MEASUREMENTS: Energy balance and lipid partitioning in the whole animal. State 3 and state 4 oxygen consumption rates in whole skeletal muscle homogenate. State 3 and state 4 oxygen consumption rates, membrane potential and uncoupling effect of palmitate in subsarcolemmal and intermyofibrillar mitochondria from skeletal muscle. RESULTS: Rats fed a high-fat diet showed an increased whole body lipid utilization. Skeletal muscle NAD-linked and lipid oxidative capacity significantly increased at the whole-tissue level, due to an increase in lipid oxidative capacity in subsarcolemmal and intermyofibrillar mitochondria and in NAD-linked activity only in intermyofibrillar ones. In addition, rats fed a high-fat diet showed an increase in the uncoupling effect of palmitate in both the mitochondrial populations. CONCLUSIONS: In young rats fed a high-fat diet, skeletal muscle contributes to enhanced whole body lipid oxidation through an increased mitochondrial capacity to use lipids as metabolic fuels, associated with a decrease in energy coupling.

The purpose of this study was to investigate the decline in rat liver mitochondria respiration found in adult rats compared to younger ones, and to find a link between this respiratory impairment and a tissue hypothyroidism state. To this end, hepatic concentration and serum levels of triiodothyronine were measured in postpubertal rats (60 days old) and adult rats (180 days old). In addition, in these rats we measured oxidative phosphorylation in homogenate together with coupled and uncoupled respiration in isolated mitochondria using succinate or durohydroquinone as substrate. We found that mitochondria from adult rats consumed less oxygen compared to younger rats due to lower electron transport chain and phosphorylating system activity. In addition, we found that in state 4 condition, mitochondria from adult rats consumed less oxygen than mitochondria from young rats. Finally, we found a decrease in liver triiodothyronine concentration in adult rats. In conclusion, the results of this study show that hepatic mitochondria in adult rats have a decreased ATP synthesis capacity and proton permeability, both consistent with the tissue hypothyroidism found in the liver of adult rats.

We have investigated the effect of 24-h fasting on basal proton leak and uncoupling protein (UCP) 3 expression at the protein level in subsarcolemmal and intermyofibrillar skeletal muscle mitochondria. In fed rats, the two mitochondrial populations displayed different proton leak, but the same protein content of UCP3. In addition, 24-h fasting, both at 24 and 29°C, induced an increase in proton leak only in subsarcolemmal mitochondria, while UCP3 content increased in both the populations. From the present data, it appears that UCP3 does not control the basal proton leak of skeletal muscle mitochondria.

The purpose of this work was to study the effect of early long-term high-fat feeding on energy balance and liver oxidative activity. To this end, rats aged about 30 d were fed a high-fat or a low-fat diet for 15, 30 or 60 d. Full energy balance and energy partitioning measurements were carried out. In addition, we measured hepatic mitochondrial and peroxisomal oxidative capacities. Serum levels of free triiodothyronine (T3) and leptin were also determined. Rats fed a high-fat diet showed an increase in metabolizable energy intake as well as in energy expenditure, while lipid gain over the whole period was lower than that expected due to a decrease in metabolic efficiency. An increase in serum free T3levels was also found in rats fed a high-fat diet after 15 and 30 d. Statistically significant correlations between serum leptin levels and body fat mass were found after 15, 30 and 60 d of high-fat feeding. Finally, no variation in hepatic mitochondrial and peroxisomal fatty acid oxidation capacity was found in rats fed a high-fat diet for 15, 30 or 60 d. In conclusion, the results of the present study show that young Wistar rats fed a high-fat diet for up to 60 d are able to counteract, at least in part, obesity development.

OBJECTIVE: To study the effects of fat feeding on fat balance and hepatic mitochondrial function in postpubertal male rats. DESIGN: Rats were fed low fat, medium fat or high fat diet for 15 days. MEASUREMENTS: Energy balance, body composition, resting metabolic rate (RMR), mitochondrial state 3 and state 4 oxygen consumption rates, succinic dehydrogenase (EC 1.3.99.1) and mitochondrial α-glycerophosphate dehydrogenase (EC 1.1.1.8) activities. RESULTS: Rats fed medium fat or high fat diet, in comparison with rats fed low fat diet, showed a significantly greater metabolisable energy intake and energy expenditure. In addition, body energy and lipid gains were significantly higher in rats fed medium fat or high fat diet than in rats fed low fat diet. Mitochondrial respiration and enzymatic activities were not affected by fat feeding. CONCLUSION: These results indicate that in postpubertal rats fed high fat diets, the increase in energy expenditure counteracts only in part the excess fat deposition. This is probably due to the impairment in regulatory responses, and enhances thermogenesis.

Energy intake, utilization, and partitioning were determined in male Wistar rats from 25 to 180 d of age. Serum free triiodothyronine, leptin, and free fatty acid concentrations were also measured. Energy balance measurements allowed us to identify a period from 25 to 90 d, characterized by a rapid body growth rate and another from 90 to 180 d, during which body growth rate slowed. From 25 to 180 d, we found decreases in daily energy intake and expenditure, which were faster before 90 d. The first period was characterized by storage of lipid and protein. In the second period, protein deposition approached zero and the excess of ingested energy was entirely stored as fat, so that age-associated obesity began to develop. The inability of rats to maintain a stable body weight after the cessation of growth of lean body mass is not due to decreased resting metabolism but rather to a partial leptin resistance. J.

We studied hepatic and skeletal muscle metabolic activity in rats fed a high-fat diet. Rats were fed a low-fat or high-fat diet for 15 days. At the end of the experimental period, full energy-balance determinations together with serum free triiodothyronine (FT 3 ), leptin, and free fatty acid (FFA) measurements were performed. In addition, we assessed fatty acid—stimulated oxygen consumption in perfused liver and in skeletal muscle homogenate. Rats fed a high-fat diet showed a significant increase in energy intake but no variation in body energy gain, due to a significant increase in energy expenditure. Serum FT 3 and FFA levels significantly increased in rats fed a high-fat diet versus rats fed a low-fat diet, while no variation was found in serum leptin levels. Perfused livers and skeletal muscle homogenates from rats fed a high-fat diet exhibited a significant increase in fatty acid-stimulated oxygen consumption. Our results suggest that the enhanced fatty acid oxidation rates in liver and skeletal muscle contribute to the maintenance of fat balance in response to increased fat intake, preventing excess fat deposition.

Changes in mitochondrial function were studied in perfused liver from rats aged 24 – 365 days. Oxygen consumption together with the rates of gluconeogenesis, urea synthesis and ketogenesis were determined. Basal mitochondrial respiration as well as the ability of the liver to synthesize glucose, urea and ketone bodies declined from 24- to 365-day-old rats. On the other hand, on transition from 24 to 60 days the liver oxidation rate of hexanoate, sorbitol and glycerol is enhanced, but not of ketone bodies or palmitate. Our results show that the transition from weaning to middle age is accompanied by defined changes in hepatic substrate oxidation. From the observed time course of the decrease in basal and substrate-stimulated oxygen consumption, it is concluded that in rat liver cells a decline in respiratory chain function, long-chain fatty acid and ketone body metabolism, gluconeogenesis and ureogenesis occurs at a relatively early life stage.

The purpose of this study was to evaluate the oxidative capacities in hepatic mitochondria isolated from prepubertal, young adult and adult rats (40, 90 and 180 days of age, respectively). In these rats, mitochondrial respiratory rates using FAD- and NAD-linked substrates as well as mitochondrial protein mass were measured. The results show that only the oxidative capacity of FAD-linked pathways significantly declined in mitochondria from 180-day-old rats compared with those from younger animals. When we consider FAD-linked respiration expressed per g liver, no significant difference was found among rats of different ages because of an increased mitochondrial protein mass found in 180-day-old rats. However, when FAD-linked and lipid-dependent respiratory rates were expressed per 100 g body weight, significant decreases occurred in 180-day-old rats. Therefore, the decrease in liver weight expressed per 100 g body weight rather than an impaired hepatic cellular activity may be the cause of body energy deficit in 180-day-old rats.

In the present study energy balance and liver respiratory activity were studied in rats fed on either a control diet or an energy-dense diet. Liver respiration was assessed both without added substrates and after the addition of hexanoate, glycerol, or sorbitol. The effect of ouabain on hexanoate-supported respiration was also determined. Metabolizable energy intake and energy expenditure increased in rats fed on an energy dense diet, but body-weight gain, as well as lipid and protein content, remained unchanged. When net energy expenditure, obtained excluding the total cost of storage, was expressed as a percentage of metabolizable energy, significant differences were found between the two groups of rats. This finding supports the presence of regulatory mechanisms in rats fed on an energy-dense diet, which are useful to counteract development of obesity. In addition, a significant increase in liver respiratory activity was found in rats fed on an energy-dense diet, both in the basal state and in that stimulated by added substrates. Na/K-pump-dependent O2consumption also increased in rats fed on an energy-dense diet. The results indicate that a greater production of metabolic heat by the liver can contribute to the increased energy expenditure found in rats fed on an energy-dense diet.

The regulatory and obligatory components of cephalic and gastrointestinal phases of the thermic effect of food (TEF) were measured in control and hypothyroid rats. A significant decrease (P Journal of Endocrinology (1996) 148, 167–174

We measured oxygen consumption in hepatocytes isolated from rats fed an energy-dense diet. We conceptually divided the respiratory pathway into blocks of reactions that produced or consumed mitochondrial membrane potential, and then measured the kinetic response of these blocks of reactions to this potential. The results show increased hepatic respiration rates in rats fed an energy-dense diet accounted for by an increased rate of the substrate oxidation reactions at any particular mitochondrial membrane potential, but no change in the kinetic response to potential of the reactions that consume it, resulting in a small rise in this potential in the rats fed an energy-dense diet. The increased hepatic oxygen consumption is in line with the increased energy expenditure that occurs in rats fed an energy-dense diet.

In this paper we have examined the effect of cold exposure on hepatic mitochondrial state 3 respiration and ATP synthesis, using succinate as the substrate, in euthyroid, hypothyroid and hyperthyroid rats. The results show that cold exposure does not elicit any variation in the above parameters in euthyroid and hyperthyroid rats, whereas when hypothyroid rats are exposed to cold, a significant increase (about +45%) occurs in state 3 respiration and ATP synthesis. We have also measured succinic dehydrogenase specific activity and uncoupled respiration during cold exposure in various thyroid states. The finding that cold exposure elicits no variation in the above parameters indicates that there is some control on ATP synthase and/or adenine nucleotide translocator. The above findings, as a whole, suggest that cold exposure acts on oxidative phosphorylation only if triiodothyronine is lacking, by controlling ATP synthase and/or adenine nucleotide translocator.