The Effects of Starvation and Refeeding on Carbohydrate and Lipid Metabolism in Vivo and in the Perfused Rat Liver

Metabolic sensing and control in mitochondria

Mitochondria are membrane-enclosed organelles with endosymbiotic origins, harboring independent genomes and a unique biochemical reaction network. To perform their critical functions, mitochondria must maintain a distinct biochemical environment and coordinate with the cytosolic metabolic networks of the host cell. This coordination requires them to sense and control metabolites and respond to metabolic stresses. Indeed, mitochondria adopt feedback or feedforward control strategies to restrain metabolic toxicity, enable metabolic conservation, ensure stable levels of key metabolites, allow metabolic plasticity, and prevent futile cycles. A diverse panel of metabolic sensors mediates these regulatory circuits whose malfunctioning leads to inborn errors of metabolism with mild to severe clinical manifestations. In this review, we discuss the logic and molecular basis of metabolic sensing and control in mitochondria. The past research outlined recurring patterns in mitochondrial metabolic sensing and control and highlighted key knowledge gaps in this organelle that are potentially addressable with emerging technological breakthroughs.

Lipid-droplet associated mitochondria promote fatty-acid oxidation through a distinct bioenergetic pattern in male Wistar rats

Mitochondria empower the liver to regulate lipid homeostasis by enabling fatty acid oxidation during starvation and lipogenesis during nutrient-rich conditions. It is unknown if mitochondria can seamlessly regulate these two distinct processes or if two discrete populations of mitochondria achieve these two functions in the liver. For the first time in the liver, we report the isolation of two distinct populations of mitochondria from male Wistar rats on an ad-libitum diet: cytoplasmic mitochondria and lipid droplet-associated mitochondria. Our studies show that while lipid droplet mitochondria exhibit higher fatty acid oxidation and are marked by enhanced levels of pACC2, MFN2, and CPT1 activity, cytoplasmic mitochondria are associated with higher respiration capacity. Notably, lipid droplet-associated mitochondria isolated from a non-alcoholic fatty liver disease (NAFLD) rat model are compromised for fatty acid oxidation. We demonstrate the importance of functional segregation of mitochondria as any aberration in lipid droplet-associated mitochondria may lead to NAFLD.

Emerging Role of Hepatic Ketogenesis in Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD), the most common chronic liver diseases, arise from non-alcoholic fatty liver (NAFL) characterized by excessive fat accumulation as triglycerides. Although NAFL is benign, it could progress to non-alcoholic steatohepatitis (NASH) manifested with inflammation, hepatocyte damage and fibrosis. A subset of NASH patients develops end-stage liver diseases such as cirrhosis and hepatocellular carcinoma. The pathogenesis of NAFLD is highly complex and strongly associated with perturbations in lipid and glucose metabolism. Lipid disposal pathways, in particular, impairment in condensation of acetyl-CoA derived from β-oxidation into ketogenic pathway strongly influence the hepatic lipid loads and glucose metabolism. Current evidence suggests that ketogenesis dispose up to two-thirds of the lipids entering the liver, and its dysregulation significantly contribute to the NAFLD pathogenesis. Moreover, ketone body administration in mice and humans shows a significant improvement in NAFLD. This review focuses on hepatic ketogenesis and its role in NAFLD pathogenesis. We review the possible mechanisms through which impaired hepatic ketogenesis may promote NAFLD progression. Finally, the review sheds light on the therapeutic implications of a ketogenic diet in NAFLD.

TAZ inhibits glucocorticoid receptor and coordinates hepatic glucose homeostasis in normal physiological states

The elucidation of the mechanisms whereby the liver maintains glucose homeostasis is crucial for the understanding of physiological and pathological states. Here, we show a novel role of hepatic transcriptional co-activator with PDZ-binding motif (TAZ) in the inhibition of glucocorticoid receptor (GR). TAZ is abundantly expressed in pericentral hepatocytes and its expression is markedly reduced by fasting. TAZ interacts via its WW domain with the ligand-binding domain of GR to limit the binding of GR to the GR response element in gluconeogenic gene promoters. Therefore, liver-specific TAZ knockout mice show increases in glucose production and blood glucose concentration. Conversely, the overexpression of TAZ in mouse liver reduces the binding of GR to gluconeogenic gene promoters and glucose production. Thus, our findings demonstrate that hepatic TAZ inhibits GR transactivation of gluconeogenic genes and coordinates gluconeogenesis in response to physiological fasting and feeding.

Effects of fasting, feeding and exercise on plasma acylcarnitines among subjects with CPT2D, VLCADD and LCHADD/TFPD

BACKGROUND

The plasma acylcarnitine profile is frequently used as a biochemical assessment for follow-up in diagnosed patients with fatty acid oxidation disorders (FAODs). Disease specific acylcarnitine species are elevated during metabolic decompensation but there is clinical and biochemical heterogeneity among patients and limited data on the utility of an acylcarnitine profile for routine clinical monitoring.

METHODS

We evaluated plasma acylcarnitine profiles from 30 diagnosed patients with long-chain FAODs (carnitine palmitoyltransferase-2 (CPT2), very long-chain acyl-CoA dehydrogenase (VLCAD), and long-chain 3-hydroxy acyl-CoA dehydrogenase or mitochondrial trifunctional protein (LCHAD/TFP) deficiencies) collected after an overnight fast, after feeding a controlled low-fat diet, and before and after moderate exercise. Our purpose was to describe the variability in this biomarker and how various physiologic states effect the acylcarnitine concentrations in circulation.

RESULTS

Disease specific acylcarnitine species were higher after an overnight fast and decreased by approximately 60% two hours after a controlled breakfast meal. Moderate-intensity exercise increased the acylcarnitine species but it varied by diagnosis. When analyzed for a genotype/phenotype correlation, the presence of the common LCHADD mutation (c.1528G > C) was associated with higher levels of 3-hydroxyacylcarnitines than in patients with other mutations.

CONCLUSIONS

We found that feeding consistently suppressed and that moderate intensity exercise increased disease specific acylcarnitine species, but the response to exercise was highly variable across subjects and diagnoses. The clinical utility of routine plasma acylcarnitine analysis for outpatient treatment monitoring remains questionable; however, if acylcarnitine profiles are measured in the clinical setting, standardized procedures are required for sample collection to be of value.

Mitochondria Bound to Lipid Droplets: Where Mitochondrial Dynamics Regulate Lipid Storage and Utilization

The isolation and biochemical characterization of lipid droplet (LD)-associated mitochondria revealed the capacity of the cell to produce and maintain distinct mitochondrial populations carrying disparate proteome and dissimilar capacities to oxidize fatty acids and pyruvate. With mitochondrial motility being a central parameter determining mitochondrial fusion, adherence to LDs provides a mechanism by which peridroplet mitochondria (PDM) remain segregated from cytoplasmic mitochondria (CM). The existence of metabolically distinct subpopulations provides an explanation for the capacity of mitochondria within the individual cell to be involved simultaneously in fatty acid oxidation and LD formation. The mechanisms that deploy mitochondria to the LD and the dysfunctions that result from unbalanced proportions of PDM and CM remain to be explored. Understanding the roles and regulation of mitochondrial tethering to LDs offers new points of intervention in metabolic diseases.

Network Modeling of Liver Metabolism to Predict Plasma Metabolite Changes During Short-Term Fasting in the Laboratory Rat

The liver—a central metabolic organ that integrates whole-body metabolism to maintain glucose and fatty-acid regulation, and detoxify ammonia—is susceptible to injuries induced by drugs and toxic substances. Although plasma metabolite profiles are increasingly investigated for their potential to detect liver injury earlier than current clinical markers, their utility may be compromised because such profiles are affected by the nutritional state and the physiological state of the animal, and by contributions from extrahepatic sources. To tease apart the contributions of liver and non-liver sources to alterations in plasma metabolite profiles, here we sought to computationally isolate the plasma metabolite changes originating in the liver during short-term fasting. We used a constraint-based metabolic modeling approach to integrate central carbon fluxes measured in our study, and physiological flux boundary conditions gathered from the literature, into a genome-scale model of rat liver metabolism. We then measured plasma metabolite profiles in rats fasted for 5–7 or 10–13 h to test our model predictions. Our computational model accounted for two-thirds of the observed directions of change (an increase or decrease) in plasma metabolites, indicating their origin in the liver. Specifically, our work suggests that changes in plasma lipid metabolites, which are reliably predicted by our liver metabolism model, are key features of short-term fasting. Our approach provides a mechanistic model for identifying plasma metabolite changes originating in the liver.

Weaning age affects growth, feed intake, gastrointestinal development, and behavior in Holstein calves fed an elevated plane of nutrition during the preweaning stage

Recent research has revealed potential advantages of feeding an elevated plane of nutrition to calves during the preweaning period. However, calves fed more nutrients preweaning may be more susceptible to depressed growth and weaning stress during the transition from liquid to solid feed. The objective of this study was to investigate the relationship between the age of weaning and feed intake, and its influence on growth, gastrointestinal development, and behavioral indicators in dairy calves fed an elevated plane of nutrition during the preweaning period. To meet this objective, 20 female Holstein calves were randomly assigned at birth to be weaned at 6 or 8 wk. Milk replacer (mixed at 150 g/L) was offered at 1.2 kg/calf per day in 2 meals until a 1-wk step-down, when meals were reduced by 50% 1 wk before weaning. Daily starter, chopped oat straw, water intake, and weekly body weights were measured until d 70 of life. To assess digestive tract development, rumen fluid, fecal, and blood samples were taken before and after weaning (d 35, 49, and 63) and analyzed for ruminal short-chain fatty acids, blood β-hydroxybutyrate, and fecal starch, respectively. Behavioral indicators of weaning stress, including vocalizing and non-nutritive oral behavior, were measured by visual observation for 1 h, 3 times per week, before the second feeding of the day during the period from 2 wk before weaning to 2 wk after weaning. The calves weaned at 8 wk compared with 6 wk had higher average daily gain for the week preweaning (0.79±0.09 vs. 0.34±0.10 kg/d) and postweaning (1.05±0.09 vs. 0.35±0.11 kg/d), and were heavier at d 70 (99.9±1.81 vs. 91.0±2.26 kg). From 5 to 8 wk of age, starter and water intakes were lower in calves weaned at 8 wk of age. However, overall starter intake did not differ during the last week of the experiment. Furthermore, calves weaned at 8 wk compared with 6 wk had higher starter intake for 1 wk preweaning (1.36±0.13 vs. 0.40±0.08 kg/d) and postweaning (2.51±0.20 vs. 1.16±0.15 kg/d). In both treatments, weaning increased ruminal short-chain fatty acids, blood β-hydroxybutyrate, and fecal starch, yet the differences between the week before and after weaning were greater for calves weaned at 6 wk compared with those weaned at 8 wk. Treatment × week relative to weaning interactions indicated that several behaviors varied between early- and later-weaned calves during the week before weaning; calves weaned at 6 wk tended to exhibit 75% more non-nutritive oral behavior and spent 55% less time ruminating, and 36% less time lying compared with calves weaned at 8 wk. Under the conditions of this study, the results suggest that calves fed an elevated plane of nutrition preweaning have higher starter intakes and average daily gain during the weaning period when weaning is extended from 6 to 8 wk of age.

Thermoneutrality modifies the impact of hypoxia on lipid metabolism

Hypoxia has been shown to rapidly increase triglycerides in mice by decreasing plasma lipoprotein clearance. However, the usual temperature of hypoxic exposure is below thermoneutrality for mice, which may increase thermogenesis and energy requirements, resulting in higher tissue lipid uptake. We hypothesize that decreased lipid clearance and ensuing hyperlipidemia are caused by hypoxic suppression of metabolism at cold temperatures and, therefore, would not occur at thermoneutrality. Twelve-week-old, male C57BL6/J mice were exposed to 6 h of 10% O₂ at the usual temperature (22°C) or thermoneutrality (30°C). Acclimation to 22°C increased lipid uptake in the heart, lungs, and brown adipose tissue, resulting in lower plasma triglyceride and cholesterol levels. At this temperature, hypoxia attenuated lipid uptake in most tissues, thereby raising plasma triglycerides and LDL cholesterol. Thermoneutrality decreased tissue lipid uptake, and hypoxia did not cause a further reduction in lipid uptake in any organs. Consequently, hypoxia at thermoneutrality did not affect plasma triglyceride levels. Unexpectedly, plasma HDL cholesterol increased. The effect of hypoxia on white adipose tissue lipolysis was also modified by temperature. Independent of temperature, hypoxia increased heart rate and glucose and decreased activity, body temperature, and glucose sensitivity. Our study underscores the importance of ambient temperature for hypoxia research, especially in studies of lipid metabolism.

Malonyl-CoA: the regulator of fatty acid synthesis and oxidation

In the catabolic state with no food intake, the liver generates ketones by breaking down fatty acids. During the nocturnal fast or longer starvation periods, this protects the brain, which cannot oxidize fatty acids. In 1977, we published a study in the JCI noting the surprising realization that malonyl-CoA, the substrate of fatty acid synthesis, was also an inhibitor of fatty acid oxidation. Subsequent experiments have borne out this finding and furthered our understanding of molecular metabolism.

Acute hypoxia induces hypertriglyceridemia by decreasing plasma triglyceride clearance in mice

Obstructive sleep apnea (OSA) induces intermittent hypoxia (IH) during sleep and is associated with elevated triglycerides (TG). We previously demonstrated that mice exposed to chronic IH develop elevated TG. We now hypothesize that a single exposure to acute hypoxia also increases TG due to the stimulation of free fatty acid (FFA) mobilization from white adipose tissue (WAT), resulting in increased hepatic TG synthesis and secretion. Male C57BL6/J mice were exposed to FiO(2) = 0.21, 0.17, 0.14, 0.10, or 0.07 for 6 h followed by assessment of plasma and liver TG, glucose, FFA, ketones, glycerol, and catecholamines. Hypoxia dose-dependently increased plasma TG, with levels peaking at FiO(2) = 0.07. Hepatic TG levels also increased with hypoxia, peaking at FiO(2) = 0.10. Plasma catecholamines also increased inversely with FiO(2). Plasma ketones, glycerol, and FFA levels were more variable, with different degrees of hypoxia inducing WAT lipolysis and ketosis. FiO(2) = 0.10 exposure stimulated WAT lipolysis but decreased the rate of hepatic TG secretion. This degree of hypoxia rapidly and reversibly delayed TG clearance while decreasing [(3)H]triolein-labeled Intralipid uptake in brown adipose tissue and WAT. Hypoxia decreased adipose tissue lipoprotein lipase (LPL) activity in brown adipose tissue and WAT. In addition, hypoxia decreased the transcription of LPL, peroxisome proliferator-activated receptor-γ, and fatty acid transporter CD36. We conclude that acute hypoxia increases plasma TG due to decreased tissue uptake, not increased hepatic TG secretion.

Vaccenic and elaidic acid equally esterify into triacylglycerols, but differently into phospholipids of fed rat liver cells

Elaidic acid (trans-9-C₁₈:₁ or trans-9) is assumed to exert atherogenic effects due to its double bond configuration. The possibility that trans-9 and vaccenic acid (trans-11-C₁₈:₁ or trans-11), its positional isomer, were biochemically equivalent and interchangeable compounds, was investigated by reference to their cis-isomers through esterification-related activities using rat liver cells and subcellular fractions. In hepatocytes, both trans-C₁₈:₁ were incorporated to the same extent in triacylglycerols, but trans-9 was more esterified than trans-11 into phospholipids (P < 0.05). Glycerol-3-phosphate acyltransferase activity in microsomes was lower with trans-11 than with trans-9, while this activity in mitochondria was ~40% greater with trans-11 than with trans-9 (P < 0.05). Activity of 2-lysophosphatidic acid acyltransferase in microsomes was of comparable extent with both trans isomers, but activity of 2-lysophosphatidylcholine acyltransferase was significantly greater with trans-9 than with trans-11 at P < 0.01. Lipoproteins secreted by hepatocytes reached equivalent levels in the presence of any isomers, but triacylglycerol production was more elevated with trans-11 than with trans-9 at P < 0.05. Cholesterol efflux from previously labelled hepatocytes was lower with trans-11 than with trans-9. When these cells were exposed to either trans-C₁₈:₁, the gene expression of proteins involved in fatty acid esterification and lipoprotein synthesis was unaffected, which indicates that the biochemical differences essentially depended on enzyme/substrate affinities. On the whole, vaccenic and elaidic acid were shown to incorporate cell phospholipids unequally, at least in vitro, which suggests they can differently affect lipid metabolic pathways in normal cells.

Rat liver mitochondrial carnitine palmitoyltransferase-I, hepatic carnitine, and malonyl-CoA: effect of starvation

Hepatic mitochondrial fatty acid oxidation and ketogenesis increase during starvation. Carnitine palmitoyltransferase I (CPT-I) catalyses the rate-controlling step in the overall pathway and retains its control over beta-oxidation under fed, starved and diabetic conditions. To determine the factors contributing to the reported several-fold increase in fatty acid oxidation in perfused livers, we measured the V(max) and K(m) values for palmitoyl-CoA and carnitine, the K(i) (and IC(50)) values for malonyl-CoA in isolated liver mitochondria as well as the hepatic malonyl-CoA and carnitine contents in control and 48 h starved rats. Since CPT-I is localized in the mitochondrial outer membrane and in contact sites, the kinetic properties of CPT-I also was determined in these submitochondrial structures. After 48 h starvation, there is: (a) a significant increase in K(i) and decrease in hepatic malonyl-CoA content; (b) a decreased K(m) for palmitoyl-CoA; and (c) increased catalytic activity (V(max)) and CPT-I protein abundance that is significantly greater in contact sites compared with outer membranes. Based on these changes the estimated increase in mitochondrial fatty acid oxidation is significantly less than that observed in perfused liver. This suggests that CPT-I is regulated in vivo by additional mechanism(s) lost during mitochondrial isolation or/and that mitochondrial oxidation of peroxisomal beta-oxidation products contribute to the increased ketogenesis by bypassing CPT-I. Furthermore, the greater increase in CPT-I protein in contact sites as compared to outer membranes emphasizes the significance of contact sites in hepatic fatty acid oxidation.

The regulation of ketogenesis

Ketone bodies accumulate in the plasma in conditions of fasting and uncontrolled diabetes. The initiating event is a change in the molar ratio of glucagon:insulin. Insulin deficiency triggers the lipolytic process in adipose tissue with the result that free fatty acids pass into the plasma for uptake by liver and other tissues. Glucagon appears to be the primary hormone involved in the induction of fatty acid oxidation and ketogenesis in the liver. It acts by acutely dropping hepatic malonyl-CoA concentrations as a consequence of inhibitory effects exerted in the glycolytic pathway and on acetyl-CoA carboxylase (EC 6.4.1.2). The fall in malonyl-CoA concentration activates carnitine acyltransferase I (EC 2.3.1.21) such that long-chain fatty acids can be transported through the inner mitochondrial membrane to the enzymes of fatty acid oxidation and ketogenesis. The latter are high-capacity systems assuring that fatty acids entering the mitochondria are rapidly oxidized to ketone bodies. Thus, the rate-controlling step for ketogenesis is carnitine acyltransferase I. Administration of food after a fast, or of insulin to the diabetic subject, reduces plasma free fatty acid concentrations, increases the liver concentration of malonyl-CoA, inhibits carnitine acyltransferase I and reverses the ketogenic process.

The nature of the growth pattern and of the metabolic response to fasting in the rat are dependent upon the dietary protein and folic acid intakes of their pregnant dams and post-weaning fat consumption

The nutritional cues which induce different phenotypes from a single genotype in developing offspring are poorly understood. How well prenatal nutrient availability before birth predicts that after birth may also determine the offspring's response to later metabolic challenge. We investigated the effect of feeding pregnant rats diets containing protein at 180 g/kg (Control) or 90 g/kg (protein-restricted, PR) and either 1 or 5 mg folic acid/kg on growth and metabolic response to fasting in their offspring, and also the effect of diets with different fat contents (40 g/kg (Fat(4)) or 100 g/kg (Fat(10))) after weaning. Offspring of dams fed the PR diet with 5 mg/kg folic acid were significantly lighter than other offspring. The PR offspring fed the Fat(4) diet had lower plasma TAG than the Control offspring, but this relationship was reversed when offspring were fed Fat(10). Increasing the folic acid content of the Control or PR maternal diets induced opposing effects on plasma TAG, NEFA, beta-hydroxybutyrate and glucose concentrations in offspring fed Fat(4). The effect was accentuated in offspring fed the Fat(10) diet such that these metabolites were increased in the Control offspring, but reduced in the PR offspring. These data show for the first time that maternal dietary folic acid intake alters offspring phenotype depending upon dietary protein intake, and that this effect is modified by fat intake after weaning. Prevention by increased folic acid intake of an altered metabolic phenotype by maternal protein-restriction may be at the expense of somatic growth.

Regulation of Lipid Flux between Liver and Adipose Tissue during Transient Hepatic Steatosis in Carnitine-depleted Rats

Rats with carnitine deficiency due to trimethylhydrazinium propionate (mildronate) administered at 80 mg/100 g body weight per day for 10 days developed liver steatosis only upon fasting. This study aimed to determine whether the transient steatosis resulted from triglyceride accumulation due to the amount of fatty acids preserved through impaired fatty acid oxidation and/or from up-regulation of lipid exchange between liver and adipose tissue. In liver, mildronate decreased the carnitine content by approximately 13-fold and, in fasted rats, lowered the palmitate oxidation rate by 50% in the perfused organ, increased 9-fold the triglyceride content, and doubled the hepatic very low density lipoprotein secretion rate. Concomitantly, triglyceridemia was 13-fold greater than in controls. Hepatic carnitine palmitoyltransferase I activity and palmitate oxidation capacities measured in vitro were increased after treatment. Gene expression of hepatic proteins involved in fatty acid oxidation, triglyceride formation, and lipid uptake were all increased and were associated with increased hepatic free fatty acid content in treated rats. In periepididymal adipose tissue, mildronate markedly increased lipoprotein lipase and hormone-sensitive lipase activities in fed and fasted rats, respectively. On refeeding, carnitine-depleted rats exhibited a rapid decrease in blood triglycerides and free fatty acids, then after approximately 2 h, a marked drop of liver triglycerides and a progressive decrease in liver free fatty acids. Data show that up-regulation of liver activities, peripheral lipolysis, and lipoprotein lipase activity were likely essential factors for excess fat deposit and release alternately occurring in liver and adipose tissue of carnitine-depleted rats during the fed/fasted transition.

Prevention of adipose tissue depletion during food deprivation in angiotensin type 2 receptor-deficient mice

Angiotensin (Ang) II is produced locally in various tissues, but its role in the regulation of tissue metabolism is still unclear. Recent studies have revealed the role of type 2 Ang II receptor (AT2R) in the control of energy homeostasis and lipid metabolism. The contribution of the AT2R to adaptation to starvation was tested using AT2R-deficient (AT2R (y)(/-)) mice. Fasted AT2R (y)(/-) mice exhibited a lower loss of adipose tissue weight associated to a decreased free fatty acid (FFA) release from stored lipids than the controls. In vitro studies show that Ang II causes an AT1R-mediated antilipolytic effect in isolated adipocytes. AT1R expression is up-regulated by fasting in both genotypes, but the increase is more pronounced in AT2R (y/-) mice. In addition, the increased muscle beta-oxidation displayed in AT2R (y/-) mice on a fed state, persists after fasting compared with wild-type mice. In liver from fed mice, AT2R deficiency did not modify the expression of genes involved in fatty acid oxidation. However, in response to fasting, the large increase of the expression of this subset of genes exhibited by wild-type mice, was impaired in AT2R (y/-) mice. Taken together, decreased lipolytic capacity and increased muscle fatty acid oxidation participate in the decreased plasma FFA observed in fasted AT2R (y/-) mice and could account for the lower FFA metabolism in the liver. These data reveal an important physiological role of AT2R in metabolic adaptations to fasting.

Activation of Liver Carnitine Palmitoyltransferase‐1 and Mitochondrial Acetoacetyl‐CoA Thiolase Is Associated with Elevated Ketone Body Levels in the ElasmobranchSqualus acanthias

Elasmobranch fishes are an ancient group of vertebrates that have unusual lipid metabolism whereby storage lipids are mobilized from the liver for peripheral oxidation largely as ketone bodies rather than as nonesterified fatty acids under normal conditions. This reliance on ketones, even when feeding, implies that elasmobranchs are chronically ketogenic. Compared to specimens sampled within 2 d of capture (recently captured), spiny dogfish Squalus acanthias that were held for 16-33 d without apparent feeding displayed a 4.5-fold increase in plasma concentration of d- beta -hydroxybutyrate (from 0.71 to 3.2 mM) and were considered ketotic. Overt activity of carnitine palmitoyltransferase-1 in liver mitochondria from ketotic dogfish was characterized by an increased apparent maximal activity, a trend of increasing affinity (reduced apparent K(m); P=0.09) for l-carnitine, and desensitization to the inhibitor malonyl-CoA relative to recently captured animals. Acetoacetyl-CoA thiolase (ACoAT) activity in isolated liver mitochondria was also markedly increased in the ketotic dogfish compared to recently captured fish, whereas no difference in 3-hydroxy-3-methylglutaryl-CoA synthase activity was found between these groups, suggesting that ACoAT plays a more important role in the activation of ketogenesis in spiny dogfish than in mammals and birds.

Caloric restriction in C57BL/6J mice mimics therapeutic fasting in humans

Background

Caloric restriction (CR) has long been recognized as a dietary therapy that improves health and increases longevity. Little is known about the persistent effects of CR on plasma biomarkers (glucose, ketone bodies, and lipids) following re-feeding in mice. It is also unclear how these biomarker changes in calorically restricted mice relate to those observed previously in calorically restricted humans.

Results

Three groups of individually housed adult female C57BL/6J (B6) mice (n = 4/group) were fed a standard rodent chow diet either: (1) unrestricted (UR); (2) restricted for three weeks to reduce body weight by approximately 15–20% (R); or (3) restricted for three weeks and then re-fed unrestricted (ad libitum) for an additional three weeks (R-RF). Body weight and food intake were measured throughout the study, while plasma lipids and levels of glucose and ketone bodies (β-hydroxybutyrate) were measured at the termination of the study. Plasma glucose, phosphatidylcholine, cholesterol, and triglycerides were significantly lower in the R mice than in the UR mice. In contrast, plasma fatty acids and β-hydroxybutyrate were significantly higher in the R mice than in the UR mice. CR had no effect on plasma phosphatidylinositol levels. While body weight and plasma lipids of the R-RF mice returned to unrestricted levels upon re-feeding, food intake and glucose levels remained significantly lower than those prior to the initiation of CR.

Conclusion

CR establishes a new homeostatic state in B6 mice that persists for at least three weeks following ad libitum re-feeding. Moreover, the plasma biomarker changes observed in B6 mice during CR mimic those reported in humans on very low calorie diets or during therapeutic fasting.

Direct comparison of the acute in vivo effects of HIV protease inhibitors on peripheral glucose disposal

The clinical use of HIV protease inhibitors (PIs) is associated with the development of peripheral insulin resistance. The incidence and degree of impaired glucose tolerance observed in treated patients vary considerably between drugs, however. To compare the ability of HIV PIs to alter peripheral glucose disposal acutely in a genetically identical model system at therapeutically relevant drug levels, healthy lean male rats previously naive to PI exposure were given ritonavir, amprenavir, lopinavir/ritonavir (4:1), or atazanavir by continuous intravenous infusion to achieve steady state drug levels of 10 or 25 muM rapidly. Under euglycemic hyperinsulinemic clamp conditions, a dose-dependent reduction in the peripheral glucose disposal rate (Rd) was observed with all the PIs except atazanavir. The rank order of sensitivity was ritonavir, lopinavir, and then amprenavir. Changes in skeletal muscle and heart 2-deoxyglucose (2-DOG) uptake correlated with reductions in Rd. All 3 of these PIs also produced significant reductions in 2-DOG uptake into primary rat adipocytes in vitro. Atazanavir had no effect on glucose uptake in vitro or in vivo. The in vivo potency of PIs to impair peripheral glucose disposal acutely correlates with the degree of insulin resistance observed in HIV-infected patients receiving these drugs. Preclinical testing of novel candidate PIs in a rodent model system may be useful in identifying the future risk of altering glucose homeostasis.

The role of the carnitine system in human metabolism

Metabolism cycles daily between the fed and fasted states. The pathways of energy production are reversible and distinct. In the anabolic (fed) state, the liver stores glucose as glycogen, and fatty acid/triglyceride synthesis is active. In the catabolic (fasted) state, the liver becomes a glucose producer, lipogenesis is slowed, and fatty acid oxidation/ketogenesis is activated. The rate-limiting step for the latter is vested in the carnitine/carnitine palmitoyltransferase (CPT) system, and the off/on regulator of this is malonyl CoA. The AMP-induced protein kinase primarily determines the concentration of malonyl CoA. Four other systems have significant influence: two on fatty acid oxidation and two on lipogenesis. Peroxisome proliferator-activated receptor gamma-1 alpha, a master regulator of metabolism, induces hepatic gluconeogenesis and fatty acid oxidation in the catabolic phase. Deficiency of stearoyl CoA desaturase, although having no role in gluconeogenesis, powerfully induces fatty acid oxidation and weight loss despite increased food intake in rodents. Major stimulators of lipogenesis are carbohydrate-responsive element binding protein and the Insig system. The malonyl CoA-regulated CPT system has been firmly established in humans. The other systems have not yet been confirmed in humans, but likely are active there as well. Activation of fatty acid oxidation has considerable clinical promise for the treatment of obesity, type 2 diabetes, steatohepatitis, and lipotoxic damage to the heart.

Pinealectomy alters adipose tissue adaptability to fasting in rats

This study investigated the effects of pinealectomy and fasting on rat adipose tissue metabolism, as well as on profiles of the hormones directly involved in its regulation (insulin, leptin, and corticosterone). Pinealectomized (PINX) and sham-operated (CONTROL) adult male Wistar rats were killed 6 weeks after surgery, in either fed or fasted (12 and 36 hours) states. Blood samples (for glucose and hormone determinations) and peri-epididymal adipocytes (for in vitro insulin-stimulated glucose uptake, oxidation, and incorporation into lipids) were collected. Pineal ablation decreased insulin-stimulated glucose uptake in adipocytes of both fed and fasted animals without affecting insulin-binding capacity. Pinealectomy attenuated the reduction in the ability to oxidize glucose in both basal and insulin-stimulated states during fasting. This alteration in the ability of adipocytes to oxidize glucose appeared together with a decrease in insulin-induced glucose incorporation into lipids in PINX animals. Additionally, pinealectomized rats showed higher corticosterone levels in both fed and fasted states, and a lower leptinemia with 36 hours of fasting, in comparison to CONTROLs. In conclusion, our data reinforce the hypothesis that the pineal gland has a role in the modulation of adipocyte metabolism, and its absence alters metabolic adaptation to fasting in rats.

Association of liver steatosis with lipid oversecretion and hypotriglyceridaemia in C57BL/6j mice fed trans-10,cis-12-linoleic acid

Conjugated linoleic acids (CLA) have recently been recognized to reduce body fat and plasma lipids in some animals. This study demonstrated that the steatosis accompanying the fat loss induced by trans-10,cis-12-C(18:2) (CLA2) and not cis-9,trans-11-C(18:2) (CLA1) isomer in C57BL/6j mice was not due to an alteration of the liver lipoprotein production that was even increased. The 3-fold decrease in plasma triacylglycerol contents and the induction of mRNA expression of low-density lipoprotein receptors concomitantly observed in CLA2-fed mice suggested an increase in the lipoprotein clearance at the level of the liver itself. CLA1 feeding produced similar but attenuated effects on triglyceridaemia only.

Control of mitochondrial beta-oxidation flux

The control of mitochondrial beta-oxidation, including the delivery of acyl moieties from the plasma membrane to the mitochondrion, is reviewed. Control of beta-oxidation flux appears to be largely at the level of entry of acyl groups to mitochondria, but is also dependent on substrate supply. CPTI has much of the control of hepatic beta-oxidation flux, and probably exerts high control in intact muscle because of the high concentration of malonyl-CoA in vivo. beta-Oxidation flux can also be controlled by the redox state of NAD/NADH and ETF/ETFH(2). Control by [acetyl-CoA]/[CoASH] may also be significant, but it is probably via export of acyl groups by carnitine acylcarnitine translocase and CPT II rather than via accumulation of 3-ketoacyl-CoA esters. The sharing of control between CPTI and other enzymes allows for flexible regulation of metabolism and the ability to rapidly adapt beta-oxidation flux to differing requirements in different tissues.

Elucidation of the mechanism by which (+)-acylcarnitines inhibit mitochondrial fatty acid transport

It is well established that medium and long chain (+)-acylcarnitines (i.e. fatty acid esters of the unnatural d-isomer of carnitine) inhibit the oxidation of long chain fatty acids in mammalian tissues by interfering with some component(s) of the mitochondrial carnitine palmitoyltransferase (CPT) system. However, whether their site of action is at the level of CPT I (outer membrane), CPT II (inner membrane), carnitine-acylcarnitine translocase (CACT, inner membrane), or some combination of these elements has never been resolved. We chose to readdress this question using rat liver mitochondria and employing a variety of assays that distinguish between the three enzyme activities. The effect on each of (+)-acetylcarnitine, (+)-hexanoylcarnitine, (+)-octanoylcarnitine, (+)-decanoylcarnitine, and (+)-palmitoylcarnitine was examined. Contrary to longstanding belief, none of these agents was found to impact significantly upon the activity of CPT I or CPT II. Whereas (+)-acetylcarnitine also failed to influence CACT, both (+)-octanoylcarnitine and (+)-palmitoylcarnitine strongly inhibited this enzyme with a similar IC(50) value ( approximately 35 microm) under the assay conditions employed. Remarkably, (+)-decanoylcarnitine was even more potent (IC(50) approximately 5 microm), whereas (+)-hexanoylcarnitine was far less potent (IC(50) >200 microm). These findings resolve a 35-year-old puzzle by establishing unambiguously that medium and long chain (+)-acylcarnitines suppress mitochondrial fatty acid transport solely through the inhibition of the CACT component. They also reveal a surprising rank order of potency among the various (+)-acylcarnitines in this respect and should prove useful in the design of future experiments in which selective blockade of CACT is desired.

Reciprocal enzymatic interference of carnitine palmitoyltransferase I and glycerol-3-phosphate acyltransferase in purified liver mitochondria

(i) Highly purified mitochondrial fractions were practically devoid of microsomal contamination and of acyl-CoA ligase activity. (ii) In mitochondria, glycerol-3-phosphate acyltransferase (GPAT) activity was supported by two enzymes, the first being very active at low palmitoyl-CoA/albumin ratios and sensitive to external agents (external form), the second being detected only at higher palmitoyl-CoA/albumin ratios and insensitive to external agents (internal form). (iii) Carnitine palmitoyltransferase I (CPT I) activity was shown to inhibit external GPAT activity only. (iv) Glycerol-3-phosphate exerted an inhibitory effect on CPT I, even when GPAT was inactive. Reciprocal interaction of CPT I and GPAT was discussed with regard to the balance existing between fatty acid oxidation and esterification metabolic pathways.

Metabolic control of food intake and estrous cycles in syrian hamsters. I. Plasma insulin and leptin

The "adipostat hypothesis" refers to the idea that circulating hormone concentrations reflect levels of body adiposity and act as signals to control food intake and reproduction. Implicit in the adipostatic hypothesis are the following two assumptions: 1) plasma levels of adipostatic hormones accurately reflect body fat content and 2) decreased plasma concentrations of adipostatic hormones necessarily result in increased food intake and inhibited reproductive processes. The present experiments are designed to test these assumptions. Fat and lean Syrian hamsters were either fasted for 12, 24, 36, or 48 h or allowed ad libitum access to food. Contrary to the first assumption, plasma leptin and insulin levels in fat hamsters dropped dramatically by 12 h after the start of a fast, with no significant change in body fat content and no postfast hyperphagia. Lean hamsters showed anestrus after a 48-h fast but not after a 24-h fast. Contrary to the second assumption of the lipostatic hypothesis, lean hamsters fasted for 24 h and then refed for the next 24 h had leptin levels that were not significantly elevated compared with those of 48-h-fasted hamsters. Thus, in adult female Syrian hamsters, plasma leptin concentrations do not accurately reflect body fat content under all conditions; normal estrous cyclicity does not necessarily require plasma leptin concentrations higher than those of fasted hamsters; and decreased plasma leptin levels do not result in increased food intake.

Short-term regulation of carnitine palmitoyltransferase I in cultured rat hepatocytes: spontaneous inactivation and reactivation by fatty acids

Liver carnitine palmitoyltransferase I (CPT I), the rate-limiting enzyme of mitochondrial beta-oxidation, rapidly loses its activity when hepatocytes are put in culture. 3-Thia fatty acids reactivate the enzyme and can increase its activity 3-4-fold in 5-10 min. Normal fatty acids are also able to stimulate CPT I, but to a limited extent, compared to 3-thia fatty acid. This activation does not affect malonyl-CoA sensitivity. CPT I in hepatocytes from both fasted and fasted-carbohydrate refed rats is inactivated and reactivated to a similar extent. Free dodecylthioacetic acid (DTA) is at least as efficient as DTA-CoA as activator. CPT I activity in isolated mitochondria is not influenced by incubation with DTA, suggesting that the regulation of CPT I depends on an extramitochondrial component(s) in the cell. It is concluded that fatty acids activate pre-existing, inactive CPT I without involvement of gene transcription and independently of malonyl-CoA.

Hypolipidaemic effects of fenofibrate are not altered by mildronate-mediated normalization of carnitine concentration in rat liver

The five-fold higher carnitine content in the liver of fenofibrate-treated rats addresses the question about the possible role of this enhancement in the hypolipidaemic effect of the drug and the underlying mechanisms. When fenofibrate was administered with mildronate (a gamma-butyrobetaine hydroxylase inhibitor) in suitable amount, the content in carnitine was found to be normalized in liver. However, triglyceride contents of liver and serum were then at least as low as in rats treated by fenofibrate only. When carnitine concentration was lowered by mildronate to the third of the normal value, a marked increase in triglycerides occurred both in liver and serum, while the five-fold increase in carnitine due to fenofibrate enhanced blood ketone body concentration with no effect on liver and serum triglycerides. Data suggest that the normal carnitine concentration is largely sufficient to meet the usual requirement for carnitine palmitoyltransferase I activity (CPT I). In rat liver, increase in mitochondrial CPT I activity and in peroxisomal fatty acid oxidation may constitute part of the hypolipidaemic effect of fenofibrate.

Hypoglycemic effects of a novel fatty acid oxidation inhibitor in rats and monkeys

Increased fatty acid oxidation contributes to hyperglycemia in patients with non-insulin-dependent diabetes mellitus. To improve glucose homeostasis in these patients, we have designed a novel, reversible inhibitor of carnitine palmitoyl-transferase I (CPT I) that potently inhibits fatty acid oxidation. SDZ-CPI-975 significantly lowered glucose levels in normal 18-h-fasted nonhuman primates and rats. In rats, glucose lowering required fatty acid oxidation inhibition of > or = 70%, as measured by beta-hydroxybutyrate levels, the end product of beta-oxidation. In cynomolgus monkeys, comparable glucose lowering was achieved with more modest lowering of beta-hydroxybutyrate levels. SDZ-CPI-975 did not increase glucose utilization by heart muscle, suggesting that CPT I inhibition with SDZ-CPI-975 would not induce cardiac hypertrophy. This was in contrast to the irreversible CPT I inhibitor etomoxir. These results demonstrate that SDZ-CPI-975 effectively inhibited fatty acid oxidation and lowered blood glucose levels in two species. Thus reversible inhibitors of CPT I represent a class of novel hypoglycemic agents that inhibit fatty acid oxidation without inducing cardiac hypertrophy.

New aspects of ketone bodies in energy metabolism of dairy cows: a review

Increased lipolysis, low insulin/glucagon ratios and malonyl-CoA concentrations are prerequisites for ketogenesis. From an aetiological viewpoint, there are two quite different types of metabolic disorders in which ketosis can occur, the hypoglycaemic-hypoinsulinaemic and the hyperglycaemic-hyperinsulinaemic type. The former, Type I, generally occurs 3-6 weeks after calving in cows whose milk secretion is so extensive that the demand for glucose exceeds the capacity for glucose production. To protect the body from hazardous protein degradation by a high rate of gluconeogenesis, this process is inhibited and the increased energy requirements are met by the elevated utilization of ketone bodies. In this strong catabolic metabolic state the plasma levels of glucose and insulin are very low, the levels of ketone bodies are high and there are small risks for fat accumulation in the liver cells. The hyperglycaemic, hyperinsulinaemic form, Type II, generally occurs earlier in lactation. An important aetiologic factor is overfeeding in the dry period, which can lead to disturbances in the hormonal adaptation of metabolism at calving with increased plasma levels of insulin and glucose and often out not always also with hyperketonaemia. If combined with stress, there may be increased lipolysis in adipose tissues, lipid synthesis and accumulation in the liver, i.e. the development of fatty liver. This hyperglycaemic form of disturbance has many similarities with the initial stage of non-insulin-dependent (Type II) diabetes in humans. It has been shown that ketone bodies inhibit protein degradation and thereby gluconeogenesis and also are able to spare glucose by inhibiting glucose utilization. They also can inhibit lipolysis and function as a regulatory safety system, replacing insulin, in situations when the activity of this hormone is low, as in Type I ketosis. Ketone bodies thus have important functions as substrates replacing glucose in many tissues and also as signal substances in the regulation of energy metabolism.

Hepatic VLDL secretion of genetically obese Zucker rats is inhibited by a high-fat diet

Hepatocytes from obese and lean Zucker rats adapted to a control (C) or a high-fat (HF) diet were prepared for the study of fatty acid (FA) uptake, partition between oxidation and esterification, and very low density lipoprotein (VLDL) production. A first 2-h kinetic study showed higher oleate uptake on a C diet by obese rat cells and an almost exclusive esterification to triacylglycerol (TG), VLDL secretion being 2.5-fold higher in obese rat cells and enhanced 1.4-fold in both genotypes in the presence of 0.7 mM oleate vs. 0.1 mM or no oleate. Fat feeding 1) decreased oleate uptake, esterification, incorporation into VLDL-TG, and mass VLDL-TG secretion and 2) abolished the VLDL-TG increase by 0.7 mM oleate. Similar but more pronounced effects were obtained in fat-fed lean animals. A second kinetic study using very short incubation times up to 1 h confirmed that fat feeding decreased oleate uptake and esterification, greatly stimulating its oxidation and production of acetoacetate (obese) or acetoacetate and beta-hydroxybutyrate (lean). Synthesis of lactate and pyruvate greatly decreased under HF feeding, remaining higher in obese rat cells. The drastic inhibition of labeled and total hepatic VLDL-TG secretion in obese and lean Zucker rats by the HF diet could be partly explained by decreased exogenous FA availability for VLDL-TG synthesis through its greater channeling toward oxidation and, indirectly, by the altered hepatocyte metabolic state.

Modulation of in vivo insulin action by dietary protein during pregnancy

Rats were provided with a standard 20% protein diet or an isocaloric 8% protein diet from day 1 of gestation and were studied on day 19 of pregnancy. Fetal numbers per dam were unchanged, but total fetal weight at day 19 of gestation was reduced by 10% (P < 0.001) in the 8% protein group. In the basal state, endogenous glucose production (Ra) and muscle glucose uptake/phosphorylation were not significantly affected by dietary protein content. The glucose infusion rate required to maintain glycemia and Ra during euglycemic-hyperinsulinemic clamp (insulin infusion rate of 4.17 mU.kg-1.min-1) were reduced in the 8% protein group by 17% (P < 0.05) and 76% (P < 0.001), respectively. Suppression of Ra by insulin was not significant in the 20% protein group. Insulin-stimulated glucose disappearance (Rd) was 24% lower (P < 0.001) in the 8% protein group (25.0 +/- 0.8 mg.min-1.kg-1) than in the 20% protein group (32.7 +/- 0.5 mg.min-1.kg-1). The overall increment in muscle glucose utilization index (mean of 6 muscles) elicited by insulin was impaired by 38.1 +/- 4.0%. Insulin suppressed nonesterified fatty acid concentration (NEFA) by 83% (P < 0.001) and plasma triacylglycerol concentration (TAG) by 67% (P < 0.05) in the 20% protein group but evoked only a 43% (P < 0.01) decline in plasma NEFA and did not significantly suppress plasma TAG in the 8% protein group.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential time course of liver and kidney glucose-6 phosphatase activity during fasting in rats

We have studied the time course of hepatic and renal microsomal glucose-6 phosphatase (Glc-6Pase) during long-term fasting in the rat. Liver microsomal Glc-6Pase increases up to 48 hr and significantly decreases after 48 hr of fasting. The following activities were determined at 0, 24, 48, 72 and 96 hr: 0.31 +/- 0.02; 0.50 +/- 0.02; 0.54 +/- 0.03; 0.44 +/- 0.03; 0.44 +/- 0.01 mumol min-1 mg protein-1, respectively (all values are means +/- SEM, n = 6). Concomitantly, kidney microsomal Glc-6Pase progressively increases throughout the fast (Vm = 0.21 +/- 0.01; 0.26 +/- 0.004; 0.30 +/- 0.01; 0.37 +/- 0.02; 0.40 +/- 0.01 mumol min-1 mg protein-1, from 0 to 96 hr, respectively). These data suggest that the differential expression of Glc-6Pase activity in the liver and the kidney during long-term fasting could have an important role in the shift from a principally hepatic gluconeogenesis to a hepatic and renal gluconeogenesis.

Functional deterioration of the liver by elevated inferior vena cava pressure: a proposed upper safety limit of pressure for maintaining liver viability in dogs

OBJECTIVE

Abnormally elevated central venous pressure is considered to be an etiological factor in the onset of acute hepatic failure following modified Fontan operation. This paper hypothesises that an increase in inferior vena cava pressure (IVCP) after such an operation has adverse effects on hepatic energy status.

DESIGN

Various degrees of venous hypertension were produced in 10 mongrel dogs by clamping the thoracic IVC with an active veno-venous shunt and varying its flow rate from 60-2.5 ml/min/kg. Arterial ketone body ratio (KBR), reflecting the hepatic mitochondrial redox state, was measured as an index of hepatic energy status.

MEASUREMENTS AND RESULTS

The lower the flow rates of the shunt, the higher the pressures of IVC and portal vein, while systolic blood pressure was maintained above 100 mmHg. CO significantly decreased when the pump speed was less than 30 ml/min/kg. KBR showed a negative correlation to IVCP as well as a positive correlation to portal blood flow (p < 0.05).

CONCLUSION

From the simple regression line obtained between IVCP and KBR, it was determined that an upper safety limit of IVCP may lie at about 27 cmH2O (20.5 mmHg), and that a IVCP of 35 cmH2O (26.6 mmHg) seems to be the critical level for maintaining liver viability.

Mechanisms involved in the coordinate regulation of strategic enzymes of glucose metabolism

In this review, we evaluate the relative regulatory importance of specific strategic enzymes (in particular glycogen synthase, acetyl-CoA carboxylase [ACC] and the pyruvate dehydrogenase complex [PDH]) for carbohydrate utilization as an anabolic precursor and as an energy substrate during the nutritional transitions between the fed and fasted states. The involvement of the specific protein kinases contributing to the inactivation of these enzymes by phosphorylation [cyclic AMP-dependent protein kinase, AMP-activated protein kinase and PDH kinase] in achieving each regulatory response is also assessed. We demonstrate a striking temporal correlation between hepatic glycogen mobilization and PDH and ACC inactivation by phosphorylation during the immediate postabsorptive period; in contrast, rates of hepatic glycogen synthesis and PDH and ACC expressed activities do not change in parallel during refeeding. The results are consistent with shifting of the primary sites of control for overall hepatic carbon flux during the fed-to-starved and starved-to-fed nutritional transitions achieved, at least in part, by a complex pattern of regulation by protein phosphorylation and metabolites which is critically dependent on the precise nutritional status. Data are also presented that demonstrate asynchronous suppression of glucose uptake/phosphorylation and pyruvate oxidation in cardiac and skeletal muscle during progressive starvation. Analogous asynchrony is observed in the reactivation of these processes in cardiac and skeletal muscle during refeeding after starvation. We provide evidence in support of the concept that selective suppression of pyruvate oxidation in oxidative muscles during early starvation and during the initial phase of refeeding is achieved because of differential sensitivity of glucose uptake/phosphorylation and pyruvate oxidation to lipid-fuel utilization. We discuss the relative importance of regulatory events governing local fatty acid production and utilization (via lipoprotein lipase and carnitine palmitoyltransferase 1, respectively) or overall fatty acid supply (dictated by events at the adipocyte) for fuel utilization by muscle during nutritional transitions. Finally, we assess the regulatory importance of glycogen synthesis in determining overall rates of glucose clearance by skeletal muscle during alimentary hyperglycemia and hyperinsulinemia.

Fuel homeostasis and carnitine metabolism in rats with secondary biliary cirrhosis

Energy metabolism is abnormal in patients and experimental animals with liver cirrhosis. To help better understand the abnormalities, fuel homeostasis and carnitine metabolism were studied in fed and 24-hr-starved rats with secondary biliary cirrhosis induced by bile duct ligation for 4 wk. Plasma ketone body concentrations were decreased by 67% in starved, bile duct-ligated rats compared with control rats. In contrast, plasma nonesterified fatty acid concentrations were not different between bile duct-ligated and control rats in the fed or the fasted state. Plasma triglyceride concentrations showed the expected decrease with starvation in control rats, but were increased with starvation in bile duct-ligated rats. Urinary excretion of dicarboxylic acids was increased in both fed and fasted bile duct ligated-rats compared with the respective control groups. Compared with control rats, hepatic total carnitine content (per gram of liver) was increased by 24% in fed and by 36% in fasted, bile duct-ligated rats. Fed, bile duct-ligated rats had an increased short-chain acylcarnitine-to-carnitine ratio in liver, plasma and urine compared with control rats. Analysis of the hepatic coenzyme A pool showed decreased coenzyme A content in fed and fasted bile duct-ligated rats compared with control rats. Hepatic long-chain acylcarnitine and long-chain acyl-coenzyme A content increased with starvation both in control and bile duct-ligated rats. The rise in plasma nonesterified fatty acid concentration and hepatic long-chain acylcarnitine and long-chain acylcoenzyme A contents with starvation in bile duct-ligated rats are consistent with unaltered hepatic availability of fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of energy balance on hepatic capacity for oleate and propionate metabolism and triglyceride secretion

The purpose of this study was to identify conditions that could decrease accumulation of triglyceride in liver, preferably by increasing hepatic secretion of triglyceride-rich lipoproteins. Hepatocytes isolated from lactating goats were incubated in vitro, and the fate of [1-14C]oleate was measured to determine hepatic capacity for various routes of long-chain fatty acid metabolism. The effect of in vivo energy balance and modifications of the nutrients present in the culture media were tested. Addition of linoleic acid, isovalerate, niacin, propionate, or propylene glycol did not affect triglyceride accumulation or secretion. Pyruvate decreased intracellular triglyceride accumulation. Changes in oxidation of oleate through manipulation of carnitine acyl transferase activity did not influence oleate esterification rate. Livers and hepatocytes isolated from goats in negative energy balance contained more lipid and triglyceride. Liver cells from goats in negative energy balance had decreased capacity for converting propionate to glucose with no change in ketogenic capacity as judged by acid soluble product formation from oleate. Hepatocytes from goats in negative energy balance retained less oleate as cell triglyceride with no change in triglyceride export, indicating a decreased net rate of esterification. Lactating goats, either in negative or positive energy balance, demonstrated the same low capacity for export of newly synthesized triglyceride as previously reported for fed wethers.

The involvement of carnitine intermediates in peroxisomal fatty acid oxidation: a study with 2-bromofatty acids

Metabolism-dependent inactivators of 3-ketothiolase I and carnitine acyltransferase I (CAT I) have been used to study the oxidation of fatty acids in intact hepatocytes. 2-Bromooctanoate inactivates mitochondrial and peroxisomal 3-ketothiolases I in a time-dependent manner. During the first 5 min of incubation, inactivation of 3-ketothiolase in mitochondria is five times faster than its inactivation in peroxisomes. Almost complete inactivation of 3-ketothiolase I in both types of organelle is achieved after incubation with 1 mM 2-bromooctanoate for 40 min. The inactivation is not affected by preincubating hepatocytes with 20 microM tetradecylglycidate (TDGA), an inactivator of CAT I, under conditions which cause greater than 95% inactivation of CAT I. 2-Bromododecanoate (1 mM) causes 60% inactivation of mitochondrial and peroxisomal 3-ketothiolases I in 40 min. These inactivations are greatly reduced by preincubating hepatocytes with 20 microM TDGA, demonstrating that 2-bromododecanoate enters both mitochondria and peroxisomes via its carnitine ester. 2-Bromopalmitate (1 mM) causes less than 5% inactivation of mitochondrial and peroxisomal 3-ketothiolases I in 40 min, but causes 95% inactivation of CAT I during this time. Incubation of hepatocytes with 10-200 microM 2-bromopalmitoyl-L-carnitine causes inactivation of mitochondrial and peroxisomal 3-ketothiolases I at similar rates. This inactivation is decreased by palmitoyl-D-carnitine during the first 5 min of incubation. Pretreating hepatocytes with 20 microM TDGA does not affect the inactivation of mitochondrial or peroxisomal 3-ketothiolase I by 2-bromopalmitoyl-L-carnitine. These results demonstrate that in intact hepatocytes, peroxisomes oxidize fatty acids of medium-chain length by a carnitine-independent mechanism, whereas they oxidize long-chain fatty acids by a carnitine-dependent mechanism.

Effects of acute starvation on vitamin A status in rats

Maintenance of vitamin A stores in the body is dependent on a number of basic metabolic processes. These processes, such as protein and carbohydrate metabolism, are disrupted in acute starvation, and, as a result, alterations in vitamin A status may result. We investigated this possibility in 8-week-old Sprague-Dawley male rats. The rats were starved for 24, 48, and 72 hr but had free access to water. At 24 hours of starvation, the plasma retinol concentration was depressed, but not significantly so. After 48 and 72 hours of starvation, however, the plasma retinol concentration decreased to less than half of the control values (61 +/- 4 vs 124 +/- 12 nmol/dl at 72 hours, mean +/- SEM, (p less than 0.005). The hepatic retinoid (retinyl esters + retinol) concentration (nmol/g liver) was increased at 24 and 48 hours of starvation compared to controls (p less than 0.05), and by 72 hours the concentration was 56% greater in starved rats than in fed controls (p less than 0.001). The total hepatic retinoid content (mumol/total liver) was decreased moderately at all periods of starvation compared to controls (p less than 0.05). In both starved and fed animals, the total hepatic content per 100 g body weight, a measure of total vitamin A reserves, was statistically the same. These results demonstrate that acute starvation in rats alters the vitamin A equilibrium between the plasma and hepatic stores without affecting the overall vitamin A reserves.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of ketone body flux in septic patients

To assess the effect of sepsis on ketone body (KB) kinetics in humans, we measured in normal and septic subjects KB appearance rate (Ra) before (initial state) and during a rise of free fatty acids (FFA) level (intravenous infusion of a triglycerides emulsion). We studied normal subjects in postabsorptive state and septic patients when receiving an hypocaloric intravenous infusion of glucose and amino acids or 12 h after its interruption. When receiving glucose and amino acids infusion, septic patients had higher glucose and insulin levels than normal subjects, and despite lower FFA concentrations (255 +/- 44 vs. 480 +/- 51 mumol/l, P less than 0.05) comparable initial KB Ra (2.50 +/- 0.10 vs. 2.48 +/- 0.30 mumol.kg-1.min-1). Triglyceride infusion increased FFA to comparable values (septic 780 +/- 130, normal 730 +/- 45 mumol/l), but KB Ra rose in septic patients only to 3.7 +/- 1.1 instead of 7.7 +/- 1.1 mumol.kg-1.min-1 as in normal subjects (P less than 0.05). Somatostatin infusion decreased the hyperinsulinemia of septic patients but did not restore a normal ketogenesis. After interruption of nutriment infusion, septic patients had normal FFA levels and only mild hyperglycemia and hyperinsulinemia. Their initial KB Ra was not modified. However, their response of KB Ra (increase to 6.27 +/- 2.0 mumol.kg-1.min-1) to raised FFA levels (842 +/- 170 mumol/l) was comparable to the response of normal subjects. In conclusion, although septic patients receiving an hypocaloric parenteral nutrition had a depressed ketogenesis they were able to restore a normal ketogenic capacity after a short-time caloric deprivation. Glucose and/or insulin appears to have a major role in this modulation of hepatic ketogenesis.

The regulation of fatty acid and branched-chain amino acid oxidation in cancer cachectic rats: a proposed role for a cytokine, eicosanoid, and hormone trilogy

Some postulates of a hypothesis concerned with the deregulation of muscle turnover by the hypoketonemia of cachectic tumor-bearing rats were examined. Plasma concentrations of ketone bodies (D-(3)-hydroxybutyrate + acetoacetate) in rats bearing the Walker 256 carcinosarcoma were reduced by 45% (P less than 0.001) whereas the concentrations of triglyceride and free fatty acids were elevated by 223% (P less than 0.001) and 335% (P less than 0.001), respectively. Parallel with the changes in plasma, the livers of tumor-bearing animals showed decreased concentrations of KB by 35% (P less than 0.05) and increased concentrations of TG and FFA by 49% (NS) and 15% (NS), respectively. In comparison with values for the control liver (fed ad libitum), the perfused liver of animals bearing the Walker 256 tumor formed 42% (P less than 0.05) and 75% (P less than 0.05) less ketone bodies and CO2, respectively, from oleate, while TG formation was enhanced by 33% (P less than 0.001). There was two- to threefold (P less than 0.001) enhancement of [1-14C]leucine oxidation in vivo by the tumor-bearing animals. The activities of branched-chain amino acid aminotransferase and branched-chain keto acid dehydrogenase were elevated by 70% (P less than 0.001) and 560% (P less than 0.001) respectively in the gastrocnemius muscle of the tumor-bearing animals. The results of the investigation supported a second proposal of the hypothesis, namely, that cancer-induced cachexia resulted in the notable elevation in the concentration of arginine vasopressin that was accompanied by parallel increases in the plasma, urine, and muscle concentrations of prostaglandin E2. The proposals of the original hypothesis have been augmented to include roles for PGE2 and the cytokine cachectin/tumor necrosis factor which may engineer all of the events depicted in the original hypothesis.