The Effects of Glucagon, Dibutyryl Cyclic Adenosine 3',5'-Monophosphate, and Concentration of Free Fatty Acid on Hepatic Lipid Metabolism

Detecting altered hepatic lipid oxidation by MRI in an animal model of MASLD

Metabolic dysfunction-associated steatotic liver disease (MASLD) prevalence is increasing annually and affects over a third of US adults. MASLD can progress to metabolic dysfunction-associated steatohepatitis (MASH), characterized by severe hepatocyte injury, inflammation, and eventual advanced fibrosis or cirrhosis. MASH is predicted to become the primary cause of liver transplant by 2030. Although the etiology of MASLD/MASH is incompletely understood, dysregulated fatty acid oxidation is implicated in disease pathogenesis. Here, we develop a method for estimating hepatic β-oxidation from the metabolism of [D15]octanoate to deuterated water and detection with deuterium magnetic resonance methods. Perfused livers from a mouse model of MASLD reveal dysregulated hepatic β-oxidation, findings that corroborate in vivo imaging. The high-fat-diet-induced MASLD mouse studies indicate that decreased β-oxidative efficiency in the fatty liver could serve as an indicator of MASLD progression. Furthermore, our method provides a clinically translatable imaging approach for determining hepatic β-oxidation efficiency.

Impaired Glucagon-Mediated Suppression of VLDL-Triglyceride Secretion in Individuals With Metabolic Dysfunction-Associated Fatty Liver Disease (MAFLD)

Individuals with metabolic dysfunction-associated fatty liver disease (MAFLD) have elevated plasma lipids as well as glucagon, although glucagon suppresses hepatic VLDL-triglyceride (TG) secretion. We hypothesize that the sensitivity to glucagon in hepatic lipid metabolism is impaired in MAFLD. We recruited 11 subjects with severe MAFLD (MAFLD+), 10 with mild MAFLD (MAFLD-), and 7 overweight control (CON) subjects. We performed a pancreatic clamp with a somatostatin analog (octreotide) to suppress endogenous hormone production, combined with infusion of low-dose glucagon (0.65 ng/kg/min, t = 0-270 min, LowGlucagon), followed by high-dose glucagon (1.5 ng/kg/min, t = 270-450 min, HighGlucagon). VLDL-TG and glucose tracers were used to evaluate VLDL-TG kinetics and endogenous glucose production (EGP). HighGlucagon suppressed VLDL-TG secretion compared with LowGlucagon. This suppression was markedly attenuated in MAFLD subjects compared with CON subjects (MAFLD+: 13% ± [SEM] 5%; MAFLD-: 10% ± 3%; CON: 36% ± 7%, P < 0.01), with no difference between MAFLD groups. VLDL-TG concentration and VLDL-TG oxidation rate increased between LowGlucagon and HighGlucagon in MAFLD+ subjects compared with CON subjects. EGP transiently increased during HighGlucagon without any difference between the three groups. Individuals with MAFLD have a reduced sensitivity to glucagon in the hepatic TG metabolism, which could contribute to the dyslipidemia seen in MAFLD patients. ClinicalTrials.gov: NCT04042142.

Profiling of RNA-binding Proteins Interacting With Glucagon and Adipokinetic Hormone mRNAs

OBJECTIVE

Glucagon in mammals and its homolog (adipokinetic hormone [AKH] in Drosophila melanogaster) are peptide hormones which regulate lipid metabolism by breaking down triglycerides. Although regulatory mechanisms of glucagon and AKH expression have been widely studied, post-transcriptional gene expression of glucagon has not been investigated thoroughly. In this study, we aimed to profile proteins binding with Gcg messenger RNA (mRNA) in mouse and Akh mRNA in Drosophila.

METHODS

Drosophila Schneider 2 (S2) and mouse 3T3-L1 cell lysates were utilized for affinity pull down of Akh and Gcg mRNA respectively using biotinylated anti-sense DNA oligoes against target mRNAs. Mass spectrometry and computational network analysis revealed mRNA-interacting proteins residing in functional proximity.

RESULTS

We observed that 1) 91 proteins interact with Akh mRNA from S2 cell lysates, 2) 34 proteins interact with Gcg mRNA from 3T3-L1 cell lysates. 3) Akh mRNA interactome revealed clusters of ribosomes and known RNA-binding proteins (RBPs). 4) Gcg mRNA interactome revealed mRNA-binding proteins including Plekha7, zinc finger protein, carboxylase, lipase, histone proteins and a cytochrome, Cyp2c44. 5) Levels of Gcg mRNA and its interacting proteins are elevated in skeletal muscles isolated from old mice compared to ones from young mice.

CONCLUSION

Akh mRNA in S2 cells are under active translation in a complex of RBPs and ribosomes. Gcg mRNA in mouse precursor adipocyte is in a condition distinct from Akh mRNA due to biochemical interactions with a subset of RBPs and histones. We anticipate that our study contributes to investigating regulatory mechanisms of Gcg and Akh mRNA decay, translation, and localization.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

The Vicious Circle of Hepatic Glucagon Resistance in Non-Alcoholic Fatty Liver Disease

A key criterion for the most common chronic liver disease—non-alcoholic fatty liver disease (NAFLD)—is an intrahepatic fat content above 5% in individuals who are not using steatogenic agents or having significant alcohol intake. Subjects with NAFLD have increased plasma concentrations of glucagon, and emerging evidence indicates that subjects with NAFLD may show hepatic glucagon resistance. For many years, glucagon has been thought of as the counterregulatory hormone to insulin with a primary function of increasing blood glucose concentrations and protecting against hypoglycemia. However, in recent years, glucagon has re-emerged as an important regulator of other metabolic processes including lipid and amino acid/protein metabolism. This review discusses the evidence that in NAFLD, hepatic glucagon resistance may result in a dysregulated lipid and amino acid/protein metabolism, leading to excess accumulation of fat, hyperglucagonemia, and increased oxidative stress contributing to the worsening/progression of NAFLD.

Glucagon Receptor Signaling and Lipid Metabolism

Glucagon is secreted from the pancreatic alpha cells upon hypoglycemia and stimulates hepatic glucose production. Type 2 diabetes is associated with dysregulated glucagon secretion, and increased glucagon concentrations contribute to the diabetic hyperglycemia. Antagonists of the glucagon receptor have been considered as glucose-lowering therapy in type 2 diabetes patients, but their clinical applicability has been questioned because of reports of therapy-induced increments in liver fat content and increased plasma concentrations of low-density lipoprotein. Conversely, in animal models, increased glucagon receptor signaling has been linked to improved lipid metabolism. Glucagon acts primarily on the liver and by regulating hepatic lipid metabolism glucagon may reduce hepatic lipid accumulation and decrease hepatic lipid secretion. Regarding whole-body lipid metabolism, it is controversial to what extent glucagon influences lipolysis in adipose tissue, particularly in humans. Glucagon receptor agonists combined with glucagon-like peptide 1 receptor agonists (dual agonists) improve dyslipidemia and reduce hepatic steatosis. Collectively, emerging data support an essential role of glucagon for lipid metabolism.

A shift toward a high-fat diet in the current metabolic paradigm: A new perspective

OBJECTIVE

Investigations into the relationship between dietary carbohydrate restriction and health are mixed. Current guidelines for nutrition promote low-fat foods and higher carbohydrate consumption for optimal health and weight loss. However, high-fat, low-carbohydrate diets are revealing both intra- and extracellular adaptations that have been shown to elicit favorable cardiometabolic changes associated with obesity. Moreover, dietary fat is associated with higher satiety levels from the hormones adiponectin, leptin, and cholecystokinin. Additionally, insulin responses from high-glycemic carbohydrates are known to alter these pathways, potentially leading to an increase in energy consumption and a possible mechanism for obesity.

CONCLUSION

There is convincing evidence of beneficial effects of controlled trials implementing high-fat, low-carbohydrate diets in both sedentary and obese individuals, but longer duration clinical trials are required to confirm this hypothesis.

Daytime restricted feeding modifies the daily regulation of fatty acid β-oxidation and the lipoprotein profile in rats

Daytime restricted feeding (2 h of food access from 12.00 to 14.00 hours for 3 weeks) is an experimental protocol that modifies the relationship between metabolic networks and the circadian molecular clock. The precise anatomical locus that controls the biochemical and physiological adaptations to optimise nutrient use is unknown. We explored the changes in liver oxidative lipid handling, such as β-oxidation and its regulation, as well as adaptations in the lipoprotein profile. It was found that daytime restricted feeding promoted an elevation of circulating ketone bodies before mealtime, an altered hepatic daily rhythmicity of 14CO2 production from radioactive palmitic acid, and an up-regulation of the fatty acid oxidation activators, the α-subunit of AMP-activated protein kinase (AMPK), the deacetylase silent mating type information regulation homolog 1, and the transcriptional factor PPARγ-1α coactivator. An increased localisation of phosphorylated α-subunit of AMPK in the periportal hepatocytes was also observed. Liver hepatic lipase C, important for lipoprotein transformation, showed a change of daily phase with a peak at the time of food access. In serum, there was an increase of LDL, which was responsible for a net elevation of circulating cholesterol. We conclude that our results indicate an enhanced fasting response in the liver during daily synchronisation to food access, which involves altered metabolic and cellular control of fatty acid oxidation as well a significant elevation of serum LDL. These adaptations could be part of the metabolic input that underlies the expression of the food-entrained oscillator.

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

Liver Enzyme Elevation associated with Total Parenteral Nutrition

A number of complications associated with total parenteral nutrition (TPN) have been identified, and methods of prevention or treatment have been developed. However, abnormal liver function continues to occur with the use of TPN, and little is known about its incidence and etiology.

Twenty-three patients, receiving TPN through the TPN program at Holy Cross Hospital from January, 1978 to May, 1978, were studied. All patients received a basic parenteral amino acid solution (Travasol®), with varying amounts of nitrogen per day, depending on nitrogen balance studies. Dextrose was supplied in quantities necessary to provide 120 percent or more of the patient's calculated basal energy expenditure (BEE). Of the 23 patients studied, elevated liver function test values were detected in two patients (8.6 percent). In both cases, the complication developed within six to eight days, and both had received calories in excess of 213 percent of their calculated BEE and had a calorie to nitrogen (kcal: N) ratio greater than 150: 1.

Mean calories as a percent of calculated BEE and kcal: N ratio were significantly higher in the patients with elevated liver function test (LFT) values than in those with normal LFT values ( p < 0.05 and p < 0.005, respectively). Following identification of the elevated LFT values, both patients were immediately placed on cyclic TPN (CyTPN). In the one patient, liver enzymes reverted toward normal after seven days of CyTPN, while the second patient required only five days. Cyclic TPN was deemed effective in the reversal of abnormal liver function in these two patients. It is speculated that the abnormal liver function was a result of fatty liver infiltration. The mechanism by which this infiltration occurs and its treatment are discussed.

Effects of Dietary Coconut Oil as a Medium-chain Fatty Acid Source on Performance, Carcass Composition and Serum Lipids in Male Broilers

This study was conducted to investigate the effects of dietary coconut oil as a medium-chain fatty acid (MCFA) source on performance, carcass composition and serum lipids in male broilers. A total of 540, one-day-old, male Arbor Acres broilers were randomly allotted to 1 of 5 treatments with each treatment being applied to 6 replicates of 18 chicks. The basal diet (i.e., R0) was based on corn and soybean meal and was supplemented with 1.5% soybean oil during the starter phase (d 0 to 21) and 3.0% soybean oil during the grower phase (d 22 to 42). Four experimental diets were formulated by replacing 25%, 50%, 75%, or 100% of the soybean oil with coconut oil (i.e., R25, R50, R75, and R100). Soybean oil and coconut oil were used as sources of long-chain fatty acid and MCFA, respectively. The feeding trial showed that dietary coconut oil had no effect on weight gain, feed intake or feed conversion. On d 42, serum levels of total cholesterol, low-density lipoprotein cholesterol, and low-density lipoprotein/high-density lipoprotein cholesterol were linearly decreased as the coconut oil level increased (p<0.01). Lipoprotein lipase, hepatic lipase, and total lipase activities were linearly increased as the coconut oil level increased (p<0.01). Abdominal fat weight/eviscerated weight (p = 0.05), intermuscular fat width (p<0.01) and subcutaneous fat thickness (p<0.01) showed a significant quadratic relationship, with the lowest value at R75. These results indicated that replacement of 75% of the soybean oil in diets with coconut oil is the optimum level to reduce fat deposition and favorably affect lipid profiles without impairing performance in broilers.

Insulin treatment of streptozotocin-induced diabetes re-establishes the patterns in carbohydrate, fat and amino acid metabolisms in growing pigs

The effects of streptozotocin (STZ) were studied in eight high-health herd-certified Yorkshire × Swedish Landrace pigs (32.5 ± 2.6 kg initial body weight [BW]), and an insulin treatment protocol was developed to re-establish their metabolisms. A single intravenous dose of 150 mg STZ/kg BW successfully induced hyperglycaemia and alterations in their fat and protein metabolisms. Within 13 h post-STZ treatment blood glucose concentration had fallen to a range of 1.3 to 4.7 mmol/L. Hypoglycaemia was promptly treated with 0.5 g glucose/kg BW intravenously. All the pigs became hyperglycaemic with blood glucose concentrations >23 mmol/L within 48 h post-STZ. Two days post-STZ serum C-peptide concentrations fell below 60 ρmol/L in all the pigs and remained below 96 ρmol/L for five weeks until the end of the study. The pigs were left untreated for one week after STZ injection. At the end of this week 13-fold and nine-fold increases in serum concentrations of triglycerides and non-esterified fatty acids, respectively, were observed. Also, at this time-point a three-fold increase in the concentration of branched-chain amino acids (BCAA) was observed, and alanine and taurine were decreased by approximately 70% and 40%, respectively. During the week when the pigs were untreated, a reduced weight gain was observed, but after the onset of insulin treatment the daily weight gain was at least as good as that of conventional high-health pigs. Then a subcutaneous treatment with short-acting insulin was initiated. The initial dose of 2/3 IU/kg BW daily, divided between two doses, was gradually increased to 1 IU/kg BW. Within three weeks, the insulin treatment restored the metabolic changes in carbohydrate, fat and protein metabolisms produced by the STZ. In conclusion, the results underscore the usefulness of this animal model in translational research as insulin treatment re-establishes the changes in carbohydrate, fat and amino acid metabolisms observed in STZ-diabetic pigs and resolves clinical signs of disease similar to those in humans.

Emerging role of AMP-activated protein kinase in endocrine control of metabolism in the liver

This review summarizes the emerging role of AMP-activated protein kinase (AMPK) in mediating endocrine regulation of metabolic fluxes in the liver. There are a number of hormones which, when acting on the liver, alter AMPK activation. Here we describe those hormones associated with activation and de-activation of AMPK and the potential mechanisms for changes in AMPK activation state. The actions of these hormones, in many cases, are consistent with downstream effects of AMPK signaling thus strengthening the circumstantial case for AMPK-mediated hormone action. In recent years, genetic mouse models have also been used in an attempt to establish the role of AMPK in hormone-stimulated metabolism in the liver. Few experiments have, however, firmly established a causal relationship between hormone action at the liver and AMPK signaling.

The metabolic actions of glucagon revisited

The initial identification of glucagon as a counter-regulatory hormone to insulin revealed this hormone to be of largely singular physiological and pharmacological purpose. Glucagon agonism, however, has also been shown to exert effects on lipid metabolism, energy balance, body adipose tissue mass and food intake. The ability of glucagon to stimulate energy expenditure, along with its hypolipidemic and satiating effects, in particular, make this hormone an attractive pharmaceutical agent for the treatment of dyslipidemia and obesity. Studies that describe novel preclinical applications of glucagon, alone and in concert with glucagon-like peptide 1 agonism, have revealed potential benefits of glucagon agonism in the treatment of the metabolic syndrome. Collectively, these observations challenge us to thoroughly investigate the physiology and therapeutic potential of insulin's long-known opponent.

In vitro metabolism of rumenic acid in bovine liver slices

Ruminant products are the major source of CLA for humans. However, during periods of fat mobilization, the liver might play an important role in CLA metabolism which would limit the availability of the latter for muscles and milk. In this context, rumenic acid (cis-9, trans-11 CLA) metabolism in the bovine liver (n = 5) was compared to that of oleic acid (n = 3) by using the in vitro liver slice method. Liver slices were incubated for 17 h in a medium containing 0.75 mM of FA mixture and 55 microM of either [1-(14)C] rumenic acid or [1-(14)C] oleic acid at 37 degrees C under an atmosphere of 95% O(2)-5% CO(2). Rumenic acid uptake by liver slices was twice (P = 0.009) that of oleic acid. Hepatic oxidation of both FA (> 50% of incorporated FA) led essentially to the production of acid-soluble products and to a lower extent to CO(2) production. Rumenic acid was partly converted (> 12% of incorporated rumenic acid) into conjugated C18:3. CLA and its conjugated derivatives were mainly esterified into polar lipids (71.7%), whereas oleic acid was preferentially esterified into neutral lipids (59.8%). Rumenic acid secretion as part of VLDL particles was very low and was one-fourth lower than that of oleic acid. In conclusion, rumenic acid was highly metabolized by bovine hepatocytes, especially by the oxidation pathway and by its conversion into conjugated C18:3 for which the biological properties need to be elucidated.

Lipolytic and metabolic response to glucagon in fasting king penguins: phase II vs. phase III

This study aims to determine how glucagon intervenes in the regulation of fuel metabolism, especially lipolysis, at two stages of a spontaneous long-term fast characterized by marked differences in lipid and protein availability and/or utilization (phases II and III). Changes in the plasma concentration of various metabolites and hormones, and in lipolytic fluxes as determined by continuous infusion of [2-3H]glycerol and [1-14C]palmitate, were examined in vivo in a subantarctic bird (king penguin) before, during, and after a 2-h glucagon infusion. In the two fasting phases, glucagon infusion at a rate of 0.025 microg. kg(-1). min(-1) induced a three- to fourfold increase in the plasma concentration and in the rate of appearance (Ra) of glycerol and nonesterified fatty acids, the percentage of primary reesterification remaining unchanged. Infusion of glucagon also resulted in a progressive elevation of the plasma concentration of glucose and beta-hydroxybutyrate and in a twofold higher insulinemia. These changes were not significantly different between the two phases. The plasma concentrations of triacylglycerols and uric acid were unaffected by glucagon infusion, except for a 40% increase in plasma uric acid in phase II birds. Altogether, these results indicate that glucagon in a long-term fasting bird is highly lipolytic, hyperglycemic, ketogenic, and insulinogenic, these effects, however, being similar in phases II and III. The maintenance of the sensitivity of adipose tissue lipolysis to glucagon could suggest that the major role of the increase in basal glucagonemia observed in phase III is to stimulate gluconeogenesis rather than fatty acid delivery.

Nitric oxide donor NOR 3 inhibits ketogenesis from oleate in isolated rat hepatocytes by a cyclic GMP-independent mechanism

Studies were conducted to clarify the effects of nitric oxide donors NOR 3 ((+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexeneamide, FK409), SIN-1 (3-morpholinosydnonimine) and SNAP (S-nitroso-N-acetylpenicillamine) on the accumulation of cGMP and cAMP and Ca2+ mobilization as well as ketogenesis from oleate in isolated rat hepatocytes. NOR 3 caused inhibition of ketogenesis from oleate along with stimulation of cGMP accumulation in rat hepatocytes, whereas SIN-1 and SNAP exerted no effect on ketogenesis despite their marked stimulation of cGMP accumulation. Although the nitric oxide trapping agent, carboxy-PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide), antagonized the stimulation by NOR 3 of cGMP accumulation, it failed to modulate the anti-ketogenic action of NOR 3. Furthermore, neither 8-bromoguanosine-3',5'-cyclic monophosphate nor N2,2'-O-dibutyrylguanosine-3',5'-cyclic monophosphate mimicked the anti-ketogenic action of NOR 3. It is concluded in the present study that NOR 3-induced inhibition of ketogenesis in rat hepatocytes is not mediated by cGMP. The present study revealed that the remaining structure of NOR 3 from which nitric oxide had been spontaneously released had no anti-ketogenic action. We first and clearly demonstrated that nitrite production was dramatically enhanced when NOR 3 was incubated in the presence of rat hepatocytes. The mechanism whereby NOR 3 inhibits ketogenesis in rat hepatocytes will be discussed.

Saturating concentration for the uptake of free fatty acids and release of ketone bodies in vivo in rat liver

The aim of this study is to determine how hepatic uptake of free fatty acids (FFA) and release of ketone bodies (KB) in vivo are influenced by plasma concentration of FFA. Anesthetized fasted rats were cannulated (aorta, portal vein, hepatic vein and inferior vena cava) and were intravenously injected with heparin (1000 U x kg(-1) body weight) or infused with lipid emulsions at rates of 0, 0.39 and 0.77 mg TG x min(-1) x rat(-1). Heparin injection elevated systemic concentration of plasma FFA significantly to 1.0 mM from 0.7 mM in the saline-injected rats. The arteriovenous (AV) gradient of FFA across the liver in the heparin-injected rats was, however, unchanged from the saline-injected rats. Hepatic release of KB in the heparin-injected rats, as indicated by the negative gradient across the liver, was found to be 83 % greater than the saline-injected rats. In the rats infused with lipid emulsions, plasma FFA concentrations were found to elevate further to 2.2 mM. However, no further increases in hepatic uptake of FFA and release of KB were found. Hepatic gradients of FFA and KB in the rats infused with lipid emulsions and injected with heparin were found to be the same. It may be concluded, therefore, that the uptake of FFA and release of KB in vivo in rat livers may be saturated at a plasma FFA concentration <1.0 mM, much lower than reported previously in in vitro studies.

Inhibition of hepatic very-low-density lipoprotein secretion in obese Zucker rats adapted to a high-protein diet

The effect of a high-protein (HP) diet on hepatic very-low-density lipoprotein (VLDL) secretion was studied in obese and lean Zucker rats. With the control (C) diet, isolated hepatocytes from obese as compared with lean rats displayed higher uptake of [1-14C]oleate 0.7 mmol/L, 95% of which was esterified to glycerolipids; greater oleate incorporation into VLDL-triacylglycerol (TG); 2.6 times higher total VLDL-TG secretion; and 11-fold higher de novo fatty acid synthesis. Adaptation to HP feeding decreased weight gains in both phenotypes and hepatocyte TG content in obese rats. Oleate uptake by hepatocytes was appreciably reduced in the obese phenotype only. Despite esterification rates similar to those for the C diet, oleate incorporation into VLDL-TG decreased by 34% and 55% in obese and lean rats, respectively. Total (mass) VLDL-TG secretion was drastically decreased by 65% and 48% in obese and lean rat hepatocytes, respectively. HP feeding combined with overnight fasting accentuated the above decreases. Fatty acid synthesis was 50% lower in cells from HP-fed obese rats, but increased 1.7-fold in lean ones. Plasma glucagon increased in both phenotypes under HP feeding, whereas plasma insulin either increased (obese) or decreased (lean), with the insulin to glucagon ratio slightly decreasing. Thus, HP feeding drastically inhibited hepatic VLDL secretion in obese and lean Zucker rats by an undefined mechanism that was apparently related neither to de novo fatty acid synthesis nor to changes in oleate partitioning between esterification and oxidation.

Advantageous effect of low-molecular-weight heparin administration on hepatic mitochondrial redox state

The effect of low-molecular-weight heparin (LMWH) on hepatic mithocondrial metabolism was compared with that of unfractionated heparin (UH) after the intravenous administration of these two kinds of heparin to normal rabbits. The magnitude of decrease in blood triglyceride levels 5 min after administration of UH (200 U/kg) was significantly greater than after LMWH (200 U/kg). Free fatty acid levels in the blood were significantly higher after this dose of UH than after LMWH. Blood total ketone body levels (acetoacetate + 3-hydroxybutyrate) 15 min after injection of 50 U/kg of UH were significantly higher than those after a dose of 50 U/kg of LMWH, and levels after 200 U/kg of UH were significantly higher than those after 200 U/kg of LMWH at 15, 30, 45 and 60 min. Enhanced ketogenesis was not noted after LMWH at any of the doses, or after UH at 3 U/kg. Arterial ketone body ratio (AKBR; acetoacetate/3-hydroxybutyrate), which reflects the hepatic mitochondrial oxidation-reduction state (NAD+/NADH), was maintained above 1.0 in all groups except in the U-200 group, while AKBR in that group was significantly decreased to 0.99 +/- 0.14 at 30 min, and further decreased to 0.80 +/- 0.08 at 60 min. These results indicate that LMWH has less effect on lipolysis than UH and does not enhance ketogenesis, resulting in less deterioration of mitochondrial redox state.

Inhibition of beta-oxidation by 3-mercaptopropionic acid produces features of Reye's syndrome in perfused rat liver

BACKGROUND/AIMS

The cause of Reye's syndrome has not been completely defined. The rate of ketogenesis in the liver is a key determinant of, and reciprocally related to, triglyceride secretion. In the present study, 3-mercaptopropionic acid (MPA), a known inhibitor of mitochondrial long-chain acyl coenzyme A (CoA) dehydrogenase, was used to investigate the relationship between ketone body production, triglyceride secretion, and triglyceride accumulation in perfused rat liver.

METHODS

Livers from fasted rats were perfused 225 minutes with or without MPA in the presence of [1-14C]oleic acid. Morphology was studied by light and electron microscopy.

RESULTS

Inhibition of fatty acid oxidation by the liver with MPA resulted in a decrease in ketone body production. Treatment with MPA caused an accumulation of small-droplet triglycerides in liver, whereas the net secretion of triglyceride ceased after an initial period of increased secretion with continued decreased ketogenesis. At the end of the perfusion period, mitochondria in the MPA group appeared to be damaged.

CONCLUSIONS

The rates of both ketogenesis and triglyceride secretion by the liver appear to be the major determinants of hepatic triglyceride content. In addition, the MPA-mediated biochemical and morphological findings are quite similar to those of Reye's syndrome.

Evidence for an impaired long-chain fatty acid oxidation and ketogenesis in Fao hepatoma cells

Fatty acid metabolism has been studied in Fao rat hepatoma cells. In basal conditions of culture, [1-14C]oleate is mainly esterified (85% of oleate uptake) in Fao cells, phospholipids being the most important esterified products (60% of oleate esterified). Addition of N6,O2'-dibutyryl-adenosine 3',5'-monophosphate (0.1 mM) in Fao cells does not change the metabolic fate of oleate whereas it induces gluconeogenesis and phosphoenolpyruvate carboxykinase mRNA accumulation. It is shown that the limitation of oleate oxidation is located at the level of the entry into mitochondria since octanoate is actively oxidized in Fao cells. Neither the activities of carnitine palmitoyltransferase (CPT) I and II nor the CPT II protein amount are affected by cAMP addition. The limitation of oleate oxidation in Fao cells results from (a) a high rate of lipogenesis and a high malonyl-CoA concentration, (b) a CPT I very sensitive to malonyl-CoA inhibition. The presence of an active oleate oxidation in mitochondria isolated from Fao cells confirms that CPT I is the limiting step of oleate oxidation. Moreover, Fao cells are unable to perform ketogenesis. This particular feature results from a specific deficiency in mitochondrial hydroxymethylglutaryl-CoA synthase protein, activity and gene expression. The metabolic characteristics observed in Fao cells could be a common feature in hepatoma cell lines with regard to the low capacity for long-chain fatty acid oxidation and ketone body production observed in the rat H4IIE and the human HepG2 cells.

Serum ketone response to glucagon as a marker of insulin dependency in diabetics

The serum ketone response to glucagon was measured in 10 patients with IDDM and 37 with NIDDM. In both groups, serum 3-hydroxybutyrate increased significantly after intravenous injection of 1 mg glucagon. The difference between the serum level of 3-hydroxybutyrate at 30 min and basal level [delta 3-OHBA(30')] was 133 +/- 25 mumol/l in the patients with IDDM, 13 +/- 8 mumol/l in those with NIDDM treated by diet alone or with oral hypoglycemic agents and 23 +/- 13 mumol/l in those with NIDDM treated with insulin. The delta 3-OHBA(30') was significantly greater in IDDM patients than in both groups of NIDDM patients (P less than 0.001). The delta 3-OHBA(30') was greater than 87 mumol/l in eighty percent of IDDM patients, but smaller than 87 mumol/l in both groups of NIDDM patients. The delta 3-OHBA(30') was correlated with the difference between the plasma level of C-peptide at 6 min and basal level [delta CPR(6')] (r = -0.540, P less than 0.001). The delta 3-OHBA(30') was not correlated with fasting plasma levels of glucose, fructosamine or hemoglobin A1c. These observations show that measurement of the serum ketone response to glucagon is a useful marker of insulin dependency. In order to determine insulin dependency, the simultaneous measurement of concentrations of ketones and C-peptide is indicated during the glucagon stimulation test.

Regulation of lipolysis and ketogenesis by norepinephrine in conscious dogs

The lipolytic and ketogenic effects of norepinephrine (NE) at levels present in the circulation or the synaptic cleft during stress were examined in the overnight-fasted conscious dog. Insulin and glucagon were maintained at basal levels while NE, at a rate of either 0.04 (n = 6) or 0.32 micrograms.kg-1.min-1 (n = 5), or saline (n = 6) was infused for 3 h. NE rose from 129 +/- 17 to 442 +/- 85 pg/ml (P less than 0.05) and 100 +/- 24 to 3,244 +/- 807 pg/ml (P less than 0.05) with the low and high infusion rates, respectively (unchanged with saline infusion). There were no significant changes in lipolysis or ketogenesis with saline infusion. Both low and high NE infusion produced sustained increases in glycerol (from 72 +/- 20 to 119 +/- 24 microM and 59 +/- 19 to 248 +/- 32 microM, respectively, both P less than 0.05), while nonesterified fatty acids (NEFA) rose from 609 +/- 85 to 952 +/- 100 and 767 +/- 140 to 2,054 +/- 199 microM (both P less than 0.05). Ketone levels and net hepatic production rose significantly only with the high NE infusion (from 88 +/- 10 to 266 +/- 46 microM and 1.30 +/- 0.26 to 7.62 +/- 1.48 mumol.kg-1.min-1, respectively, both P less than 0.05). The ratio of net hepatic ketone production to NEFA uptake rose 54% with high NE infusion. In conclusion, at circulating levels seen during stress, NE stimulates lipolysis but does not directly influence ketogenesis. At circulating levels projected to exist in the synaptic cleft during stress, NE has a potent lipolytic effect and stimulates ketogenesis.

Alpha 1-adrenergic regulation of ketogenesis in isolated rat hepatocytes

Studies were conducted to clarify the effects of noradrenaline on oleate metabolism in isolated hepatocytes from fed rats. Noradrenaline caused an inhibition of ketogenesis from oleate along with a stimulation of glucose release through alpha 1-adrenergic receptors. Anti-ketogenic action of noradrenaline was confirmed by the suppression of the formation of radioactive acid-soluble products from [1-14C]oleate in response to this agent. Noradrenaline increased the conversion of [1-14C]oleate into 14CO2 but failed to affect [1-14C]oleate esterification. When hepatocytes were incubated in a medium containing 1 mM EGTA but no Ca2+, the effects of noradrenaline on oleate oxidation were negated. On the other hand, noradrenaline-induced increase in glucose release remained unchanged even in the absence of Ca2+ in the incubation medium. Decrease in ketogenesis and increase in glucose release produced by vasopressin was completely abolished by calcium depletion. Noradrenaline caused a significant increase in cAMP levels in both the presence and absence of Ca2+, although the effect was more marked in the latter. Vasopressin did not affect it. The noradrenaline-induced increase in cAMP and glucose release in the absence of Ca2+ was also mediated by alpha 1-adrenergic receptors. These data are discussed and it is suggested that alpha 1-adrenergic agonists may control hepatic ketogenesis and glycogenolysis through two separate signal transduction mechanisms, i.e., a calcium-mobilizing system which is common with vasopressin, and a cAMP generation system which vasopressin lacks.

Effects of chronic glucagon administration on rat lipoprotein composition

Male adult rats of the Wistar strain received daily at 9 a.m. and 5 p.m. 10 micrograms of Zn-protamine glucagon (Novo) for 21 days by subcutaneous injections. Plasma levels of cholesterol, triacylglycerol and phospholipids were decreased by 47, 40 and 21%, respectively. Lipoproteins were separated by sequential ultracentrifugation. Concentrations of cholesterol, phospholipids and proteins were decreased in chylomicrons, VLDL, LDL2 (1.040-1.063 g/ml) and HDL, LDL2 being the most affected by glucagon treatment (-70%). Triacylglycerol levels were decreased only in chylomicrons and VLDL. The relative proportions of cholesterol, triacylglycerol, phospholipids and proteins in lipoproteins were virtually unchanged by glucagon, suggesting a reduced number of some lipoprotein particles in plasma. However, lipoproteins of glucagon-treated rats were depleted in cholesteryl esters, while the proportion of triacylglycerol increased in LDL and HDL. Apo E contents were decreased in plasma, LDL1 (1.006-1.040 g/ml), LDL2 and HDL, whereas apo B100 proportions increased in VLDL and LDL1 in glucagon-treated rats. Glucagon appeared to be a potent hypolipidemic agent affecting mainly the apo-E-rich lipoproteins.

Glucagon in the metabolic and nutritional management after total pancreatectomy--a case report

The effects of exogenous glucagon on a totally pancreatectomized patient were evaluated in both the acute postoperative and convalescent periods. In the acute postoperative phase, glucagon stabilized the blood glucose level and allowed the administration of an adequate amount of energy, while during the convalescent period, it proved useful in promoting the metabolism of lipids and amino acids and maintaining liver function. Moreover, we developed a glucagon aerosol which enabled frequent administration without causing major discomfort to the patient and raised the blood glucagon to satisfactory level.

Estradiol is a potent inhibitor of the hypotriglyceridemic effect of levonorgestrel in female rats

The progestin, levonorgestrel, when administered to rats intramuscularly, significantly lowered both total and very low density lipoprotein triglyceride concentrations in the blood plasma by 35-40%. This effect was readily abolished by the simultaneous intramuscular administration of estradiol benzoate. Similarly, estradiol-17 beta overcame the inhibitory effects of levonorgestrel on the incorporation of [9,10-3H]palmitate into triglycerides of freshly isolated rat hepatocytes studied in vitro. However, estradiol alone significantly raised plasma triglycerides by two-fold in vivo. Estradiol also significantly enhanced (by 9%) the incorporation of [9,10-3H]palmitate into hepatocyte triglycerides. These results suggest that the effects of estradiol on triglyceride synthesis and concentration dominate over those of levonorgestrel in the rat.

Fatty acid uptake and metabolism to ketone bodies and triacyglycerol in rat and human hepatocyte cultures is dependent on chain-length and degree of saturation. Effects of carnitine and glucagon

Rat and human hepatocyte cultures were incubated with 5 common plasma longchain fatty acids (C16-C18). Rates of fatty acid uptake were similar in rat and human hepatocytes and were of the order: 16:1 greater than 16:0; 18:2 greater than 18:1 greater than 18:0. Rates of ketogenesis were lower in human compared to rat hepatocytes. In rat hepatocytes glucagon stimulated ketogenesis only in the presence of exogenous carnitine and rates of ketogenesis were higher from unsaturated compared to corresponding saturated fatty acids. Glucagon decreased triacylglycerol secretion irrespective of the fatty acid substrate and it increased intracellular triacylglycerol accumulation. The latter effect of glucagon was more marked in the absence of carnitine supplementation.

Triglyceride-rich lipoprotein metabolism during acute hyperinsulinemia in hypertriglyceridemic humans

Chronic endogenous hyperinsulinemia is associated with increased rates of triglyceride production in humans. The effect of acute exogenous hyperinsulinemia on triglyceride production was studied in seven hypertriglyceridemic men before and during six hours of hyperinsulinemic-euglycemic clamping, and in two men before and during six hours of hyperinsulinemic-hyperglycemic clamping. Apparent triglyceride production rates were assessed qualitatively by examining the rate of decline of 3H-glycerol-labeled plasma triglyceride specific activity in the preclamp period, and again when a new steady state had occurred, during the final three hours of the clamp. During the euglycemic (91.2 +/- 3.0 mg glucose/dL plasma) clamps, plasma insulin levels were increased by 700% (0.76 +/- 0.12 to 5.3 +/- 0.29 ng/mL, P less than .001) and plasma glucagon levels decreased by 19%, compared with baseline. The apparent triglyceride production rate did not increase in five of six men during the euglycemic-hyperinsulinemic clamp, or in either man during the hyperglycemic-hyperinsulinemic clamp. During the clamp period the triglyceride declined in the plasma by 23.4 +/- 3.1%; and the apolipoprotein B by 10.5 +/- 1.5%. Hyperinsulinemia with euglycemia was also associated with a decline in the ratio of triglyceride to apolipoprotein B in the triglyceride-rich lipoproteins. This was more pronounced in very low density lipoprotein (VLDL) than in intermediate density lipoprotein (IDL). In this study, hyperinsulinemia led to a decrease in the plasma glucagon concentration. This decrease was positively correlated with the decrease in the slope of the triglyceride specific activity v time curve. Hence, the changes in triglyceride production were not due to an increase in plasma glucagon concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucagon effect on myocardial transport and utilization of energy-metabolites from the coronary microcirculation in the perfused rabbit heart

Effect of glucagon on energy-metabolite transport into cardiac muscle was studied during a single transit through the isolated rabbit heart using a rapid paired-tracer dilution method. Kinetic experiments revealed that 1.5 microM glucagon stimulated the influx of palmitate bound to 30 g/litre albumin, by increasing the V 2.3 times and increasing the Km for transport 2.4 times. Tracer uptake of D-glucose, as the only exogenous substrate provided, was increased by 80% by 1.5 microM glucagon. Myocardial utilization of [3H]-or [14C]-labelled short-chain monocarboxylic acids (L-lactate, pyruvate and acetate) was significantly reduced by glucagon, to the same degree as their unidirectional sarcolemmal transport. Inhibition of L-[14C]lactate uptake was dose-dependent and in positive correlation with myocardial lactate production. It is concluded that glucagon may regulate sarcolemmal permeability and myocardial utilization for energy-metabolites from the coronary circulation.

Alpha 1-adrenergic stimulation of ketogenesis and fatty acid oxidation is associated with inhibition of lipogenesis in rat hepatocytes

The effect of norepinephrine on fatty acid synthesis (3H2O incorporation into fatty acids), on fatty acid oxidation to CO2 and on ketogenesis was studied in isolated hepatocytes of fed rats. After incubation with norepinephrine (50 microM), lipogenesis was lower (5.7 +/- 1.1 nmoles 3H2O incorporated into fatty acids/mg dry weight/30 min) than in controls (7.5 +/- 1.7; n = 6, p less than 0.02). In contrast, (1-14C) palmitate conversion into total ketone bodies was increased to 10.9 +/- 1.8 nmoles/mg/30 min with norepinephrine, vs 8.5 +/- 1.6 in controls (p less than 0.05), and more (1-14C) palmitate was converted to 14CO2 with norepinephrine than in controls (1.48 +/- 0.10 nmoles/mg/30 min vs 1.06 +/- 0.11, p less than 0.05). The inhibitory effect of norepinephrine on lipogenesis was abolished by addition of the alpha 1-receptor blocker prazosin, but not by alpha 2 or beta-blockers. The results demonstrate that the ketogenic effect of norepinephrine is coupled with an inhibitory effect on lipogenesis which may be explained by diminished activity of acetyl-CoA carboxylase, diminished formation of malonyl-CoA and decreased activity of carnitine palmitoyl transferase I.

Effect of beta-hydroxybutyrate and acetoacetate on insulin and glucagon secretion from perfused rat pancreas

To elucidate the physiological significance of ketone bodies on insulin and glucagon secretion, the direct effects of beta-hydroxybutyrate (BOHB) and acetoacetate (AcAc) infusion on insulin and glucagon release from perfused rat pancreas were investigated. The BOHB or AcAc was administered at concentrations of 10, 1, or 0.1 mM for 30 min at 4.0 ml/min. High-concentration infusions of BOHB and AcAc (10 mM) produced significant increases in insulin release in the presence of 4.4 mM glucose, but low-concentration infusions of BOHB and AcAc (1 and 0.1 mM) caused no significant changes in insulin secretion from perfused rat pancreas. BOHB (10, 1, and 0.1 mM) and AcAc (10 and 1 mM) infusion significantly inhibited glucagon secretion from perfused rat pancreas. These results suggest that physiological concentrations of ketone bodies have no direct effect on insulin release but have a direct inhibitory effect on glucagon secretion from perfused rat pancreas.

Stimulation by acetoacetate of hepatic triacylglycerol synthesis

The factors responsible for the huge accumulation of hepatic triacylglycerols in the ketotic diabetic state are not established. Our earlier work suggested a role for ketone bodies in the increased hepatic triacylglycerol synthesis observed in the ketotic diabetic state. Isolated hepatocytes obtained from normal fed rats were incubated with sodium acetoacetate or sodium chloride (control) and [1-14C]palmitate in Krebs-albumin buffer. Acetoacetate stimulated triacylglycerol synthesis in a concentration-dependent manner without increasing palmitate uptake or inhibiting palmitate oxidation. Beta hydroxybutyrate showed no effect on palmitate esterification to triacylglycerols. Isolated hepatocytes of normal fed rats were incubated with either sodium acetoacetate or sodium chloride and the nuclear-free homogenate was incubated with [U-14C]glycero-3-phosphate and cofactors. The synthesis of triacylglycerol and the activity of the cytosolic phosphatidate phosphohydrolase were increased in the cells pre-incubated with acetoacetate. The results of this study demonstrate that the increases in triacylglycerol synthesis and the cytosolic activity of phosphatidate phosphohydrolase previously observed by us in the ketotic diabetic liver, could be reproduced in normal fed rat liver cells by incubating them with acetoacetate. The results identify acetoacetate as a potential factor, in the regulation of hepatic triacylglycerol synthesis and for hepatic accumulation of triacylglycerols observed in the ketotic diabetic state.

Increased fatty acid uptake, a factor in increased hepatic triacylglycerol synthesis in aging rats

The mechanisms responsible for the increased hepatic triacylglycerol synthesis in aging are not established. We studied [1-14C] palmitate uptake and its esterification to triacylglycerols in the isolated hepatocytes of 2-month, 10-month and 20-month-old normal rats. In all hepatocytes, palmitate uptake and its esterification were linearly related to medium palmitate concentration, but palmitate uptake and triacylglycerol synthesis by the hepatocytes of 10-month and 20-month-old rats were nearly double that observed with the cells of 2-month-old rats. These results suggest that increased fatty acid uptake by the liver cells was a contributory factor in the increased triacylglycerol synthesis observed in the liver of senescent rats. The changes in the hepatocyte leading to increased fatty acid uptake and hence increased triacylglycerol synthesis are detected as early as middle age of the rat.

Hormonal regulation of the tricarboxylic acid cycle in the isolated perfused rat liver

The effect of Ca2+-mobilizing hormones, vasopressin, angiotensin II and the alpha-adrenergic agonist phenylephrine, on the metabolic flux through the tricarboxylic acid cycle was investigated in isolated perfused rat livers. All three Ca2+-mobilizing agonists stimulated 14CO2 production and gluconeogenesis in livers of 24-h-fasted rats perfused with [2-14C]pyruvate. Prazosin blocked the phenylephrine-elicited stimulation of 14CO2 and glucose production from [2-14C]pyruvate whereas the alpha 2-adrenergic agonist, BHT-933, did not affect the rates of 14CO2 and glucose production from [2-14C]pyruvate indicating that the phenylephrine-mediated response involved alpha 1-adrenergic receptors. Phenylephrine, vasopressin and angiotensin II stimulated 14CO2 production from [2-14C]acetate in livers derived from fed rats but not in livers of 24-h-fasted rats. In livers of 24-h-fasted rats, perfused with [2-14C]acetate, exogenously added pyruvate was required for an increase in the rate of 14CO2 production during phenylephrine infusion. This last observation suggests increased pyruvate carboxylation as one of the mechanisms involved in stimulation of tricarboxylic acid cycle activity by the Ca2+-mobilizing agonists, vasopressin, angiotensin II and phenylephrine.

Cyclic AMP increases incorporation of exogenous fatty acids into triacylglycerols in hamster fibroblasts

Incorporation of various exogenous saturated or unsaturated [14C]labeled fatty acids (palmitic, stearic, oleic, linoleic and arachidonic) into triacylglycerols by hamster fibroblasts was markedly enhanced (two- to fourfold) in the presence of theophylline or dibutyryl cyclic adenosine monophosphate (dbcAMP). This effect was observed for short-term (1-6 hr) as well as long-term (15-24 hr) preincubation with dbcAMP. In the presence of sodium fluoride, a phosphoprotein phosphatase inhibitor, measurement of diacylglycerol acyltransferase (DGAT) activity in cells pretreated with dbcAMP pointed out a marked increase (3 X) in specific activity. The results suggest that DGAT activity in fibroblasts could be activated by a cAMP-dependent phosphorylation process.

A suppressive role of c-kinase for the stimulation of hepatic ketogenesis by glucagon and epinephrine

The regulatory mechanism of hepatic palmitate oxidation into ketone bodies by c-kinase has been studied in isolated hepatocytes. Glucagon and epinephrine stimulated [U-14C]palmitate oxidation to ketone bodies by 60 and 25% as early as at 1 h. The stimulatory effects were almost totally prevented by the simultaneous presence of vasopressin, phorbol 12-tetradecanoate 13-acetate (TPA), or diacylglycerol (1-oleoyl-2-acetylglycerol). When hepatocytes were treated with glucagon or epinephrine, carnitine palmitoyltransferase (CPT), a key regulatory enzyme of palmitate oxidation, was activated. This hormone-induced activation of CPT was not observed in the presence of TPA. These observations suggest that c-kinase inhibits glucagon- or epinephrine-stimulated palmitate oxidation to ketone bodies, and that this inhibition may be mediated through a covalent modification of CPT.

Mechanisms of catecholamine effects on ketogenesis

Ketogenesis may be controlled at several sites. Lipolysis with release of plasma nonesterified fatty acid (NEFA) substrate is the first step. Plasma NEFA are taken up by the liver in a concentration-dependent fashion and, after conversion to the acyl-CoA derivative, may either be reesterified or enter the mitochondria via the carnitine shuttle. After beta-oxidation the resultant acetyl-CoA may either be converted to ketone bodies that are then released into the circulation or be condensed with oxaloacetate and enter the tricarboxylic acid cycle, the third potential control point. In humans, infusion of epinephrine causes a transient two- to threefold increase in fatty acids, glycerol, and ketone bodies. Insulin levels show a small absolute increase. Norepinephrine has similar effects, although insulin levels tend to be suppressed and glucagon levels rise somewhat. If somatostatin is added simultaneously, the lipolytic and ketogenic effects are accentuated and prolonged. Dopamine, in a high dose, has no effect on ketone bodies alone but shows small increases in NEFA and ketone bodies in the presence of somatostatin and may play a modulatory role in ketogenesis. The ketogenic effect of catecholamines could thus be in the adipocyte or in the liver. Studies with perfused liver or hepatocytes showed only trivial effects on ketogenesis even with supraphysiological doses of catecholamines. Furthermore infusion studies in rats showed decreased rather than increased ketogenesis with no change in NEFA levels. The data suggest that a) there are species differences, and b) in humans epinephrine- and norepinephrine-induced increases in ketogenesis are secondary to increases in NEFA substrate supply.

Regulation of hepatic triacylglycerol synthesis and secretion

Triacylglycerols are the most concentrated storage form of energy for the mammalian organism. These lipids are synthesized and secreted by the liver and serve as a fuel for other tissues. This paper presents a brief review of the regulation of hepatic triacylglycerol synthesis and secretion. Particular attention will be given to the dissociation of the synthesis of triacylglycerols from that of the metabolically closely related nitrogenous phospholipids. Recent evidence is presented which suggests that triacylglycerol synthesis (and secretion) is regulated, at least partially, at the diacylglycerol branchpoint.

Effect of clofibrate on the metabolism of oleate in the perfused rat liver

The effect of clofibrate on the metabolism of [1-14C]- and [U-14C]oleate was examined in the perfused rat liver. Clofibrate feeding severely reduced hepatic triglyceride secretion and enhanced ketone body production. The increase in the rate of incorporation of labeled tracers into perfusate oxidation products and ketone bodies due to the clofibrate treatment was demonstrated only with [U-14C]oleate. Clofibrate strongly reduced the rate of incorporation of oleate into perfusate triglyceride, whereas that into the phospholipid fraction of the post-perfused liver doubled. In consequence, the sum of the radioactivities in esterified lipids in the perfusate and the post-perfused liver was not altered by clofibrate. A clofibrate-dependent increase in phospholipid synthesis may restrict the amount of exogenous fatty acid which is available for the formation of triglyceride-rich lipoproteins.

Sustained effect of zinc-protamin-glucagon in hyperlipidaemic patients

Is is well known from laboratory investigations and acute intervention studies that glucagon is involved in lipid metabolism. The present study was undertaken in order to evaluate a possible sustained effect of zinc-protamin-glucagon in moderately hyperlipidaemic patients. 4 patients were investigated after an acute bolus dose of glucagon 5 mg intravenously and 16 patients were given zinc-protamin-glucagon 5 mg subcutaneously, b.i.d. for 5 days. A similar degree of decrease of plasma cholesterol and triglycerides was obtained in the acute and chronic study, while the increase in fasting blood sugar and plasma insulin seen in the acute study was not present after 5 days treatment. The effect on plasma triglycerides was much more variable than the effect on plasma cholesterol. Five days after treatment the lipid concentrations had returned to the pretreatment values. The effect could not be predicted from the pretreatment lipid concentrations, neither by the type of hyperlipidaemia present. The decrease in plasma cholesterol and triglycerides were not correlated. The drug might prove useful in the long term research of lipid metabolism and the various complex hormonal interactions.