High-fat diet prevents eating response and attenuates liver ATP decline in rats given 2,5-anhydro-D-mannitol

Administration of the fructose analog 2,5-anhydro-D-mannitol (2,5-AM) stimulates eating in rats fed a low-fat diet but not in those fed a high-fat diet that enhances fatty acid oxidation. The eating response to 2,5-AM treatment is apparently triggered by a decrease in liver ATP content. To assess whether feeding a high-fat diet prevents the eating response to 2,5-AM by attenuating the decrease in liver ATP, we examined the effects of the analog on food intake, liver ATP content, and hepatic phosphate metabolism [using in vivo 31P-NMR spectroscopy (NMRS)]. Injection (intraperitoneal) of 300 mg/kg 2,5-AM increased food intake in rats fed a high-carbohydrate/low-fat diet, but not in those fed high-fat/low-carbohydrate (HF/LC) food. Liver ATP content decreased in all rats given 2,5-AM compared with saline, but it decreased about half as much in rats fed the HF/LC diet. NMRS on livers of anesthetized rats indicated that feeding the HF/LC diet attenuates the effects of 2,5-AM on liver ATP by reducing phosphate trapping. These results suggest that rats consuming a high-fat diet do not increase food intake after injection of 2,5-AM, because the analog is not sufficiently phosphorylated and therefore fails to decrease liver energy status below a level that generates a signal to eat.     

Very low-carbohydrate versus isocaloric high-carbohydrate diet in dietary obese rats

Objective: The effects of a very low‐carbohydrate (VLC), high‐fat (HF) dietary regimen on metabolic syndrome were compared with those of an isocaloric high‐carbohydrate (HC), low‐fat (LF) regimen in dietary obese rats. Research Methods and Procedures: Male Sprague‐Dawley rats, made obese by 8 weeks ad libitum consumption of an HF diet, developed features of the metabolic syndrome vs. lean control (C) rats, including greater visceral, subcutaneous, and hepatic fat masses, elevated plasma cholesterol levels, impaired glucose tolerance, and fasting and post‐load insulin resistance. Half of the obese rats (VLC) were then fed a popular VLC‐HF diet (Weeks 9 and 10 at 5% and Weeks 11 to 14 at 15% carbohydrate), and one‐half (HC) were pair‐fed an HC‐LF diet (Weeks 9 to 14 at 60% carbohydrate). Results: Energy intakes of pair‐fed VLC and HC rats were less than C rats throughout Weeks 9 to 14. Compared with HC rats, VLC rats exhibited impaired insulin and glycemic responses to an intraperitoneal glucose load at Week 10 and lower plasma triacylglycerol levels but retarded loss of hepatic, retroperitoneal, and total body fat at Week 14. VLC, HC, and C rats no longer differed in body weight, plasma cholesterol, glucose tolerance, or fasting insulin resistance at Week 14. Progressive decreases in fasting insulin resistance in obese groups paralleled concomitant reductions in hepatic, retroperitoneal, and total body fat. Discussion: When energy intake was matched, the VLC‐HF diet provided no advantage in weight loss or in improving those components of the metabolic syndrome induced by dietary obesity and may delay loss of hepatic and visceral fat as compared with an HC‐LF diet.     

2,5-Anhydro-D-mannitol increases hepatocyte sodium: transduction of a hepatic hunger stimulus?

To test the hypothesis that decreased hepatocyte ATP is transduced into a hepatic neuronal signal via a change in sodium pump activity, we examined the effect of 2,5-anhydro-D-mannitol (2,5-AM), which stimulates feeding behavior in rats, on intracellular sodium levels using 23Na nuclear magnetic resonance (NMR) spectroscopy. Isolated hepatocytes suspended in agarose beads were superfused with either 2.5 mM 2,5-AM or fructose in the presence of the paramagnetic shift reagent, thulium(III)(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylenephosphonate)). Superfusion with 2,5-AM decreased hepatocyte ATP and increased intracellular sodium levels compared with superfusion with either fructose or shift reagent alone starting within 15 min of exposure, reaching a maximum level of 120% of baseline by 30 min and declining gradually thereafter over the next 90 min. Superfusion with fructose, which also decreased hepatocyte ATP but by less than half the amount seen with 2,5-AM, had no significant effect on cellular sodium levels. The results support the hypothesis that changes in sodium pump activity could participate in transducing a hunger stimulus associated with hepatocyte energy status into a signal for hunger.     

2,5-Anhydro-D-mannitol increases hepatocyte calcium: implications for a hepatic hunger stimulus

The fructose analogue, 2,5-anhydro-D-mannitol (2,5-AM), triggers feeding in rats via a mechanism linked to its ability to trap phosphate and deplete hepatic ATP. This metabolic inhibitor is particularly useful in the study of the role of the liver in initiation of feeding as its effects are preferentially localized to the liver, and its metabolic consequences have been extensively characterized. To determine whether changes in intracellular calcium may participate in a mechanism conveying information about hepatic energy status to the nervous system, we studied the effects of 2,5-AM on intracellular calcium in isolated hepatocytes using the ratiometric indicator, fura-2. 2,5-AM elicited a marked elevation of intracellular calcium within 2-3 min of exposure that returned to baseline upon removal of the agent. Removal of external calcium failed to prevent this response, while emptying intracellular stores prevented it. These data are consistent with the hypothesis that hepatic energy status may be conveyed to the nervous system via a calcium-mediated secretion event.     

Induction of muscle thermogenesis by high-fat diet in mice: association with obesity-resistance

The obesogenic effect of a high-fat (HF) diet is counterbalanced by stimulation of energy expenditure and lipid oxidation in response to a meal. The aim of this study was to reveal whether muscle nonshivering thermogenesis could be stimulated by a HF diet, especially in obesity-resistant A/J compared with obesity-prone C57BL/6J (B/6J) mice. Experiments were performed on male mice born and maintained at 30 degrees C. Four-week-old mice were randomly weaned onto a low-fat (LF) or HF diet for 2 wk. In the A/J LF mice, cold exposure (4 degrees C) resulted in hypothermia, whereas the A/J HF, B/6J LF, and B/6J HF mice were cold tolerant. Cold sensitivity of the A/J LF mice was associated with a relatively low whole body energy expenditure under resting conditions, which was normalized by the HF diet. In both strains, the HF diet induced uncoupling protein-1-mediated thermogenesis, with a stronger induction in A/J mice. Only in A/J mice: 1) the HF diet augmented activation of whole body lipid oxidation by cold; and 2) at 30 degrees C, oxygen consumption, total content, and phosphorylation of AMP-activated protein kinase (AMPK), and AICAR-stimulated palmitate oxidation in soleus muscle was increased by the HF diet in parallel with significantly increased leptinemia. Gene expression data in soleus muscle of the A/J HF mice indicated a shift from carbohydrate to fatty acid oxidation. Our results suggest a role for muscle nonshivering thermogenesis and lipid oxidation in the obesity-resistant phenotype of A/J mice and indicate that a HF diet could induce thermogenesis in oxidative muscle, possibly via the leptin-AMPK axis.     

Effect of 2,5-anhydro-D-mannitol on membrane potential in rat hepatocyte couplets and hepatocyte monolayer cultures

The fructose analogue 2,5-anhydro-D-mannitol (2,5-AM), which depletes liver cells of ATP, has been shown to alter liver cell membrane potential (V(m)) in situ and in superfused liver slices. To study this effect of 2,5-AM on hepatocytes in more detail, patch-clamp experiments in the current-clamp mode were performed using two established models, rat hepatocyte couplets and confluent rat hepatocytes in primary culture. 2,5-AM, which has previously been shown to hyperpolarize hepatocytes in superfused liver slices and in vivo, failed to alter V(m) of hepatocyte couplets. Increasing intracellular Ca(2+) by addition of thapsigargin or ionomycin also did not evoke a change of V(m). This is most likely due to a lack of Ca(2+)-dependent K(+) channels in rat hepatocyte couplets. In contrast, 2,5-AM depolarized the cells in confluent hepatocyte monolayers. This depolarization was mimicked after inhibition of Na(+)/K(+) ATPase by ouabain. Ouabain was also able to block 2, 5-AM's effect on monolayer V(m). Thus, 2,5-AM affects the membrane potential of isolated and cultured hepatocytes in a way not comparable with cells integrated in the liver.     

Fat intake reverses the beneficial effects of low caloric intake on skeletal muscle mitochondrial H(2)O(2) production

Food restriction is the most effective modulator of oxidative stress and it is believed that a reduction in caloric intake per se is responsible for the reduced generation of reactive oxygen species (ROS) by mitochondria. Hydrogen peroxide (H(2)O(2)) generation and oxygen consumption (O(2)) by skeletal muscle mitochondria were determined in a peculiar strain of rats (Lou/C) characterized by a self-low-caloric intake and a dietary preference for fat. These rats were fed either with a standard high-carbohydrate (HC) or a high-fat (HF) diet and the results were compared to those measured in Wistar rats fed a HC diet. H(2)O(2) production was significantly reduced in Lou/C rats fed a HC diet; this effect was not due to a lower O(2) consumption but rather to a decrease in rotenone-sensitive NADH-ubiquinone oxidoreductase activity and increased expression of uncoupling proteins 2 and 3. The reduced H(2)O(2) generation displayed by Lou/C rats was accompanied by a significant inhibition of permeability transition pore (PTP) opening. H(2)O(2) production was restored and PTP inhibition was relieved when Lou/C rats were allowed to eat a HF diet, suggesting that the reduced oxidative stress provided by low caloric intake is lost when fat proportion in the diet is increased.     

The action of vasopressin and calcium on palmitate metabolism in hepatocytes and isolated mitochondria from rat liver

Vasopressin inhibits fatty acid oxidation and stimulates fatty acid esterification, glycogenolysis, and lactate production in hepatocytes from fed rats. In cells from fasted rats, the effect of the hormone on palmitate oxidation was absent, while gluconeogenesis was stimulated. The inhibitory action of vasopressin on palmitate oxidation was not due to the increased lactate production. Neither was it correlated to glycogen content or stimulation of glycogenolysis, which were restored earlier than the vasopressin effect on palmitate oxidation when previously fasted rats were refed a carbohydrate diet. The level of malonyl-CoA was moderately increased by vasopressin. Isolated mitochondria from rat liver were incubated in the presence of [U-14C]palmitate, ATP, CoA carnitine, glycerophosphate, ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid, and varying amounts of calcium. The oxidation of palmitate was inhibited when the concentration of free calcium was increased from about 0.1 to 10 microM. Simultaneously, palmitate esterification was stimulated. This effect of calcium was observed also with mitochondria from fasted rats and with octanoate as well as palmitate as the substrate. Carnitine acylation was not affected by calcium. The possibility that the observed effects of calcium on mitochondrial fatty acid utilization is part of the mechanism of action of vasopressin on hepatocyte fatty acid metabolism is discussed.     

Metabolic inhibitors synergistically decrease hepatic energy status and increase food intake

Previous studies indicate that administration of the metabolic inhibitor, 2,5-anhydro-D-mannitol (2,5-AM) or methyl palmoxirate (MP), induces feeding behavior in rats by lowering hepatic energy status. Combined treatment with these agents synergistically increases food intake. The present study was designed to investigate whether combined treatment also has a synergistic effect on hepatic energy status. Rats treated with both inhibitors increased feeding behavior compared with the controls, whereas those treated with 2, 5-AM or MP alone did not. Although 2,5-AM alone lowered hepatic ATP content regardless of MP treatment, only the combination resulted in decreases in hepatic ATP/ADP ratio and phosphorylation potential. MP treatment did not affect the uptake of 2,5-AM into liver. These results suggest that a reduction in hepatic energy status is the common triggering signal for eating behavior induced by 2,5-AM and MP and provide additional evidence for an integrated metabolic control of food intake.     

Desensitization of cyclic GMP-mediated regulation of fatty acid metabolism in hepatocytes from ethanol-fed rats

The mechanisms by which ethanol causes accumulation of hepatic triacylglycerols are complex. It has been proposed that nitric oxide/cyclic GMP signaling pathway may be involved in regulation of fatty acid metabolism in the liver. Here, we investigated if this mechanism may have a role in adaptation to ethanol consumption. Hepatocytes were isolated from rats fed with an ethanol-containing liquid diet and pair-fed control rats, and incubated with a range of concentrations of 8-bromo-cyclic GMP. In both types of cells, this cyclic GMP analog inhibited in parallel fatty acid synthesis de novo and acetyl-CoA carboxylase activity. Addition of 8-bromo-cyclic GMP also decreased the rate of palmitate esterification to triacylglycerols and phospholipids, whereas palmitate oxidation was increased. However, in all these metabolic effects, hepatocytes from ethanol-fed rats were significantly less sensitive to the addition of 8-bromo-cyclic GMP. In order to know if this may be a more general mechanism of adaptation to ethanol, we also studied the effects on glucose metabolism. Similarly, hepatocytes from ethanol-fed rats showed a decreased sensitivity in the inhibition by 8-bromo-cyclic GMP of glycogen synthesis, fatty acid synthesis and the synthesis of glycerol backbone of hepatic triacylglycerols. These data suggest that ethanol consumption induces a desensitization of the regulatory effects mediated by cyclic GMP in fatty acid metabolism, contributing to triacylglycerol accumulation in the liver.     

Crabtree effect induced by fructose in isolated hepatocytes from fed rats.

In hepatocytes isolated from fed rats, the addition of fructose caused an inhibition of respiration. In hepatocytes isolated from starved rats the Crabtree effect was not observed. No difference in oxygen uptake was found by addition of glucose to hepatocytes from fed or starved animals. The inhibition of respiration was parallel with a rise in the glycolytic flux and the oxidation of the mitochondrial respiratory carriers. The metabolic conditions in which the Crabtree effect can be operative in liver cells are discussed.     

White adipose tissue genome wide-expression profiling and adipocyte metabolic functions after soy protein consumption in rats

Obesity is associated with an increase in adipose tissue mass due to an imbalance between high dietary energy intake and low physical activity; however, the type of dietary protein may contribute to its development. The aim of the present work was to study the effect of soy protein versus casein on white adipose tissue genome profiling, and the metabolic functions of adipocytes in rats with diet-induced obesity. The results showed that rats fed a Soy Protein High-Fat (Soy HF) diet gained less weight and had lower serum leptin concentration than rats fed a Casein High-Fat (Cas HF) diet, despite similar energy intake. Histological studies indicated that rats fed the Soy HF diet had significantly smaller adipocytes than those fed the Cas HF diet, and this was associated with a lower triglyceride/DNA content. Fatty acid synthesis in isolated adipocytes was reduced by the amount of fat consumed but not by the type of protein ingested. Expression of genes of fatty acid oxidation increased in adipose tissue of rats fed Soy diets; microarray analysis revealed that Soy protein consumption modified the expression of 90 genes involved in metabolic functions and inflammatory response in adipose tissue. Network analysis showed that the expression of leptin was regulated by the type of dietary protein and it was identified as a central regulator of the expression of lipid metabolism genes in adipose tissue. Thus, soy maintains the size and metabolic functions of adipose tissue through biochemical adaptations, adipokine secretion, and global changes in gene expression.     

A comparison of lipid metabolism in hepatocytes isolated from fed and starved neonatal and adult rats.

1. The utilization of [1-14C]palmitate by hepatocytes prepared from fed and starved neonatal and adult rats has been examined by measuring isotopic incorporation into various products. 2. In cells from fed adult rats the principal products were esters (triglycerides and phospholipids) but ketone bodies were the main metabolic end products in cells from starved adult and fed and starved neonatal rats. Production of triglycerides exceeded that of phospholipids in fed adult cells whereas phospholipid formation always predominated in neonatal cells. 3. The high rate of fatty acid oxidation and hence NADH formation by neonatal cells is reflected by a lower acetoacetate--3-hydroxybutyrate ratio at the earlier stages of incubation of neonatal cells. 4. The addition of glycerol modified quantitatively the products of palmitate metabolism by adult hepatocytes but no such effects were observed with neonatal cells. 5. Compared with adult cells, neonatal hepatocytes showed very low rates of lipogenesis that were only enhanced a little by addition of lactate/pyruvate and did not show any effects of glucose concentration upon incorporation of tritium from 3H2O into lipids.     

The stimulation of erucate metabolism in isolated rat hepatocytes by rapeseed oil and hydrogenated marine oil-containing diets.

1. The metabolism of palmitate and especially of erucate was studied in hepatocytes isolated from rats fed for 3 weeks a diet containing peanut oil (diet, 1), rapeseed oil (diet 2) and partially hydrogenated marine oil (diet 3). 2. The metabolism of palmitate was not significantly influenced by the diet. The rapeseed oil diet caused 1.4 fold and 1.3 fold increase and marine oil diet 3 fold and 2.2 fold increase in the oxidation and chain-shortening respectively of [14-14C]erucic acid in isolated hepatocytes. 3. Cyanide and antimycin A did not inhibit the chain-shortening of erucate in liver cells of rats fed rapeseed oil and peanut oil. The high capacity of the chain-shortening system in hepatocytes of marine oil-fed rats was partially inhibited. 4. Inhibition of the transfer of fatty acids into the mitochondria by lowering the intracellular carnitine concentration and/or by addition of (+)-decanoyl-carnitine resulted in a very pronounced apparent stimulation of the chain-shortening of erucic acid. It is suggested that the chain-shortening system may be virtually independent of the mitochondria, unless the availability of the extramitochondria NAD+ and/or NADP+ is rate-limiting under conditions of extremely low redox potential of the mitochondria. 5. Feeding marine oil or rapeseed oil to the rats induced a 30% increase in catalase activity, a 25--30% increase in urate oxidase activity and a 50% increase in the total CoA in the liver compared to rats fed peanut oil. 6. It is suggested that the increased metabolism of erucate in hepatocytes of marine oil and rapeseed oil-fed rats may be due to the increase in ther peroxisomal beta-oxidation.     

Recovered insulin response by 2 weeks of leptin administration in high-fat fed rats is associated with restored AS160 activation and decreased reactive lipid accumulation

Leptin is an adipokine that increases fatty acid (FA) oxidation, decreases intramuscular lipid stores, and improves insulin response in skeletal muscle. In an attempt to elucidate the underlying mechanisms by which these metabolic changes occur, we administered leptin (Lep) or saline (Sal) by miniosmotic pumps to rats during the final 2 wk of a 6-wk low-fat (LF) or high-fat (HF) diet. Insulin-stimulated glucose transport was impaired by the HF diet (HF-Sal) but was restored with leptin administration (HF-Lep). This improvement was associated with restored phosphorylation of Akt and AS160 and decreased in reactive lipid species (ceramide, diacylglycerol), known inhibitors of the insulin-signaling cascade. Total muscle citrate synthase (CS) activity was increased by both leptin and HF diet, but was not additive. Leptin increased subsarcolemmal (SS) and intramyofibrillar (IMF) mitochondria CS activity. Total muscle, sarcolemmal, and mitochondrial (SS and IMF) FA transporter (FAT/CD36) protein content was significantly increased with the HF diet, but not altered by leptin. Therefore, the decrease in reactive lipid stores and subsequent improvement in insulin response, secondary to leptin administration in rats fed a HF diet was not due to a decrease in FA transport protein content or altered cellular distribution.     

Phosphorylation state of cytosolic and mitochondrial adenine nucleotides and of pyruvate dehydrogenase in isolated rat liver cells.

1. Cytosolic and mitochondrial ATP and ADP concentrations of liver cells isolated from normal fed, starved and diabetic rats were determined. 2. The cytosolic ATP/ADP ratio was 6,9 and 10 in normal fed, starved and diabetic rats respectively. 3. The mitochondrial ATP/ADP ratio was 2 in normal and diabetic rats and 1.6 in starved rats. 4. Adenosine increased the cytosolic and lowered the mitochondrial ATP/ADP ratio, whereas atractyloside had the opposite effect. 5. Incubation of the hepatocytes with fructose, glycerol or sorbitol led to a fall in the ATP/ADP ratio in both the cytosolic and the mitochondrial compartment. 6. The interrelationship between the mitochondrial ATP/ADP ratio and the phosphorylation state of pyruvate dehydrogenase in intact cells was studied. 7. In hepatocytes isolated from fed rats an inverse correlation between the mitochondrial ATP/ADP ratio and the active form of pyruvate dehydrogenase (pyruvate dehydrogenase a) was demonstrable on loading with fructose, glycerol or sorbitol. 8. No such correlation was obtained with pyruvate or dihydroxyacetone. For pyruvate, this can be explained by inhibition of pyruvate dehydrogenase kinase. 9. Liver cells isolated from fed animals displayed pyruvate dehydrogenase a activity twice that found in vivo. Physiological values were obtained when the hepatocytes were incubated with albumin-oleate, which also yielded the highest mitochondrial ATP/ADP ratio.     

The control of fatty acid metabolism in liver cells from fed and starved sheep.

Isolated liver cells prepared from starved sheep converted palmitate into ketone bodies at twice the rate seen with cells from fed animals. Carnitine stimulated palmitate oxidation only in liver cells from fed sheep, and completely abolished the difference between fed and starved animals in palmitate oxidation. The rates of palmitate oxidation to CO2 and of octanoate oxidation to ketone bodies and CO2 were not affected by starvation or carnitine. Neither starvation nor carnitine altered the ratio of 3-hydroxybutyrate to acetoacetate or the rate of esterification of [1-14C]palmitate. Propionate, lactate, pyruvate and fructose inhibited ketogenesis from palmitate in cells from fed sheep. Starvation or the addition of carnitine decreased the antiketogenic effectiveness of gluconeogenic precursors. Propionate was the most potent inhibitor of ketogenesis, 0.8 mM producing 50% inhibition. Propionate, lactate, fructose and glycerol increased palmitate esterification under all conditions examined. Lactate, pyruvate and fructose stimulated oxidation of palmitate and octanoate to CO2. Starvation and the addition of gluconeogenic precursors stimulated apparent palmitate utilization by cells. Propionate, lactate and pyruvate decreased cellular long-chain acylcarnitine concentrations. Propionate decreased cell contents of CoA and acyl-CoA. It is suggested that propionate may control hepatic ketogenesis by acting at some point in the beta-oxidation sequence. The results are discussed in relation to the differences in the regulation of hepatic fatty acid metabolism between sheep and rats.     

Does high-sucrose diet alter skeletal muscle and liver mitochondrial respiration?

A diet high in sucrose or fructose progressively impairs glucose and lipid metabolism, which leads to insulin resistance. As mitochondria are the sites of the oxidation and utilization of these substrates, we hypothesized that a high sucrose diet would alter mitochondrial respiration. Male Wistar rats were fed high-sucrose (SU) or control (CTL) diet for one week; mitochondrial respiration was investigated in mitochondria isolated from liver and both glycolytic and oxidative muscles, with pyruvate and palmitate as substrates. To test for metabolic disturbances, we measured not only glycogen content in muscles and liver, but also lactate, glucose and triglyceride blood concentrations. After one week of high-sucrose intake, we found no change in blood concentration of these variables, but glycogen content was significantly increased in liver (17.28 +/- 2.98 mg/g tissue SU vs 6.47 +/- 1.67 mg/g tissue CTL), oxidative muscle (1.59 +/- 0.21 mg/g tissue SU vs 0.70 +/- 0.24 mg/g tissue CTL) though not in glycolytic muscle (1.72 +/- 0.44 mg/g tissue SU vs 1.52 +/- 0.20 mg/g tissue CTL). State 3 mitochondrial respiration was significantly decreased in SU rats compared with CTL (p < 0.05) with pyruvate, while no change was observed with palmitate. This study shows that 1-week of high-sucrose diet altered mitochondrial pyruvate oxidation in rats and suggests that, in the context of a high-sucrose diet, impaired mitochondrial respiration could contributed to the development of insulin resistance.     

Obesity induced by a high-fat diet downregulates apolipoprotein A-IV gene expression in rat hypothalamus

Apolipoprotein A-IV (apo A-IV) is an anorectic protein produced in the intestine and brain that has been proposed as a satiety signal. To determine whether diet-induced obesity alters apo A-IV gene expression in the intestine and hypothalamus, rats were fed a high-fat (HF), low-fat (LF), or standard chow (CHOW) diet for 2, 4, 6, 8, or 10 wk. Rats fed the HF diet had significantly greater body weights than rats given the LF and CHOW diets. Intestinal and plasma apo A-IV levels were comparable across dietary groups and time. LF and CHOW rats had comparable hypothalamic apo A-IV mRNA across the course of the experiment. However, HF rats had a slow and progressive diminution in hypothalamic apo A-IV mRNA over time that became significantly lower than that of LF or CHOW rats by 10 wk. Intragastric infusion of lipid emulsion to animals that were fasted overnight significantly stimulated hypothalamic apo A-IV mRNA in LF and CHOW rats but had no effect in HF rats. These results demonstrate that chronic consumption of a HF diet significantly reduces apo A-IV mRNA levels and the response of apo A-IV gene expression to dietary lipids in the hypothalamus. This raises the possibility that dysregulation of hypothalamic apo A-IV could contribute to diet-induced obesity.     

Growth hormone-mediated breakdown of body fat: insulin and leptin responses to GH are modulated by diet composition and caloric intake in old rats.

This work was performed to elucidate whether growth hormone (GH)-mediated loss of adipose tissue and responses in plasma insulin and leptin are modulated by diet composition. 12-month-old rats were first fed a high-fat (HF) diet or a low-fat (LF) diet for 14 weeks. After that, GH or saline was administered to rat groups that were maintained on either HF or LF diets or that were switched from the HF to the LF diet. All 6 groups had free access to food. One additional saline group was pair-fed with the GH group that was switched from the HF to the LF diet. The caloric consumption of this latter group was also translated to yet another GH group receiving restricted amounts of the HF diet. GH was given in a total dose of 4 mg/kg/d for three weeks. After sacrifice, blood was collected and tissues were excised. In groups injected with saline, the weight of excised adipose tissue was 60 +/- 4.7, 41 +/- 3.8 and 50 +/- 4.5 g in animals that continued with the HF diet, LF diet, or that were switched from HF to LF, respectively. Corresponding figures after GH treatment were significantly (p < 0.05) decreased to 38 +/- 2.7, 30 +/- 2.3, and 31 +/- 2.7 g, respectively. Pair-feeding had no effect, whereas only 26 +/- 3.0 g of adipose tissue was retrieved in rats fed restricted amounts of HF diet while receiving GH. In this group, plasma insulin and leptin were also significantly (p < 0.05) depressed compared with other GH groups, especially to the group fed the unrestricted HF diet (203 +/- 35 vs. 1345 +/- 160 pmol/l and 9.3 +/- 1.2 vs. 31 +/- 4.4 micro g/l). In conclusion, this study shows that GH mediates breakdown of adipose tissue under a variety of dietary conditions, and that induction of hyperinsulinemia can be prevented if GH treatment is combined with restricted feeding of a diet which is relatively low in carbohydrates and rich in fat. This will also promote a fall of plasma leptin.