Biomedical subjects

G J Cooney

Publications and source records attributed to G J Cooney.

At least 19 recordsLinked to original sources

PubMed2004

Direct demonstration of lipid sequestration as a mechanism by which rosiglitazone prevents fatty-acid-induced insulin resistance in the rat: comparison with metformin.

AIMS/HYPOTHESIS: Thiazolidinediones can enhance clearance of whole-body non-esterified fatty acids and protect against the insulin resistance that develops during an acute lipid load. The present study used [(3)H]-R-bromopalmitate to compare the effects of the thiazolidinedione, rosiglitazone, and the biguanide, metformin, on insulin action and the tissue-specific fate of non-esterified fatty acids in rats during lipid infusion. METHODS: Normal rats were treated with rosiglitazone or metformin for 7 days. Triglyceride/heparin (to elevate non-esterified fatty acids) or glycerol (control) were then infused for 5 h, with a hyperinsulinaemic clamp being performed between the 3rd and 5th hours. RESULTS: Rosiglitazone and metformin prevented fatty-acid-induced insulin resistance (reduced clamp glucose infusion rate). Both drugs improved insulin-mediated suppression of hepatic glucose output but only rosiglitazone enhanced systemic non-esterified fatty acid clearance (plateau plasma non-esterified fatty acids reduced by 40%). Despite this decrease in plateau plasma non-esterified fatty acids, rosiglitazone increased fatty acid uptake (two-fold) into adipose tissue and reduced fatty acid uptake into liver (by 40%) and muscle (by 30%), as well as reducing liver long-chain fatty acyl CoA accumulation (by 30%). Both rosiglitazone and metformin increased liver AMP-activated protein kinase activity, a possible mediator of the protective effects on insulin action, but in contrast to rosiglitazone, metformin had no significant effect on non-esterified fatty acid kinetics or relative tissue fatty acid uptake. CONCLUSIONS/INTERPRETATION: These results directly demonstrate the "lipid steal" mechanism, by which thiazolidinediones help prevent fatty-acid-induced insulin resistance. The contrasting mechanisms of action of rosiglitazone and metformin could be beneficial when both drugs are used in combination to treat insulin resistance.

Animals↗

PubMed2003

Disassociation of muscle triglyceride content and insulin sensitivity after exercise training in patients with Type 2 diabetes.

AIM/HYPOTHESIS: We determined the effect of exercise training on insulin sensitivity and muscle lipids (triglyceride [TG(m)] and long-chain fatty acyl CoA [LCACoA] concentration) in patients with Type 2 diabetes. METHODS: Seven patients with Type 2 diabetes and six healthy control subjects who were matched for age, BMI, % body fat and VO(2)peak participated in a 3 days per week training program for 8 weeks. Insulin sensitivity was determined pre- and post-training during a 120 min euglycaemic-hyperinsulinaemic clamp and muscle biopsies were obtained before and after each clamp. Oxidative enzyme activities [citrate synthase (CS), beta-hydroxy-acyl-CoA (beta-HAD)] and TG(m) were determined from basal muscle samples pre- and post training, while total LCACoA content was measured in samples obtained before and after insulin-stimulation, pre- and post training. RESULTS: The training-induced increase in VO(2)peak (approximately 20%, p<0.01) was similar in both groups. Compared with control subjects, insulin sensitivity was lower in the diabetic patients before and after training (approximately 60%; p<0.05), but was increased to the same extent in both groups with training (approximately 30%; p<0.01). TG(m) was increased in patients with Type 2 diabetes (170%; p<0.05) before, but was normalized to levels observed in control subjects after training. Basal LCACoA content was similar between groups and was unaltered by training. Insulin-stimulation had no detectable effect on LCACoA content. CS and beta-HAD activity were increased to the same extent in both groups in response to training ( p<0.001). CONCLUSION/INTERPRETATION: We conclude that the enhanced insulin sensitivity observed after short-term exercise training was associated with a marked decrease in TG(m) content in patients with Type 2 diabetes. However, despite the normalization of TG(m )to levels observed in healthy individuals, insulin resistance was not completely reversed in the diabetic patients.

3-Hydroxyacyl CoA Dehydrogenases↗

PubMed2003

The role of intramuscular lipid in insulin resistance.

There is interest in how altered lipid metabolism could contribute to muscle insulin resistance. Many animal and human states of insulin resistance have increased muscle triglyceride content, and there are now plausible mechanistic links between muscle lipid accumulation and insulin resistance, which go beyond the classic glucose-fatty acid cycle. We postulate that muscle cytosolic accumulation of the metabolically active long-chain fatty acyl CoAs (LCACoA) is involved, leading to insulin resistance and impaired insulin signalling or impaired enzyme activity (e.g. glycogen synthase or hexokinase) either directly or via chronic translocation/activation of mediators such as a protein kinase C (particularly PKC theta and epsilon ). Ceramides and diacylglycerols (DAGs) have also been implicated in forms of lipid-induced muscle insulin resistance. Dietary lipid-induced muscle insulin resistance in rodents is relatively easily reversed by manipulations that lessen cytosolic lipid accumulation (e.g. diet change, exercise or fasting). PPAR agonists (both gamma and alpha) also lower muscle LCACoA and enhance insulin sensitivity. Activation of AMP-activated protein kinase (AMPK) by AICAR leads to muscle enhancement (especially glycolytic muscle) of insulin sensitivity, but involvement of altered lipid metabolism is less clear cut. In rodents there are similarities in the pattern of muscle lipid accumulation/PKC translocation/altered insulin signalling/insulin resistance inducible by 3-5-h acute free fatty acid elevation, 1-4 days intravenous glucose infusion or several weeks of high-fat feeding. Recent studies extend findings and show relevance to humans. Muscle cytosolic lipids may accumulate either by increased fatty acid flux into muscle, or by reduced fatty acid oxidation. In some circumstances muscle insulin resistance may be an adaptation to optimize use of fatty acids when they are the predominant available energy fuel. The interactions described here are fundamental to optimizing therapy of insulin resistance based on alterations in muscle lipid metabolism.

Acyl Coenzyme A↗

PubMed2002

Muscle long-chain acyl CoA esters and insulin resistance.

A common observation in animal models and in humans is that accumulation of muscle triglyceride is associated with the development of insulin resistance. In animals, this is true of genetic models of obesity and nutritional models of insulin resistance generated by high-fat feeding, infusion of lipid, or infusion of glucose. Although there is a strong link between the accumulation of triglycerides (TG) in muscle and insulin resistance, it is unlikely that TG are directly involved in the generation of muscle insulin resistance. There are now other plausible mechanistic links between muscle lipid metabolites and insulin resistance, in addition to the classic substrate competition proposed by Randle's glucose-fatty acid cycle. The first step in fatty acid metabolism (oxidation or storage) is activation to the long-chain fatty acyl CoA (LCACoA). This review covers the evidence suggesting that cytosolic accumulation of this active form of lipid in muscle can lead to impaired insulin signaling, impaired enzyme activity, and insulin resistance, either directly or by conversion to other lipid intermediates that alter the activity of key kinases and phosphatases. Actions of fatty acids to bind specific nuclear transcription factors provide another mechanism whereby different lipids could influence metabolism.

Acyl Coenzyme A↗

PubMed2001

Central but not peripheral glucocorticoid infusion in adrenalectomized male rats increases basal and substrate-induced insulinemia through a parasympathetic pathway.

OBJECTIVE: Glucocorticoids acting through the central nervous system are postulated to play a role in the hyperinsulinemia and increased adiposity of obesity. We investigated the role of parasympathetic activation in glucocorticoid-induced hyperinsulinemia. RESEARCH METHODS AND PROCEDURES: Plasma pancreatic polypeptide (PP) levels were used as an index of parasympathetic output. Insulinemia and plasma PP levels were measured basally and after intravenous glucose injection (300 mg/kg) in adrenalectomized male rats infused with dexamethasone (7.5 microg/kg per day) intracerebroventricularly (ICV) or subcutaneously (SC) for 3 to 6 days in the presence or absence of acute atropine blockade (1.0 mg/kg). Food intake was controlled between groups. RESULTS: Compared with normal rats, adrenalectomy decreased white adipose tissue depot weights and leptinemia, and these were restored to normal values by ICV but not SC dexamethasone infusion. Adrenalectomy significantly reduced insulinemia below normal levels, which was restored by SC dexamethasone replacement. However, ICV dexamethasone replacement increased insulinemia of adrenalectomized rats to levels higher than normal control values (basal, 500 +/- 40 pM vs. 280 +/- 40 pM; 1-minute postglucose, 2500 +/- 180 pM vs. 1240 +/- 260 pM; p < 0.0001) and increased plasma PP levels, which were correlated with insulinemia. Atropine significantly reduced plasma insulin and PP to levels similar to normal controls but had no effect in any other group. DISCUSSION: These data show that glucocorticoids act within the brain to increase insulinemia, most likely through activation of parasympathetic efferent fibers. Such an affect would contribute to the adipogenic effects of central glucocorticoids.

Adrenalectomy↗

PubMed2001

Triglycerides, fatty acids and insulin resistance--hyperinsulinemia.

There is now much interest in the mechanisms by which altered lipid metabolism might contribute to insulin resistance as is found in Syndrome X or in Type II diabetes. This review considers recent evidence obtained in animal models and its relevance to humans, and also likely mechanisms and strategies for the onset and amelioration of insulin resistance. A key tissue for development of insulin resistance is skeletal muscle. Animal models of Syndrome X (eg high fat fed rat) exhibit excess accumulation of muscle triglyceride coincident with development of insulin resistance. This seems to also occur in humans and several studies demonstrate increased muscle triglyceride content in insulin resistant states. Recently magnetic resonance spectroscopy has been used to demonstrate that at least some of the lipid accumulation is inside the muscle cell (myocyte). Factors leading to this accumulation are not clear, but it could derive from elevated circulating free fatty acids, basal or postprandial triglycerides, or reduced muscle fatty acid oxidation. Supporting a link with adipose tissue metabolism, there appears to be a close association of muscle and whole body insulin resistance with the degree of abdominal obesity. While causal relationships are still to be clearly established, there are now quite plausible mechanistic links between muscle lipid accumulation and insulin resistance, which go beyond the classic Randle glucose-fatty acid cycle. In animal models, dietary changes or prior exercise which reduce muscle lipid accumulation also improve insulin sensitivity. It is likely that cytosolic accumulation of the active form of lipid in muscle, the long chain fatty acyl CoAs, is involved, leading to altered insulin signalling or enzyme activities (eg glycogen synthase) either directly or via chronic activation of mediators such as protein kinase C. Unless there is significant weight loss, short or medium term dietary manipulation does not alter insulin sensitivity as much in humans as in rodent models, and there is considerable interest in pharmacological intervention. Studies using PPARgamma receptor agonists, the thiazolidinediones, have supported the principle that reduced muscle lipid accumulation is associated with increased insulin sensitivity. Other potent systemic lipid-lowering agents such as PPARalpha receptor agonists (eg fibrates) or antilipolytic agents (eg nicotinic acid analogues) might improve insulin sensitivity but further work is needed, particularly to clarify implications for muscle metabolism. In conclusion, evidence is growing that excess muscle and liver lipid accumulation causes or exacerbates insulin resistance in Syndrome X and in Type II diabetes; development of strategies to prevent this seem very worthwhile.

Adipose Tissue↗

PubMed2001

The role of lipids in the pathogenesis of muscle insulin resistance and beta cell failure in type II diabetes and obesity.

This review considers evidence for, and putative mechanisms of, lipid-induced muscle insulin resistance. Acute free fatty acid elevation causes muscle insulin resistance in a few hours, with similar muscle lipid accumulation as accompanies more prolonged high fat diet-induced insulin resistance in rodents. Although causal relations are not as clearcut in chronic human insulin resistant states such as obesity and type 2 diabetes, it is now recognised that muscle lipids also accumulate in these states. The classic Randle glucose-fatty acid cycle is only one of a number of mechanisms by which fatty acids might influence muscle glucose metabolism and insulin action. A key factor is seen to be accumulation of muscle long chain acyl CoAs, which could alter insulin action via several mechanisms including chronic activation of protein kinase C isoforms or ceramide accumulation. These interactions are fundamental to understanding metabolic effects of new insulin "sensitizers", e.g. thiazolidinediones, which alter lipid metabolism and improve muscle insulin sensitivity in insulin resistant states. Recent work has also pointed to a possible role of lipids in beta cell deterioration ("lipotoxicity") associated with type 2 diabetes.

Animals↗

PubMed2001

Evidence that amylin stimulates lipolysis in vivo: a possible mediator of induced insulin resistance.

The present study investigated the role of amylin in lipid metabolism and its possible implications for insulin resistance. In 5- to 7-h-fasted conscious rats, infusion of rat amylin (5 nmol/h for 4 h) elevated plasma glucose, lactate, and insulin (P <0.05 vs. control, repeated-measures ANOVA) with peak values occurring within 60 min. Despite the insulin rise, plasma nonesterified fatty acids (NEFA) and glycerol were also elevated (P < 0.001 vs. control), and these elevations (80% above basal) were sustained over the 4-h infusion period. Although unaltered in plasma, triglyceride content in liver was increased by 28% (P < 0.001) with a similar tendency in muscle (18%, P = 0.1). Infusion of the rat amylin antagonist amylin-(8-37) (125 nmol/h) induced opposite basal plasma changes to amylin, i.e., lowered plasma NEFA, glycerol, glucose, and insulin levels (all P < 0.05 vs. control); additionally, amylin-(8-37) blocked amylin-induced elevations of these parameters (P < 0.01). Treatment with acipimox (10 mg/kg), an anti-lipolytic agent, before or after amylin infusion blocked amylin's effects on plasma NEFA, glycerol, and insulin but not on glucose and lactate. We conclude that amylin could exert a lipolytic-like action in vivo that is blocked by and is opposite to effects of its antagonist amylin-(8-37). Further studies are warranted to examine the physiological implications of lipid mobilization for amylin-induced insulin resistance.

Amyloid↗

PubMed2001

Peroxisome proliferator-activated receptor (PPAR)-alpha activation lowers muscle lipids and improves insulin sensitivity in high fat-fed rats: comparison with PPAR-gamma activation.

Peroxisome proliferator-activated receptor (PPAR)-alpha agonists lower circulating lipids, but the consequences for muscle lipid metabolism and insulin sensitivity are not clear. We investigated whether PPAR-alpha activation improves insulin sensitivity in insulin-resistant rats and compared the effects with PPAR-gamma activation. Three-week high fat-fed male Wistar rats were untreated or treated with the specific PPAR-alpha agonist WY14643 or the PPAR-gamma agonist pioglitazone (both 3 mg x kg(-1) x day(-1)) for the last 2 weeks of high-fat feeding. Like pioglitazone, WY14643 lowered basal plasma levels of glucose, triglycerides (-16% vs. untreated), and leptin (-52%), and also muscle triglyceride (-34%) and total long-chain acyl-CoAs (LCACoAs) (-41%) (P < 0.05). In contrast to pioglitazone, WY14643 substantially reduced visceral fat weight and total liver triglyceride content (P < 0.01) without increasing body weight gain. WY14643 and pioglitazone similarly enhanced whole-body insulin sensitivity (clamp glucose infusion rate increased 35 and 37% and glucose disposal 22 and 15%, respectively, vs. untreated). Both agents enhanced insulin-mediated muscle glucose metabolic index (Rg') and reduced muscle triglyceride and LCACoA accumulation (P < 0.05). Although pioglitazone had more potent effects than WY14643 on muscle insulin sensitization, this was associated with its greater effect to reduce muscle LCACoA accumulation. Overall insulin-mediated muscle Rg' was inversely correlated with the content of LCACoAs (r = -0.74, P = 0.001) and with plasma triglyceride levels (r = -0.77, P < 0.001). We conclude that even though WY14643 and pioglitazone, representing PPAR-alpha and PPAR-gamma activation, respectively, may alter muscle lipid supply by different mechanisms, both significantly improve muscle insulin action in the high fat-fed rat model of insulin resistance, and this effect is proportional to the degree to which they reduce muscle lipid accumulation.

Animals↗

PubMed2000

Interaction between adrenal glucocorticoids and parasympathetic activation in mediating hyperinsulinaemia during long-term central neuropeptide Y infusion in rats.

AIMS/HYPOTHESIS: Hypothalamic neuropeptide Y is implicated in the aetiology of obesity and insulin resistance because of its hyperinsulinaemic, hyperphagic effects. We investigated the interaction of adrenal glucocorticoids and the parasympathetic nervous system in the hyperinsulinaemia caused by neuropeptide Y infusion in rats. METHODS: Neuropeptide Y was intracerebroventricularly given to normal or adrenalectomised rats for 3-6 days with pair-feeding, with or without subcutaneous dexamethasone infusion. We measured basal and intravenous glucose-induced insulinaemia and the effect of prior atropine injection. RESULTS: Neuropeptide Y increased basal plasma insulin and C-peptide concentrations (380 +/- 90 and 1000 +/- 60 pmol/1, vs 190 +/- 20 and 590 +/- 50 pmol/1 in controls, p < 0.05). Neuropeptide Y also increased the plasma concentrations of these hormones as early as 60 s after glucose injection (1630 +/- 170 and 3200 +/- 170 pmol/1 for insulin and C peptide, respectively, vs 1080 +/- 80 and 1860 +/- 130 pmol/1 in controls, p < 0.05). Atropine reversed the effect of neuropeptide Y on basal plasma insulin and C-peptide concentrations but had no effect on post-glucose plasma concentrations. The hyperinsulinaemic effects of neuropeptide Y were prevented by adrenalectomy, but were restored by dexamethasone infusion. Dexamethasone in itself did not statistically significantly increase insulinaemia in adrenalectomised rats. As in intact rats, atropine attenuated the basal hyperinsulinaemia of adrenalectomised rats that had been infused with neuropeptide Y and dexamethasone but had no effect on post-glucose hyperinsulinaemia. CONCLUSION/INTERPRETATION: These data suggest firstly that neuropeptide Y infused centrally induces basal hyperinsulinaemia in rats through glucocorticoid-dependent parasympathetic activation to the pancreas. Secondly, neuropeptide Y potentiates glucose-induced insulinaemia through a pathway dependent on adrenal glucocorticoids that cannot be reversed by short-term blockade of the increased parasympathetic tonus.

Adrenalectomy↗

PubMed2000

Long-chain acyl-CoA esters as indicators of lipid metabolism and insulin sensitivity in rat and human muscle.

Long-chain acyl-CoAs (LCACoA) are an activated lipid species that are key metabolites in lipid metabolism; they also have a role in the regulation of other cellular processes. However, few studies have linked LCACoA content in rat and human muscle to changes in nutritional status and insulin action. Fasting rats for 18 h significantly elevated the three major LCACoA species in muscle (P < 0.001), whereas high-fat feeding of rats with a safflower oil (18:2) diet produced insulin resistance and increased total LCACoA content (P < 0.0001) by specifically increasing 18:2-CoA. The LCACoA content of red muscle from rats (4-8 nmol/g) was 4- to 10-fold higher than adipose tissue (0.4-0.9 nmol/g, P < 0.001), suggesting that any contamination of muscle samples with adipocytes would contribute little to the LCACoA content of muscle. In humans, the LCACoA content of muscle correlated significantly with a measure of whole body insulin action in 17 male subjects (r(2) = 0.34, P = 0.01), supporting a link between muscle lipid metabolism and insulin action. These results demonstrate that the LCACoA pool reflects lipid metabolism and nutritional state in muscle. We conclude that the LCACoA content of muscle provides a direct index of intracellular lipid metabolism and its links to insulin action, which, unlike triglyceride content, is not subject to contamination by closely associated adipose tissue.

Acyl Coenzyme A↗

PubMed2000

Effects of individual fatty acids on glucose uptake and glycogen synthesis in soleus muscle in vitro.

Soleus muscle strips from Wistar rats were preincubated with palmitate in vitro before the determination of insulin-mediated glucose metabolism in fatty acid-free medium. Palmitate decreased insulin-stimulated glycogen synthesis to 51% of control in a time- (0-6 h) and concentration-dependent (0-2 mM) manner. Basal and insulin-stimulated glucose transport/phosphorylation also decreased with time, but the decrease occurred after the effect on glycogen synthesis. Preincubation with 1 mM palmitate, oleate, linoleate, or linolenate for 4 h impaired glycogen synthesis stimulated with a submaximal physiological insulin concentration (300 microU/ml) to 50-60% of the control response, and this reduction was associated with impaired insulin-stimulated phosphorylation of protein kinase B (PKB). Preincubation with different fatty acids (all 1 mM for 4 h) had varying effects on insulin-stimulated glucose transport/phosphorylation, which was decreased by oleate and linoleate, whereas palmitate and linolenate had little effect. Across groups, the rates of glucose transport/phosphorylation correlated with the intramuscular long-chain acyl-CoA content. The similar effects of individual fatty acids on glycogen synthesis but different effects on insulin-stimulated glucose transport/phosphorylation provide evidence that lipids may interact with these two pathways via different mechanisms.

Acyl Coenzyme A↗

PubMed2000

Acute reversal of lipid-induced muscle insulin resistance is associated with rapid alteration in PKC-theta localization.

Muscle insulin resistance in the chronic high-fat-fed rat is associated with increased membrane translocation and activation of the novel, lipid-responsive, protein kinase C (nPKC) isozymes PKC-theta and -epsilon. Surprisingly, fat-induced insulin resistance can be readily reversed by one high-glucose low-fat meal, but the underlying mechanism is unclear. Here, we have used this model to determine whether changes in the translocation of PKC-theta and -epsilon are associated with the acute reversal of insulin resistance. We measured cytosol and particulate PKC-alpha and nPKC-theta and -epsilon in muscle in control chow-fed Wistar rats (C) and 3-wk high-fat-fed rats with (HF-G) or without (HF-F) a single high-glucose meal. PKC-theta and -epsilon were translocated to the membrane in muscle of insulin-resistant HF-F rats. However, only membrane PKC-theta was reduced to the level of chow-fed controls when insulin resistance was reversed in HF-G rats [% PKC-theta at membrane, 23.0 +/- 4.4% (C); 39.7 +/- 3.4% (HF-F, P < 0.01 vs. C); 22.5 +/- 2.7% (HF-G, P < 0.01 vs. HF-F), by ANOVA]. We conclude that, although muscle localization of both PKC-epsilon and PKC-theta are influenced by chronic dietary lipid oversupply, PKC-epsilon and PKC-theta localization are differentially influenced by acute withdrawal of dietary lipid. These results provide further support for an association between PKC-theta muscle cellular localization and lipid-induced muscle insulin resistance and stress the labile nature of high-fat diet-induced insulin resistance in the rat.

Adipose Tissue↗

PubMed2000

Expression of genes involved in lipid metabolism correlate with peroxisome proliferator-activated receptor gamma expression in human skeletal muscle.

Peroxisome proliferator-activated receptor gamma (PPAR-gamma) activation in adipose tissue is known to regulate genes involved in adipocyte differentiation and lipid metabolism. However, the role of PPAR-gamma in muscle remains unclear. To examine the potential regulation of genes by PPAR-gamma in human skeletal muscle, we used semiquantitative RT-PCR to determine the expression of PPAR-gamma, lipoprotein lipase (LPL), muscle carnitine palmitoyl transferase-1 (mCPT1), fatty acid-binding protein (FABP), carnitine acylcarnitine transferase (CACT), and glucose transporter-4 (GLUT4) in freeze-dried muscle samples from 14 male subjects. These samples were dissected free of adipose and other tissue contamination, as confirmed by minimal or absent adipsin expression. Between individuals, the messenger ribonucleic acid concentration of PPAR-gamma varied up to 3-fold, whereas LPL varied up to 6.5-fold, mCPT1 13-fold, FABP 4-fold, CACT 4-fold, and GLUT4 up to 3-fold. The expression of LPL (r2 = 0.54; P = 0.003), mCPT1 (r2 = 0.42; P = 0.012), and FABP (r2 = 0.324; P = 0.034) all correlated significantly with PPAR-gamma expression in the same samples. No significant correlation was observed between the expression of CACT and PPAR-gamma or between GLUT4 and PPAR-gamma. These findings demonstrate a relationship between PPAR-gamma expression and the expression of other genes of lipid metabolism in muscle and support the hypothesis that PPAR-gamma activators such as the antidiabetic thiazolidinediones may regulate fatty acid metabolism in skeletal muscle as well as in adipose tissue.

Aged↗

PubMed2000

Acyl-CoA inhibition of hexokinase in rat and human skeletal muscle is a potential mechanism of lipid-induced insulin resistance.

There are strong correlations between impaired insulin-stimulated glucose metabolism and increased intramuscular lipid pools; however, the mechanism by which lipids interact with glucose metabolism is not completely understood. Long-chain acyl CoAs have been reported to allosterically inhibit liver glucokinase (hexokinase IV). The aim of the present study was to determine whether long-chain acyl CoAs inhibit hexokinase in rat and human skeletal muscle. At subsaturating glucose concentrations, 10 micromol/l of the three major long-chain acyl-CoA species in skeletal muscle, palmitoyl CoA (16:0), oleoyl CoA (18:1, n = 9), and linoleoyl CoA (18:2, n = 6), reduced hexokinase activity of rat skeletal muscle to 61 +/- 3, 66 +/- 7, and 57 +/- 5% of control activity (P < 0.005), respectively. The inhibition was concentration-dependent (P < 0.005) with 5 pmol/l producing near maximal inhibition. Human skeletal muscle hexokinase was also inhibited by long-chain acyl CoAs (5 pmol/l palmitoyl CoA decreased activity to 75 +/- 6% of control activity, P < 0.005). Inhibition of hexokinase in rat and human muscle by long-chain acyl CoAs was additive to the inhibition of hexokinase by glucose-6-phosphate (an allosteric inhibitor of hexokinase). This inhibition of skeletal muscle hexokinase by long-chain acyl CoA suggests that increases in intramuscular lipid metabolites could interact directly with insulin-mediated glucose metabolism in vivo by decreasing the rate of glucose phosphorylation and decreasing glucose-6-phosphate concentrations.

Acyl Coenzyme A↗

PubMed2000

Local factors modulate tissue-specific NEFA utilization: assessment in rats using 3H-(R)-2-bromopalmitate.

Insulin-resistant states are associated with accumulation of muscle lipid, suggesting an imbalance between lipid uptake and oxidation. We have employed a new fatty-acid tracer [9,10-3H]-(R)-2-bromopalmitate (3H-R-BrP) to study individual-tissue nonesterified fatty acid (NEFA) uptake in states with diminished or enhanced lipid oxidation. 3H-R-BrP was administered to conscious male Wistar rats (approximately 300 g) during fasting (5, 18, or 36 h), acute blockade of beta-oxidation (etomoxir, 15 micromol/kg), and insulin infusion (0.25 U x kg(-1) x h(-1)). Estimates of NEFA clearance rates (K(f)*) and absolute rates of uptake (R(f)*) were calculated from tissue accumulation of 3H-R-BrP products. In the basal state, NEFA uptake was dependent on the oxidative capacity of tissues: R(f)* in brown adipose tissue (BAT) > heart (HRT) > diaphragm (DPHM) > red quadriceps (RQ) > white quadriceps (WQ) > white adipose tissue (WAT). Fasting increased (P < 0.001) K(f)* in WAT but did not change NEFA clearance in other tissues. However, plasma NEFA levels were raised (P < 0.01), tending to elevate R(f)* in most tissues (P < 0.05: WAT, BAT, WQ, DPHM). Etomoxir reduced (P < 0.01) K(f)* only in oxidative tissues (BAT, RQ, DPHM, HRT). Insulin lowered plasma NEFA levels (P < 0.001) and significantly decreased R(f)* in most tissues (P < 0.05: WAT, RQ, DPHM, HRT). An increased (P < 0.05) clearance was observed in WAT, BAT, and WQ; a decrease (P < 0.01) in K(f)* was observed in HRT. This study is the first to measure tissue-specific NEFA uptake in conscious rats in the postabsorptive, fasted, and insulin-stimulated states. We have demonstrated that tissue NEFA utilization is not exclusively determined by systemic availability, but that the early steps of NEFA uptake or metabolic sequestration can also be rapidly modulated by local processes such as NEFA oxidation.

Adipose Tissue↗

PubMed1999

Chronic administration of BRL 26830A for 9 weeks improves insulin sensitivity but does not prevent weight gain in gold-thioglucose obese mice.

BRL 26830A, a beta adrenoceptor agonist, has been shown to have antiobesity and antidiabetic properties in rodents. The aim of this study was to study the effects of chronic BRL 26830A treatment (20 mg/kg/day for 9 weeks) on weight gain and the development of insulin resistance in gold-thioglucose-injected mice (GTG). BRL 26830A slowed the rate of weight gain in GTG such that mice weighed significantly less between 2 w and 7 w of treatment. However, at the time of sacrifice (9 w), there was no difference in body weight between treated and untreated GTG. The obesity-induced reduction in lipogenesis in brown adipose tissue (BAT) was increased 9 fold to greater than CON levels. However, weight and fatty acid (FA) content of BAT were reduced, suggesting increased lipid turnover and thermogenesis. Lipogenesis, FA content and fat pad weight were unchanged in white adipose tissue (WAT) and decreased in liver of GTG. Glucose tolerance was improved in both CON and GTG. Hyperglycemia, hyperinsulinemia and changes in cardiac and hepatic glucose oxidation as indicated by PDHC activity were normalized. Serum triglycerides and non-esterified fatty acids were reduced. Thus, chronic BRL 26830A treatment prevented the development of insulin resistance and attenuated weight gain, but did not prevent the development of obesity in this model.

Adipose Tissue↗

PubMed1999

Plasma insulin rise precedes rise in ob mRNA expression and plasma leptin in gold thioglucose-obese mice.

Circulating leptin levels are strongly related to the degree of adiposity, with hyperleptinemia being associated with hyperinsulinemia. In the gold thioglucose-injected mouse (GTG), hyperinsulinemia is an early abnormality in the development of insulin resistance and obesity. In this study, hyperinsulinemia occurred 1 wk post-GTG [GTG, 199 +/- 43; age-matched controls (CON), 53 +/- 5 microU/ml; P < 0.001], with leptin levels not rising until 2 wk post-GTG (CON, 3.2 +/- 0.3; GTG, 9.9 +/- 1.7 ng/ml; P < 0.001) in parallel with increases in the size of different fat pads and increased expression of ob mRNA. The ratio of serum leptin to fat pad weight was significantly higher in GTG mice 12 wk postinjection. Starvation-induced reductions in serum leptin (50%), glucose (50%), and insulin (74%) were greater than decreases in fat pad weight (18%). Adrenalectomy decreased both adiposity and serum leptin within 1 wk in both CON and GTG and altered the serum leptin level-to-fat pad weight ratio in CON. Thus hyperinsulinemia preceded increased ob expression and hyperleptinemia, which occurred in parallel with increasing adiposity, consistent with the role of leptin as an indicator of energy supplies. Changes in hormonal and nutritional status may modify this relationship.

Adipose Tissue↗