Glucose intolerance induced by a high-fat/low-carbohydrate diet in rats effects of nonesterified fatty acids

We examined the time course of effects of a high-fat/low-carbohydrate (HF/LC) diet on the impairment of glucose tolerance in rats, clarified whether insulin secretion and sensitivity were impaired by the HF/LC diet, and investigated the relationship between the increased nonesterified fatty acids (NEFA) after HF/LC diet feeding and insulin secretion and sensitivity. We found that glucose tolerance and the postglucose-loading insulin secretion were impaired after 3 and 7 d on the HF/LC diet. The glucose intolerance was accompanied by a rise in the fasting plasma NEFA level. When stimulated with 15 mmol/L of glucose, the insulin secretion was impaired in pancreatic islets from rats fed the HF/LC diet. Rats fed the HF/LC diet showed insulin resistance in vivo. The glucose-stimulated insulin secretion was inhibited in the islets following 24-h culture with palmitic acid. The 24-h infusion of palmitic acid decreased whole-body insulin sensitivity. In summary, at least 3 d on a HF/LC diet is needed to induce glucose intolerance in rats, and the impairment may be induced by decreased insulin secretion and sensitivity, which is related to the increase in the plasma NEFA level.     

The role of non-esterified fatty acids in the deterioration of glucose tolerance in Caucasian subjects: results of the Paris Prospective Study

Summary Although an increased plasma non-esterified fatty acid (NEFA) concentration has been shown to increase insulin resistance (Randle cycle), decrease insulin secretion and increase hepatic gluconeogenesis, the effect of NEFA on the deterioration of glucose tolerance has not been studied prospectively in Caucasian subjects. Therefore, we investigated whether plasma NEFA may be regarded as predictors of deterioration of glucose tolerance in subjects with normal (NGT, n = 3671) or impaired (IGT, n = 418) glucose tolerance who were participants in the Paris Prospective study. The subjects were first examined between 1967 and 1972 and underwent two 75-g oral glucose tolerance tests 2 years apart with measurements of plasma glucose, insulin and NEFA concentrations. Glucose tolerance deteriorated from NGT to IGT or non-insulin-dependent diabetes (NIDDM) in 177 subjects and from IGT to NIDDM in 32 subjects. In multivariate analysis, high fasting plasma NEFA in NGT subjects and high 2-h plasma NEFA and low 2-h plasma insulin concentrations in IGT subjects were significant independent predictors of deterioration along with older age, high fasting and 2-h plasma glucose concentrations and high iliac to thigh ratio. When subjects were divided by tertiles of plasma NEFA concentration at baseline, there was an increase in 2-h glucose concentration with increasing NEFA in the subjects who did not deteriorate, but no effect of plasma NEFA in those who deteriorated. In subjects with IGT who deteriorated compared with those who did not 2-h plasma insulin concentration was lower but there was no evidence that this resulted from an effect of plasma NEFA. Our data suggest that a high plasma NEFA concentration is a risk marker for deterioration of glucose tolerance independent of the insulin resistance or the insulin secretion defect that characterize subjects at risk for NIDDM. [Diabetologia (1997) 40: 1101–1106] Received: 11 March 1997 and in revised form: 20 May 1997     

The content of non-esterified fatty acids in rat myocardial tissue. A comparison between the Dole and Folch extraction procedures

Oliver and coworkers hypothesized that under certain circumstances NEFA (non-esterified fatty acids = FFA = free fatty acids) might be toxic for myocardial function. Unambiguous conclusions on the putative detrimental effect of intracellularly localized NEFA are hampered by contradictory values published for the NEFA content in normoxic myocardial tissue. From studies in which the assay procedures were carefully evaluated, one might conclude that the NEFA content in dog and rat myocardial tissue will not exceed 60 and 150 nmol/g wet weight, respectively. However, recently Victor and coworkers found considerably higher NEFA values in rat myocardial tissue and suggested that the low NEFA values as measured, for example, in our laboratory, resulted from incomplete extraction when the Folch medium was applied instead of the Dole mixture. Since Victor and coworkers used a modified Dole procedure as described by Hagenfeldt, we evaluated the Folch procedure as well as the original Dole technique and the modified version. Our findings indicate that the lower values found by the Folch technique are more likely to be correct. Incomplete extraction of NEFA did not occur, whereas hydrolysis and transmethylation of phospholipid fatty acids were observed in case of the (modified) Dole procedures.     

Young capillary vessels rejuvenate aged pancreatic islets

Pancreatic islets secrete hormones that play a key role in regulating blood glucose levels (glycemia). Age-dependent impairment of islet function and concomitant dysregulation of glycemia are major health threats in aged populations. However, the major causes of the age-dependent decline of islet function are still disputed. Here we demonstrate that aging of pancreatic islets in mice and humans is notably associated with inflammation and fibrosis of islet blood vessels but does not affect glucose sensing and the insulin secretory capacity of islet beta cells. Accordingly, when transplanted into the anterior chamber of the eye of young mice with diabetes, islets from old mice are revascularized with healthy blood vessels, show strong islet cell proliferation, and fully restore control of glycemia. Our results indicate that beta cell function does not decline with age and suggest that islet function is threatened by an age-dependent impairment of islet vascular function. Strategies to mitigate age-dependent dysregulation in glycemia should therefore target systemic and/or local inflammation and fibrosis of the aged islet vasculature.Aging leads to progressive decline of various homeostatic processes in mammals, including a deteriorating regulation of blood glucose levels. Pancreatic islets are small organs composed of endocrine cells that secrete the major hormones insulin, glucagon, and somatostatin, which play a key role in regulating blood glucose levels. Age-dependent dysfunction of islets and the concomitant dysregulation of blood glucose levels increase the risk for type 2 diabetes (1), which in turn contributes to other age-related chronic diseases. In general, it has been assumed that aging causes an intrinsic dysfunction of the insulin-secreting beta cells through reduced proliferative capacity and/or defective insulin secretion (1–9). However, there have been numerous reports that age-dependent impairment of glucose homeostasis is not just a result of intrinsic, age-dependent dysfunction of islets but is also caused by systemic factors. For example, islet function may be compromised by age-related increases in adiposity (10, 11) and by bloodborne factors (12), or it could be affected indirectly by age-related deficiencies in vascular remodeling (13). Thus, the replicative decline of old pancreatic beta cells can be attributed to systemic factors (12). Recent studies identified factors present in young blood that reverse age-related cognitive impairments and induce vascular remodeling and regeneration in the brain and skeletal muscle (14–16), but so far it has not been feasible to discriminate systemic influences from aging factors intrinsic to islet endocrine cells. Here we address the long-standing question of whether the age-dependent impairment of glucose homeostasis is caused by intrinsic, age-dependent dysfunction of islets or by systemic aging factors.Our strategy to discern age-related intrinsic changes in islet function was to study islets from young mature (2 mo) and aged (18 mo) mice and to follow these same groups of islets in three different environments: in vivo in the body of young and aged mice, in vitro after isolation, and again in vivo after transplantation into the anterior chamber of the eye in young mice (17). We also examined a large number of human islets from young mature and old pancreatic donors (17–65 y of age). We hypothesized that islets are affected by the systemic milieu, such that the effects the aged organism exerts on the islet can be rescued in a young organism. We characterized islet structure and function at the molecular, anatomic, and physiologic levels to distinguish intrinsic from systemic factors impinging on the islet as the organism ages. Our results reveal that aging of islets involves little intrinsic decline of beta cell function but is accompanied by malfunctioning blood vessels, suggesting that age-impaired glucose homeostasis is not caused by the intrinsic aging of beta cells but, rather, is a result of vascular aging that can be reversed by placing aged islets in a young environment.     

Effect of increased plasma non-esterified fatty acids (NEFAs) on arginine-stimulated insulin secretion in obese humans

Aims/hypothesis: We have shown previously that the increase of plasma non-esterified fatty acids for 48 h results in decreased glucose-stimulated insulin secretion in lean and non-diabetic obese subjects. It is currently not known if a prolonged increase in non-esterified fatty acids also impairs the insulin secretory response to non-glucose secretagogues. Methods: Heparin and intralipid (to increase plasma non-esterified fatty acid concentrations by about two- to fourfold) or normal saline was infused intravenously for 48 h in 14 non-diabetic obese subjects. On the third day in both studies, insulin, C-peptide, proinsulin, and insulin secretion rate were assessed in response to an intravenous arginine infusion at fasting glucose concentration and a second arginine infusion after a 60-min 11 mmol/l hyperglycaemic clamp. Results: There were no significant differences detected in acute (5 min) or total (90 min) arginine-stimulated C-peptide or insulin secretion response in the heparin-intralipid study compared with the control group at fasting glucose or during hyperglycaemia. Conclusion/interpretation: We have shown that a prolonged increase in plasma NEFA does not blunt arginine-stimulated insulin secretion or plasma insulin concentrations in non-diabetic obese subjects. These findings suggest that the previously demonstrated NEFA-induced impairment in insulin secretory response to glucose cannot be generalized for non-glucose secretagogues. [Diabetologia (2001) 44: 1989–1997] Received: 27 November 2001 and in revised form: 2 August 2001     

Stimulatory effect of increased non-esterified fatty acid concentrations on proinsulin processing in healthy humans

Aims/hypothesis. To assess the effect of increased concentrations of non-esterified fatty acids (NEFA) on proinsulin processing in healthy humans.¶Methods. We did a hyperglycaemic clamp (130 min duration, 8 mmol/l glucose, with a 5-g arginine bolus at min 120) before and after a 5-h infusion of Intralipid/heparin in 14 healthy subjects. Of the subjects eight underwent a saline control experiment. The proinsulin : insulin (PI:I) ratio immediately after the arginine bolus (122.5 to 125 min) was considered to provide an estimate for the conversion of proinsulin to insulin in the beta cell.¶Results. Concentrations of NEFA were 757 ± 86 μmol/l and 1669 ± 134 μmol/l (p < 0.001) after the 5-h infusion of saline or Intralipid, respectively. Insulin secretion rates were no different between the Intralipid and saline infusions (p = 0.73). There was no statistically significant difference for either the proinsulin concentration or the PI:I ratio during glucose stimulation alone (0 to 120 min). In response to arginine, in contrast, proinsulin remained unchanged during the saline infusion (from 31 ± 6 to 29 ± 7 pmol/l, p = 0.50) but decreased during 5 h of lipid infusion from (21 ± 3 to 15 ± 2 pmol/l, p = 0.02). The PI:I ratio in response to the arginine bolus was higher during the saline infusion (2.0 ± 0.2 % vs 1.7 ± 0.2 %, p = 0.04) but decreased during the Intralipid infusion (from 1.6 ± 0.2 % to 1.2 ± 0.1 %, p = 0.04).¶Conclusion/interpretation. The statistically significantly lower PI:I ratio in response to arginine during experimentally increased concentrations of NEFA suggests that NEFA increase the conversion of proinsulin to insulin in humans in vivo. [Diabetologia (2000) 43: 1368–1373]     

Effects of free fatty acids on insulin secretion in obesity

The prevalence of obesity in Western society has reached epidemic proportions and its aetiological role in the development of type 2 diabetes has made finding an effective treatment for the condition of crucial importance. Of the many consequences of obesity, derangements in glucose metabolism present one of the greatest problems to health. While the role of obesity in causing insulin resistance has received much attention, the effect of obesity on β‐cell failure and the consequent development of type 2 diabetes requires re‐emphasis. In this review, the current understanding of the effects of elevated free‐fatty acids on β‐cell function will be examined, including a discussion of potential mechanisms. In particular, dysregulation of biochemical pathways and alterations in key enzymes, proteins and hormones will be considered as grounds for the progression to a diabetic phenotype.     

Effects of glycogen stores and non-esterified fatty acid availability on insulin-stimulated glucose metabolism and tissue pyruvate dehydrogenase activity in the rat

Summary The effects of increased tissue glycogen stores on insulin sensitivity, and on the response of insulin-stimulated glucose utilisation to an acute elevation in plasma fatty acid levels (1.5mmol/l), were investigated in conscious rats using the hyperinsulinaemic euglycaemic clamp. Studies were performed in two groups of rats; (a) fasted 24 h; (b) fasted 4.5 h, but infused with glucose for 4 h (0.5 g/h) of this period before the clamp (fed, glucose infused rats). Clamp glucose requirement and 3-³H-glucose turnover were 20–25% lower in the fed, glucose-infused rats. In these rats, elevation of plasma fatty acid levels resulted in impaired suppression of hepatic glucose output (residual hepatic glucose output: 41±4 vs 8±6 mol·min^–1·kg^–1. p < 0.001) but did not further decrease 3-³H-glucose turnover. Elevated nonesterified fatty acid levels had no significant effect on glucose kinetics in 24 h fasted rats. In the fed glucose-infused rats, at low plasma fatty acid levels, there was no deposition of glycogen in muscle during the clamp and liver glycogen levels fell. With elevation of non-esterified fatty acid levels muscle glycogen deposition was stimulated in both groups, and there was no fall in liver glycogen during the clamps in the fed glucose-infused rats. Increased non-esterified fatty acid availability during the clamps decreased pyruvate dehydrogenase activity in liver, heart, adipose tissue and quadriceps muscle, in both groups of rats. The findings are consistent with an inhibition of glycolysis in liver, skeletal muscle and heart by increased fatty acid availability. Increased glycogen synthesis, however, compensates for decreased glycolytic flux so that glucose turnover is not decreased. When liver glycogen stores are high, an acute increase in non-esterified fatty acid availability impairs suppression of hepatic glucose output. A chronic increase in non-esteriefid fatty acid availability may lead to insulin resistance by increasing glycogen stores.     

Opposite effects of short- and long-term fatty acid infusion on insulin secretion in healthy subjects

Summary Our study investigates short- and long-term effects of infusion of non-esterified fatty acids (NEFA) on insulin secretion in healthy subjects. Twelve healthy individuals underwent a 24-h Intralipid (10% triglyceride emulsion) infusion at a rate of 0.4 ml/min with a simultaneous infusion of heparin (a bolus of 200 U followed by 0.2 U/min per kg body weight). After an overnight fast (baseline), at 6 and at 24 h of Intralipid infusion and 24 h after Intralipid discontinuation (recovery test), all subjects underwent an intravenous glucose tolerance test (iv-GTT) (25 g of glucose/min). Intralipid infusion caused a threefold rise in plasma NEFA concentrations with no difference between the 6- and the 24-h concentrations. Compared to baseline acute insulin response (AIR) (AIR=63±8 mU/l), short-term (6-h) Intralipid infusion was associated with a significant increase in AIR (86±12 mU/l p<0.01); in contrast, long-term (24-h) Intralipid delivery was associated with inhibition of AIR (31±5 mU/l) compared to baseline (p<0.001) and to the 6-h (p<0.03) triglyceride emulsion infusion. Intralipid infusion was associated with a progressive and significant decline in respiratory quotient (RQ). A positive correlation between changes in fasting plasma NEFA concentrations and AIR at the 6-h infusion (r=0.89 p<0.001) was found. In contrast, at the end of the Intralipid infusion period, changes in plasma NEFA concentrations and AIR were negatively correlated (r=–0.87 p<0.001). The recovery test showed that fasting plasma NEFA concentrations, RQ and AIR had returned to baseline values. In the control study (n=8) 0.9% NaCl infusion did not mimick the effect of Intralipid. In conclusion, our study demonstrates that short- and long-term exposures of beta cells to high plasma NEFA concentrations have opposite effects on glucose-induced insulin secretion.Abbreviations NEFA Non-esterified fatty acids - ivGTT intravenous glucose tolerance test - AIR acute insulin response - NIDDM non-insulin-dependent diabetes mellitus     

Reversibility of the acute toxic effect of Cyclosporin A on pancreatic B cells of Wistar rats

Summary. This study investigated if and to what extent the acute toxic effect of Cyclosporin A on pancreatic Wistar rat B cells is reversible. After 2 weeks of treatment rats developed marked glucose intolerance accompanied by reduced pancreatic insulin content due to a loss of B cells, diminished islet DNA synthesis and decreased B-cell insulin content. Cyclosporin A had accumulated in the pancreas. Three weeks after withdrawal of Cyclosporin A, pancreatic tissue concentrations of Cyclosporin A were still 100 times larger than in serum. Glucose tolerance, however, had already improved, associated with an increase of B-cell insulin content and apparent islet replication, and the insulin response of isolated islets was reduced. Five weeks after the withdrawal of Cyclosporin A, glucose tolerance was normal, but pancreatic insulin content and relative B-cell volume were still diminished in comparison to vehicle-treated controls. Eight weeks after withdrawal, the morphometric parameters had also been normalized. The results suggest that the loss of pancreatic B cells is caused by a toxic destruction, possibly combined with an apparent decrease of replicatory activity. The acute toxic effects of Cyclosporin A in pancreatic B cells are stepwise reversible.     

Saturated non-esterified fatty acids stimulate de novo diacylglycerol synthesis and protein kinase c activity in cultured aortic smooth muscle cells

Aims/hypothesis. Insulin resistance is linked with a cluster of multiple risk factors and excessive acceleration of atherosclerosis. The underlying mechanism is not, however, fully understood. Methods. To determine the link between insulin resistance and altered vascular function, we focused on the effect of various non-esterified fatty acids on diacylglycerol-protein kinase C pathway and mitogen-activated protein kinase activity in cultured aortic smooth muscle cells. Results.Incubation of the cells with saturated non-esterified fatty acids (200 μmol/l) for 24 h, such as palmitate or stearate, induced a significant increase in diacylglycerol concentrations by about fivefold or eightfold, respectively, whereas oleate induced a slight increase in diacylglycerol concentrations by 1.8-fold and arachidonate induced none. In addition, the increased diacylglycerol concentrations induced by palmitate were completely restored to control concentrations by triacsin C, acyl-CoA synthetase inhibitor. These results suggest that saturated non-esterified fatty acids may increase diacylglycerol concentrations through de novo pathway by stepwise acylation. In parallel with the increased diacylglycerol, incubation of the cells with saturated non-esterified fatty acids significantly induced the activation of protein kinase C and mitogen-activated protein kinase. The palmitate-induced increase in mitogen-activated protein kinase activity was restored to control concentrations by GF109203X (5 · 10^–7 mol/l), a specific protein kinase C inhibitor, suggesting a protein kinase C-dependent activation of mitogen-activated protein kinase. Conclusion/interpretation. Saturated non-esterified fatty acids induced an increase in de novo diacylglycerol synthesis and subsequent activation of protein kinase C and mitogen-activated protein kinase in cultured aortic smooth muscle cells. This could contribute to the altered vascular functions in the insulin resistant state. [Diabetologia (2001) 44: 614–620] Received: 29 September 2000 and in revised form: 11 January 2001     

Enhanced escape of non-esterified fatty acids from tissue uptake: its role in impaired insulin-induced lowering of total rate of appearance in obesity and Type II diabetes mellitus

Aims/hypothesis. To estimate non-esterified fatty acids kinetics in patients with Type II (non-insulin-dependent) diabetes mellitus and obese subjects in the postabsorptive state and during hyperinsulinaemia using non-equlibrium tracer conditions.¶Methods. We evaluated the effect of hyperinsulinaemia [euglycaemic clamp with insulin infused at 30 mU · kg^–1· h^–1 (3–4 h) and 150 mU · kg^–1· h^–1 (3 h)] on non-esterified fatty acid kinetics, traced with [¹⁴C]-palmitate using non-equlibrium tracer conditions in non-obese and obese healthy subjects and Type II diabetic patients (10 per group). Michaelis-Menten kinetics were applied for total non-esterified fatty acid disposal, which was assumed to be composed of total arterial plasma non-esterified fatty acid rate of appearance (equalling the rate of disappearance) and tissue uptake of non-esterified fatty acids derived from intravascular triglyceride hydrolysis. A model was developed to calculate the rate of escape of non-esterified fatty acids from tissue uptake and the net rate of tissue lipolysis.¶Results. Total arterial plasma non-esterified fatty acid rate of appearance was lower in non-obese healthy subjects than in the other groups at low insulin infusion (p < 0.05) and in obese Type II diabetic patients at high insulin infusion (p < 0.05). Plasma triglycerides were also lowest in non-obese healthy subjects during hyperinsulinaemia (p < 0.05 from other groups). The rate of escape from tissue uptake decreased during hyperinsulinaemia (p < 0.05 for each group) but remained higher in obese Type II diabetic patients (p < 0.05 from non-obese healthy subjects). In contrast, net rate of tissue lipolysis was not different between the groups at baseline and its decline during hyperinsulinaemia (p < 0.05 for each group) was similar in all groups.¶Conclusion/interpretation. This study challenges the view that the antilipolytic effect of insulin is impaired in Type II diabetes and obesity. We suggest that a high plasma triglyceride concentration causes a higher escape of non-esterified fatty acids from tissue uptake, leading to an impaired suppression of total arterial plasma rate of appearance during a low degree of hyperinsulinaemia in obese subjects and Type II diabetic patients and during a high degree of hyperinsulinaemia in obese Type II diabetic patients. [Diabetologia (2000) 43: 416–426]     

不同种类游离脂肪酸对大鼠胰岛素抵抗的影响

目的研究短期血浆游离脂肪酸(FFA)升高对大鼠肝脏和外周胰岛索作用的影响,比较不同饱和程度脂肪酸对大鼠肝脏和外周胰岛素抵抗的作用。方法分别给大鼠静脉输注脂肪乳(多不饱和脂肪酸) 肝素,油酸(OLE,单不饱和脂肪酸),猪油(LO,饱和脂肪酸) 肝素(LOH组)和生理盐水7 h,并在输注的最后2 h,行清醒状态高胰岛素-正血糖钳夹试验,测定血浆葡萄糖、FFA、胰岛素和C肽水平,观察不同脂肪酸对胰岛素介导的葡萄糖利用(GU)和肝葡萄糖生成(HGP)能力的影响。结果与生理盐水组比较,脂肪乳、OLE和．LOH组血浆FFA水平升高(均P＜0．01)。在钳夹状态下,所有脂肪组的HGP高于生理盐水组(均P＜0．01),而OLE组和LOH组的HGP明显高于脂肪乳组(均P＜0．01),所有脂肪组GU下降(均P＜0．01),但是不同脂肪组间GU差异无统计学意义。结论血浆脂肪酸短期升高能引起大鼠肝脏和外周胰岛素抵抗,多不饱和脂肪酸引起的肝胰岛素抵抗小于单不饱和脂肪酸或饱和脂肪酸,不同饱和程度脂肪酸诱导的周围胰岛素抵抗程度相似。     

Effects of an acute decrease in non-esterified fatty acid levels on muscle glucose utilization and forearm indirect calorimetry in lean NIDDM patients

Summary The aim of the study was to evaluate an acute decrease in NEFA levels during an oral glucose tolerance test and its effects on glucose tolerance, muscle glucose uptake and muscle indirect calorimetry in ten lean non-insulin-dependent diabetic subjects. Two 75-g oral glucose tolerance tests were performed in random order. Placebo or 250 mg acipimox (to inhibit lipolysis) were administered orally 2 h before the start of the oral glucose tolerance test. Two hours after acipimox administration (time 0), non-esterified fatty acid, glycerol and 3-hydroxybutyrate levels decreased by 84, 68 and 77% respectively, compared to basal levels. Concomitantly, muscle lipid oxidation and non-oxidative glycolysis also decreased significantly. After placebo administration, non-esterified fatty acids, glycerol and 3-hydroxybutyrate and lipid oxidation increased by 29, 28, 106 and 33%, respectively (NS vs basal levels; p<0.001 vs acipimox). There was a negative rate of net glucose storage (interpreted as glycogenolysis) during post-absorptive conditions and at time 0 after administration of both drugs. After oral glucose tolerance test, the incremental areas of blood glucose and insulin were significantly decreased by 18 and 19% after acipimox compared to placebo. In addition, the ratio between the incremental area of forearm muscle glucose uptake and the insulin levels was significantly increased by 45% during acipimox compared to placebo administration. Glucose oxidation and non-oxidative glycolysis were significantly higher while lipid oxidation was significantly lower after acipimox than after placebo. In conclusion, our study found that in lean non-insulin-dependent diabetic subjects, an acute decrease in non-esterified fatty acid levels improves glucose tolerance, muscle glucose uptake, glucose oxidation and non-oxidative glycolysis, but is unable to normalize glucose storage.Abbreviations NEFA Non-esterified fatty acids - NIDDM non-insulin-dependent diabetes mellitus - OGTT oral glucose tolerance test - FMGU forearm muscle glucose uptake - FMBF forearm muscle blood flow - M muscle     

Effects of non-esterified fatty acid availability on insulin stimulated glucose utilisation and tissue pyruvate dehydrogenase activity in the rat

Summary Fatty acids in cardiac muscle compete with glucose for oxidation, thereby inhibiting glucose utilisation. It is not clear whether a similar mechanism is important in resting skeletal muscle. We used the hyperinsulinaemic euglycaemic clamp technique in conscious rats fasted for 20 h to examine the effects of increased plasma non-esterified fatty acid levels (1 mmol/l) on glucose metabolism. Insulin was infused at 75 mU/h (plasma insulin, 2.27±0.21 g/l) or 300mU/h (16.41±0.47 g/l). An increase in non-esterified fatty acid levels decreased clamp glucose requirement and 3–³H-glucose turnover by 35% (p<0.001) when the higher insulin dose was used but there was no change at the lower dose. At both insulin infusion rates, clamp blood lactate and pyruvate responses suggested inhibition of muscle glycolysis by elevated plasma non-esterified fatty acid concentrations. Quadriceps muscle glycogen deposition during the clamps was enhanced by increased non-esterified fatty acid availability at the lower insulin dose (p<0.001) but not at the higher insulin concentration. Activation of pyruvate dehyrogenase during the clamps was partially inhibited by increased plasma non-esterified fatty acid in the heart, adipose tissue and quadriceps muscle. This was evident at both insulin levels in heart but only at the higher insulin concentration in muscle (p<0.002). The findings are consistent with an inhibition of glycolysis in skeletal muscle of mixed fibre type as a result of increased fatty acid availability. At low rates of glucose flux glycogen synthesis may compensate for decreased glycolysis so that glucose turnover is not decreased. The role of pyruvate dehydrogenase in the glucose-fatty acid cycle in muscle may depend on the prevailing plasma insulin concentration and the degree of activation of this enzyme.     

Effect of age on glucose oxidation by isolated rat islets

Summary Islets were isolated from pancreases of 2-month and 12-month-old rats, and the oxidation of¹⁴C-glucose to¹⁴CO₂ determined at various medium D-glucose concentration. Islets from 12-month-old rats oxidized significantly less glucose than those from 2-month-old rats at glucose concentrations of 150, 300, and 450 mg/dl, and this was true when islets were selected by hand or by Ficoll density gradient separation. The effect of age on glucose oxidation was seen when islets were incubated with [U-¹⁴C], [1-¹⁴C], or [6-¹⁴C] glucose. The results raise the possibility that previously reported age-related defects in glucose-stimulated insulin secretion may be secondary to the effect of age on islet glucose catabolism.     

Free fatty acids as mediators of adaptive compensatory responses to insulin resistance in dexamethasone-treated rats

BACKGROUND: Chronic low-dose dexamethasone (DEX) treatment in rats is associated to insulin resistance with compensatory hyperinsulinaemia and reduction in food intake. We tested the hypothesis that the elevation in circulating free fatty acids (FFAs) induced by DEX is the common mediator of both insulin resistance and insulin hyperproduction. METHODS: For this purpose, an anti-lipolytic agent was administered during DEX treatment to lower lipacidaemia for several hours prior to glucose and insulin tolerance tests. Leptin expression in adipose tissue (by Northern blot) and plasma leptin levels (by radioimmunoassay) were also investigated to verify whether a rise in circulating leptin could be responsible for the anorectic effect of DEX. RESULTS: Our data show that a transient pharmacological reduction of elevated plasma FFA levels abates the post-loading hyperinsulinaemia and counteracts the insulin resistance induced by DEX, supporting the hypothesis that the chronic elevation in FFAs is the common mediator of DEX-induced changes. Despite enhanced leptin expression in white adipose tissue, DEX-treated rats show no significant increase in plasma leptin levels. This suggests that the anorectic effect of DEX should be mediated, at least partially, by other factors, possibly related to the influence of concomitantly elevated plasma FFA and insulin levels on the hypothalamic centers regulating feeding. CONCLUSIONS: Our results sustain the idea that a prolonged increase in plasma FFA levels plays an important role in the adaptive regulation of glucose and energy homeostasis, not only by potentiating insulin secretion but also by providing a signal of 'nutrient abundance' capable of restraining food intake.