Inhibition of lipolysis causes suppression of endogenous glucose production independent of changes in insulin

We have shown that insulin controls endogenous glucose production (EGP) indirectly, via suppression of adipocyte lipolysis. Free fatty acids (FFA) and EGP are suppressed proportionately, and when the decline in FFA is prevented during insulin infusion, suppression of EGP is also prevented. The present study tested the hypothesis that suppression of lipolysis under conditions of constant insulin would yield a suppression of EGP. N(6)-cyclohexyladenosine (CHA) was used to selectively suppress adipocyte lipolysis during euglycemic clamps in conscious male dogs. FFA suppression by CHA caused suppression of EGP. Liposyn control experiments, which maintained FFA levels above basal during CHA infusion, completely prevented the decline in EGP, whereas glycerol control experiments, which maintained glycerol levels close to basal, did not prevent a decline in EGP. These controls suggest that the EGP suppression was secondary to the suppression of FFA levels specifically. A difference in the sensitivity of FFA and EGP suppression (FFA were suppressed approximately 85% whereas EGP only declined approximately 40%) was possibly caused by confounding effects of CHA, including an increase in catecholamine and glucagons levels during CHA infusion. Thus suppression of lipolysis under constant insulin causes suppression of EGP, despite a significant rise in catecholamines.     

Sulfonylurea therapy fails to diminish insulin resistance in type I-diabetic subjects

To assess whether extrapancreatic effects of sulfonylureas in vivo are detectable in the absence of endogenous insulin secretion, insulin sensitivity was determined in six insulin-deficient type 1-diabetic subjects. Peripheral uptake and hepatic production of glucose and lipolysis were measured during hyperinsulinemia using the euglycemic clamp technique and 3-3H-glucose infusions twice, once during a period with glibornuride treatment (50 mg b.i.d.), and once without. Hepatic glucose production decreased in diabetic subjects during hyperinsulinemia (insulin infusion of 20 mU/m2 X min; plasma free insulin levels of 40 +/- 4 mU/l) from 2.9 +/- 0.6 mg/kg min to 0.2 +/- 0.1 mg/kg X min after 120 min, and plasma free fatty acid (FFA) concentrations decreased from 1.33 +/- 0.29 to 0.38 +/- 0.08 mmol/l. Hepatic production, peripheral uptake of glucose and plasma FFA concentrations before and during hyperinsulinemia were not influenced by pretreatment with glibornuride. Compared to 8 non-diabetic subjects, type 1-diabetics demonstrated a diminished effect of hyperinsulinemia on peripheral glucose clearance (2.4 +/- 0.04 vs 4.2 +/- 0.5 ml/kg X min, P less than 0.01), whereas hepatic glucose production and plasma FFA levels were similarly suppressed by insulin. The data indicate that sulfonylurea treatment did not improve the diminished insulin sensitivity of peripheral glucose clearance in type 1-diabetic subjects; insulin action on hepatic glucose production and lipolysis was unimpaired in diabetics and remained uninfluenced by glibornuride. Thus, extrapancreatic effects of sulfonylureas in vivo are dependent on the presence of functioning beta-cells.     

Continuous 24-h nicotinic acid infusion in rats causes FFA rebound and insulin resistance by altering gene expression and basal lipolysis in adipose tissue

Nicotinic acid (NA) has been used as a lipid drug for five decades. The lipid-lowering effects of NA are attributed to its ability to suppress lipolysis in adipocytes and lower plasma FFA levels. However, plasma FFA levels often rebound during NA treatment, offsetting some of the lipid-lowering effects of NA and/or causing insulin resistance, but the underlying mechanisms are unclear. The present study was designed to determine whether a prolonged, continuous NA infusion in rats produces a FFA rebound and/or insulin resistance. NA infusion rapidly lowered plasma FFA levels (>60%, P < 0.01), and this effect was maintained for ≥5 h. However, when this infusion was extended to 24 h, plasma FFA levels rebounded to the levels of saline-infused control rats. This was not due to a downregulation of NA action, because when the NA infusion was stopped, plasma FFA levels rapidly increased more than twofold (P < 0.01), indicating that basal lipolysis was increased. Microarray analysis revealed many changes in gene expression in adipose tissue, which would contribute to the increase in basal lipolysis. In particular, phosphodiesterase-3B gene expression decreased significantly, which would increase cAMP levels and thus lipolysis. Hyperinsulinemic glucose clamps showed that insulin's action on glucose metabolism was improved during 24-h NA infusion but became impaired with increased plasma FFA levels after cessation of NA infusion. In conclusion, a 24-h continuous NA infusion in rats resulted in an FFA rebound, which appeared to be due to altered gene expression and increased basal lipolysis in adipose tissue. In addition, our data support a previous suggestion that insulin resistance develops as a result of FFA rebound during NA treatment. Thus, the present study provides an animal model and potential molecular mechanisms of FFA rebound and insulin resistance, observed in clinical studies with chronic NA treatment.     

Chylomicron metabolism in rats: kinetic modeling indicates that the particles remain at endothelial sites for minutes

Chylomicrons labeled in vivo with ¹⁴C-oleic acid (primarily in triglycerides, providing a tracer for lipolysis) and ³H-retinol (primarily in ester form, providing a tracer for the core lipids) were injected into rats. Radioactivity in tissues was followed at a series of times up to 40 min and the data were analyzed by compartmental modeling. For heart-like tissues it was necessary to allow the chylomicrons to enter into a compartment where lipolysis is rapid and then transfer to a second compartment where lipolysis is slower. The particles remained in these compartments for minutes and when they returned to blood they had reduced affinity for binding in the tissue. In contrast, the data for liver could readily be fitted with a single compartment for native and lipolyzed chylomicrons in blood, and there was no need for a pathway back to blood. A composite model was built from the individual tissue models. This whole-body model could simultaneously fit all data for both fed and fasted rats and allowed estimation of fluxes and residence times in the four compartments; native and lipolyzed chylomicrons (“remnants”) in blood, and particles in the tissue compartments where lipolysis is rapid and slow, respectively.     

No in vitro effects of fatty acids on glucose uptake, lipolysis or insulin signaling in rat adipocytes.

Elevated plasma levels of free fatty acids (FFA) can produce insulin resistance in skeletal muscle tissue and liver and, together with alterations in beta-cell function, this has been referred to as lipotoxicity. This study explores the effects of FFAs on insulin action in rat adipocytes. Cells were incubated 4 or 24 h with or without an unsaturated FFA, oleate or a saturated FFA, palmitate (0.6 and 1.5 mM, respectively). After the culture period, cells were washed and insulin effects on glucose uptake and lipolysis as well as cellular content of insulin signaling proteins (IRS-1, PI3-kinase, PKB and phosphorylated PKB) and the insulin regulated glucose transporter GLUT4 were measured. No significant differences were found in basal or insulin-stimulated glucose uptake in FFA-treated cells compared to control cells, regardless of fatty acid concentration or incubation period. Moreover, there were no significant alterations in the expression of IRS-1, PI3-kinase, PKB and GLUT4 following FFA exposure. Insulin's ability to stimulate PKB phosphorylation was also left intact. Nor did we find any alterations following FFA exposure in basal or cAMP-stimulated lipolysis or in the ability of insulin to inhibit lipolysis. The results indicate that oleate or palmitate does not directly influence insulin action to stimulate glucose uptake and inhibit lipolysis in rat fat cells. Thus, lipotoxicity does not seem to occur in the fat tissue itself.     

Mathematical models of the kinetics of glucose and insulin in plasma

On the assumption that plasma glucose and IRI curves are essentially non-periodic in character, two simple, linear, lumped-parameter mathematical models have been investigated, one consisting of a single compartment and the other of two compartments between which glucose exchanges. The two-compartment model has the stronger appeal in that it admits more readily of a variable hepatic output of glucose. Estimation of many of the rate constants involved has been effected by conforming the theoretical functions to data from intravenous glucose tolerance tests. Numerical values obtained for the rate constants associated with the two-compartment model are satisfactory, but some confusion still remains concerning the turnover time of insulin in plasma.     

Comparison of the effect of semisynthetic human insulin and porcine insulin on glucose kinetics, plasma free fatty acid and amino acid levels in man

The effect of semisynthetic human insulin on hepatic glucose output, peripheral glucose clearance, plasma levels of C-Peptide, free fatty acids and amino acids was compared with purified pork insulin using the glucose clamp technique. 8 normal overnight-fasted subjects received intravenous infusions of either human or porcine insulin at 20 mU/m2.min(-1) during 120 min achieving plasma insulin levels of approximately equal to 50 mU/l. Plasma glucose levels were maintained at euglycaemia by variable rates of glucose infusion. Hepatic glucose production measured by continuous infusion of 3-(3) H-glucose was similarly suppressed by both insulins to rates near zero. The metabolic clearance rate of glucose increased during infusion of human insulin by 120%, C-peptide levels decreased by 41% and plasma FFA concentrations fell by 74%. The respective changes during infusion of pork insulin were similar, 118%, 48% and 72%. Both insulins decreased the plasma levels of branched-chain amino acids, tyrosine, phenylalanine, methionine, serine and histidine similarly. Thus, the results demonstrate that semisynthetic human and porcine insulin are aequipotent with respect to suppression of hepatic glucose output, stimulation of glucose clearance, inhibition of insulin secretion, lipolysis and proteolysis.     

A1 adenosine receptor partial agonist lowers plasma FFA and improves insulin resistance induced by high-fat diet in rodents

There is substantial evidence in the literature that elevated plasma free fatty acids (FFA) play a role in the pathogenesis of type 2 diabetes. CVT-3619 is a selective partial A(1) adenosine receptor agonist that inhibits lipolysis and lowers circulating FFA. The present study was undertaken to determine the effect of CVT-3619 on insulin resistance induced by high-fat (HF) diet in rodents. HF diet feeding to rats for 2 wk caused a significant increase in insulin, FFA, and triglyceride (TG) concentrations compared with rats fed chow. CVT-3619 (1 mg/kg) caused a time-dependent decrease in fasting insulin, FFA, and TG concentrations. Acute administration of CVT-3619 significantly lowered the insulin response, whereas glucose response was not different with an oral glucose tolerance test. Treatment with CVT-3619 for 2 wk resulted in significant lowering of FFA, TG, and insulin concentrations in rats on HF diet. To determine the effect of CVT-3619 on insulin sensitivity, hyperinsulinemic euglycemic clamp studies were performed in C57BL/J6 mice fed HF diet for 12 wk. Glucose infusion rate was decreased significantly in HF mice compared with chow-fed mice. CVT-3619 treatment 15 min prior to the clamp study significantly (P < 0.01) increased glucose infusion rate to values similar to that for chow-fed mice. In conclusion, CVT-3619 treatment lowers FFA and TG concentrations and improves insulin sensitivity in rodent models of insulin resistance.     

Interaction between altered insulin and lipid metabolism in CEACAM1-inactive transgenic mice

Inactivation of CEACAM1 in L-SACC1 mice by a dominant-negative transgene in liver impairs insulin clearance and increases serum free fatty acid (FFA) levels, resulting in insulin resistance. The contribution of elevated FFAs in the pathogenesis of insulin resistance is herein investigated. Treatment of L-SACC1 female mice with carnitine restored plasma FFA content. Concomitantly, it normalized insulin levels without directly regulating receptor-mediated insulin internalization and prevented glucose tolerance in these mice. Similarly, treatment with nicotinic acid, a lipolysis inhibitor, restored insulin-stimulated receptor uptake in L-SACC1 mice. Taken together, these data suggest that chronic elevation in plasma FFAs levels contributes to the regulation of insulin metabolism and action in L-SACC1 mice.     

Evidence that hyperglycaemia per se does not inhibit hepatic glucose production in man

The effect of hyperglycaemia on hepatic glucose production (Ra) was investigated in nine healthy men using sequential clamp protocols during somatostatin infusion and euglycaemia (0-150 min), at plasma glucose levels of 165 mg x dl-1 (9.2 mM, 150-270 min) and during insulin infusion (1.0 mU x kg-1 x min-1, 270-360 min) in study 1 or during hypo-insulinaemia and plasma glucose levels of 220 mg x dl-1 (12.2 mM; 270-390 min) in study 2. Somatostatin decreased Ra and glucose disposal rate (Rd) but increased plasma free fatty acids (FFA) and lipid oxidation during euglycaemia. Increasing plasma glucose to 165 mg x dl-1 (9.2 mM) and hypo-insulinaemia increased Rd, but no suppressive effects on Ra, plasma FFA and lipid oxidation were observed. By contrast hyperinsulinaemia (study 1), as well as a further increase in plasma glucose (study 2), both decreased Ra. However, more pronounced hyperglycaemia increased insulin secretion despite somatostatin resulting in a fall in plasma FFA and lipid oxidation. Our data questions the accepted dogma that hyperglycaemia inhibits Ra independently of insulin action.     

Adipose tissue sensitization to insulin induced by troglitazone and MEDICA 16 in obese Zucker rats in vivo

The putative role played by insulin sensitizers in modulating adipose tissue lipolysis in the fasting state was evaluated in obese conscious Zucker rats treated with troglitazone or beta,beta'-tetramethylhexadecanedioic acid (MEDICA 16) and compared with nontreated lean and obese animals. The rates of appearance (R(a)) of glycerol and free fatty acid (FFA), primary intra-adipose reesterification, and secondary reuptake of plasma FFA in adipose fat were measured using constant infusion of stable isotope-labeled [(2)H(5)]glycerol, [2,2-(2)H(2)]palmitate, and radioactive [(3)H]palmitate. The overall lipolytic flux (R(a) glycerol) was increased 1.7- and 1.4-fold in obese animals treated with troglitazone or MEDICA 16, respectively, resulting in increased FFA export (R(a) FFA) in the troglitazone-treated rats. Primary intra-adipose reesterification of lipolysis-derived fatty acids was enhanced twofold by insulin sensitizers, whereas reesterification of plasma fatty acids was unaffected by either treatment. Despite the unchanged R(a) FFA in MEDICA 16 or the increased R(a) FFA induced by troglitazone, very low density lipoprotein production rates were robustly curtailed. Total adipose tissue reesterification, used as an estimate of glucose conversion to glyceride-glycerol, was increased 1.9-fold by treatment with the insulin sensitizers. Our results indicate that, in the fasting state, insulin sensitizers induce, in vivo, a significant activation rather than suppression of adipose tissue lipolysis together with stimulation of glucose conversion to glyceride-glycerol.     

Plasma free fatty acid transport during prolonged glucose consumption and its relationship to plasma triglyceride fatty acids in man

Plasma free fatty acid (FFA) transport in human subjects has been studied during the course of prolonged ingestion of different amounts of glucose. Compared with the fasting state, hypocaloric glucose intake resulted in marked suppression of net transport of FFA with no change in (fractional) turnover rate. There was no further suppression of net transport of FFA when the intake was increased to isocaloric or hypercaloric levels, but there was a significant increase in the (fractional) turnover rate, indicating an enhancement of clearance mechanisms. During the 20-24-hr period of fasting after isocaloric glucose consumption, the (fractional) turnover rate quickly fell to that found in the fasting individual, whereas net transport remained suppressed for much longer. This suggested that ingestion of glucose maintains an influence on lipolysis longer than on esterification. During this period of fasting after glucose administration, the contribution of plasma FFA to circulating triglyceride fatty acids increased with time and was positively and significantly correlated with the changes in the net transport of plasma FFA.     

Investigation of some theoretical models relating the concentrations of glucose and insulin in plasma

In this survey 24 possible theoretical models for the regulation of plasma glucose concentration by plasma insulin concentration have been proposed and tested. A set of data, consisting of plasma glucose concentrations at various times after an intravenous injection of glucose, was selected from the literature. An attempt was made to fit each model to the chosen data using a digital computer program and ten of these were successful. The behaviour of these ten models under different experimental conditions was compared, using a simulation technique, with further published data. At this stage six models were consistent with a steady state at the start or the end of an experiment, but only three were reasonably good at predicting all aspects of the chosen experiments. One was particularly good. Two other sets of data were taken from the literature and the six best models were fitted in order to calculate replicate values of the physiological parameters.     

Two novel prevalent polymorphisms in the hormone-sensitive lipase gene have no effect on insulin sensitivity of lipolysis and glucose disposal.

Free fatty acids released during triglyceride lipolysis play an important role in obesity-associated insulin resistance of glucose disposal. Individual sensitivity of lipolysis to the suppressive effect of insulin varies greatly among healthy subjects. It is possible that genetic factors contribute to this variation. Among the many proteins involved in the regulation of lipolysis, hormone-sensitive lipase (HSL) represents a prime candidate for genetic variants contributing to the biological variation of insulin sensitivity of lipolysis. We determined the insulin sensitivity of lipolysis (suppression of isotopically [primed-continuous infusion of d5 glycerol] measured glycerol rate of appearance) and of glucose disposal, using a three-step (n = 20) or standard (n = 53) hyperinsulinemic euglycemic clamp in 73 healthy, unrelated subjects. To assess the possible role of genetic polymorphisms, we directly sequenced the coding region of the HSL gene and the noncoding exon B from these subjects. We identified two silent mutations and three amino acid polymorphisms: Arg262Met (prevalence, 5%), Glu620Asp (prevalence, 31%) and Ser681Ile (prevalence, 22%). The latter two are located in the regulatory domain of HSL but neither had a significant impact on insulin sensitivity of lipolysis or glucose disposal (with and without adjustment for obesity and age as covariates; all P values > 0.20). We conclude that a number of genetic polymorphisms in HSL exist, some of which are highly prevalent. Neither of the polymorphisms we identified in the coding region, however, contributed measurably to the biological variation of insulin sensitivity in our lean, healthy population.     

NO-1886改善糖尿病小型猪的糖代谢

合成化合物NO-1886是一种脂蛋白脂酶活化剂,已被证明其可降低血浆TG并能升高HDLC的浓度．后又发现其还有降低高脂高蔗糖诱发糖尿病兔血浆葡萄糖浓度的作用．对高脂高蔗糖饲料喂养的小型猪脂肪细胞大小、血浆TNF—α和FFA的水平以及NO-1886对其影响进行了研究,结果发现,脂肪细胞明显肥大．血浆TNF-α和FFA以及空腹血糖水平均增高,且引起胰岛素抵抗．添加了l％NO-1886后．脂肪细胞增大被抑制,血浆TNF—α、FFA和空腹血糖的浓度均显著降低,血浆葡萄糖清除率和胰岛素分泌急性相都有了明显改善．以上结果说明,NO-1886可能通过抑制脂肪蓄积、降低血浆TNF-α和FFA的浓度而改善高脂高蔗糖饲料引起的小型猪的糖代谢紊乱．     

Thematic review series: patient-oriented research. Free fatty acid metabolism in human obesity

Adipose tissue lipolysis provides circulating FFAs to meet the body's lipid fuel demands. FFA release is well regulated in normal-weight individuals; however, in upper-body obesity, excess lipolysis is commonly seen. This abnormality is considered a cause for at least some of the metabolic defects (dyslipidemia, insulin resistance) associated with upper-body obesity. "Normal" lipolysis is sex-specific and largely determined by the individual's resting metabolic rate. Women have greater FFA release rates than men without higher FFA concentrations or greater fatty acid oxidation, indicating that they have greater nonoxidative FFA disposal, although the processes and tissues involved in this phenomenon are unknown. Therefore, women have the advantage of having greater FFA availability without exposing their tissues to higher and potentially harmful FFA concentrations. Upper-body fat is more lipolytically active than lower-body fat in both women and men. FFA released by the visceral fat depot contributes only a small percentage of systemic FFA delivery. Upper-body subcutaneous fat is the dominant contributor to circulating FFAs and the source of the excess FFA release in upper-body obesity. We believe that abnormalities in subcutaneous lipolysis could be more important than those in visceral lipolysis as a cause of peripheral insulin resistance. Understanding the regulation of FFA availability will help to discover new approaches to treat FFA-induced abnormalities in obesity.     

Modeling glucose and free fatty acid kinetics in glucose and meal tolerance test

Background

Quantitative evaluation of insulin regulation on plasma glucose and free fatty acid (FFA) in response to external glucose challenge is clinically important to assess the development of insulin resistance (World J Diabetes 1:36–47, 2010). Mathematical minimal models (MMs) based on insulin modified frequently-sampled intravenous glucose tolerance tests (IM-FSIGT) are widely applied to ascertain an insulin sensitivity index (IEEE Rev Biomed Eng 2:54–96, 2009). Furthermore, it is important to investigate insulin regulation on glucose and FFA in postprandial state as a normal physiological condition. A simple way to calculate the appearance rate (Ra) of glucose and FFA would be especially helpful to evaluate glucose and FFA kinetics for clinical applications.

Methods

A new MM is developed to simulate the insulin modulation of plasma glucose and FFA, combining IM-FSIGT with a mixed meal tolerance test (MT). A novel simple functional form for the appearance rate (Ra) of glucose or FFA in the MT is developed. Model results are compared with two other models for data obtained from 28 non-diabetic women (13 African American, 15 white).

Results

The new functional form for Ra of glucose is an acceptable empirical approximation to the experimental Ra for a subset of individuals. When both glucose and FFA are included in FSIGT and MT, the new model is preferred using the Bayes Information Criterion (BIC).

Conclusions

Model simulations show that the new MM allows consistent application to both IM-FSIGT and MT data, balancing model complexity and data fitting. While the appearance of glucose in the circulation has an important effect on FFA kinetics in MT, the rate of appearance of FFA can be neglected for the time-period modeled.

     

Effect of Acipimox on Plasma Lipids and Glucose/Insulin in Pregnant Rats

To determine how a reduction in maternal hypertriglyceridemia during late pregnancy may affect glucose/insulin relationships, pregnant and virgin rats were orally treated with acipimox, a potent antilipolytic agent. In 20-day pregnant rats receiving 80 mg of acipimox, plasma triglycerides (TG), free fatty acids (FFA), and glycerol decreased more than in virgin rats shortly after the drug (up to 7 hours), when compared with animals treated with distilled water, whereas plasma glucose level was unaffected by the treatment in either group of rats. When acipimox was given every 12 hours from day 17 to day 20 of pregnancy, plasma TG, FFA, and glycerol levels progressively increased, whereas they either decreased or did not change in virgin rats receiving the same treatment, with no effect in plasma glucose levels in either group. Fetal body weight was lower than in controls in 20-day pregnant rats that received acipimox for 3 days. On day 20 of pregnancy, 3 hours after receiving acipimox or distilled water, rats received a 2 g glucose/kg oral load and it was found that the change in plasma glucose was similar in both groups, whereas the increase in plasma insulin was greater in pregnant rats treated with acipimox. However, no difference was found in either variable after the oral glucose load in virgin rats receiving acipimox or distilled water. No differences in plasma glucose levels were found after intravenous (IV) administration of insulin in pregnant rats treated or not treated with acipimox. In conclusion, present results show that administration of acipimox during the last days of gestation inhibited lipolysis and decreased fetal weight. Over a short period of time, in pregnant rats, reductions of plasma FFA and TG after acipimox treatment improved the glucose-induced insulin release, but did not seem to have any effect in peripheral insulin resistance.     

Dual action of beta-endorphin on insulin release in genetically obese and lean mice

Intraperitoneal administration of beta-endorphin (1 mg/kg) to ob/ob mice doubled fasting plasma insulin concentrations within 30 min, while plasma glucose concentrations were unaltered. In lean mice, beta-endorphin failed to alter plasma insulin or glucose responses. In glucose-loaded ob/ob mice, beta-endorphin (1 mg/kg) reduced insulin levels at 40 min, and delayed glucose disposal. A lower dose of beta-endorphin (0.1 mg/kg) decreased plasma insulin at 90 min, with no effect on plasma glucose disposal. In lean mice, only the higher dose of beta-endorphin suppressed the glucose-stimulated rise in plasma insulin concentrations, without affecting plasma glucose. Beta-endorphin's actions were blocked by naltrexone and could not be mimicked by N-acetyl-beta-endorphin. Beta-endorphin (10(-8)M) enhanced insulin release from isolated ob/ob and lean mouse islets incubated in medium containing 6 mM glucose, but inhibited release when 20 mM glucose was present. These effects were naloxone reversible. The results indicate that 1) ob/ob mice display a greater magnitude of response in vivo to beta-endorphin's actions on insulin release compared with lean mice, 2) high concentrations of beta-endorphin exacerbate glucose disposal in ob/ob mice. 3) the prevailing glucose concentration is an important determinant of whether beta-endorphin's effects on insulin release will be stimulatory or inhibitory and 4) these actions are mediated via opiate receptors.