Free fatty acids increase basal hepatic glucose production and induce hepatic insulin resistance at different sites

To investigate the sites of the free fatty acid (FFA) effects to increase basal hepatic glucose production and to impair hepatic insulin action, we performed 2-h and 7-h Intralipid + heparin (IH) and saline infusions in the basal fasting state and during hyperinsulinemic clamps in overnight-fasted rats. We measured endogenous glucose production (EGP), total glucose output (TGO, the flux through glucose-6-phosphatase), glucose cycling (GC, index of flux through glucokinase = TGO - EGP), hepatic glucose 6-phosphate (G-6-P) content, and hepatic glucose-6-phosphatase and glucokinase activities. Plasma FFA levels were elevated about threefold by IH. In the basal state, IH increased TGO, in vivo glucose-6-phosphatase activity (TGO/G-6-P), and EGP (P < 0.001). During the clamp compared with the basal experiments, 2-h insulin infusion increased GC and in vivo glucokinase activity (GC/TGO; P < 0.05) and suppressed EGP (P < 0.05) but failed to significantly affect TGO and in vivo glucose-6-phosphatase activity. IH decreased the ability of insulin to increase GC and in vivo glucokinase activity (P < 0.01), and at 7 h, it also decreased the ability of insulin to suppress EGP (P < 0.001). G-6-P content was comparable in all groups. In vivo glucose-6-phosphatase and glucokinase activities did not correspond to their in vitro activities as determined in liver tissue, suggesting that stable changes in enzyme activity were not responsible for the FFA effects. The data suggest that, in overnight-fasted rats, FFA increased basal EGP and induced hepatic insulin resistance at different sites. 1) FFA increased basal EGP through an increase in TGO and in vivo glucose-6-phosphatase activity, presumably due to a stimulatory allosteric effect of fatty acyl-CoA on glucose-6-phosphatase. 2) FFA induced hepatic insulin resistance (decreased the ability of insulin to suppress EGP) through an impairment of insulin's ability to increase GC and in vivo glucokinase activity, presumably due to an inhibitory allosteric effect of fatty acyl-CoA on glucokinase and/or an impairment in glucokinase translocation.     

Oleate-induced decrease in hepatocyte insulin binding is mediated by PKC-delta

We have previously shown that free fatty acids (FFA) impair hepatic insulin extraction in vivo and thus generate hyperinsulinemia, a suspected risk factor for atherosclerosis and cancer. Hepatic insulin extraction is a receptor-mediated event, which is initiated by hepatocyte insulin binding. In the present study, we investigated the effect of FFA on insulin binding in freshly isolated rat hepatocytes maintained at 10 mM glucose. Hepatocyte insulin binding decreased after 1 h exposure to oleate in a concentration-dependent manner reaching a maximum (35-40%) at 125 microM. Inhibition of FFA oxidation by >90% with the carnitine palmitoyltransferase I (CPT-I) inhibitor methylpalmoxirate (MP, 30 microM) did not prevent the effect of oleate. However, when hepatocytes were treated with the PKC inhibitor bisindolylmaleimide (BIM, 1 microM) the effect of oleate was abolished. Subcellular fractionation and immunoblotting of specific PKC isoforms revealed that oleate-induced hepatic PKC-delta membrane translocation, but did not translocate-epsilon, -theta, -alpha, -betaI and -betaII. These results indicate that PKC-delta activation mediated the FFA-induced decrease in hepatocyte insulin binding under our conditions, and thus provides a mechanistic basis for FFA-induced hyperinsulinemia.     

Co-administration of etomoxir and RU-486 mitigates insulin resistance in hepatic and muscular tissues of STZ-induced diabetic rats.

Insulin resistance is a condition of central importance in a cluster of clinical disorders including diabetes mellitus, hypertension, dyslipidemia, central obesity and coronary heart disease. Despite its association with numerous health problems, the mechanism responsible for the development of this phenomenon remains to be established. A novel theory has proposed that insulin resistance in diabetes stems, at least in part, from enhanced free fatty acid (FFA) oxidation and/or excessive production of glucocorticoids (GCs). Several key predictions of this premise were subjected to experimental testing using streptozotocin (STZ)-treated rats as a model for insulin-dependent diabetes mellitus and euglycemic-hyperinsulinemic clamp technique for the in vivo measurement of insulin actions. Euglycemic clamp studies with an insulin infusion index of 5 mU/kg/min were used to measure endogenous glucose production (EGP), glucose infusion rate (GIR), glucose disposal rate (GDR) and skeletal muscle glucose utilization index (GUI). Post-absorptive basal EGP and plasma levels of glucose and free fatty acids (FFA) were elevated in the STZ diabetic rats compared to their corresponding control values. In contrast, hypoinsulinemia was evident in these animals. Steady-state GIR and GDR during euglycemic-hyperinsulinemic clamp were markedly decreased in the STZ diabetic rats. Similarly, insulin-mediated suppression of EGP and plasma FFA concentration was also impaired in these animals. GUI, a measure of 2-deoxyglucose (2-DG) uptake, was increased in response to insulin in the order of white gastrocnemus (WG), red gastrocnemus (RG), extensor digitorum longus and soleus muscles. This parallels the percentage of red fibers in these muscles. Diabetes interferes with insulin's ability to increase 2-DG uptake in all of the above muscles with the exception of WG. Nullification of the associated hyperlipidemic and hypercortisolemic states of diabetes with etomoxir (hyperlipidemic) and the glucocorticoid receptor blocker RU-486 (hypercortisolemic) ameliorated the diabetes-related impairment of the in vivo insulin action. Overall these results together with those garnered from the literature support the notion that hypercortisolemia and the enhancement of FFA oxidation are involved, at least in part, in the development of hepatic and skeletal muscle insulin resistance in poorly controlled type I diabetes.     

Mechanisms of the free fatty acid-induced increase in hepatic glucose production

The associations between obesity, insulin resistance, and type 2 diabetes mellitus are well documented. Free fatty acids (FFA), which are often elevated in obesity, have been implicated as an important link in these associations. Contrary to muscle glucose metabolism, the effects of FFA on hepatic glucose metabolism and the associated mechanisms have not been extensively investigated. It is still controversial whether FFA have substantial effects on hepatic glucose production, and the mechanisms responsible for these putative effects remain unknown. We review recent progress in this area and try to clarify controversial issues regarding the mechanisms responsible for the FFA-induced increase in hepatic glucose production in the postabsorptive state and during hyperinsulinemia.     

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.     

Effect of IGF-I on FFA and glucose metabolism in control and type 2 diabetic subjects

The effects of insulin-like growth factor I (IGF-I) and insulin on free fatty acid (FFA) and glucose metabolism were compared in eight control and eight type 2 diabetic subjects, who received a two-step euglycemic hyperinsulinemic (0.25 and 0.5 mU x kg(-1) x min(-1)) clamp and a two-step euglycemic IGF-I (26 and 52 pmol x kg(-1) x min(-1)) clamp with [3-(3)H]glucose, [1-(14)C]palmitate, and indirect calorimetry. The insulin and IGF-I infusion rates were chosen to augment glucose disposal (R(d)) to a similar extent in control subjects. In type 2 diabetic subjects, stimulation of R(d) (second clamp step) in response to both insulin and IGF-I was reduced by approximately 40-50% compared with control subjects. In control subjects, insulin was more effective than IGF-I in suppressing endogenous glucose production (EGP) during both clamp steps. In type 2 diabetic subjects, insulin-mediated suppression of EGP was impaired, whereas EGP suppression by IGF-I was similar to that of controls. In both control and diabetic subjects, IGF-I-mediated suppression of plasma FFA concentration and inhibition of FFA turnover were markedly impaired compared with insulin (P < 0.01-0.001). During the second IGF-I clamp step, suppression of plasma FFA concentration and FFA turnover was impaired in diabetic vs. control subjects (P < 0.05-0.01). CONCLUSIONS: 1) IGF-I is less effective than insulin in suppressing EGP and FFA turnover; 2) insulin-resistant type 2 diabetic subjects also exhibit IGF-I resistance in skeletal muscle. However, suppression of EGP by IGF-I is not impaired in diabetic individuals, indicating normal hepatic sensitivity to IGF-I.     

水杨酸钠对胰岛素抵抗大鼠胰岛素受体底物-1的影响

目的探讨抗炎药水杨酸钠对胰岛素抵抗大鼠胰岛素敏感性的影响及其作用机制。方法分别给大鼠静脉输注脂肪乳+肝素,脂肪乳+肝素+水杨酸钠和生理盐水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、游离脂肪酸(FFA)、胰岛素和C-肽水平,检测肝脏、肌肉中胰岛素受体底物-1(IRS-1)及307位丝氨酸磷酸化的IRS-1表达。结果输注脂肪乳大鼠葡萄糖输注率(GIR)是输注生理盐水大鼠的45%,水杨酸钠可使GIR提高1.3倍(P0.01)。脂肪乳输注组大鼠肝脏及肌肉中307位丝氨酸磷酸化的IRS-1分别为生理盐水输注组大鼠的3倍和3.8倍(P0.001),输注水杨酸钠,肝脏、肌肉307位丝氨酸磷酸化的IRS-1下降45%、20%(P0.05)。结论 FFA增高引起肝脏及肌肉中307位丝氨酸磷酸化的IRS-1水平增高,可能是导致胰岛素抵抗发生的机制之一,应用水杨酸钠,大鼠肝脏及肌肉组织中IRS-1丝氨酸磷酸化水平下降,胰岛素抵抗改善。抗炎药物水杨酸钠可能通过抑制FFA引起的IRS-1丝氨酸磷酸化,而发挥改善胰岛素抵抗的作用。     

JNK and tumor necrosis factor-alpha mediate free fatty acid-induced insulin resistance in 3T3-L1 adipocytes

Lipid infusion and high fat feeding are established causes of systemic and adipose tissue insulin resistance. In this study, we treated 3T3-L1 adipocytes with a mixture of free fatty acids (FFAs) to investigate the molecular mechanisms underlying fat-induced insulin resistance. FFA treatment impaired insulin receptor-mediated signal transduction and decreased insulin-stimulated GLUT4 translocation and glucose transport. FFAs activated the stress/inflammatory kinases c-Jun N-terminal kinase (JNK) and IKKbeta, and the suppressor of cytokine signaling protein 3, increased secretion of the inflammatory cytokine tumor necrosis factor (TNF)-alpha, and decreased secretion of adiponectin into the medium. RNA interference-mediated down-regulation of JNK blocked JNK activation and prevented most of the FFA-induced defects in insulin action. Blockade of TNF-alpha signaling with neutralizing antibodies to TNF-alpha or its receptors or with a dominant negative TNF-alpha peptide had a partial effect to inhibit FFA-induced cellular insulin resistance. We found that JNK activation by FFAs was not inhibited by blocking TNF-alpha signaling, whereas the FFA-induced increase in TNF-alpha secretion was inhibited by RNA interference-mediated JNK knockdown. Together, these results indicate that 1) JNK can be activated by FFAs through TNF-alpha-independent mechanisms, 2) activated JNK is a major contributor to FFA-induced cellular insulin resistance, and 3) TNF-alpha is an autocrine/paracrine downstream effector of activated JNK that can also mediate insulin resistance.     

Oxidative stress--mediated alterations in glucose dynamics in a genetic animal model of type II diabetes

Insulin resistance, characterized by an inexorable decline in skeletal muscle glucose utilization and/or an excessive hepatic glucose production, constitutes a major pathogenic importance in a cluster of clinical disorders including diabetes mellitus, hypertension, dyslipidemia, central obesity and coronary artery disease. A novel concept suggests that heightened state of oxidative stress during diabetes contributes, at least in part, to the development of insulin resistance. Several key predictions of this premise were subjected to experimental testing using Goto-Kakizaki (GK) rats as a genetic animal model for non-obese type II diabetes. Euglycemic-hyperinsulinemic clamp studies with an insulin infusion index of 5 mU/kg bw/min were used to measure endogenous glucose production (EGP), glucose infusion rate (GIR), glucose disposal rate (GDR) and skeletal muscle glucose utilization index (GUI). Moreover, the status of oxidative stress as reflected by the urinary levels of isoprostane and protein carbonyl formation were also assessed as a function of diabetes. Post-absorptive basal EGP and circulating levels of insulin, glucose and free fatty acid (FFA) were elevated in GK rats, compared to their corresponding control values. In contrast, steady state GIR and GDR of the hyperglycemic/hyperinsulinemic animals were reduced, concomitantly with impaired insulin's ability to suppress EGP. Insulin stimulated [3H]-2-deoxyglucose (2-DG) uptake (a measure of glucose transport activity) by various types of skeletal muscle fibers both in vivo and in vitro (isolated muscle, cultured myoblasts) was diminished in diabetic GK rats. This diabetes-related suppression of skeletal muscle glucose utilization was associated with a decrease in insulin's ability to promote the phosphorylation of tyrosine residues of insulin receptor substrate-1 (IRS-1). Similarly, the translocation of GLUT-4 from intracellular compartment to plasma membrane in response to insulin was also reduced in these animals. Oxidative stress-based markers (e.g. urinary isoprostane, carbonyl-bound proteins) were elevated as a function of diabetes. Nullification of the heightened state of oxidative stress in the GK rats with alpha-lipoic acid resulted in a partial amelioration of the diabetes-related impairment of the in vivo and in vitro insulin actions. Collectively, the above data suggest that 1) insulin resistance in GK rats occurs at the hepatic and skeletal muscle levels, 2) muscle cell glucose transport exhibited a blunted response to insulin and it is associated with a major defect in key molecules of both GLUT-4 trafficking and insulin signaling pathways, 3) skeletal muscle insulin resistance in GK rats appears to be of genetic origin and not merely related to a paracrine or autocrine effect, since this phenomenon is also observed in cultured myoblasts over several passages and finally heightened state of oxidative stress may mediate the development of insulin resistance during diabetes.     

Epinephrine effects on insulin-glucose dynamics: the labeled IVGTT two-compartment minimal model approach

The hyperglycemic effects of epinephrine (Epi) are established; however, the modulation of Epi-stimulated endogenous glucose production (EGP) by glucose and insulin in vivo in humans is less clear. Our aim was to determine the effect of exogenously increased plasma Epi concentrations on insulin and glucose dynamics. In six normal control subjects, we used the labeled intravenous glucose tolerance test (IVGTT) interpreted with the two-compartment minimal model, which provides not only glucose effectiveness (S(G)(2*)), insulin sensitivity (S(I)(2*)), and plasma clearance rate (PCR) at basal state, but also the time course of EGP. Subjects were randomly studied during either saline or Epi infusion (1.5 microg/min). Exogenous Epi infusion increased plasma Epi concentration to a mean value of 2,034 +/- 138 pmol/l. During the stable-label IVGTT, plasma glucose, tracer glucose, and insulin concentrations were significantly higher in the Epi study. The hormone caused a significant (P < 0.05) reduction in PCR in the Epi state when compared with the basal state. The administration of Epi has a striking effect on EGP profiles: the nadir of the EGP profiles occurs at 21 +/- 7 min in the basal state and at 55 +/- 13 min in the Epi state (P < 0.05). In conclusion, we have shown by use of a two-compartment minimal model of glucose kinetics that elevated plasma Epi concentrations have profound effects at both hepatic and tissue levels. In particular, at the liver site, this hormone deeply affects, in a time-dependent fashion, the inhibitory effect of insulin on glucose release. Our findings may explain how even a normal subject may have the propensity to develop glucose intolerance under the influence of small increments of Epi during physiological stress.     

Time-dependent effects of fatty acids on skeletal muscle metabolism

Increased plasma levels of free fatty acids (FFA) occur in states of insulin resistance such as type 2 diabetes mellitus, obesity, and metabolic syndrome. These high levels of plasma FFA seem to play an important role for the development of insulin resistance but the mechanisms involved are not known. We demonstrated that acute exposure to FFA (1 h) in rat incubated skeletal muscle leads to an increase in the insulin-stimulated glycogen synthesis and glucose oxidation. In conditions of prolonged exposure to FFA, however, the insulin-stimulated glucose uptake and metabolism is impaired in skeletal muscle. In this review, we discuss the differences between the effects of acute and prolonged exposure to FFA on skeletal muscle glucose metabolism and the possible mechanisms involved in the FFA-induced insulin resistance.     

Nocturnal free fatty acids are uniquely elevated in the longitudinal development of diet-induced insulin resistance and hyperinsulinemia

Obesity is strongly associated with hyperinsulinemia and insulin resistance, both primary risk factors for type 2 diabetes. It has been thought that increased fasting free fatty acids (FFA) may be responsible for the development of insulin resistance during obesity, causing an increase in plasma glucose levels, which would then signal for compensatory hyperinsulinemia. But when obesity is induced by fat feeding in the dog model, there is development of insulin resistance and a marked increase in fasting insulin despite constant fasting FFA and glucose. We examined the 24-h plasma profiles of FFA, glucose, and other hormones to observe any potential longitudinal postprandial or nocturnal alterations that could lead to both insulin resistance and compensatory hyperinsulinemia induced by a high-fat diet in eight normal dogs. We found that after 6 wk of a high-fat, hypercaloric diet, there was development of significant insulin resistance and hyperinsulinemia as well as accumulation of both subcutaneous and visceral fat without a change in either fasting glucose or postprandial glucose. Moreover, although there was no change in fasting FFA, there was a highly significant increase in the nocturnal levels of FFA that occurred as a result of fat feeding. Thus enhanced nocturnal FFA, but not glucose, may be responsible for development of insulin resistance and fasting hyperinsulinemia in the fat-fed dog model.     

不同时间脂肪乳输注对大鼠葡萄糖输注率的影响

目的观察不同时间脂肪乳输注使血浆游离脂肪酸（FFA）升高对大鼠葡萄糖输注率（GIR）的影响。方法分别给大鼠输注脂肪乳2、5、7和48 h,行高胰岛素-正血糖钳夹试验评价葡萄糖输注率,测定血浆葡萄糖、FFA。结果与生理盐水输注组比较,2 h脂肪乳输注使血浆FFA升高了2倍（P〈0.01）,GIR下降27%（P〈0.05）,脂肪乳输注5 h GIR降低了52%（P〈0.01）,7 h GIR降低56%（P〈0.01）,输注48 h脂肪乳GIR降低58%（P〈0.01）,5 h组7、h组及48 h脂肪乳输注组间GIR差异没有统计学意义。结论大鼠输注脂肪乳使FFA浓度达到基础值的3倍左右,可复制出脂毒性胰岛素抵抗的模型,而且胰岛素抵抗达到一定程度后保持恒定。     

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.     

FFA cause hepatic insulin resistance by inhibiting insulin suppression of glycogenolysis

Free fatty acids (FFA) have been shown to inhibit insulin suppression of endogenous glucose production (EGP). To determine whether this is the result of stimulation by FFA of gluconeogenesis (GNG) or glycogenolysis (GL) or a combination of both, we have determined rates of GNG and GL (with (2)H(2)O) and EGP in 16 healthy nondiabetic volunteers (11 males, 5 females) during euglycemic-hyperinsulinemic (~450 pM) clamping performed either with or without simultaneous intravenous infusion of lipid plus heparin. During insulin infusion, FFA decreased from 571 to 30 micromol/l (P < 0.001), EGP from 15.7 to 2.0 micromol x kg(-1) x min(-1) (P < 0.01), GNG from 8.2 to 3.7 micromol x kg(-1). min(-1) (P < 0.05), and GL from 7.4 to -1.7 micromol x kg(-1). min(-1) (P < 0.02). During insulin plus lipid/heparin infusion, FFA increased from 499 to 1,247 micromol/l (P < 0.001). EGP decreased 64% less than during insulin alone (-5.1 +/- 0.7 vs. -13.7 +/- 3.4 micromol x kg(-1). min(-1)). The decrease in GNG was not significantly different from the decrease of GNG during insulin alone (-2.6 vs. -4.5 micromol x kg(-1). min(-1), not significant). In contrast, GL decreased 66% less than during insulin alone (-3.1 vs. -9.2 micromol x kg(-1). min(-1), P < 0.05). We conclude that insulin suppressed EGP by inhibiting GL more than GNG and that elevated plasma FFA levels attenuated the suppression of EGP by interfering with insulin suppression of GL.     

Growth hormone-induced insulin resistance is associated with increased intramyocellular triglyceride content but unaltered VLDL-triglyceride kinetics

The ability of growth hormone (GH) to stimulate lipolysis and cause insulin resistance in skeletal muscle may be causally linked, but the mechanisms remain obscure. We investigated the impact of GH on the turnover of FFA and VLDL-TG, intramuscular triglyceride content (IMTG), and insulin sensitivity (euglycemic clamp) in nine healthy men in a randomized double-blind placebo-controlled crossover study after 8 days treatment with (A) Placebo+Placebo, (B) GH (2 mg daily)+Placebo, and (C) GH (2 mg daily)+Acipimox (250 mgx3 daily). In the basal state, GH (B) increased FFA levels (P<0.05), palmitate turnover (P<0.05), and lipid oxidation (P=0.05), but VLDL-TG kinetics were unaffected. Administration of acipimox (C) suppressed basal lipolysis but did not influence VLDL-TG kinetics. In the basal state, IMTG content increased after GH (B; P=0.03). Insulin resistance was induced by GH irrespective of concomitant acipimox (P<0.001). The turnover of FFA and VLDL-TG was suppressed by hyperinsulinemia during placebo and GH, whereas coadministration of acipimox induced a rebound increase FFA turnover and VLDL-TG clearance. We conclude that these results show that GH-induced insulin resistance is associated with increased IMTG and unaltered VLDL-TG kinetics; we hypothesize that fat oxidation in muscle tissue is an important primary effect of GH and that circulating FFA rather than VLDL-TG constitute the major source for this process; and the role of IMTG in the development of GH-induced insulin resistance merits future research.     

Free fatty acid-induced peripheral insulin resistance augments splanchnic glucose uptake in healthy humans

To investigate the effect of elevated plasma free fatty acid (FFA) concentrations on splanchnic glucose uptake (SGU), we measured SGU in nine healthy subjects (age, 44 +/- 4 yr; body mass index, 27.4 +/- 1.2 kg/m(2); fasting plasma glucose, 5.2 +/- 0.1 mmol/l) during an Intralipid-heparin (LIP) infusion and during a saline (Sal) infusion. SGU was estimated by the oral glucose load (OGL)-insulin clamp method: subjects received a 7-h euglycemic insulin (100 mU x m(-2) x min(-1)) clamp, and a 75-g OGL was ingested 3 h after the insulin clamp was started. After glucose ingestion, the steady-state glucose infusion rate (GIR) during the insulin clamp was decreased to maintain euglycemia. SGU was calculated by subtracting the integrated decrease in GIR during the period after glucose ingestion from the ingested glucose load. [3-(3)H]glucose was infused during the initial 3 h of the insulin clamp to determine rates of endogenous glucose production (EGP) and glucose disappearance (R(d)). During the 3-h euglycemic insulin clamp before glucose ingestion, R(d) was decreased (8.8 +/- 0.5 vs. 7.6 +/- 0.5 mg x kg(-1) x min(-1), P < 0.01), and suppression of EGP was impaired (0.2 +/- 0.04 vs. 0.07 +/- 0.03 mg x kg(-1) x min(-1), P < 0.01). During the 4-h period after glucose ingestion, SGU was significantly increased during the LIP vs. Sal infusion study (30 +/- 2 vs. 20 +/- 2%, P < 0.005). In conclusion, an elevation in plasma FFA concentration impairs whole body glucose R(d) and insulin-mediated suppression of EGP in healthy subjects but augments SGU.     

Differential effect of saturated and polyunsaturated fatty acids on hepatic glucose metabolism in humans

Prolonged infusions of lipid and heparin that achieve high physiological free fatty acid (FFA) concentrations inhibit hepatic (and peripheral) insulin sensitivity in humans. These infusions are composed largely of polyunsaturated fatty acids (PUFA; linoleic and linolenic). It is not known whether fatty acid composition per se affects hepatic glucose metabolism in humans. To address this issue, we examined the impact of enteral infusions of either palm oil (48% palmitic, 35% oleic, and 8% linoleic acids) or safflower oil (6% palmitic, 12% oleic, 74% linoleic acids) in 14 obese nondiabetic subjects. (2)H(2)O was administered to determine the contribution of gluconeogenesis to endogenous glucose production (EGP), and a primed continuous infusion of [6,6-(2)H]glucose was administered to assess glucose appearance. As a result of the lipid infusions, plasma FFA concentrations increased significantly in both the palm oil (507.5 +/- 47.4 to 939.3 +/- 61.3 micromol/l, P < 0.01) and safflower oil (588.2.0 +/- 43.0 to 857.8 +/- 68.7 micromol/l, P < 0.01) groups after 4 h. EGP was similar at baseline (12.4 +/- 1.8 vs. 11.2 +/- 1.0 micromol x kg FFM(-1) x min(-1)). During a somatostatin-insulin clamp, the glucose infusion rate was significantly lower (AUC glucose infusion rate 195.8 +/- 50.7 vs. 377.8 +/- 38.0 micromol/kg FFM, P < 0.01), and rates of EGP were significantly higher (10.7 +/- 1.4 vs. 6.5 +/- 1.5 micromol x kg FFM(-1) x min(-1), P < 0.01) after palm oil compared with safflower oil, respectively. Baseline rates of gluconeogenesis and glycogenolysis were also similar. However, after lipid infusion, rates of glycogenolysis were suppressed by safflower oil but not by palm oil. Thus these studies demonstrate, for the first time in humans, a differential effect of saturated fatty acids and PUFA on hepatic glucose metabolism.     

Effects of thyrotoxicosis and selective hepatic autonomic denervation on hepatic glucose metabolism in rats

Thyrotoxicosis is known to induce a broad range of changes in carbohydrate metabolism. Recent studies have identified the sympathetic and parasympathetic nervous system as major regulators of hepatic glucose metabolism. The present study aimed to investigate the pathogenesis of altered endogenous glucose production (EGP) in rats with mild thyrotoxicosis. Rats were treated with methimazole in drinking water and l-thyroxine (T(4)) from osmotic minipumps to either reinstate euthyroidism or induce thyrotoxicosis. Euthyroid and thyrotoxic rats underwent either a sham operation, a selective hepatic sympathetic denervation (Sx), or a parasympathetic denervation (Px). After 10 days of T(4) administration, all animals were submitted to a hyperinsulinemic euglycemic clamp combined with stable isotope dilution to measure EGP. Plasma triiodothyronine (T(3)) showed a fourfold increase in thyrotoxic compared with euthyroid animals. EGP was increased by 45% in thyrotoxic compared with euthyroid rats and correlated significantly with plasma T(3). In thyrotoxic rats, hepatic PEPCK mRNA expression was increased 3.5-fold. Relative suppression of EGP during hyperinsulinemia was 34% less in thyrotoxic than in euthyroid rats, indicating hepatic insulin resistance. During thyrotoxicosis, Sx attenuated the increase in EGP, whereas Px resulted in increased plasma insulin with unaltered EGP compared with intact animals, compatible with a further decrease in hepatic insulin sensitivity. We conclude that chronic, mild thyrotoxicosis in rats increases EGP, whereas it decreases hepatic insulin sensitivity. Sympathetic hepatic innervation contributes only to a limited extent to increased EGP during thyrotoxicosis, whereas parasympathetic hepatic innervation may function to restrain EGP in this condition.     

Role of PKC isoforms in glucose transport in 3T3-L1 adipocytes: insignificance of atypical PKC

To elucidate the involvement of protein kinase C (PKC) isoforms in insulin-induced and phorbol ester-induced glucose transport, we expressed several PKC isoforms, conventional PKC-alpha, novel PKC-delta, and atypical PKC isoforms of PKC-lambda and PKC-zeta, and their mutants in 3T3-L1 adipocytes using an adenovirus-mediated gene transduction system. Endogenous expression and the activities of PKC-alpha and PKC-lambda/zeta, but not of PKC-delta, were detected in 3T3-L1 adipocytes. Overexpression of each wild-type PKC isoform induced a large amount of PKC activity in 3T3-L1 adipocytes. Phorbol 12-myristrate 13-acetate (PMA) activated PKC-alpha and exogenous PKC-delta but not atypical PKC-lambda/zeta. Insulin also activated the overexpressed PKC-delta but not PKC-alpha. Expression of the wild-type PKC-alpha or PKC-delta resulted in significant increases in glucose transport activity in the basal and PMA-stimulated states. Dominant-negative PKC-alpha expression, which inhibited the PMA activation of PKC-alpha, decreased in PMA-stimulated glucose transport. Glucose transport activity in the insulin-stimulated state was increased by the expression of PKC-delta but not of PKC-alpha. These findings demonstrate that both conventional and novel PKC isoforms are involved in PMA-stimulated glucose transport and that other novel PKC isoforms could participate in PMA-stimulated and insulin-stimulated glucose transport. Atypical PKC-lambda/zeta was not significantly activated by insulin, and expression of the wild-type, constitutively active, and dominant-negative mutants of atypical PKC did not affect either basal or insulin-stimulated glucose transport. Thus atypical PKC enzymes do not play a major role in insulin-stimulated glucose transport in 3T3-L1 adipocytes.