Reduced insulin clearance contributes to the increased insulin levels after administration of glucagon-like peptide 1 in mice

Aims/hypothesis Glucagon-like peptide-1 (GLP-1) is known to be a potent stimulator of insulin secretion. However, whether GLP-1 also affects insulin clearance is not known. To explore this, we developed a technique to determine prehepatic insulin secretion in mice, based on deconvolution of plasma C-peptide concentrations. The estimated beta cell secretion was then related to plasma insulin levels to allow determination of clearance rate of endogenously produced insulin.Materials and methods Kinetic parameters of C-peptide were estimated after i.v. injection of human C-peptide (0.8 or 3 nmol/kg) or glucose (1 g/kg), either alone or together with GLP-1 (10 nmol/kg), in anaesthetised NMRI mice.Results C-peptide was distributed in two compartments (distribution volume 11.4±0.4 ml, 42±2% of which was in the accessible compartment). Fractional C-peptide clearance was 8.2±0.6% of the total distribution volume per minute. GLP-1 markedly enhanced prehepatic insulin secretion; more than 80% of prehepatic secretion occurred during the first minute after injection. Fractional clearance of endogenously released insulin after glucose was 0.66±0.11 min^–1 and this was reduced to 0.36±0.10 min^–1 by GLP-1 (p=0.04).Conclusions/interpretation It is possible to perform C-peptide deconvolution for estimating prehepatic insulin secretion in mice. GLP-1 reduces the clearance of endogenously released insulin; therefore, it may affect insulin levels by increasing prehepatic insulin secretion and by reducing insulin clearance.     

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]     

Beta oxidation in the brain is required for the effects of non-esterified fatty acids on glucose-induced insulin secretion in rats

Aims/hypothesis NEFA play a key role in the setting of insulin resistance and hyperinsulinaemia, which are both features of the prediabetic state. In addition to the direct effects on pancreas and peripheral tissues, NEFA have been reported to act via changes in autonomic nervous system activity. The present study was aimed at studying the effects of a local increase in NEFA in the brain on glucose-induced insulin secretion (GIIS) and on insulin action. We hypothesised that cerebral NEFA beta oxidation is a prerequisite for these central effects.Methods Male Wistar rats were infused with Intralipid/heparin for 24 h through the carotid artery towards the brain (IL rats), after which we performed the GIIS test, a euglycaemic–hyperinsulinaemic clamp and c-fos immunochemistry. In another series of experiments, Intralipid/heparin infusion was coupled with lateral ventricular infusion of etomoxir, a CPT1 inhibitor, which was initiated 5 days previously.Results During the infusion period, there were no changes in plasma NEFA, insulin or glucose concentrations. IL rats displayed an increased GIIS compared with control rats (C rats) infused with saline/heparin, and their liver insulin sensitivity was decreased. Furthermore, lipid infusion induced a significant decrease in c-fos-like immunoreactive neurons in medial hypothalamic nuclei, and an increase in lateral hypothalamus. Neuronal activation profile was almost normalised in IL rats infused with etomoxir, and GIIS was strongly decreased, possibly because of the concomitant normalisation of hepatic glucose output.Conclusions/interpretation These results strongly suggest that beta oxidation is required for the central effects of NEFA on GIIS.     

胰高血糖素样肽-1对糖尿病大鼠胃排空的非受体通路的影响

目的探讨室旁核(Paraventricular nucleus,PVN)内胰高血糖素样肽-1(Glucagon-like pepetide-1,GLP-1)是否存在非受体通路参与糖尿病大鼠胃排空的中枢调节。方法 40只雄性Wistar大鼠随机分为正常对照组(NC组)、糖尿病组(DM组)、GLP-1(7-36)干预组(GLP组)、Exendin(9-39)联合GLP-1干预组(E-G组)及Exendin(9-39)干预组(Exendin组)。制备糖尿病大鼠模型,PVN埋置套管并给予GLP-1(7-36)或Exendin(9-39)等药物,2周后甲基纤维素-酚红溶液测胃排空;实时荧光定量分析法检测PVN区GLP-1受体(GLP-1R)mRNA表达情况。结果注射STZ 2周后,GLP组及E-G组胃排空率较DM组、Exendin组均明显降低(P均0.05),但高于NC组(P0.05);GLP组及E-G组的下丘脑GLP-1R mRNA表达较DM组、NC组、Exendin组均明显增高(P均0.05)。GLP组胃排空率和空腹血糖较E-G组降低(P0.05);下丘脑GLP-1R mRNA表达GLP组与E-G组比较差异无统计学意义(P0.05)。结论室旁核注射GLP-1可减缓糖尿病大鼠早期的胃排空,推测GLP-1影响胃排空的中枢作用机制除促进GLP-1受体表达增加外,非受体作用通路亦发挥重要作用。     

胰高血糖素样肽-1改善非酒精性脂肪性肝病作用机制研究进展*

近期研究表明,胰高血糖素样肽-1受体激动剂可从改善肝细胞脂质代谢、抑制炎症反应、调节细胞自噬等多个环节减轻非酒精性脂肪性肝病（NAFLD）患者肝脏脂质沉积,改善组织学损伤,有望成为治疗NAFLD药物的新选择。     

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     

Glucagon-like peptide 1(GLP-1) in biology and pathology

Post-translational proteolytic processing of the preproglucagon gene in the gut results in the formation of glucagon-like peptide 1 (GLP-1). Owing to its glucose-dependent insulinotropic effect, this hormone was postulated to primarily act as an incretin, i.e. to augment insulin secretion after oral glucose or meal ingestion. In addition, GLP-1 decelerates gastric emptying and suppresses glucagon secretion. Under physiological conditions, GLP-1 acts as a part of the 'ileal brake', meaning that is slows the transition of nutrients into the distal gut. Animal studies suggest a role for GLP-1 in the development and growth of the endocrine pancreas. In light of its multiple actions throughout the body, different therapeutic applications of GLP-1 are possible. Promising results have been obtained with GLP-1 in the treatment of type 2 diabetes, but its potential to reduce appetite and food intake may also allow its use for the treatment of obesity. While rapid in vivo degradation of GLP-1 has yet prevented its broad clinical use, different pharmacological approaches aiming to extend the in vivo half-life of GLP-1 or to inhibit its inactivation are currently being evaluated. Therefore, antidiabetic treatment based on GLP-1 may become available within the next years. This review will summarize the biological effects of GLP-1, characterize its role in human biology and pathology, and discuss potential clinical applications as well as current clinical studies.     

初发2型糖尿病患者GLP-1水平变化对胰高血糖素及早相胰岛素分泌的影响

目的探讨初发2型糖尿病（T2DM）患者血胰高血糖素样肽-1（GLP-1）对胰高血糖素及早相胰岛素分泌的影响。方法以新发T2DM患者（T2DM组）、健康体检者（对照组）为研究对象,采用标准馒头餐试验,观察空腹、进餐后30 min、120 min静脉血浆GLP-1动态变化及对血浆葡萄糖、胰高血糖素、胰岛素分泌的影响。结果初发T2DM组患者馒头餐各时点GLP-1水平均分别较对照组显著降低,差异有统计学意义（P〈0.05）,馒头餐后30 min、120 min胰岛素水平显著降低（P〈0.05）,但空腹胰岛素无明显差异（P〉0.05）;而胰高血糖素则较对照组各时点显著升高,差异有统计学意义（P〈0.05）。初发T2DM组早相胰岛素分泌指数（ΔFINS30/ΔG30）显著低于对照组,差异有统计学意义（P〈0.05）。结论初发T2DM患者存在GLP-1分泌减少,GLP-1缺乏可能是T2DM患者胰岛β细胞分泌缺乏及胰高血糖素分泌过多的重要因素。     

Glucagon-like peptide 1 (GLP-1): a potent gut hormone with a possible therapeutic perspective

Glucagon-like peptide 1 (GLP-1) is a physiological incretin hormone from the lower gastrointestinal tract, partially explaining the augmented insulin response after oral compared to intravenous glucose administration in normal humans. In addition, GLP-1 also lowers glucagon concentrations, slows gastric emptying, stimulates (pro)insulin biosynthesis, and reduces food intake upon intracerebroventricular administration in animals. Therefore, GLP-1 offers some interesting perspective for the treatment of type 2, and perhaps also for type 1 diabetic patients. The other incretin hormone, gastric inhibitory polypeptide (GIP), has lost almost all its activity in type-2 diabetic patients. In contrast, GLP-1 glucose-dependently stimulates insulin secretion in type-2 diabetic patients and exogenous administration of GLP-1 ([7–37] or [7–36 amide]) in doses elevating plasma concentrations to approximately three to four times physiological postprandial levels fully normalizes fasting hyperglycaemia and reduces postprandial glycaemic increments. Due to rapid proteolytic cleavage, which results in an inactive or even antagonistic fragment, GLP-1 [9–36 amide], and to rapid elimination, the half-life of GLP-1 is too short to maintain therapeutic plasma levels for sufficient periods by subcutaneous injections of the natural peptide hormone. Current research aims to characterize GLP-1 analogues with more suitable pharmacokinetic properties than the original peptide. Given the large amount of GLP-1 present in L cells, it also appears worthwhile to search for more agents that could `mobilize' this endogenous pool of GLP-1. Received: 1 July 1998 / Accepted: 7 July 1998     

Glucagon-like peptide 1 content of intestinal tract in adult rats injected with streptozotocin either during neonatal period or 7 d before sacrifice

Glucagon-like peptide 1 (GLP-1) content of the intestinal tract was recently found to be lower in diabetes-prone BioBreeding (BBdp) rats than in the corresponding control animals (BBc rats), a finding compatible with the idea that an inflammatory intestinal state precedes insulitis in these diabetes-prone animals. This study aimed at measuring GLP-1 content of the intestinal tract both in another animal model of type 1 diabetes and in an animal model of type 2 diabetes. GLP-1 content of the jejunum, ileum, colon, and cecum was measured in male and female adult control rats and animals injected with streptozotocin (STZ) either during the neonatal period or 7 d before sacrifice. GLP-1 content of the intestinal tract was higher in type 1 diabetic rats than in control animals. Such was not the case in the type 2 diabetic rats. The findings recorded in the rats injected with STZ either during the neonatal period or later in life indicate that hyperglycemia and/or insulin deficiency do not cause a decrease in GLP-1 content of the intestinal tract. On the contrary, such a content may increase when the glucose intolerance and hypoinsulinemia are sufficiently pronounced, as was the case in the type 1 diabetic rats. These findings are thus compatible with the view that the decreased GLP-1 content of the intestinal tract in BBdp rats may result from intestinal inflammation.     

Prolonged increase of plasma non-esterified fatty acids fully abolishes the stimulatory effect of 24 hours of moderate hyperglycaemia on insulin sensitivity and pancreatic beta-cell function in obese men

Aims/hypothesis A prolonged increase of plasma NEFA impairs acute glucose-stimulated insulin secretion (GSIS) in vitro and in vivo. Our study therefore examined the combined effect of increased plasma NEFA and glucose on GSIS in humans.Methods We examined GSIS on four occasions in eight obese men during a 10 mmol/l hyperglycaemic clamp and after a 24-h infusion of (i) normal saline, (ii) intralipid and heparin to raise plasma NEFA about two-fold above basal, (iii) 20% dextrose to raise plasma glucose to about 7.5 mmol/l and (iv) intralipid and heparin combined with 20% dextrose to raise plasma NEFA and glucose.Results In study (iii) insulin sensitivity was about 20% greater than in study (i) and the disposition index was about 50% higher. Insulin sensitivity tended to be lower in study (ii) whereas the disposition index was lower than in study (i), confirming previous observations. The combination of increased plasma NEFA and glucose (study iv) reduced insulin sensitivity in comparison with study (i) and completely abolished the increase in insulin sensitivity and disposition index seen in study (iii), but did not reduce the latter to a lower value than that in the saline control study (study i).Conclusions/interpretation We showed that a prolonged increase of plasma NEFA completely abolishes the stimulatory effect of moderate hyperglycaemia on insulin sensitivity and beta-cell function in obese humans. This suggests that previous observations, showing that a prolonged increase of plasma NEFA impairs pancreatic beta-cell function, also apply to the hyperglycaemic state.Abbreviations GSIS glucose-stimulated insulin secretion - SAL saline control study - IH intralipid heparin study - GLU hyperglycaemia study - GLU-IH hyperglycaemia, intralipid and heparin study - ISR insulin secretion rate - S_I insulin sensitivity index - DI disposition index - ANCOVA analysis of covariance     

A liquid mixed meal or exogenous glucagon-like peptide 1 (GLP-1) do not alter plasma leptin concentrations in healthy volunteers

Glucagon-like peptide 1 [7–36 amide] (GLP-1) and the obese gene product (leptin) are thought to be involved in the central regulation of feeding. Both may act from the peripheral circulation to influence brain function. To study potential interactions, GLP-1 ([7–36 amide]: 0.4, 0.8 pmol kg^–1 min^–1 or placebo on separate occasions) was infused intravenously (from –30 to 240 min) into nine healthy volunteers [age 26±3 years, body mass index: 22.9±1.6 kg/m², glycated haemoglobin HbA_1c: 5.0%± 0.2% (normal: 4.0%–6.2%), creatinine: 1.1±0.1 mg/dl], and (at 0 min) a liquid test meal (50 g sucrose in 400 ml 8% amino acid, total amino acids 80 g/l) was administered via a nasogastric tube. Plasma leptin (radioimmunoassay, RIA), glucose, insulin (microparticle enzyme immunoassay), C-peptide (enzyme-linked immunosorbent assay) and GLP-1 (RIA) were measured, and statistical analysis was done with repeated-measures ANOVA and Student's t-test. Plasma leptin concentrations were 31±6 pmol/l in the basal state. They did not change within 240 min after meal ingestion nor in response to the infusion of exogenous GLP-1 [7–36 amide] (P=0.99 for the interaction of experiment and time) leading to GLP-1 mean plasma levels of 25±2 and 36±3 (basal 6±1) pmol/l. On the other hand, glucose (from basal 4.7±0.1 to 6.0±0.2 mmol/l at 15 min, P<0.05) and insulin (from basal 28±2 to 325±78 pmol/l at 45 min, P<0.05) increased clearly after the meal with placebo. In conclusion, (1) plasma leptin levels in normal human subjects show no short-term changes after feeding a liquid mixed meal and (2) do not appear to be directly influenced by physiological and pharmacological elevations in plasma GLP-1 [7–36 amide] concentrations. This does not exclude interactions at the cerebral (hypothalamic) level or on more long-term temporal scales. Received: 5 February 1997 / Accepted in revised form: 18 June 1997     

Insulin and non-esterified fatty acid relations to alimentary lipaemia and plasma concentrations of postprandial triglyceride-rich lipoproteins in healthy middle-aged men

Aims/hypothesis. Enhanced and prolonged postprandial lipaemia is related to cardiovascular disease but how postprandial lipaemia is regulated is poorly known. We therefore determined the relations of fasting insulin concentrations to fasting and postprandial lipids, lipoproteins and non-esterified fatty acids in middle-aged men.¶Methods. The subjects, 99 healthy 50-year-old men with an apolipoprotein E3/3 genotype, ate a mixed meal. The apolipoprotein B-48 and B-100 contents were determined in triglyceride-rich lipoproteins as a measure of chylomicron remnant and very low density lipoprotein particle concentrations.¶Results. Fasting plasma insulin was associated with the triglyceride response to the test meal, independently of body mass index, waist-to-hip circumference ratio, blood glucose and the insulin effect on fasting plasma triglycerides. Exaggerated and prolonged postprandial lipaemia in subjects in the upper quartile of the plasma insulin distribution was largely accounted for by large (Svedberg flotation rate > 60) very low density lipoproteins and chylomicron remnants. Insulin relations to large postprandial triglyceride-rich lipoproteins exclusively reflected the association between plasma insulin and the fasting plasma concentrations of these lipoprotein species, whereas plasma insulin and late postprandial plasma concentrations of small (Svedberg flotation rate 20–60) chylomicron remnants were related, independently of insulin influences on fasting concentrations. Strong positive relations were found between the late increases in large triglyceride-rich lipoproteins and plasma non-esterified fatty acid concentrations after 6 h.¶Conclusion/interpretation. The degree of insulin sensitivity is a major determinant of postprandial lipaemia in healthy middle-aged men and could add to the regulation of the basal production of large triglyceride-rich lipoproteins. [Diabetologia (2000) 43: 185–193]     

高脂饮食和追赶生长对大鼠胃排空及血浆胰高血糖素样肽-1浓度的影响

的病理生理变化有关.     

Non-esterified fatty acids impair insulin-mediated glucose uptake and disposition in the liver

Aims/hypothesis We investigated the effect of elevated circulating NEFA on insulin-mediated hepatic glucose uptake (HGU) and whole-body glucose disposal (M) in eight healthy male subjects.Methods Studies were performed using positron emission tomography (PET) and [¹⁸F]-2-fluoro-2-deoxyglucose ([¹⁸F]FDG) during euglycaemic hyperinsulinaemia (0–120 min) and an Intralipid/heparin infusion (IL/Hep; –90–120 min). On a different day, similar measurements were taken during euglycaemic hyperinsulinaemia and saline infusion (SAL). Graphical and compartmental analyses were used to model liver data.Results Circulating NEFA increased approximately three-fold during IL/Hep, and declined by 81±7% in the SAL study (p0.01). Both M (–28±7%) and HGU (–25±9%) were significantly lowered by NEFA elevation (p=0.004 and p=0.035 respectively). In the whole data set, the decreases in M and HGU were positively correlated (r=0.78, p=0.038). No evidence of [¹⁸F]FDG outflow was detected during the scanning time. HGU was correlated with the phosphorylation rate parameter (r=0.71, p=0.003) as derived by compartmental modelling.Conclusions/interpretation In healthy men, NEFA impair insulin-mediated HGU and whole-body glucose uptake to a similar extent. Our data suggest that multiple intracellular NEFA targets may concur to down-regulate glucose uptake by the liver.Abbreviations FDG 2-fluoro-2-deoxyglucose - HGU hepatic glucose uptake - HKi influx rate constant - M whole-body glucose uptake - PET positron emission tomography - ROI region of interest - SGU splanchnic glucose uptake - SS sum of squares     

Beneficial effects of sub-chronic activation of glucagon-like peptide-1 (GLP-1) receptors on deterioration of glucose homeostasis and insulin secretion in aging mice

Aging is associated with an increased incidence of glucose intolerance and type 2 diabetes. Glucagon-like peptide-1 (GLP-1) is an important insulinotropic peptide secreted from the gastrointestinal tract in response to nutrient absorption. The present study was designed to assess the sub-chronic glucose regulatory effects of the potent long-acting GLP-1 receptor agonist, (Val(8))GLP-1, in aging 45-49 week old mice. Daily injection of (Val(8))GLP-1 (25 n mol/kg body weight) for 12 days had no significant effect on food intake, body weight, non-fasting plasma glucose and insulin concentrations. However, after 12 days, the glycaemic response to intraperitoneal glucose was improved (P<0.05) in (Val(8))GLP-1 treated mice. In keeping with this, glucose-mediated insulin secretion was enhanced (P<0.05) and insulin sensitivity improved (P<0.05) compared to controls. These data indicate that sub-chronic activation of the GLP-1 receptor by daily treatment with (Val(8))GLP-1 counters aspects of the age-related impairment of pancreatic beta-cell function and insulin sensitivity.     

Response of incretins (GIP and GLP-1) to an oral glucose load in female and male subjects with normal glucose tolerance

The aim of this study was to analyze the blood glucose profile and the response of incretins in healthy young subjects by the 75 g oral glucose tolerance test (OGTT). We first reported that plasma glucose and GIP levels were higher in males during the early phase of the OGTT.     

GLP-1对大鼠胰岛细胞增殖及凋亡的影响

目的 观察胰升糖素样肽1（GLP-1）对大鼠胰岛细胞增殖及凋亡的影响，并探讨其作用机制。方法 （1）GLP-1与大鼠胰岛细胞瘤细胞株RINm5f共育，观察其增殖情况；（2）检测GLP-1对IL-1D诱导的RINm5f细胞凋亡的保护作用；（3）GLP-1与原代培养大鼠胰岛细胞共育1、3、5天后，检测BAX及Bcl-2基因的表达。结果（1）随着GLP-1浓度增高及刺激时间延长，RINm5f细胞A值呈剂量及时间依赖性增加；（2）GLP-1组与对照组相比RINm5f细胞凋亡率显著下降（P〈0．05）；（3）与GLP-1共育后，大鼠胰岛BAX基因的表达量无明显变化，对照组BAX基因的表达逐渐增加（P〈0．05）；而Bcl-2基因的表达量随着与GLP-1共育时间的延长呈时间依赖性增加。结论GLP-1对大鼠胰岛细胞增殖有明显促进作用；同时对胰岛细胞的凋亡有保护作用。