Differential effects of acute and extended infusions of glucagon-like peptide-1 on first- and second-phase insulin secretion in diabetic and nondiabetic humans

OBJECTIVE: The purpose of this study was to determine whether an extended infusion of the incretin hormone glucagon-like peptide 1 (GLP-1) has a greater effect to promote insulin secretion in type 2 diabetic subjects than acute administration of the peptide. RESEARCH DESIGN AND METHODS: Nine diabetic subjects and nine nondiabetic volunteers of similar age and weight were studied in identical protocols. First-phase insulin release (FPIR; the incremental insulin response in the first 10 min after the intravenous glucose bolus) and second-phase insulin release (SPIR; the incremental insulin response from 10-60 min after intravenous glucose) were measured during three separate intravenous glucose tolerance tests (IVGTTs): 1). without GLP-1 (control); 2). with acute administration of GLP-1 as a square wave starting just before glucose administration; and 3). with an extended infusion of GLP-1 for 3 h before and during the IVGTT. RESULTS: In the subjects with diabetes, FPIR was severely impaired-a defect that was only modestly improved by acute administration of GLP-1 (197 +/- 97 vs. 539 +/- 218 pmol/l. min, P < 0.05), while SPIR was substantially increased (1952 +/- 512 vs. 8072 +/- 1664 pmol/l. min, P < 0.05). In contrast, the 3-h preinfusion of GLP-1 normalized fasting hyperglycemia (7.9 +/- 0.5 vs. 5.2 +/- 0.6, P < 0.05), increased FPIR by 5- to 6-fold (197 +/- 97 vs. 1141 +/- 409 pmol/l. min, P < 0.05), and augmented SPIR significantly (1952 +/- 512 vs. 4026 +/- 851 pmol/l. min, P < 0.05), but to a lesser degree than the acute administration of GLP-1. In addition, only the 3-h GLP-1 preinfusion significantly improved intravenous glucose tolerance (K(g) control 0.61 +/- 0.04, acute infusion 0.71 +/- 0.04, P = NS; 3-h infusion 0.92 +/- 0.08%/min, P < 0.05). These findings were also noted in the nondiabetic subjects in whom acute administration of GLP-1 significantly increased SPIR relative to the control IVGTT (9439 +/- 2885 vs. 31553 +/- 11660 pmol/l. min, P < 0.001) with less effect on FPIR (3221 +/- 918 vs. 4917 +/- 1614 pmol/l. min, P = 0.075), while the 3-h preinfusion of GLP-1 significantly increased both FPIR (3221 +/- 918 vs. 7948 +/- 2647 pmol/l. min, P < 0.01) and SPIR (9439 +/- 2885 vs. 21997 +/- 9849 pmol/l. min, P < 0.03). CONCLUSIONS: Extended administration of GLP-1 not only augments glucose-stimulated insulin secretion, but also shifts the dynamics of the insulin response to earlier release in both diabetic and nondiabetic humans. The restitution of some FPIR in subjects with type 2 diabetes is associated with significantly improved glucose tolerance. These findings demonstrate the benefits of a 3-h infusion of GLP-1 on beta-cell function beyond those of an acute insulin secretagogue, and support the development of strategies using continuous or prolonged GLP-1 receptor agonism for treating diabetic patients.     

Additive glucose-lowering effects of glucagon-like peptide-1 and metformin in type 2 diabetes

OBJECTIVE: The incretin hormone glucagon-like peptide-1 (GLP-1) reduces plasma glucose in type 2 diabetic patients by stimulating insulin secretion and inhibiting glucagon secretion. The biguanide metformin is believed to lower plasma glucose without affecting insulin secretion. We conducted this study to investigate the effect of a combination therapy with GLP-1 and metformin, which could theoretically be additive, in type 2 diabetic patients. RESEARCH DESIGN AND METHODS: In a semiblinded randomized crossover study, seven patients received treatment with metformin (1,500 mg daily orally) alternating with GLP-1 (continuous subcutaneous infusion of 2.4 pmol x kg(-1) x min(-1)) alternating with a combination of metformin and GLP-1 for 48 h. Under fixed energy intake, we examined the effects on plasma glucose, insulin, C-peptide, glucagon, and appetite. RESULTS: Fasting plasma glucose (day 2) decreased from 13.9 +/- 1 (no treatment) to 11.2 +/- 0.4 (metformin) and 11.5 +/- 0.5 (GLP-1) and further decreased to 9.4 +/- 0.7 (combination therapy) (P = 0.0005, no difference between monotherapy with GLP-1 and metformin). The 24-h mean plasma glucose (day 2) decreased from 11.8 +/- 0.5 (metformin) and 11.7 +/- 0.8 (GLP-1) to 9.8 +/- 0.5 (combination) (P = 0.02, no difference between GLP-1 and metformin). Insulin levels were similar between the three regimens, but glucagon levels were significantly reduced with GLP-1 compared with metformin (P = 0.0003). Combination therapy had no additional effect on appetite scores. CONCLUSIONS: Monotherapy with GLP-1 and metformin have equal effects on plasma glucose and additive effects upon combination.     

Evaluation of beta-cell secretory capacity using glucagon-like peptide 1

OBJECTIVE: Beta-cell secretory capacity is often evaluated with a glucagon test or a meal test. However, glucagon-like peptide 1 (GLP-1) is the most insulinotropic hormone known, and the effect is preserved in type 2 diabetic patients. RESEARCH DESIGN AND METHODS: We first compared the effects of intravenous bolus injections of 2.5, 5, 15, and 25 nmol GLP-1 with glucagon (1 mg intravenous) and a standard meal (566 kcal) in 6 type 2 diabetic patients and 6 matched control subjects. Next, we studied another 6 patients and 6 control subjects and, in addition to the above procedure, performed a combined glucose plus GLP-1 stimulation, where plasma glucose was increased to 15 mmol/l before injection of 2.5 nmol GLP-1. Finally, we compared the insulin response to glucose plus GLP-1 stimulation with that observed during a hyperglycemic arginine clamp (30 mmol/l) in 8 patients and 8 control subjects. RESULTS: Peak insulin and C-peptide concentrations were similar after the meal, after 2.5 nmol GLP-1, and after glucagon. Side effects were less with GLP-1 than with glucagon. Peak insulin and C-peptide concentrations were as follows (C-peptide concentrations are given in parentheses): for patients (n = 12): meal, 277 +/- 42 pmol/l (2,181 +/- 261 pmol/l); GLP-1 (2.5 nmol), 390 +/- 74 pmol/l (2,144 +/- 254 pmol/l); glucagon, 329 +/- 50 pmol/l (1,780 +/- 160 pmol/l); glucose plus GLP-1, 465 +/- 87 pmol/l (2,384 +/- 299 pmol/l); for control subjects (n = 12): meal, 543 +/- 89 pmol/l (2,873 +/- 210 pmol/l); GLP-1, 356 +/- 51 pmol/l (2,001 +/- 130 pmol/l); glucagon, 420 +/- 61 pmol/l (1,995 +/- 99 pmol/l); glucose plus GLP-1, 1,412 +/- 187 pmol/l (4,391 +/- 416 pmol/l). Peak insulin and C-peptide concentrations during the hyperglycemic arginine clamp and during glucose plus GLP-1 injection were as follows: for patients: 475 +/- 141 pmol/l (2,295 +/- 379 pmol/l) and 816 +/- 268 pmol/l (3,043 +/- 508 pmol/l), respectively; for control subjects: 1,403 +/- 308 pmol/l (4,053 +/- 533 pmol/l) and 2,384 +/- 452 pmol/l (6,047 +/- 652 pmol/l), respectively. CONCLUSIONS: GLP-1 (2.5 nmol = 9 microg) elicits similar secretory responses to 1 mg glucagon (but has fewer side effects) and a standard meal. Additional elevation of plasma glucose to 15 mmol/l did not enhance the response further. The incremental response was similar to that elicited by arginine, but hyperglycemia had an additional effect on the response to arginine.     

Effects of 3 months of continuous subcutaneous administration of glucagon-like peptide 1 in elderly patients with type 2 diabetes

OBJECTIVE: Glucagon-like peptide 1 (GLP-1) is an insulinotropic gut hormone that, when given exogenously, may be a useful agent in the treatment of type 2 diabetes. We conducted a 3-month trial to determine the efficacy and safety of GLP-1 in elderly diabetic patients. RESEARCH DESIGN AND METHODS: A total of 16 patients with type 2 diabetes who were being treated with oral hypoglycemic agents were enrolled. Eight patients (aged 75 +/- 2 years, BMI 27 +/- 1 kg/m(2)) remained on usual glucose-lowering therapy and eight patients (aged 73 +/- 1 years, BMI 27 +/- 1 kg/m(2)), after discontinuing hypoglycemic medications, received GLP-1 delivered by continuous subcutaneous infusion for 12 weeks. The maximum dose was 120 pmol x kg(-1). h(-1). Patients recorded their capillary blood glucose (CBG) levels (four times per day, 3 days per week) and whenever they perceived hypoglycemic symptoms. The primary end points were HbA(1c) and CBG determinations. Additionally, changes in beta-cell sensitivity to glucose, peripheral tissue sensitivity to insulin, and changes in plasma ghrelin levels were examined. RESULTS: HbA(1c) levels (7.1%) and body weight were equally maintained in both groups. The usual treatment group had a total of 87 CBG measurements of 相似文献   

Improved meal-related beta-cell function and insulin sensitivity by the dipeptidyl peptidase-IV inhibitor vildagliptin in metformin-treated patients with type 2 diabetes over 1 year

OBJECTIVE: To examine the effects of dipeptidyl peptidase-IV (DPP-4) inhibition on meal-related beta-cell function and insulin sensitivity over 52 weeks in type 2 diabetes. RESEARCH DESIGN AND METHODS: In a 12-week core study, placebo (n = 51) or vildagliptin (n = 56; 50 mg OD) was added to metformin treatment (1.5-3.0 mg/day). A 40-week extension followed in 71 patients. Meal tests were performed at 0, 12, 24, and 52 weeks; glucose, insulin, and C-peptide were evaluated. RESULTS: In subjects completing 52 weeks with participation in all meal tests (n = 57), HbA(1c) (A1C) decreased in the vildagliptin/metformin group (VM group, n = 31) but increased in the placebo/metformin group (PM group, n = 26; between-group difference -1.0 +/- 0.2%; P < 0.001; baseline of all subjects combined 7.7 +/- 0.1%). Also, fasting glucose decreased in the VM group but increased in the PM group (difference -0.9 +/- 0.3 mmol/l, P = 0.016; baseline 9.8 +/- 0.3 mmol/l). Insulin secretion (postmeal suprabasal area under the 0- to 30-min C-peptide curve divided by the 30-min increase in glucose) was increased in the VM group but was reduced in the PM group (difference +0.011 +/- 0.03 pmol/l 30 min/mmol/l, P = 0.018; baseline 0.036 +/- 0.02). Insulin sensitivity during meal ingestion (oral glucose insulin sensitivity) increased in the VM group but was not altered in the PM group (difference +27 +/- 4 ml x min(-1) x m(-2), P = 0.036; baseline 246 +/- 6). Insulin secretion related to insulin sensitivity (adaptation index) increased in the VM group but decreased in the PM group (difference +3.2 +/- 1.0, P = 0.040; baseline 9.1 +/- 0.5). The change in adaptation index correlated to the change in A1C (r = -0.39, P = 0.004). CONCLUSIONS: This study presents evidence that DPP-4 inhibition by vildagliptin when added to metformin in type 2 diabetes over 52 weeks improves beta-cell function along with improved postmeal insulin sensitivity.     

A novel hyperglycaemic clamp for characterization of islet function in humans: assessment of three different secretagogues, maximal insulin response and reproducibility

BACKGROUND: Characterization of beta-cell function in humans is essential for identifying genetic defects involved in abnormal insulin secretion and the pathogenesis of type 2 diabetes. MATERIALS AND METHODS: We designed a novel test assessing plasma insulin and C-peptide in response to 3 different secretagogues. Seven lean, healthy volunteers twice underwent a 200 min hyperglycaemic clamp (10 mmol L-1) with administration of GLP-1 (1.5 pmol. kg-1. min-1) starting at 120 min and an arginine bolus at 180 min. We determined glucose-induced first and second-phase insulin secretion, GLP-1-stimulated insulin secretion, arginine-stimulated insulin response (increase above prestimulus, DeltaIarg) and the maximal, i. e. highest absolute, insulin concentration (Imax). Insulin sensitivity was assessed during second-phase hyperglycaemia. On a third occasion 6 subjects additionally received an arginine bolus at > 25 mM blood glucose, a test hitherto claimed to provoke maximal insulin secretion. RESULTS: Insulin levels increased from 46 +/- 11 pM to 566 +/- 202 pM at 120 min, to 5104 +/- 1179 pM at 180 min and to maximally 8361 +/- 1368 pM after arginine (all P < 0.001). The within subject coefficients of variation of the different secretion parameters ranged from 10 +/- 3% to 16 +/- 6%. Except for second-phase which failed to correlate significantly with DeltaIarg (r = 0.52, P = 0.23) and Imax (r = 0.75, P = 0.053) all phases of insulin secretion correlated with one another. The insulin concentration after the arginine bolus at > 25 mM glucose (n = 6) was 2773 +/- 855 pM vs. 7562 +/- 1168 pM for Imax (P = 0.003). CONCLUSION: This novel insulin secretion test elicits a distinct pattern of plasma insulin concentrations in response to the secretagogues glucose, GLP-1 and arginine and is highly reproducible and can be used for differential characterization of islet function.     

Mechanism of impaired glucose-potentiated insulin secretion in diabetic 90% pancreatectomy rats. Study using glucagonlike peptide-1 (7-37).

Chronic hyperglycemia causes a near-total disappearance of glucose-induced insulin secretion. To determine if glucose potentiation of nonglucose secretagogues is impaired, insulin responses to 10(-9) M glucagonlike peptide-1 (GLP-1) (7-37) were measured at 2.8, 8.3, and 16.7 mM glucose with the in vitro perfused pancreas in rats 4-6 wk after 90% pancreatectomy (Px) and sham-operated controls. In the controls, insulin output to GLP-1 was > 100-fold greater at 16.7 mM glucose versus 2.8 mM glucose. In contrast, the increase was less than threefold in Px, reaching an insulin response at 16.7 mM glucose that was 10 +/- 2% of the controls, well below the predicted 35-40% fractional beta-cell mass in these rats. Px and control rats then underwent a 40-h fast followed by pancreas perfusion using a protocol of 20 min at 16.7 mM glucose followed by 15 min at 16.7 mM glucose/10(-9) M GLP-1. In control rats, fasting suppressed insulin release to high glucose (by 90%) and to GLP-1 (by 60%) without changing the pancreatic insulin content. In contrast, in Px the insulin response to GLP-1 tripled in association with a threefold increase of the insulin content, both now being twice normal when stratified for the fractional beta-cell mass. The mechanism of the increased pancreas insulin content was investigated by assessing islet glucose metabolism and proinsulin biosynthesis. In controls with fasting, both fell 30-50%. In Px, the degree of suppression with fasting was similar, but the attained levels both exceeded those of the controls because of higher baseline (nonfasted) values. In summary, chronic hyperglycemia is associated with a fasting-induced paradoxical increase in glucose-potentiated insulin secretion. In Px rats, the mechanism is an increase in the beta-cell insulin stores, which suggests a causative role for a lowered beta-cell insulin content in the impaired glucose-potentiation of insulin secretion.     

Glucagon-like peptide-1 is a physiological incretin in rat.

Glucagon-like peptide-1 7-36 amide (GLP-1) has been postulated to be the primary hormonal mediator of the entero-insular axis but evidence has been indirect. The discovery of exendin (9-39), a GLP-1 receptor antagonist, allowed this to be further investigated. The IC50 for GLP-1 receptor binding, using RIN 5AH beta-cell membranes, was found to be 0.36 nmol/l for GLP-1 and 3.44 nmol/l for exendin (9-39). There was no competition by exendin (9-39) at binding sites for glucagon or related peptides. In the anaesthetized fasted rat, insulin release after four doses of GLP-1 (0.1, 0.2, 0.3, and 0.4 nmol/kg) was tested by a 2-min intravenous infusion. Exendin (9-39) (1.5, 3.0, and 4.5 nmol/kg) was administered with GLP-1 0.3 nmol/kg, or saline, and only the highest dose fully inhibited insulin release. Exendin (9-39) at 4.5 nmol/kg had no effect on glucose, arginine, vasoactive intestinal peptide or glucose-dependent insulinotropic peptide stimulated insulin secretion. Postprandial insulin release was studied in conditioned conscious rats after a standard meal. Exendin (9-39) (0.5 nmol/kg) considerably reduced postprandial insulin concentrations, for example by 48% at 15 min (431 +/- 21 pmol/l saline, 224 +/- 32 pmol/l exendin, P < 0.001). Thus, GLP-1 appears to play a major role in the entero-insular axis.     

Glucagon-like peptide 1 enhances glucose tolerance both by stimulation of insulin release and by increasing insulin-independent glucose disposal.

Glucagon-like peptide 1 [7-36 amide] (GLP-1) has been shown to enhance insulin secretion in healthy and type II diabetic humans, and to increase glucose disposal in type I diabetic patients. To further define its action on glucose kinetics, we studied six healthy subjects who received either GLP-1 (45 pmol/kg per h) or 150 mM saline on two mornings during which a modified intravenous glucose tolerance test was performed. Plasma insulin and glucose levels were analyzed using Bergman's minimal model of glucose kinetics to derive indices of insulin sensitivity (SI) and glucose effectiveness at basal insulin (SG), the latter a measure of glucose disposition independent of changes in insulin. In addition, basal insulin concentrations, the acute insulin response to glucose (AIRg), plasma glucagon levels, and the glucose disappearance constant (Kg) were measured on the days that subjects received GLP-1 or saline. Compared with saline infusions, GLP-1 increased the mean Kg from 1.61 +/- 0.20 to 2.65 +/- 0.25%/min (P = 0.022). The enhanced glucose disappearance seen with GLP-1 was in part the result of its insulinotropic effect, as indicated by a rise in AIRg from 240 +/- 48 to 400 +/- 78 pM (P = 0.013). However, there was also an increase in SG from 1.77 +/- 0.11 to 2.65 +/- 0.33 x 10(-2).min-1 (P = 0.038), which was accounted for primarily by insulin-independent processes, viz glucose effectiveness in the absence of insulin. There was no significant effect of GLP-1 on SI or basal insulin, and glucagon levels were not different during the glucose tolerance tests with or without GLP-1. Thus, GLP-1 improves glucose tolerance both through its insulinotropic action and by increasing glucose effectiveness. These findings suggest that GLP-1 has direct effects on tissues involved in glucose disposition. Furthermore, this peptide may be useful for studying the process of insulin-independent glucose disposal, and pharmacologic analogues may be beneficial for treating patients with diabetes mellitus.     

Induction of long-term glycemic control in newly diagnosed type 2 diabetic patients is associated with improvement of beta-cell function

OBJECTIVE: To investigate whether long-term optimal glycemic control can be achieved without medication by transient continuous subcutaneous insulin infusion (CSII) and the possible mechanisms responsible for this remission. RESEARCH DESIGN AND METHODS: Newly diagnosed type 2 diabetic patients (n = 138, fasting glucose >11.1mmol/l) were hospitalized and treated with CSII for 2 weeks. Intravenous glucose tolerance tests (IVGTTs) were performed, and blood glucose, HbA(1c), lipid profiles, proinsulin, insulin, and C-peptide were measured before and after CSII. Patients were followed longitudinally on diet alone after withdrawal of insulin. RESULTS: Optimal glycemic control was achieved within 6.3 +/- 3.9 days by CSII in 126 patients. The remission rates (percentages maintaining near euglycemia) at the third, sixth, twelfth, and twenty-fourth month were 72.6, 67.0, 47.1, and 42.3%, respectively. Patients who maintained glycemic control >12 months (remission group) had greater recovery of beta-cell function than those who did not (nonremission group) when assessed immediately after CSII. Homeostasis model assessment of beta-cell function (HOMA-B) and the area under the curve (AUC) of insulin during IVGTT were higher in the remission group (145.4 +/- 89.6 vs. 78.5 +/- 68.5, P = 0.002, and 1,423.4 +/- 523.2 vs. 1,159.5 +/- 476.8 pmol x l(-1) x min(-1), P = 0.044). Change in acute insulin response was also greater in the remission group than that in the nonremission group (621.8 +/- 430.4 vs. 387.3 +/- 428.8 pmol x l(-1) x min(-1), P = 0.033). CONCLUSIONS: Short-term intensive insulin therapy can induce long-term glycemic control in newly diagnosed type 2 diabetic patients with severe hyperglycemia. The improvement of beta-cell function, especially the restoration of first-phase insulin secretion, could be responsible for the remission.     

Youth type 2 diabetes: insulin resistance, beta-cell failure, or both?

OBJECTIVE: This study evaluates insulin sensitivity, pancreatic beta-cell function (BCF), and the balance between the two in youth with type 2 diabetes and assesses the relationship of diabetes duration and HbA(1c) to insulin sensitivity and BCF. RESEARCH DESIGN AND METHODS: The subjects were 14 adolescents with type 2 diabetes and 20 obese control subjects of comparable age, BMI, body composition, and puberty. Insulin sensitivity was evaluated with a 3-h hyperinsulinemic (80 mU . m(-2) . min(-1)) euglycemic clamp. First-phase insulin secretion (FPIS) and second-phase insulin secretion (SPIS) were evaluated with a 2-h hyperglycemic (12.5 mmol/l) clamp. Fasting glucose rate of appearance was determined with the use of [6,6-(2)H(2)]glucose. RESULTS: Fasting glucose rate of appearance was higher in type 2 diabetic patients than in obese control subjects (16.5 +/- 1.1 vs. 12.3 +/- 0.5 micromol . kg(-1) . min(-1); P = 0.002). Insulin sensitivity was lower in type 2 diabetic patients than in obese control subjects (1.0 +/- 0.1 vs. 2.0 +/- 0.2 micromol . kg(-1) . min(-1) per pmol/l; P = 0.001). Fasting insulin was higher in type 2 diabetic patients than in obese control subjects (289.8 +/- 24.6 vs. 220.2 +/- 18.0 pmol/l; P = 0.007), and FPIS and SPIS were lower (FPIS: 357.6 +/- 42.0 vs. 1,365.0 +/- 111.0 pmol/l; SPIS: 652.2 +/- 88.8 vs. 1,376.4 +/- 88.8 pmol/l; P < 0.001 for both). The glucose disposition index (GDI = insulin sensitivity x FPIS) was approximately 86% lower in type 2 diabetic patients than in obese control subjects. HbA(1c) correlated with FPIS (r = -0.61, P = 0.025) with no relationship to insulin sensitivity. CONCLUSIONS: Despite the impairment in both insulin sensitivity and BCF in youth with type 2 diabetes, the magnitude of the derangement is greater in BCF than insulin sensitivity when compared with that in obese control subjects. The inverse relationship between BCF and HbA(1c) may either reflect the impact of deteriorating BCF on glycemic control or be a manifestation of a glucotoxic phenomenon on BCF. Future studies in youth type 2 diabetes should target the natural course of beta-cell failure and means of retarding and/or preventing it.     

Elimination of the action of glucagon-like peptide 1 causes an impairment of glucose tolerance after nutrient ingestion by healthy baboons.

Glucagon-like peptide 1 (GLP-1) is an insulinotropic hormone released after nutrient ingestion which is known to augment insulin secretion, inhibit glucagon release, and promote insulin-independent glucose disposition. To determine the overall effect of GLP-1 on glucose disposition after a meal we studied a group of healthy, conscious baboons before and after intragastric glucose administration during infusions of saline, and two treatments to eliminate the action of GLP-1: (a) exendin-[9-39] (Ex-9), a peptide receptor antagonist of GLP-1; or (b) an anti-GLP-1 mAb. Fasting concentrations of glucose were higher during infusion of Ex-9 than during saline (4.44 +/- 0.05 vs. 4.16 +/- 0.05 mM, P < 0.01), coincident with an elevation in the levels of circulating glucagon (96 +/- 10 vs. 59 +/- 3 ng/liter, P < 0.02). The postprandial glycemic excursions during administration of Ex-9 and mAb were greater than during the control studies (Ex-9 13.7 +/- 2.0 vs. saline 10.0 +/- 0.8 mM, P = 0.07; and mAb 13.6 +/- 1.2 vs. saline 10.6 +/- 0.9 mM, P = 0.044). The increments in insulin levels throughout the absorption of the glucose meal were not different for the experimental and control conditions, but the insulin response in the first 30 min after the glucose meal was diminished significantly during treatment with Ex-9 (Ex-9 761 +/- 139 vs. saline 1,089 +/- 166 pM, P = 0.044) and was delayed in three of the four animals given the neutralizing antibody (mAb 946 +/- 262 vs. saline 1,146 +/- 340 pM). Thus, elimination of the action of GLP-1 impaired the disposition of an intragastric glucose meal and this was at least partly attributable to diminished early insulin release. In addition to these postprandial effects, the concurrent elevation in fasting glucose and glucagon during GLP-1 antagonism suggests that GLP-1 may have a tonic inhibitory effect on glucagon output. These findings demonstrate the important role of GLP-1 in the assimilation of glucose absorbed from the gut.     

Glucagon-like peptide-1 can reverse the age-related decline in glucose tolerance in rats.

Wistar rats develop glucose intolerance and have a diminished insulin response to glucose with age. The aim of this study was to investigate if these changes were reversible with glucagon-like peptide-1 (GLP-1), a peptide that we have previously shown could increase insulin mRNA and total insulin content in insulinoma cells. We infused 1.5 pmol/ kg-1.min-1 GLP-1 subcutaneously using ALZET microosmotic pumps into 22-mo-old Wistar rats for 48 h. Rat infused with either GLP-1 or saline were then subjected to an intraperitoneal glucose (1 g/kg body weight) tolerance test, 2 h after removing the pump. 15 min after the intraperitoneal glucose, GLP-1-treated animals had lower plasma glucose levels (9.04+/-0.92 mmol/liter, P < 0.01) than saline-treated animals (11.61+/-0.23 mmol/liter). At 30 min the plasma glucose was still lower in the GLP-1-treated animals (8.61+/-0.39 mmol/liter, P < 0.05) than saline-treated animals (10.36+/-0.43 mmol/liter). This decrease in glucose levels was reflected in the higher insulin levels attained in the GLP-1-treated animals (936+/-163 pmol/liter vs. 395+/-51 pmol/liter, GLP-1 vs. saline, respectively, P < 0.01), detected 15 min after glucose injection. GLP-1 treatment also increased pancreatic insulin, GLUT2, and glucokinase mRNA in the old rats. The effects of GLP-1 were abolished by simultaneous infusion of exendin [9-39], a specific antagonist of GLP-1. GLP-1 is therefore able to reverse some of the known defects that arise in the beta cell of the pancreas of Wistar rats, not only by increasing insulin secretion but also by inducing significant changes at the molecular level.     

Effect of glucagon-like peptide 1 (7-36 amide) on insulin-mediated glucose uptake in patients with type 1 diabetes

OBJECTIVE: To examine the insulinomimetic insulin-independent effects of glucagon-like peptide (GLP)-1 on glucose uptake in type 1 diabetic patients. RESEARCH DESIGN AND METHODS: We used the hyperinsulinemic-euglycemic clamp (480 pmol. m(-2) x min(-1)) in paired randomized studies of six women and five men with type 1 diabetes. In the course of one of the paired studies, the subjects also received GLP-1 at a dose of 1.5 pmol. kg(-1) x min(-1). The patients were 41 +/- 3 years old with a BMI of 25 +/- 1 kg/m(2). The mean duration of diabetes was 23 +/- 3 years. RESULTS: Plasma glucose was allowed to fall from a fasting level of approximately 11 mmol/l to 5.3 mmol/l in each study and thereafter was held stable at that level. Plasma insulin levels during both studies were approximately 900 pmol/l. Plasma C-peptide levels did not change during the studies. In the GLP-1 study, plasma total GLP-1 levels were elevated from the fasting level of 31 +/- 3 to 150 +/- 17 pmol/l. Plasma glucagon levels fell from the fasting levels of approximately 14 pmol/l to 9 pmol/l during both paired studies. Hepatic glucose production was suppressed during the glucose clamps in all studies. Glucose uptake was not different between the two studies ( approximately 40 micromol. kg(-1) x min(-1)). CONCLUSIONS: GLP-1 does not augment insulin-mediated glucose uptake in lean type 1 diabetic patients.     

Intact proinsulin and beta-cell function in lean and obese subjects with and without type 2 diabetes

OBJECTIVE: Type 2 diabetes is a heterogeneous disease in which both beta-cell dysfunction and insulin resistance are pathogenetic factors. Disproportionate hyperproinsulinemia (elevated proinsulin/insulin) is another abnormality in type 2 diabetes whose mechanism is unknown. Increased demand due to obesity and/or insulin resistance may result in secretion of immature beta-cell granules with a higher content of intact proinsulin. RESEARCH DESIGN AND METHODS: We investigated the impact of obesity on beta-cell secretion in normal subjects and in type 2 diabetic patients by measuring intact proinsulin, total proinsulin immunoreactivity (PIM), intact insulin, and C-peptide (by radioimmunoassay) by specific enzyme-linked immunosorbent assays in the fasting state and during a 120-min glucagon (1 mg i.v.) stimulation test. Lean (BMI 23.5 +/- 0.3 kg/m2) (LD) and obese (30.1 +/- 0.4 kg/m2) (OD) type 2 diabetic patients matched for fasting glucose (10.2 +/- 0.6 vs. 10.3 +/- 0.4 mmol/l) were compared with age- and BMI-matched lean (22.4 +/- 0.6 kg/m2) (LC) and obese (30.8 +/- 0.9 kg/m2) (OC) normal control subjects. RESULTS: Diabetic patients (LD vs. LC and OD vs. OC) had elevated fasting levels of intact proinsulin 6.6 +/- 1.0 vs. 1.6 +/- 0.3 pmol/l and 7.7 +/- 2.0 vs. 1.2 +/- 0.2 pmol/l; PIM: 19.9 +/- 2.5 vs. 5.4 +/- 1.0 pmol/l and 29.6 +/- 6.1 vs. 6.1 +/- 0.9 pmol/l; and total PIM/intact insulin: 39 +/- 4 vs. 15 +/- 2% and 35 +/- 5 vs. 13 +/- 2%, all P < 0.01. After glucagon stimulation, PIM levels were disproportionately elevated (PIM/intact insulin based on area under the curve analysis) in diabetic patients (LD vs. LC and OD vs. OC): 32.6 +/- 6.7 vs. 9.2 +/- 1.1% and 22.7 +/- 5.2 vs. 9.1 +/- 1.1%, both P < 0.05. Intact insulin and C-peptide net responses were significantly reduced in type 2 diabetic patients, most pronounced in the lean group. The ratio of intact proinsulin to PIM was higher in diabetic patients after stimulation in both LD versus LC: 32 +/- 3 vs. 23 +/- 2%, and OD versus OC: 28 +/- 4 vs. 16 +/- 2%, both P < 0.01. In obese normal subjects, intact proinsulin/PIM was lower both in the fasting state and after glucagon stimulation: OC versus LC: 22 +/- 3 vs. 33 +/- 3% (fasting) and 16 +/- 2 vs. 23 +/- 2% (stimulated), both P < 0.05. CONCLUSIONS: Increased secretory demand from obesity-associated insulin resistance cannot explain elevated intact proinsulin and disproportionate hyperproinsulinemia in type 2 diabetes. This abnormality may be an integrated part of pancreatic beta-cell dysfunction in this disease.     

Impaired beta-cell function in lean normotolerant former gestational diabetic women

BACKGROUND: Former gestational diabetes (fGDM) constitutes a risk condition for the development of Type 2 diabetes. Former gestational diabetes is often characterized by obesity and hyperglycaemia, which may be concomitant and independent risk factors. MATERIALS AND METHODS: To assess insulin sensitivity and beta-cell function in fGDM uncomplicated by obesity and hyperglycaemia, we studied 24 lean fGDM women and 23 control women matched for age (30.7 +/- 0.7 years, whole cohort), body mass index (22.2 +/- 0.3 kg m(-2)), and indistinguishable for plasma glucose both at fasting and at 120 min. Several insulin sensitivity and beta-cell function indices were computed: homeostasis model assessment insulin resistance index (HOMA-R), insulin sensitivity index derived from an oral glucose tolerance test (OGIS), insulinogenic index, other empirical indices of insulin secretion and beta-cell function, and indices obtained using a beta-cell model. RESULTS: Though the majority of indices, and in particular insulin sensitivity (HOMA-R: 1.35 +/- 0.13 vs. 1.65 +/- 0.14; OGIS: 492.7 +/- 6.3 vs. 496.4 +/- 9.4 mL min(-1) m(-2)), were not significantly different in the two groups, the beta-cell glucose sensitivity obtained by modelling analysis was lower in fGDM (108 +/- 14 vs. 165 +/- 22 pmol min(-1) m(-2) mM(-1), P = 0.031). CONCLUSIONS: Impairment of beta-cell glucose sensitivity may be an intrinsic risk factor in fGDM independently of obesity and hyperglycaemia. Furthermore, we have shown that modelling analysis, in contrast to the empirical parameters, may be able to detect early beta-cell alterations in fGDM women.     

Insulin and amylin release are both diminished in first-degree relatives of subjects with type 2 diabetes

OBJECTIVE: To determine whether first-degree relatives of individuals with type 2 diabetes, who are at high risk of subsequently developing hyperglycemia, manifest alterations in beta-cell function including an alteration in the co-release of insulin and amylin. RESEARCH DESIGN AND METHODS: In 30 first-degree relatives and 24 matched subjects with no family history of diabetes, beta-cell function was measured as the intravenous glucose-induced acute insulin response (AIR(g)) and acute amylin response (AAR(g)). The insulin sensitivity index (S(I)) was quantified and used to account for the role of insulin sensitivity to modulate beta-cell function (S(I) x beta-cell function). RESULTS: Fasting plasma glucose (5.3 +/- 0.1 vs. 5.1 +/- 0.1 mmol/l; means +/- SEM), immunoreactive insulin (IRI) (68 +/- 7 vs. 57 +/- 6 pmol/l) and amylin-like immunoreactivity (ALI) (5.5 +/- 0.6 vs. 4.7 +/- 0.7 pmol/l) were similar in relatives and control subjects, respectively. Relatives were insulin resistant compared with control subjects (S(I): 4.86 +/- 0.63 vs. 7.20 +/- 0.78 x 10(-5) min(-1). pmol(-1). l(-1), P = 0.01), but their AIR(g) (392 +/- 59 vs. 386 +/- 50 pmol/l) and AAR(g) (5.9 +/- 0.9 vs. 6.1 +/- 0.8 pmol/l) did not differ. When beta-cell function was determined relative to insulin sensitivity, in the first-degree relatives, both AIR(g) (S(I) x AIR(g): 1.60 +/- 0.23 vs. 2.44 +/- 0.31 x 10(-2) min(-1), P < 0.05) and AAR(g) (S(I) x AAR(g): 2.39 +/- 0.35 vs. 4.06 +/- 0.56 x 10(-4) min(-1), P < 0.05) were reduced. The molar proportion of ALI to IRI was not altered in high-risk subjects (1.75 +/- 0.16 vs. 1.71 +/- 0.15%). CONCLUSIONS: First-degree relatives of subjects with type 2 diabetes have diminished beta-cell function at a time when they are not hyperglycemic, and this reduction affects insulin and amylin responses proportionally. Thus, an altered amylin-to-insulin ratio is not likely to identify individuals at high risk of developing type 2 diabetes.     

Increasing the decrement in insulin secretion improves glucagon responses to hypoglycemia in advanced type 2 diabetes

OBJECTIVE: In advanced beta-cell failure, counterregulatory glucagon responses may be impaired due to a reduced decrement in insulin secretion during the development of hypoglycemia. The present studies were therefore undertaken to test the hypothesis that these may be improved by increasing this decrement in insulin secretion. RESEARCH DESIGN AND METHODS: Twelve subjects with type 2 diabetes who have been insulin requiring were studied as a model of advanced beta-cell failure. Glucagon responses were examined during a 90-min hypoglycemic clamp (approximately 2.8 mmol/l) on two separate occasions. On one occasion, tolbutamide was infused for 2 h before the clamp so that the decrement in insulin secretion during the induction of hypoglycemia would be increased. On the other occasion, normal saline was infused as a control. RESULTS: Before the hypoglycemic clamp, infusion of tolbutamide increased insulin secretion approximately 1.9-fold (P < 0.001). However, during hypoglycemia, insulin secretion decreased to similar rates on both occasions (P = 0.31) so that its decrement was approximately twofold greater following the tolbutamide infusion (1.63 +/- 0.20 vs. 0.81 +/- 0.17 pmol x kg(-1) x min(-1), P < 0.001). This was associated with more than twofold-greater glucagon responses (42 +/- 11 vs. 19 +/- 8 ng/l, P < 0.002) during the hypoglycemic clamp but unaltered glucagon responses to intravenous arginine immediately thereafter (449 +/- 50 vs. 453 +/- 50 ng/l, P = 0.78). CONCLUSIONS: Increasing the decrement in insulin secretion during the development of hypoglycemia improves counterregulatory glucagon responses in advanced beta-cell failure. These findings further support the concept that the impaired counterregulatory glucagon responses in advanced beta-cell failure may at least partially be due to a reduced decrement in insulin secretion.     

Effect of pioglitazone on the metabolic and hormonal response to a mixed meal in type II diabetes

We explored the mechanisms by which a 4-month, placebo-controlled pioglitazone treatment (45 mg/day) improves glycemic control in type II diabetic patients (T2D, n=27) using physiological testing (6-h mixed meal) and a triple tracer technique ([6,6-(2)H(2)]glucose infusion, (2)H(2)O and [6-(3)H]glucose ingestion) to measure endogenous glucose production (EGP), gluconeogenesis (GNG), insulin-mediated glucose clearance and beta-cell glucose sensitivity (by c-peptide modeling). Compared to sex/age/weight-matched non-diabetic controls, T2D patients showed inappropriately (for prevailing insulinemia) raised glucose production (1.05[0.53] vs 0.71[0.36]mmol min(-1) kg(ffm)(-1) pM, P=0.03) because of enhanced GNG (73.1+/-2.4 vs 59.5+/-3.6%, P<0.01) persisting throughout the meal, reduced insulin-mediated glucose clearance (6[5] vs 12[13]ml min(-1) kg(ffm)(-1) nM(-1), P<0.005), and impaired beta-cell glucose-sensitivity (27[38] vs 71[37]pmol min(-1) m(-2) mM(-1), P=0.002). Compared to placebo, pioglitazone improved glucose overproduction (P=0.0001), GNG and glucose underutilization (P=0.05) despite lower insulinemia. GNG improvement was quantitatively related to raised adiponectin. beta-cell glucose sensitivity was unchanged. In mild-to-moderate T2D, pioglitazone monotherapy decreased fasting and post-prandial glycemia, principally via inhibition of gluconeogenesis, improved hepatic and peripheral insulin resistance.     

Additive effects of glucagon-like peptide 1 and pioglitazone in patients with type 2 diabetes

OBJECTIVE: To evaluate the effect of combination therapy with pioglitazone and glucagon-like peptide (GLP)-1 in patients with type 2 diabetes. RESEARCH DESIGN AND METHODS: Eight patients with type 2 diabetes (BMI 32.7 +/- 1.3 kg/m(2) and fasting plasma glucose 13.5 +/- 1.2 mmol/l) underwent four different treatment regimens in random order: saline therapy, monotherapy with continuous subcutaneous infusion of GLP-1 (4.8 pmol x kg(-1) x min(-1)), monotherapy with pioglitazone (30-mg tablet of Actos), and combination therapy with GLP-1 and pioglitazone. The observation period was 48 h. End points were plasma levels of glucose, insulin, glucagon, free fatty acids (FFAs), and sensation of appetite. RESULTS: Fasting plasma glucose decreased from 13.5 +/- 1.2 mmol/l (saline) to 11.7 +/- 1.2 (GLP-1) and 11.5 +/- 1.2 (pioglitazone) and further decreased to 9.9 +/- 1.0 (combination) (P < 0.001). Eight-hour mean plasma glucose levels were reduced from 13.7 +/- 1.1 mmol/l (saline) to 10.6 +/- 1.0 (GLP-1) and 12.0 +/- 1.2 (pioglitazone) and were further reduced to 9.5 +/- 0.8 (combination) (P < 0.0001). Insulin levels increased during monotherapy with GLP-1 compared with monotherapy with pioglitazone (P < 0.01). Glucagon levels were reduced in GLP-1 and combination therapy compared with saline and monotherapy with pioglitazone (P < 0.01). FFAs during breakfast (area under the curve, 0-3 h) were reduced in combination therapy compared with saline (P = 0.03). Sensation of appetite was reduced during monotherapy with GLP-1 and combination therapy (P < 0.05). CONCLUSIONS: GLP-1 and pioglitazone show an additive glucose-lowering effect. A combination of the two agents may, therefore, be a valuable therapeutic approach for the treatment of type 2 diabetes.