Synergistic effect of portal glucose and glucagon-like peptide-1 to lower systemic glucose and stimulate counter-regulatory hormones.

AIMS: Glucagon-like peptide-1 (GLP-1) is an insulinotropic hormone released from the gut in response to nutrients. Besides its well-established direct effect on pancreatic beta cells, GLP-1 may also act by activating sensors in the hepatoportal area. We therefore studied the impact of putative GLP-1 sensors in the splanchnic circulation. METHODS: We infused GLP-1 into the portal vein of conscious dogs, while also infusing glucose intraportally or systemically. In the first experiment, we infused glucose intraportally, simulating portal glucose values obtained during a previous mixed-meal test, with or without co-infusion of intraportal GLP-1. In the second experiment, by infusing glucose systemically, with or without intraportal GLP-1, we investigated whether the effects of systemic glucose with or without portal GLP-1 infusion are similar to those observed in the first experiment. RESULTS: Intraportal infusion of GLP-1 and glucose significantly raised peripheral GLP-1 levels, but did not produce an insulin response different from intraportal glucose alone. However, the resulting peripheral glycaemia was significantly lower compared to glucose infusion alone, and there were elevations in glucagon, cortisol and lactate. In contrast to the portal glucose infusions, there were no significant differences in glucose, insulin, glucagon, cortisol or lactate levels between systemic glucose infusion with or without GLP-1. CONCLUSIONS/INTERPRETATION: Portal GLP-1 and portal glucose, but not systemic glucose, can produce decreased peripheral glucose levels independently of hyperinsulinaemia. This suggests that portal GLP-1 and glucose receptors mediate insulin-independent changes in peripheral glycaemia and determine a strong counter-regulatory response, as reflected by changes in glucagon and cortisol.

alpha-Glucosidase inhibition (acarbose) fails to enhance secretion of glucagon-like peptide 1 (7-36 amide) and to delay gastric emptying in Type 2 diabetic patients.

AIM: Acarbose is able to enhance GLP-1 release and delay gastric emptying in normal subjects. The effect of alpha-glucosidase inhibition on GLP-1 has been less evident in Type 2 diabetic patients. The aim of this study was to investigate the possible influence of acarbose on GLP-1 release and gastric emptying in Type 2 diabetic patients after a mixed test meal. PATIENTS AND METHODS: Ten Type 2 diabetic patients were tested with 100 mg acarbose or placebo served with a mixed meal that was labelled with 100 mg 13C-octanoic acid. Plasma concentrations of glucose, insulin, C-peptide, glucagon, GLP-1 and GIP were determined over 6 h. Gastric emptying was measured by determining breath 13CO2 using infrared absorptiometry. Statistics repeated-measures anova. RESULTS: Gastric emptying rates (t1/2: 162 +/- 45 vs. 163 +/- 62 min, P = 0.65) and plasma concentrations (increasing from approximately 12 to approximately 25 pmol/l, P = 0.37) and integrated responses of GLP-1 (P = 0.37) were not changed significantly by acarbose treatment. Postprandial plasma glucose concentrations (P < 0.0001) and their integrated responses were lowered by acarbose (by 64%; P = 0.016). The plasma concentrations of insulin and C-peptide were reduced (P = 0.007 and 0.057, respectively) by acarbose, while glucagon was not changed (P = 0.96). GIP plasma concentrations (increasing with placebo from approximately 10 to approximately 85 pmol/l and with acarbose to approximately 55 pmol/l (P < 0.0001) and their integrated responses were significantly lowered (by 43%) by acarbose (P = 0.021). After 2 weeks of acarbose treatment (50 mg t.i.d. for the first and 100 mg t.i.d. for the second week, n = 6), similar results were found. CONCLUSIONS: In hyperglycaemic Type 2 diabetic patients, ingestion of acarbose with a mixed test meal failed to enhance GLP-1 release and did not influence gastric emptying.

Absence of a memory effect for the insulinotropic action of glucagon-like peptide 1 (GLP-1) in healthy volunteers.

BACKGROUND/AIMS: The term memory effect refers to the phenomenon that B cell stimuli retain some of their insulinotropic effects after they have been removed. Memory effects exist for glucose and sulfonylureas. It is not known whether there is a B-cell memory for incretin hormones such as GLP-1. SUBJECTS/METHODS: Eight healthy young volunteers were studied on four occasions in the fasting state. In one experiment, placebo was administered (a). in three more experiments (random order), synthetic GLP-1 (7 - 36 amide) at 1.2 pmol/kg/min was administered over a period of three hours. At 0 min, a bolus of glucose was injected intravenously (0.33 g/kg body weight). GLP-1 was infused from (b). - 60 to 120 min, (c). - 210 to - 30 min, or (d). - 300 to - 120 min. Glucose (glucose oxidase), insulin, C-peptide, GLP-1, and glucagon (immunoassays) were determined. Statistical analysis was carried out by ANOVA and appropriate post hoc tests. RESULTS: GLP-1 plasma levels during the infusion periods were elevated to 89 +/- 9, 85 +/- 13, and 89 +/- 6 pmol/l (p < 0.0001 vs. placebo, 10 +/- 1 pmol/l). Glucose was eliminated faster (p < 0.0001), with an enhanced negative rebound (p = 0.014), and insulin and C-peptide increments were greater after intravenous glucose administration (p < 0.0001) if GLP-1 was administered during the injection of the glucose bolus, but not if GLP-1 had been administered until 120 or 30 min before the glucose load. There was a trend towards higher insulin concentrations (p = 0.056) five minutes after glucose with GLP-1 administered until - 30 min before the glucose load. Glucagon was suppressed by exogenous glucose, but increased significantly (p = 0.013) during the induction of reactive hypoglycemia after glucose injection during GLP-1 administration. CONCLUSION: 1). No memory effect appears to exist for insulinotropic actions of GLP-1, in line with clinical data. 2). Reactive hypoglycemia causes a prompt rise in glucagon despite pharmacological circulating concentrations of GLP-1. 3). Similar studies should be performed in Type 2-diabetic patients, because exposure to GLP-1 might recruit dormant pancreatic B cells to become glucose-competent, and this might contribute to the overall antidiabetogenic effect of GLP-1 in such patients.

[Differences in insulin secretion facilitate the differential diagnosis of insulinoma and factitious hypoglycaemia]. / Unterschiede im Insulin-Sekretionsverhalten erleichtern die Differentialdiagnose von Insulinom und Hypoglycaemia factitia.

HISTORY: A 33-year-old female nurse (married; two children; BMI 30.9 kg/m2) had recurrent episodes of symptomatic hypoglycaemia over some months. INVESTIGATIONS: Two fasting tests were terminated after 26 hours because the patient became unconscious. Improved insulin/glucose ratio was infinity and 6.1 [mU/l]/[mg/dl] (normal value < 0.5). An hyperinsulinaemic-hypoglycaemic angle "clamp test" produced a C-peptide suppression to minimally 0.26 - 0.38 nmol/l (normal value 0.06 +/- 0.01 nmol). There was no spontaneous or paradoxical burst in insulin or C-peptide concentration after either the fasting or the "clamp test". Serum analysis of sulphonylurea on several occasions documented an increase of glibenclamide above therapeutic range. TREATMENT AND COURSE: The patient denied any intake of oral antidiabetic preparations, but there were no further hypoglycaemia attacks in subsequent months. DIAGNOSIS: The demonstration of sulphonylurea in serum confirmed the diagnosis of factitious hypoglycaemia. CONCLUSION: With regard to insulin or C-peptide suppression, the results of the fasting and clamp tests are the same in factitious hypoglycaemia and insulinoma. However, under the influence of sulphonylurea drugs there are no insulin or C-peptide bursts so typical of insulinoma. In case of doubt, detection of sulphonylurea preparations in serum or urine is the only reliable way of diagnosing factitious hypoglaema due to the ingestion of sulphonylurea.

Reduced insulinotropic effect of gastric inhibitory polypeptide in first-degree relatives of patients with type 2 diabetes.

In patients with type 2 diabetes, gastric inhibitory polypeptide (GIP) has lost much of its insulinotropic activity. Whether this is similar in first-degree relatives of patients with type 2 diabetes is unknown. A total of 21 first-degree relatives, 10 patients with type 2 diabetes, and 10 control subjects (normal oral glucose tolerance) were examined. During a hyperglycemic "clamp" (140 mg/dl for 120 min), synthetic human GIP (2 pmol. kg(-1). min(-1)) was infused intravenously (30-90 min). With exogenous GIP, patients with type 2 diabetes responded with a lower increment (Delta) in insulin (P = 0.0003) and C-peptide concentrations (P < 0.0001) than control subjects. The GIP effects in first-degree relatives were diminished compared with control subjects (Delta insulin: P = 0.04; Delta C-peptide: P = 0.016) but significantly higher than in patients with type 2 diabetes (P < or = 0.05). The responses over the time course were below the 95% CI derived from control subjects in 7 (insulin) and 11 (C-peptide) of 21 first-degree relatives of patients with type 2 diabetes. In conclusion, a reduced insulinotropic activity of GIP is typical for a substantial subgroup of normoglycemic first-degree relatives of patients with type 2 diabetes, pointing to an early, possibly genetic defect.

Degradation of endogenous and exogenous gastric inhibitory polypeptide in healthy and in type 2 diabetic subjects as revealed using a new assay for the intact peptide.

Gastric inhibitory polypeptide (GIP) is susceptible to degradation, but only recently has dipeptidyl peptidase IV been identified as the enzyme responsible. Most RIAs recognize both intact GIP-(1-42) and the noninsulinotropic N-terminally truncated metabolite, GIP-(3-42), hampering measurement of plasma concentrations. The molecular nature of GIP was examined using high pressure liquid chromatography and a newly developed RIA specific for the intact N-terminus of human GIP. In healthy subjects after a mixed meal, intact GIP (N-terminal RIA) accounted for 37.0+/-2.5% of the total immunoreactivity determined by C-terminal assay. High pressure liquid chromatographic analysis of fasting samples by C-terminal assay revealed one major peak (73.8+/-2.9%) coeluting with GIP-(3-42). One hour postprandially, two major peaks were detected, corresponding to GIP-(3-42) and GIP-(1-42) (58.1+/-2.7% and 35.7+/-4.2%, respectively). GIP-(3-42) was not detected by N-terminal assay; the major peak coeluted with intact GIP (86.4+/-5.8% and 81.3+/-0.9%, 0 and 1 h, respectively). After iv infusion, intact GIP constituted 37.1+/-4.1% and 41.3+/-3.4% of the total immunoreactivity in healthy and type 2 diabetic subjects, respectively. The plasma t1/2 was shorter (P < 0.0001) when determined by N-terminal compared with C-terminal assay (7.3+/-1.0 vs. 16.8+/-1.6 and 5.2+/-0.6 vs. 12.9+/-0.9 min, healthy and diabetic subjects, respectively), and both t1/2 were shorter in the diabetic group (P < 0.05). We conclude that dipeptidyl peptidase IV is important in GIP metabolism in humans in vivo, and that an N-terminally directed assay is required for determination of plasma concentrations of biologically active GIP.