Normalization of fasting glycaemia by intravenous GLP-1 ([7-36 amide] or [7-37]) in type 2 diabetic patients

Intravenous GLP-1 [7-36 amide] can normalize fasting hyperglycaemia in Type 2 diabetic patients. Whether GLP-1 [7-37] has similar effects and how quickly plasma glucose concentrations revert to hyperglycaemia after stopping GLP-1 is not known. Therefore, 8 patients with Type 2 diabetes (5 female, 3 male; 65+/-6 years; BMI 34.3+/-7.9 kg m(-2); HbA1c 9.6+/-1.2%; treatment with diet alone (n=2), sulphonylurea (n=5), metformin (n=1)) were examined twice in randomized order. GLP-1 [7-36 amide] or [7-37] (1 pmol kg(-1)min(-1) were infused intravenously over 4 h in fasted subjects. Plasma glucose (glucose-oxidase), insulin and C-peptide (ELISA) was measured during infusion and for 4 h thereafter. Indirect calorimetry was performed. Fasting hyperglycaemia was 11.7+/-0.9 [7-36 amide] and 11.3+/-0.9 mmol l(-1) [7-37]. GLP-1 infusions stimulated insulin secretion approximately 3-fold (insulin peak 168+/-32 and 156+/-47 pmol l(-1), p<0.0001 vs basal; C-peptide peak 2.32+/-0.28 and 2.34+/-0.43 nmol l(-1), p<0.0001, respectively, with GLP-1 [7-36 amide] and [7-37]). Four hours of GLP-1 infusion reduced plasma glucose (4.8+/-0.4 and 4.6+/-0.3 mmol l(-1), p<0.0001 vs basal values), and it remained in the non-diabetic fasting range after a further 4 h (5.1+/-0.4 and 5.3+/-0.4 mmol l(-1), for GLP [7-36 amide] and [7-37], respectively). There were no significant differences between GLP-1 [7-36 amide] and [7-37] (glucose, p=0.99; insulin, p=0.99; C-peptide, p=0.99). Neither glucose oxidation nor lipid oxidation (or any other parameters determined by indirect calorimetry) changed during or after the administration of exogenous GLP-1. In conclusion, GLP-1 [7-36 amide] and [7-37] normalize fasting hyperglycaemia in Type 2 diabetic patients. Diabetes therapy (diet, sulphonyl ureas or metformin) does not appear to influence this effect. In fasting and resting patients, the effect persists during administration of GLP-1 and for at least 4 h thereafter, without rebound. Significant changes in circulating substrate concentrations (e.g. glucose) are not accompanied by changes in intracellular substrate metabolism.

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.

Overnight GLP-1 normalizes fasting but not daytime plasma glucose levels in NIDDM patients

GLP-1 (7-36 amide) normalizes fasting plasma glucose in NIDDM patients. It was the aim to study the effect of overnight intravenous GLP-1 (7-36 amide) on the following 24 h-glucose profiles. Ten NIDDM patients (7 female, 3 male; age 62 +/- 4 y., BMI (Body-Mass-Index) 29.6 +/- 3.9 kg/m2, duration 10 +/- 7 y., HbA1c 10.9 +/- 1.3% (normal 4.0-6.1%), treated with glibenclamide and/or metformin) were studied on two occasions in random order: Either GLP-1 (7-36 amide) (Saxon Biochemicals, Hannover, FRG, 1 pmol x kg(-1) x min(-1)) or placebo (0.9% NaCl with 1% human serum albumin, Behringwerke, Marburg, FRG) were infused intravenously from 22:00 to 7:00 (9 h) and plasma glucose profiles were obtained during the GLP-1 infusion and the following 24 hours. GLP-1 (7-36 amide) (plasma concentration 110 +/- 12 pmol/l) raised plasma C-peptide concentrations (p = 0.0005), suppressed glucagon (p = 0.01) and lowered plasma glucose to 5.5 +/- 0.6 and 6.3 +/- 0.4 mmol/l at 3:00 and 7:00 a.m. (vs. 10.3 +/- 0.9 and 11.3 +/- 0.6 mmol/l, p = 0.0003 and p < 0.0001, respectively, with placebo). Thereafter, starting 1 h after breakfast, no significant differences in plasma glucose, insulin, C-peptide or glucagon profiles were found between experiments with GLP-1 (7-36 amide) and placebo. Average plasma glucose concentrations over the whole 24 h period were reduced by 18% by GLP-1 administered overnight. In conclusion, (1) overnight GLP-1 (7-36 amide) normalizes fasting plasma glucose, but (2) has no sustained effect on meal-induced glucose, insulin or glucagon concentrations once its administration has been stopped. (3) Normalization of fasting plasma glucose alone does not improve daytime metabolic control in NIDDM patients on oral agents.

Prolonged and enhanced secretion of glucagon-like peptide 1 (7-36 amide) after oral sucrose due to alpha-glucosidase inhibition (acarbose) in Type 2 diabetic patients

GLP-1, an incretin hormone of the enteroinsular axis with insulinotropic and glucagonostatic activity, is secreted after nutrient ingestion. GLP-1 is mainly produced by intestinal L-cells in the lower gastrointestinal tract (GIT); simple carbohydrates are absorbed in the upper GIT and alpha-glucosidase inhibition leads to augmented and prolonged GLP-1 release in normal subjects. In a cross-over study, 100 mg acarbose or placebo was administered simultaneously with 100 g sucrose to 11 hyperglycaemic Type 2 diabetic patients poorly controlled with diet and sulphonylureas. Plasma levels of GLP-1, insulin, C-peptide, glugacon, GIP, glucose and H2-exhalation were measured over 6 h. Differences in the integrated responses over the observation period were evaluated by repeated measurement analysis of variance with fasting values used as covariates. With acarbose, sucrose reached the colon 60-90 min after ingestion as indicated by a significant increment in breath hydrogen exhalation (p = 0.005). After an early GLP-1 increment 15 min after sucrose under both conditions, GLP-1 release was prolonged in the acarbose group (p = 0.001; significant from 210 to 360 min.). Initially (0-150 min), glucose (p = 0.001), insulin (p = 0.001), and GIP (p < 0.001) were suppressed by acarbose, whereas later there were no significant differences. Glucagon levels were higher with acarbose in the last 3 h of the 6 h observation period (p = 0.02). We conclude that in hyperglycaemic Type 2 diabetic patients, ingestion of acarbose with a sucrose load leads to elevated and prolonged GLP-1 release.

Relation between gastric emptying of glucose and plasma concentrations of glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) may play a role in regulating gastric emptying. The aim of this study was to determine the relationship between gastric emptying of glucose and plasma concentrations of GLP-1. Gastric emptying of 75 g of glucose dissolved in 350 ml of water was measured by the use of scintigraphy in 12 normal volunteers. Venous blood samples for measurement of GLP-1 were obtained immediately before and for 180 min after ingestion of glucose. Plasma GLP-1 rose rapidly from a baseline of 8.5 +/- 1.2 pmol/l to 14.3 +/- 1.3 pmol/l at 10 min (p = 0.024), with a peak of 19.2 +/- 3.0 pmol/l at 30 min (p = 0.0006) after the glucose drink. The rate of gastric emptying was inversely related to the early rise in GLP-1, e.g., the 50% emptying time was related to the change in GLP-1 from baseline at 10 min (r = 0.57; p < 0.05). We conclude that there is an inverse relationship between gastric emptying of glucose and plasma GLP-1. This observation is consistent with the concept that GLP-1 is a determinant of, rather than determined by, the rate of gastric emptying.

Glucagon-like peptide 1 inhibition of gastric emptying outweighs its insulinotropic effects in healthy humans

Glucagon-like peptide 1 (GLP-1) has been shown to inhibit gastric emptying of liquid meals in type 2 diabetic patients. It was the aim of the present study to compare the action of physiological and pharmacological doses of intravenous GLP-1-(7-36) amide and GLP-1-(7-37) on gastric emptying in normal volunteers. Nine healthy subjects participated (26 +/- 3 yr; body mass index 22.9 +/- 1.6 kg/m2; hemoglobin A1C 5.0 +/- 0.2%) in five experiments on separate occasions after an overnight fast. A nasogastric tube was positioned for the determination of gastric volume by use of a dye-dilution technique (phenol red). GLP-1-(7-36) amide (0.4, 0.8, or 1.2 pmol.kg-1.min-1), GLP-1-(7-37) (1.2 pmol.kg-1.min-1), or placebo was infused intravenously from -30 to 240 min. A liquid meal (50 g sucrose, 8% amino acids, 440 ml, 327 kcal) was administered at 0 min. Glucose, insulin, and C-peptide were measured over 240 min. Gastric emptying was dose dependently slowed by GLP-1-(7-36) amide (P < 0.0001). Effects of GLP-1-(7-37) at 1.2 pmol.kg-1.min-1 were virtually identical. GLP.1 dose dependently stimulated fasting insulin secretion (-30 to 0 min) and slightly reduced glucose concentrations. After the meal (0-240 min), integrated incremental glucose (P < 0.0001) and insulin responses (P = 0.01) were reduced (dose dependently) rather than enhanced. In conclusion, 1) GLP-1-(7-36) amide or -(7-37) inhibits gastric emptying also in normal subjects, 2) physiological doses (0.4 pmol.kg-1.min-1) still have a significant effect, 3) despite the known insulinotropic actions of GLP-1-(7-36) amide and -(7-37), the net effect of administering GLP-1 with a meal is no change or a reduction in meal-related insulin responses. These findings suggest a primarily inhibitory function for GLP-1 (ileal brake mechanisms).

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 HbA1c: 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.

Glucagon-like peptide 1 and its potential in the treatment of non-insulin-dependent diabetes mellitus

Studies examining small groups of type 2-(NIDDM) diabetic patients have shown the potential of glucagon-like peptide 1 (GLP-1) to normalize fasting hyperglycaemia. Patient characteristics determining the size of the effect have not been reported. Therefore, the results of four studies were analysed. Exogenous GLP-1 was administered i.v. or s.c. in 37 type 2-diabetic patients, age 60 +/- 8 years; BMI 28.2 +/- 5.3 kg/m2; HbA1c 10.6 +/- 1.6%; diabetes duration 10 +/- 6 years, treatment with sulfonylureas, n = 33, metformin, n = 11, acarbose, n = 3. Results were analysed using repeated measures analysis of variance and multiple regression analysis. Exogenous GLP-1 lowered fasting plasma glucose within 4-5 h from 12.8 +/- 2.5 to 5.3 +/- 1.3 mmol/l (placebo: 12.8 +/- 2.3 to 10.0 +/- 2.2; p < 0.0001 for the interaction of treatment and time). Only fasting glycaemia (p = 0.0085) and the route (i.v. vs. s.c.; p = 0.05), but not gender, age, BMI, HbA1c, diabetes duration, treatment with sulfonylureas, metformin or acarbose, were significant predictors of the plasma glucose concentrations reached after the administration of GLP-1 (variation: 3.4-8.5 mmol/l). In conclusion, GLP-1 is able to normalize plasma glucose in all type 2-diabetic patients studied. This analysis underlines the great therapeutic potential of GLP-1.

A hyperinsulinaemic, sequentially eu- and hypoglycaemic clamp test to characterize autonomous insulin secretion in patients with insulinoma

To better characterize autonomous insulin secretory behaviour in insulinoma patients and to establish diagnostic criteria with high accuracy, hyper-insulinaemic, sequentially eu- and hypoglycaemic clamp tests were performed in insulinoma patients and control subjects. Ten patients with insulinoma (benign in nine, histologically proven in nine) and 10 patients with suspected episodes of hypoglycaemia, in whom thorough clinical evaluation excluded an insulinoma, were examined. Five insulinoma patients were restudied after successful extirpation of the tumour. Suppression of C-peptide during low-dose [2 pmol kg-1 min-1 (20 mU kg-1 h-1) for 90 min, plasma insulin approximately 120 pmol L-1 (20 mUL-1)] and high-dose [8 pmol kg-1 h-1 (80 mU kg-1 h-1) for 90 min, plasma insulin approximately 450 pmol L-1 (75 mU L-1)] insulin infusion under euglycaemic conditions [plasma glucose 4.4-5.0 mmol L-1 (80-90 mg dL-1)] and during high-dose insulin infusion under hypoglycaemic conditions [glucose 2-2.2 mmol L-1 (40-45 mg dL-1)] was evaluated by radioimmunoassay (RIA). Euglycaemic hyper-insulinaemia suppressed C-peptide in control subjects (P < 0.0001), whereas in insulinoma patients apparently irregular changes in C-peptide concentrations (with spontaneous or paradoxical increments, P = 0.0006 vs. controls) were observed. The combination of hyper-insulinaemia and controlled hypoglycaemia led to a nearly complete suppression of C-peptide in normal subjects (from basal, 0.76 +/- 0.08-0.06 +/- 0.01 nmol L-1; maximum observed value 0.10 nmol L-1), which was more pronounced than at the point of discontinuation of prolonged fasting (> 48 h; 0.26 +/- 0.16 nmol L-1; P = 0.005). In insulinoma patients, C-peptide remained elevated under all conditions (P = 0.51 vs. prolonged fasting). All these findings were reversible after successful surgical removal of the insulinoma. Insulinoma patients could be identified as abnormal by (a) non-suppression of C-peptide even under hyperinsulinaemic/hypoglycaemic conditions (10 out of 10 patients) and (b) irregular increments in C-peptide under conditions that led to at least partial suppression in all normal subjects (9 out of 10 patients) and/or by an apparent shift to the left of insulin secretion relative to glucose concentrations (7 out of 10 patients). Controlled exposure to hyperinsulinaemic/hypoglycaemic conditions can help to characterize autonomous secretion in insulinoma patients and may be used as a diagnostic procedure when conventional methods yield equivocal results.

Glucagon-like peptide 1 (GLP-1) as a new therapeutic approach for type 2-diabetes

Glucagon-like peptide 1 (GLP-1) is a physiological incretin hormone in normal humans explaining in part the augmented insulin response after oral versus intravenous glucose administration. In addition, GLP-1 also lowers glucagon concentrations, slows gastric emptying, stimulates (pro)insulin biosynthesis, reduces food intake upon intracerebroventricular administration in animals, and may, in addition, enhance insulin sensitivity. Therefore, GLP-1, in many aspects, opposes the Type 2-diabetic phenotype characterized by disturbed glucose-induced insulin secretory capacity, hyperglucagonaemia, moderate insulin deficiency, accelerated gastric emptying, overeating (obesity) and insulin resistance. 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 diet- and sulfonylurea-treated Type 2-diabetic patients and also in patients under insulin therapy long after sulfonylurea secondary failure. Exogenous administration of GLP-1 ([7-37] or [7-36 amide]) in doses elevating plasma concentrations to approximately 3-4 fold physiological postprandial levels fully normalizes fasting hyperglycaemia in Type 2-diabetic patients. The half life of GLP-1 is too short to maintain therapeutic plasma levels for sufficient periods by subcutaneous injections. Current research activities aim at finding GLP-1 analogues with more suitable pharmacokinetic properties than the original peptide. Another approach could be the augmentation of endogenous release of GLP-1, which is abundant in L cells of the lower small intestine and the colon. Interference with sucrose digestion using alpha-glucosidase inhibition moves nutrients into distal parts of the gastrointestinal tract and, thereby, prolongs and augments GLP-1 release. Enprostil, a prostaglandin E2 analogue, fully suppresses GIP responses, while only marginally affecting insulin secretion and glucose tolerance after oral glucose, suggesting compensatory hypersecretion of additional insulinotropic peptides, possibly including GLP-1. Given the large amount of GLP-1 present in L cells, it appears worthwhile to look for more agents that could 'mobilize' this endogenous pool of the 'antidiabetogenic' gut hormone GLP-1.

Effects of subcutaneous glucagon-like peptide 1 (GLP-1 [7-36 amide]) in patients with NIDDM

Intravenous glucagon-like peptide (GLP)-1 [7-36 amide] can normalize plasma glucose in non-insulin-dependent diabetic (NIDDM) patients. Since this is no form for routine therapeutic administration, effects of subcutaneous GLP-1 at a high dose (1.5 nmol/kg body weight) were examined. Three groups of 8, 9 and 7 patients (61 +/- 7, 61 +/- 9, 50 +/- 11 years; BMI 29.5 +/- 2.5, 26.1 +/- 2.3, 28.0 +/- 4.2 kg/m2; HbA1c 11.3 +/- 1.5, 9.9 +/- 1.0, 10.6 +/- 0.7%) were examined: after a single subcutaneous injection of 1.5 nmol/kg GLP [7-36 amide]; after repeated subcutaneous injections (0 and 120 min) in fasting patients; after a single, subcutaneous injection 30 min before a liquid test meal (amino acids 8%, and sucrose 50 g in 400 ml), all compared with a placebo. Glucose (glucose oxidase), insulin, C-peptide, GLP-1 and glucagon (specific immunoassays) were measured. Gastric emptying was assessed with the indicator-dilution method and phenol red. Repeated measures ANOVA was used for statistical analysis. GLP-1 injection led to a short-lived increment in GLP-1 concentrations (peak at 30-60 min, then return to basal levels after 90-120 min). Each GLP-1 injection stimulated insulin (insulin, C-peptide, p < 0.0001, respectively) and inhibited glucagon secretion (p < 0.0001). In fasting patients the repeated administration of GLP-1 normalized plasma glucose (5.8 +/- 0.4 mmol/l after 240 min vs 8.2 +/- 0.7 mmol/l after a single dose, p = 0.0065). With the meal, subcutaneous GLP-1 led to a complete cessation of gastric emptying for 30-45 min (p < 0.0001 statistically different from placebo) followed by emptying at a normal rate. As a consequence, integrated incremental glucose responses were reduced by 40% (p = 0.051). In conclusion, subcutaneous GLP-1 [7-36 amide] has similar effects in NIDDM patients as an intravenous infusion. Preparations with retarded release of GLP-1 would appear more suitable for therapeutic purposes because elevation of GLP-1 concentrations for 4 rather than 2 h (repeated doses) normalized fasting plasma glucose better. In the short term, there appears to be no tachyphylaxis, since insulin stimulation and glucagon suppression were similar upon repeated administrations of GLP-1 [7-36 amide]. It may be easier to influence fasting hyperglycaemia by GLP-1 than to reduce meal-related increments in glycaemia.

Comparison of hyperinsulinaemic clamp experiments using venous, 'arterialized' venous or capillary euglycaemia

It has been suggested that deviations from arterial euglycaemia during hyperinsulinaemic clamp experiments that use venous plasma glucose measurements may alter the results of such tests (glucose infusion rate, C-peptide suppression, etc.) and that 'arterialized' venous blood ('heated-hand' technique) may be suitable to circumvent these problems. Therefore, nine normal male fasting volunteers (age 25 +/- 4 years, body mass index 23.5 +/- 2.3 kg m-2) were examined three times using an insulin infusion of 1 mU kg min-1 over 120 min. Glucose was infused to maintain a concentration of 4.7 mmol I-1 (85 mg dl-1) in venous (V), 'arterialized' venous ('heated-hand' technique; HH), or capillary (C) plasma. The 'heated-hand' technique caused a rise in (rectal) body temperature of 0.3 +/- 0.1 degree C (P < 0.0001). Whereas the glucose aim was reached to a similar degree in all experiments (P = 0.36), capillary glucose concentrations differed slightly, but significantly (higher in experiments with venous and 'arterialized' venous blood specimens; P = 0.034). There were no significant differences regarding steady-state insulin concentrations (P = 0.77), glucose infusion rates (V, 7.1 +/- 0.5; HH, 7.2 +/- 0.6; C, 6.4 +/- 0.5 mg kg-1 min-1; P = 0.98), C-peptide suppression (P = 0.78), reduction in glucagon (P = 0.27) and free fatty acids (P = 0.16), and all parameters of indirect calorimetry (non-protein RQ: P = 0.67; glucose and lipid oxidation: P = 0.72 and 0.46 respectively; and energy expenditure: P = 0.42). Therefore, hyperinsulinaemic clamp experiments performed using venous, 'arterialized' venous, or capillary euglycaemia appear to be almost equally useful for the determination of insulin sensitivity and C-peptide or glucagon suppression. The elevation in body temperature that accompanies use of the 'heated-hand' technique does not noticeably influence measured metabolic parameters.

Glucagonostatic actions and reduction of fasting hyperglycemia by exogenous glucagon-like peptide I(7-36) amide in type I diabetic patients

OBJECTIVE

Glucagon-like peptide I(7-36) amide (GLP-1) is a physiological incretin hormone that, in slightly supraphysiological doses, stimulates insulin secretion, lowers glucagon concentrations, and thereby normalizes elevated fasting plasma glucose concentrations in type II diabetic patients. It is not known whether GLP-1 has effects also in fasting type I diabetic patients.

RESEARCH DESIGN AND METHODS

In 11 type I diabetic patients (HbA1c 9.1 +/- 2.1%; normal, 4.2-6.3%), fasting hyperglycemia was provoked by halving their usual evening NPH insulin dose. In random order on two occasions, 1.2 pmol . kg-1 . min-1 GLP-1 or placebo was infused intravenously in the morning (plasma glucose 13.7 +/- 0.9 mmol/l; plasma insulin 26 +/- 4 pmol/l). Glucose (glucose oxidase method), insulin, C-peptide, glucagon, GLP-1, cortisol, growth hormone (immunoassays), triglycerides, cholesterol, and nonesterified fatty acids (enzymatic tests) were measured.

RESULTS

Glucagon was reduced from approximately 8 to 4 pmol/l, and plasma glucose was lowered from 13.4 +/- 1.0 to 10.0 +/- 1.2 mmol/l with GLP-1 administration (plasma concentrations approximately 100 pmol, P < 0.0001), but not with placebo (14.2 +/- 0.7 to 13.2 +/- 1.0). Transiently, C-peptide was stimulated from basal 0.09 +/- 0.02 to 0.19 +/- 0.06 nmol/l by GLP-1 (P < 0.0001), but not by placebo (0.07 +/- 0.02 to 0.07 +/- 0.02). There was no significant effect on nonesterified fatty acids (P = 0.34), triglycerides (P = 0.57), cholesterol (P = 0.64), cortisol (P = 0.40), or growth hormone (P = 0.53).

CONCLUSIONS

Therefore, exogenous GLP-1 is able to lower fasting glycemia also in type I diabetic patients, mainly by reducing glucagon concentrations. However, this alone is not sufficient to normalize fasting plasma glucose concentrations, as was previously observed in type II diabetic patients, in whom insulin secretion (C-peptide response) was stimulated 20-fold.

Determinants of a normal (versus impaired) oral glucose tolerance after combined pancreas-kidney transplantation in IDDM patients

After successful pancreas transplantation, insulin-dependent diabetic patients are characterized by a normal or at worst impaired oral glucose tolerance (World Health Organisation criteria). It is not known which pathophysiological mechanisms cause the difference between normal and impaired oral glucose tolerance. Therefore, we studied 41 patients after successful combined pancreas-kidney transplantation using stimulation in the fasting state with oral glucose (75 g), intravenous glucose (0.33 g/kg) and glucagon bolus injection (1 mg i.v.). Glucose (glucose oxidase), insulin and C-peptide (immunoassay) were measured. Repeated-measures analysis of variance and multiple regression analysis were used to analyse the results which showed: 28 patients had a normal, and 13 patients had an impaired oral glucose tolerance. Impaired oral glucose tolerance was associated with a greatly reduced early phase insulin secretory response (insulin p < 0.0001; C-peptide p = 0.037). Age (p = 0.65), body mass index (p = 0.94), immunosuppressive therapy (cyclosporin A p = 0.84; predniso(lo)ne p = 0.91; azathioprine p = 0.60) and additional clinical parameters were not different. Reduced insulin secretory responses in patients with impaired oral glucose tolerance were also found with intravenous glucose or glucagon stimulations. Exocrine secretion (alpha-amylase in 24-h urine collections) also demonstrated reduced pancreatic function in these patients (-46%; p = 0.04). Multiple regression analysis showed a significant correlation of 120-min glucose with ischaemia time (p = 0.003) and the number of HLA-DR mismatches (p = 0.026), but not with HLA-AB-mismatches (p = 0.084). In conclusion, the pathophysiological basis of impaired oral glucose tolerance after pancreas transplantation is a reduced insulin secretory capacity. Transplant damage is most likely caused by perioperative influences (ischaemia) and by the extent of rejection damage related, for example, to DR-mis-matches.

Hyperproinsulinemia in a three-generation Caucasian family due to mutant proinsulin (Arg65-His) not associated with imparied glucose tolerance: the contribution of mutant proinsulin to insulin bioactivity

Familial hyperproinsulinemia is a genetic abnormality characterized by an increased proportion of proinsulin immunoreactivity in the circulation due to mutations affecting the posttranslational processing of proinsulin. In affected Japanese families, this has been associated with noninsulin-dependent diabetes mellitus or impaired glucose tolerance. A three-generation Caucasian family with hyperproinsulinemia was identified through unexplained hyperinsulinemia in a normal volunteer participating in a metabolic study. High pressure liquid chromatography analysis of fasting plasma revealed a major peak eluting close to the position of proinsulin. Direct sequencing of the proinsulin gene exon 3 showed a heterozygous point mutation (CGT-->CAT) resulting in the substitution of Arg-->His in position 65 (corresponding to the AC cleavage site) in the index case, his mother, and his maternal grandmother. Using specific enzyme-linked immunosorbent assay methods to quantify insulin and proinsulin (including its conversion intermediates), the impact of this mutation on B cell secretion and glucose tolerance was studied. All affected subjects had normal oral glucose tolerance. In the basal state and after oral glucose administration, their proinsulin responses were immense, but intact insulin responses were slightly reduced. However, when calculating insulin bioactivity by assuming 9% activity for mutant Arg65-->His proinsulin, responses in affected subjects were comparable to those in normal subjects. In conclusion, our data demonstrate hyperproinsulinemia in a three-generation Caucasian family due to heterozygous mutant Arg65-->His proinsulin. This was not associated with impaired glucose tolerance. These results suggest that this mutation in the heterozygous state per se does not affect glucose tolerance and that the biological activity of mutant proinsulin contributes to glucose homeostasis in this family. The association of the same mutation with impaired glucose tolerance or diabetes in previous studies may be the result of selection bias or associated conditions (e.g. the genetic background of the kindreds examined).

Release of glucagon-like peptide 1 (GLP-1 [7-36 amide]), gastric inhibitory polypeptide (GIP) and insulin in response to oral glucose after upper and lower intestinal resections

Glucagon-like peptide 1 (GLP-1[7-36 amide]) is an incretin hormone primarily synthesized in the lower gut (ileum, colon/rectum). Nevertheless, there is an early increment in plasma GLP-1 immediately after ingesting glucose or mixed meals, before nutrients have entered GLP-1 rich intestinal regions. The responsible signalling pathway between the upper and lower gut is not clear. It was the aim of this study to see, whether small intestinal resection or colonectomy changes GLP-1[7-36 amide] release after oral glucose. In eight healthy controls, in seven patients with inactive Crohn's disease (no surgery), in nine patients each after primarily jejunal or ileal small intestinal resections, and in six colonectomized patients not different in age (p = 0.10), body-mass-index (p = 0.24), waist-hip-ratio (p = 0.43), and HbA1c (p = 0.22), oral glucose tolerance tests (75 g) were performed in the fasting state. GLP-1[7-36 amide], insulin C-peptide, GIP and glucagon (specific (RIAs) were measured over 240 min.

STATISTICS

Repeated measures ANOVA, t-test (significance: p < 0.05). A clear and early (peak: 15-30 min) GLP-1[7-36 amide] response was observed in all subjects, without any significant difference between gut-resected and control groups (p = 0.95). There were no significant differences in oral glucose tolerance (p = 0.21) or in the suppression of pancreatic glucagon (p = 0.36). Colonectomized patients had a higher insulin (p = 0.011) and C-peptide (p = 0.0023) response in comparison to all other groups. GIP responses also were higher in the colonectomized patients (p = 0.0005). Inactive Crohn's disease and resections of the small intestine as well as proctocolectomy did not change overall GLP-1[7-36 amide] responses and especially not the early increment after oral glucose. This may indicate release of GLP-1[7-36 amide] after oral glucose from the small number of GLP-1[7-36 amide] producing L-cells in the upper gut rather than from the main source in the ileum, colon and rectum. Colonectomized patients are characterized by insulin hypersecretion, which in combination with their normal oral glucose tolerance possibly indicates a reduced insulin sensitivity in this patient group. GIP may play a role in mediating insulin hypersecretion in these patients.

Gastric emptying, glucose responses, and insulin secretion after a liquid test meal: effects of exogenous glucagon-like peptide-1 (GLP-1)-(7-36) amide in type 2 (noninsulin-dependent) diabetic patients

The aim of the study was to investigate whether inhibition of gastric emptying of meals plays a role in the mechanism of the blood glucose-lowering action of glucagon-like peptide-1-(7-36) amide [GLP-1-(7-36) amide] in type 2 diabetes. Eight poorly controlled type 2 diabetic patients (age, 58 +/- 6 yr; body mass index, 30.0 +/- 5.2 kg/m2; hemoglobin A1c, 10.5 +/- 1.2%) were studied in the fasting state (plasma glucose, 11.1 +/- 1.1 mmol/L). A liquid meal of 400 mL containing 8% amino acids and 50 g sucrose (327 Kcal) was administered at time zero by a nasogastric tube. Gastric volume was determined by a dye dilution technique using phenol red. In randomized order, GLP-1-(7-36) amide (1.2 pmol/kg.min; Saxon Biochemicals) or placebo (0.9% NaCl with 1% human serum albumin) was infused between -30 and 240 min. In the control experiment, gastric emptying was completed within 120 min, and plasma glucose, insulin, C-peptide, GLP-1-(7-36) amide, and glucagon concentrations transiently increased. With exogenous GLP-1-(7-36) amide (plasma level, approximately 70 pmol/L), gastric volume remained constant over the period it was measured (120 min; P < 0.0001 vs. placebo), and plasma glucose fell to normal fasting values (5.4 +/- 0.7 mmol/L) within 3-4 h, whereas insulin was stimulated in most, but not all, patients, and glucagon remained at the basal level or was slightly suppressed. In conclusion, GLP-1-(7-36) amide inhibits gastric emptying in type 2 diabetic patients. Together with the stimulation of insulin and the inhibition of glucagon secretion, this effect probably contributes to the blood glucose-lowering action of GLP-1-(7-36) amide in type 2-diabetic patients when studied after meal ingestion. At the degree observed, inhibition of gastric emptying, however, must be overcome by tachyphylaxis, reduction in dose, or pharmacological interventions so as not to interfere with the therapeutic use of GLP-1-(7-36) amide in type 2 diabetic patients.

Glucagon-like peptide 1 (7-36 amide) secretion in response to luminal sucrose from the upper and lower gut. A study using alpha-glucosidase inhibition (acarbose)

BACKGROUND

After nutrient ingestion there is an early response of glucagon-like peptide 1 (GLP-1) immunoreactivity, although GLP-1 is mainly produced in endocrine cells from the lower gut (ileum and colon/rectum), suggesting that indirect stimulation is important after the ingestion of carbohydrates that are predominantly absorbed from the upper intestine.

METHODS

To enable contact of sucrose with lower gut mucosa, sucrose was administered by mouth with and without the simultaneous ingestion of 100 mg of the alpha-glucosidase inhibitor acarbose to six normal healthy volunteers.

RESULTS

There was an early increment in GLP-1 15 min after sucrose ingestion. With acarbose, sucrose reached the colon approximately 120 min after ingestion, as indicated by an increment in breath hydrogen exhalation (p < 0.0001), and GLP-1 release was prolonged (p < 0.0001). The sucrose-related increments in glucose, insulin, C-peptide, and gastric inhibitory polypeptide (GIP) and the suppression of glucagon were only marginally affected by acarbose administration.

CONCLUSIONS

GLP-1 release appears to be influenced by indirect mechanisms (early response after sucrose) and by direct luminal contact with lower gut mucosal endocrine cells (late response with acarbose).

Both subcutaneously and intravenously administered glucagon-like peptide I are rapidly degraded from the NH2-terminus in type II diabetic patients and in healthy subjects

To fate of exogenous glucagon-like peptide I (GLP-I)(7-36) amide was studied in nondiabetic and type II diabetic subjects using a combination of high-pressure liquid chromatography (HPLC), specific radioimmunoassays (RIAs), and a sensitive enzyme-linked immunosorbent assay (ELISA), whereby intact biologically active GLP-I and its metabolites could be determined. After GLP-I administration, the intact peptide could be measured using an NH2-terminally directed RIA or ELISA, while the difference in concentration between these assays and a COOH-terminal-specific RIA allowed determination of NH2-terminally truncated metabolites. Subcutaneous GLP-I was rapidly degraded in a time-dependent manner, forming a metabolite, which co-eluted on HPLC with GLP-I(9-36) amide and had the same immunoreactive profile. Thirty minutes after subcutaneous GLP-I administration to diabetic patients (n = 8), the metabolite accounted for 88.5 +/- 1.9% of the increase in plasma immunoreactivity determined by the COOH-terminal RIA, which was higher than the levels measured in healthy subjects (78.4 +/- 3.2%; n = 8; P < 0.05). Intravenously infused GLP-I was also extensively degraded, but no significant differences were seen between the two groups. Intact GLP-I accounted for only 19.9 +/- 3.4% of the increase in immunoreactivity measured with the COOH-terminal RIA in normal subjects (n = 8), and 25.0 +/- 4.8% of the increase in diabetic subjects (n = 8), the remainder being the NH2-terminally truncated metabolite.

Pharmacokinetic, insulinotropic, and glucagonostatic properties of GLP-1 [7-36 amide] after subcutaneous injection in healthy volunteers. Dose-response-relationships

Intravenous infusions of glucagon-like peptide 1 (GLP-1) [7-36 amide] are glucose-dependently insulinotropic and glucagonostatic and normalize plasma glucose concentrations in non-insulin-dependent diabetic patients. It was the aim of this study to investigate whether subcutaneous GLP-1 [7-36 amide] also has an influence on insulin and glucagon secretion, and which doses are required for significant effects. Therefore, eight healthy volunteers (24 +/- 2 years, body mass index [BMI] 21.9 +/- 2.3 kg/m2) were studied in the fasting state on five occasions in randomized order. Placebo (0.9% NaCl with 1% human serum albumin) or GLP-1 [7-36 amide] in doses of 0.15, 0.5, 1.5 or 4.5 nmol/kg body weight (volume 1 ml or, at the highest dose, 2 ml) was administered subcutaneously. An intravenous glucose bolus (0.33 g/kg body weight) was injected 30 min later. Blood was drawn for the measurement of glucose, insulin, C-peptide, GLP-1 [7-36 amide], and glucagon using specific radioimmunoassays. There were dose-related increments in GLP-1 [7-36 amide] concentrations (p < 0.0001). However, basal values were reached again after 90-120 min. Before glucose administration, insulin (p < 0.0001) and C-peptide (p < 0.0004) increased, whereas glucagon (p = 0.0018) and glucose (p < 0.0001) decreased in a dose-dependent manner. After glucose stimulation, integrated increments in insulin (p = 0.0007) and C-peptide (p = 0.02) were augmented and kG-values increased (p < 0.0001) in a dose-related fashion. The extent of reactive hypoglycaemia was related to the GLP-1 [7-36 amide] dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological augmentation of amino acid-induced insulin secretion by GIP and GLP-I but not by CCK-8

It was the aim of this study to test insulinotropic actions of cholecystokinin octapeptide (CCK-8), gastric inhibitory polypeptide (GIP), and glucagon-like peptide I (GLP-I)-(7--36) amide at basal glucose but physiologically elevated amino acid concentrations. Therefore, in nine fasting healthy volunteers, an amino acid mixture was infused intravenously (12.6 g/h over 120 min). On separate occasions, from 30 to 120 min, placebo (0.9% NaCl-1% human serum albumin), synthetic sulfated CCK-8 (0.5 pmol.kg-1.min-1), human GIP (1 pmol.kg-1.min-1), or GLP-I-(7--36) amide (0.3 pmol.kg-1.min-1) was infused intravenously to mimic physiological increments after a meal. The amino acid infusion lead to a small increment in plasma glucose from 4.8 +/- 0.2 to 5.0 +/- 0.2 mmol/l and significantly elevated insulin and C-peptide concentrations. GIP and GLP-I-(7--36) amide further stimulated insulin (1.8-fold, P = 0.0001 and 0.004, respectively) and C-peptide (1.3-fold, P = 0.0003 and 0.013, respectively), with a subsequent slight reduction in plasma glucose (P < 0.0001). Insulin and C-peptide then decreased again in parallel. CCK-8 was without effect on insulin and C-peptide levels. In conclusion, GIP and GLP-I-(7--36) amide are not only able to interact with elevated plasma glucose but are insulinotropic also with physiologically raised amino acid concentrations. Such an interaction could play a role after the ingestion of mixed meals. Cholecystokinin, on the other hand, is not a physiological incretin also under these conditions.