Insulin and C-peptide secretory responses to glucagon in man: studies on the dose-response relationships

Pre-Meal Whey Protein Alters Postprandial Insulinemia by Enhancing β-Cell Function and Reducing Insulin Clearance in T2D

CONTEXT

Treatments that reduce postprandial glycemia (PPG) independent of stimulating insulin secretion are appealing for the management of type 2 diabetes (T2D). Consuming pre-meal whey protein (WP) reduces PPG by delaying gastric emptying and increasing plasma insulin concentrations. However, its effects on β-cell function and insulin kinetics remains unclear.

OBJECTIVE

To examine the PPG-regulatory effects of pre-meal WP by modeling insulin secretion rates (ISR), insulin clearance, and β-cell function.

METHODS

This was a single-blind, randomized, placebo-controlled, crossover design study in 18 adults with T2D (HbA1c, 56.7 ± 8.8 mmol/mol) who underwent 2 240-minute mixed-meal tolerance tests. Participants consumed WP (15 g protein) or placebo (0 g protein) 10 minutes before a mixed-macronutrient breakfast meal. PPG, pancreatic islet, and incretin hormones were measured throughout. ISR was calculated by C-peptide deconvolution. Estimates of insulin clearance and β-cell function were modeled from glucose, insulin, and ISR. Changes in PPG incremental area under the curve (iAUC; prespecified) and insulin clearance (post hoc) were measured.

RESULTS

β-cell function was 40% greater after WP (P = .001) and was accompanied with a -22% reduction in postprandial insulin clearance vs placebo (P < .0001). Both the peak change and PPG iAUC were reduced by WP (-1.5 mmol/L and -16%, respectively; both P < .05). Pre-meal WP augmented a 5.9-fold increase in glucagon and glucagon-like peptide 1 iAUC (both P < .0001), and a 1.5-fold increase in insulin iAUC (P < .001). Although the plasma insulin response was greater following WP, ISR was unaffected (P = .133).

CONCLUSION

In adults with T2D, pre-meal WP reduced PPG by coordinating an enhancement in β-cell function with a reduction in insulin clearance. This enabled an efficient postprandial insulinemic profile to be achieved without requiring further β-cell stimulation.Trial registry ISRCTN ID: ISRCTN17563146 Website link: www.isrctn.com/ISRCTN17563146.

The mediation by GLP-1 receptors of glucagon-induced insulin secretion revisited in GLP-1 receptor knockout mice

To study whether activation of GLP-1 receptors importantly contributes to the insulinotropic action of exogenously administered glucagon, we have performed whole animal experiments in normal mice and in mice with GLP-1 receptor knockout. Glucagon (1, 3 or 10 μg/kg), the GLP-1 receptor antagonist exendin 9-39 (30 nmol/kg), glucose (0.35 g/kg) or the incretin hormone glucose-dependent insulinotropic polypeptide (GIP; 3 nmol/kg) was injected intravenously or glucose (75 mg) was given orally through gavage. Furthermore, islets were isolated and incubated in the presence of glucose with or without glucagon. It was found that the insulin response to intravenous glucagon was preserved in GLP-1 receptor knockout mice but that glucagon-induced insulin secretion was markedly suppressed in islets from GLP-1 receptor knockout mice. Similarly, the GLP-1 receptor antagonist markedly suppressed glucagon-induced insulin secretion in wildtype mice. These data suggest that GLP-1 receptors contribute to the insulinotropic action of glucagon and that there is a compensatory mechanism in GLP-1 receptor knockout mice that counteracts a reduced effect of glucagon. Two potential compensatory mechanisms (glucose and GIP) were explored. However, neither of these seemed to explain why the insulin response to glucagon is not suppressed in GLP-1 receptor knockout mice. Based on these data we confirm the hypothesis that glucagon-induced insulin secretion is partially mediated by GLP-1 receptors on the beta cells and we propose that a compensatory mechanism, the nature of which remains to be established, is induced in GLP-1 receptor knockout mice to counteract the expected impaired insulin response to glucagon in these mice.

Nonaqueous, Mini-Dose Glucagon for Treatment of Mild Hypoglycemia in Adults With Type 1 Diabetes: A Dose-Seeking Study

OBJECTIVE

To evaluate mini-dose glucagon in adults with type 1 diabetes using a stable, liquid, ready-to-use preparation.

RESEARCH DESIGN AND METHODS

Twelve adults with type 1 diabetes receiving treatment with insulin pumps received subcutaneous doses of 75, 150, and 300 μg of nonaqueous glucagon. Plasma glucose, glucagon, and insulin concentrations were measured. At 180 min, subjects received insulin followed in ~60 min by a second identical dose of glucagon.

RESULTS

Mean (±SE) fasting glucose concentrations (mg/dL) were 110 ± 7, 110 ± 10, and 109 ± 9 for the 75-, 150-, and 300-μg doses, respectively, increasing maximally at 60 min by 33, 64, and 95 mg/dL (all P < 0.001). The post-insulin administration glucose concentrations were 70 ± 2, 74 ± 5, and 70 ± 2 mg/dL, respectively, with maximal increases of 19, 24, and 43 mg/dL post-glucagon administration (P < 0.02) at 45-60 min.

CONCLUSIONS

Subcutaneous, nonaqueous, ready-to-use G-Pen Mini glucagon may provide an alternative to oral carbohydrates for the management of anticipated, impending, or mild hypoglycemia in adults with type 1 diabetes.

Short-term glucagon stimulation test of C-peptide effect on glucose utilization in patients with type 1 diabetes mellitus

This work aimed to evaluate the use of a four-point glucagon stimulation test of C-peptide effect on glucose utilization in type 1 diabetic patients using a new mathematical model. A group of 32 type 1 diabetic patients and a group of 10 healthy control subjects underwent a four-point glucagon stimulation test with blood sampling at 0, 6, 15 and 30 min after 1 mg glucagon bolus intravenous administration. Pharmacokinetic and pharmacokinetic/pharmacodynamic models of C-peptide effect on glucose utilization versus area under curve (AUC) were used. A two-sample t test and ANOVA with Bonferroni correction were used to test the significance of differences between parameters. A significant difference between control and patient groups regarding the coefficient of whole-body glucose utilization and AUC C-peptide/AUC glucose ratio (p ≪ 0.001 and p = 0.002, respectively) was observed. The high correlation (r = 0.97) between modeled coefficient of whole-body glucose utilization and numerically calculated AUC C-peptide/AUC glucose ratio related to entire cohort indicated the stability of used method. The short-term four-point glucagon stimulation test allows the numerically calculated AUC C-peptide/AUC glucose ratio and/or the coefficient of whole-body glucose utilization calculated from model to be used to diagnostically identify type 1 diabetic patients.

Minimizing morbidity of hypoglycemia in diabetes: a review of mini-dose glucagon

Type 1 diabetes is a common chronic disease of childhood and one of the most difficult conditions to manage. Advances in insulin formulations and insulin delivery devices have markedly improved the ability to achieve normal glucose homeostasis. However, hypoglycemia remains the primary limiting factor in achieving normoglycemia and is a frequent complication in children with acute gastroenteritis and/or poor oral intake. In situations of impaired carbohydrate intake or absorption, glucagon therapy is the only out-of-hospital treatment option available to families and caregivers. Glucagon is recommended for the treatment of severe hypoglycemia and rapidly increases blood glucose by increasing hepatic glucose production from glycogenolysis. Mini-dose glucagon is a widely utilized off-label treatment for managing mild or impending hypoglycemia and is administered as a small subcutaneous injection. It was initially described for use in children who were unable to tolerate or absorb oral carbohydrates but not in need of advanced medical care. Yet, mini-dose glucagon may be useful in any individual with relative insulin excess. The regimen aims to prevent severe hypoglycemic episodes and is safe, effective, and easily administered by patients and caregivers in the out-of-hospital setting. By empowering patients and their families, this important tool could help to alleviate the physical, psychosocial, and financial burden evolving from impending hypoglycemia.

Arginine is preferred to glucagon for stimulation testing of β-cell function

A key aspect of research into the prevention and treatment of type 2 diabetes is the availability of reproducible clinical research methodology to assess β-cell function. One commonly used method employs nonglycemic secretagogues like arginine (arg) or glucagon (glgn). This study was designed to quantify the insulin response to arg and glgn and determine test repeatability and tolerability. Obese overnight-fasted subjects with normal glucose tolerance were studied on 4 separate days: twice using arg (5 g iv) and twice with glgn (1 mg iv). Pre- and postinfusion samples for plasma glucose, insulin, and C-peptide were acquired. Arg and glgn challenges were repeated in the last 10 min of a 60-min glucose (900 mg/min) infusion. Insulin and C-peptide secretory responses were estimated under baseline fasting glucose conditions (AIRarg and AIRglgn) and hyperglycemic (AIRargMAX AIRglgnMAX) states. Relative repeatability was estimated by intraclass correlation coefficient (ICC). Twenty-three (12 men and 11 women) subjects were studied (age: 42.4 ± 8.3 yr; BMI: 31.4 ± 2.8 kg/m²). Geometric means (95% CI) for baseline-adjusted values AIRarg and AIRglgn were 84 (75-95) and 102 (90-115) μU/ml, respectively. After the glucose infusion, AIRargMAX and AIRglgnMAX were 395 (335-466) and 483 (355-658) μU/ml, respectively. ICC values were >0.90 for AIRarg andAIRargMAX. Glucagon ICCs were 0.83, 0.34, and 0.36, respectively, although the exclusion of one outlier increased the latter two values (to 0.84 and 0.86). Both glgn and arg induced mild adverse events that were transient. Glucagon, but not arginine, induced moderate adverse events due to nausea. Taken together, arginine is preferred to glucagon for assessment of β-cell function.

Clinical and experimental pancreatic islet transplantation to striated muscle: establishment of a vascular system similar to that in native islets

OBJECTIVE

Curing type 1 diabetes by transplanting pancreatic islets into the liver is associated with poor long-term outcome and graft failure at least partly due to inadequate graft revascularization. The aim of the current study was to evaluate striated muscle as a potential angiogenic site for islet transplantation.

RESEARCH DESIGN AND METHODS

The current study presents a new experimental model that is found to be applicable to clinical islet transplantation. Islets were implanted into striated muscle and intraislet vascular density and blood flow were visualized with intravital and confocal microscopy in mice and by magnetic resonance imaging in three autotransplanted pancreatectomized patients. Mice were rendered neutropenic by repeated injections of Gr-1 antibody, and diabetes was induced by alloxan treatment.

RESULTS

Contrary to liver-engrafted islets, islets transplanted to mouse muscle were revascularized with vessel densities and blood flow entirely comparable with those of islets within intact pancreas. Initiation of islet revascularization at the muscular site was dependent on neutrophils, and the function of islets transplanted to muscle was proven by curing diabetic mice. The experimental data were confirmed in autotransplanted patients where higher plasma volumes were measured in islets engrafted in forearm muscle compared with adjacent muscle tissue through high-resolution magnetic resonance imaging.

CONCLUSIONS

This study presents a novel paradigm in islet transplantation whereby recruited neutrophils are crucial for the functionally restored intraislet blood perfusion following transplantation to striated muscle under experimental and clinical situations.

Independent measures of insulin secretion and insulin sensitivity during the same test: the glucagon-insulin tolerance test

BACKGROUND

To validate a test for independent assessment of insulin secretion and insulin sensitivity during the same occasion for metabolic studies in clinical practice, i.e. combined glucagon-stimulated C-peptide test and insulin tolerance test (GITT).

SUBJECTS AND METHODS

We measured C-peptide response to 0.5 mg of intravenous glucagon followed 30 min later by administration of 0.05 U kg(-1) insulin (insulin tolerance test, ITT). Ten subjects with normal glucose tolerance participated on different days in an ITT, glucagon-C-peptide test, ITT followed by glucagon-C-peptide test and glucagon-C-peptide test followed by ITT to establish whether and how the tests could be combined. The test was then repeated in nine patients with type 2 diabetes to investigate its reproducibility. In 20 subjects with varying degrees of glucose tolerance, the test was compared with the Botnia clamp (an intravenous glucose tolerance test combined with a euglycaemic hyperinsulinemic clamp).

RESULTS

When ITT preceded the glucagon test, C-peptide response was blunted. Therefore, we first administered glucagon and then insulin (GITT). The K(ITT) from the GITT was reproducible (CV = 13 %) and correlated strongly with the glucose disposal rate from the Botnia clamp (r = 0.87, r(2) = 0.75, P < 0.001). The C-peptide response to glucagon was reproducible (CV = 13 %). The disposition index, providing a measure of beta-cell function adjusted for insulin sensitivity, calculated from the GITT showed good discrimination between individuals with varying degrees of glucose tolerance.

CONCLUSIONS

The GITT provides simple, reproducible and independent estimates of insulin sensitivity and secretion on the same occasion for metabolic studies in individuals with normal and abnormal glucose tolerance.

Antidiabetogenic action of cholecystokinin-8 in type 2 diabetes

Cholecystokinin (CCK) is a gut hormone and a neuropeptide that has the capacity to stimulate insulin secretion. As insulin secretion is impaired in type 2 diabetes, we explored whether exogenous administration of this peptide exerts antidiabetogenic action. The C-terminal octapeptide of CCK (CCK-8) was therefore infused i.v. (24 pmol/kg x h) for 90 min in six healthy postmenopausal women and in six postmenopausal women with type 2 diabetes. At 15 min after start of infusion, a meal was served and ingested during 10 min. On a separate day, saline was infused instead of CCK-8. In both healthy subjects and subjects with type 2 diabetes, CCK-8 reduced the increase in circulating glucose after meal ingestion and potentiated the increase in circulating insulin. The ratio between the area under the curves for serum insulin and plasma glucose during the 15- to 75-min period after meal ingestion was increased by CCK-8 by 198 +/- 18% in healthy subjects (P = 0.002) and by 474 +/- 151% (P = 0.038) in subjects with type 2 diabetes. In contrast, the increase in the circulating levels of gastric inhibitory polypeptide (GIP), glucagon-like peptide-1 (GLP-1), or glucagon after meal ingestion was not significantly affected by CCK-8. The study therefore shows that CCK-8 exerts an antidiabetogenic action in both healthy subjects and type 2 diabetes through an insulinotropic action that most likely is exerted trough a direct islet effect. As at the same time, CCK-8 was infused without any adverse effects, the study suggests that CCK is a potential treatment for type 2 diabetes.

Age-related reduction in glucose elimination is accompanied by reduced glucose effectiveness and increased hepatic insulin extraction in man

This study examined whether insulin secretion, insulin sensitivity, glucose effectiveness (SG), and hepatic extraction (HE) of insulin are altered by age when glucose tolerance is normal. A frequently sampled i.v. glucose tolerance test was performed in 20 elderly (E, 10/10 male/female, all 63 yr old) and in 20 young subjects (Y, 10/10 male/female, all 27 yr old), who were similar in body mass index and 2-h blood glucose during oral glucose tolerance test. E exhibited impaired glucose elimination (i.v. tolerance index, 1.31 +/- 0.10 vs. 1.70 +/- 0.12% min-1; P = 0.019). First-phase insulin secretion and SI did not differ between the groups, whereas E had lower glucose sensitivity of second-phase insulin secretion (0.40 +/- 0.07 vs. 0.70 +/- 0.08 (pmol/L)min-2/(mmol/L), P = 0.026), lower SG, 0.017 +/- 0.002 vs. 0.025 +/- 0.002 min-1, P = 0.004), and higher HE (81.3 +/- 2.4 vs. 73.2 +/- 2.1%, P = 0.013). Across both groups, SG correlated positively with glucose tolerance index (r = 0.58, P < 0.001) and negatively with HE (r = -0.54, P < 0.001). Plasma leptin and glucagon did not change by age, whereas plasma pancreatic polypeptide (PP) was higher in E (122 +/- 18 vs. 66 +/- 6 pg/mL, P = 0.004). PP did not, however, correlate to any other parameter. We conclude that E subjects with normal oral glucose tolerance have reduced SG, impaired second-phase insulin secretion, and increased HE, whereas SI and first-phase insulin secretion seem normal. SG seems most related to age-dependent impairment of glucose elimination, whereas leptin, glucagon, and PP do not seem to contribute.

Glucose and insulin responses to glucagon injection in dairy cows with ketosis and fatty liver

The purpose of this investigation was to study the metabolic situation in clinical cases of bovine ketosis and to diagnose additional diseases. Extensive clinical examination, clinical biochemistry, haematology and fine-needle aspiration biopsy of liver was performed on 17 ketotic and eight control dairy cows in the field, and on seven hospitalized hyperketonaemic fatty liver patients. Additional findings in the ketotic group were heat (n = 7), indigestion (n = 5), endometritis (n = 2), cystic ovaries (n = 1), and mastitis (n = 1), and in the fatty liver group displaced abomasum (n = 4), abomasal ulcers (n = 3), mastitis (n = 2), laminitis (n = 1), bronchopneumonia (n = 1), and hypomagnesaemia (n = 2). There were no additional findings in the control group. Aspartate aminotransferase (AST) and creatine kinase (CK) were elevated in the ketosis and fatty liver groups. Total bilirubin, gamma-glutamyl transferase (GGT) and glutamate dehydrogenase (GD) were elevated in the fatty liver group and in some animals in the ketosis group. Total bile acid was not different between the groups. The free fatty acid/cholesterol ratio was higher in the fatty liver group compared with the control and ketosis groups. There was no or only slight fatty degeneration of the liver cells in the control and ketosis groups. Glucose and insulin preinjection concentrations and changes from basal values after glucagon injection were significantly lower in the ketosis group if compared with the control group. The responses in the fatty liver animals after glucagon injection were more heterogeneous than in the control and ketosis animals, a sign of disturbance in the metabolic adaptation, which together with high free fatty acid (FFA) levels can lead to fatty liver in cows with concurrent diseases.

Effects of different energy levels, concentrate/forage ratios and lipid supplementation to the diet on the adaptation of the energy metabolism at calving in dairy cows

A low level of energy (110 MJ ME) and a low concentrate/forage ratio (10/90) at calving resulted in low basal concentrations of glucose and insulin, but the cows had the capacity to increase the glucose level after glucagon injections. No signs of disturbances in the metabolic adaptation were observed. High intensity feeding (200 MJ ME and 50% concentrates) resulted in high basal serum insulin levels. The increase in the insulin concentrations after glucagon injections and the changes in insulin levels around calving varied widely between individual cows. The metabolic adaptation period was longer than in cows in the former group. An energy level of 170 MJ ME and variations of concentrate/forage ratios (5/95, 30/70 and 60/40) resulted in small differences in basal glucose and insulin concentrations and in response to glucagon injections. But the cows fed 60% concentrates showed signs of prolonged metabolic adaptation. Increased lipid concentrations in diets containing equal levels of energy and protein resulted in a fall in basal glucose and insulin levels and the metabolism seemed to be directed towards catabolism. Because of these metabolic effects, more needs to be known on fat supplementation if it is used in practical feeding. It is also necessary to take more interest in the effects of protein feeding on the periparturient metabolism.

Acute metabolic effects of clenbuterol in calves

The acute metabolic effects of clenbuterol were studied in calves. Clenbuterol was given intravenously at a dose of 1 microgram/kg body weight. Glucagon was used to increase insulin secretion. Pretreatment with clenbuterol did not change the glucagon-induced hyperglycaemia, but the serum levels of insulin were significantly higher. Clenbuterol showed a significant lipolytic effect. The post prandial increases in glucose and insulin were significantly higher in clenbuterol treated calves. The findings are in accordance with results from earlier studies where clenbuterol was given orally in much higher doses.

Exocrine and endocrine functional reserve in the course of chronic pancreatitis as studied by maximal stimulation tests

Thirty patients suffering from chronic alcoholic pancreatitis (18 calcified) were entered into a study of exocrine and endocrine pancreatic function based on two maximal stimulation tests, namely the secretin-cerulein test and the glucagon test with serum assays of C peptide. The glucagon test was also performed in 19 control subjects. In addition, 10 chronic pancreatitis patients and nine controls were subjected to an oral glucose tolerance test (OGTT) with serum insulin determinations. C peptide basal values were decreased only in patients with severe pancreatic exocrine insufficiency (P less than 0.001), while delta C peptide values were also reduced in patients with moderate exocrine insufficiency (P less than 0.001). Lipase output correlated very well with delta C peptide values (P less than 0.001). While serum insulin levels during OGTT and C peptide basal values showed no significant differences between the chronic pancreatitis and control groups, delta C peptide values were significantly reduced in chronic pancreatitis patients (P less than 0.02). Both endocrine and exocrine function are impaired in chronic pancreatitis, as demonstrated by maximal tests, even in early stages of the disease.

Morphological changes in the human endocrine pancreas induced by chronic excess of endogenous glucagon

The non-tumoral endocrine pancreas from a patient with elevated plasma levels of glucagon due to a malignant glucagonoma was studied immunocytochemically, ultrastructurally and morphometrically. Compared with normal pancreatic islets from control subjects, those of the pancreas from the patient with a glucagonoma showed an almost complete disappearance of A cells, a decrease in immunoreactive insulin in B cells associated with cytological features indicating enhanced synthesis and secretion of this hormone, and an increase in immunoreactive somatostatin and pancreatic polypeptide (PP) accompanied by unusually high numbers of D and PP cells. In addition, numerous B cells were found outside the islets, either forming micro-islets or scattered in the exocrine tissue (nesidioblastosis). The possible mechanisms involved in determining the changes in the secretory activity of B cells and the alterations in the cell composition of the islets are discussed.

Impaired glucose tolerance and heterogeneity of insulin responses in cows with abomasal displacement

Injections of glucagon in dairy cows resulted in a rapid and marked increase in serum insulin and also in plasma glucose. Glucagon injection appears to be a simple and useful method for testing insulin secretion and glucose release. There was a somewhat reduced insulin response 2 weeks after calving in normal cows. Cows given high intensity feeding before calving showed higher plasma glucose levels in the glucagon test 3 days post partum and showed signs of reduced sensitivity to insulin action. Cows with abomasal displacement showed impaired glucose tolerance and heterogeneity of insulin responses indicating insulin resistance and signs of Type 2 (non-insulin-dependent) diabetes mellitus.