Insulinotropic actions of exendin-4 and glucagon-like peptide-1 in vivo and in vitro

This study compares in vitro effects of exendin-4 and glucagon-like peptide (GLP)-1 on basal and glucose-stimulated insulin release from isolated rat islets and in vivo insulinotropic actions of exendin-4 and GLP-1 after an intravenous glucose challenge in rats. In static incubation of isolated islets, changing ambient glucose concentration from 3 mmol/L to 10 mmol/L stimulated insulin secretion 9.8 +/- 1.3-fold. The addition of exendin-4 or GLP-1 (1 nmol/L to 1 micromol/L) increased glucose-stimulated insulin secretion up to 5.8 +/- 1.6-fold and 3.3 +/- 1.0-fold, respectively, over basal rates (defined as no hormones added, 3 mmol/L glucose) and 19.6 +/- 2.3-fold and 15.0 +/- 3.1-fold at 10 mmol/L glucose. In dynamically perfused isolated islets exposed to 7.5 mmol/L glucose, insulin secretion increased 6.4 +/- 1.5-fold, and exendin-4 (20 nmol/L) or GLP-1 (20 nmol/L) increased this similarly by up to 13.5 +/- 2.8 and 12.7 +/- 3.9-fold,respectively. Anesthetized rats administered 5.7 mmol/kg intravenous glucose increased plasma insulin concentration 3.0-fold. Infusion of exendin-4 or GLP-1 increased this to a maximum of 7.6-fold and 5.3-fold, respectively. As with isolated islet studies, in vivo dose responses and concentration responses with exendin-4 and GLP-1 were bell-shaped. When insulinotropic effects were mapped onto the steady-state plasma concentrations associated with these infusion rates, both peptides exhibited bell-shaped concentration responses with peak insulinotropic effects occurring with plasma peptide concentrations of approximately 1 nmol/L in this model. In summary, exendin-4 and GLP-1 exhibited similar insulinotropic potencies (median effective dose [ED(50)]) when assessed on a concentration basis in in vitro and in vivo models, while exendin-4 exhibited greater efficacy (maximum response).     

Effects of glucagon-like peptide 1 on counterregulatory hormone responses, cognitive functions, and insulin secretion during hyperinsulinemic, stepped hypoglycemic clamp experiments in healthy volunteers

Glucagon-like peptide 1 (GLP-1) and analogues are being evaluated as a new therapeutic principle for the treatment of type 2 diabetes. GLP-1 suppresses glucagon secretion, which could lead to disturbances of hypoglycemia counterregulation. This has, however, not been tested. Nine healthy volunteers with normal oral glucose tolerance received infusions of regular insulin (1 mU x kg(-1) x min(-1)) over 360 min on two occasions in the fasting state. Capillary glucose concentrations were clamped at plateaus of 4.3, 3.7, 3.0, and 2.3 mmol/liter for 90 min each (stepwise hypoglycemic clamp); on one occasion, GLP-1 (1.2 pmol x kg(-1) x min(-1)) was administered i.v. (steady-state concentration, approximately 125 pmol/liter); on the other occasion, NaCl was administered as placebo. Glucagon, cortisol, GH (immunoassays), and catecholamines (radioenzymatic assay) were determined, autonomous and neuroglucopenic symptoms were assessed, and cognitive function was tested at each plateau. Insulin secretion rates were estimated by deconvolution (two-compartment model of C-peptide kinetics). At insulin concentrations of approximately 45 mU/liter, glucose infusion rates were similar with and without GLP-1 (P = 0.26). Only during the euglycemic plateau (4.3 mmol/liter), GLP-1 suppressed glucagon concentrations (4.1 +/- 0.4 vs. 6.5 +/- 0.7 pmol/liter; P = 0.012); at all hypoglycemic plateaus, glucagon increased similarly with GLP-1 or placebo, to maximum values greater than 20 pmol/liter (P = 0.97). The other counterregulatory hormones and autonomic or neuroglucopenic symptom scores increased, and cognitive functions decreased with decreasing glucose concentrations, but there were no significant differences comparing experiments with GLP-1 or placebo, except for a significant reduction of GH responses during hypoglycemia with GLP-1 (P = 0.04). GLP-1 stimulated insulin secretion only at plasma glucose concentrations of at least 4.3 mmol/liter. In conclusion, the suppression of glucagon by GLP-1 does occur at euglycemia, but not at hypoglycemic plasma glucose concentrations (< or = 3.7 mmol/liter). GLP-1 does not impair overall hypoglycemia counterregulation except for a reduction in GH responses, which is in line with other findings demonstrating pituitary actions of GLP-1. Below plasma glucose concentrations of 4.3 mmol/liter, the insulinotropic action of GLP-1 is negligible.     

Regulation of islet hormone release and gastric emptying by endogenous glucagon-like peptide 1 after glucose ingestion

BACKGROUND: Exogenous administration of glucagon-like peptide (GLP)-1 improves glucose tolerance by stimulation of insulin secretion, inhibition of glucagon secretion, and delay of gastric emptying. It is not known which of these effects is involved in the action of endogenous GLP-1 to control blood glucose. To determine the role of endogenous GLP-1 on islet cell function and gastric emptying independent of variable glycemia, we clamped blood glucose before and during glucose ingestion with and without GLP-1 receptor blockade with exendin-[9-39] (Ex-9). METHODS: There were 10 healthy subjects that participated in two experiments each, one a control and one with infusion of 750 pm/kg . min Ex-9. Subjects consumed 75 g oral glucose solution mixed with d-xylose and (13)C-glucose while their blood glucose levels were held fixed at approximately 8.9 mmol/liter. RESULTS: Plasma insulin levels during hyperglycemia alone were similar in the two studies (control, 282.5 +/- 42 vs. Ex-9, 263.8 +/- 59 pmol/liter) but were reduced by approximately 30% by Ex-9 after glucose ingestion (control, 1154 +/- 203 vs. Ex-9, 835 +/- 120 pmol/liter; P < 0.05). Blocking the action of endogenous GLP-1 caused an approximate 80% increase in postprandial glucagon concentrations. The appearance of ingested d-xylose in the blood was not affected by Ex-9, suggesting that postprandial secretion of GLP-1 has only minimal effects on gastric emptying of oral glucose. CONCLUSIONS: These findings indicate that GLP-1 is an incretin in healthy humans at modestly supraphysiological blood glucose levels. The primary effect of GLP-1 to regulate oral glucose tolerance is mediated by effects on islet hormones and not on gastric emptying.     

Differential secretion of pro-opiomelanocortin peptides by the pars distalis and pars intermedia of beagle dogs

Beagle dogs were given saline, insulin or the dopamine antagonist, haloperidol, to examine peripheral concentrations of immunoreactive (ir)-pro-opiomelanocortin (POMC) peptides resulting from pars distalis or pars intermedia stimulation. Six beagles were given each test substance on separate occasions with and without dexamethasone pretreatment. Plasma was assayed directly for glucose, ir-ACTH, ir-alpha-MSH, cortisol and, after Sephadex G-50 Fine gel filtration chromatography, for ir-beta-lipotrophin (ir-beta-LPH) and ir-beta-endorphin (ir-beta-END) content. Injection of 0.5 units insulin/kg lowered (P less than 0.01) plasma glucose from 4.9 +/- 0.3 mmol/l (mean +/- S.D., saline controls) to 2.3 +/- 0.5 mmol/l, coincident with increasing ir-ACTH (9.5 +/- 3.1 to 106 +/- 54 pmol/l), cortisol (52 +/- 27 to 221 +/- 27 nmol/l), ir-beta-LPH (not detectable to 34 +/- 18 pmol/l) and ir-beta-END (not detectable to 52 +/- 22 pmol/l). Plasma ir-alpha-MSH concentrations were not affected by insulin. Pretreatment with dexamethasone abolished the ir-ACTH, cortisol, ir-beta-LPH and ir-beta-END increases in response to 0.75 units insulin/kg. Haloperidol (1 mg/kg) increased (P less than 0.01) plasma ir-ACTH (to 103 +/- 63 pmol/l), cortisol (to 243 +/- 11 nmol/l), ir-beta-LPH (to 16 +/- 6 pmol/l), ir-beta-END (to 136 +/- 73 pmol/l) and additionally raised ir-alpha-MSH (7 +/- 8 pmol/l in saline controls to 131 +/- 80 pmol/l after haloperidol).(ABSTRACT TRUNCATED AT 250 WORDS)     

Preserved GLP-1 effects in a diabetic patient with Cushing's disease.

CONTEXT:A patient with diabetes mellitus, who participated in a study with intravenous administration of GLP-1, was later found to have Cushing's disease (markedly elevated 24 h urinary cortisol excretion and inadequate suppression of fasting cortisol with 2 mg dexamethasone). His diabetic state disappeared (2 h plasma glucose after 75 g oral glucose 159 mg/dl=IGT) after successful pituitary surgery (normal 24 h urinary cortisol excretion and adequate cortisol suppression with 2 mg dexamethasone). OBJECTIVE:The present analysis was undertaken to compare GLP-1 actions on fasting glycemia in diabetes mellitus due to Cushing's disease with GLP-1 actions in typical type 2 diabetes. DESIGN AND METHODS:GLP-1 (1.2 pmol/kg/min) and placebo had been infused into ten patients with diabetes mellitus over 4 h in the fasting state. The results from the patient with Cushing's disease (C) were compared to the data from the remaining nine patients with type 2 diabetes (D). RESULTS:Within 4 h glucose decreased from basal (C: 12.9; D: 12.9+/-0.7 mmol/l) to normal fasting values (C: 5.0; D: 4.9+/-0.4 mmol/l). The stimulation of insulin secretion and suppression of glucagon secretion was similar in the patient with Cushing's disease compared to those with type 2 diabetes. CONCLUSIONS:The insulinotropic, glucagonostatic and glucose-lowering actions of GLP-1 in a patient with diabetes mellitus due to cortisol excess were similar to actions in typical type 2 diabetes. Therefore incretin mimetics might be a novel therapeutic strategy for the treatment of glucocorticoid-induced diabetes mellitus.     

Effect of physiological hyperinsulinemia on counterregulatory hormone responses during hypoglycemia in humans.

We evaluated the effect of continuous physiological hyperinsulinemia on counterregulatory hormone responses in seven healthy subjects, each studied on two occasions. Hormone responses were measured during identical 2-h periods of hypoglycemia (plasma glucose target 3.5 mmol/L) at insulin levels of 350 pmol/L or 640 pmol/L. During hypoglycemia, there were significant (50-1400%) increases in glucagon, epinephrine, norepinephrine, GH, and cortisol which were comparable in the two groups. We further evaluated the influence of the duration of mild hyperinsulinemia on the responses in an additional group of normal subjects (n = 7). Brief (30 min) exposure to insulin was compared to a prolonged (3.5 h) insulin infusion, each followed by identical hypoglycemia. Plasma insulin (approximately 350 pmol/L) and plasma glucose (target 3.3 mmol/L) were similar in both groups. The increases in epinephrine, norepinephrine, GH, and cortisol during hypoglycemia were virtually identical in the two groups. However, the secretion of glucagon was blunted following prolonged hyperinsulinemia, increasing to levels of 249 +/- 17 ng/L in the brief studies and to only 185 +/- 20 ng/L in the prolonged studies (P < 0.005). The insulin-induced decrement in plasma amino acids were similar in the two studies and could not account for the impaired glucagon secretory response. Conclusions: 1) Brief exposure to even high physiological levels of insulin do not alter the magnitude of counterregulatory hormone secretion during hypoglycemia; 2) prolonged hyperinsulinemia results in a selective blunting of the plasma glucagon response to hypoglycemia, perhaps due to a direct suppressive effect of insulin on alpha-cell secretion.     

The insulinotropic effect of acute exendin-4 administered to humans: comparison of nondiabetic state to type 2 diabetes

Exendin-4 is a potent and long-acting agonist of the glucagon-like peptide-1 (GLP-1) receptor. GLP-1 is an insulinotropic gut peptide and is being evaluated for the regulation of plasma glucose in type 2 diabetes. The purpose of the present study was to ascertain whether exendin-4 is insulinotropic and whether it has long-lived biological effects in nondiabetic and type 2 diabetic subjects. Because incretins are glucose dependent with respect to their insulin-releasing capacity, we used the hyperglycemic glucose clamp technique to begin to address these issues in two separate protocols. In one protocol, we infused exendin-4 (0.15 pmol x kg(-1) x min(-1)) in seven nondiabetic and seven type 2 diabetic subjects during the second hour of a 5-h hyperglycemic clamp in which fasting plasma glucose was raised by 5.4 mmol/liter. The second protocol was identical to the first except that plasma glucose was allowed to fall to the fasting levels during the fourth hour and again raised by 5.4 mmol/liter during the fifth hour in four nondiabetic and four diabetic subjects. With the initiation of exendin-4 infusion at 60 min, plasma insulin response was potentiated 4- to 5-fold in both groups. Despite termination of exendin-4 at the end of the second hour, the insulin levels remained elevated for several hours and hyperglycemia was maintained. All volunteers ate a meal 5.5 h after inducing hyperglycemia. Postprandial plasma glucose, insulin, and GLP-1 did not rise in any subject, possibly because of delayed gastric emptying by exendin-4 even though its infusion had been terminated 4 h previously. We concluded that exendin-4 is a potent and long-lasting insulinotropic agent in nondiabetic and diabetic subjects.     

THE EFFECT OF OXYTOCIN INFUSION ON ADENOHYPOPHYSIAL AND ADRENAL CORTICAL RESPONSES TO INSULIN-INDUCED HYPOGLYCAEMIA

The responses of plasma adrenocorticotrophin (ACTH), cortisol, growth hormone (GH) and prolactin to insulin-induced hypoglycaemia were studied in six lean male subjects (age 22-29 years). Intravenous insulin tests were performed with and without oxytocin infusion. Blood sugar nadir occurred at the onset of symptoms (time S) with no significant differences between oxytocin and saline infusion. During the oxytocin infusion mean plasma oxytocin increased from 1.9 pmol/l to 138 pmol/l. Peak increase in plasma ACTH (oxytocin 266 +/- 54 ng/l; saline 281 +/- 43 ng/l, mean +/- SEM) was at S + 10 min while peak plasma cortisol (oxytocin 680 +/- 47 nmol/l: saline 656 +/- 40 nmol/l) was measured at S +/- 60 min, peak GH (oxytocin 96 +/- 17.8 mU/l; saline 106 +/- 18.6 mU/l) at S + 60 min and prolactin (oxytocin 1332 +/- 239 mU/l; saline 1242 +/- 273 mU/l) at S + 30 min. There were no significant differences in plasma concentrations of ACTH, cortisol, GH or prolactin between saline and oxytocin infusion. The results indicate that oxytocin has no effect on plasma ACTH, cortisol, GH and prolactin responses to insulin-induced hypoglycaemia. In particular they fail to support previous studies which suggested an inhibitory role for oxytocin in ACTH secretion.     

Glucagon-induced increase in plasma potassium levels in Type 1 (insulin-dependent) diabetic subjects

To investigate the hypothesis that in Type 1 (insulin-dependent) diabetes the increase in plasma potassium during decompensation may be due to a rise in glucagon concentrations, we have measured plasma glucose, potassium and glucagon levels in five diabetic patients during two tests with 0.154 mol/l saline or somatostatin (500 micrograms/h) performed on two successive days. The patients were maintained normoglycaemic overnight by means of a continuous insulin infusion. After insulin withdrawal during the saline infusion, glucose and potassium levels rose markedly (delta maximum: glucose, 12.0 +/- 1.5 mmol/l; potassium, 0.73 +/- 0.12 mmol/l), while glucagon showed a slight, but significant increment (delta maximum: 10.6 +/- 1.0 pmol/ml, p less than 0.05). The potassium increment was not mediated by a reduction in blood pH. Somatostatin abolished the rise in glucagon concentration and simultaneously markedly inhibited the rise in potassium and glucose levels. It is concluded that in acute insulin deficiency, glucagon could be one of the factors that contributes to hyperkalaemia.     

Inhibition of dipeptidyl peptidase-4 reduces glycemia, sustains insulin levels, and reduces glucagon levels in type 2 diabetes

The stimulation of insulin vs. inhibition of glucagon secretion in relation to the antidiabetic action of glucagon-like peptide-1 (GLP-1) is not established. Here, the influence of a 4-wk increase in circulating GLP-1 by inhibition of dipeptidyl peptidase-4 (DPP-4) on 24-h glucose and insulin and glucagon responses to breakfast was studied in subjects with dietary controlled diabetes [age: 65 +/- 8 yr (SD), body mass index: 27.3 +/- 3.3 kg/m(2), fasting plasma glucose: 9.0 +/- 1.3 mmol/liter]. Compared with placebo (n = 19), a specific DPP-4 inhibitor [(1-[[(3-hydroxy-1-adamantyl) amino] acetyl]-2-cyano-(S)-pyrrolidine) (LAF237); 100 mg daily, n = 18] reduced fasting glucose by 0.70 mmol/liter (P = 0.037), 4-h prandial glucose excursion by 1.45 mmol/liter (P < 0.001), and mean 24-h glucose by 0.93 mmol/liter (P < 0.001). Baseline and postprandial active GLP-1 were increased by LAF237. The glucagon response to breakfast was reduced by LAF237 (glucagon levels at 60 min were 88 +/- 8 pg/ml before treatment vs. 77 +/- 5 pg/ml after; P = 0.001). In contrast, the overall insulin levels were not altered. The 4-wk reduction in glucagon correlated with the reduction in 2-h glucose (r = 0.61; P = 0.008). No such association was observed for insulin. Thus, improved metabolic control by DPP-4 inhibition in type 2 diabetes is seen in association with reduced glucagon levels and, despite the lower glycemia, unaltered insulin levels.     

Defective amplification of the late phase insulin response to glucose by GIP in obese Type II diabetic patients

AIMS/HYPOTHESIS: Glucagon-like-peptide-1 (GLP-1) is strongly insulinotropic in patients with Type II (non-insulin-dependent) diabetes mellitus, whereas glucose-dependent insulinotropic polypeptide (GIP) is less effective. Our investigation evaluated "early" (protocol 1) - and "late phase" (protocol 2) insulin and C-peptide responses to GLP-1 and GIP stimulation in patients with Type II diabetes. METHODS: Protocol 1: eight Type II diabetic patients and eight matched healthy subjects received i.v. bolus injections of GLP-1(2.5 nmol) or GIP(7.5 nmol) concomitant with an increase of plasma glucose to 15 mmol/l. Protocol 2: eight Type II diabetic patients underwent a hyperglycaemic clamp (15 mmol/l) with infusion (per kg body weight/min) of either: 1 pmol GLP-1 (7-36) amide (n=8), 4 pmol GIP (n=8), 16 pmol GIP (n=4) or no incretin hormone (n=5). For comparison, six matched healthy subjects were examined. RESULTS: Protocol 1: Type II diabetic patients were characterised by a decreased "early phase" response to both stimuli, but their relative response to GIP versus GLP-1 stimulation was exactly the same as in healthy subjects [insulin (C-peptide): patients 59+/-9% (74+/-6%) and healthy subjects 62+/-5% (71+/-9%)]. Protocol 2, "Early phase" (0-20 min) insulin response to glucose was delayed and reduced in the patients, but enhanced slightly and similarly by GIP and GLP-1. GLP-1 augmented the "late phase" (20-120 min) insulin secretion to levels similar to those observed in healthy subjects. In contrast, the "late phase" responses to both doses of GIP were not different from those obtained with glucose alone. Accordingly, glucose infusion rates required to maintain the hyperglycaemic clamp in the "late phase" period (20-120 min) were similar with glucose alone and glucose plus GIP, whereas a doubling of the infusion rate was required during GLP-1 stimulation. CONCLUSION/INTERPRETATION: Lack of GIP amplification of the late phase insulin response to glucose, which contrasts markedly to the normalising effect of GLP-1, could be a key defect in insulin secretion in Type II diabetic patients.     

Leptin increases circulating glucose, insulin and glucagon via sympathetic neural activation in fasted mice.

OBJECTIVE: A number of recent studies suggest that leptin has effects on glucose metabolism and pancreatic hormone secretion. Therefore, the effect of leptin administration on circulating glucose, insulin and glucagon in fed and fasted mice was investigated. The potential contribution of the sympathetic nervous system to the effects of leptin was also examined. DESIGN: Recombinant human or murine leptin was administered intraperitoneally (300 microg/mouse per 12 h over 24 h) to fed or fasted, normal or chemically sympathectomized NMRI mice. Blood samples were collected at baseline and after 24 h. MEASUREMENTS: Plasma concentrations of glucose, insulin and glucagon. RESULTS: In the fed state (n = 24), leptin administration did not affect glucose, insulin or glucagon concentrations after 24 h. Fasting (n = 24) reduced body weight by 2.2+/-0.4 g, plasma glucose by 3.7+/-0.4 mmol/l, plasma insulin by 138+/-35 pmol/l, and plasma glucagon by 32+/-7 pg/ml. In fasted mice, human leptin (n = 24) increased plasma glucose by 1.5+/-0.2 mmol/l (P = 0.041), plasma insulin by 95+/-22 pmol/l (P = 0.018), and plasma glucagon by 16+/-3 pg/ml (P = 0.025), relative to saline-injected control animals. Murine leptin exerted similar stimulating effects on circulating glucose (+1.0+/-0.2 mmol/l, P = 0.046), insulin (+58+/-17 pmol/l, P = 0.038) and glucagon (+24+/-9 pg/ml, P = 0.018) as human leptin in fasted mice (n = 12) with no significant effect in fed mice (n = 12). Human leptin did not affect circulating glucose, insulin or glucagon in fasted mice after chemical sympathectomy with 6-hydroxydopamine (40 mg/kg iv 48 h prior to fasting; n = 12). CONCLUSION: Leptin increases circulating glucose, insulin and glucagon in 24 h fasted mice by a mechanism requiring intact sympathetic nerves.     

Alcohol and glucose counterregulation during acute insulin-induced hypoglycemia in type 2 diabetic subjects

To investigate the influence of alcohol on glucose counterregulation and recovery during acute insulin-induced hypoglycemia in type 2 diabetic subjects, 8 diet-treated type 2 diabetic subjects were examined twice after an overnight fast. A graded hyperinsulinemic (1 mU/kg/min, 60 to 195 minutes) euglycemic/hypoglycemic clamp was performed with concomitant infusion of 3-(3)H-glucose to assess glucose turnover. After a euglycemic baseline period (150 to 180 minutes), 200 mL of water was taken either alone or with alcohol (0.4 g/kg body weight). Hypoglycemia (plasma glucose nadir, 2.8 mmol/L) was subsequently induced, and the recovery period followed after discontinuation of insulin and the variable glucose infusion. On both study days, circulating concentrations of insulin and glucose were comparable. Alcohol intake markedly increased plasma lactate (area under the curve [AUC], recovery period) (244 +/- 30 v 12 +/- 4 mmol/L x 240 minutes; P = .00009) and suppressed plasma nonesterified fatty acids (NEFA) (AUC, recovery period) (95 +/- 13 v 161 +/- 18 mmol/L x 240 minutes; P = .0008). No differences were found in the counterregulatory response of catecholamines, cortisol, and growth hormone (GH). However, alcohol intake decreased peak glucagon significantly (155 +/- 12 v 200 +/- 17 pg/mL; P = .038). In diet-treated, mild type 2 diabetic subjects, alcohol does not modify recovery from insulin-induced hypoglycemia.     

Prandial regulation of ghrelin secretion in humans: does glucagon contribute to the preprandial increase in circulating ghrelin?

OBJECTIVE: Glucagon secretion is stimulated by fasting and inhibited postprandially, a pattern that mimics the secretory profiles of both ghrelin and GH. We thus hypothesized that glucagon may be a determinant of the changes in circulating ghrelin and GH that occur in relation to meals. The objective of the study was to explore this hypothesis by determining the ghrelin and GH response to a bolus of glucagon or saline in healthy subjects. SUBJECTS AND MEASUREMENTS: Nine healthy volunteers, mean age 47 years (range 33-58) and body mass index (BMI) 24 kg/m2 (range 20.9-27.6) were recruited and received either 1 mg glucagon (n = 9) or 1 ml saline (n = 6) subcutaneously on separate days between 0800 and 0830 h after an overnight fast. Venous blood was then sampled at 15-min intervals during the first hour, followed by 30-min intervals up to 4 h for glucose, insulin, GH, cortisol, somatostatin and ghrelin. RESULTS: Mean +/- SE basal ghrelin was 213.1 +/- 34.3 pmol/l and decreased significantly by 15 min after glucagon administration to 179.3 +/- 28 pmol/l (P = 0.01), then remaining suppressed relative to the basal value until 240 min after glucagon. Plasma insulin increased from a basal value of 46.7 +/- 7.7 pmol/l to a peak of 327.1 +/- 54.9 pmol/l (P < 0.0001). There was an inverse statistical relationship between the increase in insulin over the first 120 min and the decrease in ghrelin (P = 0.005), while somatostatin, GH and glucose were not significant contributors to the decrease in ghrelin (P > 0.05). Mean +/- SE basal GH was 7.3 +/- 2.9 microg/l and increased by 150 min after glucagon to a peak of 20.5 +/- 6.8 microg/l (P = 0.006). Changes in neither ghrelin nor glucose were related to the increase in GH (P = 0.7). Saline administration did not produce any significant change in ghrelin, insulin or somatostatin although the expected diurnal reduction in cortisol (P < 0.05) was observed. CONCLUSIONS: Our study found no evidence that glucagon stimulates ghrelin secretion in humans and supports the hypothesis that insulin is a negative regulator of ghrelin secretion in the postprandial state. We did not find a negative relationship between endogenous somatostatin and ghrelin despite earlier reports that exogenously administered somatostatin analogues suppress plasma ghrelin. Finally, glucagon-induced GH secretion is not mediated by an increase in plasma ghrelin.     

Effects of a V1-vasopressin antagonist on ACTH release following vasopressin infusion or insulin-induced hypoglycemia in normal men

Experimental evidence indicates that arginine vasopressin contributes to the release of adrenocorticotropic hormone under certain conditions. We studied for the first time the AVP antagonist [d(CH2)5 Tyr(Me)AVP] in 6 normal men in order to evaluate the possible role of AVP as an ACTH-releasing hormone during insulin-induced hypoglycemia. To test the agent's capacity to inhibit an ACTH release by exogenous AVP, we compared the ACTH response to an infusion of 300 ng AVP/min a. 30 min after injection of 5 micrograms/kg of the antagonist, b. after injection of placebo (0.9% NaCl). Plasma ACTH levels during AVP infusion rose from 17.2 +/- 1.6 ng/l (3.8 +/- 0.35 pmol/l) to 31.7 +/- 4.2 ng/l (7.0 +/- 0.92 pmol/l) at 40 min after injection of the antagonist, the difference to the control-group (increment from 16.5 +/- 1.2 ng/l (3.6 +/- 0.26 pmol/l) to 41.8 +/- 3.5 ng/l) (9.2 +/- 0.77 pmol/l) being significant (p less than 0.05). Peak plasma cortisol levels were 323 +/- 42 and 529 +/- 52 nmol/l, respectively (p less than 0.05). We then tested the compound in the same subjects during an insulin-induced hypoglycemia; 30 min after administration of 10 micrograms/kg of the AVP antagonist or placebo, all subjects received 0.12 IU/kg of normal insulin, thus inducing a fall of blood glucose levels below 2 mmol/l. The AVP antagonist caused a moderate but insignificant reduction of the rise in plasma ACTH and a slightly greater, significant reduction of the increment in plasma cortisol (350 +/- 19 nmol/l with antagonist and 469 +/- 90 nmol/l with placebo, p less than 0.05) during insulin-induced hypoglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Impairment of glucose tolerance in hyperthyroid cats

Intravenous glucose tolerance tests were performed in eight adult cats before and after a 4-week treatment with thyroxine. The untreated cats had a mean fasting blood glucose concentration of 7.7 +/- 0.3 mmol/l and a mean fasting insulin concentration of 88 +/- 31 pmol/l which were not significantly different from mean fasting glucose and insulin concentrations after 4 weeks of thyroxine administration (6.9 +/- 0.2 mmol/l and 101 +/- 28 pmol/l respectively). At 120 min after glucose injection, the glucose concentration in untreated cats returned to baseline concentrations as did the insulin concentration. However, in the hyperthyroid cats both glucose and insulin concentrations were significantly (P less than 0.001) higher (13.6 +/- 0.8 mmol/l and 245 +/- 17 pmol/l respectively) in comparison with the baseline and untreated cats. The t1/2 for glucose disappearance was significantly higher in the cats rendered hyperthyroid, and the glucose disposal rate constant (K) was significantly lower in this group. It is concluded that hyperthyroidism in cats leads to impairment of glucose tolerance possibly due to peripheral insulin resistance.     

Counterregulation of insulin-induced hypoglycaemia in primary hypothyroidism

Hypothyroidism has been alleged to modulate insulin action and influence the secretion of growth hormone and catecholamines. We recently investigated the influence of hypothyroidism on glucose counter-regulatory capacity and the hormonal responses to insulin-induced hypoglycaemia in 6 patients with primary hypothyroidism (age 32-52 years, TSH-values 66-200 mU/l). Hypoglycaemia was induced in the hypothyroid state and again when the subjects were euthyroid. After an overnight fast a constant rate infusion of insulin (2.4 U/h) was given for 4 h. Glucose was measured every 15 min and insulin. C-peptide, glucagon, epinephrine, norepinephrine, growth hormone and cortisol every 30 min for 5 h. During insulin infusion somewhat higher concentrations of the hormone were obtained in the hypothyroid state and simultaneously glucose levels were 0.5 mmol/l lower. As expected, basal norepinephrine levels were higher in hypothyroidism. However, no increase in circulating norepinephrine during hypoglycaemia was registered in the two experiments. The responses of counterregulatory hormones showed an enhanced response of cortisol, similar responses of growth hormone and epinephrine while the glucagon response was paradoxically impaired. Our findings suggest that hypothyroidism alters insulin metabolism, and that the glucagon response to hypoglycaemia is impaired in this condition.     

The effects of the specific serotonin antagonist ICI 169, 369 on the pituitary hormone response to insulin-induced hypoglycaemia in humans

OBJECTIVE: To examine the role of serotonin in pituitary hormone release by studying the effect of a specific 5HT2 receptor antagonist, ICI 169,369, on the ACTH, prolactin, growth hormone and AVP response to insulin-induced hypoglycaemia in healthy humans. DESIGN: A double-blind, within-subject trial using a crossover design to compare the effect of placebo with two doses of ICI 169,369 on pituitary hormone responses to insulin-induced hypoglycaemia. PATIENTS: Ten healthy subjects were studied in the low-dose (30 mg x 2) limb and 11 healthy volunteers in the high-dose (80 mg x 2) limb. MEASUREMENTS: Plasma concentrations of prolactin, growth hormone, ACTH, cortisol and AVP, and blood glucose. RESULTS: In the low-dose study, pretreatment with 30 mg ICI 169,369, 10 and 2 hours before the study, had no effect on the fall in blood glucose or the rise in plasma ACTH, prolactin, growth hormone, AVP or plasma cortisol following insulin injection, when compared with placebo. In the high-dose study the effect of a higher dose (80 mg) of ICI 169,369 on the pituitary hormone response to hypoglycaemia was compared with that of placebo. Although the fall in blood glucose was similar following drug (4.3 +/- 0.1 to 1.5 +/- 0.5 mmol/l, mean +/- SEM, P less than 0.001) and placebo (4.3 +/- 0.1 to 1.4 +/- 0.4 mmol/l, P less than 0.001), the rise in plasma AVP was lower (P less than 0.05) following pretreatment with drug (0.5 +/- 0.2 to 2.1 +/- 0.6 pmol/l, P less than 0.05) than with placebo (0.7 +/- 0.2 to 3.4 +/- 0.9 pmol/l, P less than 0.01). CONCLUSIONS: The ACTH, prolactin, growth hormone and cortisol responses were unaffected by ICI 169,369. The data are compatible with an inhibitory effect of the serotonin antagonist ICI 169,369 on the AVP, but not the ACTH, prolactin or growth hormone response to insulin-induced hypoglycaemia in humans.     

The ketosis-resistance in fibro-calculous-pancreatic-diabetes. 1. Clinical observations and endocrine-metabolic measurements during oral glucose tolerance test.

We measured circulating levels of C-peptide, pancreatic glucagon, cortisol, growth hormone and metabolites (glucose, non-esterified fatty acids, glycerol and 3-hydroxybutyrate) in fibro-calculous-pancreatic diabetic (FCPD, n = 28), insulin-dependent diabetic (IDDM, n = 28) and non-diabetic control (n = 27) subjects during an oral glucose tolerance test. There was no difference in the two diabetic groups in age (FCPD 24 +/- 2, IDDM 21 +/- 2 years, mean +/- SEM), BMI (FCPD 16.0 +/- 0.6, IDDM 15.7 +/- 0.4 kg/m2), triceps skinfold thickness (FCPD 8 +/- 1, IDDM 7 +/- 1 mm), glycaemic status (fasting plasma glucose, FCPD 12.5 +/- 1.5, IDDM 14.5 +/- 1.2 mmol/l), fasting plasma C-peptide (FCPD 0.13 +/- 0.03, IDDM 0.08 +/- 0.01 nmol/l), peak plasma C-peptide during OGTT (FCPD 0.36 +/- 0.10, IDDM 0.08 +/- 0.03 nmol/l) and fasting plasma glucagon (FCPD 35 +/- 4, IDDM 37 +/- 4 ng/l). FCPD patients, however, showed lower circulating concentrations of non-esterified fatty acids (0.73 +/- 0.11 mmol/l), glycerol (0.11 +/- 0.02 mmol/l) and 3-hydroxybutyrate (0.15 +/- 0.03 mmol/l) compared to IDDM patients (1.13 +/- 0.14, 0.25 +/- 0.05 and 0.29 +/- 0.08 mmol/l, respectively). This could be due to enhanced sensitivity of adipose tissue lipolysis to the suppressive action of circulating insulin and possibly also to insensitivity of hepatic ketogenesis to glucagon. Our results also demonstrate preservation of alpha-cell function in FCPD patients when beta-cell function is severely diminished, suggesting a more selective beta-cell dysfunction or destruction than hitherto believed.