Effect Of Gastric Inhibitory Polypeptide Research Articles

Effects of gastric inhibitory polypeptide (GIP) immunoneutralization on mouse motor coordination and memory

Gastric inhibitory polypeptide (GIP) is a regulatory peptide expressed in the mammalian upper small intestine, and both GIP and its receptor (GIPR) are expressed in the cortex and hippocampus regions of the brain as well. While learning and memory deficits have been observed in GIPR−/− mice, the effects of peripheral GIP immunoneutralization on motor-coordination, learning, and memory have not been examined. In the present study, adult GIPR−/− mice (KO) and age-matched wild-type C57BL/6 J mice (WT) received weekly vehicle PBS injections for 12 weeks, while a third group of wild-type mice were injected weekly for 12 weeks with 30 mg/kg body weight humanized GIP-mAb (AB) to assess the possibility of long-term effects of peripheral GIP antagonism on rodent memory and behavior. All mice groups then underwent a battery of tests that evaluated motor behavior, body coordination, and memory. Performance deficits in several memory studies after 12 weeks of treatment were demonstrated in KO, but not in AB or WT mice. Body coordination performance showed no significant differences among the 3 groups. A similar short-term study (3 injections over 9 days) was also conducted and the results were similar to those from the long-term study. Thus, short-term and long-term peripheral GIP antagonism by GIP-mAb did not appear to affect learning and memory in mice, consistent with the notion that the GIP-mAb does not cross the blood brain barrier. Furthermore, our studies indicate that GIP signaling in the brain appears to involve local neurocrine pathways.

Molecular Mechanism of Androgen Receptor Stimulation of Insulin Secretion in Male ß Cells

Androgen deprivation therapy increases the risk of type 2 diabetes mellitus (T2DM) in men. We previously showed that male pancreatic β-cell specific androgen receptor knockout (βARKOMIP) mice develop glucose intolerance because AR potentiates glucose-stimulated insulin secretion (GSIS) through increasing cyclic AMP (cAMP) production and amplifies the insulinotropic effect of glucagon-like peptide-1 (GLP-1). We show that male βARKOMIP mice exhibit impaired intraperitoneal (IP) glucose tolerance- because of impaired IP-GSIS- without alteration in oral glucose tolerance, suggesting that AR amplifies the islet-derived, but not the gut-derived GLP-1 to potentiate GSIS. Using male insulin-secreting β-cell line 832/3 transduced with exchange factor directly activated by a cAMP (EPAC)-based fluorescence resonance energy transfer (FRET) sensor, we observe that AR agonist dihydrotestosterone (DHT) allows GLP-1, glucagon and gastric inhibitory polypeptide (GIP) to increase cAMP production above level of the individual hormones and to a similar level to forskolin control. Accordingly, DHT increases the insulinotropic effect of forskolin, GLP-1, glucagon and GIP in mouse islets. The insulinotropic effect of DHT is abolished using EPAC and PKA inhibitors as well as rapamycin indicating that DHT amplifies the effect of GPCRs coupled to adenylate cyclase and stimulates GSIS via a cAMP/PKA/EPAC pathway and activation of mTOR. Disclosure W. Xu: None. F.B. Ashford: None. D. Hodson: None. F. Mauvais-Jarvis: Research Support; Self; Boehringer Ingelheim Pharmaceuticals, Inc..

Effects of gastric inhibitory polypeptide, glucagon-like peptide-1 and glucagon-like peptide-1 receptor agonists on Bone Cell Metabolism.

The relationship between gut and skeleton is increasingly recognized as part of the integrated physiology of the whole organism. The incretin hormones gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are secreted from the intestine in response to nutrient intake and exhibit several physiological functions including regulation of islet hormone secretion and glucose levels. A number of GLP-1 receptor agonists (GLP-1RAs) are currently used in treatment of type 2 diabetes and obesity. However, GIP and GLP-1 cognate receptors are widely expressed suggesting that incretin hormones mediate effects beyond control of glucose homeostasis, and reports on associations between incretin hormones and bone metabolism have emerged. The aim of this MiniReview was to provide an overview of current knowledge regarding the invivo and invitro effects of GIP and GLP-1 on bone metabolism. We identified a total of 30 pre-clinical and clinical investigations of the effects of GIP, GLP-1 and GLP-1RAs on bone turnover markers, bone mineral density (BMD), bone microarchitecture and fracture risk. Studies conducted in cell cultures and rodents demonstrated that GIP and GLP-1 play a role in regulating skeletal homeostasis, with pre-clinical data suggesting that GIP inhibits bone resorption whereas GLP-1 may promote bone formation and enhance bone material properties. These effects are not corroborated by clinical studies. While there is evidence of effects of GIP and GLP-1 on bone metabolism in pre-clinical investigations, clinical trials are needed to clarify whether similar effects are present and clinically relevant in humans.

Central administration of GLP-1 and GIP decreases feeding in mice

Glucagon-like peptide-1 amide (GLP-1) and gastric inhibitory polypeptide (GIP) are incretin hormones regulating energy metabolism. GLP-1 and GIP combination is suggested as a promising therapeutic strategy for treatment of obesity and diabetes. However, the neuronal mechanisms are not yet investigated. In the present study, we investigated the role of central GLP-1 and GIP in regulation of body weight homeostasis. The effect of GLP-1 with GIP on food intake, body weight, locomotor activity were determined following intracerebroventricular (ICV) administration of GLP-1 and/or GIP in mice. ICV administration of low dose GLP-1 (0.3 nmol) and GIP (1 and 3 nmol) did not change food intake. However, ICV administration of higher doses GLP-1 (1 and 3 nmol) and GIP (6 nmol) significantly decreased food intake and body weight. To investigate the synergic effect of ICV GLP-1 and GIP, subeffective dose GLP-1 (0.3 nmol) and subeffective dose GIP (1 nmol) were chosen for further co-administration study. ICV co-administration of GLP-1 and GIP significantly decreased food intake, body weight and drinking. ICV co-administration of GLP-1 and GIP significantly increased neuronal activation and pro-opiomelanocortin (POMC) expression in hypothalamic arcuate nucleus. The neuronal activation and POMC expression were observed in two distinct neuronal populations. These results provide neuronal mechanisms supporting the development of GLP-1 and GIP combination therapeutics for treatment of obesity and diabetes.

New Medications for the Treatment of Diabetes.

New Medications for the Treatment of Diabetes.

Discovery of gastric inhibitory polypeptide and its subsequent fate: Personal reflections

The present review focuses initially on experimental studies that were designed to identify acid inhibitory factors, referred to as ‘enterogastrones,’ that ultimately led to the isolation of gastric inhibitory polypeptide (GIP), a 42‐amino acid polypeptide. GIP was shown to inhibit acid secretion in animal models, as well as stimulating gastric somatostatin secretion. However, its role in human gastric physiology is unclear. Further studies showed that GIP strongly stimulated the secretion of insulin, in the presence of elevated glucose, and this ‘incretin’ action is now considered to be its most important; an alternative for the GIP acronym, glucose‐dependent insulinotropic polypeptide, was therefore introduced. In the 1970s, GIP purified by conventional chromatography was shown by high‐performance liquid chromatography to consist largely of GIP 1‐42 and GIP 3‐42. It was later shown that dipeptidyl peptidase 4 was a physiologically relevant enzyme responsible for this conversion, as well as the similar metabolism of the second incretin, glucagon‐like peptide‐1. Dipeptidyl peptidase‐4 inhibitors are currently in use as type 2 diabetes therapeutics, and studies on islet transplantation in rodent models of type 1 diabetes have shown that dipeptidyl peptidase‐4 inhibitor treatment reduces graft rejection. Additional studies on C‐terminally shortened forms of GIP have shown that GIP 1‐30 and a dipeptidyl peptidase‐4‐resistant form (D‐Ala2 GIP 1‐30) are equipotent to the intact polypeptide in vitro, and administration of D‐Ala2 GIP 1‐30 to diabetic rodents greatly improved glucose tolerance and reduced apoptotic cell death in islet β‐cells. There are probably therefore further clinically useful effects of GIP that require investigation.

Glucagon-like peptide-1 and gastric inhibitory polypeptide: new advances.

Glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) are gastrointestinal peptides that play an important role as incretin hormones in the regulation of plasma glucose and insulin secretion. GLP-1-based therapies have therefore been implemented as treatment for type 2 diabetes (T2D). The purpose of this review is to summarize novel treatment options for T2D with GLP-1-based therapies. In addition, both peptides have relevant extrapancreatic effects that have been further characterized recently and are summarized in this review. Novel findings regarding changes in GLP-1 secretion after bariatric surgery are highlighted, wherein GLP-1 plays a role in promoting body weight loss and diabetes remission. For T2D therapy, novel options with long-acting GLP-1 analogs are summarized that show a good efficacy and safety profile, also in combination with insulin as well as for obesity treatment. As GIP is not suitable for T2D therapy, extrapancreatic effects of GIP, mainly on bone metabolism that have been characterized recently, are described. These show that the activated GIP receptor is important to allow optimal bone mass and structure. This review summarizes new findings on the physiology and pathophysiology of GLP-1 and GIP and novel therapeutic aspects.

The effect of gastric inhibitory polypeptide on intestinal glucose absorption and intestinal motility in mice

Gastric inhibitory polypeptide (GIP) is released from the small intestine upon meal ingestion and increases insulin secretion from pancreatic β cells. Although the GIP receptor is known to be expressed in small intestine, the effects of GIP in small intestine are not fully understood. This study was designed to clarify the effect of GIP on intestinal glucose absorption and intestinal motility. Intestinal glucose absorption in vivo was measured by single-pass perfusion method. Incorporation of [14C]-glucose into everted jejunal rings in vitro was used to evaluate the effect of GIP on sodium-glucose co-transporter (SGLT). Motility of small intestine was measured by intestinal transit after oral administration of a non-absorbed marker. Intraperitoneal administration of GIP inhibited glucose absorption in wild-type mice in a concentration-dependent manner, showing maximum decrease at the dosage of 50nmol/kg body weight. In glucagon-like-peptide-1 (GLP-1) receptor-deficient mice, GIP inhibited glucose absorption as in wild-type mice. In vitro examination of [14C]-glucose uptake revealed that 100nM GIP did not change SGLT-dependent glucose uptake in wild-type mice. After intraperitoneal administration of GIP (50nmol/kg body weight), small intestinal transit was inhibited to 40% in both wild-type and GLP-1 receptor-deficient mice. Furthermore, a somatostatin receptor antagonist, cyclosomatostatin, reduced the inhibitory effect of GIP on both intestinal transit and glucose absorption in wild-type mice. These results demonstrate that exogenous GIP inhibits intestinal glucose absorption by reducing intestinal motility through a somatostatin-mediated pathway rather than through a GLP-1-mediated pathway.

Active immunization against (Pro3)GIP improves metabolic status in high‐fat‐fed mice

Ablation of gastric inhibitory polypeptide (GIP) receptor signalling can prevent many of the metabolic abnormalities associated with dietary-induced obesity-diabetes. The present study was designed to assess the ability of active immunization against (Pro(3))GIP to counter metabolic dysfunction associated with diet-induced obesity in high-fat-fed mice. Normal male Swiss NIH mice were injected (s.c.) once every 14 days for 98 days with complexed (Pro(3))GIP peptide, with transfer to a high-fat diet on day 21. Active immunization against (Pro(3))GIP resulted in circulating GIP antibody production and significantly (p < 0.05 p < 0.01) reduced circulating blood glucose concentrations compared to high-fat control mice from day 84 onwards. Glucose levels were not significantly different from lean controls. The glycaemic response to i.p. glucose was correspondingly improved (p < 0.01) in (Pro(3))GIP-immunized mice. Furthermore, circulating and glucose-stimulated plasma insulin levels were significantly (p < 0.01 to p < 0.001) depressed compared to high-fat control mice. Liver triglyceride, pancreatic insulin and circulating LDL-cholesterol levels were also significantly reduced in (Pro(3))GIP-immunized mice. These changes were independent of any effects on food intake or body weight. The glucose-lowering effect of native GIP was annulled in (Pro(3))GIP-immunized mice consistent with the induction of biologically effective GIP-specific neutralizing antibodies. These results suggest that immunoneutralization of GIP represents an effective means of countering the disruption of metabolic processes induced by high-fat feeding.

Effects of long-term dipeptidyl peptidase-IV inhibition on body composition and glucose tolerance in high fat diet-fed mice

Aim Glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) are major incretins associated with body weight regulation. Dipeptidyl peptidase-IV (DPP-IV) inhibitor increases plasma active GLP-1 and GIP. However, the magnitude of the effects of enhanced GLP-1 and GIP signaling by long-term DPP-IV inhibition on body weight and insulin secretion has not been determined. In this study, we compared the effects of long-term DPP-IV inhibition on body composition and insulin secretion of high fat diet (HFD)-fed wild-type (WT) and GLP-1R knockout ( GLP-1R −/−) mice. Main methods HFD-fed WT and GLP-1R −/− mice were treated with or without DPP-IV inhibitor by drinking water. Food and water intake and body weight were measured during 8 weeks of study. CT-based body composition analysis, Oral glucose tolerance test (OGTT), batch incubation study for insulin secretion and quantitative RT-PCR for expression of incretin receptors in isolated islets were performed at the end of study. Key findings DPP-IV inhibitor had no effect on food and water intake and body weight, but increased body fat mass in GLP-1R −/− mice. DPP-IV inhibitor-treated WT and GLP-1R −/− mice both showed increased insulin secretion in OGTT. In isolated islets of DPP-IV inhibitor-treated WT and GLP-1R −/− mice, glucose-induced insulin secretion was increased and insulin secretion in response to GLP-1 or GIP was preserved, without downregulation of incretin receptor expression. Significance Long-term DPP-IV inhibition may maintain body composition through counteracting effects of GLP-1 and GIP while improving glucose tolerance by increasing glucose-induced insulin secretion through the synergistic effects of GLP-1 and GIP.

Glucose-induced GIP levels in patients with insulinoma.

Gastric inhibitory polypeptide (GIP) levels during an oral glucose tolerance test (OGTT) have been determined in eight patients suffering from a single benign beta-cell adenoma. GIP hypersecretion and an absent correlation between the insulin: glucose ratio and plasma GIP concentration have been observed. Our data show that the insulinotropic effect of GIP is almost abolished in insulinoma patients. This finding may help explain blood glucose levels and the inappropriate plasma insulin response to oral glucose often observed in these patients.

Effects of gastric inhibitory polypeptide (GIP) and related analogues on glucagon release at normo- and hyperglycaemia in Wistar rats and isolated islets

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone secreted by endocrine K-cells in response to nutrient absorption. This study has utilised numerous well-characterised dipeptidyl peptidase IV-resistant GIP analogues to evaluate the glucagonotropic actions of GIP in Wistar rats and isolated rat islets. Intraperitoneal administration of GIP analogues (25 nmol/kg body weight) in combination with glucose had no effect on circulating glucagon concentrations compared to controls in Wistar rats. However, plasma glucose concentrations were significantly (p<0.05 to p<0.001) lowered by the GIP-receptor agonists, N-AcGIP, GIP(Lys37)PAL and N-AcGIP(Lys37)PAL. The GIP antagonist, (Pro3)GIP, caused a significant (p<0.05) reduction in glucagon levels following concurrent administration with saline in Wistar rats. In isolated rat islets native GIP induced a significant (p<0.01) enhancement of glucagon release at basal glucose concentrations, which was completely annulled by (Pro3)GIP. Furthermore, glucagon release in the presence of GLP-1, GIP(Lys37)PAL, N-AcGIP(Lys37)PAL and (Pro3)GIP was significantly (p<0.05 to p<0.001) decreased compared to native GIP in isolated rat islets. These data indicate a modest effect of GIP on glucagon secretion from isolated rat islets, which was not observed in vivo. However, the GIP agonists N-AcGIP, GIP(Lys37)PAL and N-AcGIP(Lys37)PAL had no effect on glucagon release demonstrating an improved therapeutic potential for the treatment of type 2 diabetes.

Pancreatic and Extrapancreatic Effects of Gastric Inhibitory Polypeptide

The hormonal factor(s) implicated as transmitters of signals from the gut to pancreatic β-cells is referred to as incretin, and gastric inhibitory polypeptide (GIP) is identified as one of the incretins. GIP is a gastrointestinal peptide hormone of 42 amino acids that is released from duodenal endocrine K-cells after absorption of glucose or fat and exerts its effects by binding to its specific receptor, the GIP receptor. By generating and characterizing mice with a targeted mutation of the GIP receptor gene, we have shown that GIP has not only an insulinotropic role, but also physiological roles on fat accumulation into adipose tissues and calcium accumulation into bone. We here propose a new acronym, GIP, for gut-derived nutrient-intake polypeptide.

Incretins and the development of type 2 diabetes

The incretin hormones gastric inhibitory polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are released in response to nutrient ingestion and potentiate glucose-stimulated insulin secretion from pancreatic beta cells. The augmentation of postprandial insulin secretion by such gastrointestinal hormones is called the incretin effect. The incretin effect is almost completely absent in patients with type 2 diabetes. This is due to 1) an approximate 15% reduction in postprandial GLP-1 secretion and 2) a near total loss of insulinotropic activity of GIP. This review article summarizes clinical studies on abnormalities in the secretion and insulinotropic effects of GIP and GLP-1 in patients with type 2 diabetes as well as in individuals at high risk. A significant proportion of first-degree relatives are characterized by a reduced insulinotropic response to exogenous GIP. Nevertheless, this phenomenon does not predispose to a more rapid deterioration in glucose tolerance or conversion to impaired glucose tolerance or diabetes. Therefore, although there are hints of early abnormalities in incretin secretion and action in prediabetic populations, it has not been proven that such phenomena are central to the pathogenesis of type 2 diabetes.

Glucagon-like peptide-1: from extract to agent. The Claude Bernard Lecture, 2005

The incretin hormones are intestinal polypeptides that enhance postprandial insulin secretion. Gastric inhibitory polypeptide (GIP) was initially thought to regulate gastric acid secretion, whereas glucagon-like peptide-1 (GLP-1) was discovered as a result of a systematic search for intestinal insulinotropic products of proglucagon gene expression. The incretin effect is markedly impaired or absent in patients with type 2 diabetes because of decreased secretion of GLP-1 and a loss of the insulinotropic effects of GIP. Metabolic control can be restored or greatly improved by administration of exogenous GLP-1, but this peptide is almost immediately degraded by dipeptidyl peptidase IV (DPP-IV), and therefore has little clinical value. DPP-IV-resistant analogues (incretin mimetics) have been identified or developed, and inhibitors of DPP-IV have also proved effective in protecting endogenous GLP-1 (and GIP) from degradation. Both principles have been tested in clinical studies. The incretin mimetics, administered by sc injection, have demonstrated lasting improvement in HbA(1)c in patients insufficiently treated with conventional oral therapy, and their use has been associated with steady weight loss for up to 2 years. The DPP-IV inhibitors, given once or twice daily by mouth, also appear to provide lasting improvement in HbA(1)c, but are weight-neutral. The first incretin mimetic has reached the market in the US, and applications for approval of the first inhibitors are expected to be filed early in 2006.

Secretion of incretin hormones and the insulinotropic effect of gastric inhibitory polypeptide in women with a history of gestational diabetes

The insulinotropic effect of gastric inhibitory polypeptide (GIP) is reduced in patients with type 2 diabetes and around 50% of their first-degree relatives under hyperglycaemic conditions. It is unknown whether this is a result of a specific defect in GIP action or of a general reduction in beta cell function. Moreover, impaired secretion of glucagon-like peptide 1 (GLP-1) has been described in patients with type 2 diabetes. Therefore, we studied the insulinotropic effect of GIP in women with previous gestational diabetes (pGDM) under euglycaemic fasting conditions and during a hyperglycaemic clamp experiment. The secretion of GIP and GLP-1 was assessed following oral glucose ingestion. On separate occasions we performed an OGTT and administered an i.v. bolus of 20 pmol GIP/kg body weight in 20 women with pGDM and 20 control women. An additional hyperglycaemic clamp experiment (140 mg/dl [7.8 mmol/l] over 120 min) with i.v. infusion of GIP (2 pmol kg(-1) min(-1); 30-90 min) was performed in 14 women in each group. Capillary and venous blood samples were drawn for the measurement of glucose (glucose oxidase), insulin, C-peptide, GIP and GLP-1 (specific immunoassays). Indices of insulin sensitivity and beta cell function were calculated. Statistical analyses were carried out using repeated measures ANOVA. Following oral glucose ingestion, plasma glucose, insulin and C-peptide concentrations increased to higher levels in the women with pGDM than in the control women (p<0.05). The women with pGDM were characterised by a higher degree of insulin resistance than the control women (p=0.007 for the Matsuda index), but showed no overt defects in glucose-stimulated insulin secretion (p=0.40 for the insulinogenic index following i.v. glucose). The secretion of GLP-1 and GIP was not different between the groups (p=0.87 and p=0.57, respectively). The insulin secretory response to GIP administration was similar in the two groups both after GIP bolus administration and during the hyperglycaemic clamp experiment (p=0.99 and p=0.88, respectively). A hyperbola-like relationship was found between the degree of insulin sensitivity (Matsuda index) and the insulin secretory response to GIP and i.v. glucose administration. These results do not support the hypothesis of an early defect in GIP action as a risk factor for subsequent development of diabetes in women with previous gestational diabetes. The inverse relationship between insulin resistance and the insulin secretory response to glucose or GIP suggests that beta cell secretory function in response to different stimuli increases adaptively when insulin sensitivity is diminished.

Stimulation of Insulin Secretion by Intravenous Bolus Injection and Continuous Infusion of Gastric Inhibitory Polypeptide in Patients With Type 2 Diabetes and Healthy Control Subjects

A reduced insulinotropic effect of gastric inhibitory polypeptide (GIP) is a characteristic of patients with type 2 diabetes. It was the aim of this study to determine the response of insulin secretion to different GIP doses administered by intravenous bolus injection and via continuous infusion in both healthy subjects and patients with type 2 diabetes. Eight patients with type 2 diabetes and eight healthy subjects participated in a 240-min hyperglycemic clamp (140 mg/dl) with intravenous infusion of placebo, GIP at a low dose, and GIP at a high dose, each administered continuously over 60 min. Boluses of placebo, 20 pmol GIP/kg, and 80 pmol GIP/kg were injected intravenously at 0, 60, and 120 min, respectively. Capillary and venous blood was drawn for glucose, insulin, C-peptide, and GIP. Plasma insulin and C-peptide concentrations were lower in patients than in control subjects during all infusion periods. GIP bolus administration evoked a significant increase in plasma insulin levels in both patients with type 2 diabetes and healthy subjects. In contrast, the continuous GIP infusion led to a weak increase in insulin secretion in both healthy subjects and type 2 diabetic patients. The dose-response relationship for the increase in insulin secretion after GIP bolus administration was similar in both groups, although at different degrees of beta-cell function. The stimulation of insulin secretion by GIP is stronger after its bolus administration than during continuous infusion. Even though the insulin secretory capacity is generally impaired in patients with type 2 diabetes, the relative sensitivity of insulin secretion to a bolus administration of GIP is almost preserved. Therefore, the existence of a specific GIP receptor defect in type 2 diabetes appears unlikely.

Effect of GIP, GLP-1, insulin and gastrin on ghrelin release in the isolated rat stomach

Ghrelin release in man depends on the macronutrient composition of the test meal. The mechanisms contributing to the differential regulation are largely unknown. To elucidate their potential role, glucagon-like peptide-1 (GLP-1), gastric inhibitory polypeptide (GIP), insulin, gastrin and somatostatin were examined on isolated rat stomach ghrelin secretion, which offers the advantage of avoiding systemic interactions. Basal ghrelin secretion was in a range that did not permit to consistently evaluate inhibiting effects. Therefore, the effect of gastrointestinal hormones and insulin was analyzed during vagal prestimulation. GLP-1(7–36)amide 10 −8 and 10 −7 M decreased ghrelin secretion significantly. In contrast, GIP 10 −8 and 10 −7 M augmented not only prestimulated, but also basal ghrelin secretion ( p<0.05). Insulin reduced ghrelin at 10 −10, 10 −8 and 10 −6 M ( p<0.05). Both gastrin 10 −8 M and somatostatin 10 −6 M also significantly inhibited ghrelin secretion. These data demonstrate that GLP-1(7–36)amide, insulin, gastrin and somatostatin are potential candidates to contribute to the postprandially observed inhibition of ghrelin secretion with insulin being the most effective inhibitor in this isolated stomach model. GIP, on the other hand, could attenuate the postprandial decrease. Because protein-rich meals do not effectively stimulate GIP release, other as yet unknown intestinal factors must be responsible for protein-induced stimulation of ghrelin release.

Gastric inhibitory polypeptide does not inhibit gastric emptying in humans

The insulinotropic gut hormone gastric inhibitory polypeptide (GIP) has been demonstrated to inhibit gastric acid secretion and was proposed to possess "enterogastrone" activity. GIP effects on gastric emptying have not yet been studied. Fifteen healthy male volunteers (23.9 +/- 3.3 yr, body mass index 23.7 +/- 2.3 kg/m(2)) were studied with the intravenous infusion of GIP (2 pmol.kg(-1).min(-1)) or placebo, each administered to the volunteers on separate occasions from -30 to 360 min in the fasting state. At 0 min, a solid test meal (250 kcal containing [(13)C]sodium octanoate) was served. Gastric emptying was calculated from the (13)CO(2) exhalation rates in breath samples collected over 360 min. Venous blood was drawn in 30-min intervals for the determination of glucose, insulin, C-peptide, and GIP (total and intact). Statistical calculations were made by use of repeated-measures ANOVA and one-way ANOVA. During the infusion, GIP rose to steady-state concentrations of 159 +/- 15 pmol/l for total and 34 +/- 4 pmol/l for intact GIP (P < 0.0001). Meal ingestion further increased GIP concentrations in both groups, reaching peak levels of 265 +/- 20 and 82 +/- 9 pmol/l for total and 67 +/- 7 and 31 +/- 9 pmol/l for intact GIP during the administration of GIP and placebo, respectively (P < 0.0001). There were no differences in glucose, insulin, and C-peptide between the experiments with the infusion of GIP or placebo. Gastric half-emptying times were 120 +/- 9 and 120 +/- 18 min (P = 1.0, with GIP and placebo, respectively). The time pattern of gastric emptying was similar in the two groups (P = 0.98). Endogenous GIP secretion, as derived from the incremental area under the curve of plasma GIP concentrations in the placebo experiments, did not correlate to gastric half-emptying times (r(2) = 0.15, P = 0.15 for intact GIP; r(2) = 0.21, P = 0.086 for total GIP). We conclude that gastric emptying does not appear to be influenced by GIP. The secretion of GIP after meal ingestion is not suppressed by its exogenous administration. The lack of effect of GIP on gastric emptying underlines the differences between GIP and the second incretin glucagon-like peptide 1.

Similar insulin secretory response to a gastric inhibitory polypeptide bolus injection at euglycemia in first-degree relatives of patients with type 2 diabetes and control subjects

Insulin secretion following the intravenous infusion of gastric inhibitory polypeptide (GIP) is diminished in patients with type 2 diabetes and at least a subgroup of their first-degree relatives at hyperglycemic clamp conditions. Therefore, we studied the effects of an intravenous bolus administration of GIP at normoglycemic conditions in the fasting state. Ten healthy control subjects were studied with an intravenous bolus administration of placebo, and of 7, 20, and 60 pmol GIP/kg body weight (BW), respectively. Forty-five first-degree relatives of patients with type 2 diabetes and 33 matched control subjects were studied with (1) a 75-g oral glucose tolerance test (OGTT) and (2) an intravenous bolus injection of 20 pmol GIP/kg BW with blood samples drawn over 30 minutes for determination of plasma glucose, insulin, C-peptide, and GIP. Statistical analysis applied repeated-measures analysis of variance (ANOVA) and Duncan’s post hoc tests. Insulin secretion was stimulated after the administration of 20 and of 60 pmol GIP/kg BW in the dose-response experiments ( P < .0001). GIP administration (20 pmol/kg BW) led to a significant rise of insulin and C-peptide concentrations in the first-degree relatives and control subjects ( P < .0001), but there was difference between groups ( P = .64 and P = .87, respectively). Also expressed as increments over baseline, no differences were apparent (Δ insulin, 7.6 ± 1.2 and 7.6 ± 1.6 mU/L, P = .99; Δ C-peptide, 0.35 ± 0.06 and 0.38 ± 0.08 ng/mL, P = .75). Integrated insulin and C-peptide responses after GIP administration significantly correlated with the respective insulin and C-peptide responses after glucose ingestion (insulin, r = 0.78, P < .0001; C-peptide, r = 0.35, P = .0015). We conclude that a reduced insulinotropic effect of GIP in first-degree relatives of patients with type 2 diabetes cannot be observed at euglycemia. Therefore, a reduced GIP-induced insulin secretion in patients with type 2 diabetes and their first-degree relatives at hyperglycemia is more likely due to a general defect of B-cell function than to a specific defect of the GIP action.