Characterisation and glucoregulatory actions of a novel acylated form of the (Pro3)GIP receptor antagonist in type 2 diabetes

In this study, we tested the biological activity of a novel acylated form of (Pro3)glucose-dependent insulinotropic polypetide [(Pro3)GIP] prepared by conjugating palmitic acid to Lys16 to enhance its efficacy in vivo by promoting binding to albumin and extending its biological actions. Like the parent molecule (Pro3)GIP, (Pro3)GIPLys16PAL was completely stable to the actions of DPP-IV and significantly (p<0.01 to p<0.001) inhibited GIP-stimulated cAMP production and cellular insulin secretion. Furthermore, acute administration of (Pro3)GIPLys16PAL also significantly (p<0.05 to p<0.001) countered the glucose-lowering and insulin-releasing actions of GIP in ob/ob mice. Daily injection of (Pro3)GIPLys16PAL (25 nmol/kg bw) in 14-18-week-old ob/ob mice over 14 days had no effect on body weight, food intake or non-fasting plasma glucose and insulin concentrations. (Pro3)GIPLys16PAL treatment also failed to significantly alter the glycaemic response to an i.p. glucose load or test meal, but insulin concentrations were significantly reduced (1.5-fold; p<0.05) after the glucose load. Insulin sensitivity was enhanced (1.3-fold; p<0.05) and pancreatic insulin was significantly reduced (p<0.05) in the (Pro3)GIPLys16PAL-treated mice. These data demonstrate that acylation of Lys16 with palmitic acid in (Pro3)GIP does not improve its biological effectiveness as a GIP receptor antagonist.     

Biological activity and antidiabetic potential of synthetic fragment peptides of glucose-dependent insulinotropic polypeptide, GIP(1-16) and (Pro3)GIP(1-16)

Glucose-dependent insulinotropic polypeptide (GIP) is a key physiological insulin releasing peptide and potential antidiabetic agent. The present study was undertaken in an attempt to develop small molecular weight GIP agonist and antagonist molecules. The bioactivity of two modified C-terminally truncated fragment GIP peptides, GIP(1-16) and (Pro3)GIP(1-16), was examined in terms of insulin secretion and glucose homeostasis using BRIN-BD11 cells and type 2 diabetic mice. In vitro insulin release studies demonstrated that GIP(1-16) and (Pro3)GIP(1-16) possessed weak GIP-receptor agonist and antagonistic properties, respectively. Intraperitoneal administration of GIP(1-16) in combination with glucose to obese diabetic (ob/ob) mice did not effect the glycaemic excursion and had a marginal effect on insulin release. GIP(1-16) was substantially less effective than the native GIP(1-42). (Pro3)GIP(1-16) administration significantly curtailed (P < 0.05) the insulinotropic and glucose lowering effects of native GIP, but was significantly less effective than (Pro3)GIP. Based on the established concept of a therapeutic benefit of GIP receptor antagonism in obesity-diabetes, ob/ob mice received once daily injection of (Pro3)GIP(1-16) for 14 days. No significant effects were observed on food intake, body weight, HbA1c, glucose tolerance, metabolic response to feeding and either insulin secretion or insulin sensitivity following prolonged (Pro3)GIP(1-16) treatment. These data demonstrate that C-terminal truncation of GIP or (Pro3)GIP yields small molecular weight GIP molecules with significantly reduced biological activity that precludes therapeutic utility.     

Effects on glucose homeostasis and insulin secretion of long term activation of the glucose-dependent insulinotropic polypeptide (GIP) receptor by N-AcGIP(LysPAL37) in normal mice

Glucose-dependent insulinotropic polypeptide (GIP) is a key hormone of the enteroinsular axis. The present study was designed to assess the metabolic effects in healthy mice of long term activation of the GIP receptor by N-AcGIP(LysPAL37), a potent long-acting GIP receptor agonist. Daily injection of N-AcGIP(LysPAL37) (25 nmol/kg body weight) for 14 days had no significant effect on food intake, body weight, glycated hemoglobin levels, non-fasting plasma glucose and insulin concentrations compared to saline treated controls. No significant differences in post-prandial plasma glucose and insulin concentrations were observed between the two groups following 15 min feeding. However, after 14 days, the glycemic response to intraperitoneal (i.p.) glucose was significantly improved in the N-AcGIP(LysPAL37) treated mice compared to controls (P < 0.01). In keeping with this, glucose-mediated insulin secretion was significantly enhanced in the N-AcGIP(LysPAL37) treated group (P < 0.05). No changes in insulin sensitivity or pancreatic insulin content of the N-AcGIP(LysPAL37) treated mice were detected. No adverse reactions were noted and the effects of N-AcGIP(LysPAL37) were reversed by 14 days cessation of treatment. These data indicate that long term activation of the GIP receptor by daily treatment with N-AcGIP(LysPAL37) improved glucose tolerance due to enhancement of pancreatic beta cell glucose responsiveness and insulin secretion.     

Characterisation and biological activity of Glu3 amino acid substituted GIP receptor antagonists

Glucose-dependent insulinotropic polypeptide (GIP) is an important gastrointestinal hormone, which regulates insulin release and glucose homeostasis, but is rapidly inactivated by enzymatic N-terminal truncation. Here we report the enzyme resistance and biological activity of several Glu(3)-substituted analogues of GIP namely; (Ala(3))GIP, (Lys(3))GIP, (Phe(3))GIP, (Trp(3))GIP and (Tyr(3))GIP. Only (Lys(3))GIP demonstrated moderately enhanced resistance to DPP-IV (p<0.05 to p<0.01) compared to native GIP. All analogues demonstrated a decreased potency in cAMP production (EC(50) 1.47 to 11.02 nM; p<0.01 to p<0.001) with (Lys(3))GIP and (Phe(3))GIP significantly inhibiting GIP-stimulated cAMP production (p<0.05). In BRIN-BD11 cells, (Lys(3))GIP, (Phe(3))GIP, (Trp(3))GIP and (Tyr(3))GIP did not stimulate insulin secretion with both (Lys(3))GIP and (Phe(3))GIP significantly inhibiting GIP-stimulated insulin secretion (p<0.05). Injection of each GIP analogue together with glucose in ob/ob mice significantly increased the glycaemic excursion compared to control (p<0.05 to p<0.001). This was associated with lack of significant insulin responses. (Ala(3))GIP, (Phe(3))GIP and (Tyr(3))GIP, when administered together with GIP, significantly reduced plasma insulin (p<0.05 to p<0.01) and impaired the glucose-lowering ability (p<0.05 to p<0.01) of the native peptide. The DPP-IV resistance and GIP antagonism observed were similar but less pronounced than (Pro(3))GIP. These data demonstrate that position 3 amino acid substitution of GIP with (Ala(3)), (Phe(3)), (Tyr(3)) or (Pro(3)) provides a new class of functional GIP receptor antagonists.     

Active immunisation against gastric inhibitory polypeptide (GIP) improves blood glucose control in an animal model of obesity-diabetes

Recent research suggests that long-term ablation of gastric inhibitory polypeptide (GIP) receptor signalling can reverse or prevent many of the metabolic abnormalities associated with dietary and genetically induced obesity-diabetes. The present study was designed to assess the sub-chronic effects of passive or active immunisation against GIP in ob/ob mice. Initial acute administration of GIP antibody together with oral glucose in ob/ob mice significantly increased the glycaemic excursion compared to controls (p<0.05). This was associated with a significant reduction (p<0.05) in the overall glucose-mediated insulin response. However, sub-chronic passive GIP immunisation was not associated with any changes in body weight, food intake or metabolic control. In contrast, active immunisation against GIP for 56 days in young ob/ob mice resulted in significantly (p<0.05) reduced circulating plasma glucose concentrations on day 56 compared to controls. There was a tendency for decreased circulating insulin in GIP immunised mice. The glycaemic response to intraperitoneal glucose was correspondingly improved (p<0.05) in mice immunised against GIP. Glucose-stimulated insulin levels were not significantly different from controls. Furthermore, insulin sensitivity was similar in mice immunised against GIP and respective controls. Overall, the results reveal that active, as opposed to passive, immunisation against GIP improves blood glucose control ob/ob mice.     

Metabolic effects of sub-chronic ablation of the incretin receptors by daily administration of (Pro3)GIP and exendin(9-39)amide in obese diabetic (ob/ob) mice

Effects of chemical ablation of the GIP and GLP-1 receptors on metabolic aspects of obesity-diabetes were investigated using the stable receptor antagonists (Pro3)GIP and exendin(9-39)amide. Ob/ob mice received a daily i.p. injection of saline vehicle, (Pro3)GIP, exendin(9-39)amide or a combination of both peptides over a 14-day period. Non-fasting plasma glucose levels were significantly (p<0.05) lower in (Pro3)GIP-treated mice compared to control mice after just 9 days of treatment. (Pro3)GIP-treated mice also displayed significantly lower plasma glucose concentrations in response to feeding and intraperitoneal administration of either glucose or insulin (p<0.05 to p<0.001). The (Pro3)GIP-treated group also exhibited significantly (p<0.05) reduced pancreatic insulin content. Acute administration of exendin(9-39)amide immediately prior to re-feeding completely annulled the beneficial effects of sub-chronic (Pro3)GIP treatment, but non-fasting concentrations of active GLP-1 were unchanged. Combined sub-chronic administration of (Pro3GIP) with exendin(9-39)amide revealed no beneficial effects. Similarly, daily administration of exendin(9-39)amide alone had no significant effects on any of the metabolic parameters measured. These studies highlight an important role for GIP in obesity-related forms of diabetes, suggesting the possible involvement of GLP-1 in the beneficial actions of GIP receptor antagonism.     

Comparison of the metabolic effects of GIP receptor antagonism and PYY(3-36) receptor activation in high fat fed mice

Glucose-dependent insulinotropic polypeptide (GIP) and peptide YY (PYY) are secreted from the intestinal K- and L-cells, respectively, following a meal. Both peptides are believed to play a key role in glucose homeostasis and energy expenditure. This study investigated the effects of daily administration of the stable and specific GIP-R antagonist, (Pro(3))GIP (25 nmol/kg) and the endogenous truncated form of PYY, PYY(3-36) (50 nmol/kg), in mice fed with a high fat diet. Daily i.p. injection of (Pro(3))GIP, PYY(3-36) or combined peptide administration over 24 days significantly (P<0.05-0.01) decreased body weight compared with saline-treated controls without change in food intake. Plasma glucose levels and glucose tolerance were significantly (P<0.05) lowered by (Pro(3))GIP treatment alone, and in combination with PYY(3-36). These changes were accompanied by a slight improvement of insulin sensitivity in all of the treatment groups. (Pro(3))GIP treatment significantly reduced plasma corticosterone (P<0.05), while combined administration with PYY(3-36) significantly lowered serum glucagon (P<0.05). No appreciable changes were observed in either circulating or glucose-stimulated insulin secretion in all treatment groups. (Pro(3))GIP-treated mice had significantly (P<0.01) lowered fasting glucose levels and an improved (P<0.05) glycemic response to feeding. These comparative data indicate that chemical ablation of GIP receptor action using (Pro(3))GIP provides an especially effective means of countering obesity and related abnormalities induced by consumption of high fat energy rich diet.     

GIP receptor antagonism reverses obesity, insulin resistance, and associated metabolic disturbances induced in mice by prolonged consumption of high-fat diet

The gut hormone gastric inhibitory polypeptide (GIP) plays a key role in glucose homeostasis and lipid metabolism. This study investigated the effects of administration of a stable and specific GIP receptor antagonist, (Pro(3))GIP, in mice previously fed a high-fat diet for 160 days to induce obesity and related diabetes. Daily intraperitoneal injection of (Pro(3))GIP over 50 days significantly decreased body weight compared with saline-treated controls, with a modest increase in locomotor activity but no change of high-fat diet intake. Plasma glucose, glycated hemoglobin, and pancreatic insulin were restored to levels of chow-fed mice, and circulating triglyceride and cholesterol were significantly decreased. (Pro(3))GIP treatment also significantly decreased circulating glucagon and corticosterone, but concentrations of GLP-1, GIP, resistin, and adiponectin were unchanged. Adipose tissue mass, adipocyte hypertrophy, and deposition of triglyceride in liver and muscle were significantly decreased. These changes were accompanied by significant improvement of insulin sensitivity, meal tolerance, and normalization of glucose tolerance in (Pro(3))GIP-treated high-fat-fed mice. (Pro(3))GIP concentrations peaked rapidly and remained elevated 24 h after injection. These data indicate that GIP receptor antagonism using (Pro(3))GIP provides an effective means of countering obesity and related diabetes induced by consumption of a high-fat, energy-rich diet.     

Alterations of glucose-dependent insulinotropic polypeptide (GIP) during cold acclimation

Cold acclimation is initially associated with shivering thermogenesis in skeletal muscle followed by adaptive non-shivering thermogenesis, particularly in brown adipose tissue (BAT). In response, hyperphagia occurs to meet increased metabolic demand and thermoregulation. The present study investigates the effects of cold (4 ± 1 °C) acclimation and hyperphagia on circulating and intestinal levels of gastric inhibitory polypeptide (GIP) in rats. Pair fed animals were used as additional controls in some experiments. Cold acclimation for 42 days significantly (p<0.01) increased daily food intake. There was no corresponding change in body weight. However, body weights of pair fed cold exposed rats were significantly (p<0.01) reduced compared to controls and ad libitum fed cold exposed rats. By day 42, non-fasting plasma glucose was increased (p<0.05) by chronic cold exposure regardless of food intake. Corresponding plasma insulin concentrations were significantly (p<0.01) lower in pair fed cold exposed rats. Circulating GIP levels were elevated (p<0.05) in ad libitum fed cold acclimated rats on days 18 and 24, but returned to normal levels by the end of the study. The glycaemic response to oral glucose was improved (p<0.01) in all cold exposed rats, with significantly (p<0.05) elevated GIP responses in ad libitum fed rats and significantly (p<0.05) reduced insulin responses in pair fed rats. In keeping with this, insulin sensitivity was enhanced (p<0.05) in cold exposed rats compared to controls. By the end of the study, cold acclimated rats had significantly (p<0.01) increased BAT mass and intestinal concentrations of GIP and GLP-1 compared to controls, independent of food intake. These data indicate that changes in the secretion and actions of GIP may be involved in the metabolic adaptations to cold acclimation in rats.     

Antagonistic effects of two novel GIP analogs, (Hyp3)GIP and (Hyp3)GIPLys16PAL, on the biological actions of GIP and longer-term effects in diabetic ob/ob mice

This study examines the actions of the novel enzyme-resistant, NH2-terminally modified GIP analog (Hyp(3))GIP and its fatty acid-derivatized analog (Hyp(3))GIPLys(16)PAL. Acute effects are compared with the established GIP receptor antagonist (Pro(3))GIP. All three peptides exhibited DPP IV resistance, and significantly inhibited GIP stimulated cAMP formation and insulin secretion in GIP receptor-transfected fibroblasts and in clonal pancreatic BRIN-BD11 cells, respectively. Likewise, in obese diabetic ob/ob mice, intraperitoneal administration of GIP analogs significantly inhibited the acute antihyperglycemic and insulin-releasing effects of native GIP. Administration of once daily injections of (Hyp(3))GIP or (Hyp(3))GIPLys(16)PAL for 14 days resulted in significantly lower plasma glucose levels (P < 0.05) after (Hyp(3))GIP on days 12 and 14 and enhanced glucose tolerance (P < 0.05) and insulin sensitivity (P < 0.05 to P < 0.001) in both groups by day 14. Both (Hyp(3))GIP and (Hyp(3))GIPLys(16)PAL treatment also reduced pancreatic insulin (P < 0.05 to P < 0.01) without affecting islet number. These data indicate that (Hyp(3))GIP and (Hyp(3))GIPLys(16)PAL function as GIP receptor antagonists with potential for ameliorating obesity-related diabetes. Acylation of (Hyp(3))GIP to extend bioactivity does not appear to be of any additional benefit.     

Characterization of the cellular and metabolic effects of a novel enzyme-resistant antagonist of glucose-dependent insulinotropic polypeptide

A novel N-terminally substituted Pro(3) analogue of glucose-dependent insulinotropic polypeptide (GIP) was synthesized and tested for plasma stability and biological activity both in vitro and in vivo. Native GIP was rapidly degraded by human plasma with only 39 +/- 6% remaining intact after 8 h, whereas (Pro(3))GIP was completely stable even after 24 h. In CHL cells expressing the human GIP receptor, (Pro(3))GIP antagonized the cyclic adenosine monophosphate (cAMP) stimulatory ability of 10(-7) M native GIP, with an IC(50) value of 2.6 microM. In the clonal pancreatic beta cell line BRIN-BD11, (Pro(3))GIP over the concentration range 10(-13) to 10(-8) M dose dependently inhibited GIP-stimulated (10(-7) M) insulin release (1.2- to 1.7-fold; P < 0.05 to P < 0.001). In obese diabetic (ob/ob) mice, intraperitoneal administration of (Pro(3))GIP (25 nmol/kg body wt) countered the ability of native GIP to stimulate plasma insulin (2.4-fold decrease; P < 0.001) and lower the glycemic excursion (1.5-fold decrease; P < 0.001) induced by a glucose load (18 mmol/kg body wt). Collectively these data demonstrate that (Pro(3))GIP is a novel and potent enzyme-resistant GIP receptor antagonist capable of blocking the ability of native GIP to increase cAMP, stimulate insulin secretion, and improve glucose homeostasis in a commonly employed animal model of type 2 diabetes.     

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.     

The reduction in hepatic insulin clearance after oral glucose is not mediated by gastric inhibitory polypeptide (GIP)

Since the C-peptide/insulin ratio is reduced after oral glucose ingestion, the incretin hormone gastric inhibitory polypeptide (GIP) has been assumed to decrease hepatic insulin extraction. It was the aim of the present study to evaluate the effects of GIP on insulin extraction. Seventy-eight healthy subjects (27 male, 51 female, 43+/-11 years) were subjected to (a). an oral glucose tolerance test and (b). an intravenous injection of 20 pmol GIP/kg body weight, with capillary and venous blood samples collected over 30 min for insulin, C-peptide and GIP (specific immunoassays). Following GIP administration, plasma concentrations of total and intact GIP reached to peak levels of 80+/-7 and 54+/-5 pmol/l, respectively (p<0.0001). The rise in insulin after oral glucose and after intravenous GIP administration significantly exceeded the rise in C-peptide (p<0.0001). Estimating insulin extraction from the total integrated insulin and C-peptide concentrations (AUCs), only the oral glucose load (p<0.0001), but not the intravenous GIP administration (p=0.18) significantly reduced insulin clearance. Therefore, insulin clearance is reduced after an oral glucose load. This effect does not appear to be mediated by GIP.     

Secretion of incretin hormones (GIP and GLP-1) and incretin effect after oral glucose in first-degree relatives of patients with type 2 diabetes

AIMS/HYPOTHESIS: Since insulin secretion in response to exogenous gastric inhibitory polypeptide (GIP) is diminished not only in patients with type 2 diabetes, but also in their normal glucose-tolerant first-degree relatives, it was the aim to investigate the integrity of the entero-insular axis in such subjects. METHODS: Sixteen first-degree relatives of patients with type 2 diabetes (4 male, 12 female, age 50+/-12 years, BMI 26.1+/-3.8 kg/m(2)) and 10 matched healthy controls (negative family history, 6 male, 4 female, 45+/-13 years, 26.1+/-4.2 kg/m(2)) were examined with an oral glucose load (75 g) and an "isoglycaemic" intravenous glucose infusion. Blood was drawn over 240 min for plasma glucose (glucose oxidase), insulin, C-peptide, GIP and glucagon-like peptide 1 (GLP-1; specific immunoassays). RESULTS: The pattern of glucose concentrations could precisely be copied by the intravenous glucose infusion (p=0.99). Insulin secretion was stimulated significantly more by oral as compared to intravenous glucose in both groups (p<0.0001). The percent contribution of the incretin effect was similar in both groups (C-peptide: 61.9+/-5.4 vs. 64.4+/-5.8%; p=0.77; insulin: 74.2+/-3.3 vs. 75.8+/-4.9; p=0.97; in first-degree relatives and controls, respectively). The individual responses of GIP and GLP-1 secretion were significantly correlated with each other (p=0.0003). The individual secretion of both GIP and GLP-1 was identified as a strong predictor of the integrated incremental insulin secretory responses as well as of the incretin effect. CONCLUSION/INTERPRETATION: Despite a lower insulin secretory response to exogenous GIP, incretin effects are similar in first-degree relatives of patients with type 2 diabetes and control subjects. This may be the result of a B cell secretory defect that affects stimulation by oral and intravenous glucose to a similar degree. Nevertheless, endogenous secretion of GIP and GLP-1 is a major determinant of insulin secretion after oral glucose.     

Glucose dependent insulinotropic polypeptide (GIP) infused intravenously is insulinotropic in the fasting state in type 2 (non-insulin dependent) diabetes mellitus

The effects of an intravenous infusion of porcine GIP on beta-cell secretion in patients with untreated type 2 diabetes mellitus have been studied. The subjects were studied on two separate days. After a 10 h overnight fast and a further 120 min basal period they were given an intravenous infusion of porcine GIP (2 pmol.kg-1.min-1) or control solution in random order from 120-140 min. Frequent plasma glucose, insulin, C-peptide and GIP measurements were made throughout and the study was continued until 200 min. Plasma glucose levels were similar throughout both tests. During the GIP infusion there was an early significant rise in insulin concentration from 0.058 +/- 0.006 nmol/l to 0.106 +/- 0.007 nmol/l (P less than 0.01) within 6 min of commencing the GIP infusion and insulin levels reached a peak of 0.131 +/- 0.011 nmol/l at 10 min (P less than 0.01). Insulin levels remained significantly elevated during the rest of the GIP infusion (P less than 0.01-0.001) and returned to basal values 20 min post infusion. No change in basal insulin values was seen during the control infusion. C-peptide levels were similarly raised during the GIP infusion and the increase was significant just 4 min after commencing the GIP infusion (P less than 0.05). GIP levels increased from 16 +/- 3 pmol/l prior to the infusion to a peak of 286 +/- 24 pmol/l 20 min later. At 4 min when a significant beta-cell response was observed GIP levels were well within the physiological range.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of independent and combined chronic metabolic effects of GIP and CB1 receptor blockade in high-fat fed mice

GIP receptor antagonism with (Pro³)GIP protects against obesity, insulin resistance, glucose intolerance and associated disturbances in mice fed high-fat diet. Furthermore, cannabinoid CB₁ receptor antagonism with AM251 reduces appetite and body weight gain in mice. The present study has examined and compared the effects of chronic daily administrations of (Pro³)GIP (25 nmol/kg body weight), AM251 (6 mg/kg body weight) and a combination of both drugs in high-fat fed mice. Daily i.p. injection of (Pro³)GIP, AM251 or combined drug administration over 22 days significantly (P < 0.05 to <0.01) decreased body weight compared with saline-treated controls. This was associated with a significant (P < 0.05 to <0.01) reduction of food intake in mice treated with AM251. Plasma glucose levels and glucose tolerance were significantly (P < 0.05) lowered by 22 days (Pro³)GIP, AM251 or combined drug treatment. These changes were accompanied by a significant (P < 0.05) improvement of insulin sensitivity in all treatment groups. In contrast, AM251 lacked effects on glucose tolerance, metabolic response to feeding and insulin sensitivity in high-fat mice when administered acutely. These data indicate that chemical blockade of GIP- or CB₁-receptor signaling using (Pro³)GIP or AM251, respectively provides an effective means of countering obesity and related abnormalities induced by consumption of high-fat energy-rich diet. AM251 lacks acute effects on glucose homeostasis and there was no evidence of a synergistic effect of combined treatment with (Pro³)GIP.     

Effects of cholecystokinin, secretin, and pancreatic polypeptide on secretion of gastric inhibitory polypeptide, insulin, and glucagon

Although the capacity of food components to cause more insulin secretion when given orally than when given intravenously is related significantly to increased plasma concentration of gastric inhibitory polypeptide (GIP), stimulated only by the oral route, questions arise as to what extent other gastrointestinal hormones modify insulin secretion either directly or by influencing the secretion of GIP. The triacontatriapeptide form of cholecystokinin (CCK₃₃), infused in dose gradients intravenously in dogs increases insulin secretion, and comparably to equimolar doses of the carboxy-terminal octapeptide of cholecystokin (CCK₈); neither compound changes fasting plasma levels of GIP or glucose. Glucagon was increased only by the largest dose of CCK₈ (0.27 ug/kg). Unlike the situation with GIP, it is not necessary to increase the plasma glucose above fasting level to obtain the insulin-releasing action of CCK. When glucose is infused intravenously (2 g in 0.5 min) at the beginning of a 15-minute infusion of CCK₈ (10 ng/kg/min), the amount of insulin release is greater than is produced by CCK₈ or glucose alone. In the same type of experiment, the infusion of GIP, in equimolar amounts as CCK₈, plus glucose causes no more insulin secretion than is stimulated by glucose alone. Secretin has only a small stimulating action on insulin release, and pancreatic polypeptide (PP) has no effect. Neither secretin nor PP affects GIP secretion, whether either is given alone, or together, or with CCK₈. Either secretin or CCK₈ inhibits oral glucose-stimulated increase in plasma GIP. These inhibitory effects are probably very much related to the hormone-induced decrease in gastric emptying, but changes in somatostatin secretion and other hormones possibly exert contributory actions. In conclusion, GIP in certain dose ranges has been reported to cause major increase in insulin secretion, but we showed that the insulin-releasing action of a small dose of glucose (2 g) infused intravenously was not augmented by GIP (44.5 ng/kg/min), although it was significantly increased by an equimolar dose of CCK₈. When plasma glucose was maintained at a fasting level, gradient equimolar dosages of CCK₈ and CCK₃₃ had comparable insulin-releasing action; GIP had no effect.     

Insulin modulates glucose-dependent insulinotropic polypeptide (GIP) secretion from enteroendocrine K cells in rats

Effects of insulin excess and deficiency on glucose-dependent insulinotropic polypeptide (GIP) was examined in rats following insulinoma transplantation or streptozotocin (STZ) administration. Over 14 days, food intake was increased (p < 0.001) in both groups of rats, with decreased body weight (p < 0.01) in STZ rats. Non-fasting plasma glucose levels were decreased (p < 0.01) and plasma insulin levels increased (p < 0.001) in insulinoma-bearing rats, whereas STZ treatment elevated glucose (p < 0.001) and decreased insulin (p < 0.01). Circulating GIP concentrations were elevated (p < 0.01) in both animal models. At 14 days, oral glucose resulted in a decreased glycaemic excursion (p < 0.05) with concomitant elevations in insulin release (p < 0.001) in insulinoma-bearing rats, whereas STZ-treated rats displayed similar glucose-lowering effects but reduced insulin levels (p < 0.01). GIP concentrations were augmented in STZ rats (p < 0.05) following oral glucose. Plasma glucose and insulin concentrations were not affected by oral fat, but fat-induced GIP secretion was particularly (p < 0.05) increased in insulinoma-bearing rats. Exogenous GIP enhanced (p < 0.05) glucose-lowering in all groups of rats accompanied by insulin releasing (p < 0.001) effects in insulinoma-bearing and control rats. Both rat models exhibited increased (p < 0.001) intestinal weight but decreased intestinal GIP concentrations. These data suggest that circulating insulin has direct and indirect effects on the synthesis and secretion of GIP.     

The biology of incretin hormones

Gut peptides, exemplified by glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are secreted in a nutrient-dependent manner and stimulate glucose-dependent insulin secretion. Both GIP and GLP-1 also promote β cell proliferation and inhibit apoptosis, leading to expansion of β cell mass. GLP-1, but not GIP, controls glycemia via additional actions on glucose sensors, inhibition of gastric emptying, food intake and glucagon secretion. Furthermore, GLP-1, unlike GIP, potently stimulates insulin secretion and reduces blood glucose in human subjects with type 2 diabetes. This article summarizes current concepts of incretin action and highlights the potential therapeutic utility of GLP-1 receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors for the treatment of type 2 diabetes.