Porcine Gastric Inhibitory Polypeptide Research Articles

The insulinotropic region of gastric inhibitory polypeptide; fragment analysis suggests the bioactive site lies between residues 19 and 30

Glucose-dependent insulinotropic polypeptide or gastric inhibitory polypeptide (GIP) is a 42 amino acid intestinal hormone, which exhibits several direct and indirect effects on fat and glucose metabolism. To determine the bioactive region(s) of the molecule, synthetic and proteolytic fragments of the hormone were generated and tested for their ability to induce a biological response in the isolated, perfused rat pancreas and stomach. A synthetic fragment corresponding to porcine GIP residues 1-30 retained strong insulinotropic activity in the isolated, perfused rat pancreas but greatly reduced somatostatinotropic activity in the isolated perfused rat stomach. A synthetic fragment corresponding to porcine GIP residues 15 to 30 was biologically inactive. However, enterokinase treatment of the synthetic 15-30 fragment restored partial insulinotropic activity in the isolated, perfused rat pancreas. The hypothesis that the restoration of biological activity was due to the enzymatic removal of the amino-terminal dipeptide (Asp-Lys) of GIP15-30 was supported by the observation that a synthetic fragment lacking these two residues was also insulinotropic. Further fractionation of the molecule generated a biologically active 19-30 fragment, suggesting that the residues necessary for the insulin response are contained within this region.

The insulinotropic region of gastric inhibitory polypeptide; fragment analysis suggests the bioactive site lies between residues 19 and 30

Glucose-dependent insulinotropic polypeptide or gastric inhibitory polypeptide (GIP) is a 42 amino acid intestinal hormone, which exhibits several direct and indirect effects on fat and glucose metabolism. To determine the bioactive region(s) of the molecule, synthetic and proteolytic fragments of the hormone were generated and tested for their ability to induce a biological response in the isolated, perfused rat pancreas and stomach. A synthetic fragment corresponding to porcine GIP residues 1-30 retained strong insulinotropic activity in the isolated, perfused rat pancreas but greatly reduced somatostatinotropic activity in the isolated perfused rat stomach. A synthetic fragment corresponding to porcine GIP residues 15 to 30 was biologically inactive. However, enterokinase treatment of the synthetic 15-30 fragment restored partial insulinotropic activity in the isolated, perfused rat pancreas. The hypothesis that the restoration of biological activity was due to the enzymatic removal of the amino-terminal dipeptide (Asp-Lys) of GIP15-30 was supported by the observation that a synthetic fragment lacking these two residues was also insulinotropic. Further fractionation of the molecule generated a biologically active 19-30 fragment, suggesting that the residues necessary for the insulin response are contained within this region.

Gastric inhibitory polypeptide and glucagon-like peptide-1(7–36) amide exert similar effects on somatostatin secretion but opposite effects on gastrin secretion from the rat stomach

Previous studies on the isolated perfused stomach have shown that gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1(7-36) amide (GLP-1(7-36) amide) stimulate release of somatostatin (somatostatin-like immunoreactivity, SLI). GIP produced a paradoxical increase in gastrin secretion, whereas GLP-1(7-36) was inhibitory. In the current study, the actions of synthetic (sp) and native (np) porcine and synthetic human (sh) GIP, GLP-1(7-36), and GLP-1(7-37) on SLI and gastrin secretion were compared using a gradient perfusion of peptide. All peptides increased SLI secretion at a threshold concentration of approximately 50 pmol/L (p < 0.05). The initial rate of increase in response to spGIP (119 +/- 39 pg/min) was greater than with other forms of GIP or GLP-1. Maximal increases obtained with the two porcine peptides did not differ. Gastrin secretion was increased by concentrations of spGIP and npGIP similar to those increasing SLI secretion, but the maximal response to shGIP was lower. In contrast to GIP-induced increases, both GLP-1(7-36) and GLP-1(7-37) suppressed gastrin secretion. It is concluded that human and porcine GIP, GLP-1(7-36), and GLP-1(7-37) all stimulate SLI secretion but with different maximal effects, and GIP stimulates gastrin secretion whereas both forms of GLP-1 inhibit gastrin secretion.

Gastric inhibitory polypeptide (GIP) binding sites in rat brain

Synthetic porcine gastric inhibitory polypeptide (GIP) was iodinated and purified by reverse-phase HPLC and used to localize saturable [ 125I]GIP binding sites by radioligand binding to frozen sections of rat brain followed by autoradiography. Saturable [ 125I]GIP binding sites were expressed in several brain regions including cerebral cortex, anterior olfactory nucleus, lateral septal nucleus, subiculum, inferior colliculus, and inferior olive. Saturable [ 125I]GIP binding was time dependent, reversible, high affinity, and specific for GIP. Scatchard analysis of equilibrium binding resulted in an estimated dissociation constant ( K d ) of 16–62 p M for the rat brain [ 125I]GIP binding sites. Peptides with amino acid sequences similar to GIP such as secretin, vasoactive intestinal polypeptide (VIP), glucagon, and peptide histidine isoleucine (PHI) only partially inhibited saturable [ 125I]GIP binding at concentrations approximately 10,000–100,000-fold higher than GIP. Saturable [ 125I]GIP binding was not observed in other rat organs surveyed such as spinal cord, pituitary, stomach, small intestine, colon, pancreas, liver, heart, or skeletal muscle. We conclude that a saturable [ 125I]GIP binding site with the pharmacological properties of an authentic GIP receptor is expressed in certain regions of the rat brain.

Stimulation of Glucagon Secretion by Gastric Inhibitory Polypeptide in Patients with Hepatic Cirrhosis and Hyperglucagonemia

Porcine gastric inhibitory polypeptide (GIP) was infused iv (120 micrograms in 60 min) in seven patients with biopsy-proven hepatic cirrhosis who had surgical porta-caval anastomoses and hyperglucagonemia in the postabsorptive state. The infusions resulted in elevation of blood levels of immunoreactive GIP into the upper range of those observed after ingestion of large mixed meals. This was accompanied by significant increments in immunoreactive glucagon (IRG) in the plasma. Similar infusions in two cirrhotic patients with surgical porta-caval anastomoses who had normal plasma IRG levels in the postabsorptive state had no effect on the plasma IRG level. Ingestion of triglyceride (60 g) in hyperglucagonemic cirrhotic patients with porta-caval anastomoses also resulted in elevation of plasma immunoreactive GIP, and this was again associated with significant elevation of the plasma IRG level. Chromatography studies showed that the increments in plasma IRG after the administration of GIP or triglyceride were largely accounted for by increases in pancreatic-type glucagon. There were no significant effects of administration of GIP or triglyceride on the blood levels of glucose or immunoreactive insulin. It is concluded that porcine GIP is glucagonotropic in patients with cirrhosis of the liver who show elevated levels of IRG in the plasma in the postabsorptive state. This effect is not due to diversion of portal blood to the systemic circulation and may be attributable to hypersensitivity of the alpha-cells to stimulation by GIP.

Suppression of insulin receptor binding by prolonged enteral or parenteral nutrient infusion in the rat: role of gastric inhibitory polypeptide

In the rat, prolonged enteral or parenteral alimentation with a high-carbohydrate diet results in hyperinsulinemia, which is substantially greater with the parenteral route. Supplementing the parenteral infusate with porcine gastric inhibitory polypeptide (GIP) to approximate plasma immunoreactive GIP levels achieved with enteral feeding further increases steady-state plasma insulin and glucose concentrations, suggesting insulin resistance. We examined the effects of sustained hyperinsulinemia elicited by continuous nutrient infusion on insulin binding to isolated rat adipocytes and the modification of this response by GIP. Compared with a baseline group, both enterally and parenterally alimented groups showed decreased insulin receptor binding affinity. However, despite substantially different steady-state plasma insulin levels, insulin binding was similar with either infusion route. Factors other than plasma insulin concentration alone therefore contribute to insulin receptor down-regulation during prolonged enteral alimentation. Supplementing the parenteral infusate with exogenous GIP resulted in a further reduction in insulin receptor affinity. Thus, adaptation to continuous nutrient infusion is characterized by insulin receptor down-regulation regardless of the route of nutrient delivery. An additional suppression of insulin receptor binding may in part be responsible for the insulin resistance elicited by prolonged exogenous GIP administration.

Role of gastric inhibitory polypeptide in postprandial hyperinsulinemia of obesity.

To assess the contribution of gastric inhibitory polypeptide (GIP) to the postprandial hyperinsulinemia of obesity, secretion rates of GIP (generated from kinetic analyses from infusions of porcine GIP) and insulin (from C-peptide applied to a validated kinetic model) to meals of 3 sizes were determined in 10 obese (5 male and 5 female) and 10 lean, sex- and age-matched healthy subjects. Although the postprandial secretion rates of GIP were greater in obese subjects (P = 0.03), postprandial concentrations of GIP were not. The latter may be explained by the greater volume of distribution of GIP in obese subjects (P = 0.036). Secretion rates and volume of distribution of GIP were correlated (r = 0.652, P less than 0.01). Despite excessive integrated postprandial (P = 0.010) insulin concentrations, insulin secretion was not significantly different between obese and lean subjects. We conclude that 1) although postprandial plasma GIP concentrations are normal, GIP secretion is increased in obesity, 2) the postprandial hyperinsulinemia of obesity is not due to excessive insulin secretion but is likely secondary to altered insulin clearance, and 3) GIP cannot account for the hyperinsulinemia of obesity through its insulinotropic action.

Effect of porcine gastric inhibitory polypeptide on β-cell function in type I and type II diabetes mellitus

The effect of highly purified natural porcine GIP on C-peptide release was examined in six type I (insulin-dependent) diabetics (IDD) with residual β-cell function, six type II (non-insulin-dependent) diabetics (NIDD), and six normal subjects. All subjects were normal weight. From −120 minutes to 180 minutes glucose or insulin was infused IV to achieve a constant plasma glucose level of 8 mmol/L. On two separate days GIP (2 pmol/kg/min) or isotonic NaCl at random were infused from 0 to 30 minutes. After 10 minutes of GIP infusion plasma IR-GIP concentrations were in the physiologic postprandial range. At 30 minutes a further increase in IR-GIP to supraphysiologic levels occurred. In all subjects plasma, C-peptide increased more after 10 minutes of GIP infusion (IDD, 0.48 ± 0.05; NIDD, 0.79 ± 0.11; normal subjects, 2.27 ± 0.29 nmol/L) than on the corresponding day with NaCl infusion (IDD, 0.35 ± 0.03; NIDD, 0.62 ± 0.08; normal subjects, 1.22 ± 0.13 nmol/L, P < .05 for all). The responses of the diabetics were significantly lower than that of the normal subjects ( P < .001 for both groups). No further increase in C-peptide occurred during the remaining 20 minutes of the GIP infusion in the diabetic subjects (IDD, 0.49 ± 0.05; NIDD, 0.83 ± 0.10 nmol/L). In the presence of a plasma glucose concentration of 8 mmol/L, physiologic concentrations of porcine GIP caused an immediate but impaired β-cell response in IDD and NIDD patients.

Potentiation of glucose-induced insulin secretion in the perfused rat pancreas by porcine GIP (gastric inhibitory polypeptide), bovine GIP, and bovine GIP(1-39).

Porcine GIP (gastric inhibitory polypeptide) potentiates glucose-induced insulin secretion under a variety of experimental conditions. Recently GIP was isolated also from bovine intestine, and found to differ from porcine GIP by having isoleucine instead of lysine in position 37. We have compared the effects of porcine GIP to that of bovine GIP and bovine GIP(1-39) on glucose-induced insulin secretion from the perfused rat pancreas. We found that porcine GIP, bovine GIP, and bovine GIP(1-39) all strongly potentiated both first and second phases of glucose-induced insulin secretion (glucose concentration 6.7 mM; polypeptide concentration 1 nM). There was no significant difference between the polypeptides with regard to the potency to potentiate glucose-induced insulin secretion. We conclude that bovine GIP, as porcine GIP, potentiates glucose-induced insulin secretion, and that the insulinotropic activity of GIP is not confined to the last three amino acids at the C-terminal end.

Insulin-dependent inhibition of hepatic glycogenolysis by gastric inhibitory polypeptide (GIP) in perfused rat liver

The effect of porcine gastric inhibitory polypeptide on hepatic glycogen metabolism was investigated in the isolated in situ perfused rat liver. Glycogenolysis was stimulated by infusion of glucagon into the portal vein (half maximal effective portal vein concentration approximately 30 pmol/l). When glucagon was infused at a final portal vein concentration of 0.5 nmol/l, simultaneous addition of insulin inhibited the glucagon-dependent glycogenolysis in a dose-dependent way (half maximal effective concentration for insulin about 2 nmol/l). Gastric inhibitory polypeptide alone at a concentration of 1 nmol/l reduced glucagon-dependent glycogenolysis only slightly. However, when infused simultaneously at low insulin concentrations (0.1 nmol/l), gastric inhibitory polypeptide suppressed hepatic glucose production dose-dependently up to 70%. The data suggest that gastric inhibitory polypeptide exerts direct metabolic effects on hepatic glycogen metabolism predominantly in a situation where insulin is simultaneously present, e.g. following ingestion of glucose.

Effects of gastric inhibitory polypeptide in the response to prolonged parenteral or enteral alimentation in rats.

To examine the effects of long-term elevation of plasma gastric inhibitory polypeptide (GIP), the responses to parenteral (PA) or enteral (EA) alimentation were studied in conscious rats with duodenal and venous cannulae. A weight-maintaining liquid diet (84% as glucose, 16% as amino acids) was infused at a constant rate for 6 days by either route, and daily blood samples were taken. A subset of animals receiving PA also received porcine GIP with the infusate (PA plus GIP; plateau plasma immunoreactive GIP, IRGIP, 610 +/- 120 pg/ml). With PA, plasma IRGIP did not change from basal levels, whereas with EA IRGIP rose to virtual plateau levels (mean 530 +/- 110 pg/ml). In the steady state, plasma immunoreactive insulin (IRI) was significantly lower with EA (mean, 153 +/- 5 microU/ml) than with PA (mean, 226 +/- 15 microU/ml), which in turn was lower than with PA plus GIP (mean, 375 +/- 23 microU/ml, P less than 0.001 by ANOVA). A similar ranking of plasma glucose levels occurred in the steady state, with means of 113 +/- 7 (EA), 126 +/- 3 (PA), and 184 +/- 9 (PA plus GIP) mg/dl (P less than 0.001 by ANOVA). To assess the response to transient hyperglycemia in the steady state, an intravenous glucose bolus was given to each group on the fifth day. Peak plasma IRI levels did not differ among the three groups; however, the glucose disappearance rate was significantly slower with PA plus GIP compared with either EA or PA.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbachol modulates GIP-mediated insulin release from rat pancreatic lobules in vitro

Rat pancreatic lobules were used to investigate the interaction of gastric inhibitory polypeptide (GIP), carbachol, glucose, and an amino acid mixture on insulin secretion. At 5 mM glucose, GIP (1.1 ng/ml) did not augment insulin secretion in the presence or absence of carbachol (5 X 10(-5)M) during a 210-min incubation. However, at 11 mM glucose, GIP did augment insulin secretion in the presence (342.5 +/- 62.0 vs. 212.5 +/- 50.5 microU . ml-1 . mg tissue-1, mean +/- SE; P less than 0.01) but not the absence (217.0 +/- 45.5 vs. 205.8 +/- 35.0 microU . ml-1 . mg tissue-1) of carbachol. During subsequent 30-min incubations, GIP was increased to a supra-physiological concentration of 11 ng/ml and again augmented insulin secretion with (65.8 +/- 10.8 vs. 27.8 +/- 2.4 microU . ml-1 . mg tissue-1 . h-1; P less than 0.001) but not without (37.2 +/- 1.8 vs. 30.2 +/- 2 microU . ml-1 . mg-1 tissue-1 . h-1) carbachol present. This GIP-mediated insulin secretion was blocked by atropine (34.8 to 1.8 vs. 37.6 +/- 1.6 microU . ml-1 . mg tissue-1 . h-1). At amino acid concentrations of 21 and 211 mM, but not 2.1 mM, GIP augmented insulin release but again only with carbachol present. In conclusion, porcine GIP augments amino acid as well as glucose-mediated insulin secretion in vitro. Furthermore, this biological action is dependent on an, as yet, unidentified cholinergic mechanism. The pathophysiological significance of the neural-hormonal interaction deserves further investigation.

Effect of Intravenously Infused Porcine Gip on Serum Insulin in Obese and Lean Subjects Studied with the Hyperglycemic Clamp Technique

To ascertain whether an altered sensitivity to gastric inhibitory polypeptide (GIP) in morbidly obese subjects can play a role in the postprandial hyperinsulinemia seen in this condition, eight obese and eight control subjects were studied with an intravenous infusion of porcine GIP. The blood glucose was maintained at 4 mmol/l above the basal level by a hyperglycemic clamp technique. Although the mean serum insulin level was higher in the obese group throughout the study, the shapes of the serum insulin curves were almost identical in the two groups after the GIP infusion. This together with the normal GIP secretion found in obese subjects question the existence of a causal relationship between an overactive entero-insular axis and the hyperinsulinemia found in these subjects.

Effects of intravenously infused porcine GIP on serum insulin, plasma C-peptide, and pancreatic polypeptide in non-insulin-dependent diabetes in the fasting state.

Eight fasting patients with non-insulin-dependent diabetes (NIDD) and six healthy controls were given an intravenous infusion of porcine gastric inhibitory polypeptide (GIP). During the GIP infusion mean plasma pancreatic polypeptide level increased significantly in both groups, whereas the mean serum insulin level increased in the NIDD group only, indicating a more important role for GIP in these patients than in healthy subjects.

A novel form of gastric inhibitory polypeptide (GIP) isolated from bovine intestine using a radioreceptor assay. Fragmentation with staphylococcal protease results in GIP1-3 and GIP4-42, fragmentation with enterokinase in GIP1-16 and GIP17-42.

A novel form of gastric inhibitory polypeptide (GIP), later also referred to as glucose-dependent insulinotropic polypeptide, has been isolated from bovine upper intestine. The purification was monitored by a recently developed radioreceptor assay, specific for GIP, using membrane preparations from hamster beta-cell tumors. A combination of ion-exchange and reverse-phase high-performance liquid chromatography was used in the isolation which resulted in homogeneous bovine GIP. Bovine GIP is, like porcine GIP, composed of 42 amino acid residues. The sequence is: Tyr-Ala-Glu-Gly-Thr-Phe-Ile-Ser-Asp-Tyr-Ser-Ile-Ala-Met-Asp-Lys-Ile-Arg- Gln-Gln - Asp-Phe-Val-Asn-Trp-Leu-Leu-Ala-Gln-Lys-Gly-Lys-Lys-Ser-Asp-Trp-Ile-His- Asn-Ile - Thr-Gln, which differs from that of the previously characterized porcine GIP by having isoleucine instead of lysine at position 37. Upon proteolytic digestion of GIP with the staphylococcal V8 protease and with enterokinase, two fragments are formed in each case, corresponding to GIP1-3, GIP4-42, and GIP1-16, GIP17-42, respectively.

Effects of gastric inhibitory polypeptide on dispersed pancreatic acinar cells from the guinea pig.

Purified natural porcine gastric inhibitory polypeptide (GIP), in high concentrations, was found to stimulate outflow of 45Ca, increase cellular accumulation of cyclic GMP and cyclic AMP and cause release of amylase from dispersed pancreatic acinar cells. Synthetic GIP increased cellular cyclic AMP levels, but did not affect outflux of calcium, cellular levels of cyclic GMP or release of amylase. The discrepancy between results with natural and synthetic preparations of porcine GIP may be explained by a possible contamination of natural GIP with cholecystokinin. Other examined pancreatic secretory stimulating peptides which induce cyclic AMP accumulation in acinar cells, also increase release of amylase. Synthetic GIP increased levels of cyclic AMP without affecting amylase release. This suggests that the correlation between amylase release and total cellular accumulation of cyclic AMP in response to GIP is not close.

Bethanechol prevents inhibition of gastric acid secretion by gastric inhibitory polypeptide.

Earlier studies of gastric inhibitory polypeptide (GIP) have shown that it is a powerful inhibitor of acid secretion in denervated gastric pouches. Other studies, however, have shown that it is a weak inhibitor in innervated stomach preparations. In this study we evaluated the inhibitory action of GIP in four dogs, each provided with both a vagally denervated (Heidenhain) pouch and a gastric fistula of the innervated stomach. The effect of intravenous infusion of pure porcine GIP (1 microgram X kg-1 X h-1) on pentagastrin dose response was studied with and without background infusion of bethanechol (25 micrograms X kg-1 X h-1). GIP powerfully inhibited pentagastrin-stimulated acid secretion from the Heidenhain pouch but not from the gastric fistula. The maximal inhibition was 76% for the Heidenhain pouch and 29% for the gastric fistula. Despite the attainment of high levels of circulating immunoreactive GIP (greater than 3,000 pg/ml) early after the onset of infusion of the peptide, inhibition occurred relatively late, suggesting that this action of GIP might be mediated by the release of some other substance(s). Background infusion of bethanechol totally abolished the inhibitory action of GIP in both the Heidenhain pouch and gastric fistula.

Natural purified porcine gastric inhibitory polypeptide (GIP) stimulates exocrine pancreas secretion due to contamination with a cholecystokinin-like substance

The effects of purified natural gastric inhibitory polypeptide-enterogastrone III (GIP-EG III) and a fraction which is further purified by high pressure liquid chromatography (GIP-HPLC) were investigated on the endocrine and exocrine isolated perfused pancreas of rats. At the dose of 5 ng/ml used for both GIP preparations, only GIP-EG III significantly stimulated volume and amylase secretion of the exocrine pancreas. The response of insulin release to stimulation by GIP-EG III or GIP-HPLC was not significantly different. In the presence of cholecystokinin-octapeptide (CCK-8) at a concentration which gave half-maximal stimulation of amylase secretion, GIP-EG III almost doubled the response of the exocrine pancreas, whereas GIP-HPLC had no additional effect. CCK-8 alone significantly increased total insulin output under hyperglycemic conditions. We conclude that porcine GIP purified by gel chromatography contains a CCK-like substance which can be removed by further purification on high pressure liquid chromatography without affecting the insulinotropic activity. Some of the reported effects of GIP could be due to contamination.

Release of Plasma Pancreatic Polypeptide in Achlorhydric Patients after Intravenous Infusion of Gastric Inhibitory Polypeptide

In order to evaluate whether gastric inhibitory polypeptide (GIP) could be responsible for the postprandial fall in serum gastrin previously observed in patients with achlorhydria, 7 achlorhydric patients were given 100 ml Lipomul (66 g triglycerides) on one occasion, and 5 of these patients were later given a 30-min intravenous infusion of porcine GIP in a dose of 1 microgram/kg. Following the Lipomul ingestion, serum gastrin fell significantly, whereas no effect on serum gastrin was seen during the intravenous GIP infusion. A small, but significant release of serum insulin was seen shortly after starting the GIP infusion, together with a significant and more sustained release of plasma pancreatic polypeptide (PP). It is concluded that GIP does not lower the serum gastrin levels in achlorhydric patients, but that GIP might participate in the intestinal phase of the PP release.

Effects of amino acids and gastric inhibitory polypeptide on insulin release in dogs.

Insulin release following intravenous administration of an amino acid solution with and without a simultaneous infusion of varying amounts of porcine gastric inhibitory polypeptide (GIP) was studied in dogs. Group I received a 10-amino acid mixture (300 mosmol/kg iv) at 16.6 ml/min for 1 h; group II, amino acid mixture plus 0.5 micrograms.kg-1.h-1 porcine GIP; group III, amino acid mixture plus 1.0 micrograms.kg-1.h-1 of GIP; group IV (a and b) received either 0.5 or 1.0 micrograms.kg-1.h-1 of GIP alone. Compared to group I, groups II and III had a greater insulin response during the first 30 min of the infusion. Group] IV (a and b) showed no insulin release. Glucose concentrations showed no significant change in all groups. From these results, it is concluded that insulin release after intravenous infusion of an amino acid mixture plus GIP is greater than after amino acids or GIP alone. It appears that this effect is more pronounced in the early phase of insulin release.