A comparison of the ability of serum and monoclonal antibodies to gastric inhibitory polypeptide to detect immunoreactive cells in the gastroenteropancreatic system of mammals and reptiles

The Human and Mouse Islet Peptidome: Effects of Obesity and Type 2 Diabetes, and Assessment of Intraislet Production of Glucagon-like Peptide-1

To characterize the impact of metabolic disease on the peptidome of human and mouse pancreatic islets, LC-MS was used to analyze extracts of human and mouse islets, purified mouse alpha, beta, and delta cells, supernatants from mouse islet incubations, and plasma from patients with type 2 diabetes. Islets were obtained from healthy and type 2 diabetic human donors, and mice on chow or high fat diet. All major islet hormones were detected in lysed islets as well as numerous peptides from vesicular proteins including granins and processing enzymes. Glucose-dependent insulinotropic peptide (GIP) was not detectable. High fat diet modestly increased islet content of proinsulin-derived peptides in mice. Human diabetic islets contained increased content of proglucagon-derived peptides at the expense of insulin, but no evident prohormone processing defects. Diabetic plasma, however, contained increased ratios of proinsulin and des-31,32-proinsulin to insulin. Active GLP-1 was detectable in human and mouse islets but 100-1000-fold less abundant than glucagon. LC-MS offers advantages over antibody-based approaches for identifying exact peptide sequences, and revealed a shift toward islet insulin production in high fat fed mice, and toward proglucagon production in type 2 diabetes, with no evidence of systematic defective prohormone processing.

Establishment of novel specific assay for short-form glucose-dependent insulinotropic polypeptide and evaluation of its secretion in nondiabetic subjects

The short‐form glucose‐dependent insulinotropic polypeptide (GIP) (1–30) is released from islet alpha cells and promotes insulin secretion in a paracrine manner in vitro. However, it is not well elucidated how GIP (1–30) is involved in glucose metabolism in vivo, since a specific assay system for GIP (1–30) has not yet been established. We first developed a sandwich enzyme‐linked immunosorbent assay (ELISA) specific for GIP (1–30) by combining a novel antibody specific to the GIP (1–30) C terminus with the common antibody against GIP N terminus. Then, we explored cross‐reactivities with incretins and glucagon‐related peptides in this ELISA. GIP (1–30) amide, but not GIP (1–42), GLP‐1, or glucagon increased absorbance in a dose‐dependent manner. We next measured plasma GIP (1–30) concentrations in nondiabetic participants (ND) during a 75‐g oral glucose tolerance test or cookie meal test (carbohydrates 75 g, lipids 28.5 g, proteins 8.5 g). Both glucose and cookie load increased GIP (1–30) concentrations in ND, but the increases were much lower than those of GIP (1–42). Furthermore, the DPP‐4 inhibitor significantly increased GIP (1–30) concentrations similarly to GIP (1–42) in ND. In conclusion, we for the first time developed an ELISA specific for GIP (1–30) and revealed its secretion in ND.

The early history of GIP 1969-2000: From enterogastrone to major metabolic hormone

This paper describes the early history of Gastric Inhibitory Polypeptide, better referred to simply as GIP, from its isolation by purification from a crude preparation of CCK-PZ (cholecystokinin/pancreozymin) to its recognition as a key play in the pathogenesis of obesity and other metabolic disorders far removed from the enterogastrone properties by which it was originally identified. Augmentation of glucose mediated insulin release, the incretin effect, was discovered soon after GIP was first isolated and only much later was its important role in the pathogenesis of obesity, through mechanism other than its insulin secretion, appreciated. Immunoassay - the method by which the concentration of GIP was measured in plasma until quite recently - was found to be flawed and to depend upon which specific epitope of the hormone an assay detected. This was especially true if it was an amino-acid sequence specific to porcine rather than human GIP. A further confounder was the discovery that much of the GIP measured by immunoassay was its biological antagonist produced by cleavage of its two N-terminal amino-acids in the circulation by the same dipeptidyl-peptidase as de-activates GLP-1. Potential use of synthetic agonistic and antagonistic GIP analogues in therapeutics was barely alluded to before year 2000.

The early history of GIP 1969-2000: From enterogastrone to major metabolic hormone

This paper describes the early history of Gastric Inhibitory Polypeptide, better referred to simply as GIP, from its isolation by purification from a crude preparation of CCK-PZ (cholecystokinin/pancreozymin) to its recognition as a key player in the pathogenesis of obesity and other metabolic disorders far removed from the enterogastrone properties by which it was originally identified. Augmentation of glucose mediated insulin release, the incretin effect, was discovered soon after GIP was first isolated and only much later was its important role in the pathogenesis of obesity, through mechanism other than insulin secretion, appreciated. Immunoassay - the only method by which the concentration of GIP was measured in plasma until quite recently - was found to be flawed and to depend upon which specific epitope of the hormone an assay detected. This was especially true if it was an amino-acid sequence specific to porcine rather than human GIP. A further confounder was the discovery that much of the GIP measured by immunoassay was its biological antagonist produced by cleavage of its two N-terminal amino-acids in the circulation by the same dipeptidyl-peptidase as de-activates GLP-1. Potential use of synthetic agonistic and antagonistic GIP analogues in therapeutics was barely alluded to before year 2000.

Caloric Restriction Paradoxically Increases Adiposity in Mice With Genetically Reduced Insulin

Antiadiposity effects of caloric restriction (CR) are associated with reduced insulin/IGF-1 signaling, but it is unclear whether the effects of CR would be additive to genetically reducing circulating insulin. To address this question, we examined female Ins1(+/-):Ins2(-/-) mice and Ins1(+/+):Ins2(-/-) littermate controls on either an ad libitum or 60% CR diet. Although Igf1 levels declined as expected, CR was unable to reduce plasma insulin levels in either genotype below their ad libitum-fed littermate controls. In fact, 53-week-old Ins1(+/-):Ins2(-/-) mice exhibited a paradoxical increase in circulating insulin in the CR group compared with the ad libitum-fed Ins1(+/-):Ins2(-/-) mice. Regardless of insulin gene dosage, CR mice had lower fasting glucose and improved glucose tolerance. Although body mass and lean mass predictably fell after CR initiation, we observed a significant and unexpected increase in fat mass in the CR Ins1(+/-):Ins2(-/-) mice. Specifically, inguinal fat was significantly increased by CR at 66 weeks and 106 weeks. By 106 weeks, brown adipose tissue mass was also significantly increased by CR in both Ins1(+/-):Ins2(-/-) and Ins1(+/+):Ins2(-/-) mice. Interestingly, we observed a clear whitening of brown adipose tissue in the CR groups. Mice in the CR group had altered daily energy expenditure and respiratory exchange ratio circadian rhythms in both genotypes. Multiplexed analysis of circulating hormones revealed that CR was associated with increased fasting and fed levels of the obesogenic hormone, glucose-dependent insulinotropic polypeptide. Collectively these data demonstrate CR has paradoxical effects on adipose tissue growth in the context of genetically reduced insulin.

Alternative form of glucose-dependent insulinotropic polypepide and its physiology

Glucose‐dependent insulinotropic polypepide (GIP) was first extracted from porcine gut mucosa and identified as “incretin” decades ago. Though early studies have shown the possible GIP isoforms by gel filtration profiles from porcine or human intestinal extracts analyzed by radioimmunoassay (RIA), GIP is currently believed to consist of 42 amino acids (GIP1‐42), which are released from gut K‐cells and promote postprandial insulin release. In fact, GIP1‐42 is usually processed from proGIP by the action of prohormone convertase (PC) 1/3 in the gut. GIP expression is occasionally found in the intestinal glucagon‐like peptide‐1‐secreting cells, suggesting gene expression of both GIP and proglucagon can co‐exist in identical cells. However, GIP1‐42 immunoreactivity is rarely found in α‐cells or other pancreatic endocrine cells of wild‐type mammals. Interestingly, we found that short‐form GIP1‐30 is expressed in and released from pancreatic α‐cells and a subset of enteroendocrine cells through proGIP processing by PC2. GIP1‐30 is also insulinotropic and modulates glucose‐stimulated insulin secretion in a paracrine manner. It is also suggested that short‐form GIP1‐30 possibly plays a crucial role for the islet development. It has not been well elucidated whether expression of GIP1‐30 is modulated in the diabetic status, and whether GIP1‐30 might have therapeutic potentials. Our preliminary data suggest that short‐form GIP1‐30 might play important roles in glucose metabolism.

K-cells and glucose-dependent insulinotropic polypeptide in health and disease

In the 1970s, glucose-dependent insulinotropic polypeptide (GIP, formerly gastric inhibitory polypeptide), a 42-amino acid peptide hormone, was discovered through a search for enterogastrones and subsequently identified as an incretin, or an insulinotropic hormone secreted in response to intraluminal nutrients. Independent of the discovery of GIP, the K-cell was identified in small intestine by characteristic ultrastructural features. Subsequently, it was realized that K-cells are the predominant source of circulating GIP. The density of K-cells may increase under conditions including high-fat diet and obesity, and generally correlates with plasma GIP levels. In addition to GIP, K-cells secrete xenin, a peptide with as of yet poorly understood physiological functions, and GIP is often colocalized with the other incretin hormone glucagon-like peptide-1 (GLP-1). Differential posttranslational processing of proGIP produces 30 and 42 amino acid versions of GIP. Its secretion is elicited by intraluminal nutrients, especially carbohydrate and fat, through the action of SGLT1, GPR40, GPR120, and GPR119. There is also evidence of regulation of GIP secretion via neural pathways and somatostatin. Intracellular signaling mechanisms of GIP secretion are still elusive but include activation of adenylyl cyclase, protein kinase A (PKA), and protein kinase C (PKC). GIP has extrapancreatic actions on adipogenesis, neural progenitor cell proliferation, and bone metabolism. However, the clinical or physiological relevance of these extrapancreatic actions remain to be defined in humans. The application of GIP as a glucose-lowering drug is limited due to reduced efficacy in humans with type 2 diabetes and its potential obesogenic effects demonstrated by rodent studies. There is some evidence to suggest that a reduction in GIP production or action may be a strategy to reduce obesity. The meal-dependent nature of GIP release makes K-cells a potential target for genetically engineered production of satiety factors or glucose-lowering agents, for example, insulin. Transgenic mice engineered to produce insulin from intestinal K-cells are resistant to diabetes induced by a beta-cell toxin. Collectively, K-cells and GIP play important roles in health and disease, and both may be targets for novel therapies.

Glucose-dependent insulinotropic polypeptide is expressed in pancreatic islet alpha-cells and promotes insulin secretion

BACKGROUND & AIMS

Glucose-dependent insulinotropic polypeptide (GIP) and the proglucagon product glucagon-like peptide-1 (GLP-1) are gastrointestinal hormones that are released in response to nutrient intake and promote insulin secretion. Interestingly, a subset of enteroendocrine cells express both GIP and GLP-1. We sought to determine whether GIP also might be co-expressed with proglucagon in pancreatic alpha-cells.

METHODS

We assessed GIP expression via reverse-transcription polymerase chain reaction, in situ hybridization, and immunohistochemistry. We developed a novel bioassay to measure GIP release from isolated islets, compared the biological activities of full-length and truncated GIP, and assessed the impact of immunoneutralization of islet GIP on glucose-stimulated insulin secretion in isolated islets.

RESULTS

GIP messenger RNA was present in mouse islets; GIP protein localized to islet alpha-cells of mouse, human, and snake pancreas, based on immunohistochemical analyses. However, using a C-terminal GIP antibody, immunoreactivity was detected in islets from prohormone convertase (PC) 2 knockout but not wild-type mice. Bioactive GIP was secreted from mouse and human islets after arginine stimulation. In the perfused mouse pancreas, GIP(1-42) and amidated GIP(1-30) had equipotent insulinotropic actions. Finally, immunoneutralization of GIP secreted by isolated islets decreased glucose-stimulated insulin secretion.

CONCLUSIONS

GIP is expressed in and secreted from pancreatic islets; in alpha-cells, PC2 processes proGIP to yield a truncated but bioactive form of GIP that differs from the PC1/3-derived form from K-cells. Islet-derived GIP promotes islet glucose competence and also could support islet development and/or survival.

Differential processing of pro-glucose-dependent insulinotropic polypeptide in gut

Glucose-dependent insulinotropic polypeptide (GIP) is a hormone released from enteroendocrine K cells in response to meals. Posttranslational processing of the precursor protein pro-GIP at residue 65 by proprotein convertase subtilisin/kexin type 1 (PC1/3) in gut K cells gives rise to the established 42-amino-acid form of GIP (GIP(1-42)). However, the pro-GIP peptide sequence contains a consensus cleavage site for PC2 at residues 52-55 and we identified PC2 immunoreactivity in a subset of K cells, suggesting the potential existence of a COOH-terminal truncated GIP isoform, GIP(1-30). Indeed a subset of mouse and human K cells display GIP immunoreactivity with GIP antibodies directed to the mid portion of the peptide, but not with a COOH-terminal-directed GIP antibody, indicative of the presence of a truncated form of GIP. This population of cells represents approximately 5-15% of the total GIP-immunoreactive cells in mice, depending on the region of intestine, and is virtually absent in mice lacking PC2. Amidated GIP(1-30) and GIP(1-42) have comparable potency at stimulating somatostatin release in the perfused mouse stomach. Therefore, GIP(1-30) represents a naturally occurring, biologically active form of GIP.

The vagus nerve, food intake and obesity

Food interacts with sensors all along the alimentary canal to provide the brain with information regarding its composition, energy content, and beneficial effect. Vagal afferents innervating the gastrointestinal tract, pancreas, and liver provide a rapid and discrete account of digestible food in the alimentary canal, as well as circulating and stored fuels, while vagal efferents, together with the sympathetic nervous system and hormonal mechanisms, codetermine the rate of nutrient absorption, partitioning, storage, and mobilization. Although vagal sensory mechanisms play a crucial role in the neural mechanism of satiation, there is little evidence suggesting a significant role in long-term energy homeostasis. However, increasing recognition of vagal involvement in the putative mechanisms making bariatric surgeries the most effective treatment for obesity should greatly stimulate future research to uncover the many details regarding the specific transduction mechanisms in the periphery and the inter- and intra-neuronal signaling cascades disseminating vagal information across the neuraxis.

Immunohistochemical distribution of glucose-dependent insulinotropic polypeptide in the adult rat brain

We have previously demonstrated that glucose-dependent insulinotropic polypeptide (GIP; gastric inhibitory polypeptide) is present in the adult rat hippocampus. This finding leads to the conclusion that all members of the secretin-glucagon family of gastrointestinal regulatory polypeptides can be found in the brain. To investigate the localization of GIP-producing cells, we used immunohistochemistry on sections of the adult rat brain. High levels of GIP immunoreactivity were observed in the olfactory bulb, hippocampus, and Purkinje cells in the cerebellum. Moreover, a moderate but distinct GIP immunoreactivity was observed in the cerebral cortex, amygdala, substantia nigra, and lateral septal nucleus as well as in several nuclei in the thalamus, hypothalamus, and brainstem. GIP immunoreactivity was frequently found to colocalize with the neuronal marker NeuN but never with the glial marker glial fibrillary acidic protein. Thus, GIP appears to be mainly neuronal to its distribution. This widespread distribution of GIP-immunoreactive cells suggests the involvement of GIP in various neuronal functions and suggests that GIP may act as a neurotransmitter or neuromodulator. This is the first characterization of the anatomical distribution of GIP-immunoreactive cells in the rat brain providing an anatomical framework for future investigations regarding the functions of GIP in the central nervous system.

Glucose-dependent insulinotropic polypeptide is expressed in adult hippocampus and induces progenitor cell proliferation

The hippocampal dentate gyrus (DG) is an area of active proliferation and neurogenesis within the adult brain. The molecular events controlling adult cell genesis in the hippocampus essentially remain unknown. It has been reported previously that adult male and female rats from the strains Sprague Dawley (SD) and spontaneously hypertensive (SHR) have a marked difference in proliferation rates of cells in the hippocampal DG. To exploit this natural variability and identify potential regulators of cell genesis in the hippocampus, hippocampal gene expression from male SHR as well as male and female SD rats was analyzed using a cDNA array strategy. Hippocampal expression of the gene-encoding glucose-dependent insulinotropic polypeptide (GIP) varied strongly in parallel with cell-proliferation rates in the adult rat DG. Moreover, robust GIP immunoreactivity could be detected in the DG. The GIP receptor is expressed by cultured adult hippocampal progenitors and throughout the granule cell layer of the DG, including progenitor cells. Thus, these cells have the ability to respond to GIP. Indeed, exogenously delivered GIP induced proliferation of adult-derived hippocampal progenitors in vivo as well as in vitro, and adult GIP receptor knock-out mice exhibit a significantly lower number of newborn cells in the hippocampal DG compared with wild-type mice. This investigation demonstrates the presence of GIP in the brain for the first time and provides evidence for a regulatory function for GIP in progenitor cell proliferation.

Multiple regulation of peptide YY secretion in the digestive tract

In the last two decades, multiple aspects of the peptide YY (PYY) secretion have been investigated. Besides fat and fatty acids, many luminal nutrients in the distal intestine appear to induce PYY release. Some studies have shown that bile acid, but not nutrients, plays a crucial role in the regulation of PYY secretion. Moreover, chyme in the proximal intestine also regulates the peptide release by indirect action through humoral and neuronal factors. Gastrin, cholecystokinin, and the vagus nerve are major candidates for mediators of these indirect actions. Several growth factors have been shown to regulate PYY synthesis in mucosa of the distal intestine. This review is aimed at presenting an overview of these recent studies on PYY secretion in the distal intestine.

K cells: a novel target for insulin gene therapy for the prevention of diabetes

Recently, gut K cells have been shown to express glucokinase, the glucose sensor of pancreatic beta cells, and transgenic mice expressing human insulin under the control of a K cell-specific promoter are resistant to diabetes development induced by the beta-cell toxin streptozotocin. These novel findings suggest that gut K cells might be a suitable target for gene therapeutic treatment of type 1 diabetes mellitus.

The origin and function of the pituitary adenylate cyclase-activating polypeptide (PACAP)/glucagon superfamily

The pituitary adenylate cyclase-activating polypeptide (PACAP)/ glucagon superfamily includes nine hormones in humans that are related by structure, distribution (especially the brain and gut), function (often by activation of cAMP), and receptors (a subset of seven-transmembrane receptors). The nine hormones include glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, glucose-dependent insulinotropic polypeptide (GIP), GH-releasing hormone (GRF), peptide histidine-methionine (PHM), PACAP, secretin, and vasoactive intestinal polypeptide (VIP). The origin of the ancestral superfamily members is at least as old as the invertebrates; the most ancient and tightly conserved members are PACAP and glucagon. Evidence to date suggests the superfamily began with a gene or exon duplication and then continued to diverge with some gene duplications in vertebrates. The function of PACAP is considered in detail because it is newly (1989) discovered; it is tightly conserved (96% over 700 million years); and it is probably the ancestral molecule. The diverse functions of PACAP include regulation of proliferation, differentiation, and apoptosis in some cell populations. In addition, PACAP regulates metabolism and the cardiovascular, endocrine, and immune systems, although the physiological event(s) that coordinates PACAP responses remains to be identified.

Gastric inhibitory polypeptide release from a tumor-derived cell line

A cell line derived from intestinal tumors of transgenic mice (STC-1) was subcloned to produce a stable line with approximately 30% immunoreactive gastric inhibitory polypeptide (irGIP)-containing cells (STC 6-14). High-performance liquid chromatography (HPLC) of STC 6-14 extracts indicated that the tumor cell-derived irGIP had the same retention time as synthetic porcine GIP-(1-42) (pGIP). Approximately 30% of the cells also contained immunoreactive somatostatin (irSS), which eluted as a single peak on HPLC, corresponding with SS-(1-14). On average, each well of extracted cells (5.0 x 10(5) cultured 4 days) contained 33.3 +/- 1.4 ng irGIP and 18.4 +/- 1.5 ng irSS. Basal release of irGIP in the presence of 5 mM glucose was 733 +/- 58 pg.ml cells-1.2h-1 (2.20 +/- 0.17% of total cell content; TCC) and doubled at 20 mM glucose (4.20 +/- 0.42% TCC). The response to glucose was augmented by addition of a SS neutralizing antibody (SOMA-10) and suppressed by 10 nM SS. Basal release of irSS in 5 mM glucose was 377 +/- 35 pg.ml cells-1.2h-1 (2.05 +/- 0.19% TCC) and was increased by glucose (> or = 15 mM) and the addition of pGIP (> or = 1 nM). The STC 6-14 cell line represents a model to study the synthesis, storage, and release of GIP and SS in a controlled environment.

Cholecystokinin release from isolated canine epithelial cells in short-term culture

Canine jejunal epithelial cells were isolated and maintained in short-term culture to study cholecystokinin (CCK) release. Sequential digestion of jejunal mucosa with collagenase and ethylenediaminetetraacetic acid was followed by counterflow elutriation to enrich CCK-containing cells. After 40 hours in culture on collagen-coated plates, 8.4% of the initially seeded cells were attached; 8.7% of them stained positive with a C-terminal CCK/gastrin antibody and 2.5% stained positive with a gastrin-specific antibody. Basal release of CCK into the culture medium amounted to 1.3% of total cell content over 105 minutes. Receptor-independent stimulation of protein kinase C by the phorbol ester beta-phorbol-12-myristate-13-acetate caused significant CCK release. The inactive form, 4 alpha-phorbol-12-myristate-13-acetate, had no effect. Activation of adenylate cyclase by 10(-5) mol/L forskolin evoked a 2.5-fold increase in CCK concentrations, which was completely abolished by 10(-8) mol/L somatostatin. L-phenylalanine stimulated CCK release at 20 and 50 mmol/L, whereas D-phenylalanine caused significant hormone output only at 50 mmol/L. L-tryptophan had no effect. Cholecystokinin release stimulated by L-phenylalanine was not influenced by the addition of either somatostatin or somatostatin antibody. In conclusion, a system of isolated canine jejunal epithelial cells was developed in short-term culture. This preparation proved suitable for the study of CCK release on a cellular basis.

Gastrin secretion from human antral G cells in culture

Receptor-dependent and -independent regulation of gastrin secretion from cultured human antral G cells was investigated. Human antral mucosal cell preparations that were enriched for G cells were obtained by sequential incubations with collagenase and ethylenediaminetetraacetic acid, centrifugal elutriation, and short-term culture. After a 2-day incubation period, gastrin- and somatostatin-containing cells accounted for 15% and 5%, respectively, of the total adhered-cell population. Forskolin, A23187, and beta-phorbol 12 myristate 13-acetate stimulated basal gastrin secretion from cultured human G cells in a concentration-dependent fashion. These results indicate that gastrin release could be mediated by elevations in cytosolic cyclic adenosine monophosphate levels, calcium influx, or activation of protein kinase C. A direct stimulatory role for bombesin- and gastrin-releasing peptide was supported by experiments showing concentration-dependent enhancement of gastrin release by bombesin from 0.01 fmol/L to 10 nmol/L. The putative bombesin antagonist [Leu13-psi-CH2NH-Leu14] bombesin augmented basal gastrin levels by itself and produced weak inhibition of bombesin-induced gastrin secretion from human antral G cells. Somatostatin potently suppressed forskolin- and bombesin-mediated gastrin release but did not significantly alter basal gastrin levels. These results suggest that bombesin and somatostatin directly activate and inhibit G-cell function via specific and sensitive receptors. Furthermore, the adenylate cyclase and phosphatidyl inositide second messenger systems seem to be intracellular mediators of gastrin secretion from human antral G cells.

Monoclonal antibodies against calcitonin. Characterization and application in light and electron microscopy immunocytochemistry

Twenty-seven monoclonal antibodies (MAbs) to synthetic human calcitonin (CT) were characterized for their reactivities with human CT peptide fragments by dot-blot analysis on nitrocellulose paper. Most of the antibodies bound to the C-terminus and fewer to the mid-region of CT. We have studied thyroid tissue specimens from several animal species after fixation in paraformaldehyde-, glutaraldehyde- or picric acid-containing mixtures and cryostat sectioning or embedment in paraffin or plastic (Epon 812 or Lowicryl 4KM) using this panel of MAbs. The site of antigen-antibody reaction was revealed either by immunoperoxidase, immunoalkaline phosphatase or by silver-enhanced immunogold staining methods. All MAbs were able to localize CT in human, rat and mouse thyroid C cells. Nineteen MAbs recognizing synthetic salmon CT and synthetic [Asu1,7]-eel CT by dot-blot, reacted with chicken ultimobranchial body C cells. One MAb recognizing native porcine CT by dot-blot, stained C cells in hog thyroid. Immunopositivity was confined to the cytoplasm and ultrastructural immunogold labelling demonstrated that cytoplasmic secretory granules were stained. Surgical specimens from human medullary thyroid carcinoma were also analysed for the presence of CT and a variable number of positive cells was found. Furthermore, Congo red-positive areas were shown to react with the MAbs. All conventional staining and immunoabsorption controls were negative. Hence, these MAbs may be suitable for use in routine immunopathological diagnosis of CT-producing tumors and for immunocytochemical localization of the three major CT variants in different animal species.

The effect of total parenteral nutrition (TPN) on gastrin release in the rat

The effect of short-term (7 days) total parenteral nutrition (TPN) on gastrin release was studied in vivo and in the isolated vascularly perfused rat stomach. The daily plasma gastrin concentration of parenterally fed rats was significantly lower than in ad lib fed control animals (53 +/- 17 pg/ml vs 159 +/- 32 pg/ml, P less than 0.05) as early as day 2 and a similar pattern was observed on days 4 and 6. The fasting plasma gastrin concentration of control animals was 2-fold greater than of the parenterally fed group (P less than 0.05). Following oral peptone, the gastrin response of TPN and control animals doubled although peak gastrin levels were greatly reduced in TPN rats. Basal gastrin release from the perfused stomachs of control rats was 2-fold greater than from TPN rats (P less than 0.05). Electrical stimulation of the vagal trunks resulted in a significantly greater elevation in gastrin secretion from control stomachs compared to TPN animals (4-fold vs. 2.4-fold increase, P less than 0.05). Quantification of the antral G-cell population revealed a significant reduction in the number of G-cell of TPN rats compared to controls (97 +/- 8 cells/mm vs 76 +/- 6 cells/mm, P less than 0.05). These results indicate that luminal nutrient stimulation is necessary for the maintenance of normal G-cell secretory activity in vivo and from the in vitro stomach. G-cell hypoplasia appears to be partially responsible for reduced gastrin output to basal and stimulated conditions after TPN.

Endocrine cells producing regulatory peptides

Recent data on the immunolocalization of regulatory peptides and related propeptide sequences in endocrine cells and tumors of the gastrointestinal tract, pancreas, lung, thyroid, pituitary (ACTH and opioids), adrenals and paraganglia have been revised and discussed. Gastrin, xenopsin, cholecystokinin (CCK), somatostatin, motilin, secretin, GIP (gastric inhibitory polypeptide), neurotensin, glicentin/glucagon-37 and PYY (peptide tyrosine tyrosine) are the main products of gastrointestinal endocrine cells; glucagon, CRF (corticotropin releasing factor), somatostatin, PP (pancreatic polypeptide) and GRF (growth hormone releasing factor), in addition to insulin, are produced in pancreatic islet cells; bombesin-related peptides are the main markers of pulmonary endocrine cells; calcitonin and CGRP (calcitonin gene-related peptide) occur in thyroid and extrathyroid C cells; ACTH and endorphins in anterior and intermediate lobe pituitary cells, alpha-MSH and CLIP (corticotropin-like intermediate lobe peptide) in intermediate lobe cells; met- and leu-enkephalins and related peptides in adrenal medullary and paraganglionic cells as well as in some gut (enterochromaffin) cells; NPY (neuropeptide Y) in adrenaline-type adrenal medullary cells, etc.. Both tissue-appropriate and tissue-inappropriate regulatory peptides are produced by endocrine tumours, with inappropriate peptides mostly produced by malignant tumours.

Effects of monoclonal antibodies to somatostatin on somatostatin-induced and intestinal fat-induced inhibition of gastric acid secretion in the rat

Cell lines producing monoclonal antibodies to somatostatin, designated S10 and S20, have recently been generated. The purpose of the present immunoneutralization study was to examine the ability of these antibodies to block the inhibitory effect of exogenous somatostatin on meal-stimulated gastric acid secretion in the innervated rat stomach and to use these antibodies as probes to determine if somatostatin is involved in intestinal fat-induced inhibition of gastric acid secretion. The plateau acid secretory response to a liver extract meal in this model was 28 +/- 2 mu Eq/30 min. Intravenous infusion of somatostatin at 2.0 micrograms/kg X h or intraduodenal oleic acid at 1.2 ml/h reduced this response to 12 +/- 1 and 14 +/- 1 mu Eq/30 min, respectively. The antibodies were given intravenously 1 h before the meal and either somatostatin or intraduodenal oleic acid infusion. S10 preinfusion returned the plateau meal responses to the levels seen with the meal alone: 25 +/- 4 and 26 +/- 1 mu Eq/30 min, respectively. S20 preinfusion had no effect, the responses being 14 +/- 1 and 16 +/- 1 mu Eq/30 min, respectively. These results demonstrate successful binding of exogenous somatostatin by S10 in vivo and reversal of intestinal fat-induced inhibition of gastric acid secretion by S10 preinfusion. It is concluded that the mechanism whereby fat in the small intestine inhibits gastric acid secretion may involve the release of somatostatin.

Immunocytochemical characterization of glucagon-immunoreactive cells using monoclonal antibodies to pancreatic glucagon

Monoclonal antibodies raised to pancreatic glucagon were tested for their ability to detect glucagon-containing endocrine cells in material processed for light and electron microscopy. Samples from man, baboon and rat were used in this investigation. Two antibodies were specific for the pancreatic islet A cells, the remainder detected both pancreatic and enteric endocrine cells. In man and baboon the glucagon-containing cells were confined to the pancreas, lower small intestine and colon. In the rat the distribution was extended to include the corpus of the stomach and the jejunum. The cells identified in the ileum and colon were of three morphological types endocrine, paracrine (type 1) with a single basal process and paracrine (type 2) with multiple small cytoplasmic processes. These antibodies also detected cells in material fixed by conventional methods for electron microscopy. The ultrastructural appearance of the baboon pancreatic glucagon-containing ultracellular secretory granules were demonstrated to be clearly distinct from those described previously in man and rat. The secretory granules averaged 330 +/- 23 nm and lacked the characteristic clear outer halo seen in the other two species.

Production and characterization of N-terminally and C-terminally directed monoclonal antibodies against pancreatic glucagon

Hybridoma technology has been successfully applied to the production of monoclonal antibodies against a variety of small soluble peptides. We report herein for the first time on the development of monoclonal antibodies to pancreatic glucagon. Twenty-three stable positive hybridomas were detected by radioimmunoassay from five separate fusions and cloned by the limiting dilution method. Four selected monoclonal antibodies were all of the IgG 2a subclass type kappa and bound to protein A. One monoclonal antibody (23.8B6) was shown to be directed toward the C-terminal region and another (23.6B4) toward the N-terminal to central region of the glucagon molecule. These antibodies did not cross-react with any of the other peptides tested. Two further monoclonal antibodies (23.4A1, 22.3A6) reacted with the C-terminal third of the glucagon molecule and showed a cross-reaction with the structurally related gastric inhibitory polypeptide of 0.7% and 9.1%, respectively. All but the C-terminal monoclonal antibody 23.8B6 showed a marked cross-reaction with ileal extracts. The N-terminally directed monoclonal antibody 23.6B4 was of sufficient avidity for use in the radioimmunoassay of pancreatic glucagon and gut glucagon-like immunoreactivity in tissue extracts, being sensitive to changes of pancreatic glucagon of 2.0 fmol/tube at a final titer of culture supernatant of 1:1.4 X 10(5). In gel permeation chromatography of intestinal extracts, two major peaks were detectable (Kav 0.27 and 0.54). The present findings show that monoclonal antibodies provide sensitive tools for detecting pancreatic glucagon and gut glucagon-like immunoreactivity. They will be valuable immunoreactants for the development of immunoradiometric assays as well as for large-scale immunoaffinity purification of gut glucagon-like immunoreactivity.

The effect of total parenteral nutrition (TPN) on the enteroinsular axis in the rat

The effect of 6 days of total parenteral nutrition (TPN) on the enteroinsular axis was studied in vivo and in vitro in the rat. During the TPN period, blood samples were taken from control and TPN animals to determine the comparative pattern of GIP release. Glucose, insulin and GIP responses to oral glucose (OGTT) were compared in TPN and control rats. The effect of glucose and GIP on insulin release from the isolated perfused pancreas of the same animals was investigated to determine if TPN altered the sensitivity of the beta cell. In conjunction with these studies the number and distribution of GIP-containing cells were compared in control and TPN animals. TPN resulted in no change in basal levels of glucose, insulin and IR-GIP. An exaggerated insulin response to OGTT occurred after TPN whereas the glucose response was reduced. The IR-GIP response to glucose was normal following TPN. The isolated perfused pancreas showed a 30% increase in insulin release in response to GIP after TPN. The insulin response to glucose appeared normal as did the number and distribution of GIP cells. Fluctuations in GIP and insulin levels in control animals were diurnal in nature, whereas IR-GIP levels in TPN animals remained near fasting levels. It was hypothesized that the increase in beta cell sensitivity to GIP may be causally connected to the exposure of the pancreas to chronically low levels of GIP during TPN.

Increased antral G-cell number and gastrin content in dogs after massive small bowel resection

The effects of massive small-bowel resection on antral gastrin tissue concentration and G-cell number have been investigated in the dog. Tissue gastrin concentrations increased significantly after resection from 16.8 +/- 2.6 ng/mg wet weight to 30.3 +/- 3.2 ng. Immunoreactive gastrin cell number also increased from 29.8 +/- 2.5 cell/mm2 to 43.1 +/- 3.0 cells/mm2. Immunocytochemistry demonstrated that the hyperplastic gastrin cells were found in small groups, with the majority of the immunoreactivity located at the luminal pole of the cells. This finding, linked to hypogastrinemia in the dogs after massive small bowel resection, suggests that some of the hyperplastic G cells may have an exocrine rather than endocrine function.

VIP-innervation of rat intestine: a developmental study with monoclonal antibodies

Four monoclonal antibodies to VIP have been generated and shown to be N-terminal specific with high affinity for VIP. VIP-containing nerve fibers and cell bodies were visible in the upper small intestine from day 1 of neonatal life. Initially the immunoreactivity was mostly in the myenteric plexus but extended into the sub-mucous plexus by day 7. From day 1 to day 7 the VIP-innervation developed both orally and caudally at a similar rate. In the stomach, the antrum showed sub-mucosal cell bodies by day 14, while in the corpus the cell bodies remained confined to the myenteric plexus. The colon showed positive fibers in the myenteric plexus at day 7 and cell bodies and fibers in the sub-mucous plexus by day 14. The size (cross-sectional area) of the individual VIP-immunoreactive cell bodies increased significantly between day 1 and day 14 with no further increase with age. At no time were immunoreactive cell bodies shown to migrate from the myenteric to the sub-mucous plexus. VIP-immunoreactive epithelial cells were not detected in the present study.

Ultrastructural localization of gastric inhibitory polypeptide (GIP) in a well characterized endocrine cell of canine duodenal mucosa

The polypeptide hormone GIP has been localized ultrastructurally by using specific, monoclonal GIP antibodies and an immunogold technique on aldehyde-osmium fixed specimens of dog duodenal mucosa. A single type of cell showing round, homogeneous, fairly osmiophilic granules with closely applied membrane and a mean size of 188 nm +/- 34 SD has been identified as the GIP cell.

Immunocytochemical localization of glucagonlike and gastric inhibitory polypeptidelike peptides in the pancreatic islets and gastrointestinal tract

The distribution of cells displaying glucagonlike or gastric inhibitory polypeptide (GIP)-like immunoreactivity was examined in the pancreatic islets and gastrointestinal tracts of rats, dogs, and humans. A-cells in the pancreatic islets in all three species were stained by antisera having regional specificity for pancreatic-type glucagon, gut-type glucagon (glicentin), or GIP. Oxyntic A-cells of the gastric mucosa in dogs and humans also were stained comparably by these three antisera. In contrast, the K- and L-cells in the intestinal mucosa of those species were stained only by antisera capable of reacting with GIP or gut-type glucagon, respectively. Tests of antibody specificity showed that the GIP antiserum did not cross-react with either pancreatic- or gut-type glucagon. Likewise, the glucagon antisera showed no cross-reactivity with GIP. Hence, these findings suggest that pancreatic and gastric A-cells contain a peptide with GIP-like immunoreactivity distinct from glucagon or glicentin per se. Although the exact basis of th GIP-like immunostaining of A-cells is unknown, it may be due to the presence of a glucagon precursor sharing certain amino-acid sequences with GIP. This hypothesis is consistent with several recent investigations showing that the processing of proglucagon molecules differs between the A- and L-cells.