Distribution and molecular forms of peptides containing somatostatin immunodeterminants in extracts from the entire gastrointestinal tract of man and pig

Mapping and quantifying neuropeptides in the enteric nervous system

Neuropeptides are a highly diverse group of signaling molecules found in the central nervous system (CNS) and peripheral organs, including the enteric nervous system (ENS). Increasing efforts have been focused on dissecting the role of neuropeptides in both neural- and non-neural-related diseases, as well as their potential therapeutic value. In parallel, accurate knowledge on their source of production and pleiotropic functions is still needed to fully understand their implications in biological processes. This review will focus on the analytical challenges involved in studying neuropeptides, particularly in the ENS, a tissue where their abundance is low, together with opportunities for further technical development.

Gap junction coupling and islet delta-cell function in health and disease

The pancreatic islets contain beta-cells and alpha-cells, which are responsible for secreting two principal gluco-regulatory hormones; insulin and glucagon, respectively. However, they also contain delta-cells, a relatively sparse cell type that secretes somatostatin (SST). These cells have a complex morphology allowing them to establish an extensive communication network throughout the islet, despite their scarcity. Delta-cells are electrically excitable cells, and SST secretion is released in a glucose- and K_ATP-dependent manner. SST hyperpolarises the alpha-cell membrane and suppresses exocytosis. In this way, islet SST potently inhibits glucagon release. Recent studies investigating the activity of delta-cells have revealed they are electrically coupled to beta-cells via gap junctions, suggesting the delta-cell is more than just a paracrine inhibitor. In this Review, we summarize delta-cell morphology, function, and the role of SST signalling for regulating islet hormonal output. A distinguishing feature of this Review is that we attempt to use the discovery of this gap junction pathway, together with what is already known about delta-cells, to reframe the role of these cells in both health and disease. In particular, we argue that the discovery of gap junction communication between delta-cells and beta-cells provides new insights into the contribution of delta-cells to the islet hormonal defects observed in both type 1 and type 2 diabetes. This reappraisal of the delta-cell is important as it may offer novel insights into how the physiology of this cell can be utilised to restore islet function in diabetes.

The endocrine effects of bitter tastant administration in the gastrointestinal system: intragastric versus intraduodenal administration

Bitter tastants are recently introduced as potential hunger-suppressive compounds, the so-called "Bitter pill." However, the literature about bitter administration lacks consistency in methods and findings. We want to test whether hunger ratings and hormone plasma levels are affected by: 1) the site of administration: intragastrically (IG) or intraduodenally (ID), 2) the bitter tastant itself, quinine hydrochloride (QHCl) or denatonium benzoate (DB), and 3) the timing of infusion. Therefore, 14 healthy, female volunteers participated in a randomized, placebo-controlled six-visit crossover study. After an overnight fast, DB (1 µmol/kg), QHCl (10 µmol/kg), or placebo were given IG or ID via a nasogastric feeding tube. Blood samples were taken 10 min before administration and every 10 min after administration for a period of 2 h. Hunger was rated at the same time points on a visual analogue scale. ID bitter administration did not affect hunger sensations, motilin, or acyl-ghrelin release compared with its placebo infusion. IG QHCl infusion tended to suppress hunger increase, especially between 50 and 70 min after infusion, simultaneously with reduced motilin values. Here, acyl-ghrelin was not affected. IG DB did not affect hunger or motilin, however acyl-ghrelin levels were reduced 50-70 minutes after infusion. Plasma values of glucagon-like peptide 1 and cholecystokinin were too low to be properly detected or to have any physiological relevance. In conclusion, bitter tastants should be infused into the stomach to reduce hunger sensations and orexigenic gut peptides. QHCl has the best potential to reduce hunger sensations, and it should be infused 60 min before food intake.NEW & NOTEWORTHY Bitter tastants are a potential new weight-loss treatment. This is a noninvasive, easy approach, which should be received with considerable enthusiasm by the public. However, literature about bitter administration lacks consistency in methods and findings. We summarize how the compound should be given based on: the site of administration, the best bitter compound to use, and at what timing in respect to the meal. This paper is therefore a fundamental step to continue research toward the further development of the "bitter pill."

Comparison of Human and Murine Enteroendocrine Cells by Transcriptomic and Peptidomic Profiling

Enteroendocrine cells (EECs) produce hormones such as glucagon-like peptide 1 and peptide YY that regulate food absorption, insulin secretion, and appetite. Based on the success of glucagon-like peptide 1-based therapies for type 2 diabetes and obesity, EECs are themselves the focus of drug discovery programs to enhance gut hormone secretion. The aim of this study was to identify the transcriptome and peptidome of human EECs and to provide a cross-species comparison between humans and mice. By RNA sequencing of human EECs purified by flow cytometry after cell fixation and staining, we present a first transcriptomic analysis of human EEC populations and demonstrate a strong correlation with murine counterparts. RNA sequencing was deep enough to enable identification of low-abundance transcripts such as G-protein-coupled receptors and ion channels, revealing expression in human EECs of G-protein-coupled receptors previously found to play roles in postprandial nutrient detection. With liquid chromatography-tandem mass spectrometry, we profiled the gradients of peptide hormones along the human and mouse gut, including their sequences and posttranslational modifications. The transcriptomic and peptidomic profiles of human and mouse EECs and cross-species comparison will be valuable tools for drug discovery programs and for understanding human metabolism and the endocrine impacts of bariatric surgery.

The relationship between N-terminal prosomatostatin, all-cause and cardiovascular mortality in patients with type 2 diabetes mellitus (ZODIAC-35)

BACKGROUND

The hormone somatostatin inhibits growth hormone release from the pituitary gland and is theoretically linked to diabetes and diabetes related complications. This study aimed to investigate the relationship between levels of the stable somatostatin precursor, N-terminal prosomatostatin (NT-proSST), with mortality in type 2 diabetes (T2DM) patients.

METHODS

In 1,326 T2DM outpatients, participating in this ZODIAC prospective cohort study, Cox proportional hazards models were used to investigate the independent relationship between plasma NT-proSST concentrations with all-cause and cardiovascular mortality.

RESULTS

Median concentration of NT-proSST was 592 [IQR 450-783] pmol/L. During follow-up for 6 [3-10] years, 413 (31%) patients died, of which 176 deaths (43%) were attributable to cardiovascular causes. The age and sex adjusted hazard ratios (HRs) for all-cause and cardiovascular mortality were 1.48 (95%CI 1.14 - 1.93) and 2.21 (95%CI 1.49 - 3.28). However, after further adjustment for cardiovascular risk factors there was no independent association of log NT-proSST with mortality, which was almost entirely attributable to adjustment for serum creatinine. There were no significant differences in Harrell's C statistics to predict mortality for the models with and without NT-proSST: both 0.79 (95%CI 0.77 - 0.82) and 0.81 (95%CI 0.77 - 0.84).

CONCLUSIONS

NT-proSST is unsuitable as a biomarker for cardiovascular and all-cause mortality in stable outpatients with T2DM.

Screening of a large panel of gastrointestinal peptide plasma levels is not adapted for the evaluation of digestive damage following irradiation

The aim of this study was to assess the potential of gastrointestinal peptide plasma levels as biomarkers of radiation-induced digestive tract damage. To this end, plasma levels of substance P, GRP, motilin, PYY, somatostatin-28, gastrin, and neurotensin were followed for up to 5 days in pigs after a 16-Gy whole-body X-irradiation, completed by a histopathological study performed at 5 days. Each peptide gave a specific response to irradiation. The plasma levels of GRP and substance P were not modified by irradiation exposure; neither were those of motilin and PYY. Concerning gastrin, a 2-3-fold increase of plasma concentration was observed in pig, which presented the most important histological alterations of the stomach. The plasma levels of somatostatin, unchanged from 1 to 4 days after irradiation, was also increased by 130% at 5 days. In contrast, a diminution of neurotensin plasma levels was noted, firstly at 1 day (-88%), and from 3 days after exposure (-50%). The present study suggested that changes in gastrin and neurotensin plasma levels were associated with structural alterations of the stomach and ileum, respectively, indicating that they may be relevant biological indicators of radiation-induced digestive damage to these segments.

Thrittene, homologous with somatostatin-28((1-13)), is a novel peptide in mammalian gut and circulation

Preprosomatostatin is a gene expressed ubiquitously among vertebrates, and at least two duplications of this gene have occurred during evolution. Somatostatin-28 (S-28) and somatostatin-14 (S-14), C-terminal products of prosomatostatin (ProS), are differentially expressed in mammalian neurons, D cells, and enterocytes. One pathway for the generation of S-14 entails the excision of Arg13-Lys14 in S-28, leading to equivalent amounts of S-28((1-12)). Using an antiserum (F-4), directed to the N-terminal region of S-28 that does not react with S-28((1-12)), we detected a peptide, in addition to S-28 and ProS, that was present in human plasma and in the intestinal tract of rats and monkeys. This F-4 reacting peptide was purified from monkey ileum; and its amino acid sequence, molecular mass, and chromatographic characteristics conformed to those of S-28((1-13)), a peptide not described heretofore. When extracts of the small intestine were measured by RIA, there was a discordance in the ratio of peptides reacting with F-4 and those containing the C terminus of ProS, suggesting sites of synthesis for S-28((1-13)) distinct from those for S-14 and S-28. This was supported by immunocytochemistry, wherein F-4 reactivity was localized in gastrointestinal (GI) endocrine cells and a widespread plexus of neurons within the wall of the distal gut while immunoreactivity to C-terminal domains of S-14 and S-28 in these neurons was absent. Further, F-4 immunoreactivity persisted in similar GI endocrine cells and myenteric neurons in mice with a targeted deletion of the preprosomatostatin gene. We believe that these data suggest a novel peptide produced in the mammalian gut, homologous with the 13 residues of the proximal region of S-28 but not derived from the ProS gene. Pending characterization of the gene from which this peptide is derived, its distribution, and function, we have designated this peptide as thrittene. Its localization in both GI endocrine cells and gut neurons suggests that thrittene may function as both a hormone and neurotransmitter.

Somatostatin-14 modulates acid-dependent inhibition of meal-stimulated gastrin via muscarinic pathways in dogs

Intraluminal antral acidification inhibits gastrin and stimulates somatostatin-14 (S-14) release, but a functional relationship in the postprandial state has not been established. To examine whether meal-stimulated S-14 mediates inhibition of gastrin release by gastric acid, the effects of omeprazole on circulating levels of S-14 separated from S-28 by gel permeation chromatography, and gastrin were measured without and with atropine in dogs. Compared to controls, pretreatment with omeprazole decreased postprandial plasma levels of S-14 and S-28 (both P<0.01) and increased gastrin (P<0.001). Atropine selectively converted the S-14 response after omeprazole to a peak sixfold increase 40 min after meal ingestion (P<0.001), which was also significantly above S-14 values after atropine alone and controls, but reduced plasma levels of S-28 and gastrin to baseline. Infusions of the somatostatin analogue, cyclo-[7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(BZL)] increased postprandial gastrin twofold above controls (P<0.05), and when administered after omeprazole reversed the inhibition of gastrin by atropine, without altering S-14 levels. In contrast, infusions of S-14, which simulated S-14 levels after omeprazole-atropine, and of [D-Trp8]-S-14, which abolished meal-stimulated S-14 responses, did not alter postprandial elevations of plasma gastrin. This study suggests that in conscious dogs muscarinic inhibitory pathways selectively regulate S-14 secretion, are amplified at neutral gastric pH and reciprocally link S-14 to gastrin secretion in the gastric phase of meal ingestion. Postprandial regulation of gastrin release by S-14 includes neurocrine interactions with muscarinic receptor activation; endocrine or paracrine regulation seem less likely.

Immunohistochemical studies of the endocrine cells within the gastro-entero-pancreatic system of Osteoglossomorpha, an ancient teleostean group

The identification and distribution of endocrine cells within the gastro-entero-pancreatic (GEP) system of five species of the Osteoglossomorpha (Osteoglossum bicirrhosum, Scleropages jardini, Pantodon buchholzi, Notopterus chitala and Gnathonemus petersii) were analyzed by immunohistochemistry. Four immunoreactive cell types were identified within the pancreatic islets (A, B, D, and F cells), using antisera directed against mammalian insulin (m-INS), somatostatins (SST-14, SST-25), and members of the pancreatic polypeptide (aPY, NPY, PYY) and glucagon (GLU, GLP) families. The B cells were located throughout the center of the islets in the five species and, in general, D cells had a similar distribution. However, immunoreactivity to anti-somatostatins varied between four of the species and G. petersii, which showed less intensely stained D cells in the islets, but greater SST immunoreactivity in both the intestinal and the stomach epithelia than in comparable epithelia of other species. For peptides of both the pancreatic polypeptide and the glucagon families, the immunoreactivity was detected at the periphery of the islets, and there was a suggestion of an interfamily colocalization of peptides in some cells. In addition, glucagon family peptides showed a scattered immunoreactivity throughout the central portion of the islets. A moderately abundant number of cells in the intestine were immunoreactive to the PP family antisera in all five species. However, immunoreactivities to GLU, GLP, SST, and m-INS antisera were variable in intestinal cells of the species. Immunoreactivity with sera raised against m-INS and PYY was also observed in the stomach of P. buchholzi. The significance of these findings is discussed in both ontogenetic and phylogenetic contexts with respect to the GEP system in actinopterygian fishes and with respect to the possibility of variable processing of prohormones in the different organs of these osteoglossomorphs.

An immunohistochemical study of the endocrine cells within the pancreas, intestine, and stomach of the gar (Lepisosteus osseus L.)

The distribution and identity of the various endocrine cell types were examined in the pancreas, stomach, and anterior intestine of the phylogenetically ancient actinopterygian, the gar (Lepisosteus osseus L.), using immunohistochemistry. Antisera used were directed against several insulins (INSs) and somatostatins (SSTs), and members of the pancreatic polypeptide (PP, aPY, NPY) and glucagon (GLUC, GLP) families. In the gar pancreas the most pronounced aggregation of islet tissue is among the exocrine acini near the union of extrahepatic common bile duct with the gastrointestinal junction. Four immunoreactive cell types were identified within well-defined islets (A, B, D, and F cells) but immunoreactive cell types were also seen isolated among the exocrine acini. Centrally located B cells were immunoreactive with mammalian and lamprey INS antisera whereas the widely dispersed D cells immunostained with anti-SST-14, -25, and -34. SST was also localized in the epithelium of the pancreatic ducts. There was a colocalization of immunoreactivity for each member of the PP and GLU families at the periphery of each islet to identify F and A cells, respectively. However, colocalization of peptides from both families is suspected for at least some cells. Although the gastric and intestinal mucosae showed a similar pattern of immunoreactivity to GLP and not GLU, they had contrasting immunoreactivity with the two INS antisera. SST immunoreactivity was restricted to the stomach, whereas three of the four PP-family peptides were only immunoreactive in the intestine. Immunoreactivity to the various antisera used in the study imply that there may be an organ-specific processing of preproinsulin, that the gar SST profile may be more similar to agnathan and bowfin rather than either elasmobranch or teleost SSTs, and that only the GLP portion of the preproglucagon gene is expressed in the gastrointestinal mucosa. Our results are consistent with other recent endocrine studies showing that the gar is a widely distinct actinopterygian.

Effect of octreotide on anal pressure and rectal compliance

PURPOSE

The somatostatin analog, octreotide, has previously been found to influence rectal sensation and may also influence anal resting pressure.

METHODS

We studied the effect of octreotide on anal resting pressure and rectal compliance in eight healthy patients. Octreotide was administered intravenously as a bolus injection in doses of 100 and 10 micrograms or as infusion of 250 micrograms/hour on separate days and compared with placebo.

RESULTS

Within one minute after a bolus injection of 100 micrograms of octreotide, anal resting pressure increased from 56 +/- 12 to 96 +/- 16 cm H2O (P < 0.005). Octrotide had no effect on rectal sensitivity or compliance measurements. Octreotide counteracted rectoanal reflex by increasing anal pressure almost to the level found with an empty rectum.

CONCLUSION

Somatostatin thus seems to contribute to the regulation of rectoanal reflex.

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.

Loxiglumide inhibits cholecystokinin stimulated somatostatin secretion and simultaneously enhances gastric acid secretion in humans

In vitro studies have demonstrated that cholecystokinin releases somatostatin from the gastric mucosa. To date, there is no information about the in vivo significance of this finding in man. Therefore, we have studied the effect of infusion of cholecystokinin resulting in plasma concentrations within the range found after meal-stimulation, on somatostatin release and on gastric acid secretion. In addition we have studied these functions during infusion of the type A cholecystokinin receptor antagonist loxiglumide. In eight healthy subjects, basal gastric acid secretion was distinctly stimulated by cholecystokinin. The effect of cholecystokinin on gastric acid secretion was markedly enhanced by loxiglumide. Cholecystokinin also significantly stimulated somatostatin output into the gastric lumen, but not into the systemic circulation. Somatostatin output into the gastric lumen during infusion of cholecystokinin was abolished by loxiglumide. The data indicate that on the one hand circulating cholecystokinin, like gastrin, stimulates gastric acid secretion probably by binding to less specific type B receptors on parietal cells that are not blocked by loxiglumide, but on the other hand that cholecystokinin, in contrast to gastrin, also inhibits gastric acid secretion probably by binding to specific type A receptors present on somatostatin producing D-cells in the gastric mucosa, that are blocked by loxiglumide.

Immunohistochemical localization of different forms of somatostatin in the gastrointestinal tract of the calf

The presence of two peptides that belong to the somatostatin family has been investigated in the calf gut. Somatostatin-14-like and Somatostatin-28-like peptides have been localized by a light microscopic immunohistochemical method. The method employed antibodies linked to colloidal gold particles that were revealed by a silver-enhancement step. Somatostatin-14-like peptide was only present in mucosal endocrine cells, which were detectable along the entire gut with the exceptions of the abomasal gastric proper glands and caecum. The cells were most abundant in cardiac and pyloric glands. Langerhans' islets also contained this type of endocrine cell. Somatostatin-28-like-immunoreactive endocrine cells were more abundant than the former cell type. They were present in the gastric proper glands and caecum where Somatostatin-14-like-immunoreactive cells were absent. They were as numerous as the former type of cell in the endocrine pancreas. The Somatostatin-28-like peptide was also detectable in the intramural nervous components of the abomasum and the intestine, in both perikarya and terminals. Our results show a possible heterogeneity of an endocrine cell type, which synthesizes and secretes somatostatin peptides. Our results also support the hypothesis that somatostatin-14 and somatostatin-28 peptides may have distinct functional roles, particularly in different species.

Somatostatin: physiology and clinical applications

Somatostatin (SOM) was originally isolated as the hypothalamic inhibitor of growth hormone release but was subsequently shown to have a widespread distribution including the gastrointestinal tract. In fact the gastrointestinal tract contains about 70% of the total body SOM. SOM has inhibitory actions on gastrointestinal exocrine and endocrine secretions, motility and blood flow. Within the gut it functions as an endocrine, paracrine, autocrine and neurocrine factor. SOM is released by a meal, and a number of neurotransmitters and regulatory peptides also influence SOM release. SOM is a key component of the gastrin-acid feedback loop as luminal acid releases SOM, which in turn has inhibitory effects on both gastrin and gastric acid. Consistent with the diverse functions of SOM, a number of different although related SOM receptors with distinct distribution patterns and intracellular mediators have been cloned and sequenced. SOM is the first of the gut regulatory peptides to have a significant therapeutic use. By inhibiting both the target cell (e.g. parietal cell) and the release of the active agent (e.g. gastrin) the therapeutic potential of SOM is magnified. To date most of the clinical experience has been with the one analogue, octreotide. This analogue has a longer half-life than SOM (hours versus minutes) but has only minimal oral activity, therefore requiring subcutaneous injections several times a day. The definite gastrointestinal applications include treatment of gastroenteropancreatic tumours. It is also becoming a favoured treatment for gastrointestinal fistulae, variceal bleeding and diarrhoea. However, octreotide has no consistent effect on tumour growth. The high density of SOM receptors on tumours has allowed localization of tumours using in vivo scintography with labelled octreotide. The sequencing of a variety of SOM receptors with different distributions and differing cellular effector systems raises the likelihood of developing SOM analogues for specific clinical applications.

Circulating somatostatin-28 is not a physiologic regulator of gastric acid production in man

Studies were designed to establish the acid inhibitory potency and plasma kinetics of somatostatin-28 (S-28) in humans and to determine whether the amount of S-28 released into the circulation after a meal is sufficient to regulate gastric acid secretion. A liquid meal induced a significant increase of S-28 (P < 0.01) whereas S-14 levels did not change. Postprandial S-28 concentrations were then mimicked by exogenous infusions and tested on basal and pentagastrin-stimulated gastric acid secretion. Expressed in terms of circulating plasma concentrations measured by specific radioimmunoassays, S-14 was 10 times more potent than S-28 in inhibiting gastric acid production. The plasma half-life of S-28 (1.86 min) was longer than that of S-14 (1.00 min) due to a slower plasma clearance rate. S-28 did neither affect basal and stimulated gastric acid secretion nor postprandial intragastric acidity. These studies suggest that postprandial plasma concentrations of S-28 are unlikely to regulate gastric acid secretion in man. They also show that S-28 is several times less potent than S-14 with respect to inhibition of gastric acid output.

Regulation of somatostatin-14 and -28 secretion by gastric acid in dogs: differential role of cholecystokinin

BACKGROUND

Prosomatostatin-derived peptides include two principle bioactive molecular forms, somatostatin 28 (S-28) and somatostatin 14 (S-14). This study examined whether there is a functional relationship between gastric acid secretion and the release of S-28 and S-14 into the circulation.

METHODS

In conscious dogs with gastric and duodenal cannulas, S-28 and S-14 responses, measured after extraction of acidified plasma and separation by gel chromatography, were evaluated by administration of nutrients and acid-inducing secretagogues without and with omeprazole.

RESULTS

Ingestion of a solid meal caused equivalent plasma elevations of S-28 and S-14, whereas infusions of histamine and gastrin selectively increased plasma S-14. Omeprazole decreased meal-stimulated S-28 (-67% +/- 8%; P < 0.01) and S-14 (-56 +/- 9%; P < 0.01) and abolished S-14 increases to histamine and gastrin. Intraduodenal perfusions of a liquid protein meal increased S-28 above S-14, comprising approximately 71% of total somatostatin-like immunoreactivity released, and omeprazole suppressed S-28 (-87% +/- 5%; P < 0.01) without influencing S-14. Similar responses occurred after exogenous cholecystokinin. Moreover, pretreatment of the intraduodenal protein meal with the cholecystokinin-A receptor antagonist MK-329 abolished increases of S-28 and S-14 and caused a further twofold increase of gastric acid (P < 0.025).

CONCLUSIONS

In the fed state, gastric acid causes direct release of S-14 from the stomach, but the acid-dependent component of S-28 secretion requires cholecystokinin as a cofactor. Negative feedback regulation between somatostatin and gastric acid secretory responses to nutrients may include S-28 modulated, in part, by cholecystokinin.

Evidence for hormonal inhibition of exocrine pancreatic function by somatostatin 28 in humans

Somatostatin 28 (S-28), originating in gastrointestinal cells, is secreted into the circulation and increases in humans after ingestion of a mixed meal. To evaluate the possibility that the increased levels of S-28 post cibum might modulate the release of enzymes and bicarbonate from the exocrine pancreas, S-28 was infused intravenously into healthy volunteers to levels seen after food intake. During S-28 infusion, the output of lipase, trypsin, amylase, and bicarbonate stimulated by either exogenous cholecystokinin octapeptide or endogenous signals from intraduodenal administration of tryptophan or a mixture of amino acids was significantly reduced. It is concluded that S-28 released from the gut during food intake modulates pancreatic exocrine function in humans.

Oxidation/reduction explains heterogeneity of pancreatic somatostatin

Somatostatin 14 (SS 14) has been isolated from pancreatic extracts, but open gel filtration immunoreactive SS often elutes in two peaks. We isolated both peaks, but upon sequence analysis only authentic SS 14 could be identified. By further gel filtration experiments it turned out that both synthetic and extractable SS appeared homogeneous at neutral pH 7.5, but showed an additional, earlier peak in acetic acid. After addition of mercaptoethanol, all of the SS eluted at this earlier position regardless of the pH. We conclude that partial reduction/oxidation of SS explains the heterogeneity.

Influence of peptides on anorectal function

Immunohistochemical and immunochemical studies on biopsies from the human rectum and anal canal have shown several regulatory peptides present in the muscle layers and the mucosa, suggesting a regulatory action on defecation. This view has been supported by studies of anorectal function in man during administration of different peptides. The physiological implications of these observations remain obscure.

Somatostatin and prosomatostatin in the retina of the rat: an immunohistochemical, in-situ hybridization, and chromatographic study

Specific antisera, raised in rabbits, against somatostatin 1-14, somatostatin 1-28, the fragment 1-12 of somatostatin 1-28, and prosomatostatin 20-36 were used for immunohistochemistry and gel filtration of the rat retina. With all antisera, immunoreactive perikarya could be located in the inner nuclear and ganglion cell layers. In the inner nuclear layer, amacrine cells with processes extending predominantly into the first sublayer of the inner plexiform layer were observed. Some processes extended also to the ganglion cell layer. In addition, somatostatin-immunoreactive interplexiform cells were present in the inner nuclear layer. In the ganglion cell layer, perikarya were found located in the midperiphery and in the far periphery of the retina. The neurons located in the midperiphery of the retina possessed a round perikaryon from which processes could be followed going into the inner plexiform layer, where they dichotomized in the third and first sublayers. The perikarya in the far periphery of the retina near the ora serrata exhibited an ovoid-shaped cell body from which processes extended horizontally in a bipolar manner in the layer itself. By use of an [35S]-labeled antisense oligonucleotide probe, in situ hybridization of the rat retina showed the presence of perikarya in the inner nuclear layer and ganglion cell layer containing mRNA encoding for prosomatostatin. Gel filtration of the retinal extracts followed by radioimmunoassay showed the presence of somatostatin 1-14, the fragment 1-12 of somatostatin 1-28, and prosomatostatin 1-64. However, somatostatin 1-28 was not detected. The results obtained in this study verify the presence of somatostatin 1-14 in the rat retina located in perikarya and processes in the inner nuclear and ganglion cell layers. The positive in-situ hybridization signals show that the intraneuronal somatostatin immunoreactivity is due to synthesis of the peptide and not uptake in the neurons. The presence of the somatostatin propeptide and fragments of this propeptide, in both intraretinal perikarya and fibers, indicate a posttranslational modification of this neuropeptide in the perikarya and the processes as well.

Distribution of two forms of somatostatin in the brain, anterior intestine, and pancreas of adult lampreys (Petromyzon marinus)

The distribution of two major immunoreactive forms of somatostatin, somatostatin-14 and somatostatin-34, within the brain, pancreas and intestine of adult lampreys, Petromyzon marinus, was identified using antisera raised against these peptides. Immunostaining of the brain is similar in juveniles and upstream migrants, and somatostatin-14 is the major somatostatin form demonstrated. A few somatostatin-34-containing cells are localized within the olfactory bulbs, thalamus and hypothalamus, but cells immunoreactive to anti-somatostatin-34 in the hypothalamus and thalamus do not co-localize somatostatin-14. Immunostaining of pinealocytes within the pineal pellucida with anti-somatostatin-14 may infer a novel function for this structure. Somatostatin-14 and somatostatin-34 are co-localized within D-cells of the cranial pancreas and caudal pancreas of juveniles and upstream migrants. Numerous somatostatin-34-immunoreactive cells are distributed within the epithelial mucosa of the anterior intestine but not all of these cells cross-react with anti-somatostatin-14. It appears that somatostatin-34 is the major somatostatin in the pancreo-gastrointestinal system of adult lampreys.

Distribution of somatostatin-14 and somatostatin-28 gastrointestinal-pancreatic cells of rats and humans

Somatostatin-14 and somatostatin-28 are biologically active peptides derived from the posttranslational cleavage of prosomatostatin. Because both peptides are found in variable concentrations in the gastrointestinal (GI) tract and pancreas, it has been contended that somatostatin-28 is either an intermediate in the processing to somatostatin-14 or a terminal product derived from prosomatostatin. To address this question, two antisera were used to recognize epitopes in two regions of somatostatin-14; one with high specificity for somatostatin-14 and the other interacting with prosomatostatin, somatostatin-28, and somatostatin-14. Distribution of these peptides was measured in extracts of pancreas and mucosa and submucosa/muscularis from the rat and human GI mucosal biopsies; the antisera were used to immunostain cells in these tissues. Extracts of human and rat intestinal mucosa contained both somatostatin-28 and somatostatin-14. By immunocytochemistry, D cells in stomach and pancreas and neural processes in the intestine, extending into the mucosal villi adjacent to endocrine cells, stained with both antisera indicating the presence of somatostatin-14, prosomatostatin, and possibly somatostatin-28. In contrast, endocrine cells in the gut reacting with antisera against somatostatin-28 did not immunostain with somatostatin-14-specific antisera. Thus, these data suggest that somatostatin-28 is the terminal peptide processed from prosomatostatin in intestinal mucosal cells, whereas somatostatin-14 is the major final product in gastric and pancreatic D cells and neurons. The localization of somatostatin-28 and somatostatin-14 in different cells in the pancreas and GI tract implies that they serve different functions.

Immunocytochemical localization and identification of prosomatostatin gene products in medullary carcinoma of human thyroid gland

Thirty-three cases of histologically proven calcitonin-positive medullary thyroid carcinoma were studied immunocytochemically for the occurrence of prosomatostatin-related peptides. Positive cells, identified with a panel of antisera raised against four different regions of the prosomatostatin molecule, were found in 100% of the tumors. Most but not all somatostatin-positive cells were also immunoreactive for calcitonin. Notably, seven patients harboring somatostatin-rich tumors revealed a more favorable clinical course. The results (1) indicate that somatostatin production is a universal concomitant of thyroid medullary carcinoma, (2) suggest that these cells are likely to produce a somatostatin precursor molecule similar to mammalian prosomatostatin, and (3) imply that somatostatin-reactive cells may have as yet unknown roles in these tumors, possibly in the realm of paracrine and autocrine regulation of cell growth.

Identification of the neurotransmitter/neuromodulator functions of the neuropeptide gastrin-releasing peptide in the porcine antrum, using the antagonist (Leu13-psi-CH2 NH-Leu14)-bombesin

We studied the effects of a new bombesin/gastrin-releasing peptide (GRP) receptor antagonist, Leu13-psi-(CH2NH)-Leu14-bombesin, on the secretion of gastrin and somatostatin and on the motor activity of isolated perfused porcine antrum in response to infusions of GRP at 10(-10) or 10(-9) mol/l and in response to electric stimulation of the vagus nerves. GRP significantly increased the secretion of gastrin and somatostatin and increased the frequency of antral contractions threefold. At 0.5 x 10(-6) mol/l the antagonist completely abolished the effects on motality and gastrin secretion and strongly inhibited the effect on somatostatin secretion. Vagus stimulation significantly increased gastrin and somatostatin secretion and increased the contraction frequency threefold. The antagonist strongly inhibited the somatostatin response, abolished the motility effects and reversed the stimulatory effect on gastrin secretion to a significant inhibition. Assuming that the antagonist interacts specifically with GRP receptors, we conclude that our data strongly support the concept that GRP-producing nerves are essential for vagally induced secretion of gastrin and somatostatin from the antrum. The GRP nerves may also play a role in the control of gastric motor activity.

A physiologic role for somatostatin 28 as a regulator of insulin secretion

Somatostatin 28 (S-28) is a peptide produced in the intestinal tract which rises in the circulation during nutrient absorption. We tested the hypothesis that S-28 regulates B-cell function by (a) studying the effects on insulin secretion of "physiologic" infusions of S-28 and (b) measuring insulin responses during elevated nutrient-stimulated endogenous S-28 levels. (a) Synthetic S-28 was infused on separate days into six healthy men at rates of 25 and 50 ng/kg per h which mimicked postprandial levels. Subjects were given a bolus of glucose (0.1 g/kg) after 120 min. Insulin responses during S-28 infusions were compared to a control study using a saline infusion in the same individuals. Glucose-stimulated insulin secretion was inhibited during the infusion of 50 ng/kg per h S-28 when compared to control (P less than 0.05). (b) Insulin secretion during elevations of endogenous S-28 was studied in healthy men who received a bolus of 2.5 g arginine (n = 14) or 25 U of secretin (n = 8) 120 min after swallowing 50 g fat, or, on a separate day, an equivalent volume of water. S-28 levels rose significantly after fat ingestion but did not change after water. Arginine and secretin-stimulated insulin secretion was inhibited following ingestion of fat compared with intake of water (P less than 0.05). Arginine-enhanced glucagon secretion was not changed by fat ingestion. We conclude that elevations in plasma S-28 levels, occurring during the postprandial state, attenuate B-cell secretion and this peptide may be a physiologic modulator of nutrient-stimulated insulin release.

Regional differences in concentrations of regulatory peptides in human colon mucosal biopsy

The study was undertaken to examine regional differences in the concentrations of five regulatory peptides in the human colonic mucosa. Biopsies were obtained during routine colonoscopy from 33 patients whose colonic mucosa was macroscopically and histologically normal. Regulatory peptides were extracted, and measured by specific radioimmunoassays. Concentrations of three peptides that are present predominantly in endocrine cells within colonic mucosa increased significantly towards the rectum: Mean concentrations of peptide YY, enteroglucagon, and somatostatin were about three times greater in the rectum than in the cecum. However, concentrations of two peptides that are present in mucosal nerve fibers diminished significantly towards the rectum: Mean rectal concentrations of vasoactive intestinal peptide and peptide histidine methionine were both about 0.6 of mean cecal concentrations. Concentrations of all five peptides were lower in biopsies taken from colonic polyps than in normal colonic mucosa. Regional differences in colonic mucosal concentrations of regulatory peptides probably reflect differences in the physiological functions of different parts of the colon.

Biochemistry and physiology of gastrointestinal somatostatin

Somatostatin, a tetradecapeptide initially isolated from the ovine hypothalamus, is widely distributed throughout the gastrointestinal tract where it may act as a hormone, local chemical messenger, or neurotransmitter to elicit many physiological actions. Release of somatostatin from D cells in the gut is regulated by mechanisms that are both dependent on and independent of cAMP. In most cases somatostatin acts to inhibit the function of its target cells. It performs this action in part via pertussis-toxin-sensitive inhibitory guanine nucleotide-binding proteins that regulate adenylate cyclase activity. Other mechanisms may involve sites of action distal to intracellular second messenger systems.

Intramural distribution of regulatory peptides in the sigmoid-recto-anal region of the human gut

The distribution of regulatory peptides was studied in the separated mucosa, submucosa and muscularis externa taken at 10 sampling sites encompassing the whole human sigmoid colon (five sites), rectum (two sites), and anal canal (three sites). Consistently high concentrations of VIP were measured in the muscle layer at most sites (proximal sigmoid: 286 (16) pmol/g, upper rectum: 269 (17), a moderate decrease being found in the distal smooth sphincter (151 (30) pmol/g). Values are expressed as mean (SE). Conversely, substance P concentrations showed an obvious decline in the recto-anal muscle (mid sigmoid: 19 (2.0) pmol/g, distal rectum: 7.1 (1.3), upper anal canal: 1.6 (0.6)). Somatostatin was mainly present in the sigmoid mucosa and submucosa (37 (9.3) and 15 (3.5) pmol/g, respectively) and showed low, but consistent concentrations in the muscle (mid sigmoid: 2.2 (0.7) pmol/g, upper anal canal: 1.5 (0.8]. Starting in the distal sigmoid colon, a distinct peak of tissue NPY was revealed, which was most striking in the muscle (of mid sigmoid: 16 (3.9) pmol/g, upper rectum: 47 (7.8), anal sphincter: 58 (14)). Peptide YY was confined to the mucosa and showed an earlier peak (upper sigmoid: 709 (186) pmol/g, mid-distal sigmoid: 1965 (484)). A clear differential distribution of regulatory peptides was thus shown in the region studied. A possible role is suggested for NPY and VIP containing nerves in the effector control of the human internal anal sphincter.

Somatostatin in the idiopathic inflammatory bowel diseases

To study the effect of mucosal inflammation on tissue concentrations of somatostatin, the distribution and concentration of somatostatin in specimens of normal and abnormal (ulcerative colitis and Crohn's disease) ileum and colon were determined by a specific radioimmunoassay. Each tissue specimen obtained at surgery was separated by microdissection into the mucosa-submucosa and the muscularis externa. Immunoreactive somatostatin was acid-extracted from each layer before measurement. Gel chromatography was used to characterize immunoreactive somatostatin measured by radioimmunoassay; somatostatin-28 was the major immunoreactive species measured in human intestine. In normal colon, concentrations of somatostatin were not related to patient age. Concentrations of immunoreactive somatostatin in the mucosa-submucosa of the descending colon were significantly decreased in ulcerative colitis and in Crohn's colitis, compared with normal colon. There was no apparent relationship between concentrations of somatostatin and the duration of inflammatory bowel disease. However, somatostatin concentrations appeared to be lower in patients with severe colitis than in patients with minimal colitis. The decrease in mucosal-submucosal concentrations of somatostatin is in agreement with previous morphologic studies, which have suggested diminished populations of endocrine cells in ulcerative colitis. The possible role of somatostatin in the colon suggests that further studies of the alteration of this gut peptide may be useful in understanding a component of the pathophysiology of idiopathic inflammatory bowel disease.

The effect of pancreatectomy and gastroenterectomy on the release of somatostatin and vasoactive intestinal polypeptide in experimental fecal peritonitis

Rats were subjected to laparotomy, to pancreatectomy, to gastroenterectomy (control groups), or to these procedures plus a septic challenge by instillation of 0.1 ml feces intraperitoneally (experimental groups). In the laparotomized controls plasma somatostatin values were significantly higher in samples from the portal vein than from the upper inferior caval vein. After both pancreatectomy and gastroenterectomy a significant fall in plasma somatostatin values was observed, and there was no significant difference between samples taken from the portal vein and the systemic circulation. An intraperitoneal septic challenge elicited a significant rise in portal plasma somatostatin in laparotomized rats, whereas this increase did not occur in pancreatectomized and gastroenterectomized animals, supporting the notion that plasma somatostatin originates from the pancreas and/or the gastrointestinal tract during septic peritonitis. No differences were detected in plasma vasoactive intestinal polypeptide (VIP) values in animals from the three control groups. However, both in laparotomized and in pancreatectomized septic animals a significant rise in plasma VIP was demonstrated in samples from the portal vein. By contrast, no such increase was observed in gastroenterectomized septic rats. Thus, the gastrointestinal tract seems to be the major source of circulating VIP during fecal peritonitis in the rat.

Localization in the gastrointestinal tract of immunoreactive prosomatostatin

Antisera against 5 different regions of the entire prosomatostatin molecule were used for immunohistochemical mapping of prosomatostatin-containing structures in the pig gastrointestinal tract, and for radioimmunological and chromatographical analysis of the products of prosomatostatin in extracts of ileal mucosa. The latter showed that the antisera were capable of identifying components containing N-terminal as well as C-terminal parts of prosomatostatin. Endocrine cells were identified with all antisera in most parts of the gastrointestinal tract, and varicose nerve fibres were observed in all parts of the small intestine but not in the stomach and the colon. The colon contained very few immunoreactive structures. Immunoreactive nerve cell bodies were found in the submucous plexus of the small intestine. All immunoreactive endocrine cells in the stomach and the duodenum and all immunoreactive nerves were stained by all 5 antisera whereas the small intestinal endocrine cells did not stain for the most N-terminal region of prosomatostatin. The results suggest that all gastrointestinal somatostatin is derived from the same precursor molecule, which, however, in the small intestinal endocrine cells is processed differently from that of the other tissues.

Circulating somatostatin. Physiological regulator of pancreatic function?

The present study was designed to determine whether somatostatin is released into the circulation in sufficient amounts to regulate exocrine and endocrine pancreatic function and to evaluate the possible role of somatostatin as a hormonal regulator of the pancreas. Mean plasma somatostatin levels (SLI) increased from 11 +/- 2 pmol liter-1 to peak concentrations of 18 +/- 2 in six healthy male volunteers after a steak meal (P less than 0.05). Infusion of somatostatin inhibited hormone-induced exocrine pancreatic secretion and suppressed cerulein-stimulated pancreatic polypeptide (PP) secretion, but did not significantly change arginine-stimulated insulin and glucagon release at mean plasma somatostatin concentrations within the range seen after a meal. The amount of somatostatin released after a meal thus was of sufficient magnitude to inhibit exocrine pancreatic function and PP release. On the other hand, basal and arginine-stimulated glucagon and insulin secretions were not significantly affected by these plasma concentrations of intravenous somatostatin suggesting that the exocrine pancreas might be more sensitive to somatostatin than the islet cells. We conclude that somatostatin in concentrations within the range seen after a meal is a potent inhibitor of stimulated acinar cell function in man. The findings support the hypothesis that somatostatin acts as a true hormonal regulator.

Effect of intragastric pH on antral gastrin and somatostatin release in anaesthetised, atropinised duodenal ulcer patients and controls

The synchronous change in the antral release of gastrin and somatostatin into a vein draining the stomach was studied during acidic and alkaline intragastric pH in six anaesthetised duodenal ulcer patients and six controls after atropinisation. No differences in the basal secretion of gastrin and somatostatin were observed among the two groups. Alkaline as well as acidic intragastric pH had no effect on the antral release of somatostatin in duodenal ulcer patients and controls. In contrast, alkaline intragastric pH was associated with a significantly higher antral gastrin release in duodenal ulcer patients than in controls. Acidic intragastric pH was associated with a significantly smaller inhibition of antral gastrin release in duodenal ulcer patients than in controls. These results suggest that atropinised anaesthetised duodenal patients release gastrin abnormally in the presence of acidic or alkaline intragastric pH and that any inverse relationship between antral gastrin and somatostatin release is uncoupled under these conditions.

Immunohistochemical detection of ganglia in the rat stomach serosa, containing neurons immunoreactive for gastrin-releasing peptide and vasoactive intestinal peptide

Ganglia, not previously described, were identified in the rat stomach serosa along the minor curvature. The ganglia consisted of varying number of cell bodies lying in clusters along or within nerve bundles. The ganglia were shown to contain GRP and VIP immunoreactive nerve fibers and cell bodies and also some NPY immunoreactive fibers, whereas they were devoid of somatostatin immunoreactivity. Nerve ligation experiments indicated that the ganglia are intrinsic to the stomach.

Identification and characterization of specific binding sites for somatostatin in cytosol of bovine cystic duct mucosa

Specific binding sites for somatostatin have been detected in cytosolic fraction of bovine cystic duct mucosa. At 37 degrees C, the interaction of 125I-Tyr11-somatostatin with cytosolic fraction was rapid, reversible, specific and saturable. At equilibrium, the binding of tracer was competitively inhibited by native peptide in the 1 nM to 2 microM range of concentrations. Scatchard analysis of binding data suggested the presence of two distinct classes of somatostatin binding sites: a class with a high affinity (Kd = 7.8 +/- 0.3 nM) and a low capacity (1.3 +/- 0.3 pmol somatostatin/mg protein) and a class with a low affinity (Kd = 129.1 +/- 2.0 nM) and a high capacity (43.5 +/- 6.7 pmol somatostatin/mg protein). The binding sites were shown to be highly specific for somatostatin since neuropeptides present in cystic duct such as Leu-enkephalin, neurotensin, substance P and vasoactive intestinal peptide did practically not show competition. These findings suggest that somatostatin could contribute to the regulation of the functions of the cystic duct mucosa in physiological and pathological conditions.

Conversion of somatostatin-28 to somatostatin-14 during maturation of epithelial cells in the porcine jejunum

Fractions of isolated epithelial cells were harvested from a segment of porcine jejunum by ten successive incubations with a chelating buffer. The cell fractions showed a progressive decrease in the activity of the brush-border enzymes, alkaline phosphatase and sucrase, with increasing incubation number but a progressive increase in the ability to incorporate labelled thymidine into DNA. Fractions enriched in cells from the crypt region (fractions 9 and 10) contained higher concentrations per mg protein of somatostatin-like immunoreactivity (1.8-fold), glucagon-like immunoreactivity (5.3-fold) and serotonin (3.0-fold) than fractions enriched in cells from the villus tip (fractions 1 and 2). Analysis of extracts of the fractions by gel filtration/radioimmunoassay showed that somatostatin-28 represented the predominant molecular form of somatostatin-like immunoreactivity in all cell fractions but the relative proportion of somatostatin-14 (and related metabolites) to somatostatin-28 was significantly higher (P less than 0.05) in fractions enriched in villus cells (fraction 1 and 2) than in fractions enriched in crypt cells (fractions 5-10). This result suggests that metabolism of somatostatin-28 to somatostatin-14 takes place during migration of the D cell from the crypt base to the villus tip. Heterogeneity in the somatostatin-14 region of the chromatograms indicates that the peptide may be further metabolized by the action of aminopeptidases.

Gastrointestinal somatostatin: distribution, secretion and physiological significance

Somatostatin-like immunoreactivity (SLI) has been found throughout the gastrointestinal tract in all species examined. In the stomach it is mainly present in endocrine-type D-cells whereas in the intestine there is also an extensive distribution in enteric neurones. In all regions of the gastrointestinal tract multiple forms of somatostatin exist. A precursor (prosomatostatin) has been partially sequenced, three forms with 20 (SS-20), 25 (SS-25) and 28 (SS-28) amino acids completely sequenced, and somatostatin-14 (SS-14) demonstrated by radioimmunoassay. Both SS-14 and SS-28 exert a wide range of actions on the gastrointestinal tract and there is strong supportive evidence for a role in the regulation of gastric acid and gastrin secretion, gastrointestinal motility and intestinal transport. Both in vivo and in vitro studies on the secretion of gastric SLI into the vasculature have shown that nutrients initiate the process but that subsequent events are regulated by a complex interplay between hormonal and neuronal pathways. GIP is one of the most potent hormonal secretagogues. In the stomach, acetylcholine, opioid peptides and substance P are probably involved in parasympathetic inhibitory pathways and gastrin releasing peptide in stimulatory pathways. The sympathetic nerves are also stimulatory. Regulation of secretion of intestinal SLI has not been so extensively studied. Although SLI is also found in the gastrointestinal lumen the significance is unclear. Despite these advances the exact route of delivery of somatostatin to its target organs is uncertain and paracrine, endocrine and neural pathways may all be involved.

The intestinal mucosa preferentially releases somatostatin-28 in pigs

We studied the molecular forms of somatostatin-like immunoreactivity (SLI), newly released from isolated perfused preparations of the porcine antrum, stomach, pancreas and upper small intestine: Perfusion effluents were concentrated by Sep-Pak C-18 adsorption, eluted with ethanol, dessicated, and subjected to gel filtration with subsequent radioimmunoassays for somatostatin-14 and N-terminal somatostatin-28 immunoreactivity. All the SLI newly released from the stomach and antrum eluted at the position of somatostatin-14, and such was also the case for more than 95% of the SLI newly released from the pancreas, while 68 -/+ 7% and 75 -/+ 8% of the SLI newly released from the isolated perfused jejunum and ileum, respectively, corresponded to somatostatin-28. By reverse phase HPLC the identity of these peptides with synhetic somatostatin-14 and -28 was established.