Circulating prosomatostatin-derived peptides. Differential responses to food ingestion

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."

Somatostatin in renal physiology and autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by progressive cyst formation, leading to growth in kidney volume and renal function decline. Although therapies have emerged, there is still an important unmet need for slowing the rate of disease progression in ADPKD. High intracellular levels of adenosine 3′,5′-cyclic monophosphate (cAMP) are involved in cell proliferation and fluid secretion, resulting in cyst formation. Somatostatin (SST), a hormone that is involved in many cell processes, has the ability to inhibit intracellular cAMP production. However, SST itself has limited therapeutic potential since it is rapidly eliminated in vivo. Therefore analogues have been synthesized, which have a longer half-life and may be promising agents in the treatment of ADPKD. This review provides an overview of the complex physiological effects of SST, in particular renal, and the potential therapeutic role of SST analogues in ADPKD.

Association of plasma somatostatin with disease severity and progression in patients with autosomal dominant polycystic kidney disease

Background

Somatostatin (SST) inhibits intracellular cyclic adenosine monophosphate (cAMP) production and thus may modify cyst formation in autosomal dominant polycystic kidney disease (ADPKD). We investigated whether endogenous plasma SST concentration is associated with disease severity and progression in patients with ADPKD, and whether plasma SST concentrations change during treatment with a vasopressin V2 receptor antagonist or SST analogue.

Methods

In this observational study, fasting concentrations of SST were measured in 127 ADPKD patients (diagnosed upon the revised Ravine criteria) by ELISA. cAMP was measured in 24 h urine by Radio Immuno Assay. Kidney function was measured (mGFR) as ¹²⁵I-iothalamate clearance, and total kidney volume was measured by MRI volumetry and adjusted for height (htTKV). Disease progression was expressed as annual change in mGFR and htTKV. Additionally, baseline versus follow-up SST concentrations were compared in ADPKD patients during vasopressin V2 receptor antagonist (tolvaptan) (n = 27) or SST analogue (lanreotide) treatment (n = 25).

Results

In 127 ADPKD patients, 41 ± 11 years, 44% female, eGFR 73 ± 32 ml/min/1.73m², mGFR 75 ± 32 ml/min/1.73m² and htTKV 826 (521–1297) ml/m, SST concentration was 48.5 (34.3–77.8) pg/ml. At baseline, SST was associated with urinary cAMP, mGFR and htTKV (p = 0.02, p = 0.004 and p = 0.02, respectively), but these associations lost significance after adjustment for age and sex or protein intake (p = 0.09, p = 0.06 and p = 0.15 respectively). Baseline SST was not associated with annual change in mGFR, or htTKV during follow-up (st. β = − 0.02, p = 0.87 and st. β = − 0.07, p = 0.54 respectively). During treatment with tolvaptan SST levels remained stable 38.2 (23.8–70.7) pg/mL vs. 39.8 (31.2–58.5) pg/mL, p = 0.85), whereas SST levels decreased significantly during treatment with lanreotide (42.5 (33.2–55.0) pg/ml vs. 29.3 (24.8–37.6), p = 0.008).

Conclusions

Fasting plasma SST concentration is not associated with disease severity or progression in patients with ADPKD. Treatment with lanreotide caused a decrease in SST concentration. These data suggest that plasma SST cannot be used as a biomarker to assess prognosis in ADPKD, but leave the possibility open that change in SST concentration during lanreotide treatment may reflect therapy efficacy.

Electronic supplementary material

The online version of this article (10.1186/s12882-018-1176-y) contains supplementary material, which is available to authorized users.

The Cells of the Islets of Langerhans

Islets of Langerhans are islands of endocrine cells scattered throughout the pancreas. A number of new studies have pointed to the potential for conversion of non-β islet cells in to insulin-producing β-cells to replenish β-cell mass as a means to treat diabetes. Understanding normal islet cell mass and function is important to help advance such treatment modalities: what should be the target islet/β-cell mass, does islet architecture matter to energy homeostasis, and what may happen if we lose a particular population of islet cells in favour of β-cells? These are all questions to which we will need answers for islet replacement therapy by transdifferentiation of non-β islet cells to be a reality in humans. We know a fair amount about the biology of β-cells but not quite as much about the other islet cell types. Until recently, we have not had a good grasp of islet mass and distribution in the human pancreas. In this review, we will look at current data on islet cells, focussing more on non-β cells, and on human pancreatic islet mass and distribution.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

Therapeutic uses of somatostatin and its analogues: Current view and potential applications

Somatostatin is an endogeneous cyclic tetradecapeptide hormone that exerts multiple biological activities via five ubiquitously distributed receptor subtypes. Classified as a broad inhibitory neuropeptide, somatostatin has anti-secretory, anti-proliferative and anti-angiogenic effects. The clinical use of native somatostatin is limited by a very short half-life (1 to 3min) and the broad spectrum of biological responses. Thus stable, receptor-selective agonists have been developed. The majority of these somatostatin therapeutic agonists bind strongly to two of the five receptor subtypes, although recently an agonist of wider affinity has been introduced. Somatostatin agonists are established in the treatment of acromegaly with recently approved indications in the therapy of neuroendocrine tumours. Potential therapeutic uses for somatostatin analogues include diabetic complications like retinopathy, nephropathy and obesity, due to inhibition of IGF-1, VEGF together with insulin secretion and effects upon the renin-angiotensin-aldosterone system. Wider uses in anti-neoplastic therapy may also be considered and recent studies have further revealed anti-inflammatory and anti-nociceptive effects. This review provides a comprehensive, current view of the biological functions of somatostatin and potential therapeutic uses, informed by the wide range of pharmacological advances reported since the last published review in 2004 by P. Dasgupta. The pharmacology of somatostatin receptors is explained, the current uses of somatostatin agonists are discussed, and the potential future of therapeutic applications is explored.

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.

The somatostatin receptor in human pancreatic β-cells

The peptide hormone somatostatin (SST) is produced in the brain, the gut, and in δ-cells in pancreatic islets of Langerhans. SST secretion from δ-cells is stimulated by glucose, amino acids, and glucagon-like peptide-1. Exogenous SST strongly inhibits the secretion of the blood glucose-regulating hormones insulin and glucagon from pancreatic β-cells and α-cells, respectively. Endogenous SST secreted from δ-cells is a paracrine regulator of insulin and glucagon secretion, although the exact physiological significance of this regulation is unclear. Secreted SST binds to specific receptors (SSTRs), which are coupled to Gi/o proteins. In both β- and α-cells, activation of SSTRs suppresses hormone secretion by reducing cAMP levels, inhibiting electrical activity, decreasing Ca²⁺ influx through voltage-gated Ca²⁺ channels and directly reducing exocytosis in a Ca²⁺ and cAMP-independent manner. In rodents, β-cells express predominantly SSTR5, whereas α-cells express SSTR2. In human islets, SSTR2 is the dominant receptor in both β- and α-cells, but other isoforms also contribute to the SST effects. Evidence from rodent models suggests that SST secretion from δ-cells is dysregulated in diabetes mellitus, which may contribute to the metabolic disturbances in this disease. SST analogues are currently used for the treatment of hyperinsulinism and other endocrine disorders, including acromegaly and Cushing's syndrome.

Somatostatin molecular variants in the vitreous fluid: a comparative study between diabetic patients with proliferative diabetic retinopathy and nondiabetic control subjects

OBJECTIVE

There is growing evidence to indicate that somatostatin could be added to the list of natural antiangiogenic factors that exist in the vitreous fluid. In addition, a deficit of intravitreous somatostatin-like immunoreactivity (SLI) has been found in diabetic patients with proliferative diabetic retinopathy (PDR). In the present study, we have determined the main molecular variants of somatostatin (somatostatin-14 and somatostatin-28) in the vitreous fluid and plasma of nondiabetic control subjects and diabetic patients with PDR. In addition, the contribution of cortistatin, a neuropeptide with strong structural similarities to somatostatin, to SLI and its levels in vitreous and plasma in both nondiabetic and diabetic patients has also been measured. RESERCH DESIGN AND METHODS: Plasma and vitreous fluid from 22 diabetic patients with PDR and 22 nondiabetic control subjects were analyzed. Somatostatin-14, somatostatin-28 and cortistatin were measured by radioimmunoassay but separation by high-performance liquid chromatography was required to measure somatostatin-14.

RESULTS

The predominant molecular form of somatostatin within the vitreous fluid was somatostatin-28 (fivefold higher than somatostatin-14 in control subjects and threefold higher in patients with PDR). Cortistatin significantly contributed to SLI and its intravitreous levels were higher than those detected in plasma (nondiabetic control subjects: 147 [102-837] vs. 78 [24-32] pg/ml; patients with PDR: 187 [87-998] vs. 62 [24-472] pg/ml; P = 0.01 for both). Intravitreous somatostatin-14 was similar in both subjects with PDR and the control group (P = 0.87). By contrast, somatostatin-28 concentration was lower in patients with PDR than in nondiabetic control subjects (350 +/- 32 vs. 595 +/- 66 pg/ml; P = 0.004).

CONCLUSIONS

Somatostatin-28 is the main molecular variant in the vitreous fluid. The intravitreous SLI deficit detected in patients with PDR is mainly due to somatostatin-28. Cortistatin is abundant in the vitreous fluid and significantly contributes to SLI. These findings could open up new strategies for PDR treatment.

Elevated circulating somatostatin levels in acromegaly

GH increases hypothalamic somatostatin (SS) synthesis and secretion but it is unknown if chronic GH excess, as found in acromegaly, may influence circulating SS levels, that are mainly of enteropancreatic source and affect several gastrointestinal functions, including motility. Circulating SS occurs in several post-translational forms including somatostatin-14 (SS-14), somatostatin-28 (SS-28) and other small peptides. The aim of the present study was to characterize the fasting and postprandial pattern of plasma circulating somatostatin in normal subjects and patients with acromegaly. Fasting total SS and SS-28 levels were measured in 32 subjects, 16 acromegalic patients with a new diagnosis (A) (8 F, 8 M, median age 48) and 16 matched healthy volunteers (C) (8 F, 8 M, median age 45). SS was also determined after a standard solid-liquid meal (550 kCal) in 24 of the subjects (12 C and 12 A). Fasting SS and SS-28 were significantly higher in acromegalic patients as compared to healthy subjects. In the former, a positive correlation was found between IGF-I and SS levels (r = 0.525 p < 0.05). Furthermore, the ratio between SS (as pmol equivalent SS-14/I) and SS-28 was higher in the acromegalic patients than in the controls (3.4 +/- 2.1 vs 2.0 +/- 1.6, p < 0.05). The postprandial SS peak, as well as the incremental area above baseline values, was similar in the patients and controls. In conclusion, fasting but not postprandial hypersomatostatinemia, mainly due to an increase in SS-14, characterizes acromegaly. Excess GH/IGF-I could be a causal factor in somatostatin hypersecretion. Conceivably this abnormality might play a role in some alterations of gastrointestinal function of acromegalic patients such as prolonged bowel transit.

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.

The innervation of the human antro-pyloric region: Organization and composition

Although the composition of the gastric innervation has been determined in animal models, relatively little known about the innervation of the human antro-pyloric region. We used immunocytochemical techniques to establish the localization and co-expression of neuropeptides and nitric oxide in the human antrum and upper duodenum. Our results demonstrate the existence of a clearly defined submucosal plexus in the antral region that is absent in rats and guinea pigs. The abundant innervation of the lamina propria contains 3 major nerve populations: VIP- and NOS-, SP- and CGRP-, and GRP-immunoreactive. For the first time, NOS-containing nerve fibers were observed throughout the length of the antral glands. Within the antrum somatostatin was confined to endocrine cells, however, at the pyloric sphincter both enteric plexi contained immunoreactive neurons and nerve fibres. Within the pyloric sphincter CGRP- and SP-immunoreactive fibres were significantly increased, correlating with the presence of large ganglia in the submucosal plexus. In conclusion, the organization and composition of the innervation of human antro-pylorus differed substantially from that reported in other mammals. The presence of an abundant mucosal innervation paralled by a well-defined submucosal plexus indicates that the functional regulation of the gastric–pyloric region will be distinct from that of smaller animal models.Key words: gastric innervation, pyloric sphincter, neuropeptides, nitric oxide, somatostatin.

Plasma somatostatin-like immunoreactivity and somatostatin-28 levels in obese men

Somatostatin (SS) impairs nutrient absorption. It has been suggested that hyposomatostatinaemia may be involved in the pathogenesis of obesity. However, data on postprandial SS-like immunoreactivity (SLI) levels in obese subjects are controversial and the levels of SS-28, the main molecular form of circulating SLI in healthy subjects, have not been determined. To characterise the fasting and postprandial plasma pattern of SLI and SS-28 in obese men, we studied eight obese men (age 24-32 yr, BMI 33-42 kg/m2), with normal glucose tolerance test and normal gastric emptying of solids, and eight healthy men (age 24-39 yr, BMI 21-24 kg/m2). Blood samples were taken at regular intervals in fasting conditions and for 2 h after a standard solid-liquid meal (2.3 MJ). Plasma SLI and SS-28 were measured by RIA. Our results showed that fasting and postprandial plasma SLI and SS-28 levels were not significantly different in healthy and obese subjects. In conclusion SS-28 is the predominant form of circulating SLI in obese subjects. The normal pattern of fasting and postprandial plasma SLI and SS-28 levels in such subjects suggests that somatostatin does not have a pathogenetic role in obesity.

Endogenous somatostatin-28 modulates postprandial insulin secretion. Immunoneutralization studies in baboons

Somatostatin-28 (S-28), secreted into the circulation from enterocytes after food, and S-14, released mainly from gastric and pancreatic D cells and enteric neurons, inhibit peripheral cellular functions. We hypothesized that S-28 is a humoral regulator of pancreatic B cell function during nutrient absorption. Consistent with this postulate, we observed in baboons a two to threefold increase in portal and peripheral levels of S-28 after meals, with minimal changes in S-14. We attempted to demonstrate a hormonal effect of these peptides by measuring their concentrations before and after infusing a somatostatin-specific monoclonal antibody (mAb) into baboons and comparing glucose, insulin, and glucagon-like peptide-1 levels before and for 4 h after intragastric nutrients during a control study and on 2 d after mAb administration (days 1 and 2). Basal growth hormone (GH) and glucagon levels and parameters of insulin and glucose kinetics were also measured. During immunoneutralization, we found that (a) postprandial insulin levels were elevated on days 1 and 2; (b) GH levels rose immediately and were sustained for 28 h, while glucagon fell; (c) basal insulin levels were unchanged on day 1 but were increased two to threefold on day 2, coincident with decreased insulin sensitivity; and (d) plasma glucose concentrations were similar to control values. We attribute the eventual rise in fasting levels of insulin to its enhanced secretion in compensation for the heightened insulin resistance from increased GH action. Based on the elevated postmeal insulin levels after mAb administration, we conclude that S-28 participates in the enteroinsular axis as a decretin to regulate postprandial insulin secretion.

Expression of peptide receptors in human endocrine tumours of the pancreas

BACKGROUND

Gut peptides are known to influence hormone release and growth of endocrine tumours of the pancreas. Although information on somatostatin receptors has been provided recently, little is known on the receptor status of other gastrointestinal hormones in such tumours.

AIMS

To analyse the spectrum of gut hormone receptors on endocrine tumours of pancreas.

SUBJECTS

Four types of endocrine tumours from eight patientS.

METHODS

The receptors for bombesin, secretin, vasoactive intestinal peptide, cholecystokinin, and somatostatin have been visualised and quantified with storage phosphor autoradiography.

RESULTS

Bombesin receptors were present in all five gastrinomas and two primary VIPomas. Secretin receptors were expressed in four primary gastrinomas and one primary VIPoma from pancreas. Vasoactive intestinal peptide receptors were identified in four primary gastrinomas and all VIPomas. Furthermore, all VIPomas expressed cholecystokinin-B (gastrin) receptors, whereas, gastrinomas did not contain cholecystokinin-B receptors. The receptors for somatostatin were detected in all gastrinomas and VIPomas. Both somatostatinoma and glucagonoma were negative for all five types of peptide receptors studied.

CONCLUSIONS

Besides somatostatin receptors, most of gastrinomas and VIPomas also express receptors for bombesin, secretin, and vasoactive intestinal peptide.

Expression of receptors for gut peptides in human pancreatic adenocarcinoma and tumour-free pancreas

Gut hormones that modulate the growth of normal pancreas may also modulate the growth of cancers originating from pancreas. This study visualized and compared the receptors for cholecystokinin (CCK), bombesin (BBS), secretin and vasoactive intestinal peptide (VIP) in tumour-free tissue sections of human pancreas (n = 10) and pancreatic ductal adenocarcinomas (n = 12) with storage phosphor autoradiography using radioligands. CCK-B receptors, present in control pancreata, were not detected in any of the pancreatic cancers. BBS receptors were visualized in control pancreata, but they were absent in 10 of 12 pancreatic cancers. In 5 of 12 pancreatic cancers, receptors for secretin were visualized, while binding for secretin was present in all tumour-free pancreata. Conversely, no specific binding of VIP was detected in control pancreata but was identified in 3 of 12 pancreatic cancer specimens. It is concluded that the expression of gut peptide receptors in pancreatic cancer differs from that in tumour-free pancreas. Receptors for these peptides are present in only a minority of pancreatic cancer specimens.

Transgenic mice overproducing islet amyloid polypeptide have increased insulin storage and secretion in vitro

To determine whether chronic overproduction of islet amyloid polypeptide alters beta-cell function, we studied a line of transgenic mice which overexpress islet amyloid polypeptide in their beta-cells. At 3 months of age, these transgenic mice had greater pancreatic content of both islet amyloid polypeptide and insulin. Further, basal and glucose-stimulated secretion of both islet amyloid polypeptide and insulin were also elevated in the perfused pancreas of the transgenic animals. These findings demonstrate that chronic overproduction and secretion of islet amyloid polypeptide are associated with increased insulin storage and enhanced secretion of insulin in vitro. This increase in insulin storage and secretion may be due to a direct effect of islet amyloid polypeptide on the beta-cell or a beta-cell adaptation to islet amyloid polypeptide-induced insulin resistance.

Cholecystokinin is a physiological regulator of gastric acid secretion in man

CCK8 is a poor stimulant of gastric acid secretion in vivo, but is equipotent to gastrin-17 (G17) in in vitro systems. To further evaluate the role of cholecystokinin (CCK) in regulating acid output in humans, dose-response curves were constructed to CCK8 or G17 (6.4-800 pmol kg-1 per h) with and without a specific CCK-A receptor antagonist (loxiglumide). During loxiglumide infusion, G17-stimulated acid output was unchanged, whereas CCK8-stimulated secretion increased significantly. Gastric somatostatin-14 release increased fivefold with CCK8 alone, but was blocked with loxiglumide administration. These data suggest that CCK8 directly stimulates acid secretion by binding to a CCK-B/gastrin receptor on parietal cells, but at the same time inhibits acid responses by stimulating gastric somatostatin release to a CCK-A receptor-mediated pathway. To test which action of CCK is relevant under physiological circumstances, the effect of loxiglumide on fasting and post-prandial acidity was measured through continuous pH-metry. After eating, gastrin levels increased fourfold compared to controls with concomitant increases in acid secretion. These results suggest that post cibum, CCK is an inhibitor of acid secretion by regulating gastrin through local somatostatin; they support the hypothesis that CCK acts as an enterogastrone.

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.

Plasma molecular forms of gastrin, neurotensin and somatostatin in pregnancy and gestational diabetes after an oral glucose load or a mixed meal

At present the physiological role of gastrin, neurotensin and somatostatin in pregnancy and gestational diabetes is scarcely known. We have measured their different molecular forms in plasma of six female controls, six normal pregnant (NP) women and six gestational diabetic (GD) women under basal conditions and 30 min after an oral glucose load (100 g) and a liquid mixed meal in order to study if their alteration could contribute to the impaired glucose tolerance in GD. Total basal concentrations of neurotensin and somatostatin were higher in GD than in controls and NP, and no change was found after the glucose load or mixed meal in GD. Neurotensin-1-13 was the main molecular form of all neurotensins at basal time in the three groups studied, being higher in GD in comparison with controls and NP. Somatostatin-1-14 was the predominant molecular form in controls and GD under basal conditions and did not show any change any change after stimuli. In NP, somatostatin-1-14 showed a significant increase following both kinds of stimuli. Total gastrin concentrations in NP and GD showed a significant increase after the glucose load, which was not observed in controls. Gastrin-17 was the main molecular form at basal time and 30 min post-stimuli in GD but not in NP and controls. We suggest that the basal elevation of neurotensin and somatostatin levels could contribute to the impaired glucose tolerance observed in gestational diabetes, as well as to the lack of post-stimuli responses for neurotensin and somatostatin in GD.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacodynamic effects of Sandostatin in the gastrointestinal tract

Somatostatin (SST) is widely distributed throughout the human gastrointestinal system. There, it is found in neurons and fibers of both the submucosal and myenteric plexus and the pancreas, and also in the D cells of the stomach, gut, and pancreatic islets. Whereas in the intestinal nervous system, duodenum, and pancreas, somatostatin-14 (SST-14) appears to be the predominant molecular form, the endocrine-type D cells of the intestine primarily contain somatostatin-28 (SST-28). SST peptides may act very differently at different sites, as hormones, paracrine substances, or neurotransmitters. Because of this complexity of action, very little is known about the physiological effects of SST in the gastrointestinal tract. In contrast, the pharmacological actions of natural synthetic SST have been thoroughly studied and have given rise to many therapeutic applications. Octreotide, an analogue with a longer half-life and higher potency, has greatly facilitated the clinical application of SST. This review deals with the pharmacological effects of octreotide on different gastrointestinal functions. The SST analogue exerts a long-lasting inhibitory action on gastric acid, pancreatic enzyme, bicarbonate secretion, and on bile flow. It also inhibits stimulated intestinal secretion, ie, the release of neuropeptides from the gut and pancreas. It can also prolong orocecum transit time and prevent gallbladder contraction. It inhibits absorption of nutrients and exerts inhibitory effects on splanchnic hemodynamics. It is because of these actions that SST has attracted so much attention in the treatment of different gastrointestinal disorders.

Somatostatin 28 and coupling of human interdigestive intestinal motility and pancreatic secretion

To determine the effects of small increases in somatostatin 28 plasma concentrations on human interdigestive gastrointestinal motility and pancreatic secretion, six fasting volunteers were intubated with gastroduodenal multilumen tubes and motility and pancreatic enzyme secretion were measured. Subjects received intravenous NaCl and somatostatin 28 at 11 and 44 pmol.kg-1.h-1 for 120 minutes or at least one interdigestive cycle. The two doses increased plasma somatostatin 28 levels within the physiological or into the supraphysiological range, respectively. Somatostatin 28 at 11 and 44 pmol.kg-1.h-1 decreased the length of the interdigestive motility cycle by 50% and 67% compared with controls, respectively (both P less than 0.002). Propagation velocity of the migrating motor complex (P less than 0.01) and plasma motilin were decreased (P less than 0.01). The smaller and larger dose decreased pancreatic enzyme outputs by 50% and 65%, respectively (P less than 0.005), but with the smaller dose, phase III-associated enzyme outputs were greater than phase I outputs. These findings suggest that small changes in somatostatin 28 plasma concentrations modulate human interdigestive motility and pancreatic enzyme output while coupling of motor and secretory events is preserved.

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.

Radioimmunoassay for salmon pancreatic somatostatin-25

A specific and sensitive radioimmunoassay (RIA) for the measurement of plasma levels of somatostatin-25 (SS-25) in salmon was developed using antisera raised against coho salmon (Oncorhynchus kisutch) SS-25. Somatostatin-25 was iodinated by the chloramine-T method and repurified on Sephadex G-25. The RIA was performed using a double antibody (goat anti-rabbit gammaglobulin as second antibody) method under disequilibrium conditions. Plasma from several salmonids (coho, chinook, rainbow trout, brook trout, arctic char, lake trout, and whitefish) as well as plasma from some nonsalmonids (sucker, bluegill) cross-reacted with the antisera; serial dilutions of plasma from rainbow trout, brook trout, chinook salmon, and coho salmon were parallel to the SS-25 standard curve. Plasma from catfish showed negligible cross-reactivity. None of the mammalian somatostatins (somatostatin-14, somatostatin-28). U II, or other pancreatic hormones (insulin, glucagon) tested showed significant cross-reactivity with the antibody in the assay system. The lowest detectable level of SS-25 was 5 pg/tube; especially reproducible results were obtained in the range of 0.15-1.20 ng/ml, which appears to be the normal range of SS-25 circulating in the plasma of salmonids. Intra- and interassay coefficients of variation were 5.7 and 12.6%, respectively. Injection of glucose into chinook salmon resulted in an elevation of plasma SS-25 titers within 30 min and was coincident with hyperglycemia.

Plasma somatostatin response to an oral mixed test meal in cirrhotic patients

Ten patients with non-alcoholic cirrhosis and ten control subjects were studied in basal conditions and after ingestion of a standard mixed test meal. Plasma somatostatin, blood glucose, plasma insulin, C-peptide and glucagon were determined before and 15, 30, 45, 60, 90, 120 and 180 min after the start of the meal. Basal somatostatin levels in patients (31.9 +/- 1.8 ng/l) were significantly higher (p less than 0.01) than in controls (12.5 +/- 0.9 ng/l). The time-course of the somatostatin secretory response after the meal was similar in the two groups, but the increase, evaluated as incremental area above baseline, was significantly smaller (p less than 0.01) in cirrhotics (804 +/- 134 ng/l per min) than in controls (1482 +/- 149 ng/l per min). Data indicate that elevated basal plasma somatostatin concentrations in cirrhosis may be consequent to elevated gastrointestinal and/or pancreatic secretion, whereas the blunted somatostatin response to the mixed test meal may derive from the hyperinsulinemia which occurs in the postprandial period.

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.

Effect of ingested carbohydrate, fat, and protein on the release of somatostatin-28 in humans

The level of somatostatin-28, a bioactive peptide derived from pro-somatostatin in gastrointestinal epithelial cells, increases in human plasma after food intake. To determine if an equivalent response occurs with individual components of a mixed meal, somatostatin-28 and prosomatostatin, somatostatin-14, and somatostatin-13, in combination, were measured in healthy men before and after intake of (a) a mixed meal (715 kcal), (b) carbohydrate (100 g equivalent glucose), (c) protein (22 and 45 g), and (d) fat (25 and 50 g). After the mixed meal, somatostatin-28 levels doubled within 120 min and gradually declined by 4 h. With carbohydrate, somatostatin-28 levels were unaltered. After 22 and 45 g of protein, somatostatin-28 increased equivalently within 60 min, representing 30% of the amount with the mixed meal. With 25 g fat, a somatostatin-28 increase similar to that with the meal was seen; this response was doubled with 50 g fat. No changes in prosomatostatin, somatostatin-14, or somatostatin-13 were observed with the mixed meal or with the separate macronutrients. The authors conclude that fat is the major stimulus for somatostatin-28 secretion in humans and hypothesize that somatostatin-28 is an inhibitor of the endocrine and exocrine pancreas during nutrient absorption.