Inhibition of the in vitro secretion of insulin by an extract of pancreatic alpha-1 cells

Somatostatin analogues for the treatment of hyperinsulinaemic hypoglycaemia

Hyperinsulinaemic hypoglycaemia (HH) is a biochemical finding of low blood glucose levels due to the dysregulation of insulin secretion from pancreatic β-cells. Under normal physiological conditions, glucose metabolism is coupled to β-cell insulin secretion so that blood glucose levels are maintained within the physiological range of 3.5-5.5 mmol/L. However, in HH this coupling of glucose metabolism to insulin secretion is perturbed so that insulin secretion becomes unregulated. HH typically occurs in the neonatal, infancy and childhood periods and can be due to many different causes. Adults can also present with HH but the causes in adults tend to be different. Somatostatin (SST) is a peptide hormone that is released by the delta cells (δ-cells) in the pancreas. It binds to G protein-coupled SST receptors to regulate a variety of location-specific and selective functions such as hormone inhibition, neurotransmission and cell proliferation. SST plays a potent role in the regulation of both insulin and glucagon secretion in response to changes in glucose levels by negative feedback mechanism. The half-life of SST is only 1-3 min due to quick degradation by peptidases in plasma and tissues. Thus, a direct continuous intravenous or subcutaneous infusion is required to achieve the therapeutic effect. These limitations prompted the discovery of SST analogues such as octreotide and lanreotide, which have longer half-lives and therefore can be administered as injections. SST analogues are used to treat different forms of HH in children and adults and therapeutic effect is achieved by suppressing insulin secretion from pancreatic β-cells by complex mechanisms. These treatments are associated with several side effects, especially in the newborn period, with necrotizing enterocolitis being the most serious side effect and hence SS analogues should be used with extreme caution in this age group.

Molecular and functional characterization of somatostatin-type signalling in a deuterostome invertebrate

Somatostatin (SS) and allatostatin-C (ASTC) are structurally and evolutionarily related neuropeptides that act as inhibitory regulators of physiological processes in mammals and insects, respectively. Here, we report the first molecular and functional characterization of SS/ASTC-type signalling in a deuterostome invertebrate—the starfish Asterias rubens (phylum Echinodermata). Two SS/ASTC-type precursors were identified in A. rubens (ArSSP1 and ArSSP2) and the structures of neuropeptides derived from these proteins (ArSS1 and ArSS2) were analysed using mass spectrometry. Pharmacological characterization of three cloned A. rubens SS/ASTC-type receptors (ArSSR1–3) revealed that ArSS2, but not ArSS1, acts as a ligand for all three receptors. Analysis of ArSS2 expression in A. rubens using mRNA in situ hybridization and immunohistochemistry revealed stained cells/fibres in the central nervous system, the digestive system (e.g. cardiac stomach) and the body wall and its appendages (e.g. tube feet). Furthermore, in vitro pharmacological tests revealed that ArSS2 causes dose-dependent relaxation of tube foot and cardiac stomach preparations, while injection of ArSS2 in vivo causes partial eversion of the cardiac stomach. Our findings provide new insights into the molecular evolution of SS/ASTC-type signalling in the animal kingdom and reveal an ancient role of SS-type neuropeptides as inhibitory regulators of muscle contractility.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

The somatostatin-secreting pancreatic δ-cell in health and disease

The somatostatin-secreting δ-cells comprise ~5% of the cells of the pancreatic islets. The δ-cells have complex morphology and might interact with many more islet cells than suggested by their low numbers. δ-Cells contain ATP-sensitive potassium channels, which open at low levels of glucose but close when glucose is elevated. This closure initiates membrane depolarization and electrical activity and increased somatostatin secretion. Factors released by neighbouring α-cells or β-cells amplify the glucose-induced effects on somatostatin secretion from δ-cells, which act locally within the islets as paracrine or autocrine inhibitors of insulin, glucagon and somatostatin secretion. The effects of somatostatin are mediated by activation of somatostatin receptors coupled to the inhibitory G protein, which culminates in suppression of the electrical activity and exocytosis in α-cells and β-cells. Somatostatin secretion is perturbed in animal models of diabetes mellitus, which might explain the loss of appropriate hypoglycaemia-induced glucagon secretion, a defect that could be mitigated by somatostatin receptor 2 antagonists. Somatostatin antagonists or agents that suppress somatostatin secretion have been proposed as an adjunct to insulin therapy. In this Review, we summarize the cell physiology of somatostatin secretion, what might go wrong in diabetes mellitus and the therapeutic potential of agents targeting somatostatin secretion or action.

The somatostatin-secreting pancreatic δ-cell in health and disease

The somatostatin-secreting δ-cells comprise ~5% of the cells of the pancreatic islets. The δ-cells have complex morphology and might interact with many more islet cells than suggested by their low numbers. δ-Cells contain ATP-sensitive potassium channels, which open at low levels of glucose but close when glucose is elevated. This closure initiates membrane depolarization and electrical activity and increased somatostatin secretion. Factors released by neighbouring α-cells or β-cells amplify the glucose-induced effects on somatostatin secretion from δ-cells, which act locally within the islets as paracrine or autocrine inhibitors of insulin, glucagon and somatostatin secretion. The effects of somatostatin are mediated by activation of somatostatin receptors coupled to the inhibitory G protein, which culminates in suppression of the electrical activity and exocytosis in α-cells and β-cells. Somatostatin secretion is perturbed in animal models of diabetes mellitus, which might explain the loss of appropriate hypoglycaemia-induced glucagon secretion, a defect that could be mitigated by somatostatin receptor 2 antagonists. Somatostatin antagonists or agents that suppress somatostatin secretion have been proposed as an adjunct to insulin therapy. In this Review, we summarize the cell physiology of somatostatin secretion, what might go wrong in diabetes mellitus and the therapeutic potential of agents targeting somatostatin secretion or action.

Glucose control of glucagon secretion-'There's a brand-new gimmick every year'

Glucagon from the pancreatic α-cells is a major blood glucose-regulating hormone whose most important role is to prevent hypoglycaemia that can be life-threatening due to the brain's strong dependence on glucose as energy source. Lack of blood glucose-lowering insulin after malfunction or autoimmune destruction of the pancreatic β-cells is the recognized cause of diabetes, but recent evidence indicates that diabetic hyperglycaemia would not develop unless lack of insulin was accompanied by hypersecretion of glucagon. Glucagon release has therefore become an increasingly important target in diabetes management. Despite decades of research, an understanding of how glucagon secretion is regulated remains elusive, and fundamentally different mechanisms continue to be proposed. The autonomous nervous system is an important determinant of glucagon release, but it is clear that secretion is also directly regulated within the pancreatic islets. The present review focuses on pancreatic islet mechanisms involved in glucose regulation of glucagon release. It will be argued that α-cell-intrinsic processes are most important for regulation of glucagon release during recovery from hypoglycaemia and that paracrine inhibition by somatostatin from the δ-cells shapes pulsatile glucagon release in hyperglycaemia. The electrically coupled β-cells ultimately determine islet hormone pulsatility by releasing synchronizing factors that affect the α- and δ-cells.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Neuropeptide Y and somatostatin inhibit insulin secretion through different mechanisms

Pancreatic β-cells regulate glucose homeostasis by secreting insulin in response to glucose elevation and G protein-coupled receptor (GPCR) activation. Neuropeptide Y (NPY) and somatostatin (SST) attenuate insulin secretion through G(i) activation of Y(1) and SSTR(1&5) receptors, respectively. The downstream pathways altered by NPY and SST are poorly understood. Thus, we investigated these underlying mechanisms. NPY and SST increase cellular redox potential, suggesting that their inhibitory effect may not be mediated through metabolic inhibition. NPY does not affect intracellular calcium ([Ca(2+)](i)) activity upon glucose stimulation, whereas SST alters this response. G(βγ)-subunit inhibition by gallein attenuates insulin secretion but does not alter metabolism or [Ca(2+)](i). mSIRK-induced G(βγ) activation does not modulate glucose metabolism but increases [Ca(2+)](i) activity and potentiates insulin release. Cotreatment with gallein and NPY or SST reduces insulin secretion to levels similar to that of gallein alone. mSIRK and NPY cotreatment potentiates insulin secretion similarly to mSIRK alone, whereas mSIRK and SST treatment decreases insulin release. The data support a model where SST attenuates secretion through G(βγ) inhibition of Ca(2+) activity, while NPY activates a Ca(2+)-independent pathway mediated by G(α). GPCR ligands signal through multiple pathways to inhibit insulin secretion, and determining these mechanisms could lead to novel diabetic therapies.

Purinergic signalling in the pancreas in health and disease

Pancreatic cells contain specialised stores for ATP. Purinergic receptors (P2 and P1) and ecto-nucleotidases are expressed in both endocrine and exocrine calls, as well as in stromal cells. The pancreas, especially the endocrine cells, were an early target for the actions of ATP. After the historical perspective of purinergic signalling in the pancreas, the focus of this review will be the physiological functions of purinergic signalling in the regulation of both endocrine and exocrine pancreas. Next, we will consider possible interaction between purinergic signalling and other regulatory systems and their relation to nutrient homeostasis and cell survival. The pancreas is an organ exhibiting several serious diseases - cystic fibrosis, pancreatitis, pancreatic cancer and diabetes - and some are associated with changes in life-style and are increasing in incidence. There is upcoming evidence for the role of purinergic signalling in the pathophysiology of the pancreas, and the new challenge is to understand how it is integrated with other pathological processes.

Cross-talk and modulation of signaling between somatostatin and growth factor receptors

The process of homo- and/or heterodimerization of G-protein coupled receptors (GPCRs) and receptor tyrosine kinase (RTK) families are crucial for implicating the fundamental properties of receptor proteins including receptor expression, trafficking, and desensitization as well as signal transduction. The members of GPCR and RTK family constitute largest cell surface receptor proteins and regulate physiological functions of cells in response to external and internal stimuli. Notably, GPCRs and RTKs play major role in regulation of several key cellular functions which are associated with several pathological conditions including cancer biology, neurodegenerative and cardiovascular diseases. The focus of this review is to highlight the recent findings on the possible cross-talk between somatostatin receptors (members of GPCR family) and growth factor receptors like epidermal growth factor receptors (members of RTK family). Furthermore, functional consequences of such an interaction in modulation of signaling pathways linked to pathological conditions specifically in cancer are discussed.

Biology of somatostatin in breast cancer

The biological effects of the neuropeptide somatostatin (SST) are mediated via a family of five somatostatin receptors (SSTRs) belonging to a family of G-protein-coupled receptors (GPCRs). SSTR regulate the secretion of hormones, growth factors, neurotransmission and cell growth in receptor-specific manner. In addition, SST plays an inhibitory role in several mammary cancer models. These effects are mediated both indirectly through inhibition of hormones and growth factors which promote tumor growth as well as directly via SSTRs present on tumor cells to inhibit mitogenic signaling of growth factor receptor kinases leading to growth arrest and induction of apoptosis. Here, we present an overview on the role of SST and its analogs in breast cancer.

Pulses of somatostatin release are slightly delayed compared with insulin and antisynchronous to glucagon

It was early proposed that somatostatin-producing delta-cells in pancreatic islets have local inhibitory effects on the release of insulin and glucagon. Recent observations that pulses of insulin and glucagon are antisynchronous make it important to examine the temporal characteristics of glucose-induced somatostatin release. Analysis of 30 s fractions from the perfused rat pancreas indicated that increase of glucose from 3 to 20 mmol/l results in initial suppression of somatostatin release followed by regular 4-5 min pulses. During continued exposure to 20 mmol/l glucose, the pulses of somatostatin overlapped those of insulin with a delay of 30 s. Somatostatin and glucagon pulses were coupled in antisynchronous fashion (phase shift 2.4+/-0.2 min), supporting the idea that the delta-cells have a local inhibitory effect on glucagon release. It was possible to remove the pulses of somatostatin and glucagon with maintenance of the insulin rhythmicity by addition of 1 micromol/l of the P2Y(1) receptor antagonist MRS 2179.

Peptides and hormesis

The article provides a broad assessment of the occurrence of hormetic-like biphasic dose-response relationships by over 30 peptides representing many major peptide classes. These peptide-induced biphasic dose responses were observed to occur in a extensive range of tissues, affecting an diverse range of biological endpoints. Despite diversity of peptides, models and endpoints, the quantitative features of the biphasic dose responses are remarkably similar with respect to the amplitude and width of the stimulatory response. These findings strongly suggest that hormetic-like biphasic dose responses represent a broadly generalizable biological phenomenon.

Homologous and heterologous asynchronicity between identified alpha-, beta- and delta-cells within intact islets of Langerhans in the mouse

1. Using laser scanning confocal microscopy to image [Ca2+]i within intact murine islets of Langerhans, we analysed the [Ca2+]i signals generated by glucose in immunocytochemically identified alpha-, beta- and delta-cells. 2. Glucagon-containing alpha-cells exhibited [Ca2+]i oscillations in the absence of glucose, which petered out when islets were exposed to high glucose concentrations. 3. Somatostatin-containing delta-cells were silent in the absence of glucose but concentrations of glucose as low as 3 mM elicited oscillations. 4. In pancreatic beta-cells, a characteristic oscillatory calcium pattern was evoked when glucose levels were raised from 3 to 11 mM and this was synchronized throughout the beta-cell population. Remarkably, [Ca2+]i oscillations in non-beta-cells were completely asynchronous, both with respect to each other and to beta-cells. 5. These results demonstrate that the islet of Langerhans behaves as a functional syncytium only in terms of beta-cells, implying a pulsatile secretion of insulin. However, the lack of a co-ordinated calcium signal in alpha- and delta-cells implies that each cell acts as an independent functional unit and the concerted activity of these units results in a smoothly graded secretion of glucagon and somatostatin. Understanding the calcium signals underlying glucagon and somatostatin secretion may be of importance in the treatment of non-insulin-dependent diabetes mellitus since both glucagon and somatostatin appear to regulate insulin release in a paracrine fashion.

The solid phase synthesis of a series of tri-substituted hydantoin ligands for the somatostatin SST5 receptor

A series of trisubstituted hydantoins has been prepared by a versatile solid phase route employing primary alcohols, amines and amino acids as the monomeric building blocks. Several compounds showed submicromolar affinity in binding assays at recombinant human somatostatin receptors.

The localization of messenger ribonucleic acids for somatostatin receptors 1, 2, and 3 in rat testis

Somatostatin (SRIF) exerts multiple inhibitory actions throughout the body by binding to specific SRIF receptors (sst). In recent years, five subtypes of SRIF receptors (sst1-5) have been cloned. In this study, 35S-labeled complementary RNA probes were used for in situ hybridization to localize the sst1-5 messenger RNAs (mRNAs) in the rat testis and examine the changes in their distribution during the cycle of the seminiferous epithelium. We found that sst 1-3 mRNAs were visualized in rat testes and were mainly localized within the seminiferous tubules. The signal for sst3 mRNA was also found in interstitial cells. sst4 and 5 mRNAs were not detected in rat testes with the method used in this study. In Sertoli cells, the most intense labeling for sst1 and 3 mRNAs was in stages IV-VII of the cycle of the seminiferous epithelium, which coincided with the lowest labeling intensity for sst2. In germ cells, sst1-3 mRNAs showed similar patterns of distribution. In these cells, sst1-3 mRNA was not observed at the early steps of spermatogenesis. Positive signals for sst1-3 mRNAs were first apparent in the pachytene spermatocytes at stage VII and last until stage XII and in the diplotene spermatocyte at stage XIII. Positive signals for sst1-3 were also detected in round spermatids at stages I-VIII. Labeling of spermatids dramatically decreased at stage IX, when these cells began their elongating changes. The presence of three sst in testis suggests that SRIF may play an essential role in testicular function.

Identification of ligand binding determinants in the somatostatin receptor subtypes 1 and 2

The somatostatin (SRIF) receptors (SSTRs) 1 and 2 bind SRIF and SRIF 28 with high affinity, although a number of synthetic hexapeptide and octapeptide analogs of SRIF bind selectively to SSTR2. Extracellular loop three and its adjoining trans-membrane-spanning regions contain elements essential for the binding of such analogs to murine SSTR2. In particular, a stretch of amino acids from residues 294-297 (FDFV) in murine SSTR2 in trans-membrane domain seven can determine affinity for the SSTR2-selective analogs. Within this region, Phe294 has previously been predicted to be essential for the binding of octapeptides (Kaupmann, K., Bruns, C., Raulf, F., Weber, H., Mattes, H., and Lubbert, H. (1995) EMBO J. 14, 727-735) based on the observation that SSTR1 can bind the octapeptide SMS-201-995 with reasonable affinity after a Ser-to-Phe conversion in the analogous region of this receptor (SSTR1S305F). We find that SSTR1S305F has low affinity for a number of SSTR2-selective hexapeptides, suggesting that these analogs have different binding requirements than SMS-201-995. A correlation is seen between the ability of SSTR1S305F to bind hexapeptide analogs and the presence of a phenylalanine, but not tyrosine, at position two in these small cyclic molecules. Thus, a single hydroxyl group in hexapeptides can play a critical role in determining receptor binding to these receptor mutants. We also find that the second extracellular loop of SSTR1 is important for the selectivity of certain SRIF agonists for binding to SSTR1. Taken together, our data indicate that there are multiple elements in the somatostatin receptors that can determine the binding affinity and selectivity of peptide analogs.

Somatostatin receptor subtypes SSTR2 and SSTR5 couple negatively to an L-type Ca2+ current in the pituitary cell line AtT-20

The somatostatin receptor subtypes SSTR2 and SSTR5 mediate distinct endocrine and exocrine functions of somatostatin and may also be involved in mediating the neuromodulatory actions of somatostatin in the brain. To investigate whether these receptors couple to voltage-sensitive Ca2+ channels, SSTR2 and SSTR5 selective agonists were tested for their effects on AtT-20 cells using whole cell patch clamp techniques. The SSTR2 selective agonist MK 678 inhibited Ca2+ currents in AtT-20 cells. The effects of MK 678 were reversible and blocked by pertussis toxin pretreatment, suggesting that SSTR2 couples to the L-type Ca2+ channels via G proteins. Other SSTR2-selective agonists, including BIM 23027 and NC8-12, were able to inhibit the Ca2+ currents in these cells. The SSTR5 selective agonist BIM 23052 also inhibited the Ca2+ currents in these cells and this effect was reversible and blocked by pertussis toxin treatment. The ability of SSTR5 to mediate inhibition of the Ca2+ current was greatly attenuated by pretreatment with the SSTR5-selective agonist BIM 23052, whereas SSTR2-mediated inhibition of the Ca2+ current was not altered by pretreatment with the SSTR2-selective agonist MK 678. Thus, the SSTR2 and SSTR5 couplings to the Ca2+ current are differentially regulated. The peptide L362,855, which we previously have shown to have high affinity for the cloned SSTR5, had minimal effects on Ca2+ currents in AtT-20 cells at concentrations up to 100 nM and did not alter the ability of MK 678 to inhibit Ca2+ currents. However, it completely antagonized the effects of the SSTR5-selective agonist BIM 23052 on the Ca2+ currents. L362,855 is an antagonist/partial agonist at SSTR5 since it can reduce Ca2+ currents in these cells at concentrations above 100 nM. L362,855 is also an antagonist/partial agonist at the cloned rat SSTR5 expressed in CHO cells since it is able to block the inhibition of cAMP accumulation induced by somatostatin at concentrations below 100 nM but at higher concentrations can inhibit cAMP formation itself. Structural analysis of L362,855 reveals that only a single hydroxyl group at residue seven in the peptide is needed to convert the compound from an antagonist/partial agonist to a full agonist at SSTR5. These studies reveal that two different somatostatin receptor subtypes, SSTR2 and SSTR5, can mediate the inhibition of an L-type Ca2+ channel in AtT-20 cells by somatostatin. The receptor subtype responses can be distinguished by selective agonists and antagonists and are regulated differently by agonist pretreatment. The inhibition of Ca2+ influx into endocrine cells and neurons may be a major cellular mechanism by which somatostatin modulates hormone and neurotransmitter release. Our results reveal that at least two receptor subtypes can mediate this cellular response.

Somatostatin

1. Somatostatin (SRIF) exerts diverse physiological actions in the body including regulation of hormone and neurotransmitter release and neuronal firing activity. Analogs of SRIF are used clinically to treat tumors and cancers and to block the hypersecretion of growth hormone in acromegaly. 2. The recent cloning of five SRIF receptor subtypes has allowed for the identification of the molecular basis of the cellular actions of SRIF. The ligand binding domains and regions involved in coupling to G proteins and cellular effector systems are being identified and the processes by which SRIF inhibits cell growth and proliferation are being established. Furthermore, subtype selective agonists have been generated which are being used to investigate the specific biological roles of each SRIF receptor subtypes. 3. Such information will be useful in developing a new generation of SRIF drugs that could be employed to treat metabolic diseases, disorders of the gut, cancer and abnormalities in the central nervous system such as epilepsy and Alzheimer's disease.

Molecular properties of somatostatin receptors

The neuropeptide somatostatin is widely distributed in the central nervous system and in peripheral tissues and may be involved in the regulation of a number of physiological functions including movement and cognition. Somatostatin may also have a role in the development of the central nervous system, in particular, the cerebellum and spinal cord. Somatostatin induces its actions by interacting with a family of membrane associated receptors. Recently, five somatostatin receptors have been cloned and referred to as SSTR1-SSTR5. The distribution of the expression of the mRNAs for these receptors are distinct but overlapping. Preliminary pharmacological analysis of these receptors may lead to the development of selective ligands at these receptors. These compounds may be useful in identifying the selective functions of these receptor subtypes. Some somatostatin analogues have antiproliferative actions and are used presently to treat carcinoids. Development of subtype selective somatostatin analogues could be helpful in further identifying somatostatin receptor-expressing tumors and in the treatment of cancer. The cloning of these receptors has now opened up the possibility of more clearly investigating the functions of somatostatin in the brain and peripheral tissues and will facilitate the generation of new somatostatin drugs that may be employed for the treatment of a number of diseases.

Technetium-99m labeled somatostatin and analogs: synthesis, characterization and in vivo evaluation

Technetium-99m complexes of somatostatin and analogs were synthesized following the introduction of sulfhydryl groups with 2-iminothiolane (Traut's Reagent). In rats the complex was taken up by the liver, kidneys, adrenals, lungs and the pancreas. Analysis of urine samples of treated rats showed that the radiochemicals have reasonably good in vivo stability. This implies that the complexes may be potentially useful for biochemical characterization of somatostatin receptors and also in scintigraphic detection of somatostatin receptor positive tumors, especially for metastatic deposits in patients on somatostatin therapy.

Neuroendocrine carcinoma of the stomach with extensive somatostatin immunoreactivity

Upper gastrointestinal tract neuroendocrine tumors producing predominantly somatostatin have thus far been described only in the duodenum; their characteristic features include the frequent presence of psammoma bodies (psammomatous somatostinomas), and the association with von Recklinghausen's neurofibromatosis. Gastric neuroendocrine tumors, on the other hand, tend to display immunoreactivity to serotonin but may include small subpopulations producing gastrin, motilin, pancreatic polypeptide, and somatostatin. In this report we describe a neuroendocrine carcinoma of the stomach with rapidly fatal outcome, displaying neurosecretory granules by electron microscopy and immunoreactivity to pan-neuroendocrine markers, ie, chromogranin and neuron-specific enolase. The only neuroendocrine regulatory peptide detected in the tumor was somatostatin, identified by immunohistochemistry in the majority of neoplastic cells. In contrast with duodenal somatostinomas, there were no psammoma bodies and no demonstrable association with von Recklinghausen's neurofibromatosis. To our knowledge this appears to be the first report of a malignant neuroendocrine tumor with diffuse somatostatin immunoreactivity.

Non-endocrine applications of somatostatin and octreotide acetate: facts and flights of fancy

Somatostatin, originally detected by Krulich and ultimately isolated by Brazeau, was initially described as a growth hormone release-inhibiting factor. Subsequent investigation into the use of native somatostatin and the development of long-acting somatostatin analogues, especially octreotide acetate, have fostered increasing uses of these compounds. Though the clinical and investigational uses of somatostatin and its analogues are varied, one central theme remains constant: the ability of these agents to suppress circulating peptide levels. This article, a review of the current non-endocrine applications of somatostatin and its analogues, covers a wide range of potential applications for somatostatin-like compounds. These include use in cirrhosis and variceal bleeding, peptic ulcer disease, pancreatic fistulas, acute and chronic pancreatitis, dumping syndrome, cancer therapy, small bowel fistulas, psoriasis, pain control, and autonomic hypotension. Somatostatin may also play a role in the development and potential treatment of neurologic disease and may have profound found influence on behavior.

Octreotide: a synthetic analog of somatostatin

Octreotide is an investigational synthetic peptide exhibiting actions similar to those of endogenous somatostatin. It has a longer half-life than the native hormone and can be administered by subcutaneous injection. Octreotide inhibits the secretion of growth hormone and numerous regulatory peptides of the gastroenteropancreatic system. Trials evaluating the clinical utility of octreotide indicate efficacy in the management of symptoms associated with acromegaly and hypersecretory neuroendocrine tumors, and in the control of nontumoral secretory diarrheas. Octreotide therapy is well tolerated. This agent should prove useful in the symptomatic control of a number of rare hypersecretory disorders.

Somatostatin: historical aspects

Somatostatin, in essence an almost universal chalone, initially described as a 14 amino-acid-long peptide that inhibits growth hormone (GH) release, has been shown to be one of a family of related peptides, ubiquitous in distribution and versatile as a paracrine factor with a potentially important role in the regulation of gut, pancreatic, and nervous system function, in addition to its well-recognized influence on the pituitary secretion of GH and thyroid-stimulating hormone. With the development of new super agonists, it has become possible to manipulate the endocrine milieu, to modify gut, pancreatic, and pituitary function, and, in the case of several diseases such as acromegaly and intractable diarrhoea, to make a significant advance in therapy.

In vitro effect of exogenous insulin on insulin secretion. Studies with glucose, leucine, arginine, aminophylline and tolbutamide

Using rat pancreatic islets and the perfused rat pancreas, the effect of exogenous insulin on insulin secretion mediated by glucose, leucine, arginine, aminophylline and tolbutamide was studied. (1) In both systems the insulin releasing capacity of glucose was inhibited by exogenous insulin. In the perfused pancreas the inhibition concerned the first and the second phase of insulin release; (2) the EC50 (half-maximal inhibitory effect of insulin on glucose-induced insulin secretion) in islets was 1.2 nM (= 160 microU/ml) and 2.8 nM (390 microU/ml) in perfused pancreas; (3) exogenous insulin also inhibited insulin release in response to leucine and arginine in the isolated islet system and in the perfused pancreas; (4) using aminophylline and tolbutamide in combination with glucose, the extent of the inhibitory effect of insulin was in the range of the inhibitory effect when glucose was used alone as stimulator in islets. Data suggest that the insulinogenic action of physiological stimulators including glucose, leucine and arginine is inhibited by exogenous insulin whereas this seems not to be the case when insulin release was stimulated by aminophylline and tolbutamide. Comparing the EC50s, isolated islets seem to be more sensitive to inhibition than the perfused pancreas when glucose was used as stimulator. As far as glucose is concerned the inhibitory effect seems to depend on the extent of its concentration and/or the extent to which the mechanism of insulin release is sensitive to stimulation. The EC50 of the inhibitory effect of exogenous insulin was in the range of dissociation constant of binding of insulin to insulin receptors of islets.

Localization of somatostatin-like immunoreactivity in the pancreatic islets of the hagfish, Myxine glutinosa and the lamprey Lampetra fluviatilis

Somatostatin-like immunofluorescence has been found by immunostaining in cells of the bile duct mucosa and pancreatic islet parenchyma of the hagfish, Myxine glutinosa, and the islet lobules of the lamprey, Lampetra fluviatilis.

Gut endocrinology: correlation of physiology and morphology

The gastrointestinal tract is a large and complex endocrine organ. Elucidation of its nature, and the actions of its individual hormonal components, has advanced in recent years following the purification and sequencing of six probable hormones and the subsequent development of radioimmunoassay techniques for their measurement and immunocytochemistry for the determination of their cellular origin. Immunoassay and immunocytochemistry are clearly complimentary to each other in providing a fuller understanding of the dynamics of endocrine function at the cellular level. This is illustrated in the examples described in this paper.