Circulating somatostatin acts on the islets of Langerhans by way of a somatostatin-poor compartment

The Regulation of Metabolic Homeostasis by Incretins and the Metabolic Hormones Produced by Pancreatic Islets

In healthy humans, the complex biochemical interplay between organs maintains metabolic homeostasis and pathological alterations in this process result in impaired metabolic homeostasis, causing metabolic diseases such as diabetes and obesity, which are major global healthcare burdens. The great advancements made during the last century in understanding both metabolic disease phenotypes and the regulation of metabolic homeostasis in healthy individuals have yielded new therapeutic options for diseases like type 2 diabetes (T2D). However, it is unlikely that highly desirable more efficacious treatments will be developed for metabolic disorders until the complex systemic regulation of metabolic homeostasis becomes more intricately understood. Hormones produced by pancreatic islet beta-cells (insulin) and alpha-cells (glucagon) are pivotal for maintaining metabolic homeostasis; the activity of insulin and glucagon are reciprocally correlated to achieve strict control of glucose levels (normoglycaemia). Metabolic hormones produced by other pancreatic islet cells and incretins produced by the gut are also crucial for maintaining metabolic homeostasis. Recent studies highlighted the incomplete understanding of metabolic hormonal synergism and, therefore, further elucidation of this will likely lead to more efficacious treatments for diseases such as T2D. The objective of this review is to summarise the systemic actions of the incretins and the metabolic hormones produced by the pancreatic islets and their interactions with their respective receptors.

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 Role of the Islet Niche on Beta Cell Structure and Function

The islets of Langerhans or pancreatic islets are pivotal in the control of blood glucose and are complex microorgans embedded within the larger volume of the exocrine pancreas. Humans can have ~3.2 million islets [1] which, to our current knowledge, function in a similar manner to sense circulating blood glucose levels and respond with the secretion of a mix of different hormones that act to maintain glucose concentrations around a specific set point [2]. At a cellular level, the control of hormone secretion by glucose and other secretagogues is well-understood [3]. The key signal cascades have been identified and many details of the secretory process are known. However, if we shift focus from single cells and consider cells within intact islets, we do not have a comprehensive model as to how the islet environment influences cell function and how the islets work as a whole. This is important because there is overwhelming evidence that the structure and function of the individual endocrine cells are dramatically affected by the islet environment [4,5]. Uncovering the influence of this islet niche might drive future progress in treatments for Type 2 diabetes [6] and cell replacement therapies for Type 1 diabetes [7]. In this review, we focus on the insulin secreting beta cells and their interactions with the immediate environment that surrounds them including endocrine-endocrine interactions and contacts with capillaries.

Insulin secretion from beta cells within intact islets: location matters

The control of hormone secretion is central to body homeostasis, and its dysfunction is important in many diseases. The key cellular steps that lead to hormone secretion have been identified, and the stimulus-secretion pathway is understood in outline for many endocrine cells. In the case of insulin secretion from pancreatic beta cells, this pathway involves the uptake of glucose, cell depolarization, calcium entry, and the triggering of the fusion of insulin-containing granules with the cell membrane. The wealth of information on the control of insulin secretion has largely been obtained from isolated single-cell studies. However, physiologically, beta cells exist within the islets of Langerhans, with structural and functional specializations that are not preserved in single-cell cultures. This review focuses on recent work that is revealing distinct aspects of insulin secretion from beta cells within the islet.

Protein-mediated interactions of pancreatic islet cells

The islets of Langerhans collectively form the endocrine pancreas, the organ that is soley responsible for insulin secretion in mammals, and which plays a prominent role in the control of circulating glucose and metabolism. Normal function of these islets implies the coordination of different types of endocrine cells, noticeably of the beta cells which produce insulin. Given that an appropriate secretion of this hormone is vital to the organism, a number of mechanisms have been selected during evolution, which now converge to coordinate beta cell functions. Among these, several mechanisms depend on different families of integral membrane proteins, which ensure direct (cadherins, N-CAM, occludin, and claudins) and paracrine communications (pannexins) between beta cells, and between these cells and the other islet cell types. Also, other proteins (integrins) provide communication of the different islet cell types with the materials that form the islet basal laminae and extracellular matrix. Here, we review what is known about these proteins and their signaling in pancreatic β -cells, with particular emphasis on the signaling provided by Cx36, given that this is the integral membrane protein involved in cell-to-cell communication, which has so far been mostly investigated for effects on beta cell functions.

Connexin-dependent signaling in neuro-hormonal systems

The advent of multicellular organisms was accompanied by the development of short- and long-range chemical signalling systems, including those provided by the nervous and endocrine systems. In turn, the cells of these two systems have developed mechanisms for interacting with both adjacent and distant cells. With evolution, such mechanisms have diversified to become integrated in a complex regulatory network, whereby individual endocrine and neuro-endocrine cells sense the state of activity of their neighbors and, accordingly, regulate their own level of functioning. A consistent feature of this network is the expression of connexin-made channels between the (neuro)hormone-producing cells of all endocrine glands and secretory regions of the central nervous system so far investigated in vertebrates. This review summarizes the distribution of connexins in the mammalian (neuro)endocrine systems, and what we know about the participation of these proteins on hormone secretion, the life of the producing cells, and the action of (neuro)hormones on specific targets. The data gathered since the last reviews on the topic are summarized, with particular emphasis on the roles of Cx36 in the function of the insulin-producing beta cells of the endocrine pancreas, and of Cx40 in that of the renin-producing juxta-glomerular epithelioid cells of the kidney cortex. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

Somatostatin secreted by islet delta-cells fulfills multiple roles as a paracrine regulator of islet function

OBJECTIVE

Somatostatin (SST) is secreted by islet delta-cells and by extraislet neuroendocrine cells. SST receptors have been identified on alpha- and beta-cells, and exogenous SST inhibits insulin and glucagon secretion, consistent with a role for SST in regulating alpha- and beta-cell function. However, the specific intraislet function of delta-cell SST remains uncertain. We have used Sst(-/-) mice to investigate the role of delta-cell SST in the regulation of insulin and glucagon secretion in vitro and in vivo.

RESEARCH DESIGN AND METHODS

Islet morphology was assessed by histological analysis. Hormone levels were measured by radioimmunoassay in control and Sst(-/-) mice in vivo and from isolated islets in vitro.

RESULTS

Islet size and organization did not differ between Sst(-/-) and control islets, nor did islet glucagon or insulin content. Sst(-/-) mice showed enhanced insulin and glucagon secretory responses in vivo. In vitro stimulus-induced insulin and glucagon secretion was enhanced from perifused Sst(-/-) islets compared with control islets and was inhibited by exogenous SST in Sst(-/-) but not control islets. No difference in the switch-off rate of glucose-stimulated insulin secretion was observed between genotypes, but the cholinergic agonist carbamylcholine enhanced glucose-induced insulin secretion to a lesser extent in Sst(-/-) islets compared with controls. Glucose suppressed glucagon secretion from control but not Sst(-/-) islets.

CONCLUSIONS

We suggest that delta-cell SST exerts a tonic inhibitory influence on insulin and glucagon secretion, which may facilitate the islet response to cholinergic activation. In addition, delta-cell SST is implicated in the nutrient-induced suppression of glucagon secretion.

Alpha-cells of the endocrine pancreas: 35 years of research but the enigma remains

Glucagon, a hormone secreted from the alpha-cells of the endocrine pancreas, is critical for blood glucose homeostasis. It is the major counterpart to insulin and is released during hypoglycemia to induce hepatic glucose output. The control of glucagon secretion is multifactorial and involves direct effects of nutrients on alpha-cell stimulus-secretion coupling as well as paracrine regulation by insulin and zinc and other factors secreted from neighboring beta- and delta-cells within the islet of Langerhans. Glucagon secretion is also regulated by circulating hormones and the autonomic nervous system. In this review, we describe the components of the alpha-cell stimulus secretion coupling and how nutrient metabolism in the alpha-cell leads to changes in glucagon secretion. The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions. We also highlight the pathophysiology of the alpha-cell and how hyperglucagonemia represents an important component of the metabolic abnormalities associated with diabetes mellitus. Therapeutic inhibition of glucagon action in patients with type 2 diabetes remains an exciting prospect.

Alterations in glucose homeostasis in SSTR1 gene-ablated mice

SSTR1 is found on the majority of human pancreatic beta cells, however, its role in insulin secretion has yet to be elucidated. In this study, we used the SSTR1 knockout mouse model to examine the role of SSTR1 in insulin secretion and glucose homeostasis in mice. Despite the reported effect of SSTR1 in inhibiting growth hormone secretion, SSTR1-/- mice had significantly reduced body weight with growth retardation. Perfusion of isolated mouse pancreata at 3 months of age demonstrated a significant increase in insulin secretion in SSTR1-/- mice compared with that of WT controls. We also found that at 3 months of age, SSTR1-/- mice had significantly decreased levels of systemic insulin secretion and were glucose intolerant. However, SSTR1 gene-ablated mice had a much higher rate of insulin clearance compared to WT mice at the same age. When challenged at 12 months of age, we found SSTR1-/- mice had increased glucose tolerance with exaggerated increase of insulin levels at the end of the experiment. Immunochemical analysis showed that the pancreatic islets of SSTR1-/- mice had significantly decreased levels of somatostatin staining and a significant decrease of SSTR5 expression. These results demonstrate that SSTR1 plays an important role in the regulation of insulin secretion in the endocrine pancreas in mice.

A reappraisal of the blood glucose homeostat which comprehensively explains the type 2 diabetes mellitus-syndrome X complex

Blood glucose concentrations are unaffected by exercise despite very high rates of glucose flux. The plasma ionised calcium levels are even more tightly controlled after meals and during lactation. This implies 'integral control'. However, pairs of integral counterregulatory controllers (e.g. insulin and glucagon, or calcitonin and parathyroid hormone) cannot operate on the same controlled variable, unless there is some form of mutual inhibition. Flip-flop functional coupling between pancreatic alpha- and beta-cells via gap junctions may provide such a mechanism. Secretion of a common inhibitory chromogranin by the parathyroids and the thyroidal C-cells provides another. Here we describe how the insulin:glucagon flip-flop controller can be complemented by growth hormone, despite both being integral controllers. Homeostatic conflict is prevented by somatostatin-28 secretion from both the hypothalamus and the pancreatic islets. Our synthesis of the information pertaining to the glucose homeostat that has accumulated in the literature predicts that disruption of the flip-flop mechanism by the accumulation of amyloid in the pancreatic islets in type 2 diabetes mellitus will lead to hyperglucagonaemia, hyperinsulinaemia, insulin resistance, glucose intolerance and impaired insulin responsiveness to elevated blood glucose levels. It explains syndrome X (or metabolic syndrome) as incipient type 2 diabetes in which the glucose control system, while impaired, can still maintain blood glucose at the desired level. It also explains why it is characterised by high plasma insulin levels and low plasma growth hormone levels, despite normoglycaemia, and how this leads to central obesity, dyslipidaemia and cardiovascular disease in both syndrome X and type 2 diabetes.

Intra-islet somatostatin regulates glucagon release via type 2 somatostatin receptors in rats

Exogenously administered somatostatin (SST) inhibits secretion of insulin and glucagon. Furthermore, it is hypothesized that islet SST regulates glucagon secretion by a local action. A number of studies utilizing SST antibodies have been performed to test this hypothesis, and their results have been conflicting. Five subtypes of SST receptor (SSTR1-5) mediate the effect of SST on target cells. In rodents, SST inhibits the release of glucagon, but not that of insulin, via SSTR2. A novel SSTR2-selective antagonist, DC-41-33, was synthesized recently. We have investigated the effects of this antagonist on arginine-stimulated glucagon and insulin release in batch incubations of isolated rat islets, perifused isolated rat islets, and isolated perfused rat pancreas. In batch incubations at 3.3 mmol/l glucose, DC-41-33 increased glucagon release in a dose-dependent manner. At the maximum dose tested (2 micro mol/l), DC-41-33 enhanced the glucagon response by 4.3- to 5-fold. Similarly, this compound increased arginine-induced glucagon release in perifused islets at 3.3 mmol/l glucose (2.8-fold) and perfused pancreas at 3.3 and 5.5 mmol/l glucose (2.5- and 2.3-fold, respectively). In the two latter experimental systems, DC-41-33 had no significant effect on insulin release. In conclusion, our results strongly support the hypothesis that islet SST inhibits glucagon secretion via a local action.

Pancreatic somatostatin inhibits insulin secretion via SSTR-5 in the isolated perfused mouse pancreas model

INTRODUCTION

The function of pancreatic somatostatin in insulin secretion is controversial, and the receptor(s) mediating such event has not been exclusively investigated.

AIM AND METHODOLOGY

To differentiate the specific role of SSTR5 in the mouse pancreas, we generated a mouse SSTR5 gene ablation model. Mice homozygous for the deletion (SSTR5-/-) and wild type (WT) littermate controls underwent whole pancreas perfusion to determine the effect of SSTR5 gene ablation on glucose-stimulated insulin secretion. The perfusion was done with and without octreotide added to the infusion buffer. Furthermore, pancreatic somatostatin was immunoneutralized by using a potent somatostatin monoclonal antibody to determine whether pancreatic somatostatin regulates insulin secretion in these mice.

RESULTS

Results showed that at 3 months of age, there were no alterations in insulin secretion compared with WT controls. However, glucose-stimulated insulin secretion was significantly enhanced in 12-month-old SSTR5-/- mice compared with WT controls. The addition of octreotide to the perfusion significantly suppressed insulin secretion in WT controls, while it had no effect on SSTR5-/- mice. Immunoneutralization of pancreatic somatostatin resulted in enhanced glucose-stimulated insulin secretion in WT controls, but decreased levels of insulin secretion in SSTR5-/- mice.

CONCLUSION

These results suggest that, in the mouse, pancreatic somatostatin regulates insulin secretion through SSTR5, and that the effect is age-specific.

Gliclazide directly inhibits arginine-induced glucagon release

Arginine-stimulated insulin and somatostatin release is enhanced by the sulfonylurea gliclazide. In contrast, gliclazide inhibits the glucagon response. The aim of the present study was to investigate whether this inhibition of glucagon release was mediated by a direct suppressive effect of gliclazide or was secondary to the paracrine effect of released somatostatin. To eliminate the paracrine effects of somatostatin, we first perfused isolated rat pancreata with a medium supplemented with 23% of the standard calcium content. Second, we perifused isolated rat islets with a novel and highly specific antagonist of type 2 somatostatin receptor, DC-41-33 (2 micro mol/l), which fully antagonizes the suppressive somatostatin effect on rat A cells. Gliclazide (30 micro mol/l) inhibited glucagon release by 54% in the perfusion experiments, whereas the somatostatin response was nearly abolished. In islet perifusions with DC-41-33, arginine-induced glucagon release was inhibited by 66%. We therefore concluded that gliclazide inhibits glucagon release by a direct action on the pancreatic A cell.

Assessment of the role of interstitial glucagon in the acute glucose secretory responsiveness of in situ pancreatic beta-cells

Glucagon is a potent stimulator of insulin release in the presence of a permissive glucose concentration, activating beta-cells in vitro via both glucagon- and glucagon-like peptide-1 (GLP-1)-receptors. It is still unclear whether locally released glucagon amplifies the secretory responsiveness of neighboring beta-cells in the intact pancreas. The present study investigates this question in the perfused pancreas by examining the effects of antagonists for glucagon receptors ([des-His(1),des-Phe(6),Glu(9)]glucagon-NH(2), 10 micromol/l) and GLP-1-receptors [exendin-(9-39)-NH(2), 1 micromol/l] on the insulin secretory response to glucose. The specificity of both antagonists was demonstrated by their selective interaction with glucagon-receptor signaling in rat hepatocytes and GLP-1-receptor signaling in Chinese hamster lung (CHL) fibroblasts. In purified rat beta-cells, the glucagon-receptor antagonist (10 micromol/l) inhibited the effect of 1 nmol/l glucagon upon glucose-induced insulin release by 78 plus minus 6%. In the perfused rat pancreas, neither of these antagonists inhibited the potent secretory response to 20 mmol/l glucose, although they effectively suppressed the potentiating effect of, respectively, an infusion of glucagon (1 nmol/l) or GLP-1 (1 nmol/l) on insulin release. When endogenous glucagon release was enhanced by isoproterenol (100 nmol/l), no amplification was seen in the simultaneous or subsequent insulin secretory response to glucose. It is concluded that, at least under the present selected conditions, the glucose-induced insulin release by the perfused rat pancreas seems to occur independent of an amplifying glucagon signal from neighboring alpha-cells.

Multiple polypeptide hormone expression in pancreatic islet cell carcinomas derived from phosphoenolpyruvatecarboxykinase-SV40 T antigen transgenic rats

Transgenic rats carrying a PEPCK-SV40 large T-antigen (TAg) transgene rapidly develop numerous pancreatic islet cell neoplasms, the cells of which express TAg. Although many of the larger neoplasms contain relatively undifferentiated cells, many tumors contain areas of well-differentiated cells with abundant endoplasmic reticulum (ER) and secretory granules for endocrine hormones like those observed in normal pancreatic islets. In the well-differentiated lesions, glucagon-producing alpha-cells, insulin-producing beta-cells, and somatostatin-producing delta-cells are readily identifiable morphologically under the electron microscope. Beta-cells were observed in all normal and hyperplastic islets, and nests of these cells were scattered throughout the larger neoplasms. These nests varied from small clusters of epithelium-like cells that stain intensely for insulin, to sheets of small, basophilic cells that stain more diffusely for the hormone. Alpha-cells were also present in all of the normal and hyperplastic islets, but in larger hyperplastic islets, the peripheral localization was absent. Larger neoplasms contained many nests of glucagon-expressing cells, as well as scattered glucagon-producing single cells. Delta-cells were rarely observed in the hyperplastic islets and in the neoplasms. Blood-glucose levels were unaltered in the transgenic animals relative to their nontransgenic litter mates. Thus although these islet cell neoplasms express several polypeptide hormones, there is no obvious clinical effect of such expression in vivo.

Quantification of insulin release from implantable polymer-based delivery systems and augmentation of therapeutic effect with simultaneous release of somatostatin

Insulin injections control diabetes mellitus but do not reproduce physiologic regulation. Polymer-based controlled-release technology has enabled us to demonstrate: that the controlled release of insulin from polymer matrices can indeed be used to control diabetes mellitus but does so at the expense of hyperinsulinemia and hypoglycemia; and that somatostatin can be delivered in similar fashion, so as to provide glucose homeostasis in a more physiologic range, at lower insulin levels and at somatostatin doses below those used in intermittent infusion studies; and, that microgram quantities of a drug can be delivered successfully in vivo with intact biological function and in a manner that can be monitored continuously. In the present study the simultaneous polymer-matrix-controlled release of insulin with somatostatin extended glycemic control in diabetic rats. Eleven rats received subcutaneous polymer matrix implants containing insulin alone and 11 rats received implants containing insulin and somatostatin. Plasma and urinary glucose control were improved in both groups. Glucose concentrations in the insulin alone group remained depressed for 5 days until insulin release from the matrices declined below 11.6 units/kg/day. When somatostatin was delivered at 0.75-1.1 micrograms/kg/day together with insulin, plasma glucose control persisted for 12 days until insulin release decreased below 3.6 units/kg/day. It is our hope that further experiments regarding the potential role of both controlled-release devices and somatostatin will be performed to provide continuing therapeutic alternatives to the insulin-dependent diabetic. This is also the first in vivo demonstration of the simultaneous release of two biologically active peptide hormones from polymer matrices. The use of the polymer matrix systems may not only have profound effects on the ambulatory care of diabetes but might also permit the investigation of the synergistic effects of other families of compounds.

Monoclonal antibodies to somatostatin receptor of rat brain

Murine Monoclonal antibodies (MAbs) to the rat brain somatostatin (SRIF) receptor were produced. Sp 2/0 myeloma cells were fused with splenocytes of Balb/c mice immunized with the soluble rat brain SRIF receptor which was partially purified by gel-filtration chromatography. Screening by radioligand ([125I-Tyr11]SRIF-14) binding inhibition assay yielded three stable cell lines producing IgG1, IgM, or IgA antibody. Autoradiographic study of the polyacrylamide gel electrophoresed under nondenaturing conditions revealed that these MAbs inhibited the ligand binding to the receptor, regardless of their incubation with the receptor prior to the ligand binding. The results suggest that the MAbs produced are the antibodies to the ligand binding site of the receptor, and bind to the receptor in competition with the ligand.

Insulin-cancer relationships: possible dietary implication

Insulin is a major anabolic hormone in mammals and its involvement in malignancies is well documented. An attempt is made to classify experimental and human cancers into four groups, according to the way the tumors are affected by, or interact with, insulin. Such an approach provides a better understanding of the dietary effects on tumorigenesis. Since human cancers are of the insulin-producing/secreting or insulin-dependent types, it is suggested that screening of individuals for blood insulin level and reducing the insulin status by dietary means may lead to a decreased risk of cancer. Anti-insulin drugs may be useful as supplements to therapeutic treatment.

Inhibition of gastrin-stimulated growth of gastrointestinal tumour cells by octreotide and the gastrin/cholecystokinin receptor antagonists, proglumide and lorglumide

The rat pancreatic cell line, AR42J possessed high-affinity gastrin and somatostatin receptors and its growth was stimulated by physiological gastrin-17 concentrations between 5 x 10(-11) mol/l and 10(-9) mol/l as measured by [75Se]selenomethionine uptake. The somatostatin analogue, octreotide (2 x 10(-7) to 2 x 10(-11) mol/l), reduced this stimulated growth. Gastrin-stimulated AR42J growth was also inhibited by proglumide (3 x 10(-4) mol/l) and lorglumide (3 x 10(-5) mol/l) at maximal G17 concentrations of 5 x 10(-11) and 10(-10) mol/l, respectively, and the analogues competed with [125I] gastrin-17 (5 x 10(-10) mol/l) for binding to gastrin receptors on AR42J (50% inhibitory concentrations, less than or equal to 10(-3) mol/l and 4 x 10(-6) mol/l, respectively. Octreotide reduced the basal growth of the human gastric cell line, MKN45G, (which is associated with intracellular gastrin immunoreactivity) in serum-free medium to 73% of control at a concentration of 2 x 10(-8) mol/l, which was reversed by gastrin-17 (10(-10) mol/l). Lorglumide (3 x 10(-5) mol/l) also reduced the basal growth to 30% of control, which was reversed to 78% by 10(-5) mol/l gastrin. Proglumide had no effect on the basal growth of MKN45G.

Vectorial insulin secretion by pancreatic beta-cells

Morphological studies of pancreatic beta-cells have suggested the presence of discrete sensory and secretory domains. In the present study we now provide functional evidence by demonstrating polarity of insulin release by HIT-T15 cells. A significant diffusion barrier across a twin chamber culture system was verified in the presence of confluent HIT-T15 cells. When stimulated with sulphonylurea, ionophore or high potassium, insulin was preferentially released into the lower chamber irrespective of whether secretagogues were added to the upper or lower chambers. Vectorial insulin secretion may be a significant determinant of islet hormone paracrine interactions in the maintenance of glucose homeostasis.

Changes in the pancreatic A-, B- and D-cell populations during development of diabetes in spontaneously diabetic Chinese hamsters of the Asahikawa colony (CHAD)

We investigated the pathological changes in pancreatic islets during the development of diabetes in spontaneously diabetic Chinese hamsters of the Asahikawa colony (CHAD), using morphometric analysis and specific immunocytochemical methods. We also investigated the relationships between changes in islet cell composition and the hormonal changes in the plasma and pancreas. Plasma and pancreatic insulin levels were significantly lower in diabetic hamsters than in pre-diabetic hamsters. However, plasma insulin levels in the pre-diabetic hamsters were significantly higher than those in the hamsters from the non-diabetic control strain, although the pancreatic insulin content in the pre-diabetics was significantly lower than that in the non-diabetics. Since even a severely diabetic CHAD is alive for many months after the onset of the disease without injections of insulin, its clinical course seems to be close to that of type 2 human diabetes. In contrast, plasma and pancreatic glucagon levels were significantly higher in diabetic hamsters than in non-diabetics and pre-diabetics. There were significantly positive correlations between plasma and pancreatic insulin, and plasma and pancreatic glucagon levels in CHAD (P less than 0.01). On the other hand, no significant differences in the pancreatic somatostatin content were found among the non-diabetics, pre-diabetics, and severe diabetics. Significant correlations were found between plasma and pancreatic hormone levels (except for somatostatin) and the advance of diabetes in CHAD (P less than 0.01). Morphometric analysis by planimeter revealed that islets in the severe diabetics were 25% smaller than in the pre-diabetics. Significantly less B-cell area within the diabetic islets was found when compared with the non-diabetic and pre-diabetic islets. Significantly larger A- and D-cell areas within the diabetic islets were found compared with the non-diabetic and pre-diabetic islets. There was a significant correlation between the areas of the three types of cell within the islets and the severity of diabetes (P less than 0.01). It is suggested, therefore, that the pancreatic islet function in CHAD is closely associated with the morphologic changes in islet endocrine cells. The elevation of plasma and pancreatic glucagon levels and the marked increase of the A-cell area within the islets from severely diabetic CHAD may reveal an absolute increase of A-cell numbers.

Binding and internalization of somatostatin, insulin, and glucagon by cultured rat islet cells

The pathways by which islet B, A, and D cells bind and internalize homologous (self) and heterologous (other) islet hormones were compared. [125I-Tyr]Somatostatin-14 (S-14), 125I-insulin, and 125I-glucagon were incubated with monolayer cultures of neonatal rat islet cells. Tissues were processed for quantitative electron microscopic autoradiography by the probability circle method coupled to morphometry. For all three radioligands and all three cell types surface labeling was rapidly followed by internalization of the radioligands into endocytotic vesicles. The further intracellular movement of the ligand occurred in a time- and temperature-related manner and depended on whether it was homologous or heterologous for the cell in question. Thus [125I-Tyr]S-14 in B and A cells, 125I-insulin in A and D cells, and 125I-glucagon in B and D cells were rapidly transferred from endocytotic vesicles to lysosomal structures. By contrast, [125I-Tyr]S-14 in D cells, 125I-insulin in B cells, and 125I-glucagon in A cells showed poor progression from endocytotic vesicles to downstream vesicular structures. We conclude that (a) each of the three radioligands is internalized by islet cells in a time- and temperature-dependent manner; (b) after initial internalization the further intracellular progression of the endocytosed radioligand occurs freely in cells heterologous for the radioligand but poorly in cells homologous for the radioligand; and (c) binding and endocytosis can be uncoupled from lysosomal degradation of ligand.

Sensitivity of rat pancreatic A and B cells to somatostatin

Islet A and B cells were purified from the rat pancreas and examined for their respective sensitivity to somatostatin. Both somatostatin-14 (S14) and -28 (S28) inhibited glucagon and insulin release through direct interactions with the corresponding cell types. A dose-dependent suppression of the secretory activities was paralleled by a reduction in cellular cyclic AMP formation with similar ED50 values for both actions. The somatostatin effects on pancreatic hormone release may thus be mediated via an inhibition of adenylate cyclase activity. In pancreatic A cells, S14 and S28 were equally potent inhibitors with ED50 values ranging from 2 x 10(-12) to 2 x 10(-11) mol/l. Pancreatic B cells exhibited a similar sensitivity to S28 as the A cells (ED50 of 2 to 5 x 10(-11) mol/l), but not to S14 (ED50 of 2 x 10(-9) mol/l). Extrapolation of these in vitro sensitivities of islet A and B cells to the in vivo situation suggests that both cell types can respond to circulating S28 levels and that A cells are sensitive to both locally and distally released S14. Islet B cells appear insensitive to the normal peripheral S14 levels but could respond to locally released somatostatin. The marked difference in the sensitivities of islet A and B cells to S14 suggest that these cell types are equipped with different somatostatin receptors. This notion was further supported by the cell-selective actions of the synthetic S14 analogues [D-Trp8, D-Cys14]S14 and desAsn5[D-Trp8, D-Ser13]S14.

The anterograde and retrograde infusion of glucagon antibodies suggests that A cells are vascularly perfused before D cells within the rat islet

We have suggested that the order of cellular vascular perfusion within the islet is important in the regulation of islet hormone secretion. Anatomically, the A and D cells appear to be randomly dispersed throughout the mantle. Although islet capillary blood flow is known to be from the B-cell core to the A- and D-cell mantle, it has not yet been established whether the cells of the mantle may influence one another vascularly. Rat pancreata were perfused in vitro anterogradely and retrogradely with or without glucagon antibody in order to determine the order of cellular perfusion and interaction between the A and D cells in the islet mantle. Anterograde infusion of glucagon antibody did not affect insulin secretion, but rapidly decreased somatostatin secretion -46 +/- 8%, (p less than 0.005). Retrograde infusion of glucagon antibody decreased insulin secretion (-27 +/- 8%, p less than 0.005) but had no effect upon somatostatin secretion. This study not only confirms a core to mantle islet perfusion but also establishes that the A cell precedes the D cell in the terms of vascular perfusion. Thus within the islet, vascular borne insulin regulates the release of glucagon, which in turn, regulates the release of somatostatin. Somatostatin is vascularly neutral owing to its downstream position in the sequence (B to A to D) of cellular perfusion.

Somatostatin-gastrin interactions in the rat stomach

Low concentrations of somatostatin and gastrin within or slightly above the range of physiologically circulating levels were perfused in the isolated, vascularly perfused rat stomach preparation. Somatostatin at 10 and 50 pg/ml significantly inhibited acetylcholine-stimulated gastrin secretion by 26% and 45%, respectively, whereas perfusion of 50 and 500 pg/ml exogenous gastrin did not modify gastric somatostatin secretion. Perfusion of somatostatin-antiserum significantly increased gastrin release by 235%. It is concluded that (1) somatostatin is a powerful inhibitor of the gastrin cell under in vitro conditions; the data are in accordance with a concept that endogenous somatostatin could act as a true hormone; (2) the secretory activity of the somatostatin cell is not significantly affected by circulating gastrin.

The condensing vacuole of exocrine cells is more acidic than the mature secretory vesicle

A number of intracellular, membrane-bound compartments in both the endocytic and exocytic pathways of eukaryotic cells have an acidic internal pH. In endocrine cells, the mature secretory vesicle has an acidic pH; secretory vesicles isolated from exocrine cells, however, appear to have a neutral pH. Recently we have used a newly developed immunocytochemical technique to map low-pH compartments in insulin-secreting islet cells with the electron microscope and find that during the maturation of the secretory vesicle there is a progressive acidification of these vesicles that begins as soon as the trans Golgi condensing vacuoles form. Now we have used this technique to examine two exocrine cells: the pancreatic acinar cell and the parotid serous cell. In both cell types, the trans Golgi condensing vacuoles are acidic and accumulate the low-pH probe to the same extent as condensing vacuoles of insulin-secreting islet cells. Unlike insulin-secreting cells, however, maturation of the granules is accompanied by a return of luminal pH to near neutrality. Therefore, although the pH of storage granules in exocrine and endocrine cells is different, the pH of the condensing vacuoles in both cells is acidic.

Insulinotropin: glucagon-like peptide I (7-37) co-encoded in the glucagon gene is a potent stimulator of insulin release in the perfused rat pancreas

Insulin secretion is controlled by a complex set of factors that include not only glucose but amino acids, catecholamines, and intestinal hormones. We report that a novel glucagon-like peptide, co-encoded with glucagon in the glucagon gene is a potent insulinotropic factor. The glucagon gene encodes a proglucagon that contains in its sequence glucagon and additional glucagon-like peptides (GLPs). These GLPs are liberated from proglucagon in both the pancreas and intestines. GLP-I exists in at least two forms: 37 amino acids GLP-I(1-37), and 31 amino acids, GLP-I(7-37). We studied the effects of synthetic GLP-Is on insulin secretion in the isolated perfused rat pancreas. In the presence of 6.6 mM glucose, GLP-I(7-37) is a potent stimulator of insulin secretion at concentrations as low as 5 X 10(-11) M (3- to 10-fold increases over basal). GLP-I(1-37) had no effect on insulin secretion even at concentrations as high as 5 X 10(-7) M. The earlier demonstration of specific liberation of GLP-I(7-37) in the intestine and pancreas, and the magnitude of the insulinotropic effect at such low concentrations, suggest that GLP-I(7-37) participates in the physiological regulation of insulin secretion.

Physiological and pathophysiological aspects of somatostatin

Somatostatin is found in the D-cells of organs that are exclusively responsible for the digestion, absorption, and metabolism of ingested nutrients. D-cells apparently release their secretory products both into the interstitial space (paracrine action) and into the circulation (endocrine action). Ingestion of all three basic nutrients--fat, carbohydrate, and particularly protein--elicits a significant increase in peripheral vein plasma somatostatin levels in dogs and humans. Acidification of a meal stimulates somatostatin release in dogs. Vagal, cholinergic, and adrenergic mechanisms exert a species-dependent effect on somatostatin release. Gut hormones also participate in the regulation of postprandial somatostatin release, and endogenous opioids have an effect that depends on the composition of the meal. Stimulation of postprandial somatostatin release by H2-receptor agonists and prostaglandins has been reported. Insulin inhibits and glucagon stimulates somatostatin release. Elevated levels of circulating glucose reduce the somatostatin response, an effect that cannot be entirely explained by the parallel augmentation of insulin secretion. Circulating nutrients also modify the effect of gut hormones on D-cell function. The physiological action of somatostatin is an inhibitory effect on virtually all gastrointestinal and pancreatic exocrine and endocrine functions. Secretory and/or motor activities are attenuated, thereby preventing an exaggerated and overshooting response. Alterations of tissue somatostatin content and plasma somatostatin levels have been observed in obesity and suggest that somatostatin deficiency may be a pathogenic factor. The observed changes of somatostatin may be secondary to alterations of other functions; nevertheless, hyposomatostatinaemia might facilitate nutrient assimilation.

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.

Pathways of protein secretion in eukaryotes

Protein secretion from cells can take several forms. Secretion is constitutive if proteins are secreted as fast as they are synthesized. In regulated secretion newly synthesized proteins destined for secretion are stored at high concentration in secretory vesicles until the cell receives an appropriate stimulus. When both constitutive and regulated protein secretion can take place in the same cell a mechanism must exist for sorting the correct secretory protein into the correct secretory vesicle. The secretory vesicle must then be delivered to the appropriate region of plasma membrane. Transfection of DNA encoding foreign secretory proteins into regulated secretory cells has provided insight into the specificity of sorting into secretory vesicles.

Nonrandom positioning of Golgi apparatus in pancreatic B cells

The Golgi apparatus has a polarized distribution in a variety of cell types, and in certain epithelia its intracellular position correlates with the direction of vectorial transport and release of secretory products. We have studied quantitatively the orientation of the Golgi apparatus in pancreatic islet B cells by examining its location with respect to the juxtacapillary face of the cell. This was done a) at the light microscope level on semithin sections of osmium-impregnated islets, and b) at the ultrastructural level on thin sections of intact pancreas. The data obtained with both methods show that in pancreatic B cells the Golgi apparatus is preferentially located in the position opposite to the juxtacapillary face of the cell. These results suggest a structural polarization of B cells, which may be important for the coordinate function of islets of Langerhans.

Secretory patterns of glucoregulatory hormones in prehepatic circulation of dogs

The temporal organization of patterns of secretion of insulin, glucagon, and somatostatin was studied in basal conditions in normal or pancreatectomized dogs fitted with an indwelling hepatic portal catheter. Portal and peripheral blood samples were collected at a 7.5- or 15-min frequency, which covered the medium range of the ultradian period. The raw data were studied using spectral analyses employing fast Fourier transformation (FFT) techniques. The results indicate that in normal dogs: 1) endogenous physiological periodicities, statistically significant at a level of alpha = 0.05 (i.e., 95% confidence interval), were found to exist for the three pancreatic hormones in portal blood between 0.54 and 1.78 h/cycle and in peripheral blood between 0.59 and 1.84 h/cycle; 2) the portal levels of the hormones are significantly higher than peripheral ones; and, 3) whereas pancreatic hormones oscillated, glucose was found to maintain a steady level. In pancreatectomized dogs, no regular rhythm was detected. Thus, whereas endogenous periodicities exist for the secretion of pancreatic hormones in the normal dog, in the pancreatectomized dog the extrapancreatic glucagon and somatostatin are secreted in nonperiodic, randomly occurring pulses.

Tissue-specific posttranslational processing of pre-prosomatostatin encoded by a metallothionein-somatostatin fusion gene in transgenic mice

The somatostatins are neuropeptides of 14 and 28 amino acids that inhibit the release of growth hormone and other hypophyseal and gastrointestinal peptides. These neuropeptides are cleaved posttranslationally from a common precursor, pre-prosomatostatin. We report here the production and processing of pre-prosomatostatin by transgenic mice carrying a metallothionein-somatostatin fusion gene. The most active site of somatostatin production, as determined by hormone concentrations in the tissues, is the anterior pituitary, a tissue that does not normally synthesize somatostatin-like peptides. Anterior pituitary processed pre-prosomatostatin almost exclusively to the two biologically active peptides, somatostatin-14 and somatostatin-28, whereas the liver and kidney synthesized much smaller quantities of predominantly a 6000 dalton somatostatin-like peptide. The growth of the transgenic mice was normal despite high plasma levels of the somatostatin-like peptides. These studies indicate that proteases which cleave prosomatostatin to somatostatin-28 and somatostatin-14 are not specific to tissues that normally express somatostatin.

A unique control mechanism in the regulation of insulin secretion. Secretagogue-induced somatostatin receptor recruitment

In this study, we have correlated the translocation of somatostatin (SRIF) receptors from the cell interior to the plasma membrane with the ability of SRIF to inhibit insulin release. Islets were perifused with glucose (30, 100, 165, 200, or 300 mg/dl) in the presence of sodium isethionate. Sodium isethionate inhibits insulin release, but not the recruitment of SRIF receptors. Thus, the recruitment of SRIF receptors to the surface membrane continued without the lysis of secretion vesicles. SRIF binding rose from 3.75 +/- 0.16 to 6.46 +/- 0.28 fmol/10 islets as glucose concentration increased. Sodium isethionate was then removed, islets perifused with low glucose (30 mg/dl), and challenged with 400 microM isobutylmethylxanthine (IBMX) with or without SRIF (5 micrograms/ml). In the islets perifused with high glucose concentration, IBMX lysed a greater number of vesicles and caused enhanced release of insulin. The greater the number of secretion vesicles marginated to the plasma membrane by glucose, the greater the response to IBMX. Colchicine (1 mM) prevented secretion vesicle migration and this potentiation effect of higher concentrations of glucose was eliminated. In experiments with IBMX and SRIF, the degree of inhibition of IBMX-induced insulin release by SRIF was proportional to the magnitude of SRIF binding to these islets. SRIF inhibited insulin release by 20 microU/100 islets initially perifused with low glucose (30 mg/dl) and by 875 microU/100 islets perifused with high glucose (300 mg/dl). The maximal effect of SRIF was observed when its binding reached a level of 5.4 fmol/10 islets. We conclude that inhibition of insulin release by SRIF is proportional to the SRIF receptor concentration, and that translocation of SRIF receptors during exocytosis plays an important role in paracrine regulation of insulin secretion by rendering the islets more sensitive to SRIF.

Effects of cysteamine and antibody to somatostatin on islet cell function in vitro. Evidence that intracellular somatostatin deficiency augments insulin and glucagon secretion

In this study we have characterized the effects of cysteamine (CHS) on the cellular content and release of immunoreactive somatostatin (S-14 LI), insulin (IRI), and glucagon (IRG) from monolayer cultures of neonatal rat islets. Incubation of cultures with 0.1-10 mM CHS for 1 h led to an apparent, dose-dependent reduction of cellular S-14 LI that was 50% of control at 0.3 mM, 87% at 1 mM, and 95% at 10 mM. IRI content was unaffected by CHS up to 1 mM, but at 10 mM 90% loss of IRI occurred. All concentrations were without effect on IRG content. The loss of S-14 LI and IRI was completely reversible with time, but with different recovery rates for the two hormones (48 h for S-14 LI, and 72 h for IRI). Released S-14 LI rose progressively with increasing doses of CHS from 21 +/- 2.5 pg/ml per hour to 41 +/- 1.4 pg/ml per hour at CHS concentrations of 5 mM and 10 mM. IRI and IRG secretion were both also significantly enhanced (by 55% and 88%, respectively), despite the elevated medium S-14 LI. Since CHS reduced cellular S-14 LI but augmented medium S-14 LI, the relative effects of CHS (1 mM) and immunoneutralization with antibody to S-14 LI on IRI and IRG secretion were tested. Anti S-14 LI alone stimulated basal IRG (67%) but not IRI. Cultures rendered S-14 LI deficient with both CHS and anti-S-14 LI exhibited threefold and 2.3-fold potentiation of IRG and IRI secretions, respectively, greater than that expected from the separate effects of the two agents. Increasing medium glucose from 2.8 mM to 16.7 mM stimulated IRI release by 86% and suppressed IRG by 53%. CHS (1 mM) and anti-S-14 LI further augmented stimulated IRI release, by 30%; although 16.7 mM glucose suppression of IRG was still maintained under these conditions, the quantitative IRG response was significantly greater. These results suggest that CHS induces an apparent loss of islet S-14 LI, and at high doses, of IRI as well, but has no effect on A cells. Complete islet S-14 LI deficiency augments IRI and IRG secretion over a wide range of glucose concentrations, suggesting a physiological role of D cells on B cell and A cell regulation. D cell modulation of B cells requires cellular but not extracellular S-14 LI, being mediated probably though direct intracellular communication, whereas the A cells seem to be regulated by both direct contact as well as through locally secreted S-14 LI.

Evidence for polarization of plasma membrane domains in pancreatic endocrine cells

Polarization of plasma membrane domains is an essential feature of secretory epithelial cells from exocrine glands. The surface of exocrine cells (a typical example is the acinar cell of the pancreas) is separated into an apical domain, where secretion occurs by exocytosis, and a basolateral domain, which senses variations of the internal milieu and is enriched with receptors for various hormones and secretagogues. It is unknown whether secretion is polarized in endocrine cells (except for thyroid follicular cells, which are organized into cavitary structures). To determine whether distinct plasma membrane domains exist in endocrine cells, we infected monolayer cultures of pancreatic endocrine cells with enveloped RNA viruses known to bud selectively from either the apical or basolateral domain in polarized epithelial cells. This asymmetrical budding is thought to reflect the polarized nature of the infected cells, as in non-polarized cells such as fibroblasts, the same viruses bud nonselectively from the entire cell surface. We show here that influenza virus and vesicular stomatitis virus (VSV) emerge asymmetrically from cultured pancreatic islet cells; this represents the first evidence for polarization of plasma membrane domains in pancreatic endocrine cells.

Pancreatic somatostatin

This is a review of pancreatic somatostatin which is limited in its scope and therefore focuses upon some selected issues. Throughout the entire review the same basic questions recur: Why do islets contain somatostatin? What is the physiological role of somatostatin and what does this peptide have to do with diabetes? Clear answers to these questions do not emerge, but a number of hunches are explored. The review provides a very brief look at somatostatin secretion, a discussion of the potential interactions which islet D cells might have with other islet cell types, consideration of how knowledge of islet anatomy may help us understand the D cell, and finally some comments about what happens to the D cell in diabetes and fasting.