Somatostatin analogs inhibit somatostatin release

Urocortin3: Local inducer of somatostatin release and bellwether of beta cell maturity

Urocortin 3 (UCN3) is a peptide hormone expressed in pancreatic islets of Langerhans of both human alpha and human beta cells and solely in murine beta cells. UCN3 signaling acts locally within the islet to activate its cognate receptor, corticotropin releasing hormone receptor 2 (CRHR2), which is expressed by delta cells, to potentiate somatostatin (SST) negative feedback to reduce islet cell hormone output. The functional importance of UCN3 signaling in the islet is to modulate the amount of SST tone allowing for finely tuned regulation of insulin and glucagon secretion. UCN3 signaling is a hallmark of functional beta cell maturation, increasing the beta cell glucose threshold for insulin secretion. In doing so, UCN3 plays a relevant functional role in accurately maintaining blood glucose homeostasis. Additionally, UCN3 acts as an indicator of beta cell maturation and health, as UCN3 is not expressed in immature beta cells and is downregulated in dedifferentiated and dysfunctional beta cell states. Here, we review the mechanistic underpinnings of UCN3 signaling, its net effect on islet cell hormone output, as well as its value as a marker for beta cell maturation and functional status.

Input-output signal processing plasticity of vagal motor neurons in response to cardiac ischemic injury

Vagal stimulation is emerging as the next frontier in bioelectronic medicine to modulate peripheral organ health and treat disease. The neuronal molecular phenotypes in the dorsal motor nucleus of the vagus (DMV) remain largely unexplored, limiting the potential for harnessing the DMV plasticity for therapeutic interventions. We developed a mesoscale single-cell transcriptomics data from hundreds of DMV neurons under homeostasis and following physiological perturbations. Our results revealed that homeostatic DMV neuronal states can be organized into distinguishable input-output signal processing units. Remote ischemic preconditioning induced a distinctive shift in the neuronal states toward diminishing the role of inhibitory inputs, with concomitant changes in regulatory microRNAs miR-218a and miR-495. Chronic cardiac ischemic injury resulted in a dramatic shift in DMV neuronal states suggestive of enhanced neurosecretory function. We propose a DMV molecular network mechanism that integrates combinatorial neurotransmitter inputs from multiple brain regions and humoral signals to modulate cardiac health.

Alpha-, Delta- and PP-cells: Are They the Architectural Cornerstones of Islet Structure and Co-ordination?

Islet non-β-cells, the α- δ- and pancreatic polypeptide cells (PP-cells), are important components of islet architecture and intercellular communication. In α-cells, glucagon is found in electron-dense granules; granule exocytosis is calcium-dependent via P/Q-type Ca(2+)-channels, which may be clustered at designated cell membrane sites. Somatostatin-containing δ-cells are neuron-like, creating a network for intra-islet communication. Somatostatin 1-28 and 1-14 have a short bioactive half-life, suggesting inhibitory action via paracrine signaling. PP-cells are the most infrequent islet cell type. The embryologically separate ventral pancreas anlage contains PP-rich islets that are morphologically diffuse and α-cell deficient. Tissue samples taken from the head region are unlikely to be representative of the whole pancreas. PP has anorexic effects on gastro-intestinal function and alters insulin and glucagon secretion. Islet architecture is disrupted in rodent diabetic models, diabetic primates and human Type 1 and Type 2 diabetes, with an increased α-cell population and relocation of non-β-cells to central areas of the islet. In diabetes, the transdifferentiation of non-β-cells, with changes in hormone content, suggests plasticity of islet cells but cellular function may be compromised. Understanding how diabetes-related disordered islet structure influences intra-islet cellular communication could clarify how non-β-cells contribute to the control of islet function.

Somatostatin receptor subtypes in human type 2 diabetic islets

OBJECTIVES

Somatostatin inhibits hormone release through 5 G protein-coupled somatostatin receptors (sst1-sst5). However, the role of somatostatin in islet physiology is not fully known. The immunoreactivity to sst1 to sst5 in normal human endocrine pancreas has been described. The present study reports the expression of sst1 to sst5 in human pancreatic islets with type 2 diabetes mellitus.

METHODS

Pancreatic autopsy specimens from individuals with type 2 diabetes mellitus and matched controls were double immunostained to demonstrate sst1 to sst5 in the major islet cell types.

RESULTS

Most apparent differences in type 2 diabetic islets were the lack of sst1 and sst4 in glucagon cells and sst1-3 and 4 in somatostatin cells, whereas minor changes were demonstrated in insulin cells. The pancreatic polypeptide cells showed a reversed staining pattern in diabetic islets compared with the controls.

CONCLUSIONS

In type 2 diabetes mellitus, the sst pattern differed from that of the controls in somatostatin, pancreatic polypeptide, and glucagon cells, to a minor extent in insulin cells. It is unclear whether the changes in sst patterns are primarily due to the diabetes or secondary to metabolic disturbances. However, this study may be the basis for further functional studies to evaluate the role of sst1 to sst5 in the diabetic state.

Cholecystokinin and Somatostatin Negatively Affect Growth of the Somatostatin-RIN-14B Cells

With the exclusive presence of the pancreatic CCK-2 receptors on the pancreatic delta cells of six different species, this study was undertaken to determine the role of cholecystokinin and gastrin on growth of these somatostatin (SS) cells. For this study, the SS-RIN-14B cells were used in culture and their growth was evaluated by cell counting. Results. To our surprise, we established by Western blot that these RIN cells possess the two CCK receptor subtypes, CCK-1 and CCK-2. Occupation of the CCK-1 receptors by caerulein, a CCK analog, led to inhibition of cell proliferation, an effect prevented by a specific CCK-1 receptor antagonist. Occupation of the CCK-2 receptors by the gastrin agonist pentagastrin had no effect on cell growth. Proliferation was not affected by SS released from these cells but was inhibited by exogenous SS. Conclusions. Growth of the SS-RIN-14B cells can be negatively affected by occupation of their CCK-1 receptors and by exogenous somatostatin.

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.

sst1 mRNA is the prominent somatostatin receptor mRNA in the rat gastrointestinal tract: reverse transcription polymerase chain reaction and in situ-hybridization study

The inhibitory peptide hormone somatostatin and its receptors (sst1-sst5) regulate many physiological functions in the gastrointestinal tract. In an attempt to correlate the various effects of somatostatin in gastrointestinal physiology to individual sst subtypes sst1-sst5, mRNAs have been localized by semiquantitative reverse transcription polymerase chain reaction amplification and in situ hybridization of sst1 and sst3 in the rat alimentary tract. sst1-sst4 mRNAs were found throughout the gastrointestinal tract, sst1 mRNA being more abundant than sst2 and much more abundant than sst3 and sst4 mRNAs. sst5 transcripts were at the detection threshold. sst1 and sst3 mRNAs are present in enterocytes and enteric neurons suggesting a role of these subtypes in the somatostatin-mediated inhibition of acetylcholine release from myenteric neurons and of secretomotor neuron activity in the submucous plexus. The presence of sst3 mRNA in smooth muscle cells points to an additional role of this receptor in regulating gut motility.

Expression of NMDA receptor-1 (NR1) and huntingtin in striatal neurons which colocalize somatostatin, neuropeptide Y, and NADPH diaphorase: a double-label histochemical and immunohistochemical study

The subset of striatal neurons which colocalize SS/NPY/NADPH-d are selectively resistant to neurodegeneration in Huntington's Disease (HD) and to excitotoxic cell death induced experimentally with NMDA receptor (NMDAR) agonists. Here we have analyzed the expression of immunoreactive NMDAR-1 (NR1) subunit (as an index of NMDAR protein) and of huntingtin (the normal product of the HD gene) in primary cultures of rat striatum to see if differential expression of the two antigens in the subset of SS/NPY/NADPH-d and other striatal neurons can explain their selective resistance or vulnerability. Double-label histochemical and immunocytochemical studies were carried out using conventional and confocal laser scanning microscopy to characterize the cellular and subcellular expression of NR1 and SS, or NPY or bNOS, together with NADPH-d histochemistry. The percentages of cultured striatal neurons that were positive for NADPH-d, SS, NPY, bNOS, and NRI were, respectively, 3.8, 8.4, 10.2, 5.1, and 80%. The majority of striatal NADPH-d neurons coexpressed SS and NPY; 17% of SS-producing neurons were strongly positive for NR1; the remaining cells (approximately 80%) exhibited only weak NR1 expression. Comparable data were obtained for NPY-positive neurons, 15% of which colocalized NR1 strongly and 70-80% weakly. By double-label immunofluorescence, huntingtin was nonselectively expressed in virtually all striatal neurons including SS/NPY/NADPH-d neurons. These results show that the majority of striatal SS/NPY/NADPH-d neurons express NR1. The relative abundance of NR1 in SS/NPY/NADPH-d neurons, however, varies between a small subset of neurons that are receptor rich and the remainder that express low levels only and may determine susceptibility to NMDAR-mediated neurotoxicity. Huntingtin is nonselectively expressed in virtually all striatal neurons and does not appear to be a determinant of the selective resistance of normal striatal SS/NPY/NADPH-d neurons to NMDA toxicity.

Secretory and biosynthetic responses of gastrin and somatostatin to acute changes in gastric acidity

The activity of gastric parietal cells in terms of hydrochloric acid (HCl) secretion is regulated by the interaction of stimulatory substances (e.g. gastrin) and inhibitors (e.g. somatostatin) acting in an endocrine and paracrine mode, as well as luminal factors. In the present study the following parameters were measured: the synthesis (mRNA), storage (tissue peptide concentration) and secretion (plasma peptide concentration) of somatostatin and gastrin following short-term treatment of rats with pentagastrin (acid stimulant), secretin, omeprazole (reduces gastric acidity by inactivating gastric H/K ATPase) and the somatostatin analogue octreotide (reduces gastric acidity by inhibiting both the parietal cell and gastrin). The mRNA coding for H/K ATPase and carbonic anhydrase II (CA II), the two enzymes responsible for the generation of hydrogen ions from the parietal cell, were also quantitated. In response to octreotide, somatostatin peptide and mRNA levels in the fundus rose to 180 +/- 16% (P < 0.001) and 1073 +/- 356% (P < 0.05) of control, respectively. In contrast, octreotide caused a decrease in antral somatostatin peptide and its mRNA did not change significantly. No significant changes in synthesis, secretion or storage of gastrin were observed except for omeprazole induced hypergastrinaemia (580 +/- 76%, P < 0.001). H/K ATPase and CA II mRNA were largely unaffected except for an increase in CA II mRNA following octreotide and a decrease in H/K ATPase mRNA after pentagastrin. These data support the concept of the differential control of antral and fundic somatostatin synthesis and provide evidence for a regulatory loop by which somatostatin can influence its own synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional characterization of somatostatin receptors expressed on hamster glucagonoma cells

We characterized somatostatin receptors expressed in hamster glucagonoma INR1G9 cells and the effects of somatostatin on glucagon secretion, proglucagon gene expression, and the adenosine 3',5'-cyclic monophosphate (cAMP)-dependent signal-transduction cascade. 125I-labeled somatostatin was displaced by somatostatin-14 and somatostatin-28 with a dissociation constant of 2 nmol/l. Stable GTP analogues decreased binding of 125I-somatostatin to its receptors, suggesting an interaction of somatostatin receptors with G proteins. Chemical cross-linking of 125I-somatostatin to its receptor revealed a molecular mass of the ligand-receptor complex of 47 kDa. Somatostatin inhibited forskolin-stimulated activation of adenylate cyclase [2.5 microM forskolin (161%) + 1 microM somatostatin (128%); P < 0.05] and protein kinase A [10 microM forskolin (143%) + 1 microM somatostatin (114%); P < 0.05] but did not influence basal activities of these enzymes. Forskolin-induced stimulation of cAMP generation was reduced by somatostatin [2.5 microM forskolin (306%) + 1 microM somatostatin (145%); P < 0.05]. Somatostatin inhibited forskolin, theophylline, and arginine stimulation of glucagon secretion. Basal as well as forskolin-, theophylline-, and isobutyl methylxanthine-induced proglucagon gene expression was significantly reduced by somatostatin. Our data show that, in INR1G9 cells, somatostatin receptors are at least in part coupled to the adenylate cyclase system. Somatostatin is a potent negative regulator of both basal and forskolin-stimulated proglucagon gene expression. The interaction with forskolin occurs at the level of adenylate cyclase. The effect of somatostatin on basal proglucagon gene transcription is most probably mediated by an unrelated second messenger system. Somatostatin may influence several functions of the pancreatic A cell.

Molecular pharmacology of somatostatin receptors

The neuropeptide somatostatin (SRIF) is widely expressed in the brain and in the periphery in two main forms, SRIF-14 and SRIF-28. Similarly, the presence of SRIF receptors throughout the whole body has been reported. SRIF produces a variety of effects including modulation of hormone release (e.g. GH, glucagon, insulin), of neurotransmitter release (e.g. acetylcholine, dopamine, 5-HT), and its own release is modulated by many neurotransmitters. SRIF affects cognitive and behavioural processes, the endocrine system, the gastrointestinal tract and the cardiovascular system and also has tumor growth inhibiting effects. Initially, two classes of SRIF receptors have been proposed on the basis of biochemical and functional studies. However, the recent cloning of five putative SRIF receptor subtypes which belong to the G-protein coupled receptor superfamily suggests that SRIF mediates its various effects via a whole family of receptors. Here we review, in this new context, the molecular pharmacology of the SRIF receptor subtypes present in the brain and in the periphery, and address the question of nomenclature of SRIF receptors.

Cysteamine-induced reduction in tissue somatostatin immunoreactivity is associated with alterations in somatostatin mRNA

The drug cysteamine (CHS) induces a profound loss of somatostatin-14 (SS-14) biological and immunological (SS-14 LI) activity from somatostatin cells in vivo and in vitro. The present study was designed to determine (i) whether CHS induced loss of somatostatin is accompanied by secondary increases in SS-mRNA perhaps through loss of autoinhibition of somatostatin cells; (ii) whether CHS exerts additional direct effects on SS gene regulation. CHS was administered to rats in vivo or applied in vitro to primary cultures of rat islet cells, rat islet somatostatin-producing tumor cells (1027 B2), and endogenous or in vitro synthesized SS-mRNA. In vivo administration of CHS led to 80% reduction in tissue SSLI by 4 h. These changes were accompanied by significant alterations in SS-mRNA that were both tissue-specific and time-dependent. The pattern in brain and intestine was typified by a significant 60% increase in SS-mRNA at 2 h followed by a gradual reduction to approximately 55% of control at 8 h. Stomach showed a significant 95% increase in SS-mRNA at 4 h followed by a 37% decrease by 8 h. Pancreatic SS-mRNA displayed a sustained 25-65% reduction for 8 h. Pretreatment of islet cell cultures with CHS reproduced the in vivo findings with pancreas viz. decreased SSLI (80-90% of control) accompanied by a parallel reduction in SS-mRNA (40-50% of control) sustained from 2-72 h. CHS also induced a reduction in immunoreactive insulin and insulin mRNA in cultured islet cells. As with normal islet cells, CHS treatment of 1027 B2 islet tumor cells led to a profound and sustained decrease in SSLI and SS-mRNA. These changes occurred in the absence of any alteration in intracellular cAMP levels. CHS was without effect when incubated directly with SS-mRNA isolated from 1027 B2 cells or with in vitro synthesized SS-mRNA. We conclude that in addition to its effect on SSLI, CHS also induces time- and tissue-dependent alterations in SS-mRNA. The mechanism of CHS action on SS-mRNA is complex and may involve both an indirect effect secondary to loss of somatostatin autoinhibition (to account for SS-mRNA increases) and/or a direct inhibition of SS gene expression (to explain SS-mRNA reduction). The precise site of direct CHS action on SS gene regulation remains to be defined.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence against autocrine feedback regulation of cholecystokinin secretion in man

To determine whether exogenous cholecystokinin (CCK) inhibits endogenous CCK release, cholecystokinin-8S (CCK-8S) was infused intravenously during continuous intraduodenal stimulation of endogenous CCK by a meal. CCK was measured in plasma by 2 region-specific radioimmunoassays employing antibodies T204 and 1703. AB T204 binds to carboxy-terminal CCK peptides containing the sulphated tyrosyl region, including CCK-8S, and AB 1703 to carboxy-terminal CCK peptides containing at least 14 amino acid residues. Meal-stimulated plasma CCK concentrations remained elevated during the entire infusion period. CCK-8S infusion further increased meal-stimulated plasma CCK concentrations, when measured with AB T204, while meal-stimulated plasma CCK concentrations were not suppressed by CCK-8S infusion, when measured with AB 1703. It is concluded that meal-stimulated endogenous CCK release is not affected by the effects of intravenously administered CCK-8S. These data suggest that autocrine feedback regulation of CCK release is not operative in man.

The endocrine pancreas of glucagon- and somatostatin-immunized rabbits. II. Electron microscopy

An active or passive immunization against hormones and the subsequent neutralization of hormones by circulating antibodies is a valuable tool for the identification of hormonal action. To recognize presumed local (autocrine, paracrine) effects exerted by pancreatic hormones, the endocrine pancreas of rabbits was investigated electron-microscopically after long-term immunization against glucagon or somatostatin. Glucagon immunization resulted in hyperplasia and hypertrophy of glucagon- (A-) cells and in their increased metabolic activities: They showed prominent nucleoli, increased amounts of endoplasmic reticulum, Golgi areas, and mitochondria. These changes were paralleled by alterations in secretion granules (increased size, decreased hormonal content), increased numbers of lysosomes (crinophagic bodies), and an increment of the filamentous system. Basically, these findings point to an autocrine regulation of A-cells. Following somatostatin immunization, somatostatin- (D-) cells were hyperplastic but unchanged in their metabolic state. Instead, insulin-(B-) cells and A-cells exhibited equivalents of increased cellular activities (parameters, see above). This stimulation most probably is caused by cancelled paracrine (inhibitory) effects of somatostatin. The changes observed after both immunizations were differently expressed in morphologically heterogeneous islet types (size, angioarchitecture, cellular composition, microtopology of the various cell types). It is concluded, therefore, that the regulation of islets is not uniform. Autocrine and paracrine effects exerted by islet hormones are of different significance in individual islets, or they interfere differently with other regulatory signals.

Experience in rat pancreas transplantation at Meikai University

A review of rat pancreatic transplantation conducted at Meikai University is presented. It was found that pancreas transplantation normalized the endocrine based metabolic disturbances of diabetes. Arginine-induced serum insulin, glucagon, and somatostatin responses in the grafted pancreas were similar to those in normal pancreas. Urine amylase was found to be a more sensitive marker in graft rejection as compared with blood glucose using a urinary drainage model. The value of pancreas transplantation in diabetic nephropathy was dependent upon the timing of the transplantation; performed early in the course of diabetes (less than 4 months, post-diabetes), it was able to reverse the nephropathy.

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.

Selective binding of somatostatin-14 and somatostatin-28 to islet cells revealed by quantitative electron microscopic autoradiography

Quantitative electron microscopic autoradiography was used for comparing the binding of labeled somatostatin-14 (S-14) and somatostatin-28 (S-28 section) to islet cells. Monolayer cultures of rat islet cells were incubated with [125I-Tyr11]S-14 (S-14 section) or [125I-Leu8, D-Trp22, Tyr25]S-28 (S-28 section) in the presence or absence of excess unlabeled peptides. Autoradiographic grains (ARG) associated with individual islet cells were identified and expressed as the mean number per B, A, and D cells. Specific ARG associated with S-14 were found over B and A cells. S-28 section-related specific ARG were concentrated over B, A, as well as D cells. The highest density of S-14 section labeling occurred over A cells, which under conditions of maximum labeling (37 degrees C for 60 min) contained five times as many ARG as did B cells. By contrast, under the same incubation conditions, the labeling density with S-28 section was maximal over B cells, which contained four and five times as many grains as A and D cells, respectively. These observations show preferential association of S-14 section with the A cell and S-28 section with the B cel provide strong evidence for the existence of separate binding sites for S-14 section and S-28 section on A and B cells, respectively, which presumably mediate the previously reported glucagon selective inhibitory effect of S-14 and the insulin-selective action of S-28.

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.

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.

Evidence of a paracrine role for pancreatic somatostatin in vivo

Somatostatin (SS) in the D cells of the pancreatic islets has been hypothesized to tonically inhibit the secretion of glucagon and insulin from the neighboring A and B cells. To test this hypothesis directly, a nonimmunoreactive analogue of somatostatin [( D-Ala5-D-Trp8]SS) was infused intravenously at 0.55-17 micrograms/min into anesthetized dogs to suppress the secretion of pancreatic somatostatin and observe the effects of that suppression on glucagon and insulin release. Infusions of this analogue into anesthetized dogs at both a low dose (1.7 micrograms X min-1 X 30 min iv, n = 7) and at a medium dose (5.5 micrograms X min-1 X 30 min iv, n = 7) suppressed the release of somatostatin-like immunoreactivity (SLI) from the in situ canine pancreas by 31 +/- 10% of base line (P less than 0.025) and 45 +/- 6% of base line (P less than 0.0005), respectively. These doses increased glucagon secretion markedly (by 179 +/- 39 and 250 +/- 60% of base line, both P less than 0.005) and increased insulin secretion moderately (by 35 +/- 17 and 62 +/- 27% of base line, respectively, both P less than 0.05). The highest dose of analogue (17 micrograms/min, n = 9) produced less stimulation of glucagon release (delta = +95 +/- 35% of basal, P less than 0.025) and marked inhibition of insulin release (delta = -61 +/- 9% of basal, P less than 0.0005) despite a larger inhibition of pancreatic SLI release (delta = -84 +/- 3% of basal, P less than 0.0005).(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative electron microscopic autoradiography of insulin, glucagon, and somatostatin binding sites on islets

After monolayer cultures of rat islets were exposed to [(125)I]insulin,[(125)I]glucagon, and [(125)I]tyrosinyl somatostatin, specific autoradiographic grains associated with each radioactively labeled ligand were found on B, A, and D cells. The density of labeling of the B, A, and D cells with each labeled ligand correlated well with the known actions of the three hormones on each of the islet cells.

Dose-dependent inhibitory and non-inhibitory action of somatostatin on insulin release in rats

The dose-dependent effect of intravenously infused synthetic somatostatin-14 on basal and postprandial insulin and gastrin release was assessed in anesthetized rats. Infusion of 1 ng . kg-1 . min-1 elicited a significant reduction of basal and postprandial insulin levels compared to the saline control groups. At 15 ng . kg-1 . min-1 basal insulin was not affected but postprandial insulin levels were still significantly reduced. At 30 ng . kg-1 . min-1 neither basal nor stimulated insulin levels were affected. At the highest concentration of 120 ng . kg-1 . min-1 basal and postprandial insulin levels were suppressed similar to lowest infusion rate of 1 ng . kg-1 . min-1. Basal gastrin levels were significantly reduced only at the highest rate of 120 ng . kg-1 . min-1. A significant reduction of postprandial gastrin levels was observed at 15 ng . kg-1 . min-1 and all higher infusion rates employed. Measurements of plasma somatostatin-like immunoreactivity (SLI) demonstrated that plasma SLI levels during the lowest infusion rate of 1 ng . kg-1 . min-1 were not different from the controls. No significant rise of plasma SLI levels was observed in response to the test meal. The higher infusion rates elicited a dose-dependent increase in plasma SLI levels. These data demonstrate that in rats somatostatin exerts a biological effects on insulin release at very low doses while certain greater infusion rates hae no suppressive effect. Gastrin secretion is inhibited in a more linear pattern.

Coordinate, equimolar secretion of smaller peptide products derived from pro-ACTH/endorphin by mouse pituitary tumor cells

The secretion of peptide products derived from pro-ACTH/endorphin was examined with several radioimmunoassays and with polyacrylamide gel analyses of immunoprecipitates of radioactively labeled peptides. In studies using a mouse pituitary tumor cell line the accumulation of each of the four molecular forms of adrenocorticotropic hormone (ACTH) in tissue culture medium was shown to be a linear function of time. No evidence for self inhibition of secretion by accumulated, secreted peptides (i.e., ultra-short feedback) was found. Furthermore, synthetic human ACTH and synthetic camel beta-endorphin did not alter secretion of peptides when added to the culture medium at levels up to 10,000 times physiological. Stimulation of the release of ACTH-, endorphin-, lipotropin-, and 16k fragment immunoreactive material by norepinephrine was fully blocked by cobalt; by this criterion, stimulated release was calcium dependent. All the smaller molecules derived from the pro-ACTH/endorphin common precursor were secreted in equimolar amounts under all circumstances tested, within the precision of these studies (+/- 11%). Norepinephrine and cobalt did not significantly alter the secretion of pro-ACTH/endorphin and ACTH biosynthetic intermediate. The stimulation of secretion by norepinephrine and inhibition of secretion by cobalt was restricted to the lower molecular weight products derived from pro-ACTH/endorphin: glycosylated and nonglycosylated ACTH(1-39); beta-lipotropin, beta-endorphin, and gamma-lipotropin; and 16k fragment.

Paradoxical elevation of growth hormone by intraventricular somatostatin: possible ultrashort-loop feedback

Somatostatin, the growth hormone-inhibiting factor, when microinjected into the third ventricle of the rat brain, paradoxically induced the release of growth hormone. A pituitary site of action having been ruled out, this result supports the concept that exogenous somatostatin within the hypothalamus acts either to suppress the release of somatostatin from somatostatin-containing neurons, possibly via an ultrashort-loop feedback mechanism, or to augment release of hypothalamic growth hormone-releasing factor, thereby inducing a release of growth hormone. Injection of somatostatin into the third ventricle also decreased plasma concentrations of luteinizing hormone, follicle-stimulating hormone, and thyroid-stimulating hormone, probably by inhibiting the release of luteinizing hormone-releasing factor and thyrotropin-releasing factor.