Effects of prostaglandin E2 and D2 on gastric somatostatin and gastrin secretion

The effects of PGE2 and PGD2 on gastric somatostatin and gastrin releases were investigated using the isolated perfused rat stomach. In the presence of 5.5 mM glucose, the infusion of PGE2 elicited a significant augmentation in somatostatin release, but suppressed gastrin secretion from the perfusate. On the other hand, PGD2 did not affect somatostatin release, although the gastrin secretion decreased significantly, the same as after PGE2 infusion. These results suggest that PGE2 and PGD2 may be important in the regulation of gastric endocrine function, but that PGD2 does not affect gastric somatostatin secretion.     

The effect of somatostatin on release and insulinotropic action of gastric inhibitory polypeptide.

Studies were carried out in conscious dogs in which the effect of intravenous somatostatin on immunoreactive gastric inhibitory polypeptide (IR-GIP) release was investigated. In addition, the inhibitory action of somatostatin on the insulin response to pure porcine GIP was assessed. Intravenous administration of somatostatin resulted in a delayed IR-GIP and immunoreactive insulin (IRI) response to oral glucose. Somatostatin also delayed the IR-GIP response to the ingestion of fat. In both types of experiments, initial depression of IRI levels was followed by a sharp rise in IRI release. Intravenous infusion of somatostatin produced 80% inhibition of the IRI response to pure porcine GIP. It was concluded that somatostatin inhibits the physiological release of IR-GIP and the insulinotropic action of exogenous porcine GIP.     

Acute and transient effects of stress on immunoreactive somatostatin cell bodies and fibers in the preoptic-anterior hypothalamus and median eminence of female rats

Stress in rats causes acute release of hypothalamic somatostatin (SS) in median eminence (ME) that induces a marked and prolonged suppression of growth hormone (GH) secretion. This was evidenced by immunocytochemistry (ICC) and radioimmunoassay (RIA) in the present study. Adult female rats were decapitated under nonstress or for 30, 60, 120 and 180 min after 15 min leg restraint stress. The rabbit anti-SS was used to detect SS-14 and SS-28 containing cell bodies with ICC in preoptic-anterior hypothalamus (PO-AH). At 30, 60, 120 min after stress, there was marked decrease in the number and size of subsets of SS cell bodies. RIA demonstrated striking increase in SS in ME and significant decrease in GH of the portal blood. The most reproducible changes in cell bodies involved subsets of PeV neurons. Interestingly, these changes were largely reversed by 180 min. The results of the study demonstrate that stress cause acute changes in PO-AH, SS system and it appears that stress affects both SS synthesis and the secretion.     

Evaluation ofin vivo insulin action and glucose metabolism in milk-fed rats

Milk diet has long been recommended in the management of gastrointestinal pathologies. Since milk feeding represents a high fat-low carbohydrate diet and it is acknowledged that insulin resistance is one of the consequences of high fat feeding, it is important to know whether or not chronic milk feeding leads to an impairment of the insulin-mediated glucose metabolism. To examine this question, adult female rats were given raw cow's milk (50% of total calories as lipids) for 18 days. They were compared to rats raised in parallel and fed the standard laboratory diet (15% of total calories as lipids). At the end of the 18 day period, body weight, daily caloric intake, basal plasma glucose and insulin levels in the milk-fed rats were similar to those in the control rats.In vivo insulin action was assessed with the euglycemichyperinsulinemic clamp technique in anesthetized animals. These studies were coupled with the 2-deoxyglucose technique allowing a measurement of glucose utilization by individual tissues. In the milk fed rats: 1) the basal rate of endogenous glucose production was significantly (p<0.01) reduced (by 20%); 2) their hepatic glucose production was however normally suppressed by hyperinsulinemia; 3) their basal glucose utilization rate was significantly (p<0.01) reduced (by 20%); 4) their glucose utilization rate by the whole-body mass or by individual tissues was normally increased by hyperinsulinemia. These results indicate that insulin action in adult rats is not grossly altered after chronic milk-feeding, at least under the present experimental conditions.     

Regulatory effects of protein S-acylation on insulin secretion and insulin action

Post-translational modifications (PTMs) such as phosphorylation and ubiquitination are well-studied events with a recognized importance in all aspects of cellular function. By contrast, protein S-acylation, although a widespread PTM with important functions in most physiological systems, has received far less attention. Perturbations in S-acylation are linked to various disorders, including intellectual disability, cancer and diabetes, suggesting that this less-studied modification is likely to be of considerable biological importance. As an exemplar, in this review, we focus on the newly emerging links between S-acylation and the hormone insulin. Specifically, we examine how S-acylation regulates key components of the insulin secretion and insulin response pathways. The proteins discussed highlight the diverse array of proteins that are modified by S-acylation, including channels, transporters, receptors and trafficking proteins and also illustrate the diverse effects that S-acylation has on these proteins, from membrane binding and micro-localization to regulation of protein sorting and protein interactions.     

Evidence for a role of endogenous opiates in postprandial somatostatin release

Previously, we have demonstrated the effects of exogenously administered opiates on somatostatin release in dogs and therefore the present study was designed to determine the effect of endogenous opiates via naloxone-induced opiate receptor blockade on somatostatin release. Additionally, plasma insulin and pancreatic polypeptide (PP) levels were determined in response to intragastrically instilled protein, carbohydrate and fat test meals in a group of eight conscious dogs. To all test meals either naloxone (4 mg) or saline was added. The rise of plasma somatostatin levels in response to liver extract, sucrose and fat was attenuated significantly by naloxone. Naloxone had no effect on the rise of postprandial plasma insulin and PP levels. The present data demonstrate that endogenous opiates have a stimulatory effect on postprandial somatostatin release in dogs which indicates a tight interaction that might be of relevance for nutrient homeostasis.     

Modulation of acetylcholine-induced secretion of gastric bombesin-like immunoreactivity by cholinergic and histamine H2-receptors, somatostatin and intragastric pH

Recently we have shown the release of bombesin-like immunoreactivity (BLI) from the isolated perfused rat stomach. In these experiments we have shown that BLI secretion is stimulated by acetylcholine. Gastric inhibitory peptide (GIP) exerts an inhibitory effect which is dependent on the intraluminal pH. The present study was designed to examine further the exact cholinergic mechanisms and to study the interaction between cholinergic and histaminergic mechanisms as well as the effect of the intraluminal pH. Acetylcholine elicited a dose-dependent increase in BLI and gastrin secretion (10(-6) M and 2 X 10(-6)M), whereas somatostatin release was suppressed at luminal pH 7. Blockade of muscarinic cholinergic receptors by atropine (10(-5)M) and nicotinic cholinergic receptors by hexamethonium (10(-5) M) abolished the effect of acetylcholine on all three peptides. Reduction of the intraluminal pH to 2 also abolished acetylcholine-induced stimulation of BLI and gastrin secretion and the inhibition of somatostatin secretion. Changes of intraluminal pH per se had no effect on the secretion of either peptide. Somatostatin (10(-7) M) reduced both BLI and gastrin secretion during stimulation with acetylcholine. The addition of the H2-receptor antagonist cimetidine (10(-5) M) abolished the effect of both doses of acetylcholine on BLI and somatostatin secretion and also the effect of the lower dose of acetylcholine (10(-6) M) on gastrin secretion during luminal pH 7. At luminal pH 2 cimetidine did not alter BLI and somatostatin secretion in response to acetylcholine, however, gastrin release was augmented in the presence of cimetidine. These data demonstrate that the effect of acetylcholine on BLI, gastrin, and somatostatin secretion is mediated by muscarinic and nicotinic cholinergic receptors and also by histamine H2-receptors. Somatostatin inhibits cholinergically induced BLI secretion. The cholinergic effects on BLI, somatostatin and gastrin secretion are abolished during an acidic intragastric pH. In this isolated perfused rat stomach model the inhibitory effect of intraluminal acid on gastrin secretion is, at least in part, mediated by H2-receptors. This suggests that the secretion of bombesin, a potential peptidergic neurotransmitter is modulated by neural, endocrine and local tissue factors and also by alterations of intragastric pH.     

Fluidity of insulin action

Unlike the intensive research in pursuit of understanding the molecular mechanisms of insulin signaling and resistance to its biological action associated most significantly with obesity and type 2 diabetes, the influence of the plasma membrane on insulin sensitivity has been intermittently studied over the years—mainly because it was thought that mediators of insulin action, such as the insulin receptor and the insulin-responsive glucose transporter GLUT4, localize more or less uniformly in the lipids that form cell membranes. Recent insights into membrane physiology suggest that the plasma membrane impacts the function of membrane proteins mediating insulin action. Furthermore, membrane disturbances may be the basis of insulin resistance. Relevant insulin signal transduction data in terms of plasma membrane and insulin resistance are the focus of this review. The discussion visits the cell membrane hypothesis of insulin resistance that suggests insulin action could be related to changes in cell membrane properties.     

Effects of phorbol esters on insulin receptor function and insulin action in hepatocytes: evidence for heterogeneity

We investigated the effect of phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator on insulin receptors and insulin action in freshly isolated and primary cultures of rat hepatocytes. PMA (1 x 10^–7 M) did not alter insulin receptor numbers or affinity either acutely or chronically but within 60 minute inactivated insulin stimulated tyrosine kinase of the insulin receptor. PKC activation inhibitied insulin (1 x 10^–7M) stimulation of glycogen and lipid synthesis with a decrease or no change in basal glycogenesis and lipogenesis respectively. However, PKC activation did not alter insulin stimulated or basal amino acid transport even though PCK activation inhibited insulin stimulation of the insulin. receptor tyrosine kinase. Thus, within one tissue, PKC activation has differential effect on insulin action depending on which pathway is examined. Furthermore, insulin stimulation of the insulin receptor tyrosine kinase may not be a necessary step for all insulin signaling pathways.     

Insulin, glucagon and somatostatin localization in the pancreas of metamorphosed Xenopus laevis

The current study was designed to determine if insulin, glucagon and somatostatin-containing cells are present in the pancreas of adult Xenopus laevis. Localization methods utilized included cytochemical aldehyde fuchsin (AF) staining as well as the immunochemical peroxidase antiperoxidase (PAP) procedure for light microscopy. The results show numerous large clusters of AF-positive cells within a network of highly vascularized acinar tissue. PAP immunochemical localization with insulin antibody on adjacent sections demonstrates positive immunoreactivity to AF-positive cell groups and also the presence of immunoreactive insulin (IRI). Cells exhibiting this immunoreactivity are located in the central region of the islet-like structures. Serial sections not only show PAP immunoreactivity for IRI, but also for immunoreactive glucagon (IRG) and immunoreactive somatostatin (IRS) in the same islet-like structure. IRG and IRS-containing cells are situated around the periphery of the islet-like structures, surrounding the central core of IRI-containing cells. Antibody specificity was confirmed by homologous and heterologous antigen immuno-absorbance assays, as well as incubation of adjacent sections in preimmune sera. Based on this data we conclude that: the distribution of cells of the endocrine pancreas of metamorphosed Xenopus laevis is similar to that of many mammals and certain urodeles. Given the apparent specificity of the antigen-antibody reactions, it appears that Xenopus insulin, glucagon and somatostatin are structurally conserved.     

Unmasking of the inhibitory action of glucose on insulin release after alpha 2-adrenergic activation

Insulin release in response to glucose was measured after culture of islets from ob/ob-mice in a Ca2+-deficient medium. The stimulatory effect of 20 mM glucose disappeared after addition of 1 microM L-epinephrine, and it was reversed into inhibition when the medium contained 0.1 to 10 microM clonidine. Glucose inhibited insulin release also after activation of the alpha 2-adrenoceptors with B-HT 933, whereas blocking of these receptors with idazoxan removed glucose inhibition in the presence of clonidine. It is concluded that alpha 2-adrenergic activation provides an efficient means of unmasking the inhibitory component in the action of glucose on insulin release.     

Interaction of somatostatin and calcium in regulating insulin release from isolated pancreatic islets of rats.

The effect of synthetic somatostatin on insulin release was studied in vitro by using isolated islets of rats. Somatostatin, with concentrations from 10 ng/ml to 10μg/ml, inhibited insulin release induced by 16.7 mM glucose. Insulin release elicited by 10 μg/ml glucagon or 2 mM dibutyryl cyclic AMP was likewise inhibited by 100ng/ml somatostatin. By raising the calcium concentration of the incubation medium to 6 mM, glucose-induced insulin release was fully restored even in the presence of somatostatin.However, the same maneuver only partially counteracted the somatostatin inhibition of dibutyryl cyclic AMP-induced insulin release. These results suggest the involvement of calcium mobilization process in the inhibitory action of somatostatin.     

Association between dopamine and somatostatin receptor expression and pharmacological response to somatostatin analogues in acromegaly

Acromegaly is a hormonal disorder resulting from excessive growth hormone (GH) secretion frequently produced by pituitary adenomas and consequent increase in insulin‐like growth factor 1 (IGF‐I). Elevated GH and IGF‐I levels result in a wide range of somatic, cardiovascular, endocrine, metabolic and gastrointestinal morbidities. Somatostatin analogues (SSAs) form the basis of medical therapy for acromegaly and are currently used as first‐line treatment or as second‐line therapy in patients undergoing unsuccessful surgery. However, a considerable percentage of patients do not respond to SSAs treatment. Somatostatin receptors (SSTR1‐5) and dopamine receptors (DRD1‐5) subtypes play critical roles in the regulation of hormone secretion. These receptors are considered important pharmacological targets to inhibit hormone oversecretion. It has been proposed that decreased expression of SSTRs may be associated with poor response to SSAs. Here, we systematically examine SSTRs and DRDs expression in human somatotroph adenomas by quantitative PCR. We observed an association between the response to SSAs treatment and DRD4, DRD5, SSTR1 and SSTR2 expression. We also examined SSTR expression by immunohistochemistry and found that the immunohistochemical detection of SSTR2 in particular might be a good predictor of response to SSAs.     

Effect of bombesin and gastrin-releasing peptide on the release of gastric inhibitory polypeptide and insulin in rats

The objective of this study was to determine whether bombesin- or gastrin-releasing peptide-induced release of insulin occurs before or after the release of gastric inhibitory polypeptide (GIP) in rats. The present results demonstrate that GIP release occurs before insulin release and suggest that bombesin-like peptides and GIP interact to stimulate insulin secretion.     

Insulinotropic and gastrin-releasing action of gastrin-releasing peptide (GRP)

The effect of intravenous administration of gastrin-releasing peptide ( GRP ) on serum gastrin and insulin levels was studied in ad libitum fed and 24-h fasted rats. Administration of GRP (55 micrograms/kg body weight) caused a significant (P less than 0.05) elevation in serum gastrin levels at 10, 30, 60, and 120 min in the rats fed ad libitum, whereas in the fasted rats, gastrin levels rose significantly only at 10 min. GRP did not cause insulin release in fasted rats, but in the fed rats, it led to a significant elevation in serum insulin levels at 10 and 30 min, in comparison to controls. GRP appears to have an insulinotropic action in addition to a gastrin-releasing effect.     

猕猴发育过程中肠肝组织生长抑素及其受体表达演变规律

探讨在猕猴发育过程中生长抑素(somatostatin,SST)及其受体在肠肝组织的演变规律。通过手术途径获得胚胎6月、新生2 d、新生45 d和成年猕猴的回肠、肝脏、门静脉和外周血等标本。应用放射免疫分析法测定各标本中的SST含量; 通过免疫组化方法观察SST在肠、肝组织内的分布;利用原位杂交检测SST受体2(somatostatin receptor 2,SSTR2)的表达。结果显示:(1)胚胎6月的猕猴,小肠内SST含量为(27．3±16．6)ng／mg蛋白;黏膜隐窝处SST呈弱阳性染色,肌层 SST染色阴性。在发育过程中,小肠内SST含量逐渐增加,成年期时达最高水平(120．1±35．3)ng／mg蛋白,较胚胎6月显著增加(P<0．01)。(2)成年小肠黏膜隐窝处及肌间神经丛SST呈强阳性染色。(3)胚胎6月,小肠粘膜上皮可见大量SSTR2 表达,成年时SSTR2表达下调,且主要定位于腺上皮隐窝处;胚胎及新生期肌层SSTR2染色阴性,成年时小肠肌间神经丛则可见阳性SSTR2染色。(4)肝脏在发育过程中SST及SSTR2含量逐渐降低;发育的各个时期,小肠组织的SST含量均明显高于肝脏组织含量,门静脉SST水平也始终高于外周血。总之,位于小肠黏膜隐窝处的SST和SSTR2随着发育逐渐增加,来自肠道的SST进入门静脉后迅速被降解。SST阳性的肠肌间神经丛仅在发育成熟后才出现。     

生长抑素受体2的小鼠时空组织表达谱

生长抑素(somatostatin, SST)作为一种抑制性多肽激素,在多种生物过程中发挥重要的功能。生长抑素受体2 (somatostatin receptor 2, SSTR2)作为生长抑素表达最广泛的受体在多种组织中表达,但其表达的具体细胞类型尚不清楚。本研究在小鼠不同发育阶段的多种组织中鉴定了SSTR2蛋白表达的细胞类型。通过多色免疫荧光在小鼠胚胎期15.5 d、出生后1 d、7 d、15 d、3个月和6个月的脑、骨、肺、肠道、皮肤、胃、脾和肾等组织中检测了Sstr2基因的表达。结果发现Sstr2在不同发育阶段的多种组织的特定细胞类型中表达,包括脑神经元和星形胶质细胞,骨的间充质基质细胞、造血细胞和B细胞,肺的巨噬细胞、Ⅱ型肺泡上皮细胞和气道纤毛细胞,肠道的上皮细胞和神经元,皮肤的毛囊细胞,胃体的上皮细胞,脾的造血干细胞、造血祖细胞和神经纤维,肾的肾小管上皮细胞等。本研究确定了小鼠多组织不同发育阶段Sstr2表达的细胞类型,为生长抑素与生长抑素受体2的生理功能研究提供了新的线索。     

Protein phosphatase-1 and insulin action

Protein Phosphatase-1 (PP-1) appears to be the key component of the insulin signalling pathway which is responsible for bridging the initial insulin-simulated phosphorylation cascade with the ultimate dephosphorylation of insulin sensitive substrates. Dephosphorylations catalyzed by PP-1 activate glycogen synthase (GS) and simultaneously inactivate phosphorylase a and phosphorylase kinase promoting glycogen synthesis. Our in vivo studies using L6 rat skeletal muscle cells and freshly isolated adipocytes indicate that insulin stimulates PP-1 by increasing the phosphorylation status of its regulatory subunit (PP-1_G). PP-1 activation is accompanied by an inactivation of Protein Phosphatase-2A (PP-2A) activity. To gain insight into the upstream kinases that mediate insulin-stimulated PP-1_G phosphorylation, we employed inhibitors of the ras/MAPK, PI3-kinase, and PKC signalling pathways. These inhibitor studies suggest that PP-1_G phosphorylation is mediated via a complex, cell type specific mechanism involving PI3-kinase/PKC/PKB and/or the ras/MAP kinase/Rsk kinase cascade. cAMP agonists such as SpcAMP (via PKA) and TNF- (recently identified as endogenous inhibitor of insulin action via ceramide) block insulin-stimulated PP-1_G phosphorylation with a parallel decrease of PP-1 activity, presumably due to the dissociation of the PP-1 catalytic subunit from the regulatory G-subunit. It appears that any agent or condition which interferes with the insulin-induced phosphorylation and activation of PP-1, will decrease the magnitude of insulin's effect on downstream metabolic processes. Therefore, regulation of the PP-1_G subunit by site-specific phosphorylation plays an important role in insulin signal transduction in target cells. Mechanistic and functional studies with cell lines expressing PP-1_G subunit site-specific mutations will help clarify the exact role and regulation of PP-1_G site-specific phosphorylations on PP-1 catalytic function.