Impaired glucose homeostasis in mice lacking the alpha1b-adrenergic receptor subtype

To assess the role of the alpha1b-adrenergic receptor (AR) in glucose homeostasis, we investigated glucose metabolism in knockout mice deficient of this receptor subtype (alpha1b-AR-/-). Mutant mice had normal blood glucose and insulin levels, but elevated leptin concentrations in the fed state. During the transition to fasting, glucose and insulin blood concentrations remained markedly elevated for at least 6 h and returned to control levels after 24 h whereas leptin levels remained high at all times. Hyperinsulinemia in the post-absorptive phase was normalized by atropine or methylatropine indicating an elevated parasympathetic activity on the pancreatic beta cells, which was associated with increased levels of hypothalamic NPY mRNA. Euglycemic clamps at both low and high insulin infusion rates revealed whole body insulin resistance with reduced muscle glycogen synthesis and impaired suppression of endogenous glucose production at the low insulin infusion rate. The liver glycogen stores were 2-fold higher in the fed state in the alpha1b-AR-/- compared with control mice, but were mobilized at the same rate during the fed to fast transition or following glucagon injections. Finally, high fat feeding for one month increased glucose intolerance and body weight in the alpha1b-AR-/-, but not in control mice. Altogether, our results indicate that in the absence of the alpha1b-AR the expression of hypotalamic NPY and the parasympathetic nervous activity are both increased resulting in hyperinsulinemia and insulin resistance as well as favoring obesity and glucose intolerance development during high fat feeding.     

Pancreatic Alpha-Cell Dysfunction Contributes to the Disruption of Glucose Homeostasis and Compensatory Insulin Hypersecretion in Glucocorticoid-Treated Rats

Glucocorticoid (GC)-based therapies can cause insulin resistance (IR), glucose intolerance, hyperglycemia and, occasionally, overt diabetes. Understanding the mechanisms behind these metabolic disorders could improve the management of glucose homeostasis in patients undergoing GC treatment. For this purpose, adult rats were treated with a daily injection of dexamethasone (1 mg/kg b.w., i.p.) (DEX) or saline as a control for 5 consecutive days. The DEX rats developed IR, augmented glycemia, hyperinsulinemia and hyperglucagonemia. Treatment of the DEX rats with a glucagon receptor antagonist normalized their blood glucose level. The characteristic inhibitory effect of glucose on glucagon secretion was impaired in the islets of the DEX rats, while no direct effects were found on α-cells in islets that were incubated with DEX in vitro. A higher proportion of docked secretory granules was found in the DEX α-cells as well as a trend towards increased α-cell mass. Additionally, insulin secretion in the presence of glucagon was augmented in the islets of the DEX rats, which was most likely due to their higher glucagon receptor content. We also found that the enzyme 11βHSD-1, which participates in GC metabolism, contributed to the insulin hypersecretion in the DEX rats under basal glucose conditions. Altogether, we showed that GC treatment induces hyperglucagonemia, which contributes to an imbalance in glucose homeostasis and compensatory β-cell hypersecretion. This hyperglucagonemia may result from altered α-cell function and, likely, α-cell mass. Additionally, blockage of the glucagon receptor seems to be effective in preventing the elevation in blood glucose levels induced by GC administration.     

Modulation of A and B cell functions by tolbutamide and arginine in the pancreas of thiamine-deficient rats

The effects of administration of glucose orally and tolbutamide or arginine intravenously on insulin and glucagon secretion and blood glucose level were studied in normal and thiamine-deficient rats. In thiamine deficiency, insulin secretion and glucose tolerance were impaired during glucose ingestion. Tolbutamide decreased the blood glucose level in both control and thiamine-deficient rats but its stimulatory effect on insulin secretion was minimal in thiamine-deficient rats unlike the control animals. Arginine did not alter substantially the blood glucose or insulin in thiamine-deficient rats, whereas it increased the insulin level in control rats. The fasting plasma glucagon level was high in thiamine deficiency. Tolbutamide increased the plasma glucagon in control rats, but did so only marginally in thiamine-deficient rats. Arginine also increased the glucagon secretion throughout the period of study in control rats. In thiamine-deficient rats the glucagon secretion was pronounced only after 20 min of arginine administration. These results suggest that an unimpaired glucose metabolism is a prerequisite to induce proper insulin secretion. Only proper insulin secretion can check the glucagon secretion rather than the increased glucose level. Hypoglycemia can induce glucagon secretion independent of the insulin level.     

Overfeeding during lactation modulates insulin and leptin signaling cascade in rats' hearts

Insulin has been described as a potential mediator of intrinsic responses to the nutritional state in the heart due to its effects on cardiac metabolism, mainly on glucose transport. It has been demonstrated that leptin can act through some components of the insulin-signaling cascade. We investigated the association between overfeeding during lactation and alterations of insulin and leptin signaling in the heart. In summary, we analyzed a feasible cross-talk between insulin and leptin through the study of some key proteins of their cascades in the heart. In order to study the effect of overfeeding on these cascades, Wistar rats were overfed through litter size reduction to only three pups. At 10 and 21 days of life, key proteins such as insulin receptor, leptin receptor, PI3-kinase, JAK2, STAT3, and GLUT4 were measured by Western blotting. Furthermore, the pups' weight and the plasma levels of insulin, leptin and glucose were determined. Overfed animals were overweight, had high insulin and leptin plasma levels, and displayed an activation of insulin and leptin cascade, leading to an increased translocation of GLUT4. We suggest that overfeeding during lactation probably alters cardiac metabolism, through the activation of a modulated cross-talk between leptin and insulin cascades.     

Modulation of carbohydrate metabolism and peptide hormones by soybean isoflavones and probiotics in obesity and diabetes

Soybean and its isoflavones have been shown to have beneficial effects on carbohydrate and lipid metabolism and on renal function. Probiotics may potentiate the beneficial effects of isoflavones by converting the inactive isoflavone glycoside to aglycones, which are biologically active, thereby producing a synergistic effect. We therefore studied the effects of soybean isoflavones in the presence and absence of probiotics on glucose and triglyceride metabolism and the peptide hormones involved in their metabolism. Lean and obese SHR/N-cp rats were fed AIN-93 diets containing 0.1% soybean isoflavone mixture, 0.1% probiotics mixture or both. Plasma was analyzed for glucose, triglycerides, parameters of renal function and peptide hormones -- insulin, leptin, glucagon and ACTH -- that are involved in glucose and lipid metabolism. Isoflavones given alone lowered plasma glucose in both phenotypes while triglyceride was decreased only in lean animals. Isoflavones also lowered aspartate amino transferase and alanine amino transferase in both phenotypes. Isoflavones had significant effect on plasma insulin, leptin and glucagon in lean rats but not in obese rats. Thus, our data show that in lean animals, isoflavones have hypoglycemic and hypolipidemic effect, and the effect is mediated by changes in peptide hormones. When lipid levels are very high as in obese rats, isoflavones fail to lower plasma triglyceride levels. Probiotics do not appear to enhance the effect of isoflavones.     

Leptin induces insulin-like signaling that antagonizes cAMP elevation by glucagon in hepatocytes

Although many effects of leptin are mediated through the central nervous system, leptin can regulate metabolism through a direct action on peripheral tissues, such as fat and liver. We show here that leptin, at physiological concentrations, acts through an intracellular signaling pathway similar to that activated by insulin in isolated primary rat hepatocytes. This pathway involves stimulation of phosphatidylinositol 3-kinase (PI3K) binding to insulin receptor substrate-1 and insulin receptor substrate-2, activation of PI3K and protein kinase B (AKT), and PI3K-dependent activation of cyclic nucleotide phosphodiesterase 3B, a cAMP-degrading enzyme. One important function of this signaling pathway is to reduce levels of cAMP, because leptin-mediated activation of both protein kinase B and phosphodiesterase 3B is most marked following elevation of cAMP by glucagon, and because leptin suppresses glucagon-induced cAMP elevation in a PI3K-dependent manner. There is little or no expression of the long form leptin receptor in primary rat hepatocytes, and these signaling events are probably mediated through the short forms of the leptin receptor. Thus, leptin, like insulin, induces an intracellular signaling pathway in hepatocytes that culminates in cAMP degradation and an antagonism of the actions of glucagon.     

干预胰高血糖素信号通路治疗糖尿病的研究进展

胰高血糖素是胰岛素最重要的拮抗激素,其从胰岛α细胞合成后分泌入血,与靶组织的胰高血糖素受体结合,激活靶信号通路,生成环一磷酸腺苷(cAMP),促进糖原分解和糖异生,升高血糖.愈来愈多的研究显示,通过抑制α细胞产生和分泌胰高血糖素、中和血循环胰高血糖素、胰高血糖素受体拮抗剂、抑制胰高糖素受体基因表达等干预胰高血糖素的信号通路的措施有可能成为治疗糖尿病的新方法.     

Effect of leptin on insulin, glugacon and somatostatin secretion in the perfused rat pancreas.

We have investigated the effect of rat leptin as well as the 22-56 fragment of this molecule on pancreatic hormone secretion in the perfused rat pancreas. In pancreases from fed rats, leptin failed to alter the insulin secretion elicited by glucose, arginine or tolbutamide, but inhibited the insulin response to both CCK-8 and carbachol, secretagogues known to act on the B-cell by increasing phospholipid turnover. This insulinostatic effect was also observed with the 22-56 leptin fragment. In pancreases obtained from 24-hour fasted rats, no effect of leptin on carbachol-induced insulin output was found, perhaps as a consequence of depressed B-cell phospholipid metabolism. Leptin did not influence glucagon or somatostatin release. Our results do not support the concept of leptin as a major regulator of B-cell function. Leptin inhibition of carbachol-induced insulin output might reflect a restraining effect of this peptide on the cholinergic stimulation of insulin release.     

Hyperglycemia after protein ingestion concurrent with injection of a GLP-1 receptor agonist in rats: a possible role for dietary peptides

Protein ingestion after injection of the glucagon-like peptide-1 receptor agonist Exendin-4 (Ex-4) causes hyperglycemia in rats. The objectives of this study were to determine the components of protein digestion responsible for this effect and to associate it with changes in the concentrations of other metabolites and hormones. Two experiments were conducted. In the first experiment, food-deprived rats were gavaged with intact whey (WP) or albumin protein, their hydrolysates, amino acid mixtures (1 g/2.5 ml), or water 5 min after injection of either PBS or Ex-4 (0.5 microg/rat). Tail vein blood was analyzed for glucose over 2 h. In the second experiment, food-deprived rats were gavaged with WP with or without Ex-4. Groups of conscious rats were killed by decapitation either before, or at selected times after gavage. Plasma concentrations of glucose, amino acids, free fatty acids (FFA), glycerol, insulin, glucagon, and leptin were measured. In experiment 1, blood glucose was higher when intact proteins and protein hydrolysates, but not amino acid mixtures, were given with than without Ex-4 (P < 0.05). In experiment 2, concentrations of glucose, FFA, and the ratio of tyrosine to branched-chain amino acid were higher (P < 0.01), but leptin and essential amino acid concentrations were lower (P < 0.05), and insulin, glucagon, and glycerol were similar when WP was given with or without Ex-4. We conclude that the hyperglycemia caused by the administration of Ex-4 concurrently with dietary protein arises from the action of peptides released during digestion and their interaction with Ex-4 in the regulation of glucose, fatty acid, and amino acid metabolism.     

The pancreatic beta cell is a key site for mediating the effects of leptin on glucose homeostasis

The hormone leptin plays a crucial role in maintenance of body weight and glucose homeostasis. This occurs through central and peripheral pathways, including regulation of insulin secretion by pancreatic beta cells. To study this further in mice, we disrupted the signaling domain of the leptin receptor gene in beta cells and hypothalamus. These mice develop obesity, fasting hyperinsulinemia, impaired glucose-stimulated insulin release, and glucose intolerance, similar to leptin receptor null mice. However, whereas complete loss of leptin function causes increased food intake, this tissue-specific attenuation of leptin signaling does not alter food intake or satiety responses to leptin. Moreover, unlike other obese models, these mice have reduced fasting blood glucose. These results indicate that leptin regulation of glucose homeostasis extends beyond insulin sensitivity to influence beta cell function, independent of pathways controlling food intake. These data suggest that defects in this adipoinsular axis could contribute to diabetes associated with obesity.     

Effects of biotin deficiency on pancreatic islet morphology, insulin sensitivity and glucose homeostasis

Several studies have revealed that physiological concentrations of biotin are required for the normal expression of critical carbohydrate metabolism genes and for glucose homeostasis. However, the different experimental models used in these studies make it difficult to integrate the effects of biotin deficiency on glucose metabolism. To further investigate the effects of biotin deficiency on glucose metabolism, we presently analyzed the effect of biotin deprivation on glucose homeostasis and on pancreatic islet morphology. Three-week-old male BALB/cAnN Hsd mice were fed a biotin-deficient or a biotin-control diet (0 or 7.2 μmol of free biotin/kg diet, respectively) over a period of 8 weeks. We found that biotin deprivation caused reduced concentrations of blood glucose and serum insulin concentrations, but increased plasma glucagon levels. Biotin-deficient mice also presented impaired glucose and insulin tolerance tests, indicating defects in insulin sensitivity. Altered insulin signaling was linked to a decrease in phosphorylated Akt/PKB but induced no change in insulin receptor abundance. Islet morphology studies revealed disruption of islet architecture due to biotin deficiency, and an increase in the number of α-cells in the islet core. Morphometric analyses found increased islet size, number of islets and glucagon-positive area, but a decreased insulin-positive area, in the biotin-deficient group. Glucagon secretion and gene expression increased in islets isolated from biotin-deficient mice. Our results suggest that biotin deficiency promotes hyperglycemic mechanisms such as increased glucagon concentration and decreased insulin secretion and sensitivity to compensate for reduced blood glucose concentrations. Variations in glucose homeostasis may participate in the changes observed in pancreatic islets.     

The effect of glucagon antibodies on plasma glucose and insulin levels

The actions of glucagon and insulin are interrelated as the two hormones have opposite physiological effects and the secretion of insulin is regulated, at least in part, by the level of glucagon. We have found that rabbits which are immunized against glucagon have normal fasting levels of blood glucose but a lowered level of insulin. These rabbits are also able to rapidly utilize intravenously injected glucose butwith a much lower plasma level of insulin. These results demonstrate that in the presence of glucagon antibodies, normal blood sugar levels can be maintained with a reduced supply of insulin. It is suggested that this finding may be useful in the treatment of diabetes.     

Circadian patterns of plasma leptin,insulin and glucose concentration in the toad-headed lizard Phrynocephalus versicolor

To understand the circadian rhythms of the glucose metabolism and related hormones in desert reptiles, we studied the daily changes in plasma leptin, insulin, and glucose in the toad-headed lizard, Phrynocephalus versicolor, in the semi-desert of the Alashan Plateau. Blood samples were obtained from groups of 20 lizards (about equal sex ratio for each group) at 4-h intervals throughout a 24-h period. Leptin, insulin, and glucose levels were determined using ELISA, RIA, and the glucose oxidase method, respectively. Ambient temperature and humidity were also measured when the blood samples were taken. We found that: (1) there was a significant difference in plasma leptin concentrations at different times of day; (2) no significant circadian rhythm could be measured for plasma insulin and glucose; (3) ambient temperature, humidity, and body condition had no significant effect on plasma leptin, insulin, and glucose levels; and (4) there were significant correlations between plasma glucose concentrations and leptin concentrations 8 h later, and between insulin concentrations and leptin concentration 8–12 h later. These results suggested that the secretions of leptin and insulin played an important role in glucose metabolism in reptiles in an arid environment.     

Glucagon and regulation of glucose metabolism

As a counterregulatory hormone for insulin, glucagon plays a critical role in maintaining glucose homeostasis in vivo in both animals and humans. To increase blood glucose, glucagon promotes hepatic glucose output by increasing glycogenolysis and gluconeogenesis and by decreasing glycogenesis and glycolysis in a concerted fashion via multiple mechanisms. Compared with healthy subjects, diabetic patients and animals have abnormal secretion of not only insulin but also glucagon. Hyperglucagonemia and altered insulin-to-glucagon ratios play important roles in initiating and maintaining pathological hyperglycemic states. Not surprisingly, glucagon and glucagon receptor have been pursued extensively in recent years as potential targets for the therapeutic treatment of diabetes.     

Eicosapentaenoic fatty acid increases leptin secretion from primary cultured rat adipocytes: role of glucose metabolism

Eicosapentaenoic acid (EPA), one of the n-3 polyunsaturated fatty acids, has been shown to stimulate leptin mRNA expression and secretion in 3T3-L1 cells. However, other studies have reported inhibitory effects of EPA on leptin expression and secretion in vivo and in vitro. To determine the direct effects of EPA on basal and insulin-stimulated leptin secretion, isolated rat adipocytes were incubated with EPA in the absence and presence of insulin. EPA (10, 100, and 200 microM) increased basal leptin gene expression and secretion (+43.8%, P < 0.05; +71.1%, P < 0.01; and +73.7%, P < 0.01, respectively). EPA also increased leptin secretion in the presence of 1.6 nM insulin; however, the effect was less pronounced than in the absence of it. Because adipocyte glucose and lipid metabolism are involved in the regulation of leptin production, the metabolic effects of this fatty acid were also examined. EPA (200 microM) increased basal glucose uptake in isolated adipocytes (+50%, P < 0.05). Anaerobic metabolism of glucose, as assessed by lactate production and proportion of glucose metabolized to lactate, has been shown to be inversely correlated to leptin secretion and was decreased by EPA in both the absence and presence of insulin. EPA increased basal glucose oxidation as determined by the proportion of (14)C-labeled glucose metabolized to CO(2). Lipogenesis ((14)C-labeled glucose incorporation into triglyceride) was decreased by EPA in the absence of insulin, whereas lipolysis (glycerol release) was unaffected. The EPA-induced increase of basal leptin secretion was highly correlated with increased glucose utilization (r = +0.89, P < 0.01) and inversely related to the anaerobic glucose metabolism to lactate. EPA's effect on insulin-stimulated leptin secretion was not related to increased glucose utilization but was inversely correlated with anaerobic glucose metabolism to lactate (r = -0.84, P < 0.01). Together, the results suggest that EPA, like insulin, stimulates leptin production by increasing the nonanaerobic/oxidative metabolism of glucose.     

Control of glucose balance in the perfused rat liver by the parasympathetic innervation

Electrical stimulation of the nerve bundles around the hepatic artery and the portal vein activates both the sympathetic and parasympathetic liver nerves; the sympathetic effects clearly predominate. Parasympathetic effects were therefore studied in the rat liver perfused in situ by perivascular nerve stimulation in the presence of both an alpha- and a beta-blocker. In the presence of the alpha-blocker phentolamine and the beta-blocker propranolol all sympathetic nerve effects were prevented; the remaining parasympathetic stimulation had no influence on the basal glucose and lactate metabolism nor on the hemodynamics. Insulin alone, with both alpha- and beta-blockade, provoked a small, parasympathetic nerve stimulation in the presence of insulin a more pronounced enhancement of glucose utilization. In the presence of an alpha- and beta-blocker perivascular nerve stimulation antagonized the glucagon stimulated glucose release, but did not affect lactate exchange. The nerve effect was abolished by the parasympathetic antagonist atropine. Acetylcholine or insulin, with both an alpha- and beta-blocker present, mimicked the effects of nerve stimulation antagonizing the glucagon-stimulated glucose release. Nerve stimulation in the presence of insulin was more effective than either stimulus alone. The present results show that in rat liver stimulation of the parasympathetic hepatic nerves has direct effects on glucose metabolism synergistic with insulin and antagonistic to glucagon.     

Effects of acute and chronic endotoxin treatment on glucagon and insulin receptors on rat liver plasma membranes

The effects of acute and chronic endotoxin treatment on the plasma levels of insulin and glucagon and their binding to rat liver plasma membranes were examined. Both acute and chronic endotoxin administration increased plasma glucagon levels and decreased the glucagon to insulin molar ratio. Acute, but not chronic, endotoxin decreased blood glucose and insulin levels. Glucagon binding was increased in membranes prepared from the acutely treated rats. However, in membranes obtained from rats treated chronically with endotoxin, only insulin binding was increased. The increases in the binding of both insulin and glucagon were the result of increases in receptor sites.     

Insulin as a physiological modulator of glucagon secretion

Glucose homeostasis is regulated primarily by the opposing actions of insulin and glucagon, hormones that are secreted by pancreatic islets from beta-cells and alpha-cells, respectively. Insulin secretion is increased in response to elevated blood glucose to maintain normoglycemia by stimulating glucose transport in muscle and adipocytes and reducing glucose production by inhibiting gluconeogenesis in the liver. Whereas glucagon secretion is suppressed by hyperglycemia, it is stimulated during hypoglycemia, promoting hepatic glucose production and ultimately raising blood glucose levels. Diabetic hyperglycemia occurs as the result of insufficient insulin secretion from the beta-cells and/or lack of insulin action due to peripheral insulin resistance. Remarkably, excessive secretion of glucagon from the alpha-cells is also a major contributor to the development of diabetic hyperglycemia. Insulin is a physiological suppressor of glucagon secretion; however, at the cellular and molecular levels, how intraislet insulin exerts its suppressive effect on the alpha-cells is not very clear. Although the inhibitory effect of insulin on glucagon gene expression is an important means to regulate glucagon secretion, recent studies suggest that the underlying mechanisms of the intraislet insulin on suppression of glucagon secretion involve the modulation of K(ATP) channel activity and the activation of the GABA-GABA(A) receptor system. Nevertheless, regulation of glucagon secretion is multifactorial and yet to be fully understood.     

Localization and role of leptin in the thyroid gland of the lizard Podarcis sicula (Reptilia, Lacertidae)

Leptin, the product of the ob gene, is a hormone secreted by adipocytes that regulates food intake and energy expenditure. The hypothalamus-pituitary-thyroid axis is markedly influenced by the metabolism status, being suppressed during food deprivation. The present study was designed to ascertain whether (1) lizard thyroid gland expresses the long form of leptin receptor (Ob-Rb) and (2) the leptin administration affects the thyroid gland activity in this species (and to verify whether leptin plays a similar role in reptiles as observed in the other vertebrates). The presence of leptin receptor in the thyroid gland of Podarcis sicula was demonstrated by immunohistochemical technique (avidin-biotin-peroxidase complex--ABC method). The role of leptin in the control of thyroid gland activity was studied in vivo using light microscopy (LM) technique coupled to a specific radioimmunoassay for thyroid-stimulating hormone (TSH) and thyroid hormones (T4 and T3). Leptin (0.1 mg/100 g body wt)/day increased T4 and T3 release for 3 days but decreased the plasma concentration of TSH; using LM clear signs of stimulation in the thyroid gland were observed. These findings suggest that systemic administration of leptin stimulates the morphophysiology of the thyroid gland in the lizard through a direct mechanism involving Ob-Rb.