Type 2 diabetes plasma insulin levels high,january 9 gospel,foods to help with prediabetes - Review


What is Insulin Resistance?Insulin resistance (IR) is the condition in which normal amounts of insulin are inadequate to produce a normal insulin response from fat, muscle and liver cells. Science, Technology and Medicine open access publisher.Publish, read and share novel research.
Insulin resistance in fat cells reduces the effects of insulin and results in elevated hydrolysis of stored triglycerides in the absence of measures which either increase insulin sensitivity or which provide additional insulin. Regulation of glucose-stimulated insulin secretion by nutrients, hormones and neurotransmitters.
Increased mobilization of stored lipids in these cells elevates free fatty acids in the blood plasma. Glucose enters the cell by glucose transporters (GLUT2 in rodents, GLUT1 in humans) and is then phosphorylated for its metabolism through glycolysis and oxidation.
Therefore, maintaining glucose levels within a normal range is essential for life in vertebrates. Insulin resistance in muscle cells reduces glucose uptake (and so local storage of glucose as glycogen), whereas insulin resistance in liver cells results in impaired glycogen synthesis and a failure to suppress glucose production.
The generation of ATP by glycolysis, the Krebs cycle and the respiratory chain closes the ATP-sensitive K+ channel (KATP), allowing sodium (Na+) entry without balance. Apoptotic beta-cells undergoing secondary necrosis may release beta-cell antigens, which would activate the antigen presenting cells.
Glucose homeostasis in the organism is tightly regulated by insulin, a hormone that acts on the major glucose metabolic tissues such as muscle, liver and adipose tissue. Elevated blood fatty-acid concentrations (associated with insulin resistance and diabetes mellitus Type 2), reduced muscle glucose uptake, and increased liver glucose production all contribute to elevated blood glucose concentration. These two events depolarizethe membrane and open voltage-dependent T-type calcium (Ca2+) and sodium (Na+) channels. Glucose metabolism by the Krebs Cycle also renders a series of metabolic coupling factors that may initiate and sustain insulin secretion. Insulin’s main effects include promoting glucose uptake, glycogen synthesis in the liver and muscle, triglyceride formation to be stored in adipocytes, and protein synthesis. Na+ and Ca2+ entry further depolarizes the membrane and L-type and maybe other voltage-dependent calcium channels (VDCC) open.
These metabolic coupling factors participate in mitochondrial shuttles, involving NADPH, pyruvate, malate, citrate, isocitrate, acyl-CoAs, and glutamate. When T cells reencounter the islet-antigens, they are retained in the islet, releasing inflammatory factors and inducing insulitis. Insulin secretion is held by the pancreatic beta-cells, and it is modulated by glucose levels. This activation increases intracellular Ca2+ ([Ca2+]i), which leads to fusion of insulin-containing secretory granules with the plasma membrane and the first phase insulin secretion.
Signaling pathways that contribute to maintaining or increasing glucose-stimulated insulin secretion include PKA and PKC.
Inflammatory cytokines activate transcription factors NF?? and STAT-1, which decrease PDX1 and GLUT1 expression, leading to insufficient insulin production and secretion. Insufficient insulin secretion and consequent impairment of insulin’s actions lead to Diabetes Mellitus.Diabetes is a group of metabolic diseases characterized by hyperglycemia, caused by a defect on insulin production, insulin action or both. A sustained second phase of insulin secretion is held when the granules from the readily releasable pool are converted to the immediately releasable pool, an ATP-dependent process termed “priming”.
Glucagon, glucagon-Like peptide 1 (GLP-1), and glucose-dependent insulinotropic peptide (GIP) act through PKA pathway, while acetylcholine and cholecystokinine act through the PKC pathway. Type 1 diabetes in particular is due to an autoimmune destruction of the insulin producing pancreatic beta-cell, which usually leads to absolute insulin deficiency (ADA 2009). Fatty acids may contribute to insulin secretion through the PKC pathway through formation of diacylglycerol (DAG) or through protein acylation. This type of diabetes accounts for 5-10% of the total cases of diabetes worldwide, and although its onset is commonly during childhood and adolescence, it can occur at any age, even during late adulthood. Aminoacids may stimulate insulin release by increasing ATP production from the Krebs Cycle, by membrane depolarization, or by participating in intracellular calcium increase.
As the loss of beta-cells is determinant for the development of overt type 1 diabetes, understanding beta-cell’s normal physiology, namely insulin secretion, and how it may be affected during the progression of this disease is essential. Moreover, the development of new therapeutic interventions for type 1 diabetes, such as islet transplantation, beta cell maintenance and replacement, or stem cell therapy, requires a profound knowledge of how the presence of different nutrients and signals may regulate insulin secretion and beta-cell mass.
In this chapter we aim to review the mechanisms involved in normal beta-cell function and beta-cell mass regulation, and how this function may be modulated by glucose, nutrients and signals in the beta-cell milieu. We also review how these mechanisms may be affected by the onset and progression of type 1 diabetes.
In an ''insulin-resistant'' person, normal levels of insulin do not have the same effect on muscle and adipose cells, with the result that glucose levels stay higher than normal.
To compensate for this, the pancreas in an insulin-resistant individual is stimulated to release more insulin.
Normal function of the beta-cell - glucose stimulated insulin secretionThe pancreas is an endocrine and exocrine gland. The elevated insulin levels have additional effects (see insulin) which cause further biological effects throughout the body.The most common type of insulin resistance is associated with a collection of symptoms known as metabolic syndrome. The exocrine portion corresponds to acinar tissue, responsible for secreting digestive enzymes into the pancreatic juice, while the endocrine portion comprises the pancreatic islets, which consist of several cell types secreting different hormones: -cells (insulin), -cells (glucagon), -cells (somatostatin), PP-cells (pancreatic polypeptide) and -cells (ghrelin). This is often seen when hyperglycemia develops after a meal, when pancreatic I?-cells are unable to produce sufficient insulin to maintain normal blood sugar levels (euglycemia).
The inability of the I?-cells to produce sufficient insulin in a condition of hyperglycemia is what characterizes the transition from insulin resistance to Type 2 diabetes mellitus.Various disease states make the body tissues more resistant to the actions of insulin. Recent research is investigating the roles of adipokines (the cytokines produced by adipose tissue) in insulin resistance. Beta-cells are responsible for secreting insulin in response to rises in blood nutrient levels during the postprandial state. The process by which glucose promotes insulin secretion requires its sensing and metabolism by the beta-cell, a process called glucose-stimulated insulin secretion. Exercise reverses this process in muscle tissue, but if left unchecked, it can spiral into insulin resistance.Elevated blood levels of glucose a€” regardless of cause a€” leads to increased glycation of proteins with changes (only a few of which are understood in any detail) in protein function throughout the body. Insulin is secreted in a pulsatile and biphasic fashionGlucose-stimulated insulin secretion is biphasic and pulsatile (Stagner, J.I.
With respect to visceral adiposity, a great deal of evidence suggests two strong links with insulin resistance. First, unlike subcutaneous adipose tissue, visceral adipose cells produce significant amounts of proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-a), and Interleukins-1 and -6, etc.
In numerous experimental models, these proinfammatory cytokines profoundly disrupt normal insulin action in fat and muscle cells, and may be a major factor in causing the whole-body insulin resistance observed in patients with visceral adiposity.
The secretory pulses of beta-cells are associated with synchronous Ca2+ oscillations in response to glucose stimulus (Bergsten, P. Second, visceral adiposity is related to an accumulation of fat in the liver, a condition known as nonalcoholic fatty liver disease (NAFLD). 1994), and they have been suggested to be coupled to glycolysis oscillations of the beta cell (Kar, S. In this case, the production of antibodies against insulin leads to lower-than-expected glucose level reductions (glycemia) after a specific dose of insulin. In vitro and in vivo studies have demonstrated that insulin may modulate the shift of Mg from extracellular to intracellular space.
Intracellular Mg concentration has also been shown to be effective in modulating insulin action (mainly oxidative glucose metabolism), offset calcium-related excitation-contraction coupling, and decrease smooth cell responsiveness to depolarizing stimuli. Poor intracellular Mg concentrations, as found in Type 2 diabetes mellitus and in hypertensive patients, may result in a defective tyrosine-kinase activity at the insulin receptor level and exaggerated intracellular calcium concentration. Both events are responsible for the impairment in insulin action and a worsening of insulin resistance in noninsulin-dependent diabetic and hypertensive patients.
By contrast, in T2DM patients daily Mg administration, restoring a more appropriate intracellular Mg concentration, contributes to improve insulin-mediated glucose uptake.
The benefits deriving- from daily Mg supplementation in T2DM patients are further supported by epidemiological studies showing that high daily Mg intake are predictive of a lower incidence of T2DM. Shortly after glucose stimulus, a first burst of insulin secretion occurs, followed by a decrease in the rate of secretion. A second sustained phase of insulin secretion can be observed just after this decrease, which can continue for up to several hours until euglycemia is achieved (Curry, D.L.
An American study has shown that glucosamine (often prescribed for joint problems) may cause insulin resistance.Insulin resistance has also been linked to PCOS (polycystic ovary syndrome) as either causing it or being caused by it. Mechanisms involved in the first phase of insulin secretion - the triggering pathwayThe first phase of glucose-stimulated insulin secretion is a multistep process that requires transport and oxidation of glucose, electrophysiological changes and fusion of insulin-containing secretory granules with the beta-cell plasma membrane (Figure 1). Glucose enters the cell by facilitated diffusion mediated by glucose transporters (GLUT2 in rodents, GLUT1 in humans). This enzyme plays a critical role in glucose-stimulated insulin secretion and is considered the glucosensor of the pancreatic beta cell. Insulin resistance has certainly risen in step with the increase in sugar consumption and the substantial commercial usage of HFCS since its introduction to the food trades; the effect may also be due to other parallel diet changes however. Due to its kinetic characteristics, glucokinase is a determining factor for glucose phosphorylation (Matschinsky, F.M. The generation of ATP by glycolysis, the Krebs cycle and the respiratory chain leads to closure of the ATP-sensitive K+ channel (KATP), a hetero-octamer comprised of four subunits of the sulphonylurea 1 receptor (SUR1) and four subunits of the inwardly rectifying K+ channel Kir6.2 (Aguilar-Bryan, L. CellularAt the cellular level, excessive circulating insulin appears to be a contributor to insulin resistance via down-regulation of insulin receptors.
Since the usual instances of Type 2 insulin resistance are distinct from pathological over production of insulin, this does not seem to be the typical cause of the insulin resistance leading to Type 2 diabetes mellitus, the largest clinical issue connected with insulin resistance.
The presence of insulin resistance typically precedes the diagnosis of Types 2 diabetes mellitus, however, and as elevated blood glucose levels are the primary stimulus for insulin secretion and production, habitually excessive carbohydrate intake is a likely contributor. These two events depolarize the membrane to a range that allows the opening of voltage-dependent T-type calcium (Ca2+) and sodium (Na+) channels. Additionally, some Type 2 cases require so much external insulin that this down-regulation contributes to total insulin resistance.Inflammation also seems to be implicated in causing insulin resistance. Their activation triggers action potentials that increase in intracellular Ca2+ ([Ca2+]i) (Hiriart, M. PKC Theta inhibits Insulin Receptor Substrate (IRS) activation and hence prevents glucose up-take in response to insulin. MolecularInsulin resistance has been proposed at a molecular level to be a reaction to excess nutrition by superoxide dismutase in cell mitochondria that acts as a antioxidant defense mechanism.
Together with calcium mobilized from intracellular stores, this Ca2+ increase leads to fusion of insulin-containing secretory granules with the plasma membrane and the release of insulin into the circulation (Rorsman, P. It is also based on the finding that insulin resistance can be rapidly reversed by exposing cells to mitochondrial uncouplers, electron transport chain inhibitors, or mitochondrial superoxide dismutase mimetics.GeneticIndividual variability is a cause with an inherited component, as sharply increased rates of insulin resistance and Type 2 diabetes are found in those with close relatives who have developed Type 2 diabetes. DiseaseSub-clinical Cushing's syndrome and hypogonadism (low testosterone levels) seem to be the major insulin resistance causes .Recent research and experimentation has uncovered a non-obesity related connection to insulin resistance and Type 2 diabetes. It has long been observed that patients who have had some kinds of bariatric surgery have increased insulin sensitivity and even remission of Type 2 diabetes. Following glucose metabolism, the rate-limiting-step for the first phase lies in the rate of signal transduction between sensing the rise in [Ca2+]i and exocytosis of the immediately releasable granules (Straub, S.G. This suggested similar surgery in humans, and early reports in prominent medical journals (January 8) are that the same effect is seen in humans, at least the small number who have participated in the experimental surgical program.
The speculation is that some substance is produced in that portion of the small intestine which signals body cells to become insulin resistant. If the producing tissue is removed, the signal ceases and body cells revert to normal insulin sensitivity.


Mechanisms involved in the second phase insulin secretion - the amplifying pathwayThe existence of a second phase of insulin secretion was first reported in the 1960s. 1968) observed that, in total pancreas perfusion with glucose, insulin release showed an early and rapid increase at 2 min after glucose infusion, peaking at 4 min. A second or “slow” phase, characterized by an increasing rate of insulin secretion was sustained during the whole period of glucose infusion. On the other hand, when the pancreas was perfused with tolbutamide, a sulfonylurea that blocks the potassium channels, only the first rapid release peak was observed, suggesting this biphasic insulin secretion is only generated in glucose-stimulated insulin secretion (Curry, D.L. Both metformin and the thiazolidinediones improve insulin resistance, but are only approved therapies for type 2 diabetes, not insulin resistance, ''per se''. By contrast, growth hormone replacement therapy may be associated with increased insulin resistance.Metformin has become one of the more commonly prescribed medications for insulin resistance, and currently a newer drug, exenatide (marketed as Byetta), is being used.
It was until the 1990s that evidence of mechanisms for glucose-stimulated insulin secretion independent of ionic action (i.e. Exenatide has not been approved except for use in diabetics, but often improves insulin resistance by the same mechanism as it does diabetes. It also has been used to aid in weight loss for diabetics and those with insulin resistance, and is being studied for this use as well as for weight loss in people who have gained weight while on antidepressants. The ''Diabetes Prevention Program'' showed that exercise and diet were nearly twice as effective as metformin at reducing the risk of progressing to type 2 diabetes.Many people with insulin resistance currently follow the lead of some diabetics, and add cinnamon in therapeutic doses to their diet to help control blood sugar. Since then, the concept of a rapid first phase glucose-stimulated insulin secretion, caused by a triggering pathway (or KATP-dependent mechanism), followed by a sustained second phase due to an amplifying pathway (or KATP-independent mechanism) has developed (Aizawa, T.
Biphasic insulin secretion has been explained by the existence of different pools of insulin-containing granules inside the beta cell (Aizawa, T. There is a reserve pool of granules located in the cytoplasm which accounts for approximately 94% of the total granules, and a releasable pool of granules which are docked to the plasma membrane.
It has been suggested that the docked granules have different ability to be released and therefore constitute two subsets, the readily releasable pool, and the immediately releasable pool.
The granules from the immediately releasable pool are the first to be secreted in response to intracellular Ca2+ increase during the triggering pathway, leading to the first phase of insulin secretion. At the lowest point of secretion in between the two phases, the granules from the readily releasable pool are converted to the immediately releasable pool, an ATP-dependent process termed “priming”.
This priming has been suggested to be the rate-limiting step for exocytosis, and the target process for signals involved in the amplifying pathway that leads to the sustained second phase of insulin secretion (Straub, S.G. Given the glucose-stimulated nature of biphasic insulin secretion and the ATP-dependence of priming, most of these signals are proposed to be derived from glucose metabolism. Transcription factors regulating beta cell functionTranscription factors in the beta-cell act in a cooperative manner, forming transcriptional networks, to induce not only insulin expression, but also the expression of other genesFigure 1.Mechanism of biphasic glucose-stimulated insulin secretion. Some of these factors include PDX-1, HNF4?, MAFA, FOXA2 and NeuroD1 (Lazo-de-la-Vega-Monroy, M.L.
PDX-1 is one of the most important transcription factors regulating the insulin gene transcription.
Many of the target genes for pdx1 are crucial for glucose-induced insulin secretion, such as glucose transporter glut2 (Ahlgren, U. PDX1 plays a role in the maintenance and proliferation of beta-cells as well (Holland, A.M. Itsoverexpression in diabetic mice (Irs2 knockouts) participates in beta-cell mass recovery and helps ameliorate glucose tolerance (Kushner, J.A. PDX1 decrease has also been associated with apoptosis and reduced expression of the anti-apoptotic genes BclXL and Bcl-2 (Johnson, J.D. 2006), defects in post-translational processing of insulin, inhibition of GLP-1 receptor expression (Wang, H.
However, glucose metabolism can also render a series of signals, or metabolic coupling factors, that may initiate and sustain the second phase of insulin secretion, presumably by favoring mobilization of the insulin granules form the reserve pool and the replenishment of the immediately releasable pool of insulin granules.
Some of these metabolic coupling factors participate in mitochondrial shuttles, involving NADPH, pyruvate, malate, citrate, isocitrate, acyl-CoAs, and glutamate (Jitrapakdee, S.
There are also various signaling pathways that, when activated, may contribute to maintaining or increasing glucose-stimulated insulin secretion, including the CaMKII (Calcium-Calmodulin-Dependent Protein Kinase II), PKA (Protein Kinase A), PKC (Protein Kinase C) and PKG (Protein kinase G) pathways. Mitochondrial signallingThe role of mitochondria in the second phase of glucose-induced insulin secretion has been established by several studies in cell lines and humans (Jitrapakdee, S. There is even evidence of an uncommon subform of diabetes, mitochondrial diabetes, where mutations in mitochondrial DNA causepancreatic beta-cell dysfunction (Maechler, P. Pyruvate, the end product of glycolysis, plays an important role in this process, as it participates in several cycles whose final products constitute amplifying signals for insulin secretion. Particularly, NADPH, GTP, Malonyl-CoA, long-chain acyl-CoA, and glutamate have been suggested to sustain insulin secretion, although the exact mechanisms by which they have their effects remain to be elucidated (Jitrapakdee, S. 2010).Once entering the mitochondria, pyruvate may be either converted to Acetyl-CoA by pyruvate dehydrogenase, or carboxylated to oxalacetate by pyruvate carboxylase, and therefore enter the Krebs cycle (Figure 2). Notably, there is a high expression of pyruvate carboxylase in the pancreatic islets comparable to that in gluconeogenic tissues, but islets lack phosphoenolpyruvate carboxykinase (PEPCK), the first enzyme in the glyconeogenic pathway (MacDonald, M.J. Moreover, several studies have correlated pyruvate carboxylation with insulin secretion (Han, J. As the pancreatic islet is not a lipogenic tissue, the fact that acetyl-CoA activity is high in this tissue may indicate that malonyl-CoA can also act as a metabolic coupling factor for insulin secretion (Prentki, M.
Isocitrate, for example, is converted to ?-ketoglutarate by the NADP-dependent isocitrate dehydrogenase, rendering NADPH. Glutamate has been suggested to be another metabolic coupling factor for insulin secretion, possibly by entering insulin secretory granules and promoting exocytosis (Maechler, P. Finally, GTP may be produced by an isoform of the succinyl-CoA synthetase, which catalyzes the conversion of succinyl-CoA to succinate in the TCA cycle. Calcium signaling and calcium-calmodulin-dependent protein kinase II (CaMKII)As noted earlier, glucose-stimulated insulin secretion is a Ca2+-mediated process.
The increase of cytosolic calcium inside the beta-cell must be sensed and transduced in order to exert a secretory response. Besides being localized at the insulin secretory granules, CaMKII phosphorylates proteins involved in the secretory machinery, including synapsin I (Matsumoto, K. Insulin release is then suggested to be modulated by CaMK II by mobilizing the secretory granules toward the cell membrane by MAP-2 phosphorylation and by potentially regulating the docking or priming mechanisms via VAMP and synapsin I protein phosphorylation. Since CaM kinase II remains active after glucose stimulation, it is suggested as a mechanism of readily releasable pool replenishment. The G-protein coupled signaling pathways: PKA and PKCThe guanyl-nucleotide-binding (GTP) protein system or G-protein coupled system plays an important role on insulin secretion. Depending on the type of G? subunit present, these signals will activate or inhibit Adenylate Cyclase (G?s and G?i subunits respectively).
When the Adenylate Cyclase is activated in the beta-cell, it converts ATP in cyclic AMP (cAMP), which in turn can activate the cAMP-dependent protein kinase (PKA) and the Rap guanine nucleotide exchange factor (GEF) 4 or Epac2.
PKA will phosphorylate several proteins, including L-type voltage-dependent calcium channels and proteins from the exocytotic machinery, increasing sustained insulin secretion (Ammala, C.
Epac2 has been shown to favor insulin secretion by increasing the size of the reserve pool and facilitating the recruitment of the granules to the plasma membrane (Shibasaki, T.
The insulin gene itself has cAMP response elements in its promoter that modulate insulin transcription in response to this nucleotide (Melloul, D. 1979), while ligands that decrease adenylate cyclase activity affect insulin secretion in a negative way (Jones, P.M. Hormones and neurotransmitters mostly act on insulin secretion by this pathway (see below).Phospholipase C (PLC) is the other effector protein regulated by G-protein coupled receptors in the beta-cell.
PLC activation cleaves phosphoinositides into two second messengers, inositol 1,4,5-trisphosphate (IP3), involved in Ca2+ release from the endoplasmic reticulum, and diacylglycerol (DAG). PKC phosphorylates the KATP channels and the voltage-dependent Ca2+ channels and mobilize the secretory vesicles (Doyle, M.E.
Both nutrients and neurotransmitters may act through PKC activation, albeit by different mechanisms.
It has been proposed that nutrients may activate atypical isoforms of PKC (-?, -?, and –?) by a non-identified mechanism independent of DAG, while the typical isoforms (-?, -?, -?, and -?) of PKC (Protein Kinase C) are activated by DAG (Jones, P.M. Calcium increases the activity of calcium-dependent nitric oxide synthases, a key step in the synthesis of cGMP by soluble guanylyl cyclase(cGC). Calcium may also decrease cGMP synthesis by activating a calcium-dependent phosphodiesterase (PDE1).
On the other hand, protein kinase G (PKG), an enzyme activated by cGMP, may phoshporylate different targets and modulate intracellular calcium concentration, primarily closing KATP channels (Soria, B. Although several studies have pointed to a role of sGC and cGMP on insulin secretion (Laychock, S.G. It has also been shown that PKG activity is necessary to increase ATP content in response to cGMP (Vilches-Flores, A. Nutrient modulation of insulin secretionBeta-cells may be considered fuel sensors, as they are continually monitoring and responding to nutrient concentration in the circulation in order to secrete insulin and therefore, regulate glucose homeostasis. Given that meals are composed by multiple nutrients, it is important to examine the interplay between glucose-sensing in the beta-cell and other dietary nutrients, such as amino acids, fatty acids and vitamins. Insulin secretion in response to fatty acidsWhile it would appear that free fatty acids do not stimulate insulin secretion in the absence of glucose, there is a substantial body of evidence that they are essential for glucose-stimulated insulin secretion (Salehi, A. It has been proposed that, in the presence of glucose, fatty acid oxidation is inhibited, due to formation of malonyl-CoA by acetyl-CoA carboxylase. This permits the accumulation of long-chain acyl-CoA in the cytosol that then stimulate insulin secretion directly or through the formation of other lipid compounds such as diacylglycerol and various phospholipids (Nolan, C.J. The effects of fatty acids on glucose-stimulated insulin secretion are directly correlated with chain length and the degree of unsaturation, where long-chain fatty acids (such as palmitate or linoleate) acutely improve insulin release, however, chronic increase of long-chain fatty acids reduce insulin release in response to glucose stimulation (Newsholme, P.
Insulin secretion in response to amino acidsIn addition to fatty acid involvement in glucose-stimulated insulin secretion, amino acids derived from dietary proteins and those released from intestinal epithelial cells, in combination with glucose; stimulate insulin secretion, in vivo.
Amino acids individually are poor insulin secretagogues and a relatively small number of amino acids promote or synergistically enhance glucose stimulated insulin release from pancreatic beta-cells(Newsholme, P. Glutamine and alanine are quantitatively the most abundant amino acids in blood and extracellular fluids and therefore might be the most relevant to insulin secretion (Newsholme, P. Alanine increase ATP production in islet beta-cells, an event that has potential to promote the K+ATP channel triggering pathway. Alanine is also one of the electrogenic amino acids, being co-transported with Na+ so that its import depolarizes the plasma membrane and promotes Ca2+ influx, events that trigger insulin secretion (McClenaghan, N.H. Although glutamine is rapidly transported and metabolized by islets, it does not promote insulin secretion by itself or enhance glucose-stimulated insulin secretion, but can elicit insulin release in the presence of leucine (Newsholme, P.
It is believed that this is because leucine activates glutamic dehydrogenase, which then increases the capacity of glutamine to contribute to anaplerosis via alpha-ketoglutarate (Newsholme, P.
2007a).Similarly as glucose-stimulated insulin release, leucine acts by generating ATP thought its metabolism, thus causing closure of ATP-sensitive potassium channels, membrane depolarization via opening of the L-voltage-dependent calcium channels, leading to calcium influx and increased cytoplasmic calcium concentrations.
Furthermore, leucine acutely stimulates insulin secretion by serving as both metabolic fuel and allosteric activator of glutamate dehydrogenase, resulting in conversion of glutamate to 2-ketoglutarate, a compound that has been proposedto be a common mediator of glucose, amino acid, and organic acid insulin secretion (Odegaard, M.L. Additionally, transamination of leucine to ?-ketoisocaproate and entry into TCA cycle via acetyl-CoA can contribute to ATP generation by increasing the oxidation rate of the amino acid and thus stimulation of insulin secretion.Other amino acids also stimulate insulin secretion by elevating cytosolic calcium concentration, although their mechanisms are achieved independently of ATP generation.
Positive charged amino acids such as arginine, lysine and histidine, elicit insulin secretion by beta-cell inward transport of positive charge, triggering depolarization of cytoplasm membrane, and influx of extracellular calcium (Newsholme, P.
Vitamin AVitamin A is found in the organism either as retinol, retinal or retinoic acid forms. Retinoic acid is the active form, and the majority of its effects involve the activation of ligand-dependent transcription factors from the superfamily of hormonal nuclear receptors. Two of these receptors are known: the retinoic acid receptors (RARs) and the rexinoid receptors (RXRs).
These can bind as heterodimers to specific DNA sequences named Retinoic Acid Response Elements, (RAREs) in the promoters of their target genes, or interact with other receptors such as Vitamin D receptors (VDRs), thyroid hormone receptors and PPARs (Peroxisome Proliferation Activating Receptors).


1987) and retinoic acid increases insulin secretion in cultured islets (Cabrera-Valladares, G. 1999), presumably by its stimulatory effect on pancreatic glucokinase expression and activity (Cabrera-Valladares, G. It can also be obtained from food in the form of ergocalcipherol (vitamin D2) or cholechalcipherol (vitamin D3). When UVB radiation is absorbed through the skin, 7-dehydrocholesterol reserves form the pre-vitamin D3, which is transformed into vitamin D3 (1,25(OH2)D3 ) in a further process, by the action of the 25(OH2)D3 hydroxylase (Holick, M.F. Vitamin D acts on Vitamin D receptors (VDRs), which are either in the nucleus or in the membrane, rendering two different mechanisms of action, genomic, and non-genomic (rapid response) (Norman, A.W. It has been suggested that increases in cytosolic Ca2+, a non-genomic effect of vitamin D, can increase insulin secretion (Norman, A.W. Unrelated to this classic role, pharmacological concentrations of biotin regulate gene expression at both the transcriptional and the translational level (Rodriguez-Melendez, R.
2005), and have a wide repertoire of effects on systemic processes such as development (Watanabe, T.
We have found that biotin stimulates insulin and pancreatic glucokinase expression(Romero-Navarro, G. 1999), an enzyme that plays an important role in glucose homeostasis regulating insulin secretion in response to changes in blood glucose concentrations. Our group found that biotin concentrations of 10 to 1000 nM augmented glucokinase activity and mRNA abundance in cultured rat pancreatic islets (Romero-Navarro, G. A similar stimulatory effect on pancreatic glucokinase was observed in the insulinoma RIN 1046-38 cell line (Borboni, P.
1999) have revealed that glucose-stimulated insulin secretion increases in response to acute exposure to pharmacological doses of biotin in either primary cultured islets (Romero-Navarro, G.
In isolated pancreatic islets, using blockers and inhibitors of different signaling pathways, we have discovered that the induction of glucokinase mRNA and the increase on insulin secretion by biotin involves guanylate cyclase and PKG activation, which triggers ATP production (Vilches-Flores, A. 2009).Although the acute effect of biotin on in vitro insulin secretion has been well documented, further studies addressing the effect of this vitamin on in vivo models, resembling the actual doses and periods of treatment currently recommended for diabetes treatment, need to be done.
Other modulatory signals of insulin secretion - hormones and neurotransmittersInsulin secretion in response to the plasmatic concentration of glucose can be increased or decreased by several hormones (including insulin itself) and neurotransmitters via activation of their membrane receptors on the beta-cells(Flat, P.R. The G protein receptors and adenylate cyclase pathway are responsible for mediating most of these effects.
The adenylate cyclase pathway may be activated by some neurotransmitters, like acetylcholine, and hormones like GLP-1. GLP-1 is also an important factor for insulin synthesis and secretion, having a trophic effect on the beta-cells as well (Baggio, L.L. Other modulating pathways are activated in the beta-cells in response to oxidative stress caused by high glucose levels, like the JNK pathway, which ablates insulin synthesis and interferes with its action (Kaneto, H. Insulin and the beta-cell autocrine signaling Various studies have shown an autocrine role of insulin on beta-cell function and survival (Aikin, R. In this process, insulin binding to tyrosine-kinase receptors located in the beta-cell promotes the receptor’s autophosphorylation, catalyzing subsequent tyrosine phosphorylation of other proteins like IRS (IRS1 and IRS2). Once phosphorylated, these proteins interact with signaling molecules, which results in a phosphorylation cascade where PI3K, PDK and Akt are sequentially activated.
In human islets, insulin has a positive effect on insulin production at the transcriptional level, as well as on beta-cell proliferation (Persaud, S.J. Insulin secretion in response to glucagonGlucagon is considered the contrarregulatory hormone of insulin, as its systemic actions are contrary to the ones exerted by insulin.
Paradoxically, it has been shown that glucagon stimulates insulin secretion both in rats (Kawai, K. Glucagon induces a transient increase in plasma insulin up to 1 mg glucagon concentrations, and this increase is seen before glucose levels rise (Ahren, B. There is evidence that the positive effect of glucagon on insulin secretion is mediated by activation of glucagon receptors in the beta-cells (Kawai, K. Effects of incretins on insulin secretion Incretins are hormones secreted in the postprandial state by the enteroendocrine cells in the gut. Two incretins have been described GIP (glucose-dependent insulinotropic peptide) and GLP-1 (glucagon-like peptide-1) (Brubaker, P.L. GLP-1 is released rapidly into the circulation after oral nutrient ingestion, and its secretion occurs in a biphasic pattern starting with an early (within10–15 min) phase that is followed by a longer (30 –60 min) second phase (Herrmann, C.
Incretin-receptor activation leads to activation of adenylate cyclase and elevation of cAMP. Its actions include stimulation of glucose-dependent insulin secretion, induction of beta-cell proliferation, and enhanced resistance to islet cells apoptosis (Brubaker, P.L. Both GIP and GLP-1 are cleaved and inactivated by the enzyme dipeptidyl peptidase 4 (DPP4).
The rapid degradation of GLP-1 by DPP4 has led to the development of degradation-resistant GLP-1–receptor agonists and dipeptidyl peptidase-4 inhibitors, in order to increase the incretin effects. Neurotransmitters in the regulation of insulin secretion Besides nutrients, neurohormonal signals such as autonomic innervation can markedly modulate glucose-stimulated insulin secretion. Islets are thoroughly innervated by autonomic nerves, which contain an extensive variety of neuropeptide transmitters.
Increased sympathetic activity affects insulin secretion in situations of stress, exercise and trauma. Activation of parasympathetic nerves before and during feeding by the smell, taste and digestive tract, along with incretin hormones derived from the gut are responsible for enhancing insulin response to meals.Parasympathetic neurotransmitters that stimulate insulin secretion include acetylcholine, vasoactive intestinal polypeptide and gastrin-releasing polypeptide.
Sympathetic neurotransmitters inhibit insulin release; these include norepinephrine, galanin and neuropeptide Y.
The enteroinsular axis, mediated by incretin hormones, explains why the insulin response to an ingested nutrient load is greater than when the same load is given parenterally. Gastrointestinal hormones such as gastric inhibitory peptide, glucagon-like peptide-1 (7-36) and cholecystokinin exert physiological relevant insulinotrophic effects (Flatt, P.R. In particular glucagon-like peptide-1 (7-36) has attracted attention by its potential role in the treatment of diabetes (see above). There are at least three potential sites were insulin can be modulated by hormones, peptides and neurotransmitters. Firstly, these may affect the ion channels that regulate membrane potential and calcium influx. Secondly, they may influence the mobilization of intracellular calcium stores, mainly the endoplasmic reticulum, and therefore cytosolic calcium concentration.
Thirdly, they may modify the calcium sensibility of the contractile protein interactions that lead to the release of the insulin secretory granules (Flatt, P.R. The two better known targets of hormones, peptides and neurotransmitters within the beta-cell are related to adenylate cyclase and phospholipase C. Activation of adenylate cyclase produces cyclic adenosine monophosphate (cAMP), which inhibits calcium sequestration within intracellular stores. Activation of cAMP-dependent protein kinase (PKA) results in phosphorylation of intracellular proteins that enhance calcium sensitization.
PKA also promotes phosphorylation of voltage-dependent calcium channels thereby increasing calcium influx (Flatt, P.R.
Phospholipase C activation cleaves phosphatidylinosistol in the membrane producing inositol-1,4,5 triphosphate wich in turn inhibits calcium sequestration into the endoplasmic reticulum, while the adjacent cleavage product, diacylglycerol activates protein kinase C. Similarly to the effects of adenylate cyclase signaling pathway, activation of phospholipase C alters insulin secretion by mechanisms related to calcium sensitivity and protein phosphorylation (Flatt, P.R. Beta-cell massBesides a correct beta-cell function, the organism’s beta-cell mass is also important for maintaining adequate insulin production and secretion.
Beta-cell mass is determined by cell number as well as cell size, and it increases progressively during fetal, neonatal and growth periods in the life of an organism, reaching a plateau during adulthood and decaying gradually with age (Ackermann, A.M. Diverse processes participate in increasing and maintaining the beta cell mass, such as neogenesis (newly forming of cells from precursors), proliferation (cell replication), beta-cell size increase (hypertrophy), and apoptosis (cell death) (Ackermann, A.M. 2008), the participation of neogenesis during post-natal and adult beta cell mass is limited (Dor, Y. 2000) the mainly responsible mechanisms for post-natal beta cell expansion (Ackermann, A.M. The organism is also capable of modifying beta-cell mass depending on its insulin requirements. In insulin resistance states, such as pregnancy and obesity, beta-cell mass is increased (Rhodes, C.J.
Nevertheless, some of the factors regulating this process have been identified, such as growth factors (growth hormone, lactogens, insulin, insulin-like growth factors), incretins, cell cycle proteins, and transcription factors (PDX-1) (Ackermann, A.M.
Although many of the molecular regulators of postnatal beta-cell mass and beta-cell turnover have been identified in rodent models, it has been observed that human beta-cells’ ability to proliferate under the same signals is very restricted compared to rodent ones (Parnaud, G. Moreover, in humans, beta-cell proliferation has suggested to occur only until early adulthood, as proliferation studies in humans have shown that there is no beta-cell replication after the first 30 years of life (Perl, S.
Beta-cell failure and death in type 1 DMOvert hyperglycemia and therefore, the onset of type 1 diabetes occurs when 70-80% of the beta-cell mass is gone.
But the progressive loss of beta-cells is suggested to occur slowly over several years (Cnop, M. This progressive damage may also account for a reduction of the first-phase insulin secretion seen in patients positive to islet cell antibodies but who had not developed hyperglycemia yet (Srikanta, S. Nevertheless, the rate of beta-cell destruction in type 1 diabetes patients is variable and so can be the first manifestations of the disease. While some patients, mainly children and teenagers, may present ketoacidosis as first sign of diabetes, others (usually adults) could show modest fasting hyperglycemia, which may not evolve to severe hyperglycemia nor ketoacidosis for several years due to remaining function of the beta-cell (ADA 2009). Regardless this variable nature, type 1 diabetes progression after the initiation of the autoimmune response may be divided in two different phases: insulitis and overt diabetes (Mathis, D. Apoptosis of the beta-cell is present even in the initiation and, evidently, both in insulitis and diabetes.
These observations suggest that the beta-cell has a more important role in the pathophysiology of the disease than previously thought (Eizirik, D.L. It has been proposed that beta-cell death possibly participates in the initiation of the autoimmune response, particularly in autoantigen presentation (Filippi, C.M.
2000), may undergo physiological periods of apoptosis, particularly during the perinatal period. Moreover, viral infections or inflammatory cytokines may induce accumulation of misfolded proteins, causing ER stress, which can also lead to beta-cell apoptosis (Eizirik, D.L. When these T cells reencounter the islet-antigens, they are retained in the islet, triggering the inflammatory process or insulitis (Mathis, D.
Beta-cells themselves are capable of producing chemokines and cytokines in response to inflammatory factors such as IL-1? and IFN? (Cardozo, A.K. 2003), a process mediated by activation of the transcription factors NF?? and STAT-1 (Cardozo, A.K. This cytokines, besides promoting beta-cell death, can contribute to the recruitment and activation immune cells (Eizirik, D.L. Once insulitis is established, selective destruction of the beta-cells occur mainly by two proposed mechanisms: a recognition-linked mechanism and activation-linked mechanism. The former involves direct recognition of the beta-cell antigens by cytotoxic T-cells, while the latter is caused by exposure of soluble mediators secreted by T-cells that induce beta-cell death (Cnop, M.



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