Normal Insulin Secretion Research Articles

Addition of Insulin to Oral Therapy in Patients with Type 2 Diabetes

A majority of individuals with type 2 diabetes will eventually require exogenous insulin therapy to achieve or maintain glycemic control. This review provides practical recommendations for adding insulin therapy for patients with type 2 diabetes whose glucose levels are inadequately controlled with oral medications. We used a systematic review of MEDLINE to retrieve relevant articles from 1990 to 2004 using the search terms insulin therapy, combination oral therapy, glycemic control, insulin analogs, insulin glargine, and basal insulin, which we supplemented with a review of clinical practice guidelines from the American Diabetes Association and the American Association of Clinical Endocrinologists. Type 2 diabetes mellitus is becoming more common in the United States and is likely to increase in prevalence as obesity, a risk factor for type 2 diabetes, likewise increases. Treatment often begins with oral monotherapy, but after 3 years of treatment, more than half of patients will require more than one pharmacological agent, and eventually most patients will require insulin. Adding insulin to oral therapy at an earlier stage in treatment provides improved glycemic control without promoting increased hypoglycemia or weight gain, lowers the risk of microvascular complications by 25%, and reduces the amount of insulin patients require. Various insulin preparations, including the newer analog insulins, with different onsets and durations of action are available to help meet individual patients' dosing needs. The addition of insulin to oral antidiabetic therapy can improve glycemic control. Newer insulin analogs can emulate normal physiologic insulin secretion and potentially limit diabetes-related comorbidity.

Physiologic Insulin Replacement in Type 2 Diabetes

The purpose of this article is to review physiologic insulin replacement using a basal-prandial approach and focus on the role of postprandial glucose control. In addition, therapeutic options for optimizing glycemic control are described, including diabetes education, currently available agents, treatment regimens, and strategies. This article reviews the benefits of treating with a basal-prandial insulin approach and focuses on post-prandial glucose-targeted therapies, including rapid-acting insulin analogs. Data from landmark clinical trials in type 2 diabetes suggest that intensive glycemic control can reduce the frequency and severity of diabetes complications. A basal-prandial insulin regimen combines a long-acting insulin analog with a rapid-acting insulin to mimic normal physiologic insulin secretion, providing a flexible tool to enhance patient self-management. Strategies that include patient education, improved health care provider/patient communication, and basal-prandial insulin treatment can help patients achieve good glycemic control. The optimal setting for successful diabetes treatment is a team approach with the health care provider, nurse/certified diabetes educator, and patient as essential members.

Functional MR Microimaging of Pancreatic β-Cell Activation

The increasing incidence of diabetes and the need to further understand its cellular basis has resulted in the development of new diagnostic and therapeutic techniques. Nonetheless, the quest to noninvasively ascertain beta-cell mass and function has not been achieved. Manganese (Mn)-enhanced MRI is presented here as a tool to image beta-cell functionality in cell culture and isolated islets. Similar to calcium, extracellular Mn was taken up by glucose-activated beta-cells resulting in 200% increase in MRI contrast enhancement, versus nonactivated cells. Similarly, glucose-activated islets showed an increase in MRI contrast up to 45%. Although glucose-stimulated Ca influx was depressed in the presence of 100 microM Mn, no significant effect was seen at lower Mn concentrations. Moreover, islets exposed to Mn showed normal glucose sensitivity and insulin secretion. These results demonstrate a link between image contrast enhancement and beta-cell activation in vitro, and provide the basis for future noninvasive in vivo imaging of islet functionality and beta-cell mass.

The role of insulin analogues in the current treatment of diabetes mellitus

Ever since insulin was discovered by Banting and Best in 1921, all further researches in this field had been conducted with one goal: to find new insulin molecules which would provide better glycemic control with fewer side effects i.e. to mimic endogenous physiological insulin secretion. In healthy individuals, endogenous insulin secretion can be classified as basal (which provides basal glucose homeostasis) and stimulated (as a response to a meal). Conventional insulin preparations--human insulin, have time-action profiles that cannot fully immitate endogenous insulin secretion, thus leading to postprandial hyperglicemia and high glycemic oscilations during the day. RAPID-ACTING ANALOGUES: Rapid acting analogues should have a time-action profile with onset of less than one hour, duration less than four hours, hypoglycemic potency equal or greater than that of human insulin, and similar effects in all patients. Two rapid action analogues, lispro and aspart are available. The ideal basal insulin should provide slow and constant absorption, long half-life that would provide once daily dosing (or every other day), and peakless effect. Insulin glargine led to solubility at pH 4 and to slow absorption in neutral pH environment. Insulin detemir is a soluble insulin analogue with neutral pH and affinity to bind to serum albumin, thus gaining prolonged action. The mitogenic influence of insulin is due to the affinity to bind to IGF-I receptors. Following two-year administration of glargine in mice and rats, systemic carcinogenic potential was not found, though there were reports of hepatocellular carcinomas, which are frequently found in these animals. In the last two decades, many trials have shown that unsatisfactory glycemic control leads to chronic complications in both types of diabetes. Using basal glucose level, postprandial glycemy and HbA1c as metabolic parameters, it has been proven that only strict glycemic control can lower the risk of developing complications. Discovery of insulin analogues (both rapid acting and basal) enables physicians to provide better glycoregulation and less hypoglycemic incidents to their patients.

Stable agonist of glucose-dependent insulinotropic polypeptide (GIP) restores pancreatic beta cell glucose responsiveness but not glucose intolerance in aging mice

Glucose tolerance progressively declines with age whilst the onset of type two diabetes increases dramatically. Glucose-dependent insulinotropic polypeptide (GIP) potentiates glucose-induced insulin secretion. The present study was designed to assess the insulinotropic effects of a potent long-acting GIP receptor agonist, N-AcGIP(LysPAL 37), in aging mice. In older mice, body weights, basal plasma glucose and insulin concentrations were significantly higher than in young mice ( P<0.05 to P<0.001). Intraperitoneal injection of glucose alone (18 mmol/kg body weight) revealed a significantly lower ( P<0.05) insulin response in older mice, which was accompanied by impaired glucose tolerance ( P<0.05). Normal glucose-mediated insulin secretion was restored in N-AcGIP(LysPAL 37) treated older mice. However the glycaemic excursion remained significantly impaired in older mice ( P<0.05), suggestive of impaired insulin action. Native GIP had a similar overall effect in younger and older mice. These data indicate that N-AcGIP(LysPAL 37) is able to counter the age-related deterioration of pancreatic beta cell glucose sensitivity and insulin secretion.

ATP-sensitive potassium channel (KATPchannel) expression in the normal canine pancreas and in canine insulinomas

BackgroundPancreatic beta cells express ATP-sensitive potassium (KATP) channels that are needed for normal insulin secretion and are targets for drugs that modulate insulin secretion. The KATP channel is composed of two subunits: a sulfonylurea receptor (SUR 1) and an inward rectifying potassium channel (Kir6.2). KATP channel activity is influenced by the metabolic state of the cell and initiates the ionic events that precede insulin exocytosis. Although drugs that target the KATP channel have the expected effects on insulin secretion in dogs, little is known about molecular aspects of this potassium channel. To learn more about canine beta cell KATP channels, we studied KATP channel expression by the normal canine pancreas and by insulin-secreting tumors of dogs.ResultsPancreatic tissue from normal dogs and tumor tissue from three dogs with histologically-confirmed insulinomas was examined for expression of KATP channel subunits (SUR1 and Kir6.2) using RT-PCR. Normal canine pancreas expressed SUR1 and Kir6.2 subunits of the KATP channel. The partial nucleotide sequences for SUR1 and Kir6.2 obtained from the normal pancreas showed a high degree of homology to published sequences for other mammalian species. SUR1 and Kir6.2 expression was observed in each of the three canine insulinomas examined. Comparison of short sequences from insulinomas with those obtained from normal pancreas did not reveal any mutations in either SUR1 or Kir6.2 in any of the insulinomas.ConclusionCanine pancreatic KATP channels have the same subunit composition as those found in the endocrine pancreases of humans, rats, and mice, suggesting that the canine channel is regulated in a similar fashion as in other species. SUR1 and Kir6.2 expression was found in the three insulinomas examined indicating that unregulated insulin secretion by these tumors does not result from failure to express one or both KATP channel subunits.

Plasma non-esterified fatty acids: why are we not measuring them routinely?

Non-esteri¢ed fatty acids (NEFAs), also known as free fatty acids (FFAs), are released from the hydrolysis of triglyceride in adipose tissue. Their concentration in plasma is usually in the range 100 mmol/L--1mmol/L. NEFAs are a normal part of metabolic adaptability during daily life. Their concentrations in plasma fall after eachmeal as insulin suppresses adipose tissue lipolysis, and rise during the night. As this occurs other tissues, notably skeletal muscle, adapt their substrate utilization accordingly. There is an emerging view that the ability of skeletal muscle (and possibly other tissues) to alter its metabolism appropriately for the prevailing substrate supply is associated with insulin sensitivity and good ‘metabolic health’. This capacity has been termed ‘metabolic £exibility’. A quick glance at current literature on insulin resistance, type 2 diabetes or cardiovascular disease (CVD), however, will give quite a diierent impression of the role of NEFAs. They are increasingly portrayed as the villains in a web of metabolic disturbances that link these diierent conditions in the metabolic syndrome. This view of NEFAs as ‘baddies’ could be said to start with the description of the glucose--fatty acid cycle in 1963. Over the subsequent years, elevated NEFA concentrations have been associated with impaired glucose utilization, impaired suppression of glucose production, impaired insulin secretion (although fatty acids are in fact crucial for normal insulin secretion), impaired endothelial function and vascular reactivity, accelerated hepatic very-low-density lipoprotein-triglyceride secretion, activation of cholesteryl-ester transfer protein (contributing to the dyslipidaemia of insulin resistance), and almost everything else that is bad metabolically. It is rather easy, on this basis, to make a convincing story linking elevated NEFA concentrations with most features of the metabolic syndrome, and certainly with increased risk of type 2 diabetes and CVD. NEFA concentrations are typically elevated as adipose tissue accumulates, especially with abdominal obesity. This is a tempting explanation for the many adverse eiects of abdominal obesity. If all this is true, then measurement of NEFAconcentrations should be highly informative about the risk of type 2 diabetes and CVD.Why, then, are plasma NEFA concentrations notmeasuredmore regularly in clinical practice? After all, a simple enzymatic method has been available for their measurement for several decades: we are no longer dependent upon solvent extraction and solvent-phase titration. In fact, the literature on NEFA as a risk factor is far less convincing than one might at ¢rst expect. Elevated NEFAconcentrations have been shown to be a riskmarker for the future development of type 2 diabetes in several prospective studies in various ethnic groups, but they have come out far less strongly, in fact hardly at all, as a marker for future CVD. Only in the QueŁ bec cardiovascular study were elevated NEFA concentrations found to be a risk marker for future ischaemic heart disease. In the Paris prospective study, elevated NEFA concentrations were predictive of later development of hypertension and, unexpectedly, of cancer, but not of CVD mortality. So why is it that elevated NEFA concentrations do not strongly predict CVD, other than that they have not often been measured in prospective studies? The ¢rst reason is statistical. It is similar to the reason that plasma triglyceride concentrations do not independently predict CVD risk in many studies. Plasma triglyceride concentrations are very variable from day to day in any one individual, giving a high imprecision of estimate of ‘usual’ concentration from a single fasting sample. In contrast, HDL-cholesterol concentrations, which are fairly closely inversely related to plasma triglyceride, are far more stable from day to day. Thus in a multivariate analysis, low HDL-cholesterol will appear to be a far stronger risk marker for CVD than triglyceride concentrations. The day-to-day coe⁄cient of variation of fasting plasma triglyceride concentrations is typically 20%. But for NEFA concentrations, the ¢gure is about twice that (43% in the study byWidjaja et al.). Thus, statistically, NEFA concentrations are not good markers for epidemiology. Beyond that, it is in any case unlikely that NEFA concentrations would appear as independent markers of CVD risk. As argued above, elevated NEFA concentrations are likely to be linked to CVD through other mechanisms that are themselves well-recognized risk markers, such as plasma triglyceride and lowHDL-cholesterol, impaired endothelial function, pro-thrombotic

A Novel Insulin Delivery Algorithm in Rats With Type 1 Diabetes: The Fading Memory Proportional‐Derivative Method

An algorithm designed to automatically control insulin delivery was tested in rats with Type 1 diabetes. This nonlinear algorithm included a fading memory component of proportional and derivative errors in order to simulate normal insulin secretion. Error-weighting functions for the proportional and derivative terms were used with a performance index designed for error adaptation. In the first version of the algorithm, the proportional gain was adaptively varied. In the second version, a low rate of basal insulin delivery was adaptively varied. Six 6-h studies with each version were conducted using frequent blood sampling and intravenous insulin delivery. In Version 2 studies, blood glucose levels during the last two hours were well-controlled and significantly lower than in Version 1 (118 +/- 2.0 vs. 130 +/- 2.9 mg/dL). Neither version produced hypoglycemia. Future research using this algorithm needs to focus on automated glucose sensing in combination with insulin delivery.

Targeted Inactivation of Hepatocyte Growth Factor Receptor c-met in β-Cells Leads to Defective Insulin Secretion and GLUT-2 Downregulation Without Alteration of β-Cell Mass

Overexpression of hepatocyte growth factor (HGF) in the beta-cell of transgenic mice enhances beta-cell proliferation, survival, and function. In the current studies, we have used conditional ablation of the c-met gene to uncover the physiological role of HGF in beta-cell growth and function. Mice in which c-met is inactivated in the beta-cell (MetCKO mice) display normal body weight, blood glucose, and plasma insulin compared with control littermates. In contrast, MetCKO mice displayed significantly diminished glucose tolerance and reduced plasma insulin after a glucose challenge in vivo. This impaired glucose tolerance in MetCKO mice was not caused by insulin resistance because sensitivity to exogenous insulin was similar in both groups. Importantly, in vitro glucose-stimulated insulin secretion in MetCKO islets was decreased by approximately 50% at high glucose concentrations compared with control islets. Furthermore, whereas insulin and glucokinase expression in MetCKO islets were normal, GLUT-2 expression was decreased by approximately 50%. These changes in beta-cell function in MetCKO mice were not accompanied by changes in total beta-cell mass, islet morphology, islet cell composition, and beta-cell proliferation. Interestingly, however, MetCKO mice display an increased number of small islets, mainly single and doublet beta-cells. We conclude that HGF/c-met signaling in the beta-cell is not essential for beta-cell growth, but it is essential for normal glucose-dependent insulin secretion.

Loss of Connexin36 Channels Alters β-Cell Coupling, Islet Synchronization of Glucose-Induced Ca2+ and Insulin Oscillations, and Basal Insulin Release

Normal insulin secretion requires the coordinated functioning of beta-cells within pancreatic islets. This coordination depends on a communications network that involves the interaction of beta-cells with extracellular signals and neighboring cells. In particular, adjacent beta-cells are coupled via channels made of connexin36 (Cx36). To assess the function of this protein, we investigated islets of transgenic mice in which the Cx36 gene was disrupted by homologous recombination. We observed that compared with wild-type and heterozygous littermates that expressed Cx36 and behaved as nontransgenic controls, mice homozygous for the Cx36 deletion (Cx36(-/-)) featured beta-cells devoid of gap junctions and failing to exchange microinjected Lucifer yellow. During glucose stimulation, islets of Cx36(-/-) mice did not display the regular oscillations of intracellular calcium concentrations ([Ca(2+)](i)) seen in controls due to the loss of cell-to-cell synchronization of [Ca(2+)](i) changes. The same islets did not release insulin in a pulsatile fashion, even though the overall output of the hormone in response to glucose stimulation was normal. However, under nonstimulatory conditions, islets lacking Cx36 showed increased basal release of insulin. These data show that Cx36-dependent signaling is essential for the proper functioning of beta-cells, particularly for the pulsatility of [Ca(2+)](i) and insulin secretion during glucose stimulation.

Basal Insulin Therapy in Type 2 Diabetes

Patients with type 2 diabetes mellitus are usually treated initially with oral antidiabetic agents, but as the disease progresses, most patients eventually require insulin to maintain glucose control. Optimal insulin therapy should mimic the normal physiologic secretion of insulin and minimize the risk of hypoglycemia. This article discusses the role of insulin therapy in patients with type 2 diabetes, emphasizing long-acting insulin agents designed to approximate physiologic basal insulin secretion and provide control over fasting plasma glucose. Clinical trials of recently developed long-acting insulins are reviewed herein, with emphasis on studies that combined basal insulin with oral agents or with short-acting insulins in a basal-bolus approach. The normal physiologic pattern of insulin secretion by pancreatic beta cells consists of a sustained basal insulin level throughout the day, superimposed after meals by relatively large bursts of insulin that slowly decay over 2 to 3 hours (bolus insulin). Basal support with long-acting insulin is a key component of basal-bolus therapy for patients with diabetes who require insulin with or without the addition of oral agents. Newer long-acting agents such as insulin glargine provide a steadier and more reliable level of basal insulin coverage and may have significant advantages over traditional long-acting insulins as part of a basal-bolus treatment strategy.

Pancreatic β-Cell Lipoprotein Lipase Independently Regulates Islet Glucose Metabolism and Normal Insulin Secretion

Lipid and glucose metabolism are adversely affected by diabetes, a disease characterized by pancreatic beta-cell dysfunction. To clarify the role of lipids in insulin secretion, we generated mice with beta-cell-specific overexpression (betaLPL-TG) or inactivation (betaLPL-KO) of lipoprotein lipase (LPL), a physiologic provider of fatty acids. LPL enzyme activity and triglyceride content were increased in betaLPL-TG islets; decreased LPL enzyme activity in betaLPL-KO islets did not affect islet triglyceride content. Surprisingly, both betaLPL-TG and betaLPL-KO mice were strikingly hyperglycemic during glucose tolerance testing. Impaired glucose tolerance in betaLPL-KO mice was present at one month of age, whereas betaLPL-TG mice did not develop defective glucose homeostasis until approximately five months of age. Glucose-simulated insulin secretion was impaired in islets isolated from both mouse models. Glucose oxidation, critical for ATP production and triggering of insulin secretion mediated by the ATP-sensitive potassium (KATP) channel, was decreased in betaLPL-TG islets but increased in betaLPL-KO islets. Islet ATP content was not decreased in either model. Insulin secretion was defective in both betaLPL-TG and betaLPL-KO islets under conditions causing calcium-dependent insulin secretion independent of the KATP channel. These results show that beta-cell-derived LPL has two physiologically relevant effects in islets, the inverse regulation of glucose metabolism and the independent mediation of insulin secretion through effects distal to membrane depolarization.

High-Dose Glibenclamide Can Replace Insulin Therapy Despite Transitory Diarrhea in Early-Onset Diabetes Caused by a Novel R201L Kir6.2 Mutation

Recently, mutations in the gene KCNJ11 encoding the Kir 6.2 subunit of the ATP-sensitive K+ channel (KATP channel) have been described in patients with permanent neonatal diabetes (1). The KATP channel complex is an aggregate of four subunits of the Kir6.2 inward rectifier channel plus four regulatory units known as the sulfonylurea receptor (SUR1). In pancreatic β-cells, glucose metabolism leads to a rapid rise in intracellular ATP levels, leading to closure of the KATP channel. The resultant cell depolarization is critical for normal insulin secretion. Sulfonylureas are able to close the KATP channel by interacting with SUR1 via an ATP-independent mechanism. As a result, four young patients carrying mutations in the Kir6.2 channel have been treated with sulfonylureas (2,3) and withdrawn from insulin. To test whether …

Reduced PDX-1 expression impairs islet response to insulin resistance and worsens glucose homeostasis

In type 2 diabetes mellitus, insulin resistance and an inadequate pancreatic beta-cell response to the demands of insulin resistance lead to impaired insulin secretion and hyperglycemia. Pancreatic duodenal homeodomain-1 (PDX-1), a transcription factor required for normal pancreatic development, also plays a key role in normal insulin secretion by islets. To investigate the role of PDX-1 in islet compensation for insulin resistance, we examined glucose disposal, insulin secretion, and islet cell mass in mice of four different genotypes: wild-type mice, mice with one PDX-1 allele inactivated (PDX-1+/-, resulting in impaired insulin secretion), mice with one GLUT4 allele inactivated (GLUT4+/-, resulting in insulin resistance), and mice heterozygous for both PDX-1 and GLUT4 (GLUT4+/-;PDX-1+/-). The combination of PDX-1 and GLUT4 heterozygosity markedly prolonged glucose clearance. GLUT4+/-;PDX-1+/- mice developed beta-cell hyperplasia but failed to increase their beta-cell insulin content. These results indicate that PDX-1 heterozygosity (approximately 60% of normal protein levels) abrogates the beta-cell's compensatory response to insulin resistance, impairs glucose homeostasis, and may contribute to the pathogenesis of type 2 diabetes.

Over-expression of sterol-regulatory-element-binding protein-1c (SREBP1c) in rat pancreatic islets induces lipogenesis and decreases glucose-stimulated insulin release: modulation by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR).

Accumulation of intracellular lipid by pancreatic islet beta-cells has been proposed to inhibit normal glucose-regulated insulin secretion ('glucolipotoxicity'). In the present study, we determine whether over-expression in rat islets of the lipogenic transcription factor SREBP1c (sterol-regulatory-element-binding protein-1c) affects insulin release, and whether changes in islet lipid content may be reversed by activation of AMPK (AMP-activated protein kinase). Infection with an adenovirus encoding the constitutively active nuclear fragment of SREBP1c resulted in expression of the protein in approx. 20% of islet cell nuclei, with a preference for beta-cells at the islet periphery. Real-time PCR (TaqMan) analysis showed that SREBP1c up-regulated the expression of FAS (fatty acid synthase; 6-fold), acetyl-CoA carboxylase-1 (2-fold), as well as peroxisomal-proliferator-activated receptor-gamma (7-fold), uncoupling protein-2 (1.4-fold) and Bcl2 (B-cell lymphocytic-leukaemia proto-oncogene 2; 1.3-fold). By contrast, levels of pre-proinsulin, pancreatic duodenal homeobox-1, glucokinase and GLUT2 (glucose transporter isoform-2) mRNAs were unaltered. SREBP1c-transduced islets displayed a 3-fold increase in triacylglycerol content, decreased glucose oxidation and ATP levels, and a profound inhibition of glucose-, but not depolarisation-, induced insulin secretion. Culture of islets with the AMPK activator 5-amino-4-imidazolecarboxamide riboside decreased the expression of the endogenous SREBP1c and FAS genes, and reversed the effect of over-expressing active SREBP1c on FAS mRNA levels and cellular triacylglycerol content. We conclude that SREBP1c over-expression, even when confined to a subset of beta-cells, leads to defective insulin secretion from islets and may contribute to some forms of Type II diabetes.

Differentiating the effects of Cx36 and E-cadherin for proper insulin secretion of MIN6 cells

Connexins have been implicated in many cell functions, even though in most cases it is still unclear whether these functions may actually be mediated by other proteins that are secondarily affected by connexin changes. Secretory systems provide useful models in which to tackle this central question. Primary pancreatic β-cells and insulin-producing lines are connected by gap junction channels made of Cx36. Using stable transfection of an antisense Cx36 cDNA, we have previously obtained clones of MIN6 cells that featured a markedly reduced expression of Cx36 and impaired insulin secretion. Here, we first show that this change also resulted in loss of E-cadherin and occludin expression, thus preventing the attribution of the secretory defects to a specific type of membrane protein. To investigate this question, we have now restored the expression of either Cx36 or E-cadherin in the Cx36 antisense-transfected cells. We show that a lentivirus-mediated transduction efficiently restored Cx36 expression in MIN6 cells and allowed for expression of variable levels of this protein. We further document that adequate but not excessive levels of Cx36 allowed for recover of normal insulin secretion in response to various secretagogues. Finally, we demonstrate that restoration of normal E-cadherin expression was not able to achieve the same secretory correction. The data demonstrate that Cx36, but not E-cadherin, is necessary to control specific steps of β-cell secretion.

Type 2 diabetes: treat to target.

Traditionally, practitioners have reserved insulin therapy for patients with type 2 diabetes until diet, exercise, and treatment with oral agents have failed to maintain glycemic control. Increasing evidence, however, supports advancing insulin therapy earlier in treating diabetes, not only to normalize glycemic control and emulate normal physiologic insulin secretion, but also to delay or prevent disease-associated comorbidity.

Fatty acid metabolism and insulin secretion in pancreatic beta cells.

Increases in glucose or fatty acids affect metabolism via changes in long-chain acyl-CoA formation and chronically elevated fatty acids increase total cellular CoA. Understanding the response of pancreatic beta cells to increased amounts of fuel and the role that altered insulin secretion plays in the development and maintenance of obesity and Type 2 diabetes is important. Data indicate that the activated form of fatty acids acts as an effector molecule in stimulus-secretion coupling. Glucose increases cytosolic long-chain acyl-CoA because it increases the "switch" compound malonyl-CoA that blocks mitochondrial beta-oxidation, thus implementing a shift from fatty acid to glucose oxidation. We present arguments in support of the following: (i) A source of fatty acid either exogenous or endogenous (derived by lipolysis of triglyceride) is necessary to support normal insulin secretion; (ii) a rapid increase of fatty acids potentiates glucose-stimulated secretion by increasing fatty acyl-CoA or complex lipid concentrations that act distally by modulating key enzymes such as protein kinase C or the exocytotic machinery; (iii) a chronic increase of fatty acids enhances basal secretion by the same mechanism, but promotes obesity and a diminished response to stimulatory glucose; (iv) agents which raise cAMP act as incretins, at least in part, by stimulating lipolysis via beta-cell hormone-sensitive lipase activation. Furthermore, increased triglyceride stores can give higher rates of lipolysis and thus influence both basal and stimulated insulin secretion. These points highlight the important roles of NEFA, LC-CoA, and their esterified derivatives in affecting insulin secretion in both normal and pathological states.

Hexachlorobenzene impairs glucose metabolism in a rat model of porphyria cutanea tarda: a mechanistic approach.

Hexachlobenzene (HCB), one of the most persistent environmental pollutants, induces porphyria cutanea tarda (PCT). The aim of this work was to analyze the effect of HCB on some aspects of glucose metabolism, particularly those related to its neosynthesis in vivo. For this purpose, a time-course study on gluconeogenic enzymes, pyruvate carboxylase (PC), phosphoenolpyruvate carboxykinase (PEPCK), glucose-6-phosphatase (G-6-Pase) and on pyruvate kinase (PK), a glycolytic enzyme, was carried out. Plasma glucose and insulin levels, hepatic glycogen, tryptophan contents, and the pancreatic insulin secretion pattern stimulated by glucose were investigated. Oxidative stress and heme pathway parameters were also evaluated. HCB treatment decreased PC, PEPCK, and G-6-Pase activities. The effect was observed at an early time point and grew as the treatment progressed. Loss of 60, 56, and 37%, respectively, was noted at the end of the treatment when a considerable amount of porphyrins had accumulated in the liver as a result of drastic blockage of uroporphyrinogen decarboxylase (URO-D) (95% inhibition). The plasma glucose level was reduced (one-third loss), while storage of hepatic glucose was stimulated in a time-dependent way by HCB treatment. A decay in the normal plasma insulin level was observed as fungicide intoxication progressed (twice to four times lower). However, normal insulin secretion of perifused pancreatic Langerhans islets stimulated by glucose during the 3rd and 6th weeks of treatment did not prove to be significantly affected. HCB promoted a time-dependent increase in urinary chemiluminiscence (fourfold) and hepatic malondialdehide (MDA) content (fivefold), while the liver tryptophan level was only raised at the longest intoxication times. These results would suggest that HCB treatment does not cause a primary alteration in the mechanism of pancreatic insulin secretion and that the changes induced by the fungicide on insulin levels would be an adaptative response of the organism to stimulate gluconeogenesis. They showed for the first time that HCB causes impairment of the gluconeogenic pathway. Therefore, the reduced levels of glucose would thus be the consequence of decreased gluconeogenesis, enhanced glucose storage, and unaffected glycolysis. The impairment of gluconeogenesis (especially for PEPCK) and the related variation in glucose levels caused by HCB treatment could be a consequence of the oxidative stress produced by the fungicide. Tryptophan adds its effect to this decrease in the higher phases of HCB intoxication, where its levels overcome the control values possibly owing to the drastic decline of URO-D. This derangement of carbohydrates leads porphyric hepatocytes to have lower levels of free glucose. These results contribute to our understanding of the protective and modulatory effect that diets rich in carbohydrates have in hepatic porphyria disease.

Visualizing Superoxide Production in Normal and Diabetic Rat Islets of Langerhans

Oxygen free radicals have been implicated in beta-cell dysfunction and apoptosis associated with type 1 and type 2 diabetes mellitus. The roles of free radicals in diabetes have thus far been defined indirectly by monitoring oxidative tissue damage and the effects of antioxidants, free radical scavengers, and overexpression of superoxide dismutase. We employed the superoxide-mediated oxidation of hydroethidine to ethidium to dynamically and directly assess the relative rates of mitochondrial superoxide anion generation in isolated islets in response to glucose stimulation. Superoxide content of isolated islets increased in response to glucose stimulation. We next compared the oxyradical levels in Zucker lean control and Zucker diabetic fatty rat islets by digital imaging microfluorometry. The superoxide content of Zucker diabetic fatty islets was significantly higher than Zucker lean control islets under resting conditions, relatively insensitive to elevated glucose concentrations, and correlated temporally with a decrease in glucose-induced hyperpolarization of the mitochondrial membrane. Importantly, superoxide levels were elevated in islets from young, pre-diabetic Zucker diabetic fatty animals. Overproduction of superoxide was associated with perturbed mitochondrial morphology and may contribute to abnormal glucose signaling found in the Zucker diabetic fatty model of type 2 diabetes mellitus.