Plasma Insulin Levels In Response Research Articles

Role of Adipose Triglyceride Lipase and Lipolysis in the Regulation of Insulin Secretion: Study in β-Cell-Specific ATGL-Deficient Mice

Previous studies have suggested that lipolysis-derived lipid signalling molecules play a role in the regulation of glucose-stimulated insulin secretion. To directly assess the role of lipolysis of β-cell endogenous lipid stores in glucose signalling for insulin secretion, β-cell-specific adipose triglyceride lipase (ATGL)-deficient mice (KO) were generated. β-cell-specific tamoxifen-inducible ATGL deletion in MIP-Cre-ER/ATGL-LoxP mice (KO) was compared to tamoxifen-injected-MIP-Cre-ER and -ATGL-LoxP mice (controls). Two weeks after tamoxifen treatment in 8-week-old mice, ATGL protein level was dramatically decreased in KO islets from both sexes. KO male mice showed a decrease in body weight gain compared to the controls, which became significant 6 weeks after tamoxifen injection. Fasting plasma levels of free fatty acids and triglycerides were unchanged in KO mice of both sexes. Male KO mice showed a 60% decrease in insulinemia without altered glycemia in fed and fasted states. They also showed a 70% reduction in plasma insulin levels in response to a glucose challenge and a trend toward glucose intolerance. However, despite similar ATGL deletion in female KO mice, none of these changes in glucose homeostasis and insulin secretory response were observed. These results demonstrate that lipolysis via ATGL is involved in glucose-stimulated insulin secretion, at least in male mice. Female mice are often protected from diabetes in various animal models due to estrogens and a number of compensatory processes. We are currently studying the compensatory mechanisms that occur in KO female mice as they may reveal signalling and compensatory pathways involved in insulin secretion.

The Wnt signaling pathway effector TCF7L2 controls gut and brain proglucagon gene expression and glucose homeostasis.

The type 2 diabetes risk gene TCF7L2 is the effector of the Wnt signaling pathway. We found previously that in gut endocrine L-cell lines, TCF7L2 controls transcription of the proglucagon gene (gcg), which encodes the incretin hormone glucagon-like peptide-1 (GLP-1). Whereas peripheral GLP-1 stimulates insulin secretion, brain GLP-1 controls energy homeostasis through yet-to-be defined mechanisms. We aim to determine the metabolic effect of a functional knockdown of TCF7L2 by generating transgenic mice that express dominant-negative TCF7L2 (TCF7L2DN) specifically in gcg-expressing cells. The gcg-TCF7L2DN transgenic mice showed reduced gcg expression in their gut and brain, but not in pancreas. Defects in glucose homeostasis were observed in these mice, associated with attenuated plasma insulin levels in response to glucose challenge. The defect in glucose disposal was exacerbated with high-fat diet. Brain Wnt activity and feeding-mediated hypothalamic AMP-activated protein kinase (AMPK) repression in these mice were impaired. Peripheral injection of the cAMP-promoting agent forskolin increased brain β-cat Ser675 phosphorylation and brain gcg expression and restored feeding-mediated hypothalamic AMPK repression. We conclude that TCF7L2 and Wnt signaling control gut and brain gcg expression and glucose homeostasis and speculate that positive cross-talk between Wnt and GLP-1/cAMP signaling is an underlying mechanism for brain GLP-1 in exerting its metabolic functions.

Knockdown of NPY Expression in the Dorsomedial Hypothalamus Promotes Development of Brown Adipocytes and Prevents Diet-Induced Obesity

Hypothalamic neuropeptide Y (NPY) has been implicated in control of energy balance, but the physiological importance of NPY in the dorsomedial hypothalamus (DMH) remains unclear. Here we report that knockdown of NPY expression in the DMH by adeno-associated virus-mediated RNAi reduced fat depots in rats fed regular chow and ameliorated high-fat diet-induced hyperphagia and obesity. DMH NPY knockdown resulted in development of brown adipocytes in inguinal white adipose tissue through the sympathetic nervous system. This knockdown increased uncoupling protein 1 expression in both inguinal fat and interscapular brown adipose tissue (BAT). Consistent with the activation of BAT, DMH NPY knockdown increased energy expenditure and enhanced the thermogenic response to a cold environment. This knockdown also increased locomotor activity, improved glucose homeostasis, and enhanced insulin sensitivity. Together, these results demonstrate critical roles of DMH NPY in body weight regulation through affecting food intake, body adiposity, thermogenesis, energy expenditure, and physical activity.

Deletion of Glutamate Dehydrogenase in ß-Cells Abolishes Part of the Insulin Secretory Response Not Required for Glucose Homeostasis

Insulin exocytosis is regulated in pancreatic ss-cells by a cascade of intracellular signals translating glucose levels into corresponding secretory responses. The mitochondrial enzyme glutamate dehydrogenase (GDH) is regarded as a major player in this process, although its abrogation has not been tested yet in animal models. Here, we generated transgenic mice, named betaGlud1(-/-), with ss-cell-specific GDH deletion. Our results show that GDH plays an essential role in the full development of the insulin secretory response. In situ pancreatic perfusion revealed that glucose-stimulated insulin secretion was reduced by 37% in betaGlud1(-/-). Furthermore, isolated islets with either constitutive or acute adenovirus-mediated knock-out of GDH showed a 49 and 38% reduction in glucose-induced insulin release, respectively. Adenovirus-mediated re-expression of GDH in betaGlud1(-/-) islets fully restored glucose-induced insulin release. Thus, GDH appears to account for about 40% of glucose-stimulated insulin secretion and to lack redundant mechanisms. In betaGlud1(-/-) mice, the reduced secretory capacity resulted in lower plasma insulin levels in response to both feeding and glucose load, while body weight gain was preserved. The results demonstrate that GDH is essential for the full development of the secretory response in beta-cells. However, maximal secretory capacity is not required for maintenance of glucose homeostasis in normo-caloric conditions.

The physiology and pharmacology of incretins in type 2 diabetes mellitus

Two incretin hormones, glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide (GLP)‐1, help control blood glucose levels by affecting plasma insulin levels in response to oral glucose intake – this is known as the incretin effect. Incretins increase plasma insulin by a number of mechanisms, most of which are mediated through GIP and GLP‐1 receptors (GIPR and GLP‐1R respectively). Incretins act through the pancreas to stimulate insulin release and can also act at the cellular level by increasing insulin gene transcription and insulin biosynthesis. Incretins can also lower plasma glucose through non‐insulin routes including inhibition of gastric emptying and reduced food intake. In type 2 diabetes, the incretin effect is reduced; as a result, insulin secretion is reduced and plasma glucose levels rise. However, the incretin effect can be restored in such individuals by the administration of exogenous incretins. Intravenous GLP‐1 can stimulate insulin secretion, reduce glucagon secretion and normalize fasting plasma glucose in individuals with type 2 diabetes who were previously poorly controlled. The therapeutic potential of such agents is currently being explored.

Impaired pancreatic beta cell function in the fetal GK rat. Impact of diabetic inheritance.

The Goto-Kakisaki (GK) rat is a genetic model of non-insulin-dependent diabetes. At 21.5 d of age we found that GK fetuses had an increased plasma glucose concentration, a decreased plasma insulin level, and a reduced pancreatic beta cell mass. To investigate the beta cell function during fetal life we used a hyperglycemic clamp protocol applied to the mothers, which allowed us to obtain a steady-state hyperglycemia in the corresponding fetuses. At variance, with Wistar (W) fetuses, plasma insulin concentration in GK fetuses did not rise in response to hyperglycemia. In contrast, GK fetal pancreas released insulin in response to glucose in vitro to the same extent as W fetal pancreas. Such a discrepancy between the in vivo and in vitro results suggests that the lack of pancreatic reactivity to glucose as seen in vivo is extrinsic to the fetal GK beta cell. Finally, the importance of gestational hyperglycemia was investigated by performing crosses between GK and W rats. Fetuses issued from crosses between W mother and GK father or GK mother and W father had a beta cell mass close to normal values and were still able to increase their plasma insulin levels in response to hyperglycemia in vivo. Our data suggest that hyperglycemia in utero does not influence the severity of the decrease of the beta cell mass or the lack of the insulin secretory response to glucose in the fetal GK rat. Moreover they indicate that conjunction of GK genes originating from both parents is necessary in order for these defects to be fully expressed.

Impaired glucose tolerance in recipients of an intraperitoneally implanted microencapsulated islet allograft is caused by the slow diffusion of insulin through the peritoneal membrane

I NTRAPERITONEAL transplantation of microencapsulated islets has been reported to restore normoglycemia both in chemically induced and in autoimmune diabetic animal models,‘-5 and recently also in man.6 But a previous report from our laboratory showed that glucose tolerance remains disturbed in spite of normoglycemia, and that there is no increase of plasma insulin levels in response to a physiologic stimulus such as the intake of a meal5 This may be the consequence of several factors such as a gradual decrease of the number of viable and functioning islets after transplantation as a consequence of fibrotic overgrowth of the capsules, or a slow release of insulin in response to a glycemic stimulus as a consequence of overly large capsule diameters.’ The transplantation site as such may be another factor. Previous studies on the kinetics of intraperitoneal insulin absorption report a short delay between the intraperitoneal administration of insulin and rise of plasma insulin levels. *-‘i However, these studies were often performed with insulin bolus injections in unphysiologically high doses which may well be associated with a more rapid absorption of insulin than should be expected with a gradual release of insulin from encapsulated pancreatic islets. Therefore, the present study investigates whether the intraperitoneal implantation site as such interferes with optimal transport kinetics between the islets and the bloodstream.

Glucose and Insulin Response to Carbohydrate Foods in Male Endurance Athletes

The purpose of the study was to determine the effect of three carbohydrate(CHO) foods on plasma glucose and insulin response in trained endurance athletes. Nine, male subjects consumed one of three CHO foods—applesauce(AS), graham crackers(GC) or orange juice(OJ). Test foods were prepared using a formula of 1 gram of CHO per kilogram of body weight. Two days prior to each trial, subjects controlled their dietary intake and exercise output. The composition of the test diet, as a percentage of total calories, was 45% CHO, 15% protein and 40% fat. Ten hours prior to each trial subjects refrained from consuming food and beverages with the exception of water and from engaging in exercise. On the day of the trial, the test food was consumed in six minutes(min), with an additional two min allowed for the consumption of 240ml of water. Subjects then rested quietly for the next 90minutes. Blood samples were collected immediately before the ingestion of the food and at 15,30,45,60,75 and 90min postprandial. Mean glucose concentrations were greatest for OJ at 15min and GC at 30min (113mg/dl). A similar pattern was observed for plasma insulin—47 uU/Lat 30min and 40 uU/L at 15min for GC and OJ respectively. Other than at 15min, there was no significant difference between the glucose means for the three meals. At 15min OJ elicited a significantly greater response than AS. There were no significant differences between the plasma insulin means at baseline or 15min. At 30, 45 and 60minutes, mean plasma insulin response for GC was elevated when compared to AS. Similarly, at 30min GC produced a significantly greater insulin response than OJ. The results suggest that the ingestion of OJ elevates glucose to a greater degree than AS or GC 15min postprandial and that a significant difference exists between mean plasma glucose levels of OJ and AS. At 30min, plasma insulin levels in response to GC ingestion are elevated and are significantly greater than either OJ or AS. This response continues for AS at 45 and 60min but not for OJ. These data add to the growing body of knowledge regarding endurance athletes’ glycemic response to carbohydrate foods.

Glucose and insulin response to carbohydrate foods in male endurance athletes

The purpose of the study was to determine the effect of three carbohydrate(CHO) foods on plasma glucose and insulin response in trained endurance athletes. Nine, male subjects consumed one of three CHO foods—applesauce(AS), graham crackers(GC) or orange juice(OJ). Test foods were prepared using a formula of 1 gram of CHO per kilogram of body weight. Two days prior to each trial, subjects controlled their dietary intake and exercise output. The composition of the test diet, as a percentage of total calories, was 45% CHO, 15% protein and 40% fat. Ten hours prior to each trial subjects refrained from consuming food and beverages with the exception of water and from engaging in exercise. On the day of the trial, the test food was consumed in six minutes(min), with an additional two min allowed for the consumption of 240 ml of water. Subjects then rested quietly for the next 90 minutes. Blood samples were collected immediately before the ingestion of the food and at 15,30,45,60,75 and 90 min postprandial. The highest mean glucose response was for GC at 30 min(120 mg/dl). At 15 min, OJ exhibited the highest mean glucose (113 mg/dl). A similar pattern was observed for plasma insulin—47 uU/L at 30 min and 40 uU/L at 15 min for GC and OJ respectively. Other than at 15 min, there was no significant difference between the glucose means for the three meals. At 15 min OJ elicited a significantly greater response than AS. There were no significant differences between the plasma insulin means at baseline or 15 min. At 30, 45 and 60 minutes, mean plasma insulin response for GC was elevated when compared to AS. Similarly, at 30 min GC produced a significantly greater insulin response than OJ. The results suggest that the ingestion of OJ elevates glucose to a greater degree than AS or GC 15 min postprandial and that a significant difference exists between mean plasma glucose levels of OJ and AS. At 30 min, plasma insulin levels in response to GC ingestion are elevated and are significantly greater than either OJ or AS. This response continues for AS at 45 and 60 min but not for OJ. These data add to the growing body of knowledge regarding endurance athletes' glycemic response to carbohydrate foods.

Atrial natriuretic peptide (ANP) acts via specific binding sites on cGMP system of rat pancreatic islets without affecting insulin release

Atrial natriuretic peptide (ANP) has been shown to increase plasma insulin levels in vivo and to act on various target cells as a potent stimulator of the cGMP system. It has, therefore, been investigated whether ANP has a direct insulinotropic effect mediated by specific binding sites and by affecting the cGMP system in isolated rat pancreatic islets. Unlabelled ANP inhibited 125I-ANP binding in a concentration-related manner (Kd1 and Kd2 = 0.02 and 11.2 nM, Bmax1 and Bmax2 = 0.0147 and 0.0328 pmoles per 1 mg protein). ANP was able to augment cGMP levels in islets, but was not able to enhance insulin secretion at various glucose concentrations. Since the role of cGMP for the glucose-mediated insulin release is controversial, in addition to ANP M&B 22,948 (a cGMP phosphodiesterase inhibitor) was investigated to evaluate the possible role of cGMP for insulin release more precisely. Like ANP M&B 22,948 increased cGMP levels but did not affect insulin release. The data indicate no direct insulinotropic effect of ANP, although ANP binding sites are present on rat pancreatic islets and question the claimed role of cGMP for insulin secretion in general. Therefore, the recently observed in vivo elevation of plasma insulin levels in response to ANP is rather an indirect than a direct effect.

Vitamin D3 improves impaired glucose tolerance and insulin secretion in the vitamin D-deficient rat in vivo.

It has previously been shown in this laboratory that vitamin D3 is essential for normal insulin secretion from the perfused rat pancreas. In this present study, the influence of vitamin D status on insulin secretion in vivo was investigated. Intravenous glucose tolerance tests were performed on conscious vitamin D-deficient rats (-D), vitamin D-replete rats fed ad libitum (+D AL), and vitamin D-replete rats pair fed to the D-deficient animals (+D PF). Vitamin D deficiency, easily recognizable by low daily dietary intake and depressed plasma calcium levels, was found to impair plasma glucose clearance as characterized by an elevated KG value (representing a function of the area beneath the tolerance curve). KG values for the +D AL, +D PF, and -D groups were 504 +/- 15, 480 +/- 46, and 641 +/- 28, respectively. The increase in KG corresponded to a significant reduction in glucose-mediated insulin secretion as compared to the +D animals. This difference appeared not to be related to the increase caloric intake associated with vitamin D repletion, since +D rats which had been pair fed to the -D animals also exhibited restored plasma insulin levels in response to glucose. Plasma phosphorus concentrations were comparable in all three groups, and thus this parameter is also unlikely to be a contributory factor in the observed phenomenon. Additional experiments were conducted to evaluate the involvement of hypocalcemia in the observed impaired glucose tolerance. Normalization of plasma calcium levels (from 4.8 mg/100 ml to 9.6/100 ml) of the -D rats, by dietary calcium and phosphorus manipulation, failed to improve glucose clearance (KG for -D normocalcemic rats = 639 +/- 61) or insulin secretion. These results support the concept that vitamin D plays a physiological role in insulin secretion, acting, at least in part, independently of nutritional factors and the prevailing plasma calcium and phosphorus concentrations.

Studies of the human liver insulin receptor in noninsulin-dependent diabetes mellitus.

The insulin binding characteristics and the structural components of the insulin receptor were studied in the purified liver plasma membranes from seven patients with noninsulin-dependent diabetes (NIDDM) and seven control subjects. In comparison to the controls, diabetic subjects had a 65% reduction in plasma insulin levels in response to an oral glucose load. Specific insulin binding by liver membranes from diabetic patients was, however, twofold greater than the binding activity by membranes from control subjects. This alteration resulted largely from an increase in the number of insulin receptors and a modest increase in receptor binding affinity. Holo (nonreduced) receptor species of similar molecular weights were detected by an affinity labeling technique in the two membrane preparations and sulfhydryl reduction demonstrated an insulin binding subunit of 125,000 mol wt. Overall, these results show that the hepatic insulin resistance of NIDDM cannot be explained by a deficiency in insulin binding.

Insulin release after acute hydrocortisone treatment in mice

This investigation examined a potential mechanism for the inhibition of insulin release following an acute dose of hydrocortisone sodium succinate (HC). Hydrocortisone (300 mg/kg intraperitoneally) elevated plasma glucose levels ( P ≤ 0.05) when administered to male Swiss-Webster mice, without altering plasma insulin levels. This results in a significantly lower insulinogenic index ( P ≤ 0.05). Hydrocortisone suppressed the glucose-stimulated insulin levels ( P ≤ 0.05) following an intravenous glucose challenge (2 g/kg) in both fed and fasted mice. Pretreatment with chlorisondamine (EC) and phentolamine (PT) did not alter the HC-induced hyperglycemia but did result in higher plasma insulin levels in response to the higher glucose levels. Adrenalectomy did not prevent the HC-depressed insulin response to hyperglycemia. This is consistent with the hypothesis that an acute dose of HC may indirectly suppress insulin release by central activation of the sympathetic nerves at the pancreatic islets.

Increases in plasma insulin levels in response to electrical stimulation of the dorsal motor nucleus of the vagus nerve.

In order to investigate the physiological counterpart of the anatomical finding showing that the dorsal motor nucleus of the vagus nerve (DMX) is a source of efferent vagal fibers innervating the pancreas, unilateral electrical stimulation using monopolar electrodes (50 microA, 30 Hz, 0.2 msec) at a glycemia of 150 mg/100 ml was performed in normal anesthetized rats. DMX stimulation resulted in rapid (within 1 min) rise in plasma insulin levels (greater than or equal to 200%). Stimulation of the nucleus of tractus solitarius, anatomically connected to DMX, also produced a 50% increase in insulinemia. The effect of DMX stimulation was almost completely abolished by atropine pretreatment or acute bilateral subdiaphragmatic vagotomy. The effect of DMX stimulation was not potentiated by the alpha-adrenergic blocker (infusion of phentolamine) indicating that no inhibitory fiber was recruited during DMX stimulation. It is concluded that DMX is connected to the endocrine pancreas exclusively via vagal fibers and has a role in neurally mediated insulin release.

SUPPRESSION OF INSULIN RELEASE BY FISH-INSULIN-INDUCED HYPOGLYCÆMIA WITH REFERENCE TO THE DIAGNOSIS OF INSULINOMAS

Plasma-human-insulin levels during fish-insulin-induced hypoglycæmia have been measured using a specific antiserum. In normal and obese subjects there was a marked decrease in plasma-human-insulin levels, falling to < 1·5 μU. per ml. when the plasma-glucose becomes 35 mg. per 100 ml. In a patient with an insulinoma there was diminished suppression of plasma-insulin levels in response to hypoglycæmia. Stimulation tests are not reliable means of diagnosing insulinomas, and fish insulin provides a dynamic suppression test.

Changes in plasma insulin during ethanol-induced hypoglycemia

Intravenous ethanol infusions following a 36-hr fast have been carried out on nonobese and obese nondiabetic subjects, a group of nonobese mild diabetic patients, and two patients with suspected insulinomas. After a 36-hr fast, ethanol induces moderate hypoglycemia, which was, in normal subjects, associated with a fall in plasma insulin to values of < 1 μU/ml. Expressed in relation to the plasma glucose, the plasma insulin levels decreased by a mean of 65% for a 10 mg 100 ml fall in the plasma glucose. Plasma insulin levels in obese subjects fell markedly during the hypoglycemia, the per cent fall relative to the fall in plasma glucose being similar to that in normal subjects. Nonobese, mildly diabetic subjects had an absolute fall in insulin levels similar to that in normal subjects, but when expressed in terms of the initial insulin levels and the fall in plasma glucose, they had significantly decreased suppression of insulin levels with a mean fall of 29%/10 mg/100 ml fall in plasma glucose. Two normal subjects with α-adrenergic blockade had little suppression of plasma insulin levels in response to hypoglycemia. Both patients with suspected insulinomas had markedly diminished suppression of plasma insulin levels during ethanol-induced hypoglycemia, and erratic variation in plasma insulin concentrations, which were not associated with changes in plasma glucose concentration. An ethanol infusion is useful in excluding a suspected insulinoma, and may be useful in establishing the diagnosis in patients in whom the diagnosis is in doubt.

Increase in plasma immunoreactive insulin following administration of insulin to the gastrointestinal tract of rabbits.

Changes in plasma insulin levels in response to enterai administration of insulin have been studied in rabbits. Insulin, when administered to the stomach, did not cause hypoglycemia at doses up to 150 U./kg. When insulin at doses over 40 U./kg. was infused to the upper jejunum via an indwelling catheter, there was a slight but significant increase in plasma insulin levels, followed by a significant fall in blood glucose. A considerable increase in plasma insulin was observed when insulin at doses over 20 U./kg. was infused to the Thiry-Velia loop of the jejunum in which no pancreatic secretion entered. With a dose of 100 U./kg., the mean plasma insulin rose promptly to the peak of 241 μU./ml., followed by a severe hypoglycemia. As judged by hypoglycemie and immunological responses, insulin can be absorbed from the intestine in a physiologically active form, but the fraction absorbed is relatively small.

An unusual form of diabetes mellitus in ataxia telangiectasia.

Five of eight patients with documented ataxia telangiectasia had an unusual form of diabetes mellitus, characterized by marked hyperglycemia, resistance to ketosis, absence of glycosuria and markedly elevated plasma insulin levels in response to the administration of glucose or tolbutamide. Moreover, the blood glucose response to both exogenous and endogenous insulin was decreased, signifying insulin resistance similar to that found in pregnancy, acromegaly and obesity. In addition, the five subjects with diabetes had evidence of hepatic disease, which has not previously been reported in ataxia telangiectasia. Although the etiology of the hepatic dysfunction is not known, the immunologic deficiency commonly present in this disease may be responsible. Carbohydrate intolerance and the hypersecretion of insulin have been attributed to hepatic disease in some patients, or to certain lesions of the central nervous system in others, but it is not known if the diabetic state in ataxia telangiectasia results from dysfunction of either of these systems.

The effect of adrenergic blocking agents on plasma insulin and blood glucose during urethan or epinephrine induced hyperglycemia

Blood glucose and immunoreactive plasma insulin levels were measured in rats during hyperglycemia induced by urethan or epinephrine. The effect of adrenergic blocking agents was studied. Urethan induced hyperglycemia was prevented by acute bilateral adrenalectomy. The hyperglycemic effect of urethan was blocked significantly by phentolamine, N-isopropyl-methoxamine and Dichloro-isoproterenol, and that of epinephrine by phentolamine and N-isopropyl-methoxamine. There was no rise in plasma insulin levels in response to urethan or epinephrine induced hyperglycemia. The inhibition of insulin release in response to hyperglycemia was significantly reversed both in urethan and epinephrine treated rats, by phentolamine only (an α-adrenergic blocking agent). It is suggested that urethan induced hyperglycemia is induced by epinephrine. Endogenous epinephrine, as well as exogenous epinephrine, inhibits the insulin secretory response to hyperglycemia by stimulation of α-receptors. The inhibition of insulin release in response to hyperglycemia is not the major cause of the hyperglycmeia induced by urethan or epinephrine since some adrenergic blocking agents can inhibit the hyperglycemia without increasing plasma insulin levels.