Transhepatic absorption and biliary excretion of insulin

The application of insulin to the liver in rats is followed by an increase of the insulin concentration in the bile. The pathway of insulin from the liver surface to the bile may include a secretory process by the hepatic cells, or it may bypass the hepatic cells, using direct anatomical pathways from blood and lymph to bile. The concentration of insulin in arterial and venous blood, in lymph, and in bile was measured following application of insulin to the liver surface and following peritoneal or intravenous administration. The results confirm that insulin is absorbed from the surface of the liver, but the glucose modulating effect was less effective than after intravenous administration. The insulin concentration in bile was increased after insulin administration by all routes, with the highest and most prolonged increases found after intraperitoneal administration. The results suggest that following transhepatic and intravenous administration, insulin reaches the bile without passing through the liver cells.     

β-Conglycinin Lowers Very-Low-Density Lipoprotein-Triglyceride Levels by Increasing Adiponectin and Insulin Sensitivity in Rats

The relationship between insulin sensitivity and the plasma triglyceride-lowering effect induced by β-conglycinin was investigated. Male Wistar rats (19 weeks old) were fed diets containing casein, soy protein isolate, or β-conglycinin for 4 weeks. In oral glucose administration, the β-conglycinin-fed rats showed a significant decrease in the area under the glucose curve (0–60 min) as compared with the casein-fed rats. The hypoglycemic effect was significantly higher in the β-conglycinin-fed rats than in the casein-fed rats at 30 min after intraperitoneal insulin injection. The liver sterol regulatory element-binding-protein-1 mRNA expression level was significantly lower and the plasma adiponectin concentration was significantly higher in the β-conglycinin-fed rats than in the casein-fed rats. The hypotriglyceridemic effect of β-conglycinin depended on a significant decrease in the concentration of very-low-density-lipoprotein triglycerides. These results indicate that β-conglycinin increases adiponectin levels and improves glucose tolerance. The ability of β-conglycinin to lower plasma lipid levels might be due to increased insulin sensitivity of the liver.     

Sub-chronic administration of the 11beta-HSD1 inhibitor, carbenoxolone, improves glucose tolerance and insulin sensitivity in mice with diet-induced obesity

We have examined the metabolic effects of daily administration of carbenoxolone (CBX), a naturally occurring 11beta-hydroxysteroid dehydrogenase (11beta-HSD1) inhibitor, in mice with high fat diet-induced insulin resistance and obesity. Eight-week-old male Swiss TO mice placed on a synthetic high fat diet received daily intraperitoneal injections of either saline vehicle or CBX over a 16-day period. Daily administration of CBX had no effect on food intake, but significantly lowered body weight (1.1- to 1.2-fold) compared to saline-treated controls. Non-fasting plasma glucose levels were significantly decreased (1.6-fold) by CBX treatment on day 4 and remained lower throughout the treatment period. Circulating plasma corticosterone levels were not significantly altered by CBX treatment. Plasma glucose concentrations of CBX-treated mice were significantly reduced (1.4-fold) following an intraperitoneal glucose load compared with saline controls. Similarly, after 16-day treatment with CBX, exogenous insulin evoked a significantly greater reduction in glucose concentrations (1.4- to 1.8-fold). 11beta-HSD1 gene expression was significantly down-regulated in liver, whereas glucocorticoid receptor gene expression was increased in both liver and adipose tissue following CBX treatment. The reduced body weight and improved metabolic control in mice with high fat diet-induced obesity upon daily CBX administration highlights the potential value of selective 11beta-HSD1 inhibition as a new route for the treatment of type 2 diabetes and obesity.     

Late effects of postnatal administration of monosodium glutamate on insulin action in adult rats

Early postnatal administration of monosodium glutamate (MSG) to rats induces obesity, hyperinsulinemia and hyperglycemia in adulthood, thus suggesting the presence of insulin resistance. We therefore investigated the effects of insulin on glucose transport and lipogenesis in adipocytes as well as insulin binding to specific receptors in the liver, skeletal muscle and fat tissues. An increase of plasma insulin, glucose and leptin levels was found in 3-month-old rats treated with MSG during the postnatal period. The attenuation of insulin stimulatory effect on glucose transport was observed in MSG-treated rats. Despite the lower basal and insulin-stimulated glucose uptake, the incorporation of glucose into lipids was significantly higher in MSG-treated rats, suggesting a shift in glucose metabolism towards lipid synthesis in fat tissue. Insulin binding to plasma membranes from the liver, skeletal muscle and adipocytes was decreased in MSG-treated rats. This is in agreement with the lower insulin effect on glucose transport in these animals. Furthermore, a decreased amount of GLUT4 protein was found in adipocytes from MSG-treated obese rats. The results demonstrated an attenuation of insulin effect on glucose transport due to a lower insulin binding and lower content of GLUT4 protein in MSG-treated rats. However, the effect of insulin on lipogenesis was not changed. Our results indicated that early postnatal administration of MSG exerts an important effect on glucose metabolism and insulin action in adipocytes of adult animals.     

Regulation of glycogenolysis in the liver of the newborn rat in vivo inhibitory effect of glucose

Newborn rats were injected immediately after delivery with glucose or glucose plus mannoheptulose, and the time-courses of liver glycogen, plasma glucose, insulin and glucagon concentration were studied. The administration of glucose prevented both liver glycogenolysis and the increase in plasma glucagon concentration which normally occurs immediately after delivery. In addition, the administration of glucose prevented the decrease of plasma glucose and insulin concentration which normally occurs during the first hour of extrauterine life. Supplementation of glucose with mannoheptulose prevented the increase of plasma insulin concentrations caused by the administration of glucose; liver glycogenolysis, however, was not stimulated in these circumstances. The increase in the rate of glycogenolysis caused by the administration of glucagon was prevented in newborn rats previously treated with glucose. These results suggest that glucose exerts an inhibitory effect on the stimulation of neonatal liver glycogenolysis by glucagon.     

Effect of copper or insulin in diabetic copper-deficient rats

The effects of copper and insulin on lipogenesis and glucose tolerance were studied using diabetic, copper-deficient rats. Diabetes was induced by intraperitoneal injection of 50 mg streptozotocin/kg body weight to rats fed a sucrose-copper deficient diet for 7 weeks. Five days later the rats were injected intraperitoneally with [14C]glucose with either saline, insulin, copper, or copper plus insulin. The disappearance of serum [14C]glucose at 30, 60, and 120 min postinjection and the incorporation of [14C]glucose into lipid of epididymal fat 2 hr after administration were determined. The combined effect of copper and insulin significantly decreased peak blood glucose at 30 min and increased the incorporation of [14C]glucose into lipid in the epididymal fat pad when compared to either copper or insulin alone. The enhancement of glucose utilization may be due to a formation of a more stable complex which will increase insulin binding and/or decrease its degradation.     

Effect of parathyroid hormone on the increase in serum glucose and insulin levels after a glucose load to thyroparathyroidectomized rats.

Thyroparathyroidectomy (TPTX) caused a significant increase in serum glucose and a corresponding fall in serum calcium in both fed and fasted rats. The increase in serum glucose, induced by TPTX, was markedly potentiated by a single intraperitoneal administration of calcium (2 mg/100 g BW) which caused a significant elevation of serum calcium in thyroparathyroidectomized rats. Parathyroid hormone (PTH; 20 U/100 g BW) administered subcutaneously to thyroparathyroidectomized rats, caused a significant decrease in serum glucose (0.1 g/100 g BW) to sham-operated rats significantly increased both serum glucose and insulin. The rise of serum glucose produced by a glucose load was markedly potentiated by TPTX, but the increase in serum insulin was not promoted significantly. The administration of PTH decreased both serum glucose and insulin levels increased by a glucose load to thyroparathyroidectomized rats, in a dose-dependent manner. The administration of calcitonin (80 MRC mU/100 g BW) significantly prevented the effect of PTH to decrease serum glucose after a glucose load to thyroparathyroidectomized rats, and calcitonin increased serum insulin. These results suggest that the effect of PTH on serum glucose does not involve insulin secretion.     

Beta-glycosphingolipids improve glucose intolerance and hepatic steatosis of the Cohen diabetic rat

A link between altered levels of various gangliosides and the development of insulin resistance was described in transgenic mice. Naturally occurring glycosphingolipids were shown to exert immunomodulatory effects in a natural killer T (NKT) cell-dependent manner. This study examined whether glycosphingolipid-induced modulation of the immune system may reduce pancreatic and liver steatosis and stimulate insulin secretion in the Cohen diabetes-sensitive (CDS) rat, a lean model of non-insulin-resistant, nutritionally induced diabetes. Four groups of CDS rats fed a diabetogenic diet were treated with daily intraperitoneal injections of glycosphingolipids beta-glucosylceramide, beta-lactosylceramide, a combination of both (IGL), or vehicle (PBS) for up to 45 days. Immune modulation was assessed by fluorescence-activated cell sorting analysis of intrahepatic and intrasplenic lymphocytes. Steatosis was assessed by MRI imaging and histological examination of liver and pancreas, Blood glucose and plasma insulin concentrations were assessed during an oral glucose tolerance test. Administration of glycosphingolipids, particularly IGL, increased intrahepatic trapping of CD8 T and NKT lymphocytes. Pancreatic and liver histology were markedly improved and steatosis was reduced in all treated groups compared with vehicle-treated rats. Insulin secretion was restored after glycosphingolipid treatment, resulting in improved glucose tolerance. The immunomodulatory effect of beta-glycosphingolipids improved the beta-cell function of the hyperglycemic CDS rat. Thus our results suggest a role for the immune system in the pathogenesis of diabetes in this model.     

Blood glucose and serum insulin levels in lean and genetically obese mice.

Fed and 24 hour fasted lean and genetically obese mice (ob/ob) were given a fixed glucose load per gm body weight by intraperitoneal and intragastric administration. Intraperitoneal glucose injection into the obese mice produced a prolonged elevated blood glucose level with a concomitant significant decrease of circulating insulin. Possible interpretations of this observation are discussed. In those obese animals in which glucose was administered intragastrically the fed obese mice had a blood glucose concentration of 450-500 mg% for a period of one hour but there was no increase in circulating insulin, however, in the fasted obese mice in which the glucose concentration was about 350 mg% for one hour, there was a significant increase in the circulating insulin levels. The fed and fasted lean mice showed normal glucose tolerance curves and the expected increase in circulating insulin following either intraperitoneal orintragastric glucose loads. It is concluded that hyperglycaemia in the ob/ob mice is unlikely to be the principal cause of hyperinsulinaemia.     

Anabolic regulation of gluconeogenesis by insulin in isolated rat hepatocytes

The role of substrate availability in the regulation of gluconeogenesis in isolated rat hepatocytes was studied using [U-14C]alanine as a tracer in the presence of different concentrations of L-alanine in the incubation medium. At low alanine concentrations (0.5 mM) insulin decreased the 14C incorporation into the glucose pool and increased the incorporation of tracer carbons into the protein and lipid pools and into CO2. The net radioactivity lost from the glucose pool was only a small percentage of the total increase in the activity of the protein, lipid, CO2, or glycogen pools, supporting the notion that the effect of insulin in diminishing gluconeogenesis is secondary to its effects on pathways using pyruvate. At higher concentrations of alanine (2.5, 5.0, and 10.0 mM) in the incubation medium insulin increased the movement of alanine carbons into protein and glucose. This suggests that at higher substrate concentrations the ability of the liver to synthesize proteins is overwhelmed and the pyruvate carbons are forced into the gluconeogenesis pathway. These results were further confirmed by using [U-14C]lactate. The increases in observed specific activity of glucose following insulin administration would not be possible if insulin acted by affecting the activity of any enzyme directly involved in the formation or utilization of pyruvate, most of which have been proposed as sites of insulin action. Data presented show that insulin "inhibits" gluconeogenesis by affecting a change in substrate availability.     

Improvement of insulin resistance by panax ginseng in fructose-rich chow-fed rats.

In an attempt to probe a new target for handling insulin resistance, we used Panax ginseng root to screen the effect on insulin resistance induced by fructose-rich chow in rats. Insulin action on glucose disposal rate was measured using the glucose-insulin index, which is the product of the areas under the curve of glucose and insulin during the intraperitoneal glucose tolerance test. Oral administration of Panax ginseng root (125.0 mg/kg) into rats three times daily for three days after receiving fructose-rich chow for four weeks reversed the increased glucose-insulin index, indicating that Panax ginseng root has the ability to improve insulin sensitivity. In addition, the plasma glucose concentrations in rats repeatedly treated with Panax ginseng root were not elevated as markedly as those of the vehicle-treated group during the fructose-rich chow-feeding period. Also, the time in which the plasma glucose-lowering response to tolbutamide (10.0 mg/kg, i. p.) receded in fructose-rich chow fed rats was markedly delayed by repeated Panax ginseng root treatment compared to the vehicle-treated group. The plasma glucose-lowering activity of tolbutamide is believed to depend on the secretion of endogenous insulin, which is widely used as an indicator of insulin resistance development. Thus, it provided supportive data that oral administration of Panax ginseng root could delay the development of insulin resistance in rats. In conclusion, our results suggest that oral administration of Panax ginseng root improves insulin sensitivity and may be used as an adjuvant therapy for treating diabetic patients with insulin resistance.     

Increase of insulin sensitivity by stevioside in fructose-rich chow-fed rats.

The intake of dietary fructose has undergone a marked increase around the world, especially the developed countries, in recent times. Stevioside, a glycoside contained in the leaves of Stevia rebaudiana Bertoni (Compositae), was used to screen the effect induced by a diet containing 60% fructose on insulin resistance in rats. Single oral administration of stevioside for 90 min decreased plasma glucose concentrations in a dose-dependent manner in rats receiving fructose-rich chow for four weeks. In addition, insulin action on glucose disposal rate was measured using the glucose-insulin index, the product of the areas under the curve of glucose, and insulin during the intraperitoneal glucose tolerance test. Oral administration of stevioside (5.0 mg/kg) in rats given four weeks of fructose-rich chow for 90 min reversed the value of glucose-insulin index, indicating that stevioside has the ability to improve insulin sensitivity in this insulin-resistant animal model. Time for the loss of plasma glucose lowering response to tolbutamide (10.0 mg/kg, i. p.) in fructose-rich chow fed rats was also markedly delayed by repeated stevioside treatment three times daily compared to the vehicle-treated group. The plasma glucose-lowering activity of tolbutamide was introduced to account for varying levels of endogenous insulin secretion, and is widely used as the indicator of insulin resistance development. Thus, it provided the supportive data that repeated oral administration of stevioside delayed the development of insulin resistance in rats on a high-fructose diet. Increased insulin sensitivity by stevioside administration was further identified using the plasma glucose-lowering action of exogenous insulin in streptozotocin-induced diabetic rats (STZ-diabetic rats). Oral administration of stevioside at 0.2 mg/kg three times daily into STZ-diabetic rats for ten days increased the response to exogenous insulin. Taken together, this demonstrated that oral administration of stevioside improves insulin sensitivity, and seems suitable as an adjuvant for diabetic patients and/or those that consume large amounts of fructose.     

Mechanism of action of insulin in diabetic patients: a dose-related effect on glucose production and utilisation

Six insulin-requiring diabetics were studied after insulin had been withheld for 24 hours. On three separate occasions each received a two-hour infusion of insulin at a low dose (2·6 U/h) and a high dose (10·6 U/h) and an infusion of saline as control. The rates of production and utilisation of glucose were measured isotopically. The rate of fall of plasma glucose concentration was faster on the high-dose infusion of insulin than on the low, whereas the fall in plasma free fatty acids, glycerol, and keton bodies was the same on both insulin infusions. The mechanism whereby the two rates of insulin administration lowered plasma glucose concentration differed: during the low-dose infusion the decrease in the glucose concentration was produced entirely by a fall of hepatic glucose output, whereas during the high-dose insulin infusion the glucose concentration fell because both the rate of glucose production fell and the rate of glucose utilisation rose. In all experiments there was a direct relation between a fall in serum potassium concentration and the fall in plasma glucose concentration irrespective of the mechanism that reduced the glucose concentration.These results indicate that in uncontrolled diabetics low-dose insulin infusions lower the blood glucose concentration entirely by reducing glucose production from the liver and that the effect of insulin on potassium transport is independent of its effect on glucose uptake.     

Portal GLP-1 administration in rats augments the insulin response to glucose via neuronal mechanisms

The incretin glucagon-like peptide-1 (GLP-1)-(7---36) amide is an important factor in prandial glucose homeostasis. Findings that GLP-1 is rapidly inactivated led to the hypothesis that the target of GLP-1 is close to the site of release. To investigate whether the target tissue is located in the hepatoportal system, we administered GLP-1 with glucose into the portal vein of rats and compared this with peripheral GLP-1 administration (jugular vein) and studied the effects of blockers of the nervous system. Portal GLP-1 augmented the insulin response to a portal glucose bolus by 81% (P < 0.01) and markedly improved the glucose disposal rate (P < 0.05). Peripheral administration of GLP-1 produced a similar augmentation of the insulin response (88%) and of the glucose disposal rate. However, only the effect of portal GLP-1 on insulin secretion was blocked by the ganglionic blocker chlorisondamine. The data suggest that prandial beta-cell stimulation by GLP-1 is evoked via a neural reflex triggered in the hepatoportal system. Because absorbed nutrients and GLP-1 first appear in the portal system, this mechanism may constitute a major pathway of GLP-1 action during meals.     

Effect of isoproterenol on the plasma C-peptide and insulin levels in humans

There are conflicting reports on the effect of stimulation of the beta-adrenergic receptors on insulin removal by the liver. It was, therefore, the aim of the present study to clarify that problem. Four experiments have been carried out on a group of 8 healthy female volunteers: (1) isoproterenol was infused intravenously, (2) glucose was infused intravenously, (3) isoproterenol was infused with glucose, and (4) infusion of isoproterenol and glucose was preceded by administration of propranolol (the beta-adrenergic blocking agent). The concentration of C-peptide and insulin was determined in plasma from the antecubital vein. It has been found that stimulation of the beta-adrenergic receptors with isoproterenol reduces insulin removal by the human liver. This effect of isoproterenol is prevented by blockade of the beta-adrenergic receptors with propranolol.     

Long-term continuous intraperitoneal insulin treatment in brittle diabetes.

Attempts to achieve a fair metabolic equilibrium in a young woman with brittle diabetes by continuous subcutaneous, intramuscular, and continuous intravenous administration of insulin were unsuccessful. Continuous intraperitoneal administration of insulin through a permanently inserted polyethylene catheter connected to an open-loop peristaltic pump led to an appreciable improvement in mean blood glucose concentration, mean amplitude of glycaemic excursions, and M value and to normalisation of intermediate metabolic products. The peritoneal catheter was well tolerated for over 120 days without appreciable adverse effects. This case suggests that long-term intraperitoneal administration of insulin is a feasible therapeutic approach in the management of brittle diabetes.     

Abnormal glucose metabolism in heterozygous mutant mice for a type I receptor required for BMP signaling

BMPRIA and its high‐affinity ligand BMP4 have recently been shown to be expressed in the β‐cells of the pancreas. Here, we report the abnormalities of heterozygous mice for Bmpr1a in glucose metabolism during the course of intraperitoneal glucose tolerance test. The heterozygous mice had increased blood glucose levels throughout the first 2.5 h after the administration of glucose. Analysis of glucose‐stimulated insulin secretion (GSIS) indicates that insulin secretion in the heterozygous mice is compromised, and induction of secreted insulin by stimulation is substantially lower compared with the wild‐type controls. No apparent abnormalities in pancreas, thyroid, and liver were seen upon histological examination. Real‐time PCR results of selected genes showed an increase in the mRNA level of Ins1 and Ins2 in the heterozygous group. These results indicate that the glucose‐sensing pathway in these heterozygous mice is altered because of the heterozygosity in Bmpr1a. Together, our data suggest that BMP signaling through BMPRIA plays an important role in glucose metabolism and possibly working through the GSIS pathway. genesis 47:385–391, 2009. © 2009 Wiley‐Liss, Inc.     

Bile salts promote the absorption of insulin from the rat colon

The absorption of insulin mixed with sodium deoxycholate (DOC) or sodium cholate from the rectal mucosa of diabetic and non-diabetic rats was measured by the effect on blood glucose levels. Blood sugar was lowere by 50% one hour after administration of 12 u soluble insulin mixed with 1–10 mg/ml DOC, or 2–20 mg/ml sodium cholate. There was a linear correlation between the reduction in blood glucose and the amount of insulin administered (1–64 units) when mixed with 5 mg/ml DOC. Radioimmuno-assay of plasma insulin showed an increase from 16.2 μu/ml to 3335 muuu/ml after rectal administration of 12 u soluble insulin. We conclude that insulin when mixed with bile salts can be absorbed by the intestine to reach the circulation in a biologically active form.     

Role of hepatic STAT3 in brain-insulin action on hepatic glucose production

STAT3 regulates glucose homeostasis by suppressing the expression of gluconeogenic genes in the liver. The mechanism by which hepatic STAT3 is regulated by nutritional or hormonal status has remained unknown, however. Here, we show that an increase in the plasma insulin concentration, achieved either by glucose administration or by intravenous insulin infusion, stimulates tyrosine phosphorylation of STAT3 in the liver. This effect of insulin was mediated by the hormone's effects in the brain, and the increase in hepatic IL-6 induced by the brain-insulin action is essential for the activation of STAT3. The inhibition of hepatic glucose production and of expression of gluconeogenic genes induced by intracerebral ventricular insulin infusion was impaired in mice with liver-specific STAT3 deficiency or in mice with IL-6 deficiency. These results thus indicate that IL-6-STAT3 signaling in the liver contributes to insulin action in the brain, leading to the suppression of hepatic glucose production.     

Effects of Y. pestis mouse toxin on carbohydrate metabolism in rats

Effects of intravenous Y. pestis mouse toxin (LD50) injection on glucose, lactate glucagon, insulin blood levels and cAMP liver content in dynamics of intoxication development were studied. Hypoglycemia, observed 2 hours after toxin administration seems not to be due to the enhanced glucose utilization in peripheral tissues because insulin blood level during this period was decreased and lactate concentration has not been changed. Glucagon content by 2-5 hour of shock was strong elevated. Proposal is made that Y. pestis mouse toxin might induce carbohydrate metabolism alterations via direct liver glucose synthesising enzymes inhibition rather than cAMP-dependent glycogenolysis and gluconeogenesis regulation disturbances in this organ.