Effects of tolbutamide on insulin binding to isolated fat cells of the rat

The effects of sulfonylurea glyburide on superoxide dismutase, catalase, and glutathione peroxidase activities in the brain tissue of streptozotocin-induced diabetic rat

OBJECTIVES

A member of the second-generation sulfonylureas, glyburide (GLY; glibenclamide) provides an effective therapy for patients with type 2 diabetes. It stimulates pancreatic insulin secretion, suggesting that it is effective in the treatment of type 2 diabetes primarily by elevating the circulating insulin levels. However, experimental evidences have indicated that sulfonylureas have also had an extrapancreatic effect, which may directly contribute toward maintaining blood glucose homeostasis during diabetes.

METHODS

In this study, we administrated GLY to streptozotocin-induced diabetic rats and determined the effects of such treatment on activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) from brain tissue.

RESULTS AND DISCUSSION

Brain CAT and GPx activities were not significantly different in the diabetic group compared to controls (P>.05), but the SOD activity was significantly reduced in the diabetic group compared to controls (P<.001). GLY treatment of 4 weeks had restored the SOD and CAT enzyme activities in diabetic rat brain (P<.05). In addition, high blood glucose levels of untreated diabetic rats were decreased following the GLY treatment (P<.01). Administration of GLY to diabetic rats restored the diabetes-induced changes, suggesting that GLY could restore the brain SOD and CAT activities.

Effect of the sulfonylurea glyburide on glycogen synthase activity in alloxan-induced diabetic rat adipocytes

1. The effect of glyburide (glibenclamide) treatment in vivo on the adipose tissue glycogen synthase activity of type II diabetic rats has been studied. 2. Three week treatment of diabetic animals with glyburide (5 mg/kg orally, in saline) increased adipose glycogen synthase activity and decreased blood glucose levels. 3. These results demonstrate that the sulfonylurea glyburide is capable of exerting direct insulin-like effect on adipose glycogen-synthase activity of type II diabetic rats in vivo.

Effect of the sulfonylurea glyburide on glutathione and glutathione peroxidase activity in alloxan-induced diabetic rat hepatocytes

1. The effect of glyburide treatment on glutathione peroxidase activity and glutathione levels of non-insulin diabetic rats has been studied. 2. Hepatic glutathione and glutathione peroxidase concentrations were significantly reduced in diabetic animals. 3. Glyburide treatment of diabetic rats for 4 weeks corrected the changes on the glutathione levels observed in diabetic liver. 4. High blood glucose levels of untreated diabetic rats were decreased following glyburide treatment as well. 5. Administration of glyburide to diabetic rats reversed the diabetes-induced changes suggesting that glyburide may directly increase liver glutathione concentrations.

Effect of the sulfonylurea glyburide on superoxide dismutase activity in alloxan-induced diabetic rat hepatocytes

In the present study we administrated glyburide (glibenclamide) to type 2 (NIDDM) diabetic rats and determined the effect of such treatment on liver superoxide dismutase (SOD) activity. Hepatic SOD activity was significantly reduced in diabetic animals. Glyburide treatment of diabetic rats for 4 weeks corrected the changes observed in diabetic liver. In addition, blood glucose levels of untreated diabetic rats decreased following glyburide treatment. Administration of glyburide to diabetic rats reversed the diabetes-induced changes, suggesting that glyburide may directly increase liver SOD enzyme activity.

Increase in insulin binding and inhibition of the decrease in the phospholipid content of human term placental homogenates in culture by the sulfonylurea glipizide

Term placental explants were cultivated for 48 hr without (control) and with various concentrations of glipizide. Maximum binding of [125I]-insulin in the control samples was decreased after 12 and 24 hr returning to initial values after 48 hr. In the presence of glipizide the binding was generally higher, reaching 180% (557 and 1000 nmol/L) of the corresponding control value (P < 0.01) after 48 hr owing to the presence of nearly 3-fold more (P < 0.05) receptors than in the untreated controls. Tissue cholesterol content was almost unaffected whereas both the phospholipid content and the corresponding phospholipid-to-cholesterol ratios were markedly, and in a time-dependent manner, increased by glipizide as compared to the controls. This was due to decreasing cholesterol and phospholipid concentrations in the controls during the time of culture as compared to initial values, and also to unchanged levels in glipizide-treated cultures. We conclude that glipizide affects placental insulin receptors and the phospholipid content of the tissue.

Effects of sulfonylureas on adipocyte and skeletal muscle insulin action in patients with non-insulin-dependent diabetes mellitus

The effect of glibenclamide treatment on insulin action in isolated fat cells was studied in eight moderately obese patients with non-insulin-dependent diabetes mellitus (NIDDM). Insulin receptor binding and the effect of insulin on glucose transport and lipogenesis were determined before and after 3 months of glibenclamide therapy. At the end of the treatment period, mean daytime plasma glucose concentrations were reduced (10.8 +/- 0.4 versus 7.0 +/- 0.3 mmol/L, p less than 0.001) whereas mean daytime plasma insulin level was increased (40 +/- 12 versus 71 +/- 9 mU/L, p less than 0.001). Adipocyte insulin receptor binding as well as basal glucose transport and metabolism were unaffected by drug treatment. In contrast, insulin-stimulated glucose transport and lipogenesis were both significantly enhanced (p less than 0.05). These findings are comparable to those of another study involving seven moderately obese subjects with NIDDM who had biopsies of the lateral vastus muscle taken for measurement of insulin receptor function and glycogen synthase activity before and during 2 months of gliclazide treatment. In that study insulin receptors purified with wheatgerm agglutinin showed unchanged insulin binding and receptor kinase activity. Moreover, gliclazide had no impact on maximal glycogen synthase activity. However, under physiologic hyperinsulinemic conditions gliclazide therapy was associated with an increased sensitivity of glycogen synthase for its allosteric activation by glucose-6-phosphatase (p less than 0.04). In conclusion, sulfonylurea treatment of NIDDM enhances insulin-stimulated peripheral glucose utilization in part through a potentiation of insulin action on adipose tissue glucose transport and lipogenesis and skeletal muscle glycogen synthase.

Pharmacokinetic-pharmacodynamic relationships of oral hypoglycaemic agents. An update

Oral hypoglycaemic drugs, sulphonylureas and biguanides, occupy an important place in the treatment of Type II (non-insulin-dependent) diabetic patients who fail to respond satisfactorily to diet therapy and physical exercise. Although the precise mechanisms of action of these compounds are still poorly understood, there is sufficient agreement that sulphonylureas have both pancreatic and extrapancreatic effects, whereas biguanides have predominantly extrapancreatic actions. By using labelled compounds or measuring the circulating concentrations, the main pharmacokinetic properties of oral hypoglycaemic agents have been assessed and, in some cases, their pharmacokinetic-pharmacodynamic relationships have been evaluated. A correlation between diabetes control and plasma sulphonylurea or biguanide concentrations is generally lacking at the steady-state, with the possible exception of long-acting agents; after either oral or intravenous dosing, the reduction of plasma glucose is usually related to the increased circulating drug concentrations. The toxic effects of oral hypoglycaemic drugs are more frequent in the elderly and in the presence of conditions that may lead to drug accumulation or potentiation (increased dosage, use of long-acting compounds, hepatic and renal disease, interaction with other drugs); however, a relationship between toxic effects and drug plasma levels has been reported only for biguanides.

Effect of glyburide on glycogen metabolism in cultured rat hepatocytes

Sulfonylurea agents decrease hepatic glucose production and fasting glucose levels in type II diabetic patients without changing fasting insulin concentrations. This raises the possibility that these drugs may act directly on hepatic carbohydrate metabolism. Cultured rat hepatocytes were used to test this hypothesis. To ascertain whether this in vitro system was suitable to demonstrate an effect of sulfonylurea agents (eg, the well-documented insulin-potentiating action), we initially measured the effect of glyburide (2 micrograms/mL) on insulin-stimulated net glucose-14C incorporation into glycogen. Glyburide increased sensitivity to insulin (ie, shifted the dose-response curve to the left) without affecting either responsiveness or insulin binding. Thus, the ED50 was significantly lowered (8.4 v 15.2 ng/mL), whereas the percent increase (181% v 170%) over the basal level, specific tracer insulin binding (5.3% v 5.1% per mg protein), and the Scatchard plots were similar. Since an effect of sulfonylurea agents could be demonstrated in this system, and the glycogen pathways supply 75% of hepatic glucose production after an overnight fast, we next measured the direct effect of glyburide (2 micrograms/mL) on glycogen storage and breakdown. Glycogenolysis was assessed by measuring the breakdown of prelabeled glycogen (from galactose-14C) and glycogen synthesis by the incorporation of glucose-C14 into glycogen. Glyburide significantly inhibited glycogenolysis and stimulated glycogen synthesis. Furthermore, glyburide significantly stimulated glycogen synthase while glycogen phosphorylase was unaffected. In conclusion, glyburide directly inhibited glycogenolysis, stimulated glycogen synthesis and glycogen synthase, and potentiated the action of insulin on glycogen synthesis at a postbinding site in cultured rat hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship between the pharmacokinetics and pharmacodynamic effects of oral hypoglycaemic drugs

Oral hypoglycaemic drugs have widely differing pharmacokinetic properties. Possible pharmacodynamic benefits include greater efficacy and fewer adverse effects. In general, it has not been possible to demonstrate unequivocal differences in clinical efficacy between the sulphonylureas during long term use, although there are clear differences in potency. These differences have been emphasised to the extent that the term 'second-generation' has been used for the most potent sulphonylureas, but there is little to suggest that potency is of any therapeutic significance. Trials to study differences in efficacy have rarely been of acceptable design. They have often used fixed doses of drugs, begging the question of whether true potency ratios have been established for chronic treatment. They have rarely involved substantial numbers of patients in double-blind crossover studies with a suitable washout period. Trials which show that there is a clear relationship between drug concentrations in blood and drug effects (whether therapeutic effects or adverse effects such as severe hypoglycaemia) are generally lacking. Qualitative and semiquantitative analysis of adverse effects supports the concept that drugs with a long half-life (e.g. chlorpropamide), renally excreted active metabolites (e.g. acetohexamide) or unusual properties (e.g. glibenclamide, which accumulates progressively in islet tissue) are more likely to cause prolonged hypoglycaemia, which may be fatal. The major adverse effect of treatment with biguanides is lactic acidosis, and this probably occurs more commonly in patients treated with phenformin than those treated with metformin because of pharmacogenetic variation in phenformin metabolism. The available evidence therefore favours the use of drugs with a short elimination half-life which are extensively metabolised and which have no active metabolites.

Characterization of the sulfonylurea-induced potentiation of the insulin response in cultured 3T3 adipocytes

Studies were carried out to determine the role of sulfonylureas in the regulation of insulin-sensitive hexose uptake in cultured 3T3 adipocytes. Exposure (0-72 hr) of cells to the sulfonylurea-derivative tolbutamide (0.05-0.3 mg/ml) induced a time- and concentration-dependent potentiation of the stimulatory effect of insulin on hexose uptake (500 vs 340%). The effect was maximal within 24 hr and completely reversible. It was strictly limited to the presence of insulin. Basal hexose uptake and insulin binding were not affected by the drug. High concentrations of the agent (greater than 0.3 mg/ml) induced a decrease in insulin response, suggesting a concentration optimum. Lineweaver-Burk analysis of uptake data revealed that the potentiating effect of tolbutamide was due to enhancement of the insulin-induced increase in apparent Vmax, i.e. in the number or activity of hexose transporters. This enhancement was inhibited by cycloheximide (1 microgram/ml), indicating involvement of protein synthesis in the induction of the effect. It is concluded that sulfonylureas act by influencing synthesis of protein(s) which potentiate the effect of insulin on hexose uptake.

Insulin-potentiating action of gliclazide (diamicron)

Normal rats were given a daily dose of gliclazide (8 mg/kg) for a period of 6 days. Twenty-four hours after the treatment period, the treated animals had a significant increase in the initial rate of glucose utilization during intravenous glucose tolerance tests when compared to the controls. Insulin-stimulated 3-0-Methylglucose transport and glucose oxidation were potentiated in adipocytes prepared from gliclazide-treated rats, indicating that improved glycemic control can, at least partly, be attributed to extrapancreatic effects. The drug treatment did not induce any changes in total insulin binding, implicating that the extrapancreatic effects of this hypoglycemic drug are mediated through a post-binding site.

Dual actions of sulfonylureas and glyburide. Receptor and post-receptor effects

Glyburide and other sulfonylureas consistently enhance receptor binding in cells from patients with non-insulin-dependent diabetes mellitus, whereas no effects and mixed effects have been demonstrated in cells from patients with insulin-dependent diabetes mellitus and in normal cells, respectively. These findings indicate that the experimental model may be critical in demonstrating sulfonylurea effects on receptor binding. Postbinding function studies have shown a definite enhancement of peripheral glucose metabolism by sulfonylurea drugs; such post-receptor changes have not clearly correlated with receptor binding alterations. Studies using mouse-cultured myocytes indicate that both glyburide and tolazamide have stimulatory effects on glucose uptake, whereas only glyburide caused an increase in receptor binding. The data suggest a major and widespread post-receptor function for the sulfonylurea drugs, particularly glyburide, possibly mediated through pathways similar but not identical to insulin pathways. The direct receptor effects, in contrast, are possibly more tissue-specific and/or disease-dependent. In non-insulin-dependent diabetes mellitus, these drugs exert clinical efficacy by acting through both mechanisms.

Fat cells: model system to investigate molecular mechanism(s) of sulfonylurea-potentiated glucose transport

The cellular mechanism of action of sulfonylureas may vary depending upon the exact nature of the drug, the tissue or cell type in question, and the status of the subject from which it was obtained. In adipocytes from patients with type II non-insulin-dependent diabetes, there is increasing evidence indicating that sulfonylureas ameliorate a post-receptor defect in insulin action by potentiating the insulin-stimulated glucose transport normally seen in these cells. Studies undertaken to elucidate the molecular mechanism of this potentiation investigated the effects of a 48-hour incubation with glyburide (2 micrograms/ml) on the recruitment of glucose carriers from microsomal storage pools to the plasma membrane. With the use of cytochalasin B, a potent competitive inhibitor of glucose transport, glucose-sensitive cytochalasin B binding was studied in basal and insulin-stimulated adipocytes from control and sulfonylurea-treated tissue. The data indicated that sulfonylurea treatment did not affect the total glucose-sensitive cytochalasin B binding capacity of adipocyte membranes. It did, however, increase the insulin-induced recruitment of the glucose carrier from the microsome to the plasma membrane by 27 to 31 percent. This suggests that the molecular mechanism of sulfonylurea-enhanced insulin-stimulated glucose transport is the recruitment of glucose transporters from an intracellular microsomal storage pool to the plasma membrane.

Effects of sulfonylureas on the actions of insulin and insulin-mimickers: potentiation of stimulated hexose transport in adipocytes

The sulfonylurea glyburide, a 'second-generation' oral hypoglycemic compound, was studied in vitro in order to determine its cellular mechanism of action in adipocytes prepared from cultured rat epididymal fat tissue. Glyburide treatment (1 microgram/ml) for 20 h did not alter insulin receptor number or affinity, or down-regulation by insulin. Biologic responses of these cells were measured in the presence of insulin or the oxidants Vitamin K5 and H2O2, which have insulin-like activity, but do not act through the binding portion of the receptor. 2-Deoxyglucose uptake was not significantly changed by exposure to glyburide alone. However, the sulfonylurea increased the insulin-stimulated or insulin-mimicker-activated uptake by approximately 30%. Insulin-stimulated glucose oxidation was also potentiated when glucose transport was rate limiting for metabolism. These findings extend our earlier observation that in adipose tissue the primary cellular mechanism of action of sulfonylureas is to potentiate insulin-stimulated hexose transport, and that this process may account for their hypoglycemic activity.

Trial of sulfonylurea in combination with insulin in the therapy of diabetes type I and II. Evidence against a primary extrapancreatic receptor effect

Recently in vitro evidence has been presented that sulfonylurea derivatives exert their chronic extrapancreatic effect by increasing the number of cellular insulin receptors. To ascertain if this receptor effect holds in vivo, we performed a randomized double-blind study on 21 type I (0.3 ng/ml residual C-peptide secretory capacity after glucose/glibenclamide stimulation), and on 19 insulin treated type II (2.0 ng/ml C-peptide) diabetics. The patients received for six weeks 10 mg/d of glibenclamide in addition to insulin. Insulin binding was initially lower in type II (4.7 +/- 0.75% per 10(7) monocytes and 6.39 +/- 1.08% per 4.5 X 10(9) erythrocytes) than in type I diabetic patients (5.1 +/- 0.48% and 7.95 +/- 0.88% respectively) and in 12 normal subjects (5.25 +/- 0.48 and 8.1 +/- 0.94% respectively). Glibenclamide normalized the number of monocyte receptors (from 4.14 to 5.49 X 10(4) sites/cell) in type II patients, but was without effect in type I diabetics. Blood glucose was significantly reduced (240 to 182 mg/dl; p = 0.02) in the type II group with a concomitant decrease in glycosylated hemoglobin from 12.4 to 10.5% (p = 0.01). Most of the effect occurred during the first week of treatment. Glibenclamide was the more effective the worse the initial metabolic state (r = -0.93; p = 0.001). Erythrocyte insulin receptors decreased markedly in both groups, perhaps due to a sulfonyl urea-induced change in erythrocyte plasma survival time. It is concluded that sulfonylurea treatment is a valuable adjunct in reducing the insulin resistance in insulin treated type II diabetics. The effect depends on the availability of endogenous insulin, thus exhibiting only partly extrapancreatic character.(ABSTRACT TRUNCATED AT 250 WORDS)

Extrapancreatic action of the sulphonylurea gliquidone: post-receptor effect on insulin-stimulated glycogen synthesis in rat hepatocytes in primary culture

The effects of a sulphonylurea, gliquidone, on insulin binding and the insulin induced rate of glycogen synthesis, were studied in rat hepatocytes in primary culture. Hepatocytes were cultured for 48 h. During the second 24 h of this period, the hepatocytes were incubated with or without gliquidone (5 mg/l). The binding of 125I-insulin and the insulin stimulation of glycogen synthesis from 14C-glucose were measured. Gliquidone influenced neither insulin binding nor the basal rate of glycogen synthesis, but it did enhance the effect of insulin on glycogen synthesis. Responsiveness was increased by gliquidone at all insulin concentrations used (10-10,000 mU/l); at 1000 mU/l the drug increased glycogen synthesis from 310 to 430% above the basal rate. Half-maximal stimulation was reached in control cells at an insulin concentration of 95 mU/l and in gliquidone-treated cells at 94 mU/l, which indicates unchanged insulin sensitivity. Based on these experiments with cultured rat hepatocytes it appears that the extrapancreatic action of gliquidone is not mediated by an effect on insulin binding.

Effects of theophylline on insulin receptors and insulin action in the adipocyte

The effects of theophylline on insulin receptors and insulin action in isolated rat adipocytes were studied. Theophylline reduced insulin binding by a decrease of receptor affinity. As concentration-response curves revealed, the effect was paralleled by a reduction of the cellular ATP content. Basal as well as insulin-stimulated glucose transport (2-deoxyglucose and 3-O-methylglucose uptake) were inhibited by much smaller theophylline concentrations (0.15-0.6 mM) than those necessary to reduce insulin binding and to lower ATP levels (1-4.8 mM), or to stimulate lipolysis (0.3-2.4 mM). Insulin fully antagonized the effect of theophylline on lipolysis but failed to reverse the inhibition of glucose transport completely. The results suggest that (a) theophylline impairs insulin action at a post-receptor level and, at higher concentrations, by a decrease of receptor binding, (b) the reduction of insulin receptor affinity probably reflects ATP depletion of the adipocyte, and (c) the xanthine inhibits glucose transport independently from its effects on lipolysis.

Reversible reduction of insulin receptor affinity by ATP depletion in rat adipocytes

The effects of the metabolic inhibitor NaN3 on insulin receptors in isolated rat fat-cells were investigated. The agent reduced insulin binding in parallel to a decrease of the ATP content of cells. Both effects were observed in the same concentration range of NaN3, and were fully reversible. According to the binding curves the affinity rather than the number of receptors was reduced. Kinetic experiments revealed an increased dissociation rate of the insulin-receptor complex. The effects outlasted cell disruption, since the receptor affinity was still lowered in plasma membranes obtained from NaN3-treated cells. Thus an inhibition of insulin internalization could not account for the observed effects. It is suggested that the observed ATP-dependence of insulin receptor affinity reflects a reversible structural alteration of the receptor, or of some closely related membrane protein.