Effects of glucagon and insulin on lipolysis and ketogenesis in sheep.
The hepatic and portal productions of acetoacetate and beta-hydroxybutyrate and lipolysis were studied in normal and insulin-controlled alloxan-diabetic sheep. Since hyperinsulinemia is associated with glucagon administration, the latter group of sheep were used to maintain constant plasma insulin levels. After control values were obtained glucagon was infused intraportally at 90 mug/hr for two hours. The ketone body production by portal drained viscera was not significantly affected by glucagon. In alloxanized sheep, glucagon significantly (P less than 0.01) increased net hepatic production of acetoacetate (from -0.54 +/- 0.08 to 0.46 +/- 0.07 g/hr). Lipolysis also increased. However, in the normal sheep, hyperinsulinemia prevented any stimulatory effect of glucagon on hepatic ketogenesis and lipolysis. Therefore, while glucagon appears capable of stimulating ketogenesis andlipolysis, these effects are readily suppressed by insulin. (+info)
Effect of diabetes and aminoguanidine therapy on renal advanced glycation end-product binding.
BACKGROUND: Advanced glycation end-products (AGEs) have been implicated in the pathogenesis of diabetic nephropathy, and aminoguanidine (AG) has been shown to decrease the accumulation of AGEs in the diabetic kidney. METHODS: This study investigates changes in AGE binding associated with diabetes in the rat kidney using in vitro and in vivo autoradiographic techniques. Male Sprague-Dawley rats were randomized into control and diabetic groups with and without AG treatment and were sacrificed after three weeks. Frozen kidney sections (20 microm) were incubated with [125I]-AGE-RNase or [125I]-AGE-BSA. To localize the AGE binding site, in vivo autoradiography was performed by injection of 15 microCi of [125I]-AGE-BSA into the abdominal aorta of the rat. RESULTS: Low-affinity binding sites specific for AGEs in the renal cortex (IC50 = 0.28 microm) were detected by in vitro autoradiography. There was a significant increase in [125I]-AGE binding in the diabetic kidney, which was prevented by AG treatment. Emulsion autoradiography revealed that binding was localized primarily to proximal tubules in the renal cortex. Renal AGE levels, as assessed by fluorescence or by radioimmunoassay, were increased after three weeks of diabetes. This increase was attenuated by AG therapy. CONCLUSIONS: AGE binding sites are present within the proximal tubules of the kidney and appear to be modulated by endogenous AGE levels. It remains to be determined if these binding sites represent receptors involved in clearance of AGEs or are linked to pathogenic pathways that lead to the development of diabetic nephropathy. (+info)
Treatment of streptozotocin-induced diabetic rats with vanadate and phlorizin prevents the over-expression of the liver insulin receptor gene.
Administration of vanadate, an insulinomimetic agent, has been shown to normalize the increased number of insulin receptors in the liver of streptozotocin-induced diabetic rats. In the present study, the effects of vanadate on various steps of expression of the liver insulin receptor gene in diabetic rats have been analyzed and compared with those of phlorizin, a glucopenic drug devoid of insulinomimetic properties. Livers of rats killed 23 days after streptozotocin injection showed a 30-40% increase in the number of cell surface and intracellular insulin receptors, a 50-90% increase in the levels of 9.5 and 7.5 kb insulin receptor mRNA species, and a 20% decrease in the relative abundance of the A (exon 11-) insulin receptor mRNA isotype. Daily administration of vanadate or phlorizin from day 5 to day 23 prevented the increase in insulin receptor number and mRNA level, and vanadate treatment also normalized receptor mRNA isotype expression. Unlike observations in vivo, vanadate and phlorizin differentially affected the expression of the insulin receptor gene in Fao hepatoma cells. Vanadate treatment (0.5 mmol/l for 4 h) decreased the levels of the 9.5 and 7.5 kb insulin receptor transcripts by at least twofold, without affecting the relative abundance of the A insulin receptor mRNA isotype. In contrast, phlorizin treatment (5 mmol/l for 4 h) slightly increased or did not affect the levels of the 9.5 and 7.5 kb insulin receptor transcripts respectively, and increased by twofold the relative expression of the A insulin receptor mRNA isotype. It is suggested that, although mediated in part by a reversal of hyperglycemia, normalization of liver insulin receptor gene expression by vanadate treatment in diabetic rats may also involve a direct inhibitory effect of this drug on gene expression. (+info)
Novel peroxisome proliferator-activated receptor (PPAR) gamma and PPARdelta ligands produce distinct biological effects.
The peroxisome proliferator-activated receptors (PPARs) include three receptor subtypes encoded by separate genes: PPARalpha, PPARdelta, and PPARgamma. PPARgamma has been implicated as a mediator of adipocyte differentiation and the mechanism by which thiazolidinedione drugs exert in vivo insulin sensitization. Here we characterized novel, non-thiazolidinedione agonists for PPARgamma and PPARdelta that were identified by radioligand binding assays. In transient transactivation assays these ligands were agonists of the receptors to which they bind. Protease protection studies showed that ligand binding produced specific alterations in receptor conformation. Both PPARgamma and PPARdelta directly interacted with a nuclear receptor co-activator (CREB-binding protein) in an agonist-dependent manner. Only the PPARgamma agonists were able to promote differentiation of 3T3-L1 preadipocytes. In diabetic db/db mice all PPARgamma agonists were orally active insulin-sensitizing agents producing reductions of elevated plasma glucose and triglyceride concentrations. In contrast, selective in vivo activation of PPARdelta did not significantly affect these parameters. In vivo PPARalpha activation with WY-14653 resulted in reductions in elevated triglyceride levels with minimal effect on hyperglycemia. We conclude that: 1) synthetic non-thiazolidinediones can serve as ligands of PPARgamma and PPARdelta; 2) ligand-dependent activation of PPARdelta involves an apparent conformational change and association of the receptor ligand binding domain with CREB-binding protein; 3) PPARgamma activation (but not PPARdelta or PPARalpha activation) is sufficient to potentiate preadipocyte differentiation; 4) non-thiazolidinedione PPARgamma agonists improve hyperglycemia and hypertriglyceridemia in vivo; 5) although PPARalpha activation is sufficient to affect triglyceride metabolism, PPARdelta activation does not appear to modulate glucose or triglyceride levels. (+info)
Streptozotocin (STZ), a glucose analogue known to induce diabetes in experimental animals, causes DNA strand breaks and subsequent activation of poly(ADPribose) polymerase (Parp). Because Parp uses NAD as a substrate, extensive DNA damage will result in reduction of cellular NAD level. In fact, STZ induces NAD depletion and cell death in isolated pancreatic islets in vitro. Activation of Parp therefore is thought to play an important role in STZ-induced diabetes. In the present study, we established Parp-deficient (Parp-/-) mice by disrupting Parp exon 1 by using the homologous recombination technique. These mice were used to examine the possible involvement of Parp in STZ-induced beta-cell damage in vivo. The wild-type (Parp+/+) mice showed significant increases in blood glucose concentration from 129 mg/dl to 218, 370, 477, and 452 mg/dl on experimental days 1, 7, 21, and 60, respectively, after a single injection of 180 mg STZ/kg body weight. In contrast, the concentration of blood glucose in Parp-/- mice remained normal up to day 7, slightly increased on day 21, but returned to normal levels on day 60. STZ injection caused extensive necrosis in the islets of Parp+/+ mice on day 1, with subsequent progressive islet atrophy and loss of functional beta cells from day 7. In contrast, the extent of islet beta-cell death and dysfunction was markedly less in Parp-/- mice. Our findings clearly implicate Parp activation in islet beta-cell damage and glucose intolerance induced by STZ in vivo. (+info)
Inhibition of advanced glycation endproduct formation by acetaldehyde: role in the cardioprotective effect of ethanol.
Epidemiological studies suggest that there is a beneficial effect of moderate ethanol consumption on the incidence of cardiovascular disease. Ethanol is metabolized to acetaldehyde, a two-carbon carbonyl compound that can react with nucleophiles to form covalent addition products. We have identified a biochemical modification produced by the reaction of acetaldehyde with protein-bound Amadori products. Amadori products typically arise from the nonenzymatic addition of reducing sugars (such as glucose) to protein amino groups and are the precursors to irreversibly bound, crosslinking moieties called advanced glycation endproducts, or AGEs. AGEs accumulate over time on plasma lipoproteins and vascular wall components and play an important role in the development of diabetes- and age-related cardiovascular disease. The attachment of acetaldehyde to a model Amadori product produces a chemically stabilized complex that cannot rearrange and progress to AGE formation. We tested the role of this reaction in preventing AGE formation in vivo by administering ethanol to diabetic rats, which normally exhibit increased AGE formation and high circulating levels of the hemoglobin Amadori product, HbA1c, and the hemoglobin AGE product, Hb-AGE. In this model study, diabetic rats fed an ethanol diet for 4 weeks showed a 52% decrease in Hb-AGE when compared with diabetic controls (P < 0.001). Circulating levels of HbA1c were unaffected by ethanol, pointing to the specificity of the acetaldehyde reaction for the post-Amadori, advanced glycation process. These data suggest a possible mechanism for the so-called "French paradox," (the cardioprotection conferred by moderate ethanol ingestion) and may offer new strategies for inhibiting advanced glycation. (+info)
The influence of NO synthase inhibitor and free oxygen radicals scavenger--methylene blue--on streptozotocin-induced diabetes in rats.
The excessive production of nitric oxide (NO) and the subsequent increase of local oxidative stress is suggested as one of the pathophysiological mechanisms of streptozotocin-induced diabetes. It was reported that the administration of NO synthase inhibitors partially attenuated the development of streptozotocin-induced diabetes and reduced hyperglycaemia. Here we have studied the influence of methylene blue, which combines the properties of NO synthase inhibitor with antioxidant effects. The experiments were performed on male rats divided into four groups: control, diabetic (single dose of 70 mg of streptozotocin/kg i.p.), methylene blue (50 mg/kg in the food) and diabetic simultaneously fed with methylene blue. After 45 days the experiments were discontinued by decapitation. Serum glycaemia, glycated haemoglobin and oxidative stress parameters (plasma malondialdehyde concentration and erythrocyte superoxide dismutase activity) were significantly higher in the diabetic group. Simultaneous methylene blue administration partially reduced glycaemia and glycated haemoglobin, but did not decrease oxidative stress. We conclude that NO synthase inhibitor methylene blue partially attenuates the development of streptozotocin-induced diabetes in male rats, but does not reduce the development of oxidative stress in the diabetic group. (+info)
Alterations of heart function and Na+-K+-ATPase activity by etomoxir in diabetic rats.
To examine the role of changes in myocardial metabolism in cardiac dysfunction in diabetes mellitus, rats were injected with streptozotocin (65 mg/kg body wt) to induce diabetes and were treated 2 wk later with the carnitine palmitoyltransferase inhibitor (carnitine palmitoyltransferase I) etomoxir (8 mg/kg body wt) for 4 wk. Untreated diabetic rats exhibited a reduction in heart rate, left ventricular systolic pressure, and positive and negative rate of pressure development and an increase in end-diastolic pressure. The sarcolemmal Na+-K+-ATPase activity was depressed and was associated with a decrease in maximal density of binding sites (Bmax) value for high-affinity sites for [3H]ouabain, whereas Bmax for low-affinity sites was unaffected. Treatment of diabetic animals with etomoxir partially reversed the depressed cardiac function with the exception of heart rate. The high serum triglyceride and free fatty acid levels were reduced, whereas the levels of glucose, insulin, and 3,3',-5-triiodo-L-thyronine were not affected by etomoxir in diabetic animals. The activity of Na+-K+-ATPase expressed per gram heart weight, but not per milligram sarcolemmal protein, was increased by etomoxir in diabetic animals. Furthermore, Bmax (per g heart wt) for both low-affinity and high-affinity binding sites in control and diabetic animals was increased by etomoxir treatment. Etomoxir treatment also increased the depressed left ventricular weight of diabetic rats and appeared to increase the density of the sarcolemma and transverse tubular system to normalize Na+-K+-ATPase activity. Therefore, a shift in myocardial substrate utilization may represent an important signal for improving the depressed cardiac function and Na+-K+-ATPase activity in diabetic rat hearts with impaired glucose utilization. (+info)