Metformin improves vascular function in insulin-resistant rats

Metformin delays the manifestation of diabetes and vascular dysfunction in Goto-Kakizaki rats by reduction of mitochondrial oxidative stress

BACKGROUND AND AIM

This study was undertaken to test the hypothesis that hyperglycaemia induces the generation of reactive oxygen species (ROS) by mitochondria and that the oxidative stress thereby exerted is diminished by treatment with metformin. As a parameter of mitochondrial ROS formation, the activity of mitochondrial aconitase activity was determined using Goto-Kakizaki (GK) rats as model of type 2 diabetes.

METHODS

In parallel with the development of diabetes (glucose, insulin), the generation of oxidative stress was determined in aortic tissue, heart and kidney of GK rats by measurement of lipid peroxides, oxidized proteins (carbonyl activity) and mitochondrial aconitase activity. Vascular activity was determined in aortae by measuring the endothelium-dependent vasodilatation in response to acetylcholine, and vasoconstriction in response to phenylephrine.

RESULTS

At the age of 12-14 weeks, blood glucose levels rose dramatically from 7.5 up to 16.2 mM, indicating the manifestation of an overt diabetes. In addition, the glucose tolerance was impaired. The increase in blood glucose was not accompanied by changes in plasma insulin. Whereas the lipid peroxides in plasma only showed a tendency to increase, the amount of oxidized proteins (carbonyl moieties) increased from 4.6 to 10.9 micromol/mg protein (2.4 fold). In addition, the lipid peroxides in tissue were increased. Mitochondrial aconitase activity was reduced in the aorta and kidney, but not in the heart of diabetic animals. Treatment with metformin nearly normalized the hyperglycaemia and prevented the rise in carbonyl, tissue lipid peroxides and the fall in aconitase activity. Whereas the endothelium-dependent vasodilatation was not affected by the diabetes, the reaction of aortae in response to phenylephrine was strongly enhanced, changes which were prevented by treatment with metformin.

CONCLUSIONS

These observations provide in vivo evidence that the generation of ROS plays an important role in the onset of diabetes and the development of vascular dysfunction in GK rats with type 2 diabetes.

Activation of the AMP-activated kinase by antidiabetes drug metformin stimulates nitric oxide synthesis in vivo by promoting the association of heat shock protein 90 and endothelial nitric oxide synthase

Metformin, one of most commonly used drugs for the treatment of type 2 diabetes, improves vascular endothelial functions and reduces cardiovascular events in patients with type 2 diabetes, although its mechanisms remain unknown. The current study aimed to elucidate how metformin improves endothelial functions. Exposure of cultured bovine aortic endothelial cells (BAECs) to clinically relevant concentrations of metformin (50-500 micromol/l) dose-dependently increased serine-1179 (Ser1179) phosphorylation (equal to human Ser1179) of endothelial nitric oxide (NO) synthase (eNOS) as well as its association with heat shock protein (hsp)-90, resulting in increased activation of eNOS and NO bioactivity (cyclic GMP). These effects of metformin were mimicked or completely abrogated by adenoviral overexpression of a constitutively active 5'-AMP-activated kinase (AMPK) mutant or a kinase-inactive AMPK-alpha, respectively. Furthermore, administration of metformin as well as 5-aminoimidazole-4-carboxamide ribonucleoside, an AMPK agonist, significantly increased eNOS Ser1179 phosphorylation, NO bioactivity, and coimmunoprecipitation of eNOS with hsp90 in wild-type C57BL6 mice but not in AMPK-alpha1 knockout mice, suggesting that AMPK is required for metformin-enhanced eNOS activation in vivo. Finally, incubation of BAECs with clinically relevant concentrations of metformin dramatically attenuated high-glucose (30 mmol/l)-induced reduction in the association of hsp90 with eNOS, which resulted in increased NO bioactivity with a reduction in overexpression of adhesion molecules and endothelial apoptosis caused by high-glucose exposure. Taken together, our results indicate that metformin might improve vascular endothelial functions in diabetes by increasing AMPK-dependent, hsp90-mediated eNOS activation.

Efeitos da metformina na disfunção endotelial da circulação renal induzida pela hiperglicemia em coelhos não diabéticos

OBJETIVO: Avaliar o efeito da metformina (Met) na disfunção endotelial da circulação renal de coelhos não diabéticos induzida por concentrações de glicose usualmente observadas em pacientes com diabetes tipo 2 (DM2) em tratamento ambulatorial. METODOLOGIA: Rins isolados de coelhos não diabéticos foram expostos por 3h a concentrações normais (100mg/dl) e elevadas (270mg/dl) de glicose na presença ou ausência de Met (100µM). Os níveis de glicose utilizados correspondem à mediana da glicemia pós-prandial (270mg/dl) obtida 2h após o café da manhã em 780 pacientes com DM2 atendidos em nosso serviço. A reatividade vascular (RV) dependente do endotélio (DE) foi avaliada com acetilcolina (ACh) e independente do endotélio (IE) com nitroprussiato de sódio (NPS). RESULTADO: Houve redução significativa na vasodilatação DE no grupo com infusão elevada de glicose em comparação ao controle (redução máxima na pressão de perfusão de respectivamente 25 ± 3 vs. 41 ± 3%; p< 0,01). No grupo de infusão com concentrações elevadas de glicose associada à infusão contínua de Met, a resposta vasodilatadora DE foi restaurada sem haver diferença significativa com o grupo controle (redução da pressão de perfusão respectivamente de 43 ± 1,5% e 41 ± 3%, p> 0,05). A Met não alterou a vasodilatação induzida pela ACh na presença de níveis normais de glicose. Finalmente, a vasodilatação renal induzida por NPS não foi modificada pela infusão conjunta de glicose e Met. CONCLUSÃO: Níveis de glicose observados em pacientes com DM2 em tratamento ambulatorial são capazes de provocar alterações agudas na RV no modelo experimental estudado, sendo estes efeitos totalmente abolidos pela Met. Os mecanismos envolvidos nesta ação protetora da Met merecem investigações específicas.

Metformin reduces blood pressure and restores endothelial function in aorta of streptozotocin-induced diabetic rats

Effect of metformin treatment on blood pressure, endothelial function and oxidative stress in streptozotocin (STZ)-induced diabetes in rats was studied. In vitro effect of metformin on vascular reactivity to various agonist in the presence of metformin in untreated nondiabetic and STZ-diabetic rats were also studied. Sprague-Dawley rats were randomized into nondiabetic and STZ-diabetic groups. Rats were further randomized to receive metformin (150 mg/kg) or vehicle for 4 weeks. Metformin treatment reduced blood pressure without having any significant effect on blood glucose level in STZ-diabetic rats. Enhanced phenylephrine (PE)-induced contraction and impaired acetylcholine (Ach)-induced relaxation in STZ-diabetic rats were restored to normal by metformin treatment. Enhanced Ach-induced relaxation in metformin-treated STZ-diabetic rats was blocked due to pretreatment with 100 microM of Nomega-nitro-L-arginine-methyl ester (L-NAME) or 10 microM of methylene blue but not 10 microM of indomethacin. Metformin treatment significantly increased antioxidant enzymes and reduced lipid peroxidation in STZ-diabetic rats. In vitro studies in aortic rings of untreated nondiabetic and STZ-diabetic rats showed that the presence of higher concentration of metformin (1 mM and 10 mM) significantly reduced PE-induced contraction and increased Ach-induced relaxation. Metformin per se relaxed precontracted aortic rings of untreated nondiabetic and STZ-diabetic rats in a dose-dependent manner. Pretreatment with L-NAME or removal of endothelium blocked metformin-induced relaxation at lower concentration (up to 30 microM) but not at higher concentration (above 30 microM). Metformin-induced relaxation was blocked in the presence of 1 mM of 4-aminopyridine, or 1 mM of tetraethylammonium but not in the presence of 100 microM of barium ion or 10 microM of glybenclamide. The restored endothelial function along with direct effect of metformin on aortic rings and reduced oxidative stress contributes to reduced blood pressure in STZ-diabetic rats. From the present study, it can be concluded that metformin administration to STZ-diabetic rats lowers blood pressure, and restores endothelial function.

Metformin treatment restores the altered microvascular reactivity in neonatal streptozotocin-induced diabetic rats increasing NOS activity, but not NOS expression

Abnormalities in vascular function are well recognized in diabetes. Hyperglycemia may be central to the pathogenesis of vascular dysfunction but is not certain whether improvements in glycaemic control will improve vascular function. The effects of metformin, an antidiabetic agent that improves insulin sensitivity and glycaemic control, on the microvascular reactivity have not been reported in neonatal streptozotocin-induced (n-STZ) diabetes. Diabetes was induced by STZ injection (160 mg/kg, ip) in neonates (2-day-old) Wistar rats. n-STZ diabetic rats were treated with metformin (300 mg/kg, 15 d, by gavage). Using intravital microscopy the changes in mesenteric arteriolar and venular diameters were determined in anesthetized control and n-STZ diabetic rats, before and after topical application of endothelium-dependent vasodilator agents, mediators or not of the inflammatory response, and endothelium-independent vasodilator agent. We also determined the total nitric oxide synthase (NOS) activity (conversion of L-arginine to citrulline) and endothelial(e), inducible(i), and neuronal(n) NOS expression (using polymerase chain reaction after reverse transcription of the mRNAs into cDNAs) in the mesentery of metformin-treated n-STZ diabetic and vehicle-treated n-STZ diabetic and control rats. Although metformin treatment did not correct the high glycaemic levels and the impaired glucose tolerance, the reduced vasodilator responses and total NOS activity in n-STZ diabetic rats were corrected by the treatment. Neither diabetes nor metformin treatment altered the expression of the three NOS isoforms. We concluded that metformin restores the reduced response to vasodilator agents, independently of the correction of the metabolic alterations. Improvement of total NOS activity might be in part responsible for the correction.

Safety and tolerability of pioglitazone, metformin, and gliclazide in the treatment of type 2 diabetes

This analysis compares the safety and tolerability of pioglitazone (a thiazolidinedione), metformin (a biguanide), and gliclazide (a sulfonylurea). Data collected from four 1-year, double-blind studies comparing treatment of over 3700 patients with type 2 diabetes with pioglitazone, metformin, or gliclazide have been combined to provide comparative tolerability and safety profiles. All treatments were well tolerated with approximately 6% of patients withdrawing from treatment because of side-effects. The side-effects profile varied between treatments, with pioglitazone being associated with edema, metformin with gastrointestinal side-effects, and gliclazide with hypoglycemia. Cardiovascular outcome was similar with all treatments, with no excess reports of cardiac failure with pioglitazone treatment. Both pioglitazone and gliclazide resulted in mean weight gain, whilst with metformin there was mean weight loss. Mean liver enzyme values decreased with pioglitazone and to a lesser extent with metformin. With gliclazide, mean liver enzyme values increased. The expected small decreases in mean hemoglobin and hematocrit seen with pioglitazone also occurred with metformin and to a lesser degree with gliclazide. The results show that all three drugs are safe, but that tolerability profiles vary. Each treatment provides an alternative therapy for type 2 diabetes, dependent on the particular needs of individual patients.

Metformin improves endothelial function in patients with metabolic syndrome

BACKGROUND

Metabolic Syndrome (MS) is associated with impaired endothelial function and increased cardiovascular risk. Insulin resistance is a key feature of MS and plays an important role in the pathogenesis of endothelial dysfunction. Aim of the present study was to evaluate the effect of metformin on endothelial function and insulin resistance, assessed by the homeostasis model (HOMA-IR, homeostasis model assessment-insulin resistance), in patients with MS.

METHODS

Sixty-five subjects (37 men and 28 women, mean age 54 +/- 6 years) with MS were allocated to receive metformin 500 mg twice daily (n = 32) or placebo (n = 33) for 3 months. Before and after treatment we assessed endothelial function, using flow-mediated dilatation of the brachial artery, and HOMA-IR.

RESULTS

Patients who received metformin demonstrated statistically significant improvement in endothelium-dependent vasodilation compared with those treated with placebo (from 7.4 +/- 2.1% to 12.4 +/- 1.9% vs. 7.3 +/- 2.5% to 6.9 +/- 2.7%, P = 0.0016, metformin vs. placebo respectively), without significant effect on endothelium-independent response to sublingual glyceryl trinitrate (P =0.32). Metformin improved insulin resistance compared with placebo group (HOMA-IR from 3.39 to 2.5 vs. 3.42 to 3.37; 26% reduction in HOMA-IR, P = 0.01). An association between the improvement in insulin resistance and the improvement in endothelial function (r = -0.58, P = 0.0016) was found.

CONCLUSION

Metformin improves both endothelial function and insulin resistance in patients with MS. These findings support the central role of insulin resistance in the development of endothelial dysfunction and the role of metformin for the treatment of patients with MS.

Metabolic syndrome therapy: prevention of vascular injury by antidiabetic agents

More than 65 million Americans are currently obese. Type 2 diabetes mellitus, frequently seen in obese subjects, affects 17 million adults in the United States, with a continuous and alarmingly increasing rate. To prevent development of diabetes in those who are at high risk, it is recommended to optimize meal planning and enhance physical activity to make sustained weight reduction possible. In addition to lifestyle changes, various oral antidiabetic agents are available, with diverse mechanisms of action. Some target defective insulin secretion (sulphonylureas, benzoic acid derivatives) or glucose absorption (glycosidase inhibitors), whereas others target insulin resistance (metformin, thiazolidinediones). Patients with metabolic syndrome and diabetes have an increased risk for cardiovascular disease linked to a higher prevalence of hypertension, dyslipidemia, microalbuminuria, and altered hemostasis--parameters that may be modified by antidiabetic agents. In this article, we review the oral agents used to treat type 2 diabetes and the metabolic syndrome, and their effects on vascular tissue.

Metformin prevents the impairment of endothelium-dependent vascular relaxation induced by high glucose challenge in rabbit isolated perfused kidneys

High glucose concentrations are involved in the development of diabetic-associated vascular complications. We have previously reported that acute high glucose challenge, corresponding to post-prandial glycemia levels observed in patients with type 2 diabetes, blunts ACh-induced endothelium-dependent relaxation of the renal circulation of non-diabetic rabbits. Isolated perfused kidneys from non-diabetic rabbits were acutely exposed (3 h) to normal (5.5 mM--control group) or high (15 mM) D-glucose concentrations in the presence or absence of a continuous infusion of metformin (20 or 100 microM). Renal vascular reactivity was evaluated with endothelium-dependent (acetylcholine, ACh) and -independent (sodium nitroprusside, SNP) vasodilating agents. ACh-induced maximal renal vasodilation was reduced by high glucose infusion (15 mM) in comparison to the control group (25+/-3% and 41+/-3% respectively; P<0.01), being restored to 41+/-4% and 43+/-2% by a simultaneous 3-h infusion of 20 or 100 microM of metformin respectively (P>0.05). Perfusion of the kidneys with the angiotensin II-converting enzyme inhibitor captopril (10 microM) also significantly prevented the deleterious effects of high glucose challenge in the renal circulation. The use of a continuous infusion of N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM) did not affect the protective effect of metformin in the renal circulation (39+/-4%; P>0.05), while tetraethylammonium (TEA, 10 mM) partially blunted this effect (33+/-4, P<0.01). Renal vasodilation induced by SNP was not modified by simultaneous infusion of high glucose and/or metformin. It is concluded that the impairment of ACh-induced endothelium-dependent renal vasodilation observed after acute exposure to high glucose concentrations is abolished by metformin administration. These alterations of renal vascular reactivity can be accounted for, at least in part, by the activation of the renal renin-angiotensin system during hyperglycemia. The protective effects of metformin present some EDHF-dependent component and are not related to metabolic pathways dependent on nitric oxide.

Metformin prevents glucose-induced protein kinase C-beta2 activation in human umbilical vein endothelial cells through an antioxidant mechanism

Hyperglycemia determines the vascular complications of diabetes through different mechanisms: one of these is excessive activation of the isoform beta2 of protein kinase C (PKC-beta2). Metformin, a widely used antidiabetic agent, is associated with decreased cardiovascular mortality in obese type 2 diabetic patients. Therefore, we assessed the role of metformin in glucose-induced activation of PKC-beta2 and determined the mechanism of its effect in human umbilical venous endothelial cells grown to either normo- (5 mmol/l) or hyperglycemia (10 mmol/l) and moderately and acutely exposed to 25 mmol/l glucose. We studied PKC-beta2 activation by developing adenovirally expressed chimeras encoding fusion protein between green fluorescent protein (GFP) and conventional beta2 isoform (PKC-beta2-GFP). Glucose (25 mmol/l) induced the translocation of PKC-beta2-GFP from the cytosol to the membrane in cells grown to hyperglycemia but not in those grown in normal glucose medium. Metformin (20 micromol/l) prevented hyperglycemia-induced PKC-beta2-GFP translocation. We also assessed oxidative stress under the same conditions with a 4-((9-acridine-carbonyl)amino)-2,2,6,6-tetramethylpiperidin-oxyl,free radical (TEMPO-9-AC) fluorescent probe. We observed significantly increased radical oxygen species production in cells grown in hyperglycemia medium, and this effect was abolished by metformin. We show that in endothelial cells, metformin inhibits hyperglycemia-induced PKC-beta2 translocation because of a direct antioxidant effect. Our data substantiate the findings of previous large intervention studies on the beneficial effect of this drug in type 2 diabetic patients.

Differential effects of sodium tungstate and vanadyl sulfate on vascular responsiveness to vasoactive agents and insulin sensitivity in fructose-fed rats

High fructose feeding induces insulin resistance, impaired glucose tolerance, and hypertension in rats and mimics most of the features of the metabolic syndrome X. The effects of a 6-week treatment with the transition metals administered in drinking water, vanadium (VOSO4·5H2O, 0.75 mg/mL) or tungsten (Na2O4W, 2 g/mL), were investigated on the reactivity to norepinephrine (NEPI) or acetylcholine (ACh) of thoracic aorta rings isolated from fructose (60%) or standard chow fed rats. Maximal effect (Emax) and pD2(–log EC50) values were determined in each case in the presence or absence of endothelium, while the degree of insulin resistance was determined using the euglycemic hyper insulinemic glucose clamp technique. Aortic segments isolated from 6-week fructose-fed animals were characterized by NEPI hyperresponsiveness (increase in Emax) and endothelium-dependent NEPI supersensitivity (increase in pD2) without any change in the reactivity to ACh. Vanadium or tungsten administered in fructose-fed animals prevented both hypertension and NEPI hyperresponsiveness, while vanadium, but not tungsten, reduced NEPI supersensitivity. Vanadium, but not tungsten, increased the relaxing activity of ACh, both in control and fructose-fed animals. Insulin resistance associated with high fructose feeding was reversed by vanadium but not by tungsten treatment. The differential effects of the two transition metals on vascular responsiveness to NEPI or ACh may be explained by their differential effects on insulin sensitivity.Key words: vanadium, tungsten, aorta, hypertension, fructose, glucose clamp.

Activation of the AMP-activated protein kinase by the anti-diabetic drug metformin in vivo. Role of mitochondrial reactive nitrogen species

Metformin, one of the most commonly used drugs for the treatment of type II diabetes, was recently found to exert its therapeutic effects, at least in part, by activating the AMP-activated protein kinase (AMPK). However, the site of its action, as well as the mechanism to activate AMPK, remains elusive. Here we report how metformin activates AMPK. In cultured bovine aortic endothelial cells, metformin dose-dependently activated AMPK in parallel with increased detection of reactive nitrogen species (RNS). Further, either depletion of mitochondria or adenoviral overexpression of superoxide dismutases, as well as inhibition of nitric-oxide synthase, abolished the metformin-enhanced phosphorylations and activities of AMPK, implicating that activation of AMPK by metformin might be mediated by the mitochondria-derived RNS. Furthermore, administration of metformin, which increased 3-nitrotyrosine staining in hearts of C57BL6, resulted in parallel activation of AMPK in the aorta and hearts of C57BL6 mice but not in those of endothelial nitric-oxide synthase (eNOS) knockout mice in which metformin had no effect on 3-nitrotyrosine staining. Because the eNOS knockout mice expressed normal levels of AMPK-alpha that was activated by 5-aminoimidazole-4-carboxamide riboside, an AMPK agonist, these data indicate that RNS generated by metformin is required for AMPK activation in vivo. In addition, metformin significantly increased the co-immunoprecipitation of AMPK and its upstream kinase, LKB1, in C57BL6 mice administered to metformin in vivo. Using pharmacological and genetic inhibitors, we found that inhibition of either c-Src or PI3K abolished AMPK that was enhanced by metformin. We conclude that activation of AMPK by metformin might be mediated by mitochondria-derived RNS, and activation of the c-Src/PI3K pathway might generate a metabolite or other molecule inside the cell to promote AMPK activation by the LKB1 complex.

Metformin in obstetric and gynecologic practice: a review

Metformin is a common treatment for women who have insulin resistance manifesting as type 2 diabetes or polycystic ovarian syndrome (PCOS). With an increasing number of these patients conceiving, it is expected that the use of metformin in and around the time of pregnancy will increase. This article reassesses the mechanisms, safety, and clinical experience of metformin use in obstetrics and gynecology. Metformin is an attractive therapeutic option because administration is simple, hypoglycemia rare, and weight loss promoted. There is a large volume of research supporting the use of metformin treatment in diabetes mellitus, androgenization, anovulation, infertility, and recurrent miscarriage. Although metformin is known to cross the placenta, there is, as yet, no evidence of teratogenicity. Metformin has an array of complex actions, accounting for the varied clinical roles, many of which are still to be fully evaluated. Much research is still needed.

Antiatherogenic properties of metformin: the experimental evidence

Cardiovascular disease (CVD) is the major determining factor of morbidity and mortality in type 2 diabetic patients. The established relationship between type 2 diabetes and atherosclerosis has fueled suggestions that anti-diabetic drugs with beneficial effects on CV risk factors may help attenuate the atherosclerotic process in diabetic patients. Metformin is a hypoglycaemic agent widely used in the management of type 2 diabetes. In addition to its insulin-sensitising action, this drug has favourable effects on various CV risk factors and reduces macrovascular complications in obese type 2 diabetic patients. This review summarises in vivo and in vitro experimental evidence on the antiatherogenic properties of metformin.

Do effects on blood pressure contribute to improved clinical outcomes with metformin?

Hyperinsulinaemia and hypertension commonly coexist, and a large body of evidence points to a common pathogenesis based on the presence of underlying insulin resistance (the "insulin hypothesis" of hypertension). Metformin improves insulin sensitivity in liver and muscle as its primary antihyperglycaemic mechanism of action, and intensive glycaemic management with metformin significantly reduced the risk of macrovascular diabetic complications in the UK Prospective Diabetes Study. The clinical outcome benefits in the metformin group included a significant reduction in the risk of stroke (- 41% vs + 14% with sulphonylurea or insulin treatment, p=0.032), which is well known to be highly sensitive to changes in blood pressure. Furthermore, a placebo-controlled study has shown that metformin significantly improved endothelial function, a key regulator of vascular tone and blood pressure, in type 2 diabetic patients. However, clinical studies have shown that metformin treatment is not associated with clinically relevant reductions in blood pressure in man. These apparently conflicting observations are difficult to reconcile. Either the beneficial vascular actions of metformin involve physiological systems not involved in the control of blood pressure, or counter-regulatory mechanisms prevent beneficial effects of metformin on the vasculature being translated into a clinically meaningful antihypertensive effect. Further research will be required to resolve this paradox.

Microcirculation in insulin resistance and diabetes: more than just a complication

The microvascular bed is an anatomical entity which is governed by specific, highly regulated mechanisms which are closely adapted to the specific function of each vascular segment. Among those, small arteriolar vasomotion and capacity of small vessels to constrict in response to physical and humoral stimuli play a major role. Other processes of importance for the adequacy of nutritive perfusion are haemorheological properties of whole blood and red cells, adhesiveness of leukocytes and capillary permeability. This review provides some description of these phenomena, how they impact on organ function and how they appear in diabetes. Metformin, as a unique example among the drug arsenal, exerts various effects preferentially at the level of smallest vessels (arterioles, capillaries, venules). This review summarises our actual knowledge and includes several new data showing its high potential for reducing microvascular dysfunction. Most of these unique properties have also been demonstrated in non-diabetic animals or humans, suggesting they are intrinsic to the drug and not secondary to diabetic metabolic improvement. A particular focus is put on the relevance of metformin's capacity to stimulate slow wave arteriolar vasomotion and improve functional capillary density, whereby nutritive flow can be re-established. Finally, the implication of microcirculation in other aspects of insulin resistance and diabetes, such as macroangiopathy and metabolic control, is discussed and strengthens the concept of a broad involvement of microvascular dysfunction in these diseases as well as the potential interest of introducing adapted treatment early in the history of a patient's diabetes.

Cardioprotection with pioglitazone is abolished by nitric oxide synthase inhibitor in ischemic rabbit hearts--comparison of the effects of pioglitazone and metformin

BACKGROUND

The effects of two drugs representing different classes of antidiabetic pharmacology (pioglitazone, a thiazolidinedione; and metformin, a biguanide) on the myocardial metabolism in the ischemia are poorly understood.

METHODS

To test the hypothesis that cardioprotection of pioglitazone and metformin is associated with nitric oxide (NO), we studied the high energy phosphate metabolism by 31P-nuclear magnetic resonance (NMR) in isolated rabbit hearts. Forty-five minutes of continuous normothermic global ischemia was carried out. Pioglitazone or metformin was administered at the beginning, 60 min prior to the global ischemia, with or without the nitric oxide synthase inhibitor, L-NAME, administered 5 min or 60 min prior to the ischemia. In the first experiment, whether NO was produced or not by administration of pioglitazone, for the prevention of myocardial ischemic injury, was investigated. Hearts of male Japanese white rabbits were divided into 4 experimental groups: the control (C) group, the P group consisting of pioglitazone treatment, the P + L5 group consisting of pioglitazone treatment with L-NAME (5 min before ischemia), and the P + L60 group consisting of pioglitazone treatment with L-NAME (60 min before ischemia). In the next experiment, a comparison between the effects of pioglitazone and metformin in preventing ischemic injury were studied. The hearts were divided into 4 experimental groups: the control (C) group, the P group consisting of pioglitazone treatment, the P + L5 group consisting of pioglitazone treatment with L-NAME (5 min before ischemia), the M group consisting of metformin treatment, and the M + L5 group consisting of metformin treatment with L-NAME (5 min before ischemia).

RESULTS

In the first experiment, the decrease in adenosine triphosphate (ATP) during ischemia was significantly inhibited in the P group in comparison with the C group (P < 0.01). However, the decrease in ATP was not inhibited in the P + L5 group during ischemia. In contrast, in the P + L60 group, the decrease in ATP was not inhibited during a part of ischemia. In the next experiment, a comparison between the effects of pioglitazone and metformin in preventing ischemic injury was studied. As a result of administration of either pioglitazone or metformin, there was no difference between groups with and without L-NAME.

CONCLUSION

These results suggest that pioglitazone has a significant beneficial effect on improving the myocardial energy metabolism during ischemia. This cardioprotection may be dependent on nitric oxide (NO) synthase during ischemia more than preischemia. Furthermore, the present findings suggest that both pioglitazone and metformin have equal cardioprotective effects mediated by NO on myocardial ischemic injury in rabbits.

Pathophysiological implications of insulin resistance on vascular endothelial function

BACKGROUND

Insulin resistance is a key component of the insulin resistance syndrome and is a crucially important metabolic abnormality in Type 2 diabetes. Insulin-resistant individuals are at significantly increased risk of cardiovascular disease, although the underlying mechanisms remain incompletely understood. The endothelium is thought to play a critical role in maintaining vascular homeostasis, a process dependent on the balance between the production of nitric oxide, superoxide and other vasoactive substances. Endothelial dysfunction has been demonstrated in insulin-resistant states in animals and humans and may represent an important early event in the development of atherosclerosis. Insulin resistance may be linked to endothelial dysfunction by a number of mechanisms, including disturbances of subcellular signalling pathways common to both insulin action and nitric oxide production. Other potential unifying links include the roles of oxidant stress, endothelin, the renin angiotensin system and the secretion of hormones and cytokines by adipose tissue. Lifestyle measures and drug therapies which improve insulin sensitivity and ameliorate endothelial dysfunction may be important in delaying the progression to overt cardiovascular disease in at risk individuals.

METHODS

We conducted a literature search using Medline, restricted to articles published in the English language between 1966 and the present, and reviewed bibliographies of relevant articles. An initial search strategy employing combinations of the MeSH terms: insulin resistance; endothelium, vascular; insulin; nitric oxide or hyperinsulinaemia produced over 300 references. Focused searches using keywords relevant to the molecular aspects of endothelial function and insulin signalling, and lifestyle or pharmacological interventions relevant to insulin resistance or endothelial function, produced over 300 further references. Abstracts of all references were screened before selecting those relevant to this review.

Involvement of organic cation transporter 1 in the lactic acidosis caused by metformin

Biguanides are a class of drugs widely used as oral antihyperglycemic agents for the treatment of type 2 diabetes mellitus, but they are associated with lactic acidosis, a lethal side effect. We reported previously that biguanides are good substrates of rat organic cation transporter 1 (Oct1; Slc22a1) and, using Oct1(-/-) mice, that mouse Oct1 is responsible for the hepatic uptake of a biguanide, metformin. In the present study, we investigated whether the liver is the key organ for the lactic acidosis. When mice were given metformin, the blood lactate concentration significantly increased in the wild-type mice, whereas only a slight increase was observed in Oct1(-/-) mice. The plasma concentration of metformin exhibited similar time profiles between the wild-type and Oct1(-/-) mice, suggesting that the liver is the key organ responsible for the lactic acidosis. Furthermore, the extent of the increase in blood lactate caused by three different biguanides (metformin, buformin, and phenformin) was compared with the abilities to reduce oxygen consumption in isolated rat hepatocytes. When rats were given each of these biguanides, the lactate concentration increased significantly. This effect was dose-dependent, and the EC(50) values of metformin, buformin, and phenformin were 734, 119, and 4.97 microM, respectively. All of these biguanides reduced the oxygen consumption by isolated rat hepatocytes in a concentration-dependent manner. When the concentration required to reduce the oxygen consumption to 75% of the control value (from 0.40 to 0.29 micromol/min/mg protein) was compared with the EC(50) value obtained in vivo, a clear correlation was observed among the three biguanides, suggesting that oxygen consumption in isolated rat hepatocytes can be used as an index of the incidence of lactic acidosis.

Cardioprotection by metformin is abolished by a nitric oxide synthase inhibitor in ischemic rabbit hearts

We investigated the effects of metformin on myocardial metabolism during ischemia by 31P-nuclear magnetic resonance (NMR) in isolated rabbit hearts. Metformin was administered 60 min prior to induction of global ischemia, or in combination with a nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), at 5 min or 60 min prior to the ischemia. Normothermic global ischemia was then carried out for 45 min. Twenty-eight hearts were divided into 4 experimental groups consisting of 7 hearts each: a control (C) group; an M group receiving metformin treatment alone; an M+L (5) group receiving metformin treatment with L-NAME at 5 min before ischemia; and an M+L (60) group receiving metformin treatment with L-NAME at 60 min before ischemia. During ischemia, the decrease in adenosine triphosphate (ATP) was significantly inhibited in the M group in comparison with the C group (p < 0.01). However, this preservation of ATP in the M group was inhibited in the M+L (5) group during ischemia. In contrast, in the M+L (60) group, this preservation of ATP in the M group was not inhibited during, but not at the end of, ischemia. These results suggest that metformin has a significant beneficial effect for improving the myocardial energy metabolism during myocardial ischemia. This cardioprotection may be more dependent on nitric oxide synthase during ischemia than during pre-ischemia.

Pharmacotherapy of diabetes mellitus: implications for the prevention and treatment of cardiovascular disease

Diabetes mellitus in adults is associated with an increased risk of premature vascular disease and a higher mortality rate. The presence of other risk factors, often seen in diabetic patients, such as systemic hypertension, augments the rate of vascular diseases. Evidence is growing that tight control of hyperglycemia using insulin and/or oral hypoglycemic agents will modify this risk. More aggressive control of concomitant hypertension and/or hyperlipidemia is also required. Diabetic patients who have myocardial infarctions do worse than nondiabetic patients. Various strategies to improve outcomes include the use of tight blood glucose control, and various coronary interventions are currently under clinical study.

Low-dose dexamethasone in the rat: a model to study insulin resistance

The main aim of this study was to set up a new animal model to study insulin resistance. Wistar rats (6 or 7 per group) received the following for 4 wk in experiment 1: 1) vehicle, 2) 2 microg/day subcutaneous dexamethasone, 3) metformin (400 mg x kg(-1) x day(-1) os), and 4) dexamethasone plus metformin. In experiment 2 the rats received the following: 1) vehicle, 2) dexamethasone, 3) dexamethasone plus arginine (2%; as substrate of the nitric oxide synthase for nitric oxide production) in tap water, and 4) dexamethasone plus isosorbide dinitrate (70 mg/kg; as direct nitric oxide donor) in tap water. Insulin sensitivity was significantly reduced by dexamethasone already at week 1, before the increase in blood pressure (day 15) and without significant changes in body weight compared with vehicle. Dexamethasone-treated rats had significantly higher triglycerides, hematocrit, and insulin, whereas serum total nitrates/ nitrites were lower compared with vehicle. The concomitant treatment with metformin minimized all the described effects of dexamethasone. In experiment 2, only isosorbide dinitrate was able to prevent the observed dexamethasone-induced metabolic, hemodynamic, and insulin sensitivity changes. Chronic low-dose subcutaneous dexamethasone (2 microg/day) is a useful model to study the relationships between insulin resistance and blood pressure in the rat, and dexamethasone might decrease insulin sensitivity and increase blood pressure through an endothelium-mediated mechanism.

Involvement of organic cation transporter 1 in hepatic and intestinal distribution of metformin

Metformin, a biguanide, is widely used as an oral hypoglycemic agent for the treatment of type 2 diabetes mellitus. The purpose of the present study was to investigate the role of organic cation transporter 1 (Oct1) in the disposition of metformin. Transfection of rat Oct1 cDNA results in the time-dependent and saturable uptake of metformin by the Chinese hamster ovary cell line with K(m) and V(max) values of 377 microM and 1386 pmol/min/mg of protein, respectively. Buformin and phenformin, two other biguanides, were also transported by rOct1 with a higher affinity than metformin: their K(m) values were 49 and 16 microM, respectively. To investigate the role of Oct1 in the disposition of metformin, the tissue distribution of metformin was determined in Oct1 gene-knockout mice after i.v. administration. Distribution of metformin to the liver in Oct1(-/-) mice was more than 30 times lower than that in Oct1(+/+) mice, and can be accounted for by the extracellular space. Distribution to the small intestine was also decreased in Oct1(-/-) mice, whereas that to the kidney as well as the urinary excretion profile showed only minimal differences. In conclusion, the present findings suggest that Oct1 is responsible for the hepatic uptake as well as playing a role in the intestinal uptake of metformin, whereas the renal distribution and excretion are mainly governed by other transport mechanism(s).

Obesity, atherosclerosis and the vascular endothelium: mechanisms of reduced nitric oxide bioavailability in obese humans

It is now well established that obesity is an independent risk factor for the development of coronary artery atherosclerosis. The maintenance of vascular homeostasis is critically dependent on the continued integrity of vascular endothelial cell function. A key early event in the development of atherosclerosis is thought to be endothelial cell dysfunction. A primary feature of endothelial cell dysfunction is the reduced bioavailability of the signalling molecule nitric oxide (NO), which has important anti atherogenic properties. Recent studies have produced persuasive evidence showing the presence of endothelial dysfunction in obese humans NO bioavailability is dependent on the balance between its production by a family of enzymes, the nitric oxide synthases, and its reaction with reactive oxygen species. The endothelial isoform (eNOS) is responsible for a significant amount of the NO produced in the vascular wall. NO production can be modulated in both physiological and pathophysiological settings, by regulation of the activity of eNOS at a transcriptional and post-transcriptional level, by substrate and co-factor provision and through calcium dependent and independent signalling pathways. The present review discusses general mechanisms of reduced NO bioavailability including factors determining production of both NO and reactive oxygen species. We then focus on the potential factors responsible for endothelial dysfunction in obesity and possible therapeutic interventions targetted at these abnormalities.

Reversal of diabetes-induced rat graft transplant coronary artery disease by metformin

BACKGROUND

Induction of diabetes in rat heterotopic heart transplantation models leads to an accelerated form of severe transplant coronary artery disease (TxCAD). We undertook this study to determine whether treatment of diabetes with metformin would favorably affect TxCAD.

METHODS

Heterotopic abdominal heart transplantation was performed in rat isograft and allograft models. After transplantation, diabetes was induced with streptozotocin. Fifty percent of the animals received metformin at 500 mg/kg twice daily. We quantitatively assessed TxCAD using histologic sections of harvested hearts at 30 and 60 days with computer-assisted morphometry. We compared vessels in the first tertile of the area distribution with vessels in the last tertile.

RESULTS

Fasting glucose levels in metformin-treated animals were 161 +/- 45 mg/dl compared with 400 +/- 120 mg/dl (p < 0.05) in untreated rats. Treatment with metformin led to decreased diabetes-induced TxCAD in the larger vessels. This effect was sustained during the study course in the isografts but not in the allografts. Treatment with metformin did not prevent progression of TxCAD in the smaller vessels at 60 days.

CONCLUSIONS

Metformin reduced luminal occlusion and severe TxCAD in the larger vessels but did not alter the course of TxCAD in the smaller vessels. These results may have therapeutic implications for patients.

Improved endothelial function with metformin in type 2 diabetes mellitus

OBJECTIVES

This study was designed to assess the effect of metformin on impaired endothelial function in type 2 diabetes mellitus.

BACKGROUND

Abnormalities in vascular endothelial function are well recognized among patients with type 2 (insulin-resistant) diabetes mellitus. Insulin resistance itself may be central to the pathogenesis of endothelial dysfunction. The effects of metformin, an antidiabetic agent that improves insulin sensitivity, on endothelial function have not been reported.

METHODS

Subjects with diet-treated type 2 diabetes but without the confounding collection of cardiovascular risk factors seen in the metabolic syndrome were treated with metformin 500 mg twice daily (n = 29) or placebo (n = 15) for 12 weeks. Before and after treatment, blood flow responses to intraarterial administration of endothelium-dependent (acetylcholine), endothelium-independent (sodium nitroprusside) and nitrate-independent (verapamil) vasodilators were measured using forearm plethysmography. Whole-body insulin resistance was assessed on both occasions using the homeostasis model (HOMA-IR).

RESULTS

Subjects who received metformin demonstrated statistically significant improvement in acetylcholine-stimulated flows compared with those treated with placebo (p = 0.0027 by 2-way analysis of variance), whereas no significant effect was seen on nitroprusside-stimulated (p = 0.27) or verapamil-stimulated (p = 0.40) flows. There was a significant improvement in insulin resistance with metformin (32.5% reduction in HOMA-IR, p = 0.01), and by stepwise multivariate analysis insulin resistance was the sole predictor of endothelium-dependent blood flow following treatment (r = -0.659, p = 0.0012).

CONCLUSIONS

Metformin treatment improved both insulin resistance and endothelial function, with a strong statistical link between these variables. This supports the concept of the central role of insulin resistance in the pathogenesis of endothelial dysfunction in type 2 diabetes mellitus. This has important implications for the investigation and treatment of vascular disease in patients with type 2 diabetes mellitus.

Metformin treatment corrects vascular insulin resistance in hypertension

OBJECTIVE

In states of insulin resistance, the vasorelaxant actions of insulin are blunted, which may contribute towards the development of increased vascular tone/hypertension and reduced glucose uptake. To examine whether treating insulin resistance in hypertension restores the vascular actions of insulin, we studied the long-term effects of metformin on the contractile responses of isolated aortas from control and insulin-resistant, hyperinsulinaemic fructose-hypertensive rats in the presence and absence of insulin.

DESIGN AND METHODS

Sprague Dawley rats were divided into control, control metformin-treated, fructose and fructose metformin-treated groups (n = 8 per group). The treated groups received metformin (500 mg/kg per day for 6 weeks), following which isometric responses to noradrenaline (NA) and angiotensin II (A-II) were examined in thoracic aortas in the presence and absence of insulin (100 mU/ml for 2 h) using isolated organ-bath apparatus. In addition, endothelium-dependent and independent vascular responses to acetylcholine (ACh) and sodium nitroprusside (SNP) were also studied.

RESULTS

Metformin treatment prevented the development of fructose-induced insulin resistance, hyperinsulinaemia and hypertension. Insulin attenuated the contractile responses to NA and A-II in control rat aortas; however, blood vessels from untreated fructose rats were refractory to insulin-induced vasodilation. Strikingly, long-term metformin treatment restored the vasodepressor actions of insulin in fructose rats. Metformin did not affect the contractile responses to NA or A-II in either control or fructose rats. In addition, metformin treatment restored ACh-induced endothelium-dependent vasorelaxation in aortas from fructose rats without affecting SNP-induced relaxation.

CONCLUSIONS

These data show, for the first time, that long-term metformin treatment corrects vascular insulin resistance and improves endothelium-dependent vasorelaxation in hypertension. These effects appear to be secondary to metformin-induced improvements in metabolic derangements (versus a direct vascular action of metformin). Improving the vascular effects of insulin may serve to decrease peripheral tone, attenuate blood pressure and improve insulin sensitivity.