Inhibitory action of insulin-sensitizing agents on calcium channels in smooth muscle cells from resistance arteries of guinea-pig

Metformin Shortens Prolonged QT Interval in Diabetic Mice by Inhibiting L-Type Calcium Current: A Possible Therapeutic Approach

The incidence and mortality of cardiovascular disease in diabetic patients are 2-3 times higher than those in non-diabetic patients. Abnormal function of the L-type calcium channel in myocardial tissue might result in multiple cardiac disorders such as a prolonged QT interval. Therefore, QT prolongation is an independent risk factor of cardiovascular disease in patients with diabetes mellitus. Metformin, a hypoglycemic agent, is widely known to effectively reduce the occurrence of macrovascular diseases. The aim of the present study was to evaluate the effect of metformin on prolonged QT interval and to explore potential ionic mechanisms induced by diabetes. Diabetic mouse models were established with streptozotocin and an electrocardiogram was used to monitor the QT interval after 4 weeks of metformin treatment in each group. Action potential duration (APD) and L-type calcium current (I_Ca-L) were detected by patch-clamp in isolated mice ventricular cardiomyocytes and neonatal cardiomyocytes of mice. The expression levels of CACNA1C mRNA and Cav1.2 were measured by real-time PCR, western blot and immunofluorescence. A shortened QT interval was observed after 4 weeks of metformin treatment in diabetic mice. Patch-clamp results revealed that both APD and I_Ca-L were shortened in mouse cardiomyocytes. Furthermore, the expression levels of CACNA1C mRNA and Cav1.2 were decreased in the metformin group. The same results were also obtained in cultured neonatal mice cardiomyocytes. Overall, these results verify that metformin could shorten a prolonged QT interval by inhibiting the calcium current, suggesting that metformin may play a role in the electrophysiology underlying diabetic cardiopathy.

The effect of interbrain synchronization in gesture observation: A fNIRS study

INTRODUCTION

Gestures characterize individuals' nonverbal communicative exchanges, taking on different functions. Several types of research in the neuroscientific field have been interested in the investigation of the neural correlates underlying the observation and implementation of different gestures categories. In particular, different studies have focused on the neural correlates underlying gestures observation, emphasizing the presence of mirroring mechanisms in specific brain areas, which appear to be involved in gesture observation and planning mechanisms.

MATERIALS AND METHODS

Specifically, the present study aimed to investigate the neural mechanisms, through the use of functional Near-Infrared Spectroscopy (fNIRS), underlying the observation of affective, social, and informative gestures with positive and negative valence in individuals' dyads composed by encoder and decoder. The variations of oxygenated (O2Hb) and deoxygenated (HHb) hemoglobin concentrations of both individuals were collected simultaneously through the use of hyperscanning paradigm, allowing the recording of brain responsiveness and interbrain connectivity.

RESULTS

The results showed a different brain activation and an increase of interbrain connectivity according to the type of gestures observed, with a significant increase of O2Hb brain responsiveness and interbrain connectivity and a decrease of HHb brain responsiveness for affective gestures in the dorsolateral prefrontal cortex (DLPFC) and for social gestures in the superior frontal gyrus (SFG). Furthermore, concerning the valence of the observed gestures, an increase of O2Hb brain activity and interbrain connectivity was observed in the left DLPFC for positive affective gestures compared to negative ones.

CONCLUSION

In conclusion, the present study showed different brain responses underlying the observation of different types of positive and negative gestures. Moreover, interbrain connectivity calculation allowed us to underline the presence of mirroring mechanisms involved in gesture-specific frontal regions during gestures observation and action planning.

Affective, Social, and Informative Gestures Reproduction in Human Interaction: Hyperscanning and Brain Connectivity

Gestural communication characterizes daily individuals' interactions in order to share information and to modify others' behavior. Social neuroscience has investigated the neural bases which support recognizing of different gestures. The present research, through the use of the hyperscanning approach, that allows the simultaneously recording of the activity of two or more individuals involved in a joint action, aims to investigate the neural bases of gestural communication. Moreover, by using hyperscanning paradigm we explore the inter-brain connectivity between two inter-agents, the one who performed the gesture (encoder) and the one who received it (decoder), with functional Near-infrared Spectroscopy (fNIRS) during the reproduction of affective, social and informative gestures with positive and negative valence. Result showed an increase in oxygenated hemoglobin concentration (O2Hb) and inter-brain connectivity in the dorsolateral prefrontal cortex (DLPFC) for affective gestures, in the superior frontal gyrus (SFG) for social gestures and the frontal eye fields (FEF) for informative gestures, for both encoder and decoder. Furthermore, it emerged that positive gestures activate more the left DLPFC, with an increase in inter-brain connectivity in DLPFC and SFG. The present study revealed the relevant function of the type and valence of gestures in affecting intra- and inter-brain connectivity.

Metformin and Vascular Diseases: A Focused Review on Smooth Muscle Cell Function

Metformin has been used in diabetes for more than 60 years and has excellent safety in the therapy of human type 2 diabetes (T2D). There is growing evidence that the beneficial health effects of metformin are beyond its ability to improve glucose metabolism. Metformin not only reduces the incidence of cardiovascular diseases (CVD) in T2D patients, but also reduces the burden of atherosclerosis (AS) in pre-diabetes patients. Vascular smooth muscle cells (VSMCs) function is an important factor in determining the characteristics of the entire arterial vessel. Its excessive proliferation contributes to the etiology of several types of CVD, including AS, restenosis, and pulmonary hypertension. Current studies show that metformin has a beneficial effect on VSMCs function. Therefore, this review provides a timely overview of the role and molecular mechanisms by which metformin acts through VSMCs to protect CVD.

Relationships Between Ion Channels, Mitochondrial Functions and Inflammation in Human Aging

Aging is often associated with a loss of function. We believe aging to be more an adaptation to the various, and often continuous, stressors encountered during life in order to maintain overall functionality of the systems. The maladaptation of a system during aging may increase the susceptibility to diseases. There are basic cellular functions that may influence and/or are influenced by aging. Mitochondrial function is amongst these. Their presence in almost all cell types makes of these valuable targets for interventions to slow down or even reserve signs of aging. In this review, the role of mitochondria and essential physiological regulators of mitochondria and cellular functions, ion channels, will be discussed in the context of human aging. The origins of inflamm-aging, associated with poor clinical outcomes, will be linked to mitochondria and ion channel biology.

Metformin and sulodexide restore cardiac microvascular perfusion capacity in diet-induced obese rats

Background

Disturbances in coronary microcirculatory function, such as the endothelial glycocalyx, are early hallmarks in the development of obesity and insulin resistance. Accordingly, in the present study myocardial microcirculatory perfusion during rest and stress was assessed following metformin or sulodexide therapy in a rat model of diet-induced obesity. Additionally, the effect of degradation of the glycocalyx on myocardial perfusion was assessed in chow-fed rats.

Methods

Rats were fed a high fat diet (HFD) for 8 weeks and were divided into a group without therapy, and groups that received the anti-diabetic drug metformin or the glycocalyx-stabilizing drug sulodexide in their drinking water during the last 4 weeks of the feeding period. Myocardial microvascular perfusion was determined using first-pass perfusion MRI before and after adenosine infusion. The effect of HFD on microcirculatory properties was also assessed by sidestream darkfield (SDF) imaging of the gastrocnemius muscle. In an acute experimental setting, hyaluronidase was administered to chow-fed control rats to determine the effect of enzymatical degradation of the glycocalyx on myocardial perfusion.

Results

HFD-rats developed central obesity and insulin sensitivity was reduced as evidenced by the marked reduction in insulin-induced phosphorylation of Akt in both cardiac and gastrocnemius muscle. We confirmed our earlier findings that the robust increase in myocardial perfusion in chow-fed rats after an adenosine challenge (+56%, p = 0.002) is blunted in HFD rats (+8%, p = 0.68). In contrast, 4-weeks treatment with metformin or sulodexide partly restored the increase in myocardial perfusion during adenosine infusion in HFD rats (+81%, p = 0.002 and +37%, p = 0.02, respectively). Treating chow-fed rats acutely with hyaluronidase, to enzymatically degrade the glyocalyx, completely blunted the increase in myocardial perfusion during stress.

Conclusions

In early stages of HFD-induced insulin resistance myocardial perfusion becomes compromised, a process that can be countered by treatment with both metformin and sulodexide. The adverse effect of acute glycocalyx degradation and protective effect of long-term sulodexide administration on myocardial perfusion provides indirect evidence, suggesting a role for the glycocalyx in preserving coronary microvascular function in pre-diabetic animals.

Sildenafil does not enhance but rather attenuates vasorelaxant effects of antidiabetic agents

Type 2 diabetic men commonly experience erectile dysfunction for which phosphodiesterase-5 (PDE5) inhibitors like sildenafil (Viagra) are often recommended. By preventing degradation of cyclic guanosine monophosphate (cGMP) in vascular smooth muscle, these inhibitors also enhance arterial vasorelaxant effects of nitric oxide donors (which stimulate cGMP synthesis). In the present work, we confirmed this enhancing effect after co-administration of sildenafil with nitroprusside to freshly-isolated rat tail arterial tissues. However, in the same tissues we also observed that sildenafil does not enhance but rather attenuates vasorelaxant effects of three commonly-used antidiabetic drugs, i.e. the biguanide metformin and the thiazolidinediones pioglitazone and rosiglitazone. Indeed, sildenafil completely blocked vasorelaxant effects of low concentrations of these drugs. In addition, we found that this same novel anti-vasorelaxant interaction of sildenafil with these agents was abolished by either 1) omitting extracellular glucose or 2) inhibiting specific smooth muscle glycolytic pathways; pathways known to preferentially utilize extracellular glucose to fuel certain adenosine triphosphate (ATP)-dependent ion transporters: e.g. ATP-sensitive K channels, sarcoplasmic reticulum Ca-ATPase, plasma membrane Ca-ATPase and Na/K-ATPase. Accordingly, we suspect that altered activity of one or more of these ion transporters mediates the observed attenuating (anti-vasorelaxant) interaction of sildenafil with the antidiabetic drugs. The present results are relevant because hypertension is so common and difficult to control in Type 2 diabetes. The present data suggest that sildenafil might interfere with the known antihypertensive potential of metformin and the thiazolidinediones. However, they do not suggest that it will interact with them to cause life-threatening episodes of severe hypotension, as can occur when it is co-administered with nitrates.

Thiazolidinedione insulin sensitizers alter lipid bilayer properties and voltage-dependent sodium channel function: implications for drug discovery

The thiazolidinediones (TZDs) are used in the treatment of diabetes mellitus type 2. Their canonical effects are mediated by activation of the peroxisome proliferator–activated receptor γ (PPARγ) transcription factor. In addition to effects mediated by gene activation, the TZDs cause acute, transcription-independent changes in various membrane transport processes, including glucose transport, and they alter the function of a diverse group of membrane proteins, including ion channels. The basis for these off-target effects is unknown, but the TZDs are hydrophobic/amphiphilic and adsorb to the bilayer–water interface, which will alter bilayer properties, meaning that the TZDs may alter membrane protein function by bilayer-mediated mechanisms. We therefore explored whether the TZDs alter lipid bilayer properties sufficiently to be sensed by bilayer-spanning proteins, using gramicidin A (gA) channels as probes. The TZDs altered bilayer elastic properties with potencies that did not correlate with their affinity for PPARγ. At concentrations where they altered gA channel function, they also altered the function of voltage-dependent sodium channels, producing a prepulse-dependent current inhibition and hyperpolarizing shift in the steady-state inactivation curve. The shifts in the inactivation curve produced by the TZDs and other amphiphiles can be superimposed by plotting them as a function of the changes in gA channel lifetimes. The TZDs’ partition coefficients into lipid bilayers were measured using isothermal titration calorimetry. The most potent bilayer modifier, troglitazone, alters bilayer properties at clinically relevant free concentrations; the least potent bilayer modifiers, pioglitazone and rosiglitazone, do not. Unlike other TZDs tested, ciglitazone behaves like a hydrophobic anion and alters the gA monomer–dimer equilibrium by more than one mechanism. Our results provide a possible mechanism for some off-target effects of an important group of drugs, and underscore the importance of exploring bilayer effects of candidate drugs early in drug development.

Evidence of direct smooth muscle relaxant effects of the fibrate gemfibrozil

Fibrates are commonly employed to treat abnormal lipid metabolism via their unique ability to stimulate peroxisome proliferator-activated receptor alpha (PPARalpha). Interestingly, they also decrease systemic arterial pressure, despite recent evidence that PPAR alpha may contribute to expression of renin and related hypertension. Yet, mechanisms responsible for their potential antihypertensive activity remain unresolved. Rapid decreases in arterial pressure following bolus intravenous injections of bezafibrate strongly suggest they may relax arterial smooth muscle directly. But since bezafibrate is highly susceptible to photodegradation in aqueous media, it has never been critically tested for this possibility in vitro with isolated arterial smooth muscle preparations. Accordingly, we tested gemfibrozil which is resistant to photodegradation. We examined it over a therapeutically-relevant range (50-400 microM) for both acute and delayed relaxant effects on contractions of the isolated rat tail artery; contractions induced by either depolarizing its smooth muscle cell membranes with high potassium or stimulating its membrane-bound receptors with norepinephrine and arginine-vasopressin. We also examined these same gemfibrozil levels for effects on spontaneously-occurring phasic rhythmic contractile activity, typically not seen in arteries under in vitro conditions but commonly exhibited by smooth muscle of uterus, duodenum and bladder. We found that gemfibrozil significantly relaxed all induced forms of contraction in the rat tail artery, acutely at the higher test levels and after a delay of a few hours at the lower test levels. The highest test level of gemfibrozil (400 microM) also completely abolished spontaneously-occurring contractile activity of the isolated uterus and duodenum and markedly suppressed it in the bladder. This is the first evidence that a fibrate drug can directly relax smooth muscle contractions, either induced by various contractile agents or spontaneously-occurring. These findings are particularly relevant to both the recently renewed concern over the impact of fibrates on hypertension and a new understanding of their gastrointestinal side effects.

Alteration of vascular reactivity in diabetic human mammary artery and the effects of thiazolidinediones

Vascular reactivity was investigated in endothelium-denuded human internal mammary artery (IMA) rings from type 2 diabetic patients. It was also investigated whether insulin sensitizer thiazolidinedione drugs, pioglitazone and rosiglitazone, can directly affect the reactivity of IMA. Using organ bath techniques, cumulative concentration-response curves to phenylephrine (PE), KCl, cromakalim (CRO) and sodium nitroprusside (SNP) were constructed in diabetic and non-diabetic IMA rings. Means of maximal responses (% Emax) and pEC50 values (sensitivity) were compared. Emax values and the sensitivity to PE and KCl were increased while KATP-channel-mediated relaxations were reduced significantly in diabetic rings compared with non-diabetic rings (n = 5–12, P < 0.05). No changes were observed for SNP responses (n = 5, P > 0.05). Incubations with pioglitazone (1 and 10 μM) and rosiglitazone (1 and 20 μM), for 30 min, did not affect KATP-channel-mediated relaxations (n = 5 each, P > 0.05). Pioglitazone partly inhibited pre-contractions of PE and KCl at 10 μM, rosiglitazone did not. Vascular dysfunction observed in diabetic IMA may be of specific importance since they are widely used as coronary bypass material. Thiazolidinedione drugs may not worsen arterial dilatation through KATP channels in ischaemic or hypoxic insults in diabetic patients who are prone to such conditions. Pioglitazone has vasorelaxant property in the grafts.

Effects of chronic PPAR-agonist treatment on cardiac structure and function, blood pressure, and kidney in healthy sprague-dawley rats

PPAR-γ agonists have been associated with heart failure (HF) in diabetic patients. These incidences have been reported mostly in patient populations who were at high risk for HF or had pre-existing impaired cardiovascular function. However, whether there are similar effects of these agents in subjects with no or reduced cardiovascular pathophysiology is not clear. In this study, the effects of chronic treatment with PD168, a potent peroxisome proliferator activated receptor (PPAR) subtype-γ agonist with weak activity at PPAR-α, and rosiglitazone (RGZ), a less potent PPAR-γ agonist with no PPAR-α activity, were evaluated on the cardiovascular-renal system in healthy male Sprague-Dawley (SD) rats by serial echocardiography and radiotelemetry. Rats were treated with vehicle (VEH), PD168, @ 10 or 50 mg/kg.bw/day (PD-10 or PD-50, resp.) or RGZ @ 180 mg/kg.bw/day for 28 days (n = 10/group). Relative to VEH, RGZ, and both doses of PD168 resulted in a significant fall in blood pressure. Furthermore, RGZ and PD168 increased plasma volume (% increase from baseline) 18%, 22%, and 48% for RGZ, PD-10, and PD-50, respectively. PD168 and RGZ significantly increased urinary aldosterone excretion and heart-to-body weight ratio relative to VEH. In addition, PD168 significantly decreased (10-16%) cardiac ejection fraction (EF) and increased left ventricular area (LVA) in systole (s) and diastole (d) in PD-10 and -50 rats. RGZ significantly increased LVAd; however, it did not affect EF relative to VEH. In conclusion, chronic PPAR-γ therapy may predispose the cardiorenal system to a potential sequela of structural and/or functional changes that may be deleterious with regard to morbidity and mortality.

Thiazolidinedione drugs block cardiac KATP channels and may increase propensity for ischaemic ventricular fibrillation in pigs

AIMS/HYPOTHESIS

Opening of ATP-sensitive potassium (K(ATP)) channels during myocardial ischaemia shortens action potential duration and is believed to be an adaptive, energy-sparing response. Thiazolidinedione drugs block K(ATP) channels in non-cardiac cells in vitro. This study determined whether thiazolidinedione drugs block cardiac K(ATP) channels in vivo.

METHODS

Experiments in 68 anaesthetised pigs determined: (1) effects of inert vehicle, troglitazone (10 mg/kg i.v.) or rosiglitazone (0.1 or 1.0 mg/kg i.v.) on epicardial monophasic action potential (MAP) during 90 min low-flow ischaemia; (2) effects of troglitazone, rosiglitazone or pioglitazone (1 mg/kg i.v.) on response of MAP to intracoronary infusion of a K(ATP) channel opener, levcromakalim; and (3) effects of inert vehicle, rosiglitazone (1 mg/kg i.v.) or the sarcolemmal K(ATP) blocker HMR-1098 on time to onset of ventricular fibrillation following complete coronary occlusion.

RESULTS

With vehicle, epicardial MAP shortened by 44+/-9 ms during ischaemia. This effect was attenuated to 12+/-8 ms with troglitazone and 6+/-6 ms with rosiglitazone (p<0.01 for both vs vehicle), suggesting K(ATP) blockade. Intracoronary levcromakalim shortened MAP by 38+/-10 ms, an effect attenuated to 12+/-8, 13+/-4 and 9+/-5 ms during co-treatment with troglitazone, rosiglitazone or pioglitazone (p<0.05 for each), confirming K(ATP) blockade. During coronary occlusion, median time to ventricular fibrillation was 29 min in pigs treated with vehicle and 6 min in pigs treated with rosiglitazone or HMR-1098 (p<0.05 for both vs vehicle), indicating that K(ATP) blockade promotes ischaemic ventricular fibrillation in this model.

CONCLUSIONS/INTERPRETATION

Thiazolidinedione drugs block cardiac K(ATP) channels at clinically relevant doses and promote onset of ventricular fibrillation during severe ischaemia.

Interference with PPAR gamma function in smooth muscle causes vascular dysfunction and hypertension

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor that plays a critical role in metabolism. Thiazolidinediones, high-affinity PPARgamma ligands used clinically to treat type II diabetes, have been reported to lower blood pressure and provide other cardiovascular benefits. Some mutations in PPARgamma (PPARG) cause type II diabetes and severe hypertension. Here we tested the hypothesis that PPARgamma in vascular muscle plays a role in the regulation of vascular tone and blood pressure. Transgenic mice expressing dominant-negative mutations in PPARgamma under the control of a smooth-muscle-specific promoter exhibit a loss of responsiveness to nitric oxide and striking alterations in contractility in the aorta, hypertrophy and inward remodeling in the cerebral microcirculation, and systolic hypertension. These results identify PPARgamma as pivotal in vascular muscle as a regulator of vascular structure, vascular function, and blood pressure, potentially explaining some of the cardioprotective effects of thiazolidinediones.

Physiology, Genetics, and Cardiovascular Disease: Focus on African Americans

The disproportionate impact of cardiovascular disease in African Americans is well recognized. Not only do risk factors such as obesity occur at a higher rate in the African-American community, but this population experiences a greater mortality from cardiovascular disease than their white counterparts. The cardiovascular system is regulated in part by two opposing mediators linking the risk factors of obesity, vascular dysfunction, and diabetes. One of these mediators--angiotensin II--increases blood pressure, impairs endothelial function, decreases peroxisome proliferator activated-receptor gamma, and is proinflammatory, growth stimulating, profibrotic, and proatherogenic. The other mediator, peroxisome proliferator activated-receptor gamma, lowers blood pressure, improves endothelial function, decreases angiotensin II type 1 receptor function, and is anti-inflammatory, growth-inhibiting, antifibrotic, and antiatherogenic. Genotypic variants have been discovered that affect the functioning of both of these important systems. Some of these variants--like some genotypic variants discovered in the adrenergic system--occur with different frequencies in African Americans than in Americans of European descent and may help to explain racial/ethnic differences in susceptibility to cardiovascular disease and aspects of the response to treatment. Recognition of these genotypic differences may permit the development of therapies tailored to individual patients.

Importance de l’activation des PPARgamma dans la pharmacologie de la rigidité artérielle

Specific treatment of age-related aortic wall arteriosclerosis and stiffening is lacking. As anti-inflammatory ligands for peroxisome proliferator-activated receptor-gamma have beneficial effects on the arterial wall in atherosclerosis, we investigated whether chronic pioglitazone treatment protects against another form of vascular wall disease, arteriosclerosis. In a rat model of elastocalcinotic arteriosclerosis (hypervitaminosis D and nicotine, VDN), we evaluated whether pioglitazone attenuated arteriosclerosis and its consequences, aortic wall rigidity, increased aortic pulse pressure and left ventricular hypertrophy. In VDN rats, medial calcification was associated with monocyte/macrophage infiltration and induction of TNF-alpha and IL-1B. Pioglitazone decreased TNF-alpha and IL-1B mRNA expression, blunted aortic wall calcification and prevented fragmentation of elastic fibers. Pioglitazone reduced aortic wall stiffness, aortic pulse pressure and left ventricular hypertrophy. Our results may be clinically relevant in elderly patients suffering from aortic wall stiffening and isolated systolic hypertension.

The insulin sensitiser pioglitazone does not influence skin microcirculatory function in patients with type 2 diabetes treated with insulin

AIMS/HYPOTHESIS

Insulin resistance is associated with abnormal microvascular function. Treatment with insulin sensitisers may provoke oedema, suggesting microvascular effects. The mechanisms underlying the peripheral oedema observed during glucose-lowering treatment with thiazolidinediones are unclear. Therefore we examined the effect of pioglitazone on microvascular variables involved in oedema formation.

METHODS

Subjects (40-80 years) with type 2 diabetes and on insulin were randomised to 9 weeks of pioglitazone therapy (30 mg/day; n=14) or placebo (n=15). The following assessments were performed at baseline and 9 weeks: microvascular filtration capacity; isovolumetric venous pressure; capillary pressure; capillary recruitment following venous or arterial occlusion; postural vasoconstriction; and maximum blood flow. A number of haematological variables were also measured including vascular endothelium growth factor (VEGF), IL-6 and C-reactive protein (CRP).

RESULTS

Pioglitazone did not significantly influence any microcirculatory variable as compared with placebo (analysis of covariance [ANCOVA] for microvascular filtration capacity for the two groups, p=0.26). Mean VEGF increased with pioglitazone (61.1 pg/ml), but not significantly more than placebo (9.76 pg/ml, p=0.94). HbA(1c) levels and the inflammatory markers IL-6 and CRP decreased with pioglitazone compared with placebo (ANCOVA: p=0.009, p=0.001 and p=0.004, respectively).

CONCLUSIONS/INTERPRETATION

Pioglitazone improved glycaemic control and inflammatory markers over 9 weeks but had no effect on microcirculatory variables associated with oedema or insulin resistance in type 2 diabetic patients treated with insulin.

PPAR gamma and human metabolic disease

The nuclear receptor family of PPARs was named for the ability of the original member to induce hepatic peroxisome proliferation in mice in response to xenobiotic stimuli. However, studies on the action and structure of the 3 human PPAR isotypes (PPARalpha, PPARdelta, and PPARgamma) suggest that these moieties are intimately involved in nutrient sensing and the regulation of carbohydrate and lipid metabolism. PPARalpha and PPARdelta appear primarily to stimulate oxidative lipid metabolism, while PPARgamma is principally involved in the cellular assimilation of lipids via anabolic pathways. Our understanding of the functions of PPARgamma in humans has been increased by the clinical use of potent agonists and by the discovery of both rare and severely deleterious dominant-negative mutations leading to a stereotyped syndrome of partial lipodystrophy and severe insulin resistance, as well as more common sequence variants with a much smaller impact on receptor function. These may nevertheless have much greater significance for the public health burden of metabolic disease. This Review will focus on the role of PPARgamma in human physiology, with specific reference to clinical pharmacological studies, and analysis of PPARG gene variants in the abnormal lipid and carbohydrate metabolism of the metabolic syndrome.

Peroxisome proliferator-activated receptor-gamma and its agonists in hypertension and atherosclerosis : mechanisms and clinical implications

Cardiovascular diseases are the leading cause of morbidity and mortality in the US. Proper management and/or prevention of atherosclerosis and hypertension, two complex and chronic disorders, would significantly reduce the risk for cardiovascular events such as myocardial infarction and stroke, but this requires an understanding of the mechanisms underlying their development and progression. Whereas a great deal has been learned and applied toward the management of these disorders, especially hypertension, morbidity and mortality remains unacceptably high, most likely because there are disease-causing mechanisms that have yet to be fully recognized. Understanding these disease mechanisms is necessary so that novel management strategies can be developed. One of these novel mechanisms centers on peroxisome proliferator-activated receptor (PPAR)-gamma. PPAR-gamma is a member of the nuclear receptor superfamily of ligand-activated transcription factors known to play a role in glucose homeostasis and adipocyte differentiation and, more recently, has been shown to have anti-inflammatory, antiatherogenic, and antihypertensive effects. Thiazolidinediones, a class of drugs used in the treatment of type 2 diabetes mellitus, are high-affinity ligands for PPAR-gamma. In this review, the anti-inflammatory, anti-atherosclerotic, and anti-hypertensive mechanisms by which PPAR-gamma and its agonists are thought to exert protective effects on the cardiovascular system are discussed. Ongoing clinical trials using PPAR-gamma activators for the management of cardiovascular diseases, especially in patients with type 2 diabetes mellitus, are summarized.

Effect of thiazolidinediones on equilibrative nucleoside transporter-1 in human aortic smooth muscle cells

Thiazolidinediones are a new class of anti-diabetic agents which increase insulin sensitivity by binding to the peroxisome proliferator-activated receptor gamma (PPAR(gamma)) and stimulating the expression of insulin-responsive genes involved in glucose and lipid metabolism. These drugs also have vasodilatory and anti-proliferative effects on vascular smooth muscle cells. However the mechanisms for these actions are not fully understood. Adenosine is a vasodilator and a substrate of equilibrative nucleoside transporters (ENT). The present study studied the effects of three thiazolidinediones, troglitazone, pioglitazone and ciglitazone, on ENT1 in the human aortic smooth muscle cells (HASMCs). Although incubating HASMCs for 48h with thiazolidinediones had no effect on ENT1 mRNA and protein levels, troglitazone acutely inhibited [3H]adenosine uptake and [3H]NBMPR binding of HASMCs with IC50 values of 2.35+/-0.35 and 3.99+/-0.57microM, respectively. The effect of troglitazone on ENT1 was PPAR(gamma)-independent and kinetic studies revealed that troglitazone was a competitive inhibitor of ENT1. In contrast, pioglitazone and ciglitazone had minimal effects on [3H]adenosine uptake by HASMCs. Troglitazone differs from pioglitazone and ciglitazone in that its side-chain contains a Vitamin E moiety. The difference in structure of troglitazone did not account for its inhibitory effect on ENT1 because Vitamin E did not inhibit [3H]adenosine uptake by HASMCs. Using the nucleoside transporter deficient PK15NTD cells stably expressing ENT1 and ENT2, it was found that troglitazone inhibited ENT1 but had no effect on ENT2. From these results, it is suggested that troglitazone may enhance the vasodilatory effect of adenosine by inhibiting ENT1. Pharmacologically, troglitazone is a novel inhibitor of ENT1.

Pioglitazone Improves Aortic Wall Elasticity in a Rat Model of Elastocalcinotic Arteriosclerosis

Specific treatment of age-related aortic wall arteriosclerosis and stiffening is lacking. Because ligands for peroxisome proliferator-activated receptor γ have beneficial effects on the arterial wall in atherosclerosis, via an antiinflammatory mechanism, we investigated whether long-term pioglitazone (Pio) treatment protects against another form of vascular wall disease, arteriosclerosis. We evaluated, in a rat model of elastocalcinotic arteriosclerosis (hypervitaminosis D and nicotine [VDN]), whether Pio (3 mg · kg −1 per day for 1.5 month PO) attenuated arteriosclerosis and its consequences: aortic wall rigidity, increased aortic pulse pressure, and left ventricular hypertrophy. In VDN rats, medial calcification was associated with monocyte/macrophage infiltration and induction of tumor necrosis factor α and interleukin 1β. Pio increased nuclear peroxisome proliferator-activated receptor γ immunostaining in the aortic wall, decreased tumor necrosis factor α ( P <0.05 versus VDN Pio − ), tended to decrease interleukin 1β mRNA expression ( P =0.08 versus VDN Pio − ), blunted aortic wall calcification (271±69, P <0.05 versus VDN Pio − 562±87 μmol · g −1 dry weight) and prevented fragmentation of elastic fibers (segments per 10 000 μm 2 : 8.4±0.3; P <0.05 versus VDN Pio − 10.5±0.6). Pio reduced aortic wall stiffness (elastic modulus/wall stress: 4.8±0.6; P <0.05 versus VDN Pio − 10.0±1.6), aortic pulse pressure (30±2 mm Hg; P <0.05 versus VDN Pio − 39±4) and left ventricular hypertrophy (1.58±0.05 g · kg −1 ; P <0.05 versus VDN Pio − 1.76±0.06). In conclusion, long-term Pio treatment attenuates aortic wall elastocalcinosis and, thus, lowers aortic wall stiffness, aortic pulse pressure, and left ventricular hypertrophy.

Novel PPARγ Agonists GI 262570, GW 7845, GW 1929, and Pioglitazone Decrease Calcium Channel Function and Myogenic Tone in Rat Mesenteric Arteries

Novel non-thiazolidinedione, tyrosine-derived peroxisome proliferator-activated receptor gamma agonists, GI 262570, GW 7845, GW 1929, developed by GlaxoSmithKline (GSK) along with pioglitazone and nisoldipine, were studied on currents through L-type voltage-dependent calcium channels (VDCC) in freshly isolated smooth muscle cells from mesenteric arteries, and on the diameter of pressurized mesenteric arteries in vitro. Using Ba2+ (10 mmol/l) as the charge carrier through VDCC, the half-inhibition constants (IC50) for GI 262570, GW 7845, GW 1929, and pioglitazone were 2.0 +/- 0.5, 3.0 +/- 0.5, 5.0 +/- 0.7, and 10.0 +/- 0.8 mumol/l, respectively. For arterial diameter measurements the IC50 values for GI 262570, GW 7845, GW 1929, and pioglitazone were 2.4, 4.1, 6.3, and 13.9 mumol/l, respectively. Each GSK compound and pioglitazone was effective at inhibiting VDCC and relaxing pressurized arteries, suggesting that the vasodilation of resistance arteries could be explained by the inhibition of calcium entry through VDCC.

Peroxisome proliferator-activated receptor-gamma calls for activation in moderation: lessons from genetics and pharmacology

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a prototypical member of the nuclear receptor superfamily and integrates the control of energy, lipid, and glucose homeostasis. PPARgamma can bind a variety of small lipophilic compounds derived from metabolism and nutrition. These ligands, in turn, determine cofactor recruitment to PPARgamma, regulating the transcription of genes in a variety of metabolic pathways. PPARgamma is the main target of the thiazolidinedione class of insulin-sensitizing drugs, which are currently a mainstay of therapy for type 2 diabetes. However, this therapy has a number of side effects. Here, we review the clinical consequences of PPARgamma polymorphisms in humans, as well as several studies in mice using general or tissue-specific knockout techniques. We also discuss the recent pharmacological literature describing a variety of new PPARgamma partial agonists and antagonists, as well as pan-PPAR agonists. The results of these studies have added to the understanding of PPARgamma function, allowing us to hypothesize a general mechanism of PPARgamma action and speculate on future trends in the use of PPARgamma as a target in the treatment of type II diabetes.

Attenuated Inhibition of L-type Calcium Currents by Troglitazone in Fructose–Fed Rat Cardiac Ventricular Myocytes

We recently reported that troglitazone, an insulin-sensitizing agent, inhibited l-type Ca current (ICa,L) more effectively in streptozotocin (STZ)-induced diabetic ventricular myocytes than in age-matched control myocytes. However, whether this agent would effectively inhibit ICa,L in an animal model of hyperinsulinemia is unknown. Using whole-cell voltage-clamp techniques, ICa,L was measured in ventricular myocytes isolated from 12 to 16 weeks on fructose-enriched feeding and age-matched control rats. Under control conditions, fructose-fed myocytes did not differ from control myocytes in membrane capacitance, current density, or voltage-dependent properties of ICa,L. Troglitazone inhibited ICa,L in both control and fructose-fed myocytes in a concentration-dependent manner. However, this inhibition was less in fructose-fed than in control myocytes; the half-maximum inhibitory concentrations of troglitazone measured at a holding potential of -50 mV were 16.9 and 9.8 micromol/L, respectively. Contrary to the STZ-induced diabetic rat, the suppressive effect of troglitazone on cardiac ventricular ICa,L was attenuated in fructose-fed rats. Persistent elevation of plasma insulin concentration may play a role in these processes.

Glucocorticoid-mediated mineralocorticoid receptor activation and hypertension

PURPOSE OF REVIEW

Traditionally, the mineralocorticoid receptor was thought to be activated by the mineralocorticoid hormone aldosterone, and to exhibit its main action on epithelia by promoting renal sodium retention, potassium excretion and inducing hypertension upon excessive activation. Recently, evidence appeared that mineralocorticoid receptors are expressed in nonepithelial cells and activated by endogenous glucocorticoids including cortisol. Therefore, the prereceptor regulation of cortisol access to the mineralocorticoid receptors by 11beta-hydroxysteroid dehydrogenase enzymes (11beta-HSDs), a mechanism absent in most nonepithelial cells, appears to be relevant for disease states with cortisol-induced mineralocorticoid action. The present review focuses on direct and indirect effects attributable to mineralocorticoid receptor activation by glucocorticoids.

RECENT FINDINGS

The determination of the intracellular topology of 11beta-HSD1, facing the endoplasmic reticulum lumen, and 11beta-HSD2, facing the cytoplasm, suggests that 11beta-HSD1 acts as a prereceptor mechanism in the local activation of glucocorticoid receptors, whereas 11beta-HSD2 controls mineralocorticoid receptors by interacting with the receptor in the absence of aldosterone. Downregulation of 11beta-HSD2 was observed with various stimuli including hypoxia, shear stress, angiotensin II and tumor necrosis factor alpha. The corresponding signal transcription pathways and some relevant transcription factors have been identified. Renal sodium retention in liver cirrhosis, nephrotic syndrome and hypoxia have been linked to 11beta-HSD2 reduced activity. Overexpression of 11beta-HSD1 specifically in adipose tissue in mice caused central obesity, a metabolic syndrome and hypertension due to increased intracellular cortisol concentrations. Peroxisome proliferator-activated receptor gamma agonists reduce 11beta-HSD1 activity and diminish the intracellular availability of cortisol, an effect accompanied by a decline in blood pressure. Three individuals with loss-of-function mutations of peroxisome proliferator-activated receptor gamma developed early hypertension. A potential mechanism might be glucocorticoid dependent mineralocorticoid receptor-mediated downregulation of endothelial nitric oxide synthase.

SUMMARY

Recently, mineralocorticoid receptor antagonists have been used in the randomized aldactone evaluation study (RALES) with spironolactone, the eplerenone post-AMI heart failure efficacy and survival study (EPHESUS), and in severe and postmyocardial infarct heart failure, respectively. These investigations cannot be understood on the basis of the present physiological knowledge and underscore the relevance of focusing on mineralocorticoid receptor activation by ligands other than aldosterone.

Peroxisome proliferator-activated receptor γ: Its role in metabolic syndrome

Here we review PPARgamma function in relation to human adipogenesis, insulin sensitization, lipid metabolism, blood pressure regulation and prothrombotic state to perhaps provide justification for this nuclear receptor remaining a key therapeutic target for the continuing development of agents to treat human metabolic syndrome.

PPARγ ligands attenuate mesangial contractile dysfunction in high glucose

BACKGROUND

To elucidate the regulation of peroxisome proliferator-activated receptor gamma (PPARgamma) and its roles in mesangial cells, we examined the expression of PPARgamma1 and effects of its ligands on cell phenotypes and angiotensin II-induced contractile response in cultured rat mesangial cells under a high (20 mmol/L) glucose condition.

METHODS

The effects of tumor necrosis factor alpha (TNFalpha), protein kinase C (PKC) activation, antisense DNA for PPARgamma1, PPARgamma ligands and PD98059 were examined in mesangial cells cultured in either 5 mmol/L or 20 mmol/L glucose. The expressions of PPARgamma1 protein and alpha-smooth muscle actin (alphaSMA) as a marker of phenotype of cells were determined by Western blot. The expression of PPARgamma1 mRNA was determined by a reverse transcription-polymerase chain reaction method. The reduction of cell surface area in response to angiotensin II was measured by microscope to determine cellular contraction.

RESULTS

PKC activation, TNFalpha, and 20 mmol/L glucose decreased PPARgamma1 at both protein and mRNA levels, which was inhibited by PD98059, a specific inhibitor of mitogen-activated protein kinase (MAPK). Decreases of PPARgamma1 protein and contractile response and an increase of alphaSMA occurred simultaneously in the cells treated with 20 mmol/L glucose after 5 days, which were attenuated to the normal levels by PPARgamma ligands. The antisense DNA also induced the decrease of PPARgamma1 protein, contractile dysfunction, and increase of alphaSMA.

CONCLUSION

MAPK suppresses PPARgamma1 at the transcriptional level, and the reduction of PPARgamma1 in cultured rat mesangial cells under the high glucose condition induces phenotypic change and loss of contractile function. PPARgamma ligands recover both reductions of PPARgamma 1 protein and contractile response.

Attenuation by 4-aminopyridine of delayed vasorelaxation by troglitazone

Troglitazone and other thiazolidinediones (TZDs) are thought to relax arterial smooth muscle by directly inhibiting calcium channels in smooth muscle cell membranes. However, until recently such inhibition was only examined acutely, ie, within only seconds or minutes after administration of these agents to arterial smooth muscle preparations. Recently, a novel experiment was reported in which troglitazone caused a 2-phase relaxation of perfused resistance arteries, namely, an acute relaxation (within the first 20 minutes of treatment), which was blocked by a nonselective calcium channel blocker and a delayed relaxation (after 2 hours), which was not. We sought to determine if any of the 4 major potassium (K) channels in vascular smooth muscle play a role in the delayed relaxation. We incubated vascular contractile rings prepared from ventral tail arteries of rats with physiological buffer containing either 0 or 4 micromol/L troglitazone for 3 hours (4 micromol/L is typical of plasma levels from diabetic patients). Different K channel inhibitors (1 mmol/L 4-aminopyridine [4AP]; 1 mmol/L tetraethylammonium [TEA]; 5 micromol/L glyburide; 20 micromol/L barium) were coadministered with each level of troglitazone in additional preparations. Then these arterial rings were contracted with either norepinephrine (NE), arginine vasopressin (AVP), or high-K buffer. All contractions were significantly relaxed by troglitazone (P <.05). Only 4AP significantly attenuated troglitazone's relaxation of NE and AVP contractions (P <.05), though not high-K-induced contractions. TEA, glyburide, and barium had no such influence. Thus, for both adrenergic (NE) and nonadrenergic (AVP) contractions, the delayed arterial vasorelaxation by troglitazone may be mediated at least in part by activation of 4AP-sensitive K channels. Furthermore, the specific subtype of the channels involved is most likely those bound in the outer cell membrane where their effectiveness in terms of mediating relaxation would depend on an intact transmembrane K ion gradient.

PPAR(gamma) agonist rosiglitazone improves vascular function and lowers blood pressure in hypertensive transgenic mice

The peroxisome proliferator activated receptor (PPARgamma) agonist rosiglitazone has been reported to yield cardiovascular benefits in patients by a mechanism that is not completely understood. We tested whether oral rosiglitazone (25 mg/kg per day, 21 days) treatment improves blood pressure and vascular function in a transgenic mouse expressing both human renin and human angiotensinogen transgenes (R(+)A(+)). Rosiglitazone decreased systolic (138+/-5 versus 128+/-5 mm Hg) and mean blood pressure (145+/-5 versus 126+/-7 mm Hg) of R(+)A(+) mice as measured by tail-cuff and indwelling carotid catheters, respectively. Relaxation of carotid arteries to acetylcholine and authentic nitric oxide, but not papaverine, was impaired in R(+)A(+) mice when compared with littermate controls (RA(-)). There were no effects of rosiglitazone on RA(-) mice; however, relaxation to acetylcholine (49+/-10 versus 82+/-9% at 100 micromol/L) and nitric oxide (51+/-11 versus 72+/-6% at 10 micromol/L) was significantly improved in treated R(+)A(+) mice. Rosiglitazone treatment of R(+)A(+) mice did not alter the expression of genes, including endothelial nitric oxide synthase (eNOS), angiotensin 1 receptors, and preproendothelin-1, nor did it alter the levels of eNOS or soluble guanylyl cyclase protein. In separate studies, carotid arteries from R(+)A(+) and RA(-) mice relaxed in a concentration-dependent manner to rosiglitazone, suggesting possible PPARgamma-independent effects in the vasculature. This response was not inhibited with the nitric oxide synthase inhibitor N(omega)-nitro-l-arginine methyl ester (200 micromol/L) or the PPARgamma antagonist bisphenol A diglycidyl ether; 4,4'-isopropylidenediphenol diglycidyl ether (100 micromol/L). These data suggest that in addition to potential genomic regulation caused by PPARgamma activation, the direct effect of rosiglitazone in blood vessels may contribute to the improved blood pressure and vessel function.

Deleterious effects of acute treatment with a peroxisome proliferator-activated receptor-gamma activator in myocardial ischemia and reperfusion in pigs

Thiazolidinediones exert electrophysiologic effects in noncardiac cells in vitro, but to date there have been no reports of effects on cardiac rhythm. We previously demonstrated that chronic pretreatment with a thiazolidinedione peroxisome proliferator-activated receptor (PPAR)-gamma activator, troglitazone, improves recovery of left ventricular (LV) function and substrate metabolism after ischemia and reperfusion, without causing arrhythmias. In this study, we determined whether similar salutary effects are achieved with acute treatment with troglitazone. Anesthetized pigs underwent 90 min of regional LV ischemia and 90 min of reperfusion. Fifteen pigs were treated with troglitazone (10 mg/kg load, 5 mg. kg(-1). h(-1) infusion i.v.) beginning 1 h before ischemia. Seven pigs received corresponding vehicle. Plasma troglitazone concentration (mean 5 microg/ml) was similar to that achieved in clinical use of this agent. Before ischemia, acute troglitazone treatment had no effect on LV function, electrocardiogram, or substrate utilization. During ischemia or reperfusion, eight pigs in the troglitazone group died of ventricular fibrillation, compared with no pigs in the vehicle group (P < 0.05). Pigs that developed ventricular fibrillation had shorter QT intervals than survivors of either group. Among survivors, neither LV function nor substrate utilization differed between groups. Acute treatment with troglitazone increases susceptibility to ventricular fibrillation during myocardial ischemia and reperfusion. Whether thiazolidinediones have proarrhythmic potential in clinical use requires further investigation.

Troglitazone: the discovery and development of a novel therapy for the treatment of Type 2 diabetes mellitus

Prior to the introduction of troglitazone, it had been more than 30 years since the last significant improvement in antidiabetic therapy. In view of the pressing need for more effective oral agents for the treatment of Type 2 diabetes mellitus, troglitazone was granted priority review by the FDA and was launched in the USA in 1997. The first of the thiazolidinedione insulin sensitizing agents, troglitazone was quickly followed by rosiglitazone and pioglitazone. The glitazones proved to be effective not only in lowering blood glucose, but also to have beneficial effects on cardiovascular risk. Troglitazone was subsequently withdrawn because of concerns about hepatotoxicity, which appears to be less of a problem with rosiglitazone and pioglitazone. Recent insights into the molecular mechanism of action of the glitazones, which are ligands for the peroxisome proliferator-activated receptors, open the prospect of designing more effective, selective and safer antidiabetic agents. This document will review the history of troglitazone from discovery through clinical development.

Enhanced inhibition of L-type calcium currents by troglitazone in streptozotocin-induced diabetic rat cardiac ventricular myocytes

1. Troglitazone, an insulin-sensitizing agent shown to improve cardiac function in both experimental animals and patients with diabetes, inhibits voltage-dependent L-type Ca(2+) currents (I(Ca,L)) in cardiac myocytes, which may underlie its cardioprotective effects. However, inhibition by troglitazone of I(Ca,L) in diabetic cardiac myocytes has not been characterized. 2. Using whole-cell voltage-clamp techniques, I(Ca,L) was measured in ventricular myocytes isolated from 4-6 weeks streptozotocin (STZ)-induced diabetic rats and age-matched control rats. 3. Under control conditions with CsCl internal solution, diabetic myocytes did not differ from control myocytes in membrane capacitance, current density or voltage-dependent properties of I(Ca,L). 4. Troglitazone decreased amplitude of I(Ca,L) in both control and diabetic myocytes in a concentration-dependent manner. This inhibition was more potent in diabetic than in control myocytes; half-maximum inhibitory concentrations of troglitazone measured at a holding potential of -50 mV were 4.3 and 9.5 micromol l(-1), respectively. 5. Troglitazone at 5 micromol l(-1) did not significantly influence the voltage dependency of steady-state inactivation or the inactivation time course of I(Ca,L) in either control or diabetic myocytes. 6. Since troglitazone inhibits I(Ca,L) more effectively in STZ-induced diabetic ventricular myocytes, this agent may prevent cardiac dysfunction in diabetes.

Intracellular angiotensin II stimulates voltage-operated Ca(2+) channels in arterial myocytes

Although the presence of intracellular angiotensin II (Ang II) and of Ang II-binding sites has been reported, their roles in cell function have not been fully clarified. The purpose of the present study was to test the hypothesis that intracellular Ang II modifies voltage-operated Ca(2+) channels in vascular smooth muscle. Ca(2+) channel currents were recorded in guinea pig mesenteric arterial myocytes with the whole-cell patch-clamp method. Intracellular dialysis of Ang II increased the amplitudes of Ca(2+) channel current (133 +/- 9% of the control with 10 nmol/L Ang II, n=16). Concomitant dialysis of the Ang II type 1 receptor antagonist, CV-11974 (1 micromol/L, n=11), but not the bath application of this drug, suppressed this Ang II action. In contrast, the dialysis of the Ang II type 2 receptor antagonist, PD123319 (1 micromol/L, n=5), failed to affect the Ang II action. Dialysis of either a phospholipase C inhibitor (U-73122, 10 micromol/L, n=5) or protein kinase C inhibitors (calphostin C, 100 nmol/L, n=5; protein kinase C inhibitor peptide-[19-36], 1 micromol/L, n=5) suppressed the Ang II action. Dialysis of KT5720 (100 nmol/L, n=5), an inhibitor of cAMP-dependent protein kinase, did not affect the Ang II action. Intracellular dialysis of angiotensin I (10 nmol/L) enhanced Ca(2+) channel currents (13 3 +/- 8%, n=6), which were sensitive to intracellular enalaprilat (1 micromol/L, n=5) or CV-11974 (n=5). These results suggest that intracellular Ang II has a stimulating action on voltage-operated Ca(2+) channels in vascular smooth muscle, possibly through intracellular binding sites similar to the Ang II type 1 receptor, which are associated with phospholipase C and protein kinase C.

Comparative actions of insulin sensitizers on ion channels in vascular smooth muscle

Thiazolidinedione and isoxazolidinedione insulin sensitizers activate peroxisome proliferator-activated receptor gamma (PPAR gamma). Some thiazolidinediones modify ion channels in smooth muscles; however, the mechanism by which their actions occur has not been clarified. We, thus, examined the effects of three thiazolidinediones (troglitazone, pioglitazone, and rosiglitazone) and isoxazolidinedione (JTT-501), as well as an intrinsic ligand for PPAR gamma, 15-deoxy-Delta(12,14) prostaglandin J(2) (prostaglandin J(2)), on voltage-operated Ca(2+) currents (I(Ca)), voltage-dependent K(+) currents (I(Kv)), and Ca(2+)-activated K(+) currents (I(Kca)), to clarify whether a thiazolidinedione structure or PPAR gamma activation is related to their actions on ion channels. The whole-cell patch clamp method was used to record currents in smooth muscle cells from guinea-pig mesenteric arteries. Thiazolidinediones inhibited I(Ca) in a dose-dependent manner (troglitazone>pioglitazone=rosiglitazone). Troglitazone (> or =1 microM) and rosiglitazone (100 microM), but not pioglitazone, inhibited I(Kv). Rosiglitazone (> or =10 microM) enhanced, troglitazone (> or =1 microM) inhibited, and pioglitazone did not affect I(Kca). A high concentration of JTT-501 (100 microM) inhibited I(Ca), I(Kv), and I(Kca) to a similar extent. Prostaglandin J(2) enhanced I(Kca), but affected neither I(Ca) nor I(Kv). In summary, the three thiazolidinediones and isoxazolidinedione act differently on Ca(2+) and K(+) channels in vascular smooth muscle. The action of thiazolidinediones on I(Ca) could be attributed to specific regions of the molecules and not to activation of PPAR gamma. Involvement of PPAR gamma activation in the stimulation of I(Kca) is possible but should be tested further.

Troglitazone corrects metabolic changes but not vascular dysfunction in dietary-obese rats

Insulin resistance has been attributed to the defect in vascular function associated with obesity, type 2 diabetes and dyslipidaemia. We have investigated vascular effects of chronic (3-week) administration of troglitazone on female Wistar rats with moderate dietary obesity. Compared with lean controls, untreated obese rats had significantly higher body weights, fat pad masses, plasma triglycerides, free fatty acids and leptin levels (for all P < 0.01). These metabolic changes were corrected by troglitazone treatment. In mesenteric arteries, responses to noradrenaline or KCl were similar in all groups. However, in noradrenaline-preconstricted arteries, vasorelaxations to acetylcholine and insulin were significantly (50-60% less than in lean, P < 0.001) attenuated in both untreated and troglitazone-treated obese rats. Relaxations to sodium nitroprusside showed similar but lesser impairment in both untreated and troglitazone-treated obese animals. Our data show that although troglitazone markedly improved obesity-induced metabolic changes, it failed to correct vascular dysfunction associated with obesity in female Wistar rats.

Peroxisome proliferator-activated receptor gamma activators downregulate angiotensin II type 1 receptor in vascular smooth muscle cells

BACKGROUND

Peroxisome proliferator-activated receptor gamma (PPARgamma) activators, such as troglitazone (Tro), not only improve insulin resistance but also suppress the neointimal formation after balloon injury. However, the precise mechanisms have not been determined. Angiotensin II (Ang II) plays crucial roles in the pathogenesis of atherosclerosis, hypertension, and neointimal formation after angioplasty. We examined the effect of PPARgamma activators on the expression of Ang II type 1 receptor (AT(1)-R) in cultured vascular smooth muscle cells (VSMCs).

METHODS AND RESULTS

AT(1)-R mRNA and AT(1)-R protein levels were determined by Northern blot analysis and radioligand binding assay, respectively. Natural PPARgamma ligand 15-deoxy-Delta(12,14)-prostaglandin J(2), as well as Tro, reduced the AT(1)-R mRNA expression and the AT(1)-R protein level. The PPARgamma activators also reduced the calcium response of VSMCs to Ang II. PPARgamma activators suppressed the AT(1)-R promoter activity measured by luciferase assay but did not affect the AT(1)-R mRNA stability, suggesting that the suppression occurs at the transcriptional level.

CONCLUSIONS

PPARgamma activators reduced the AT(1)-R expression and calcium response to Ang II in VSMCs. Downregulation of AT(1)-R may contribute to the inhibition of neointimal formation by PPARgamma activators.

Troglitazone improves recovery of left ventricular function after regional ischemia in pigs

BACKGROUND

This study determined whether treatment of normal (nondiabetic) pigs with the insulin-sensitizing agent troglitazone improves recovery of left ventricular (LV) function after acute ischemia and whether such effects are associated with altered myocardial substrate metabolism.

METHODS AND RESULTS

Juvenile pigs (n=6) were treated with troglitazone (75 mg. kg(-1). d(-1) PO) for 8 weeks. Untreated pigs (n=8) served as controls. Under anesthetized, open-chest conditions, pigs underwent 90 minutes of moderate regional LV ischemia and 90 minutes of reperfusion. Regional LV function was assessed with subendocardial sonomicrometry crystals. Fasting plasma insulin and free fatty acid levels were lower in troglitazone-treated pigs than in untreated pigs, whereas blood glucose did not differ between groups. These findings suggest that treatment enhanced systemic insulin sensitivity. Baseline hemodynamics and regional LV function did not differ between groups. After ischemia and reperfusion, systolic function (external work) of the ischemic region recovered to 44+/-6% of baseline in troglitazone-treated pigs versus 18+/-6% of baseline in untreated pigs (P<0.05). Regional diastolic function (maximum rate of wall expansion) recovered to 78+/-7% of baseline in treated pigs versus 52+/-7% of baseline in untreated pigs (P<0.05). Recovery of global LV systolic and diastolic function was also significantly greater in treated pigs. Myocardial glucose uptake did not differ between groups under any condition; however, net myocardial lactate uptake after reperfusion was 7 times greater in troglitazone-treated pigs than in untreated pigs, suggesting that treatment enhanced myocardial carbohydrate oxidation after reperfusion.

CONCLUSIONS

In nondiabetic pigs, chronic troglitazone treatment improves recovery of LV systolic and diastolic function after acute ischemia.

Inhibitory action of troglitazone, an insulin-sensitizing agent, on the calcium current in cardiac ventricular cells of guinea pig

We investigated the effects of troglitazone, a new orally active hypoglycemic agent, on the voltage-dependent L-type Ca2+ current in single cardiac ventricular myocytes of guinea pigs by the whole-cell voltage clamp technique. Troglitazone blocked the Ca2+ currents in a concentration-dependent manner. The inhibitory effect was more potent at the holding potential (HP) of - 50 mV than at - 80 mV. The half-maximum inhibiting concentration (IC50) of troglitazone was 0.8 microM with the Hill coefficient of 0.84 at -50 mV HP. In contrast, the IC50 value was higher than 10 microM at -80 mV HP. These results suggest that troglitazone at therapeutic concentrations inhibit the Ca2+ channels and may exert cardioprotective effects in diabetic conditions.

Metformin improves vascular function in insulin-resistant rats

This study assessed the effect of metformin treatment on insulin, mean arterial pressure (MAP), and endothelial function in insulin-resistant (IR) rats. In addition, we assessed the direct effect of metformin in vitro. Sprague-Dawley rats were randomized to control (n=28) or IR (n=28) groups. Rats were further randomized to receive metformin (300 mg/kg) or placebo for 2 weeks. MAP and insulin were measured. Subsequently, a third-order branch of the superior mesenteric artery was isolated, and endothelial function was assessed. Specifically, dose-response experiments of acetylcholine (ACh) with or without N-nitro-L-arginine (LNNA) were performed. For in vitro experiments, mesenteric arteries were removed from untreated control and IR rats and treated with metformin (100 micromol/L) before ACh+/-LNNA. MAP and insulin levels were improved in IR-metformin compared with IR-placebo rats. Maximal relaxation (E(max)) to ACh was enhanced in IR-metformin (92+/-2%) compared with IR-placebo rats (44+/-4%) (P<0.05). Relaxation in response to ACh+LNNA was greater in IR-metformin (33+/-4%) than in IR-placebo rats (12+/-4%) but remained depressed compared with control rats (E(max)=68+/-5%). The control group was not affected by metformin. In vitro treatment of arteries with metformin in response to ACh produced results similar to those in the experiments with metformin-treated rats. Although metformin improves metabolic abnormality in IR rats, this action does not appear to mediate its effect on vascular function. Both in vivo and in vitro metformin improved ACh-induced relaxation in IR rats to control levels, apparently through nitric oxide-dependent relaxation. These data suggest that metformin improves vascular function through a direct mechanism rather than by improving metabolic abnormalities.

Dominant negative mutations in human PPARgamma associated with severe insulin resistance, diabetes mellitus and hypertension

Thiazolidinediones are a new class of antidiabetic agent that improve insulin sensitivity and reduce plasma glucose and blood pressure in subjects with type 2 diabetes. Although these agents can bind and activate an orphan nuclear receptor, peroxisome proliferator-activated receptor gamma (PPARgamma), there is no direct evidence to conclusively implicate this receptor in the regulation of mammalian glucose homeostasis. Here we report two different heterozygous mutations in the ligand-binding domain of PPARgamma in three subjects with severe insulin resistance. In the PPARgamma crystal structure, the mutations destabilize helix 12 which mediates transactivation. Consistent with this, both receptor mutants are markedly transcriptionally impaired and, moreover, are able to inhibit the action of coexpressed wild-type PPARgamma in a dominant negative manner. In addition to insulin resistance, all three subjects developed type 2 diabetes mellitus and hypertension at an unusually early age. Our findings represent the first germline loss-of-function mutations in PPARgamma and provide compelling genetic evidence that this receptor is important in the control of insulin sensitivity, glucose homeostasis and blood pressure in man.

Therapeutic index for rosiglitazone in dietary obese rats: separation of efficacy and haemodilution

1. The blood glucose-lowering efficacy of rosiglitazone (RSG) and the mechanisms of associated weight gain were determined in dietary obese rats (DIOs). DIO and chow-fed rats received RSG 0.3-30 mg kg-1 daily for 21 days. 2. In DIOs, plasma glucose and insulin concentrations were reduced by RSG at dosages of 3 and 10 mg kg-1, respectively. Homeostasis model assessment (HOMA) indicated the threshold for a reduction of insulin resistance was 1 mg kg-1. Neither glucose nor insulin levels were affected by treatment in chow-fed rats. 3. RSG 0.3 mg kg-1 lowered free fatty acids (FFAs) in DIOs, whereas for plasma triglycerides (TGs), the threshold was 3 mg kg-1. By contrast, the threshold for reducing packed red cell volume (PCV) and increasing cardiac mass was 10 mg kg-1. Thus, the therapeutic index for RSG in DIOs was >3 and < or = 10. 4. Energy intake and weight gain increased in treated DIOs (by 20% and 50 g, at 30 mg kg-1) and chow-fed rats (by 25% and 35 g, at 30 mg kg-1). In DIOs, these increases coincided with falls in plasma leptin (40% lower at 30 mg kg-1) and insulin (43% lower at 30 mg kg-1). By contrast, in chow-fed rats, weight gain and hyperphagia occurred without changes in either leptin or insulin. However, reductions in FFAs below 0.4 - 0.3 mM were associated with hyperphagia and weight gain in DIO and chow-fed rats. 5. We conclude that increased energy intake and body weight did not attenuate the improved metabolism evoked by RSG in DIO rats, and that insulin action was enhanced at a dose >3 fold below the threshold for causing haemodilution and cardiac hypertrophy in DIO rats.

Pharmacological peroxisome proliferator-activated receptorgamma ligands: emerging clinical indications beyond diabetes

The discovery of peroxisome proliferator-activated receptor gamma (PPARgamma) as the molecular target for antidiabetic thiazolidinediones has heralded a new era in the approach to understanding the pathophysiology of insulin resistance and its relationship to cardiovascular disease. However, the subsequent discovery of PPARgamma-dependent modulation of immune function and the cell cycle has led to a new paradigm in the approach to treating proliferative, inflammatory diseases. Moreover, PPARgamma agonists can promote apoptosis, block angiogenesis and inhibit pathological remodelling in a variety of malignant and non-malignant pathological states. These findings imply that the pharmacological modulation of this key nuclear transcription factor and its co-factors could be important tools in understanding the relationships between multigenic diseases, and pave the way to a focused interventional approach in their treatment. With the availability of the PPARgamma protein crystal structure, the ligand binding domain co-ordinates and a better knowledge of the interaction of PPARgamma with co-factor assemblies, libraries of simple synthetic organic PPARgamma ligands can be constructed. High throughput screening can identify the best candidates for targeting cellular phenotypic transition, cell cycle control, inflammation and apoptosis. Instead of single agents for single pathologies, one can envisage the development of multifunctional therapeutic agents that target the multiple cellular processes that contribute to multifactorial diseases such as diabetes, hypertension, atherosclerosis, psoriasis and other inflammatory diseases, and carcinogenesis. The considerable potential of PPARgamma ligands in the treatment of diseases other than diabetes is the subject of this review.

Troglitazone and pioglitazone attenuate agonist-dependent Ca2+ mobilization and cell proliferation in vascular smooth muscle cells

1. The effects of troglitazone and pioglitazone on agonist-induced Ca2+ mobilization and cell proliferation were studied using fluorescent Ca2+ indicator fura-2 AM and incorporation of [3H]-thymidine in rat aortic smooth muscle cells. The patch clamp techniques were also employed. 2. Vasopressin and platelet-derived growth factor-BB (PDGF) caused a transient elevation in [Ca2+]i by Ca2+ mobilization from intracellular stores, followed by a sustained rise due to Ca2+ entry. Nicardipine partly inhibited the sustained phase, but La3+ completely abolished it. 3. Troglitazone and pioglitazone did not significantly affect the transient rise elicited by these agonists, but preferentially inhibited the sustained phase of [Ca2+]i. 4. Under voltage clamp conditions, troglitazone and pioglitazone inhibited voltage-dependent L-type Ca2+ current (ICa.L). They also inhibited nonselective cation channels (Icat) elicited by vasopressin in a concentration-dependent manner. The half maximal inhibitory concentrations of troglitazone on ICa.L and Icat were 4.6 and 5.7 microM, respectively. On the other hand, nifedipine and nicardipine did not inhibit Icat. 5. Vasopressin and PDGF increased incorporation of [3H]-thymidine, and nifedipine and nicardipine partly suppressed it. However, the inhibitory effects of La3+ and exclusion of extracellular Ca2+ were more potent than the Ca2+ blocking agents. Troglitazone and pioglitazone also inhibited it concentration-dependently. 6. These results suggest that troglitazone and pioglitazone preferentially inhibited agonist (vasopressin and PDGF)-induced Ca2+ entry and proliferation in rat vascular smooth muscle cells, where the inhibitory effects of thiazolidinediones on ICa.L and Icat might be partly involved. Thus, thiazolidinediones may exert hypotensive and antiatherosclerotic effects.

Sodium-potassium pump current in smooth muscle cells from mesenteric resistance arteries of the guinea-pig

1. The Na+-K+ pump current was studied in smooth muscle cells from mesenteric resistance arteries of guinea-pigs by the use of the perforated patch-clamp technique in the presence of blockers for various ion channels and exchangers. 2. When the Na+ concentration in the pipette solution ([Na+]i) was 50 mM, an increase in the extracellular K+ concentration ([K+]o) from 0 to 10 mM caused an outward current. Both the removal of K+ from the bath solution and the application of 10 microM ouabain abolished this current. Thus, this K+-induced and ouabain-sensitive current was considered to be the Na+-K+ pump current. 3. The amplitude of the Na+-K+ pump current increased as the membrane potential was made more positive until around 0 mV, while the amplitude saturated at more positive potentials than 0 mV. 4. An increase in [K+]o or [Na+]i amplified the Na+-K+ pump current. For [K+]o, the binding constant (Kd) was 1.6+/-0.3 mM and the Hill coefficient (nH) was 1.1+/-0.2 (n = 6). For [Na+]i, Kd was 22+/-5 mM and nH was 1.7+/-0.5 (n = 4-19). 5. The presence of various monovalent cations other than Na+ in the bath solution also evoked the Na+-K+ pump current. The order of potency was K+ >= Rb+ > Cs+ >> Li+. 6. Ouabain inhibited the Na+-K+ pump current in a dose-dependent manner with a Kd of 0.35+/-0.03 microM and an nH of 1.2+/-0.1 (n = 6-8). 7. The Na+-K+ pump current increased as temperature increased. The temperature coefficient (Q10; 26-36 C) was 1.87 (n = 9). 8. In summary the present study characterized for the first time the Na+-K+ pump current in vascular smooth muscle cells by the use of the voltage-clamp method. The use of this method should provide essential information for Na+,K+-ATPase-mediated changes in the cell functions of vascular smooth muscle cells.

Troglitazone inhibits voltage-dependent calcium currents in guinea pig cardiac myocytes

BACKGROUND

It has been suggested that intracellular Ca2+ overload in cardiac myocytes leads to the development of diabetic cardiomyopathy. Troglitazone, an insulin-sensitizing agent, is a promising therapeutic agent for diabetes and has been shown to prevent diabetes-induced myocardial changes. To elucidate the underlying mechanism of troglitazone action on cardiac myocytes, the effects of troglitazone on voltage-dependent Ca2+ currents were examined and compared with classic Ca2+ antagonists (verapamil and nifedipine).

METHODS AND RESULTS

Whole-cell voltage-clamp techniques were applied in single guinea pig atrial myocytes. Under control conditions with CsCl internal solution, the voltage-dependent Ca2+ currents consisted of both T-type (ICa,T) and L-type (ICa,L) Ca2+ currents. Troglitazone effectively reduced the amplitude of ICa,L in a concentration-dependent manner. Troglitazone also suppressed ICa,T, but the effect of troglitazone on ICa,T was less potent than that on ICa,L. The current-voltage relationships for ICa,L and the reversal potential for ICa,L were not altered by troglitazone. The half-maximal inhibitory concentration of troglitazone on ICa,L measured at a holding potential of -40 mV was 6.3 micromol/L, and 30 micromol/L troglitazone almost completely inhibited ICa,L. Troglitazone 10 micromol/L did not affect the time courses for inactivation of ICa,L and inhibited ICa,L mainly in a use-independent fashion, without shifting the voltage-dependency of inactivation. This effect was different from those of verapamil and nifedipine. Troglitazone also reduced isoproterenol- or cAMP-enhanced ICa,L.

CONCLUSIONS

These results demonstrate that troglitazone inhibits voltage-dependent Ca2+ currents (T-type and L-type) and then antagonizes the effects of isoproterenol in cardiac myocytes, thus possibly playing a role in preventing diabetes-induced intracellular Ca2+ overload and subsequent myocardial changes.

Differential effects of insulin-sensitizers troglitazone and rosiglitazone on ion currents in rat vascular myocytes

Insulin-sensitizing thiazolidinediones such as troglitazone and pioglitazone have been shown to lower blood pressure in vivo and cause vasorelaxation in vitro. Rosiglitazone (BRL 49653) is a novel thiazolidinedione which has been reported not to cause vasoleraxation. We therefore compared the effects of troglitazone and rosiglitazone on Ca2+ and K+ currents in rat aorta and pulmonary artery smooth muscle cells. Currents were recorded with the conventional whole cell patch clamp technique. Both drugs reduced the voltage-gated (L-type) Ca2+ current in rat aorta cells, with half-maximal current inhibition by troglitazone and rosiglitazone at 2 and 10 microM, respectively. Troglitazone, 2 microM and rosiglitazone, 20 microM caused a similar hyperpolarizing shift of 12 mV in the potential-dependence of Ca2+ current availability. Troglitazone (20 microM) produced a marked block of the tetraethylammonium- and paxilline-sensitive Ca2+ activated K+ current, while rosiglitazone (20 microM and 60 microM) slightly enhanced this current. Rat pulmonary artery smooth muscle cells have a prominent delayed rectifier K+ current. Troglitazone produced a potent block of this current (half-maximal inhibition at <1 microM), while rosiglitazone caused a smaller inhibition at 10 and 60 microM. These results show that troglitazone has relatively potent blocking effects on a wide variety of ion currents in vascular smooth muscle cells. Rosiglitazone exerts less potent, but similar effects on the Ca2+ current and delayed rectifier K+ current, but it enhances the Ca2+ activated K+ current. reserved.

Acute vasorelaxant effects of metformin and attenuation by stimulation of sympathetic agonist release

We recently discovered 1) that intravenous injection of the antidiabetic drug metformin in the rat rapidly reduces arterial pressure elevations maintained by the alpha-adrenoceptor agonist phenylephrine (PE) and 2) that direct administration of metformin to isolated rat tail arterial tissue rings rapidly relaxes PE-induced contractions. To further characterize this potential direct vasodilator action, we examined effects of metformin on contractions induced not only by PE but also by norepinephrine (NE) and by nonadrenergic agonists (5-hydroxytryptamine, 5HT; arginine vasopressin, AVP). Also, because the rat tail artery contains abundant adrenergic nerve endings we conducted these tests not only in arterial rings with nerve endings intact but in rings in which they had been removed by pretreatment with 6-hydroxydopamine. In intact rings, metformin at levels from approximately 0.2 to 20 mmol/L rapidly relaxed half-maximal contractions induced by PE and NE similarly and to a markedly greater degree than contractions induced by 5-HT (p<0.05). Metformin did not relax AVP-induced contractions. In addition, removal of adrenergic nerve endings facilitated metformin's relaxant effects (p<0.05). Thus, the acute vasodilator action of metformin appears 1) to be selectively more powerful on arterial smooth muscle contractions induced adrenergically versus nonadrenergically and 2) to be buffered by a possible metformin-induced release of endogenous NE from adrenergic nerve endings. Such results were not seen during relaxation produced by either the calcium channel inhibitor nifedipine or the nitrovasodilator nitroprusside suggesting that metformin's effects are mediated by other mechanisms.