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

Ferroptosis mechanisms and regulations in cardiovascular diseases in the past, present, and future

Cardiovascular diseases (CVDs) are the main diseases that endanger human health, and their risk factors contribute to high morbidity and a high rate of hospitalization. Cell death is the most important pathophysiology in CVDs. As one of the cell death mechanisms, ferroptosis is a new form of regulated cell death (RCD) that broadly participates in CVDs (such as myocardial infarction, heart transplantation, atherosclerosis, heart failure, ischaemia/reperfusion (I/R) injury, atrial fibrillation, cardiomyopathy (radiation-induced cardiomyopathy, diabetes cardiomyopathy, sepsis-induced cardiac injury, doxorubicin-induced cardiac injury, iron overload cardiomyopathy, and hypertrophic cardiomyopathy), and pulmonary arterial hypertension), involving in iron regulation, metabolic mechanism and lipid peroxidation. This article reviews recent research on the mechanism and regulation of ferroptosis and its relationship with the occurrence and treatment of CVDs, aiming to provide new ideas and treatment targets for the clinical diagnosis and treatment of CVDs by clarifying the latest progress in CVDs research.

Oxidative stress and inflammation as central mediators of atrial fibrillation in obesity and diabetes

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in humans. Several risk factors promote AF, among which diabetes mellitus has emerged as one of the most important. The growing recognition that obesity, diabetes and AF are closely intertwined disorders has spurred major interest in uncovering their mechanistic links. In this article we provide an update on the growing evidence linking oxidative stress and inflammation to adverse atrial structural and electrical remodeling that leads to the onset and maintenance of AF in the diabetic heart. We then discuss several therapeutic strategies to improve atrial excitability by targeting pathways that control oxidative stress and inflammation.

Diabetes mellitus and atrial remodeling: mechanisms and potential upstream therapies

Atrial fibrillation (AF) is the most common cardiac arrhythmia in clinical practice, and its prevalence has increasing substantially over the last decades. Recent data suggest that there is an increased risk of AF among the patients with diabetes mellitus (DM). However, the potential molecular mechanisms regarding DM-related AF and diabetic atrial remodeling are not fully understood. In this comprehensive review, we would like to summarize the potential relationship between diabetes and atrial remodeling, including structural, electrical, and autonomic remodeling. Also, some upstream therapies, such as thiazolidinediones, probucol, ACEI/ARBs, may play an important role in the prevention and treatment of AF. Therefore, large prospective randomized, controlled trials and further experimental studies should be challengingly continued.

Diabetes mellitus and atrial fibrillation: Pathophysiological mechanisms and potential upstream therapies

Diabetes mellitus (DM) represents one of the most important risk factors for atrial fibrillation (AF) while AF is a strong and independent marker of overall mortality and cardiovascular morbidity in diabetic patients. Autonomic, electrical, electromechanical, and structural remodeling, including oxidative stress, connexin remodeling and glycemic fluctuations seem to be implicated in AF pathophysiology in the setting of DM. The present review highlights the association between DM and AF, provides a comprehensive overview of the responsible pathophysiological mechanisms and briefly discusses potential upstream therapies for DM-related atrial remodeling.

Peroxisome proliferator-activated receptors modulate cardiac dysfunction in diabetic cardiomyopathy

Cardiovascular disease (CVD) is the leading cause of morbidity and mortality among patients with diabetes mellitus (DM). Chronic inflammation and derangement of myocardial energy and lipid homeostasis are common features of DM. The transcription factors of peroxisome proliferator-activated receptors (PPARs) belong to the nuclear receptor superfamily, which are important in regulating energy and lipid homeostasis. There are three PPAR isoforms, α, γ, and δ, and their roles have been increasingly recognized to be important in CVD. These three isoforms are expressed in the heart and play pivotal roles in myocardial lipid metabolism, as well as glucose and energy homeostasis, and contribute to extra metabolic roles with effects on inflammation and oxidative stress. Moreover, regulation of PPARs may have significant effects on cardiac electrical activity and arrhythmogenesis. This review describes the roles of PPARs and their agonists in DM cardiomyopathy, inflammation, and cardiac electrophysiology.

Effects of pioglitazone on cardiac ion currents and action potential morphology in canine ventricular myocytes

Despite its widespread therapeutical use there is little information on the cellular cardiac effects of the antidiabetic drug pioglitazone in larger mammals. In the present study, therefore, the concentration-dependent effects of pioglitazone on ion currents and action potential configuration were studied in isolated canine ventricular myocytes using standard microelectrode, conventional whole cell patch clamp, and action potential voltage clamp techniques. Pioglitazone decreased the maximum velocity of depolarization and the amplitude of phase-1 repolarization at concentrations ≥3 μM. Action potentials were shortened by pioglitazone at concentrations ≥10 μM, which effect was accompanied with significant reduction of beat-to-beat variability of action potential duration. Several transmembrane ion currents, including the transient outward K(+) current (Ito), the L-type Ca(2+) current (ICa), the rapid and slow components of the delayed rectifier K(+) current (IKr and IKs, respectively), and the inward rectifier K(+) current (IK1) were inhibited by pioglitazone under conventional voltage clamp conditions. Ito was blocked significantly at concentrations ≥3 μM, ICa, IKr, IKs at concentrations ≥10 μM, while IK1 at concentrations ≥30 μM. Suppression of Ito, ICa, IKr, and IK1 has been confirmed also under action potential voltage clamp conditions. ATP-sensitive K(+) current, when activated by lemakalim, was effectively blocked by pioglitazone. Accordingly, action potentials were prolonged by 10 μM pioglitazone when the drug was applied in the presence of lemakalim. All these effects developed rapidly and were readily reversible upon washout. In conclusion, pioglitazone seems to be a harmless agent at usual therapeutic concentrations.

Chemical proteomics-based analysis of off-target binding profiles for rosiglitazone and pioglitazone: clues for assessing potential for cardiotoxicity

Drugs exert desired and undesired effects based on their binding interactions with protein target(s) and off-target(s), providing evidence for drug efficacy and toxicity. Pioglitazone and rosiglitazone possess a common functional core, glitazone, which is considered a privileged scaffold upon which to build a drug selective for a given target--in this case, PPARγ. Herein, we report a retrospective analysis of two variants of the glitazone scaffold, pioglitazone and rosiglitazone, in an effort to identify off-target binding events in the rat heart to explain recently reported cardiovascular risk associated with these drugs. Our results suggest that glitazone has affinity for dehydrogenases, consistent with known binding preferences for related rhodanine cores. Both drugs bound ion channels and modulators, with implications in congestive heart failure, arrhythmia, and peripheral edema. Additional proteins involved in glucose homeostasis, synaptic transduction, and mitochondrial energy production were detected and potentially contribute to drug efficacy and cardiotoxicity.

Interaction between Ca(2+) channel blockers and isoproterenol on L-type Ca(2+) current in canine ventricular cardiomyocytes

AIM

The aim of this work was to study antagonistic interactions between the effects of various types of Ca(2+) channel blockers and isoproterenol on the amplitude of L-type Ca(2+) current in canine ventricular cells.

METHODS

Whole-cell version of the patch clamp technique was used to study the effect of isoproterenol on Ca(2+) current in the absence and presence of Ca(2+) channel-blocking agents, including nifedipine, nisoldipine, diltiazem, verapamil, CoCl(2) and MnCl(2) .

RESULTS

Five micromolar Nifedipine, 1 μM nisoldipine, 10 μM diltiazem, 5 μM verapamil, 3 mM CoCl(2) and 5 mM MnCl(2) evoked uniformly a 90-95% blockade of Ca(2+) current in the absence of isoproterenol. Isoproterenol (100 nM) alone increased the amplitude of Ca(2+) current from 6.8 ± 1.3 to 23.7 ± 2.2 pA/pF in a reversible manner. Isoproterenol caused a marked enhancement of Ca(2+) current even in the presence of nifedipine, nisoldipine, diltiazem and verapamil, but not in the presence of CoCl(2) or MnCl(2) .

CONCLUSION

The results indicate that the action of isoproterenol is different in the presence of organic and inorganic Ca(2+) channel blockers. CoCl(2) and MnCl(2) were able to fully prevent the effect of isoproterenol on Ca(2+) current, while the organic Ca(2+) channel blockers failed to do so. This has to be born in mind when the effects of organic Ca(2+) channel blockers are evaluated either experimentally or clinically under conditions of increased sympathetic tone.

Peroxisome proliferator-activated receptor γ decouples fatty acid uptake from lipid inhibition of insulin signaling in skeletal muscle

Peroxisome proliferator-activated receptor γ (PPARγ) is expressed at low levels in skeletal muscle, where it protects against adiposity and insulin resistance via unclear mechanisms. To test the hypothesis that PPARγ directly modulates skeletal muscle metabolism, we created two models that isolate direct PPARγ actions on skeletal myocytes. PPARγ was overexpressed in murine myotubes by adenotransfection and in mouse skeletal muscle by plasmid electroporation. In cultured myotubes, PPARγ action increased fatty acid uptake and incorporation into myocellular lipids, dependent upon a 154 ± 20-fold up-regulation of CD36 expression. PPARγ overexpression more than doubled insulin-stimulated thymoma viral proto-oncogene (AKT) phosphorylation during low lipid availability. Furthermore, in myotubes exposed to palmitate levels that inhibit insulin signaling, PPARγ overexpression increased insulin-stimulated AKT phosphorylation and glycogen synthesis over 3-fold despite simultaneously increasing myocellular palmitate uptake. The insulin signaling enhancement was associated with an increase in activating phosphorylation of phosphoinositide-dependent protein kinase 1 and a normalized expression of palmitate-induced genes that antagonize AKT phosphorylation. In vivo, PPARγ overexpression more than doubled insulin-dependent AKT phosphorylation in lipid-treated mice but did not augment insulin-stimulated glucose uptake. We conclude that direct PPARγ action promotes myocellular storage of energy by increasing fatty acid uptake and esterification while simultaneously enhancing insulin signaling and glycogen formation. However, direct PPARγ action in skeletal muscle is not sufficient to account for the hypoglycemic actions of PPARγ agonists during lipotoxicity.

Can the electrophysiological action of rosiglitazone explain its cardiac side effects?

Recent large clinical trials found an association between the antidiabetic drug rosiglitazone therapy and increased risk of cardiovascular adverse events. The aim of this report is to elucidate the cardiac electrophysiological properties of rosiglitazone (R) on isolated rat and murine ventricular papillary muscle cells and canine ventricular myocytes using conventional microelectrode, whole cell voltage clamp, and action potential (AP) voltage clamp techniques. In histidine-decarboxylase knockout mice as well as in their wild types R (1-30 µM) shortened AP duration at 90% level of repolarization (APD(90)) and increased the AP amplitude (APA) in a concentration-dependent manner. In rat ventricular papillary muscle cells R (1-30 µM) caused a significant reduction of APA and maximum velocity of depolarization (V(max)) which was accompanied by lengthening of APD(90). In single canine ventricular myocytes at concentrations ≥10 µM R decreased the amplitude of phase-1 repolarization, the plateau potential and reduced V(max). R suppressed several ion currents in a concentration-dependent manner under voltage clamp conditions. The EC(50) value for this inhibition was 25.2±2.7 µM for the transient outward K(+ ) current (I(to)), 72.3±9.3 µM for the rapid delayed rectifier K(+ ) current (I(Kr)), and 82.5±9.4 µM for the L-type Ca(2+ ) current (I(Ca)) with Hill coefficients close to unity. The inward rectifier K(+ ) current (I(K1)) was not affected by R up to concentrations of 100 µM. Suppression of I(to), I(Kr), and I(Ca) has been confirmed under action potential voltage clamp conditions as well. The observed alterations in the AP morphology and densities of ion currents may predict serious proarrhythmic risk in case of intoxication with R as a consequence of overdose or decreased elimination of the drug, particularly in patients having multiple cardiovascular risk factors, such as elderly diabetic patients.

Effects of rosiglitazone on the configuration of action potentials and ion currents in canine ventricular cells

BACKGROUND AND PURPOSE

In spite of its widespread clinical application, there is little information on the cellular cardiac effects of the antidiabetic drug rosiglitazone in larger experimental animals. In the present study therefore concentration-dependent effects of rosiglitazone on action potential morphology and the underlying ion currents were studied in dog hearts.

EXPERIMENTAL APPROACH

Standard microelectrode techniques, conventional whole cell patch clamp and action potential voltage clamp techniques were applied in enzymatically dispersed ventricular cells from dog hearts.

KEY RESULTS

At concentrations ≥10 µM rosiglitazone decreased the amplitude of phase-1 repolarization, reduced the maximum velocity of depolarization and caused depression of the plateau potential. These effects developed rapidly and were readily reversible upon washout. Rosiglitazone suppressed several transmembrane ion currents, concentration-dependently, under conventional voltage clamp conditions and altered their kinetic properties. The EC(50) value for this inhibition was 25.2 ± 2.7 µM for the transient outward K(+) current (I(to)), 72.3 ± 9.3 µM for the rapid delayed rectifier K(+) current (I(Kr)) and 82.5 ± 9.4 µM for the L-type Ca(2+) current (I(Ca) ) with Hill coefficients close to unity. The inward rectifier K(+) current (I(K1)) was not affected by rosiglitazone up to concentrations of 100 µM. Suppression of I(to), I(Kr), and I(Ca) was confirmed also under action potential voltage clamp conditions.

CONCLUSIONS AND IMPLICATIONS

Alterations in the densities and kinetic properties of ion currents may carry serious pro-arrhythmic risk in case of overdose with rosiglitazone, especially in patients having multiple cardiovascular risk factors, like elderly diabetic patients.

Effects of long-term pioglitazone treatment on peripheral and central markers of aging

BACKGROUND

Thiazolidinediones (TZDs) activate peroxisome proliferator-activated receptor gamma (PPARgamma) and are used clinically to help restore peripheral insulin sensitivity in Type 2 diabetes (T2DM). Interestingly, long-term treatment of mouse models of Alzheimer's disease (AD) with TZDs also has been shown to reduce several well-established brain biomarkers of AD including inflammation, oxidative stress and Abeta accumulation. While TZD's actions in AD models help to elucidate the mechanisms underlying their potentially beneficial effects in AD patients, little is known about the functional consequences of TZDs in animal models of normal aging. Because aging is a common risk factor for both AD and T2DM, we investigated whether the TZD, pioglitazone could alter brain aging under non-pathological conditions.

METHODS AND FINDINGS

We used the F344 rat model of aging, and monitored behavioral, electrophysiological, and molecular variables to assess the effects of pioglitazone (PIO-Actos(R) a TZD) on several peripheral (blood and liver) and central (hippocampal) biomarkers of aging. Starting at 3 months or 17 months of age, male rats were treated for 4-5 months with either a control or a PIO-containing diet (final dose approximately 2.3 mg/kg body weight/day). A significant reduction in the Ca(2+)-dependent afterhyperpolarization was seen in the aged animals, with no significant change in long-term potentiation maintenance or learning and memory performance. Blood insulin levels were unchanged with age, but significantly reduced by PIO. Finally, a combination of microarray analyses on hippocampal tissue and serum-based multiplex cytokine assays revealed that age-dependent inflammatory increases were not reversed by PIO.

CONCLUSIONS

While current research efforts continue to identify the underlying processes responsible for the progressive decline in cognitive function seen during normal aging, available medical treatments are still very limited. Because TZDs have been shown to have benefits in age-related conditions such as T2DM and AD, our study was aimed at elucidating PIO's potentially beneficial actions in normal aging. Using a clinically-relevant dose and delivery method, long-term PIO treatment was able to blunt several indices of aging but apparently affected neither age-related cognitive decline nor peripheral/central age-related increases in inflammatory signaling.

Eicosapentaenoic acid inhibits voltage-gated sodium channels and invasiveness in prostate cancer cells

BACKGROUND AND PURPOSE

The voltage-gated Na(+) channels (Na(v)) and their corresponding current (I(Na)) are involved in several cellular processes, crucial to metastasis of cancer cells. We investigated the effects of eicosapentaenoic (EPA), an omega-3 polyunsaturated fatty acid, on I(Na) and metastatic functions (cell proliferation, endocytosis and invasion) in human and rat prostate cancer cell lines (PC-3 and Mat-LyLu cells).

EXPERIMENTAL APPROACH

The whole-cell voltage clamp technique and conventional/quantitative real-time reverse transcriptase polymerase chain reaction analysis were used. The presence of Na(v) proteins was shown by immunohistochemical methods. Alterations in the fatty acid composition of phospholipids after treatment with EPA and metastatic functions were also examined.

KEY RESULTS

A transient inward Na(+) current (I(Na)), highly sensitive to tetrodotoxin, and Na(V) proteins were found in these cells. Expression of Na(V)1.6 and Na(V)1.7 transcripts (SCN8A and SCN9A) was predominant in PC-3 cells, while Na(V)1.7 transcript (SCN9A) was the major component in Mat-LyLu cells. Tetrodotoxin or synthetic small interfering RNA targeted for SCN8A and SCN9A inhibited metastatic functions (endocytosis and invasion), but failed to inhibit proliferation in PC-3 cells. Exposure to EPA produced a rapid and concentration-dependent suppression of I(Na). In cells chronically treated (up to 72h) with EPA, the EPA content of cell lipids increased time-dependently, while arachidonic acid content decreased. Treatment of PC-3 cells with EPA decreased levels of mRNA for SCN9A and SCN8A, cell proliferation, invasion and endocytosis.

CONCLUSION AND IMPLICATIONS

Treatment with EPA inhibited I(Na) directly and also indirectly, by down-regulation of Na(v) mRNA expression in prostate cancer cells, thus inhibiting their metastatic potential.

Function and role of voltage-gated sodium channel NaV1.7 expressed in aortic smooth muscle cells

Voltage-gated Na(+) channel currents (I(Na)) are expressed in several types of smooth muscle cells. The purpose of this study was to evaluate the expression of I(Na), its functional role, pathophysiology in cultured human (hASMCs) and rabbit aortic smooth muscle cells (rASMCs), and its association with vascular intimal hyperplasia. In whole cell voltage clamp, I(Na) was observed at potential positive to -40 mV, was blocked by tetrodotoxin (TTX), and replacing extracellular Na(+) with N-methyl-d-glucamine in cultured hASMCs. In contrast to native aorta, cultured hASMCs strongly expressed SCN9A encoding Na(V)1.7, as determined by quantitative RT-PCR. I(Na) was abolished by the treatment with SCN9A small-interfering (si)RNA (P < 0.01). TTX and SCN9A siRNA significantly inhibited cell migration (P < 0.01, respectively) and horseradish peroxidase uptake (P < 0.01, respectively). TTX also significantly reduced the secretion of matrix metalloproteinase-2 6 and 12 h after the treatment (P < 0.01 and P < 0.05, respectively). However, neither TTX nor siRNA had any effect on cell proliferation. L-type Ca(2+) channel current was recorded, and I(Na) was not observed in freshly isolated rASMCs, whereas TTX-sensitive I(Na) was recorded in cultured rASMCs. Quantitative RT-PCR and immunostaining for Na(V)1.7 revealed the prominent expression of SCN9A in cultured rASMCs and aorta 48 h after balloon injury but not in native aorta. In conclusion, these studies show that I(Na) is expressed in cultured and diseased conditions but not in normal aorta. The Na(V)1.7 plays an important role in cell migration, endocytosis, and secretion. Na(V)1.7 is also expressed in aorta after balloon injury, suggesting a potential role for Na(V)1.7 in the progression of intimal hyperplasia.

The potential role of thiazolidinediones in atrial fibrillation

Thiazolidinediones (TZDs) represent insulin sensitizing drugs that are being increasingly used for the treatment of type 2 diabetes. These agents have also pleiotropic properties that possibly contribute to their favorable cardiovascular effects. In particular, TZDs have anti-inflammatory and anti-oxidant potential while they modulate cardiovascular remodeling. On the other hand, atrial electrical and structural remodeling constitutes the substrate for atrial fibrillation (AF) development and perpetuation. Of note, inflammation and oxidative stress have been recently implicated in the pathogenesis of AF while non-channel blocking drugs with pleiotropic properties, including anti-inflammatory and anti-oxidant, seem to favorably affect atrial remodeling. It is therefore reasonable to assume that TZDs may have a role in the management of AF. Despite some limited observations, no study to date has examined the effect of TZDs therapy on AF development. In addition, the role of these agents in atrial remodeling has not been clarified yet.

Effect of dexamethasone on voltage-gated Na+ channel in cultured human bronchial smooth muscle cells

Voltage-gated Na(+) channel (I(Na)) encoded by SCN9A mRNA is expressed in cultured human bronchial smooth muscle cells. We investigated the effects of dexamethasone on I(Na), by using whole-cell voltage clamp techniques, reverse transcriptase/polymerase chain reaction (RT-PCR), and quantitative real-time RT-PCR. Acute application of dexamethasone (10(-6) M) did not affect I(Na). However, the percentage of the cells with I(Na) was significantly less in cells pretreated with dexamethasone for 48 h, and the current-density of I(Na) adjusted by cell capacitance in cells with I(Na) was also decreased in cells treated with dexamethasone. RT-PCR analysis showed that alpha and beta subunits mRNA of I(Na) mainly consisted of SCN9A and SCN1beta, respectively. Treatment with dexamethasone for 24-48 h inhibited the expression of SCN9A mRNA. The inhibitory effect of dexamethasone was concentration-dependent, and was observed at a concentration higher than 0.1 nM. The effect of dexamethasone on SCN9A mRNA was not blocked by spironolactone, but inhibited by mifepristone. The inhibitory effects of dexamethasone on SCN9A mRNA could not be explained by the changes of the stabilization of mRNA measured by using actinomycin D. These results suggest that dexamethasone inhibited I(Na) encoded by SCN9A mRNA in cultured human bronchial smooth muscle cells by inhibiting the transcription via the glucocorticoid receptor.

Diabetic cardiomyopathy

This article focuses on advances in the understanding of the pathogenesis and treatment of diabetic cardiomyopathy. Patients with diabetes are at an elevated risk for heart failure, and comorbid heart failure confers an increased risk of morbidity and mortality. Diabetic cardiomyopathy and to apply proven lifesaving therapies in all heart failure patients, including those with diabetes, in the absence of contraindications or intolerance.

Molecular and pharmacological characteristics of transient voltage-dependent K+ currents in cultured human pulmonary arterial smooth muscle cells

The A-type voltage-dependent K(+) current (I(A)) has been identified in several types of smooth muscle cells including the pulmonary artery (PA), but little is known about the pharmacological and molecular characteristics of I(A) in human pulmonary arterial smooth muscle cells (hPASMCs). We investigated I(A) expressed in cultured PASMCs isolated from the human main pulmonary artery, using patch-clamp techniques, reverse transcriptase-polymerase chain reaction (RT-PCR), quantitative real-time RT-PCR and immunocytochemical studies. With high EGTA and ATP in the pipette, the outward currents were dominated by a transient K(+) current (I(A)), followed by a relatively small sustained outward current (I(K)). I(A) was inhibited by 4-aminopyridine (4-AP) concentration-dependently, and could be separated pharmacologically into two components by tetraethylammonium (TEA) sensitivity. A component was sensitive to TEA, and the second component was insensitive to TEA. I(A) was inhibited by blood depressing substrate (BDS)-II, a specific blocker of K(V)3.4 subunit, and phrixotoxin-II, a specific blocker of K(V)4.2 and 4.3. Flecainide inhibited I(A) concentration-dependently, but it inhibited it preferentially in the presence of TEA (TEA-insensitive I(A)). Systematic screening of expression of K(V) genes using RT-PCR showed the definite presence of transcripts of the I(A)-encoding genes for K(V)3.4, K(V)4.1, K(V)4.2 and K(V)4.3 as well as the I(K)-encoding genes for K(V)1.1, K(V)1.5 and K(V)2.1. The real-time RT-PCR analysis showed that the relative abundance of the encoding genes of I(A) alpha-subunit and K(V) channel-interacting proteins (KChIPs) was K(V)4.2 > K(V)3.4 > K(V)4.3 (long) > K(V)4.1, and KChIP3 >> KChIP2, respectively. The presence of K(V)3.4, K(V)4.2 and K(V)4.3 proteins was also demonstrated by immunocytochemical studies, and confirmed by immunohistochemical staining using intact human PA sections. These results suggest that I(A) in cultured hPASMCs consists of two kinetically and pharmacologically distinct components, probably K(V)3.4 and K(V)4 channels.

Acute and chronic effects of eicosapentaenoic acid on voltage-gated sodium channel expressed in cultured human bronchial smooth muscle cells

This study investigated acute and chronic effects of eicosapentaenoic acid (EPA) on voltage-gated Na+ current (I(Na)) expressed in cultured human bronchial smooth muscle cells (hBSMCs). The whole-cell voltage clamp technique and quantitative real-time RT-PCR analysis were applied. The alterations in the fatty acid composition of phospholipids after treatment with EPA were also examined. Extracellular application of EPA produced a rapid and concentration-dependent suppression of tetrodotoxin-sensitive I(Na) with the half-maximal inhibitory concentration of 2 microM. After washing out EPA with albumin, I(Na) returned to the control level. Similar inhibitory effects were observed regarding other fatty acids (docosahexaenoic, arachidonic, stearic, and oleic acids), but EPA was the most potent inhibitor. The effect of EPA on I(Na) was not blocked by nordihydroguaiaretic acid and indometacin, and was accompanied by a significant shift of the steady-state inactivation curve to more negative potentials. In cells chronically treated with EPA, the EPA content of the cell lipid fraction (mol%) increased time-dependently, while arachidonic acid (AA) decreased, resulting in an increase of EPA to AA ratio. Then, the level of mRNA (SCN9A) encoding I(Na) decreased significantly. These results provide novel evidence that EPA not only rapidly inhibits I(Na), but also reduces the mRNA levels of the Na+ channel after cellular incorporation of EPA in cultured hBSMCs.

Preventing heart failure in patients with diabetes

Diabetic cardiomyopathy is characterized by a prominent interstitial fibrosis. Postulated etiologies include microangiopathy, autonomic neuropathy, and metabolic factors. A common root of these pathologies is hyperglycemia or hyperinsulinemia, both of which are prominent in type 2 diabetes mellitus, which has the highest incidence of cardiovascular morbidity and mortality. The relative importance of each factor is a matter of debate; it is likely that both of these factors in addition to the concomitant risk factors seen in diabetics (dyslipidemias, hypertension, obesity, coagulation abnormalities) contribute to the spectrum of myocardial disease in diabetes. A discussion of these contributive pathologies and the hyperglycemia and hyperinsulinemia that underlie them is the subject of this review. Treatment methodologies to control the development of such pathology also are discussed.

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.

Voltage-gated sodium channel expressed in cultured human smooth muscle cells: involvement of SCN9A

Voltage-gated Na(+) channel (I(Na)) is expressed under culture conditions in human smooth muscle cells (hSMCs) such as coronary myocytes. The aim of this study is to clarify the physiological, pharmacological and molecular characteristics of I(Na) expressed in cultured hSMCs obtained from bronchus, main pulmonary and coronary artery. I(Na), was recorded in these hSMCs and inhibited by tetrodotoxin (TTX) with an IC(50) value of approximately 10 nM. Reverse transcriptase/polymerase chain reaction (RT-PCR) analysis of mRNA showed the prominent expression of transcripts for SCN9A, which was consistent with the results of real-time quantitative RT-PCR. These results provide novel evidence that TTX-sensitive Na(+) channel expressed in cultured hSMCs is mainly composed of Na(v)1.7.

Raloxifene acutely suppresses ventricular myocyte contractility through inhibition of the L-type calcium current

1. The selective oestrogen (ER) receptor modulator, raloxifene, is widely used in the treatment of postmenopausal osteoporosis, but may also possess cardioprotective properties. We investigated whether it directly suppresses myocyte contractility through Ca(2+) channel antagonism in a similar way to 17beta-oestradiol. 2. Cell shortening and Ca(2+) transients were measured in single guinea-pig ventricular myocytes field-stimulated (1 Hz, 37 degrees C) in a superfusion chamber. Electrophysiological recordings were performed using single electrode voltage-clamp. 3. Raloxifene decreased cell shortening (EC(50) 2.4 microm) and the Ca(2+) transient amplitude (EC(50) 6.4 microm) in a concentration-dependent manner. At a concentration of 1 microm, raloxifene produced a 33+/-2% (mean+/-s.e.m) and 24+/-2% reduction, respectively (P<0.001, n=14 for both parameters). 4. These inhibitory actions were not observed in myocytes that had been incubated with the specific antagonist, ICI 182,780 (10 microm) (n=11). 5. Raloxifene (1 microm) shortened action potential durations at 50 and 90% repolarisation (P<0.05 and <0.001, respectively; n=27) and decreased peak L-type Ca(2+) current by 45%, from -5.1+/-0.5 pA/pF to -2.8+/-0.3 pA/pF (P<0.001, n=18). 6. Raloxifene did not significantly alter sarcoplasmic reticulum Ca(2+) content, as assessed by integrating the Na(+)/Ca(2+) exchanger currents following rapid caffeine application. 7. The present study provides evidence for direct inhibitory actions of raloxifene on ventricular myocyte contractility, mediated through Ca(2+) channel antagonism.

Inhibitory effects of carvedilol on calcium channels in vascular smooth muscle cells

Carvedilol has hypotensive effects and inhibits agonist-induced cell proliferation of vascular smooth muscle and then prevents vascular remodeling. However, the basic mechanisms have not been clarified. We examined the effects of carvedilol on [Ca2+]i mobilization and voltage-dependent L-type Ca2+ current (ICa.L) in vascular smooth muscle cells, and compared them with metoprolol. [Ca2+]i was measured using fura-2 AM and patch clamp techniques in rat embryonic aortic smooth muscle cells (A7r5). In the presence of extracellular Ca2+, vasopressin and endothelin-1 increased [Ca2+]i due first to the Ca2+ release from store sites, and subsequently Ca2+ entry. Carvedilol did not inhibit the Ca2+ release, but significantly suppressed the sustained rise due to Ca2+ entry concentration-dependently. Nilfedipine and nicardipine (10 microM) partly inhibited the sustained rise, but carvedilol inhibited it more effectively than the Ca2+ channel blockers. Under voltage clamp conditions, carvedilol (0.2-10 microM) reversibly inhibited the ICa.L concentration-dependently without any changes in the current-voltage relationships of ICa.L. Carvedilol shifted the steady-state inactivation for ICa.L to more negative potentials and inhibited ICa.L in a voltage-dependent manner. In addition, carvedilol did not inhibit Ca2+ release from store sites induced by thapsigargin, but significantly inhibited the sustained rise due to capacitative Ca2+ entry unrelated to ICa.L. In contrast, metoprolol did not mimic these effects of carvedilol. These results provide evidence that carvedilol inhibits ICa.L and may also inhibit the channels for agonist (vasopressin and endothelin-1)-induced Ca2+ entry in vascular smooth muscle cells, which might contribute to the vasorelaxing and antiproliferative effects of carvedilol.

Troglitazone administration limits infarct size by reduced phosphorylation of canine myocardial connexin43 proteins

Troglitazone, an antidiabetic thiazolidinedione, has been shown to have a scavenging effect on reactive oxygen species, which can modulate expression of connexin43. The study purpose was to evaluate whether troglitazone provides cardioprotection and to assess whether the cardioprotection is associated with an attenuated expression of connexin43 at the border of infarction in a canine model of acute myocardial infarction. Vehicle or troglitazone (1, 5, and 50 mg/kg; n = 14 for each group) was given intravenously 15 min before the coronary artery occlusion. Among the survivors, infarct size was significantly larger in the control than in the supplemented groups. There was a significantly lower infarct size in the high-dose group compared with that in the low-dose group (15 +/- 7% vs. 23 +/- 10% of the risk region in the low-dose group, P = 0.04). Reperfusion caused a significant elevation in superoxide anions as measured by lucigenin-derived chemiluminescence, which was significantly inhibited in animals treated with troglitazone. Connexin43 underwent dephosphorylation in response to ischemia-reperfusion measured by Western blot in control hearts at the border zone; these changes were significantly enhanced by troglitazone administration. Confocal microscopy confirmed the changes of junctional complexes. The magnitude of infarct size positively correlated with the magnitude of phosphorylated connexin43 expression assessed by Western blot analysis (r = 0.73, P < 0.0001). This result demonstrated that the cardioprotective effect of troglitazone as an antioxidant may be associated with reduced phosphorylation of myocardial connexin43 protein.

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.

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.

Simple techniques suitable for student use to record action potentials from the frog heart

Demonstrating action potentials during class experiments is very educational for science students. It is not easy, however, to obtain a stable intracellular recording of action potentials from the conventionally used skeletal muscle cells, because the tip of a glass microelectrode often comes out or breaks due to muscle contraction. Here, I present a much simpler recording method using a flexible polyethylene electrode with a wide orifice (approximately 1 mm) for a bullfrog heart beating on automaticity. Extracellular recordings of action potentials (electrocardiogram) can be obtained by placing an electrode on the cardiac surface, and transmembrane potentials can be obtained by rupturing the membrane with negative pressure, i.e., whole cell configuration. Once attached to the heart by suction, the polyethylene electrode does not easily come off during contraction of the heart. Perfusion of the heart via the postcaval vein offers us opportunities for observing the effects of either changing ionic compositions of solutions or applying drugs. The techniques shown here provide a simple and convenient way to perform a variety of class experiments.

Troglitazone enhances glucose uptake induced by alpha-adrenoceptor stimulation via phosphatidylinositol 3-kinase in rat heart

1. Thiazolidinedione-derived agents have been reported to act as insulin sensitizers by augmenting insulin-dependent stimulation of phosphatidylinositol 3-kinase (PI3K) activity in a specific manner. It has been suggested that alpha-adrenoceptor stimulation mediates glucose uptake through PI3K in the heart. 2. To elucidate whether the thiazolidinedione-derived agent troglitazone (TRO) affects glucose uptake induced by alpha-adrenoceptor stimulation through PI3K, the rate of glucose uptake was quantified from the rate of accumulation of sugar phosphate (d[SP]/dt) using [(31)P] nuclear magnetic resonance spectroscopy after substitution of glucose with 2-deoxyglucose in rat perfused heart. Hearts were stimulated with 100 micromol/L phenylephrine plus 10 micromol/L propranolol (alpha-adrenoceptor stimulation), or 1 micromol/L isoproterenol plus 10 micromol/L phentolamine (beta-adrenoceptor stimulation). 3. The d[SP]/dt in the alpha- and beta-adrenoceptor-stimulated groups (0.45 +/- 0.06 and 0.42 +/- 0.04 micromol/min per g, respectively) was higher than that of the control group (0.27 +/- 0.02 micromol/min per g; P < 0.01). The addition of 2 microg/mL troglitazone to alpha-adrenoceptor stimulation augmented d[SP]/dt (0.72 +/- 0.08 micromol/min per g; P < 0.05 vs the alpha-adrenoceptor-stimulated group), which was effectively blocked by 3 micromol/L wortmannin (0.35 +/- 0.06 micromol/min per g; P < 0.01 vs troglitazone + alpha-adrenoceptor stimulation group). However, addition of troglitazone to beta-adrenoceptor stimulation did not alter d[SP]/dt (0.33 +/- 0.02 micromol/min per g; P = NS vs the beta-adrenoceptor-stimulated group). 4. These results indicate that troglitazone acutely enhances alpha-adrenoceptor stimulation on glucose uptake through a PI3K-dependent pathway, thus possibly improving glucose utilization in a catecholamine-released state.

Ca(2+)-sensitizing effect is involved in the positive inotropic effect of troglitazone

1. Troglitazone, an insulin sensitizing agent, has a direct positive inotropic effect. However, the mechanism of this effect remains unclear. Thus, we examined the inotropic effect of troglitazone while focusing on intracellular Ca2+ handling. 2. Troglitazone significantly increased peak isovolumic left ventricular pressure (LVP(max)), peak rate of rise of LVP (dP/dt(max)), peak rate of fall of LVP (dP/dt(min)) in isolated rat hearts perfused at a constant coronary flow and heart rate. This inotropic effect of troglitazone was not inhibited by pretreatment with carbachol (muscarine receptor agonist), H89 (protein kinase A inhibitor), U73122 (phospholipase C inhibitor), H7 (protein kinase C inhibitor), verapamil (L-type Ca2+ channel antagonist), thapsigargin (Ca(2+)-adenosine triphosphatase inhibitor) or ryanodine (ryanodine receptor opener). 3. Radioimmunoassay showed that the cyclic adenosine monophosphate concentration in the left ventricle was not increased by troglitazone. 4. Whole-cell patch clamp analysis revealed that troglitazone had no effect on inward Ca2+ currents in cardiomyocytes. 5. In fura-2 loaded perfused rat hearts, troglitazone exerted its positive inotropic effect without increasing Ca2+ concentration. 6. These results suggest that neither the inward Ca2+ currents nor Ca2+ handling in the sarcoplasmic reticulum was involved in the inotropic effect of troglitazone. Furthermore, troglitazone exerted its positive inotropic effect without affecting the intracellular concentration of Ca2+. 7. In conclusion, the positive inotropic effect of troglitazone is mediated by a sensitization of Ca2+.

Improvement of left ventricular diastolic dynamics in prediabetic stage of a type II diabetic rat model after troglitazone treatment

Troglitazone, an oral antidiabetic agent, has hypoglycemic effects in insulin-resistant animal models and humans. This study was conducted to evaluate its effect on left ventricular diastolic dynamics of a spontaneous diabetic (DM) rat model. Twenty DM rats and 20 age-matched nonDM rats were used, and 10 of each group were treated with troglitazone as a 0.2% food admixture for 10 weeks. At 5 and 15 weeks of age, Doppler echocardiography and M-mode echocardiography were performed. Troglitazone treatment significantly improved the left ventricular diastolic dynamics of DM rats: deceleration time (msec) of early diastolic inflow decreased significantly (treated 52 +/- 3 vs untreated 64 +/- 5, p = 0.0002), and peak velocity of early transmitral inflow (cm/sec) increased significantly (treated 96 +/- 7 vs untreated 86 +/- 8, p = 0.0216). The data suggest that troglitazone improves left ventricular diastolic dynamics of a DM rat model at prediabetic stage.

Survival activity of troglitazone in rat motoneurones

Troglitazone (TGZ), an antidiabetic drug that improves insulin-resistance in the peripheral tissues, was tested for neurotrophic activity in motoneurones and other neurones in culture. In rat motoneurones, TGZ had a remarkable effect on survival, which was comparable or superior to that of brain-derived neurotrophic factor, a known potent neurotrophic factor for rat motoneurones. However, TGZ did not promote the survival of sensory, sympathetic, septal or hippocampal neurones. The effect of TGZ on motoneurones was additive to that of insulin-like growth factor-I and both activities were inhibited by phosphatidylinositol 3-kinase (PI3-kinase) inhibitors, wortmannin and LY294002, suggesting the involvement of the activation of PI3-kinase in the activity of TGZ. Pioglitazone, another antidiabetic drug structurally similar to TGZ, did not show any activity, indicating that the agonistic activity of TGZ for peroxisome proliferator-activated receptor-gamma is not involved in the survival activity. Chromanol, an antioxidant moiety of TGZ, showed little or no survival activity. These results indicate specific neurotrophic activity of TGZ for motoneurones through the activation of PI3-kinase and support the applicability of TGZ for the treatment of motor neurone diseases such as amyotrophic lateral sclerosis.

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.