The effect of streptozotocin-induced diabetes in rats on cardiac contractile proteins

Improvement of Cardiac Function and Subcellular Defects Due to Chronic Diabetes upon Treatment with Sarpogrelate

In order to investigate the subcellular mechanisms underlying the beneficial effects of sarpogrelate-a 5-HT2A receptor antagonist-on diabetic cardiomyopathy, diabetes was induced in rats by injecting streptozotocin (65 mg/kg). Diabetic animals were treated with or without sarpogrelate (5 mg/kg daily) for 6 weeks; diabetic animals were also treated with insulin (10 units/kg daily) for comparison. Elevated plasma levels of glucose and lipids, depressed insulin levels, hemodynamic alterations and cardiac dysfunction in diabetic animals were partially or fully attenuated by sarpogrelate or insulin treatment. Diabetes-induced changes in myocardial high-energy phosphate stores, as well as depressed mitochondrial oxidative phosphorylation and Ca2+-uptake activities, were significantly prevented by these treatments. Reductions in sarcolemma Na+-K+ ATPase, Na+-Ca2+ exchange, Ca2+-channel density and Ca2+-uptake activities were also attenuated by treatments with sarpogrelate and insulin. In addition, decreases in diabetes-induced sarcoplasmic reticulum Ca2+-uptake, Ca2+-release and Ca2+-stimulated ATPase activities, myofibrillar Mg2+-ATPase and Ca2+-stimulated ATPase activities, and myosin Mg2+-ATPase and Ca2+-ATPase activities were fully or partially prevented by sarpogrelate and insulin treatments. Marked alterations in different biomarkers of oxidative stress, such as malondialdehyde, superoxide dismutase and glutathione peroxidase, in diabetic hearts were also attenuated by treating the animals with sarpogrelate or insulin. These observations suggest that therapy with sarpogrelate, like that with insulin, may improve cardiac function by preventing subcellular and metabolic defects as a consequence of a reduction in oxidative stress.

Emerging roles of protein O-GlcNAcylation in cardiovascular diseases: Insights and novel therapeutic targets

Cardiovascular diseases (CVDs) are the leading cause of death globally. While the major focus of pharmacological and non-pharmacological interventions has been on targeting disease pathophysiology and limiting predisposing factors, our understanding of the cellular and molecular mechanisms underlying the pathogenesis of CVDs remains incomplete. One mechanism that has recently emerged is protein O-GlcNAcylation. This is a dynamic, site-specific reversible post-translational modification of serine and threonine residues on target proteins and is controlled by two enzymes: O-linked β-N-acetylglucosamine transferase (OGT) and O-linked β-N-acetylglucosaminidase (OGA). Protein O-GlcNAcylation alters the cellular functions of these target proteins which play vital roles in pathways that modulate vascular homeostasis and cardiac function. Through this review, we aim to give insights on the role of protein O-GlcNAcylation in cardiovascular diseases and identify potential therapeutic targets in this pathway for development of more effective medicines to improve patient outcomes.

Current Status and Potential Therapeutic Strategies for Using Non-coding RNA to Treat Diabetic Cardiomyopathy

Diabetic cardiomyopathy (DMCM) is the leading cause of mortality and morbidity among diabetic patients. DMCM is characterized by an increase in oxidative stress with systemic inflammation that leads to cardiac fibrosis, ultimately causing diastolic and systolic dysfunction. Even though DMCM pathophysiology is well studied, the approach to limit this condition is not met with success. This highlights the need for more knowledge of underlying mechanisms and innovative therapies. In this regard, emerging evidence suggests a potential role of non-coding RNAs (ncRNAs), including micro-RNAs (miRNAs), long non-coding RNAs (lncRNAs), and circular RNAs (circRNAs) as novel diagnostics, mechanisms, and therapeutics in the context of DMCM. However, our understanding of ncRNAs’ role in diabetic heart disease is still in its infancy. This review provides a comprehensive update on pre-clinical and clinical studies that might develop therapeutic strategies to limit/prevent DMCM.

Increased Sestrin3 Contributes to Post-ischemic Seizures in the Diabetic Condition

Seizures are among the most common neurological sequelae of stroke, and diabetes notably increases the incidence of post-ischemic seizures. Recent studies have indicated that Sestrin3 (SESN3) is a regulator of a proconvulsant gene network in human epileptic hippocampus. But the association of SESN3 and post-ischemic seizures in diabetes remains unclear. The present study aimed to reveal the involvement of SESN3 in seizures following transient cerebral ischemia in diabetes. Diabetes was induced in adult male mice and rats via intraperitoneal injection of streptozotocin (STZ). Forebrain ischemia (15 min) was induced by bilateral common carotid artery occlusion, the 2-vessel occlusion (2VO) in mice and 4-vessel occlusion (4VO) in rats. Our results showed that 59% of the diabetic wild-type mice developed seizures after ischemia while no seizures were observed in non-diabetic mice. Although no apparent cell death was detected in the hippocampus of seizure mice within 24 h after the ischemic insult, the expression of SESN3 was significantly increased in seizure diabetic mice after ischemia. The post-ischemic seizure incidence significantly decreased in SESN3 knockout mice. Furthermore, all diabetic rats suffered from post-ischemic seizures and non-diabetic rats have no seizures. Electrophysiological recording showed an increased excitatory synaptic transmission and intrinsic membrane excitability in dentate granule cells of the rat hippocampus, together with decreased I _A currents and Kv4.2 expression levels. The above results suggest that SESN3 up-regulation may contribute to neuronal hyperexcitability and seizure generation in diabetic animals after ischemia. Further studies are needed to explore the molecular mechanism of SESN3 in seizure generation after ischemia in diabetic conditions.

Role of CCR2 in the Development of Streptozotocin-Treated Diabetic Cardiomyopathy

CCR2 has been proven to play an important role in diabetes. However, the role of CCR2 in diabetic cardiomyopathy has not been examined. In this study, we investigated the effects of cardiac CCR2 on diabetic cardiomyopathy. We created a model of streptozotocin (STZ)-induced diabetic cardiomyopathy. Expression of CCR2 was upregulated in the hearts of STZ-induced diabetic mice. CCR2 knockout significantly improved STZ-induced cardiac dysfunction and fibrosis. Moreover, deletion of CCR2 inhibited STZ-induced apoptosis and the production of STZ-induced reactive oxygen species in the heart. CCR2 knockout resulted in M2 polarization in hearts of STZ-treated mice. Treatment with a CCR2 inhibitor reversed hyperglycemia-induced cardiac dysfunction in db/db mice. These results suggest that CCR2-induced inflammation and oxidative stress in the heart are involved in the development of diabetic cardiomyopathy and that CCR2 could be a novel target for therapy.

Impact of diabetes mellitus on the contractile properties of the left and right atrial myofilaments

OBJECTIVES

The incidence of diabetes mellitus in patients with ischaemic cardiomyopathy is increasing. To evaluate the impact of diabetes mellitus on contractility, we examined the calcium-induced force in left and right atrial myofilaments of patients with and without diabetes.

METHODS

We included 149 patients (106 without diabetes, 43 with diabetes), scheduled for elective coronary artery bypass grafting from August 2016 to June 2017. The left and right atria were excised and prepared for skinned fibre measurements (pCa-force curve). The unit for the force measurements is Millinewton (mN). Comprehensive demographic data as well as echocardiographic findings of the patients were collected.

RESULTS

We observed a significant decrease of left atrial force values in patients with diabetes, averaged over all calcium concentrations (patients with diabetes 0.50 ± 0.19 mN vs 0.68 ± 0.23 mN in patients without diabetes, P = 0.002) as well as in right atrial fibres (patients with diabetes 0.35 ± 0.17 mN vs 0.47 ± 0.21 mN in patients without diabetes, P = 0.005). There was a significant influence of repeated measurements (of the calcium concentrations) on force in left atrial myofilaments (P < 0.001). There was also a significant impact of diabetes on the force values of the different calcium concentrations in left atrial myofilaments (P 0.002). In right atrial myofilaments we also found a significant influence of repeated measurements (of the calcium concentrations) on force (P < 0.001). Additionally the impact of diabetes on the force values was significant (P = 0.005).

CONCLUSIONS

We demonstrated that diabetes mellitus has a significantly negative impact on calcium-induced force development in left and right atrial myofilaments.

Evaluation of tissue Doppler ultrasonographic and strain imaging for assessment of myocardial dysfunction in dogs with type 1 diabetes mellitus

OBJECTIVE To investigate cardiac structural and functional changes by tissue Doppler imaging (TDI) and strain imaging in dogs with spontaneous type 1 diabetes mellitus. ANIMALS 30 client-owned dogs, of which 10 had normotensive type 1 diabetes mellitus and 20 were healthy. PROCEDURES All dogs underwent physical examination, laboratory analyses, standard echocardiography, and TDI. RESULTS On TDI and strain imaging, transmitral peak early diastolic velocity (E)-to-tissue Doppler-derived peak early diastolic velocity at basal segment (E') of septum ratio, E:lateral E' ratio, and septal tissue Doppler-derived peak late diastolic velocity at basal segment (A') were significantly higher and the septal E':A' ratio and lateral longitudinal strain were significantly lower for diabetic dogs than for control dogs. Furthermore, in diabetic dogs, serum glucose and fructosamine concentrations after a 12-hour period of food withholding were positively correlated with regional systolic functional variables (septal and lateral longitudinal strain) and left ventricular filling pressure indices (E:septal E' and E:lateral E' ratios) but were negatively correlated with diastolic functional variables (E:transmitral peak late diastolic velocity and septal and lateral E':A' ratios). CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that myocardial function in diabetic dogs may be altered before the development of clinical heart-associated signs and that the change may be more readily detected by TDI and strain imaging than by conventional echocardiography. In addition, findings indicated that hyperglycemia could have detrimental effects on myocardial function, independent of hypertension, other cardiac diseases, and left ventricular hypertrophy, in dogs with type 1 diabetes.

Molecular mechanism of lipid-induced cardiac insulin resistance and contractile dysfunction

Long-chain fatty acids are the main cardiac substrates from which ATP is generated continually to serve the high energy demand and sustain the normal function of the heart. Under healthy conditions, fatty acid β-oxidation produces 50-70% of the energy demands with the remainder largely accounted for by glucose. Chronically increased dietary lipid supply often leads to excess lipid accumulation in the heart, which is linked to a variety of maladaptive phenomena, such as insulin resistance, cardiac hypertrophy and contractile dysfunction. CD36, the predominant cardiac fatty acid transporter, has a key role in setting the heart on a road to contractile dysfunction upon the onset of chronic lipid oversupply by translocating to the cell surface and opening the cellular 'doors' for fatty acids. The sequence of events after the CD36-mediated myocellular lipid accumulation is less understood, but in general it has been accepted that the excessively imported lipids cause insulin resistance, which in turn leads to contractile dysfunction. There are several gaps of knowledge in this proposed order of events which this review aims to discuss. First, the molecular mechanisms underlying lipid-induced insulin resistance are not yet completely disclosed. Specifically, several mediators have been proposed, such as diacylglycerols, ceramides, peroxisome proliferator-activated receptors (PPAR), inflammatory kinases and reactive oxygen species (ROS), but their relative contributions to the onset of insulin resistance and their putatively synergistic actions are topics of controversy. Second, there are also pieces of evidence that lipids can induce contractile dysfunction independently of insulin resistance. Perhaps, a more integrative view is needed, in which several lipid-induced pathways operate synergistically or in parallel to induce contractile dysfunction. Unraveling of these processes is expected to be important in designing effective therapeutic strategies to protect the lipid-overloaded heart.

Influence of Mitoxantrone on the Syntheses of Dna and Proteins of Mouse Tissues

In view of the structural similarity of mitoxantrone to anthracyclines and its ability to intercalate into DNA, we studied its influence on the synthetic processes of DNA and proteins in CD-1 mice tissues. By studying at the DNA level the impairment of 2H-thymidine incorporation and its return to normal, it was found that bone marrow and spleen showed similar behavior, i.e., a rapid return to normal, which occurred before bone marrow cell number and spleen weight returned to basal values. At the cardiac level, the incorporation values of precursors into DNA, reduced by treatment with mitoxantrone, came back very slowly to the control ones. Hepatic DNA showed a lower sensitivity to mitoxantrone. Analysis of 3H-leucine incorporation into three protein fractions of the heart showed that the contractile proteins were the most responsive fractions to mitoxantrone treatment. Experiments on CD-1 mice treated repeatedly with mitoxantrone revealed that the antitumor drug, at the cumulative dose of 8 mg/kg i.v., induced alterations in myocardiac morphology similar qualitatively to those induced by doxorubicin, although smaller quantitatively.

Inhibition of HIF-1α decreases expression of pro-inflammatory IL-6 and TNF-α in diabetic retinopathy

PURPOSE

Recent studies demonstrate that pro-inflammatory cytokines (PICs, i.e. IL-1β, IL-6 and TNF-α) in retinal tissues are likely involved in the development of diabetic retinopathy (DR). In this report, we particularly examined contributions of hypoxia inducible factor subtype 1α (HIF-1α) to the expression of PICs and their receptors in diabetic retina.

METHODS

Streptozotocin (STZ) was systemically injected to induce hyperglycaemia in rats. ELISA and Western blot analysis were employed to determine the levels of HIF-1α and PICs as well as PIC receptors in retinal tissues of control rats and STZ rats.

RESULTS

The levels of retinal HIF-1α were significantly increased in STZ rats 4-10 weeks after induction of hyperglycaemia as compared with control animals. With increasing HIF-1α retinal PICs including IL-1β, IL-6 and TNF-α, their respective receptors, namely IL-1R, IL-6R and TNFR1, were also elevated in STZ rats. Moreover, inhibition of HIF-1α by injection of 2-methoxyestradiol (2-MET) significantly decreased the amplified expression IL-6, TNF-α, IL-6R and TNFR1 in diabetic retina, but did not modify IL-1β pathway. In addition, we examined protein expression of Caspase-3 indicating cell apoptosis in the retina of STZ rats after infusing 2-MET, demonstrating that 2-MET attenuated an increase in Caspase-3 evoked by STZ.

CONCLUSION

Hypoxia inducible factor subtype 1α (HIF-1α) activated in diabetic retina is likely to play a role in regulating pathophysiological process via IL-6 and TNF-α mechanism. This has pharmacological implications to target specific HIF-1α, IL-6 and TNF-α signalling pathway for dysfunction and vulnerability related to DR.

Electrophysiological basis of metabolic-syndrome-induced cardiac dysfunction

Myocardial contractility is controlled by intracellular Ca2+ cycling with the contribution of sarcoplasmic reticulum (SR). In this study, we aimed to investigate the role of altered SR function in defective regulation of intracellular Ca2+ levels in rats with metabolic syndrome (MetS) induced by a 16-week high-sucrose drinking-water diet. Electric-field stimulated transient intracellular Ca2+ changes in MetS cardiomyocytes exhibited significantly reduced amplitude (∼30%) and prolonged time courses (2-fold), as well as depressed SR Ca2+ loading (∼55%) with increased basal Ca2+ level. Consistent with these data, altered ryanodine receptor (RyR2) function and SERCA2a activity were found in MetS cardiomyocytes through Ca2+ spark measurements and caffeine application assay in a state in which sodium calcium exchanger was inhibited. Furthermore, tetracaine application assay results and hyperphosphorylated level of RyR2 also support the “leaky RyR2” hypothesis. Moreover, altered phosphorylation levels of phospholamban (PLN) support the depressed SERCA2a-activity thesis and these alterations in the phosphorylation of Ca2+-handling proteins are correlated with altered protein kinase and phosphatase activity in MetS cardiomyocytes. In conclusion, MetS-rat heart exhibits altered Ca2+ signaling largely due to altered SR function via changes in RyR2 and SERCA2a activity. These results point to RyR2 and SERCA2a as potential pharmacological targets for restoring intracellular Ca2+ homeostasis and, thereby, combatting dysfunction in MetS-rat heart.

Abrogation of Nrf2 impairs antioxidant signaling and promotes atrial hypertrophy in response to high-intensity exercise stress

BACKGROUND

Anomalies in myocardial structure involving myocyte growth, hypertrophy, differentiation, apoptosis, necrosis etc. affects its function and render cardiac tissue more vulnerable to the development of heart failure. Although oxidative stress has a well-established role in cardiac remodeling and dysfunction, the mechanisms linking redox state to atrial cardiomyocyte hypertrophic changes are poorly understood. Here, we investigated the role of nuclear erythroid-2 like factor-2 (Nrf2), a central transcriptional mediator, in redox signaling under high intensity exercise stress (HIES) in atria.

METHODS

Age and sex-matched wild-type (WT) and Nrf2(-/-) mice at >20 months of age were subjected to HIES for 6 weeks. Gene markers of hypertrophy and antioxidant enzymes were determined in the atria of WT and Nrf2(-/-) mice by real-time qPCR analyses. Detection and quantification of antioxidants, 4-hydroxy-nonenal (4-HNE), poly-ubiquitination and autophagy proteins in WT and Nrf2(-/-) mice were performed by immunofluorescence analysis. The level of oxidative stress was measured by microscopical examination of di-hydro-ethidium (DHE) fluorescence.

RESULTS

Under the sedentary state, Nrf2 abrogation resulted in a moderate down regulation of some of the atrial antioxidant gene expression (Gsr, Gclc, Gstα and Gstµ) despite having a normal redox state. In response to HIES, enlarged atrial myocytes along with significantly increased gene expression of cardiomyocyte hypertrophy markers (Anf, Bnf and β-Mhc) were observed in Nrf2(-/-) when compared to WT mice. Further, the transcript levels of Gclc, Gsr and Gstµ and protein levels of NQO1, catalase, GPX1 were profoundly downregulated along with GSH depletion and increased oxidative stress in Nrf2(-/-) mice when compared to its WT counterparts after HIES. Impaired antioxidant state and profound oxidative stress were associated with enhanced atrial expression of LC3 and ATG7 along with increased ubiquitination of ATG7 in Nrf2(-/-) mice subjected to HIES.

CONCLUSIONS

Loss of Nrf2 describes an altered biochemical phenotype associated with dysregulation in genes related to redox state, ubiquitination and autophagy in HIES that result in atrial hypertrophy. Therefore, our findings direct that preserving Nrf2-related antioxidant function would be one of the effective strategies to safeguard atrial health.

Impact of Comorbidities on Acute Injury and Recovery in Preclinical Stroke Research: Focus on Hypertension and Diabetes

Human ischemic stroke is very complex, and no single preclinical model can comprise all the variables known to contribute to stroke injury and recovery. Hypertension, diabetes, and hyperlipidemia are leading comorbidities in stroke patients. The use of predominantly young adult and healthy animals in experimental stroke research has created a barrier for translation of findings to patients. As such, more and more disease models are being incorporated into the research design. This review highlights the major strengths and weaknesses of the most commonly used animal models of these conditions in preclinical stroke research. The goal is to provide guidance in choosing, reporting, and executing appropriate disease models that will be subjected to different models of stroke injury.

Challenges and issues with streptozotocin-induced diabetes - A clinically relevant animal model to understand the diabetes pathogenesis and evaluate therapeutics

Streptozotocin (STZ) has been extensively used over the last three decades to induce diabetes in various animal species and to help screen for hypoglycemic drugs. STZ induces clinical features in animals that resemble those associated with diabetes in humans. For this reason STZ treated animals have been used to study diabetogenic mechanisms and for preclinical evaluation of novel antidiabetic therapies. However, the physiochemical characteristics and associated toxicities of STZ are still major obstacles for researchers using STZ treated animals to investigate diabetes. Another major challenges in STZ-induced diabetes are sustaining uniformity, suitability, reproducibility and induction of diabetes with minimal animal lethality. Lack of appropriate use of STZ was found to be associated with increased mortality and animal suffering. During STZ use in animals, attention should be paid to several factors such as method of preparation of STZ, stability, suitable dose, route of administration, diet regimen, animal species with respect to age, body weight, gender and the target blood glucose level used to represent hyperglycemia. Therefore, protocol for STZ-induced diabetes in experimental animals must be meticulously planned. This review highlights specific skills and strategies involved in the execution of STZ-induced diabetes model. The present review aims to provide insight into diabetogenic mechanisms of STZ, specific toxicity of STZ with its significance and factors responsible for variations in diabetogenic effects of STZ. Further this review also addresses ways to minimize STZ-induced mortality, suggests methods to improve STZ-based experimental models and best utilize them for experimental studies purported to understand diabetes pathogenesis and preclinical evaluation of drugs.

Removal of Abnormal Myofilament O-GlcNAcylation Restores Ca2+ Sensitivity in Diabetic Cardiac Muscle

Contractile dysfunction and increased deposition of O-linked β-N-acetyl-d-glucosamine (O-GlcNAc) in cardiac proteins are a hallmark of the diabetic heart. However, whether and how this posttranslational alteration contributes to lower cardiac function remains unclear. Using a refined β-elimination/Michael addition with tandem mass tags (TMT)-labeling proteomic technique, we show that CpOGA, a bacterial analog of O-GlcNAcase (OGA) that cleaves O-GlcNAc in vivo, removes site-specific O-GlcNAcylation from myofilaments, restoring Ca(2+) sensitivity in streptozotocin (STZ) diabetic cardiac muscles. We report that in control rat hearts, O-GlcNAc and O-GlcNAc transferase (OGT) are mainly localized at the Z-line, whereas OGA is at the A-band. Conversely, in diabetic hearts O-GlcNAc levels are increased and OGT and OGA delocalized. Consistent changes were found in human diabetic hearts. STZ diabetic hearts display increased physical interactions of OGA with α-actin, tropomyosin, and myosin light chain 1, along with reduced OGT and increased OGA activities. Our study is the first to reveal that specific removal of O-GlcNAcylation restores myofilament response to Ca(2+) in diabetic hearts and that altered O-GlcNAcylation is due to the subcellular redistribution of OGT and OGA rather than to changes in their overall activities. Thus, preventing sarcomeric OGT and OGA displacement represents a new possible strategy for treating diabetic cardiomyopathy.

Sex-specific vascular responses of the rat aorta: effects of moderate term (intermediate stage) streptozotocin-induced diabetes

Hyperglycemia affects male and female vascular beds differently. We have previously shown that 1 week after the induction of diabetes with streptozotocin (STZ), male and female rats exhibit differences in aortic endothelial function. To examine this phenomenon further, aortic responses were studied in male and female rats 8 weeks after the induction of diabetes (intermediate stage). Endothelium-dependent vasodilation (EDV) to acetylcholine (ACh) was measured in phenylephrine (PE) pre-contracted rat aortic rings. Concentration response curves to PE were generated before and after L-NAME, a nitric oxide synthase (NOS) inhibitor. Furthermore, mRNA expression of endothelial nitric oxide synthase (eNOS) and NADPH oxidase subunit (Nox1) were determined. At 8 weeks, diabetes impaired EDV to a greater extent in female than male aortae. Furthermore, the responsiveness to PE was significantly enhanced only in female diabetic rats, and basal NO, as indicated by the potentiation of the response to PE after L-NAME, was reduced in female diabetic rat aortae to the same levels as in males. In addition, eNOS mRNA expression was decreased, while the Nox1 expression was significantly enhanced in diabetic female rats. These results suggest that aortic function in female diabetic rats after 8 weeks exhibits a more prominent impairment and that NO may be involved.

Contractile apparatus dysfunction early in the pathophysiology of diabetic cardiomyopathy

Diabetes mellitus significantly increases the risk of cardiovascular disease and heart failure in patients. Independent of hypertension and coronary artery disease, diabetes is associated with a specific cardiomyopathy, known as diabetic cardiomyopathy (DCM). Four decades of research in experimental animal models and advances in clinical imaging techniques suggest that DCM is a progressive disease, beginning early after the onset of type 1 and type 2 diabetes, ahead of left ventricular remodeling and overt diastolic dysfunction. Although the molecular pathogenesis of early DCM still remains largely unclear, activation of protein kinase C appears to be central in driving the oxidative stress dependent and independent pathways in the development of contractile dysfunction. Multiple subcellular alterations to the cardiomyocyte are now being highlighted as critical events in the early changes to the rate of force development, relaxation and stability under pathophysiological stresses. These changes include perturbed calcium handling, suppressed activity of aerobic energy producing enzymes, altered transcriptional and posttranslational modification of membrane and sarcomeric cytoskeletal proteins, reduced actin-myosin cross-bridge cycling and dynamics, and changed myofilament calcium sensitivity. In this review, we will present and discuss novel aspects of the molecular pathogenesis of early DCM, with a special focus on the sarcomeric contractile apparatus.

Targeting caveolin-3 for the treatment of diabetic cardiomyopathy

Diabetes is a global health problem with more than 550 million people predicted to be diabetic by 2030. A major complication of diabetes is cardiovascular disease, which accounts for over two-thirds of mortality and morbidity in diabetic patients. This increased risk has led to the definition of a diabetic cardiomyopathy phenotype characterised by early left ventricular dysfunction with normal ejection fraction. Here we review the aetiology of diabetic cardiomyopathy and explore the involvement of the protein caveolin-3 (Cav3). Cav3 forms part of a complex mechanism regulating insulin signalling and glucose uptake, processes that are impaired in diabetes. Further, Cav3 is key for stabilisation and trafficking of cardiac ion channels to the plasma membrane and so contributes to the cardiac action potential shape and duration. In addition, Cav3 has direct and indirect interactions with proteins involved in excitation-contraction coupling and so has the potential to influence cardiac contractility. Significantly, both impaired contractility and rhythm disturbances are hallmarks of diabetic cardiomyopathy. We review here how changes to Cav3 expression levels and altered relationships with interacting partners may be contributory factors to several of the pathological features identified in diabetic cardiomyopathy. Finally, the review concludes by considering ways in which levels of Cav3 may be manipulated in order to develop novel therapeutic approaches for treating diabetic cardiomyopathy.

Mechanisms of subcellular remodeling in heart failure due to diabetes

Diabetic cardiomyopathy is not only associated with heart failure but there also occurs a loss of the positive inotropic effect of different agents. It is now becoming clear that cardiac dysfunction in chronic diabetes is intimately involved with Ca(2+)-handling abnormalities, metabolic defects and impaired sensitivity of myofibrils to Ca(2+) in cardiomyocytes. On the other hand, loss of the inotropic effect in diabetic myocardium is elicited by changes in signal transduction mechanisms involving hormone receptors and depressions in phosphorylation of various membrane proteins. Ca(2+)-handling abnormalities in the diabetic heart occur mainly due to defects in sarcolemmal Na(+)-K(+) ATPase, Na(+)-Ca(2+) exchange, Na(+)-H(+) exchange, Ca(2+)-channels and Ca(2+)-pump activities as well as changes in sarcoplasmic reticular Ca(2+)-uptake and Ca(2+)-release processes; these alterations may lead to the occurrence of intracellular Ca(2+) overload. Metabolic defects due to insulin deficiency or ineffectiveness as well as hormone imbalance in diabetes are primarily associated with a shift in substrate utilization and changes in the oxidation of fatty acids in cardiomyocytes. Mitochondria initially seem to play an adaptive role in serving as a Ca(2+) sink, but the excessive utilization of long-chain fatty acids for a prolonged period results in the generation of oxidative stress and impairment of their function in the diabetic heart. In view of the activation of sympathetic nervous system and renin-angiotensin system as well as platelet aggregation, endothelial dysfunction and generation of oxidative stress in diabetes and blockade of their effects have been shown to attenuate subcellular remodeling, metabolic derangements and signal transduction abnormalities in the diabetic heart. On the basis of these observations, it is suggested that oxidative stress and subcellular remodeling due to hormonal imbalance and metabolic defects play a critical role in the genesis of heart failure during the development of diabetic cardiomyopathy.

Streptozotocin-induced diabetes prolongs twitch duration without affecting the energetics of isolated ventricular trabeculae

BACKGROUND

Diabetes induces numerous electrical, ionic and biochemical defects in the heart. A general feature of diabetic myocardium is its low rate of activity, commonly characterised by prolonged twitch duration. This diabetes-induced mechanical change, however, seems to have no effect on contractile performance (i.e., force production) at the tissue level. Hence, we hypothesise that diabetes has no effect on either myocardial work output or heat production and, consequently, the dependence of myocardial efficiency on afterload of diabetic tissue is the same as that of healthy tissue.

METHODS

We used isolated left ventricular trabeculae (streptozotocin-induced diabetes versus control) as our experimental tissue preparations. We measured a number of indices of mechanical (stress production, twitch duration, extent of shortening, shortening velocity, shortening power, stiffness, and work output) and energetic (heat production, change of enthalpy, and efficiency) performance. We calculated efficiency as the ratio of work output to change of enthalpy (the sum of work and heat).

RESULTS

Consistent with literature results, we showed that peak twitch stress of diabetic tissue was normal despite suffering prolonged duration. We report, for the first time, the effect of diabetes on mechanoenergetic performance. We found that the indices of performance listed above were unaffected by diabetes. Hence, since neither work output nor change of enthalpy was affected, the efficiency-afterload relation of diabetic tissue was unaffected, as hypothesised.

CONCLUSIONS

Diabetes prolongs twitch duration without having an effect on work output or heat production, and hence efficiency, of isolated ventricular trabeculae. Collectively, our results, arising from isolated trabeculae, reconcile the discrepancy between the mechanical performance of the whole heart and its tissues.

Morphology and Contractility of Cardiac Myocytes in Early Stages of Streptozotocin-Induced Diabetes Mellitus in Rats

Diabetic cardiomyopathy is the leading cause of mortality in type 1 diabetes. Thus study of cardiomyocyte morphology and function during early stages of diabetes using modern analytical methods is of critical importance. Therefore, using confocal microscopy, we determined metric parameters, volumes and contractility, with calcium transients in isolated left-ventricular myocytes at one week after induction of diabetes in rats. Myocyte volume analysis from 3D confocal scans was performed using an automated contour detection algorithm that took the actual shape of the myocytes into account. We showed a significant reduction in myocyte volume in diabetic animals. We also showed a significant reduction in length and width but not in thickness of the myocytes, which suggests disproportional reorganization of the structure of the heart tissue during short-term diabetes. From a functional point of view, we observed a significant decrease in cell shortening at a stimulation frequency of 0.5 Hz. This was accompanied by a decrease in calcium transient amplitude. Together, these data suggest that impaired calcium handling is one of the factors that contributes to the observed decrease in myocyte shortening during early stages of streptozotocin-induced diabetes in rats.

Acetyl-l-carnitine and oxfenicine on cardiac pumping mechanics in streptozotocin-induced diabetes in male Wistar rats

INTRODUCTION

In the treatment of patients with diabetes, one objective is an improvement of cardiac metabolism to alleviate the left ventricular (LV) function. For this study, we compared the effects of acetyl-l-carnitine (one of the carnitine derivatives) and of oxfenicine (a carnitine palmitoyltransferase-1 inhibitor) on cardiac pumping mechanics in streptozotocin-induced diabetes in male Wistar rats, with a particular focus on the pressure-flow-volume relationship.

METHODS

Diabetes was induced by a single tail vein injection of 55 mg kg(-1) streptozotocin. The diabetic animals were treated on a daily basis with either acetyl-L-carnitine (1 g L(-1) in drinking water) or oxfenicine (150 mg kg(-1) by oral gavage) for 8 wk. They were also compared with untreated age-matched diabetic controls. LV pressure and ascending aortic flow signals were recorded to calculate the maximal systolic elastance (E max) and the theoretical maximum flow (Q max). Physically, E max reflects the contractility of the myocardium as an intact heart, whereas Q max has an inverse relationship with the LV internal resistance.

RESULTS

When comparing the diabetic rats with their age-matched controls, the cardiodynamic condition was characterized by a decline in E max associated with the unaltered Q max. Acetyl-l-carnitine (but not oxfenicine) had reduced cardiac levels of malondialdehyde in these insulin-deficient animals. However, treating with acetyl-l-carnitine or oxfenicine resulted in an increase in E max, which suggests that these 2 drugs may protect the contractile status from deteriorating in the diabetic heart. By contrast, Q max showed a significant fall after administration of oxfenicine, but not with acetyl-L-carnitine. The decrease in Q max corresponded to an increase in total vascular resistance when treated with oxfenicine.

CONCLUSIONS

Acetyl-l-carnitine, but not oxfencine, optimizes the integrative nature of cardiac pumping mechanics by preventing the diabetes-induced deterioration in myocardial intrinsic contractility associated with unaltered LV internal resistance.

MicroRNA-301a mediated regulation of Kv4.2 in diabetes: identification of key modulators

Diabetes is a metabolic disorder that ultimately results in major pathophysiological complications in the cardiovascular system. Diabetics are predisposed to higher incidences of sudden cardiac deaths (SCD). Several studies have associated diabetes as a major underlying risk for heart diseases and its complications. The diabetic heart undergoes remodeling to cope up with the underlying changes, however ultimately fails. In the present study we investigated the changes associated with a key ion channel and transcriptional factors in a diabetic heart model. In the mouse db/db model, we identified key transcriptional regulators and mediators that play important roles in the regulation of ion channel expression. Voltage-gated potassium channel (Kv4.2) is modulated in diabetes and is down regulated. We hypothesized that Kv4.2 expression is altered by potassium channel interacting protein-2 (KChIP2) which is regulated upstream by NFkB and miR-301a. We utilized qRT-PCR analysis and identified the genes that are affected in diabetes in a regional specific manner in the heart. At protein level we identified and validated differential expression of Kv4.2 and KChIP2 along with NFkB in both ventricles of diabetic hearts. In addition, we identified up-regulation of miR-301a in diabetic ventricles. We utilized loss and gain of function approaches to identify and validate the role of miR-301a in regulating Kv4.2. Based on in vivo and in vitro studies we conclude that miR-301a may be a central regulator for the expression of Kv4.2 in diabetes. This miR-301 mediated regulation of Kv4.2 is independent of NFkB and Irx5 and modulates Kv4.2 by direct binding on Kv4.2 3′untranslated region (3′-UTR). Therefore targeting miR-301a may offer new potential for developing therapeutic approaches.

Cannabinoid 1 receptor promotes cardiac dysfunction, oxidative stress, inflammation, and fibrosis in diabetic cardiomyopathy

Endocannabinoids and cannabinoid 1 (CB(1)) receptors have been implicated in cardiac dysfunction, inflammation, and cell death associated with various forms of shock, heart failure, and atherosclerosis, in addition to their recognized role in the development of various cardiovascular risk factors in obesity/metabolic syndrome and diabetes. In this study, we explored the role of CB(1) receptors in myocardial dysfunction, inflammation, oxidative/nitrative stress, cell death, and interrelated signaling pathways, using a mouse model of type 1 diabetic cardiomyopathy. Diabetic cardiomyopathy was characterized by increased myocardial endocannabinoid anandamide levels, oxidative/nitrative stress, activation of p38/Jun NH(2)-terminal kinase (JNK) mitogen-activated protein kinases (MAPKs), enhanced inflammation (tumor necrosis factor-α, interleukin-1β, cyclooxygenase 2, intracellular adhesion molecule 1, and vascular cell adhesion molecule 1), increased expression of CB(1), advanced glycation end product (AGE) and angiotensin II type 1 receptors (receptor for advanced glycation end product [RAGE], angiotensin II receptor type 1 [AT(1)R]), p47(phox) NADPH oxidase subunit, β-myosin heavy chain isozyme switch, accumulation of AGE, fibrosis, and decreased expression of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA2a). Pharmacological inhibition or genetic deletion of CB(1) receptors attenuated the diabetes-induced cardiac dysfunction and the above-mentioned pathological alterations. Activation of CB(1) receptors by endocannabinoids may play an important role in the pathogenesis of diabetic cardiomyopathy by facilitating MAPK activation, AT(1)R expression/signaling, AGE accumulation, oxidative/nitrative stress, inflammation, and fibrosis. Conversely, CB(1) receptor inhibition may be beneficial in the treatment of diabetic cardiovascular complications.

Diabetes-induced alterations in the extracellular matrix and their impact on myocardial function

Diabetes is an increasing public health problem that is expected to escalate in the future due to the growing incidence of obesity in the western world. While this disease is well known for its devastating effects on the kidneys and vascular system, diabetic individuals can develop cardiac dysfunction, termed diabetic cardiomyopathy, in the absence of other cardiovascular risk factors such as hypertension or atherosclerosis. While much effort has gone into understanding the effects of elevated glucose or altered insulin sensitivity on cellular components within the heart, significant changes in the cardiac extracellular matrix (ECM) have also been noted. In this review article we highlight what is currently known regarding the effects diabetes has on both the expression and chemical modification of proteins within the ECM and how the fibrotic response often observed as a consequence of this disease can contribute to reduced cardiac function.

Lineage restricted progenitors for the repopulation of decellularized heart

The severe shortage of available donor hearts necessitates the development of other options for heart replacement. Recent results underline the promise of the decellularized organ approach in engineering a functional heart. However, little is known so far regarding the ability of decellularized heart ECM to differentiate embryonic stem cells or committed progenitor cells. In the present work, we compared the differentiation potential of human embryonic stem cells (hESCs) and human mesendodermal cells (hMECs) derived from hESCs, in decellularized hearts under static culture. Expression of various cardiac specific markers such as cTnT, Nkx-2.5, Myl2, Myl7, Myh6 and CD31 was elucidated by gene expression, immunostaining and flow cytometry. Both hMECs and hESCs upregulated expression of cardiac markers upon differentiation, but they exclusively expressed genes for myosin light chain (Myl2, Myl7) and myosin heavy chain (Myh6), respectively. To enhance the differentiation ability of the stem/progenitor cells in the acellular constructs, they were implanted subcutaneously in SCID mice. Immunostaining of the explants revealed the persistence of cardiac marker expressing cells, but which lacked beating function. Our results indicate that the intact extracellular matrix components and preserved mechanical properties of the decellularized heart had directed differentiation of the stem/progenitor cells into the cardiac lineage.

Carbonylation of myosin heavy chains in rat heart during diabetes

Cardiac inotropy progressively declines during diabetes mellitus. To date, the molecular mechanisms underlying this defect remain incompletely characterized. This study tests the hypothesis that ventricular myosin heavy chains (MHC) undergo carbonylation by reactive carbonyl species (RCS) during diabetes and these modifications contribute to the inotropic decline. Male Sprague-Dawley rats were injected with streptozotocin (STZ). Fourteen days later the animals were divided into two groups: one group was treated with the RCS blocker aminoguanidine for 6 weeks, while the other group received no treatment. After 8 weeks of diabetes, cardiac ejection fraction, fractional shortening, left ventricular pressure development (+dP/dt) and myocyte shortening were decreased by 9%, 16%, 34% and 18%, respectively. Ca(2+)- and Mg(2+)-actomyosin ATPase activities and peak actomyosin syneresis were also reduced by 35%, 28%, and 72%. MHC-alpha to MHC-beta ratio was 12:88. Mass spectrometry and Western blots revealed the presence of carbonyl adducts on MHC-alpha and MHC-beta. Aminoguanidine treatment did not alter MHC composition, but it blunted formation of carbonyl adducts and decreases in actomyosin Ca(2+)-sensitive ATPase activity, syneresis, myocyte shortening, cardiac ejection fraction, fractional shortening and +dP/dt induced by diabetes. From these new data it can be concluded that in addition to isozyme switching, modification of MHC by RCS also contributes to the inotropic decline seen during diabetes.

Amino acid supplementation counteracts metabolic and functional damage in the diabetic rat heart

We aimed to assess whether a specific mixture of amino acid (AA) supplements counteracts the metabolic and functional changes in the streptozotocin (STZ)-induced diabetic rat heart model. Adult male Wistar rats were divided into 6 groups (n = 10 each) and treated for 43 days: nondiabetic controls, nondiabetic rats given an AA mixture (0.1 g/kg per day), diabetic rats (induced with 65 mg/kg STZ given intraperitoneally), diabetic rats given AAs, diabetic rats given insulin (5 IU/day given subcutaneously), and diabetic rats given insulin plus AAs. During treatment, glycemia and insulinemia levels were measured in all groups. Changes in enzyme (reduced nicotinamide adenine dinucleotide-dehydrogenase, cytochrome c oxidase) activities and myosin heavy chain (MHC) composition were measured in the left ventricle. In 5 rats contractile function was assessed by measuring maximal shortening velocity of skinned ventricular trabeculae and the expression of translational regulator mammalian target of rapamycin (mTOR) was also found. STZ-induced diabetes was associated with reduced myocardial contractility, overall loss of oxidative capacity, a shift toward a slower MHC phenotype, and decreased mTOR tissue content. All of these changes appeared to be reversible with insulin. AA supplements partially restored the myocardial and oxidative dysfunction and also increased mTOR tissue content. The combination of insulin and AAs did not have a synergistic effect on either enzymatic or functional profiles. We conclude that AA supplements may contribute to restoring the oxidative and contractile dysfunction of diabetic rat hearts, probably through an mTOR-insulin independent mechanism.

Elevated catalase and heme oxygenase-1 may contribute to improved postischaemic cardiac function in long-term type 1 diabetes

1. Although increased oxidative stress has been shown repeatedly to be implicated in diabetes, the cardiovascular anti-oxidant state and heart response to ischaemia in long-term Type 1 diabetes remain largely unknown. The present study was designed to observe heart tolerance to ischaemia-reperfusion and endogenous anti-oxidants in the cardiovascular system in long-term hyperglycaemic rats. 2. Hearts from Sprague-Dawley rats surviving up to 6 months with streptozocin-induced severe hyperglycaemia (blood glucose > 20 mmol/L) were isolated and subjected to global ischaemia and reperfusion. Cardiac function, electrocardiogram and anti-oxidants in the myocardium and aorta were examined. In addition, the morphology of the myocardial mitochondria and the in vitro function of aortic vessels were assessed. 3. Hearts from diabetic rats demonstrated lower baseline heart function but had higher postischaemic coronary flow and left ventricular developed pressure compared with their respective controls (P < 0.05). In addition, hearts from diabetic animals had fewer arrhythmias (P < 0.01) and lower left ventricular end-diastolic pressure during reperfusion (P < 0.05). Higher catalase and heme oxygenase-1 content was found in the aorta and myocardium from diabetic rats (P < 0.01). In aortas from diabetic animals, acetylcholine-induced vasodilatation was enhanced and was approximately 15% after inhibition of nitric oxide synthase, compared with 0% in controls. The 15% relaxation was abrogated by heme oxygenase blockade. Mitochondria from the myocardium of diabetic rats showed significant increases in both size and number (P < 0.05). 4. Hearts of long-term Type 1 diabetic rats demonstrated improved recovery of postischaemic cardiac function and reduced reperfusion arrhythmia. Hyperglycaemia may enhance cardiovascular anti-oxidant capacity and mitochondrial neogenesis, which renders the heart resistant to ischaemia and oxidative injury.

Aminoguanidine prevents the impairment of cardiac pumping mechanics in rats with streptozotocin and nicotinamide-induced type 2 diabetes

BACKGROUND AND PURPOSE

Aminoguanidine (AG), an inhibitor of advanced glycation endproducts, has been shown to prevent arterial stiffening and cardiac hypertrophy in streptozotocin (STZ) and nicotinamide (NA)-induced type 2 diabetes in rats. Our aims were to examine whether AG produced benefits on cardiac pumping mechanics in the STZ and NA-treated animals in terms of maximal systolic elastance (E(max)) and theoretical maximum flow (Q(max)).

EXPERIMENTAL APPROACH

After induction of type 2 diabetes, rats received daily injections of AG (50 mg kg(-1), i.p.) for 8 weeks and were compared with age-matched, untreated, diabetic controls. Left ventricular (LV) pressure and ascending aortic flow signals were recorded to calculate E(max) and Q(max), using the elastance-resistance model. Physically, E(max) reflects the contractility of the myocardium as an intact heart, whereas Q(max) has an inverse relationship with the LV internal resistance.

KEY RESULTS

Both type 2 diabetes and AG affected E(max) and Q(max), and there was an interaction between diabetes and AG for these two variables. The E(max) and Q(max) were reduced in rats with type 2 diabetes, but showed a significant rise after administration of AG to these diabetic rats. Moreover, the increase in Q(max) corresponded to a decrease in total peripheral resistance of the systemic circulation when the STZ and NA-induced diabetic rats were treated with AG.

CONCLUSIONS AND IMPLICATIONS

AG therapy prevented not only the contractile dysfunction of the heart, but also the augmentation in LV internal resistance in rats with STZ and NA-induced type 2 diabetes.

Blockade of the renin-angiotensin system attenuates sarcolemma and sarcoplasmic reticulum remodeling in chronic diabetes

Although the defects in the sarcolemma (SL) and sarcoplasmic reticulum (SR) membranes are known to be associated with cardiac dysfunction in chronic diabetes, very little information regarding the mechanisms of these membrane abnormalities is available in the literature. For this reason, rats were treated daily for 8 weeks with and without enalapril, an angiotensin-converting enzyme inhibitor, or losartan, an angiotensin receptor antagonist, 3 days after inducing diabetes with an injection of streptozocin. Treatment of diabetic animals with both enalapril and losartan attenuated alterations in cardiac function and the left ventricular redox potential without any changes in the increased plasma glucose or reduced plasma insulin levels. The SL Na+-K+ ATPase, Ca2+ pump, Na+-dependent Ca2+-uptake, Ca2+-channel density, and low-affinity Ca2+-binding activities were depressed whereas Ca2+ ecto-ATPase activity was increased in the diabetic heart. Furthermore, the SR Ca2+-release and Ca2+-pump activities in the diabetic hearts were decreased without any changes in the Mg2+-ATPase activity. These alterations in SL and SR membranes in diabetic animals were partly prevented by treatments with enalapril and losartan. The results suggest that the activation of the renin-angiotensin system plays an important role in diabetes-induced changes in SL and SR membranes as well as cardiac function.

Exercise training improves cardiac performance in diabetes: in vivo demonstration with quantitative cine-MRI analyses

Diabetic cardiomyopathy is a distinct myocardial complication of the catabolic state of untreated insulin-dependent diabetes mellitus in the streptozotocin-induced diabetic rat. Exercise training has long been utilized as an effective adjunct to pharmacotherapy in the management of the diabetic heart. However, the in vivo functional benefit(s) of the training programs on cardiac cycle events in diabetes are poorly understood. In this study, we used three groups of Sprague-Dawley rats (sedentary control, sedentary diabetic, and exercised diabetic) to assess the effects of endurance training on the left ventricular (LV) cardiac cycle events in diabetes. At the end of 9 wk of exercise training, noninvasive cardiac functional evaluation was performed by using high-resolution magnetic resonance imaging (9.4 T). An ECG-gated cine imaging protocol was used to capture the LV cardiac cycle events through 10 equally incremented phases. The cardiac cycle phase volumetric profiles showed favorable functional changes in exercised diabetic group, including a prevention of decreased end-diastolic volume and attenuation of increased end-systolic volume that accompanies sedentary diabetes. The defects in LV systolic flow velocity, acceleration, and jerk associated with sedentary diabetes were restored toward control levels in the trained diabetic animals. This magnetic resonance imaging study confirms the prevailing evidence from earlier in vitro and in vivo invasive procedures that exercise training benefits cardiac function in this model of diabetic cardiomyopathy despite the extreme catabolic state of the animals.

Endothelin-1 receptor blockade prevented the electrophysiological dysfunction in cardiac myocytes of streptozotocin-induced diabetic rats

Diabetes mellitus is complicated with the development of cardiac contractile dysfunction and electrical instability, which contributes to high morbidity and mortality in diabetic patients. This study examined the possible roles of enhanced endothelin-1 (ET-1) on diabetes-induced alterations in ventricular myocyte electrophysiology. Type 1 diabetic rats were induced by single dose injection of streptozotocin (STZ) and treated with or without ET-1 receptor antagonist bosentan for 8 wk before myocyte isolation. Action potential, outward K+ currents, and inward Ca2+ currents in ventricular myocytes were recorded using whole-cell patch clamp technique. STZ-injected rats exhibited hyperglycemia, reduced body weight gain, and elevated plasma ET-1 concentration, indicative of diabetes induction. Ventricular myocytes isolated from diabetic rats exhibited prolonged action potential and reduced all three types of outward K+ currents. Resting membrane potential, height of action potential, and L-type Ca2+ current were not altered in diabetic myocytes. In vivo chronic treatment of diabetic rats with bosentan significantly augmented K+ currents and reversed action potential prolongation in ventricular myocytes. On the other hand, bosentan treatment had no detectable effect on the electrophysiological properties in control myocytes. In addition, bosentan had no effect on Ltype Ca2+ currents in both control and diabetic myocytes. Our data suggest that altered electrophysiological properties in ventricular myocytes were largely resulted from augmented ET-1 system in diabetic animals.

Molecular defects in cardiac myofibrillar proteins due to thyroid hormone imbalance and diabetesThis paper is a part of a series in the Journal's "Made in Canada" section. The paper has undergone peer review

The heart very often becomes a victim of endocrine abnormalities such as thyroid hormone imbalance and insulin deficiency, which are manifested in a broad spectrum of cardiac dysfunction from mildly compromised function to severe heart failure. These functional changes in the heart are largely independent of alterations in the coronary arteries and instead reside at the level of cardiomyocytes. The status of cardiac function reflects the net of underlying subcellular modifications induced by an increase or decrease in thyroid hormone and insulin plasma levels. Changes in the contractile and regulatory proteins constitute molecular and structural alterations in myofibrillar assembly, called myofibrillar remodeling. These alterations may be adaptive or maladaptive with respect to the functional and metabolic demands on the heart as a consequence of the altered endocrine status in the body. There is a substantial body of information to indicate alterations in myofibrillar proteins including actin, myosin, tropomyosin, troponin, titin, desmin, and myosin-binding protein C in conditions such as hyperthyroidism, hypothyroidism, and diabetes. The present article is focussed on discussion how myofibrillar proteins are altered in response to thyroid hormone imbalance and lack of insulin or its responsiveness, and how their structural and functional changes explain the contractile defects in the heart.

Effects of diabetes on ryanodine receptor Ca release channel (RyR2) and Ca2+ homeostasis in rat heart

The defects identified in the mechanical activity of the hearts from type 1 diabetic animals include alteration of Ca2+ signaling via changes in critical processes that regulate intracellular Ca2+ concentration. These defects result partially from a dysfunction of cardiac ryanodine receptor calcium release channel (RyR2). The present study was designed to determine whether the properties of the Ca2+ sparks might provide insight into the role of RyR2 in the altered Ca2+ signaling in cardiomyocytes from diabetic animals when they were analyzed together with Ca2+ transients. Basal Ca2+ level as well as Ca2+-spark frequency of cardiomyoctes isolated from 5-week streptozotocin (STZ)-induced diabetic rats significantly increased with respect to aged-matched control rats. Ca2+ transients exhibited significantly reduced amplitude and prolonged time courses as well as depressed Ca2+ loading of sarcoplasmic reticulum in diabetic rats. Spatio-temporal properties of the Ca2+ sparks in cardiomyocytes isolated from diabetic rats were also significantly altered to being almost parallel to the changes of Ca2+ transients. In addition, RyR2 from diabetic rat hearts were hyperphosphorylated and protein levels of both RyR2 and FKBP12.6 depleted. These data show that STZ-induced diabetic rat hearts exhibit altered local Ca2+ signaling with increased basal Ca2+ level.

Renin-angiotensin blockade attenuates cardiac myofibrillar remodelling in chronic diabetes

Previous studies have shown that the renin-angiotensin system (RAS) is activated in diabetes and this may contribute to the subcellular remodelling and heart dysfunction in this disease. Therefore, we examined the effects of RAS blockade by enalapril, an angiotensin-converting enzyme inhibitor, and losartan, an angiotensin receptor AT1 antagonist, on cardiac function, myofibrillar and myosin ATPase activity as well as myosin heavy chain (MHC) isozyme expression in diabetic hearts. Diabetes was induced in rats by a single injection of streptozotocin (65 mg/kg; i.v.) and these animals were treated with and without enalapril (10 mg/kg/day; oral) or losartan (20 mg/kg/day; oral) for 8 weeks. Enalapril or losartan prevented the depressions in left ventricular rate of pressure development, rate of pressure decay and ventricular weight seen in diabetic animals. Both drugs also attenuated the decrease in myofibrillar Ca2+-ATPase, Mg2+-ATPase and myosin ATPase activity seen in diabetic rats. The diabetes-induced increase in beta-MHC content and gene expression as well as the decrease in alpha-MHC content and mRNA levels were also prevented by enalapril and losartan. These results suggest the occurrence of myofibrillar remodelling in diabetic cardiomyopathy and provide evidence that the beneficial effects of RAS blockade in diabetes may be associated with attenuation of myofibrillar remodelling in the heart.

Effect of insulin and angiotensin II receptor subtype-1 antagonist on myocardial remodelling in rats with insulin-dependent diabetes mellitus

OBJECTIVES

To assess the role of insulin or an angiotensin II receptor antagonist (losartan), or both, in preventing cardiomyocyte damage in rats suffering from insulin-dependent diabetes mellitus (IDDM), and to correlate it with insulin receptor modulation at the cardiomyocyte, coronary endothelium and skeletal muscle cell level.

DESIGN

Animals were divided into groups of normal rats, diabetic rats, and diabetic rats given insulin, each subdivided into a control group and an experimental group treated with losartan.

METHODS

The animals were killed 1 month after enrollment to the study. Perfusion of the heart with iodine-125-labelled insulin was carried out for all the groups and the binding kinetics of insulin to its receptors on the coronary endothelial cells and the cardiomyocytes were determined using a physical/mathematical model. In addition, tissue samples from the heart and intercostal skeletal muscle were snap frozen and used for histological, indirect immunofluorescence and western blot analysis.

RESULTS

Cardiac muscle from diabetic animals exhibited diffuse cardiomyopathic changes consisting of widespread vacuolation, loss of striation and cellular hypertrophy, which were reduced and even prevented by treatment with insulin and losartan. In addition, losartan seemed to mediate the upregulation of insulin receptor density on cardiomyocytes and skeletal muscle, and increase insulin receptor affinity at the coronary endothelial site. Finally, treatment with losartan induced a significant decrease in glucose concentrations in the diabetic group compared with the appropriate controls.

CONCLUSIONS

Addition of losartan to the standard insulin treatment in non-hypertensive animals with IDDM offers new benefits concerning cardiac protection and prevention of damage. This may be attributed, in part, to insulin receptor density and sensitization.

Changes in crossbridge mechanical properties in diabetic rat cardiomyopathy

Diabetes mellitus is associated with an increased risk of heart failure, resulting from a specific cardiomyopathy independent of coronary atherosclerosis. It is not yet established whether altered myocardial function is related to changes in molecular mechanics of myosin. Accordingly, we investigated the total number, single force and kinetics of myosin crossbridges (CB) in a rat model of streptozotocin-induced diabetic cardiomyopathy. Experiments were conducted on left ventricular papillary muscles from male diabetic (D) Wistar (n = 16) and age-matched control (C) rats (n = 15). Mechanical indices including the maximum unloaded shortening velocity V(max) and the maximum total isometric tension normalized per cross-sectional area TF(max) were determined. Using A. F. Huxley's equations, we calculated the total cycling CB number per mm(2) Psi, the elementary force per single CB Pi, the maximum values of the rate constant for CB attachment f(1) and detachment g(1) and g(2), and the turnover rate of myosin ATPase per site k(cat). The D rats exhibited a 25% decrease in TF(max) and a 34% decrease in V(max) as compared to C. This contractile dysfunction was associated with a significant reduction in Psi (9.0 +/- 1.6 in D versus 11.4 +/- 1.9 10(9)mm(-2) in C, P < 0.001) without significant change in Pi (6.1 +/- 0.8 in D versus 6.3 +/- 0.9 pN in C, NS). In the 2 groups, TF(max) correlated positively with Psi (r = 0.76, P < 0.001 and r = 0.64, P < 0.01, in D and C respectively) but no relationship was found between TF(max) and Pi. As compared to C, D showed lower values of f(1), g(1) and g(2), and a slower turnover rate of myosin ATPase. Thus, present data suggested that the cardiac contractile impairment observed in streptozotocin-induced diabetic rat cardiomyopathy was mainly related to a decrease in active CB total number and CB kinetics alterations without significant change in CB single force.

Left ventricular diastolic dysfunction: an early sign of diabetic cardiomyopathy?

The existence of a diabetic cardiomyopathy has been proposed as evidence has accumulated for the presence of myocardial dysfunction in diabetic patients in the absence of ischemic, valvular or hypertensive heart disease. Diastolic dysfunction has been described as an early sign of this diabetic heart muscle disease preceding the systolic damage. Abnormalities in diastolic performance have been first demonstrated by cardiac catheterisation and subsequently by mainly using echocardiography. The pathogenesis of this left ventricular dysfunction is not clearly understood. Microangiopathy, increased extracellular collagen deposition, or abnormalities in calcium transport alone or in combination are considered to be associated with this dysfunction. The relationship between diastolic dysfunction and glycemic control is still a matter of debate. Some epidemiological and clinical arguments suggest that diastolic abnormalities may contribute to the high morbidity and mortality among diabetic patients. However, the prognostic importance of subclinical diastolic dysfunction and the possibilities for intervention are not fully known. Eventually, despite numerous studies, evidence of an intrinsic diastolic dysfunction in diabetes mellitus remains questionable. Indeed, quite contradictory results have been reported. They have been obtained in small, inhomogeneous populations, with sometimes confounding factors, using various echocardiographic indices with known limitations. Also, further studies using more refined techniques for the evaluation of diastolic function are needed, as a prerequisite, to unequivocally relate diabetes mellitus to a specific cardiomyopathy.

Dominant negative 14-3-3 promotes cardiomyocyte apoptosis in early stage of type I diabetes mellitus through activation of JNK

14-3-3 family members are dimeric, phosphoserine binding proteins that regulate signal transduction, apoptotic, and checkpoint control pathways. Recently, cardiomyocyte apoptosis has been characterized in type I diabetes mellitus. In order to study the molecular mechanism underlying diabetes-induced cardiomyocyte apoptosis, we examined the role of 14-3-3 protein and MAPK pathways in transgenic mice with cardiac specific expression of dominant negative 14-3-3eta (DN-14-3-3). p38 MAPK was highly activated 1, 28, and 56 days after diabetes induction by streptozotocin, whereas peak JNK activation was found on day 3 and decreased afterwards. In contrast, ERK1/2 were not activated in diabetic myocardium. Cardiomyocyte apoptosis was peaked on day 3 and decreased on 7, 28, and 56 days. p38 MAPK and JNK activation as well as cardiomyocyte apoptosis were greatly increased in DN-14-3-3 mice relative to non-transgenic mice. Moreover, we found a significant correlation between JNK activation and apoptosis in diabetic myocardium. These results indicate for the first time that 14-3-3 protein plays a critical anti-apoptotic role in diabetic myocardium by inhibiting the JNK pathway.

Mitochondria respiration and susceptibility to ischemia-reperfusion injury in diabetic hearts

Cardiovascular complications are the primary cause of death for diabetic patients. Clinical and experimental observation has showed the development of dysfunctional cardiomyopathy as one of the main complications of diabetes. Whether the cardiomyopathy results from an increased susceptibility of cardiac tissue to ischemic insult or from a specific functional defect of cardiac mitochondria is a controversial issue. The investigation of possible functional defect in cardiac mitochondria from diabetic rats indicates a decline in state 3 respiration only in animals presenting a marked decrease in body weight. Mitochondria from rats presenting a level of hyperglycemia similar to diabetic animals but not the marked weight loss typical of the latter group show no decline in state 3 respiration, the values being indistinguishable from those of control mitochondria. Mitochondria from hyperglycemic rats, however, show a 15-20% increase in state 4 oxygen consumption but only when glutamate is used as energetic substrate, as compared to a 40-50% increase in state 4 respiration in mitochondria from diabetic rats under similar experimental conditions. This phenomenon is unrelated to diabetes duration, as it is observed at 2 as well as 8 weeks after diabetes onset. Taken together, these data argue against hyperglycemia per se being a direct cause of the decline in state 3 oxygen consumption observed in cardiac mitochondria of type-I diabetic rats and indicate that differences exist in cardiac mitochondrial function in rats generically labeled as diabetic. These differences can contribute to explain discrepancies in experimental results reported by various groups in the field and provide an additional parameter to be taken into consideration in evaluating the varying sensitivity of diabetic hearts to ischemia-reperfusion injury.

Gender differences in myosin heavy chain-beta and phosphorylated phospholamban in diabetic rat hearts

The objective of this study was to determine whether a gender difference exists in myosin heavy chain (MHC) isoform or sarcoplasmic reticulum protein levels in diabetic rat hearts. As is the case with normal rodent hearts, all four chambers of the control rat hearts expressed almost 100% MHC-alpha. In 6-wk diabetic rats, MHC-beta expression in ventricles of males was significantly greater (78 +/- 7%) than in females (50 +/- 5%). The cardiac sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a) protein level was decreased and the phospholamban (PLB) protein level was increased in the left ventricle of diabetic rats, but there was no difference between male and female diabetic rats. The phosphorylated PLB level was decreased more in male than in female diabetic rats. Insulin treatment completely normalized blood glucose level, cardiac SERCA2a and PLB protein levels, and the decrease in MHC-beta levels in both male and female diabetic rats. Insulin treatment completely normalized serum insulin and almost completely normalized phosphorylation of PLB at serine 16 in male diabetic rats. Although insulin treatment completely normalized serum insulin levels in male diabetic rats, in females it only partially normalized serum insulin levels. Also, insulin treatment almost completely normalized phosphorylation of PLB at threonine 17 in female diabetic rats; however, the increase was significantly greater than that identified for insulin-treated male diabetic rats. We conclude that higher levels of MHC-beta and dephosphorylated PLB may contribute to more contractile dysfunction in male than in female diabetic rat hearts, and that phosphorylation of PLB at threonine 17 is more responsive to insulin in female diabetic rat hearts.

Mg2+-dependent ATPase activity in cardiac myofibrils from the insulin-resistant JCR:LA-cp rat

There is a great deal of information presently available documenting a cardiomyopathic condition in insulin-deficient models of diabetes. Less information is available documenting a similar status in non insulin-dependent models of diabetes. We have studied the functional integrity of the myofibrils isolated from hearts of JCR:LA rats. The JCR:LA rat is hyperinsulinemic, hyperlipidemic, glucose intolerant and obese. As such, it carries many of the characteristics found in humans with non insulin-dependent diabetes mellitus. These animals also have many indications of heart disease. However, it is not clear if the hearts suffer from vascular complications or are cardiomyopathic in nature. We examined Mg2+-dependent myofibrillar ATPase in hearts of JCR:LA-cp/cp rats and their corresponding control animals (+/?) and found no significant differences (P> 0.05). This is in striking contrast to the depression in this activity exhibited by cardiac myofibrils isolated from insulin-deficient models of diabetes. Our data demonstrate that myofibrillar functional integrity is normal in JCR:LA-cp rats and suggest that these hearts are not in a cardiomyopathic state. Insulin status may be critical in generating a cardiomyopathic condition in diabetes.

Protein kinase C-delta modulates apoptosis induced by hyperglycemia in adult ventricular myocytes

We evaluated the direct effect of hyperglycemia on apoptosis of adult rat ventricular myocytes (ARVM) in vitro. Hyperglycemia (16.5 mM) for 24 h increased apoptosis by greater than threefold (48.2 +/- 4.4%, by the TdT-mediated dUTP nick-end labeling method) compared with baseline (14.7 +/- 2.5%). Hyperosmolarity with mannitol (11.0 mM) in the presence of 5.5 mM glucose also increased apoptosis by approximately twofold of baseline. Both glucose and mannitol treatment resulted in the membrane translocation of protein kinase C (PKC)-delta, and the activation of PKC-delta was confirmed by immune complex kinase assay. PKC-delta-specific translocation inhibitor peptide (deltaV1-1) attenuated only apoptosis induced by hyperglycemia but not by mannitol. A PKC-epsilon-specific translocation inhibitor peptide (epsilonV1-1) affected neither type of apoptosis. Moderate overexpression of PKC-delta by adenovirus gene transfer prevented the antiapoptotic effect of deltaV1-1. Furthermore, deltaV1-1 attenuated the production of reactive oxygen species (ROS) by glucose. Taken together, our results indicate that increased ROS production regulated by PKC-delta is in part responsible for the induction of apoptosis by hyperglycemia and that apoptosis by hyperglycemia is mechanistically different from that by hyperosmolarity.

Systolic elastance and resistance in the regulation of cardiac pumping function in early streptozotocin-diabetic rats

We determined the roles of maximal systolic elastance (E(max)) and theoretical maximum flow ((max)) in the regulation of cardiac pumping function in early streptozotocin (STZ)-diabetic rats. Physically, E(max) can reflect the intrinsic contractility of the myocardium as an intact heart, and (max) has an inverse relation to the systolic resistance of the left ventricle. Rats given STZ 65 mg/kg i.v. (n = 17) were divided into two groups, 1 week and 4 weeks after induction of diabetes, and compared with untreated age-matched controls (n = 15). Left ventricular (LV) pressure and ascending aortic flow signals were recorded to calculate E(max) and (max), using the elastance-resistance model. After 1 or 4 weeks, STZ-diabetic animals show an increase in effective LV end-diastolic volume (V(eed)), no significant change in peak isovolumic pressure (P(iso)(max)), and a decline in effective arterial volume elastance (E(a)). The maximal systolic elastance E(max) is reduced from 751.5 +/- 23.1 mmHg/ml in controls to 514.1 +/- 22.4 mmHg/ml in 1- and 538.4 +/- 33.8 mmHg/ml in 4-week diabetic rats. Since E(max) equals P(iso)(max)/V(eed), an increase in V(eed) with unaltered P(iso)(max) may primarily act to diminish E(max) so that the intrinsic contractility of the diabetic heart is impaired. By contrast, STZ-diabetic rats have higher theoretical maximum flow (max) (40.9 +/- 2.8 ml/s in 1- and 44.5 +/- 3.8 ml/s in 4-week diabetic rats) than do controls (30.7 +/- 1.7 ml/s). There exists an inverse relation between (max) and E(a) when a linear regression of (max) on E(a) is performed over all animals studied (r = 0.65, p < 0.01). The enhanced (max) is indicative of the decline in systolic resistance of the diabetic rat heart. The opposing effects of enhanced (max) and reduced E(max) may negate each other, and then the cardiac pumping function of the early STZ-diabetic rat heart could be preserved before cardiac failure occurs.

Non-invasive magnetic resonance imaging assessment of myocardial changes and the effects of angiotensin-converting enzyme inhibition in diabetic rats

A non-invasive cine magnetic resonance imaging (MRI) technique was developed to allow, for the first time, detection and characterization of chronic changes in myocardial tissue volume and the effects upon these of treatment by the angiotensin-converting enzyme (ACE) inhibitor captopril in streptozotocin (STZ)-diabetic male Wistar rats. Animals that had been made diabetic at the ages of 7, 10 and 13 weeks and a captopril-treated group of animals made diabetic at the age of 7 weeks were scanned. The findings were compared with the results from age-matched controls. All animal groups (n = 4 animals in each) were consistently scanned at 16 weeks. Left and right ventricular myocardial volumes were reconstructed from complete data sets of left and right ventricular transverse sections which covered systole and most of diastole using twelve equally incremented time points through the cardiac cycle. The calculated volumes remained consistent through all twelve time points of the cardiac cycle in all five experimental groups and agreed with the corresponding post-mortem determinations. These gave consistent myocardial densities whose values could additionally be corroborated by previous reports, confirming the validity of the quantitative MRI results and analysis. The myocardial volumes were conserved in animals whose diabetes was induced at 13 weeks but were significantly increased relative to body weight in animals made diabetic at 7 and 10 weeks. Captopril treatment, which was started immediately after induction of diabetes, prevented the development of this relative hypertrophy in both the left and right ventricles. We have thus introduced and validated quantitative MRI methods in a demonstration, for the first time, of chronic myocardial changes in both the right and left ventricles of STZ-diabetic rats and their prevention by the ACE inhibitor captopril.

Altered K(+) channel gene expression in diabetic rat ventricle: isoform switching between Kv4.2 and Kv1.4

Expression of voltage-gated K(+) channels encoding the K(+) independent transient outward current in the streptozocin-induced diabetic (DM) rat ventricle was studied to determine the basis for slowed cardiac repolarization in diabetes mellitus. Although hypertrophy was not detected in diabetic rats at 12 wk after streptozocin treatment, ventricular Kv4.2 mRNA levels decreased 41% relative to nondiabetic controls. Kv1.4 mRNA levels increased 179% relative to controls, whereas Kv4.3 mRNA levels were unaffected. Immunohistochemistry and Western blot analysis of the diabetic heart showed that the density of the Kv4.2 protein decreased, whereas Kv1.4 protein increased. Thus isoform switching from Kv4.2 to Kv1.4 is most likely the mechanism underlying the slower kinetics of transient outward K(+) current observed in the diabetic ventricle. Brain Kv1.4, Kv4.2, or Kv4.3 mRNA levels were unaffected by diabetes. Myosin heavy chain (MHC) gene expression was altered with a 32% decrease in alpha-MHC mRNA and a 259% increase in beta-MHC mRNA levels in diabetic ventricle. Low-dose insulin-like growth factor-II (IGF-II) treatment during the last 6 of the 12 wk of diabetes (DM + IGF) protected against these changes in MHC mRNAs despite continued hyperglycemia and body weight loss. IGF-II treatment did not change K(+) channel mRNA levels in DM or control rat ventricles. Thus IGF treatment may prevent some, but not all, biochemical abnormalities in the diabetic heart.

Influence of secretin and L-NAME on vascular permeability in the coronary circulation of intact and diabetic rats

The permeability in the intact and diabetic rat coronary circulation after administration of secretin (3.0 micromol/kg i.v.), an inhibitor of NOS (nitric oxide synthase), and L-NAME (N(G)-nitro-L-arginine-methyl ester hydrochloride) (1 mg/kg i.v.), and both substances given together, were studied. To measure protein extravasation Evans blue dye was used as a marker of vascular permeability. The vascular permeability of the left ventricle did not differ in intact and diabetic rats. In the diabetes state increased permeability of atria was observed. Administration of secretin did not influence the coronary vascular permeability in either the intact or the diabetic rats. L-NAME increased the atria permeability and did not change left ventricle permeability. In diabetes, injection of L-NAME caused a decrease in the permeability in both the atria and left ventricle. In intact rats secretin diminished the L-NAME effect in the atria. In diabetic rats co-administration of secretin+L-NAME increased the permeability of the atria and left ventricle, but L-NAME administered alone decreased them. Secretin modified the effect of L-NAME on coronary permeability in intact and diabetic rats.