Fructose Modulates Cardiomyocyte Excitation-Contraction Coupling and Ca2+ Handling In Vitro

BACKGROUND: High dietary fructose has structural and metabolic cardiac impact, but the potential for fructose to exert direct myocardial action is uncertain. Cardiomyocyte functional responsiveness to fructose, and capacity to transport fructose has not been previously demonstrated. OBJECTIVE: The aim of the present study was to seek evidence of fructose-induced modulation of cardiomyocyte excitation-contraction coupling in an acute, in vitro setting. METHODS AND RESULTS: The functional effects of fructose on isolated adult rat cardiomyocyte contractility and Ca²⁺ handling were evaluated under physiological conditions (37°C, 2 mM Ca²⁺, HEPES buffer, 4 Hz stimulation) using video edge detection and microfluorimetry (Fura2) methods. Compared with control glucose (11 mM) superfusate, 2-deoxyglucose (2 DG, 11 mM) substitution prolonged both the contraction and relaxation phases of the twitch (by 16 and 36% respectively, p<0.05) and this effect was completely abrogated with fructose supplementation (11 mM). Similarly, fructose prevented the Ca²⁺ transient delay induced by exposure to 2 DG (time to peak Ca²⁺ transient: 2 DG: 29.0±2.1 ms vs. glucose: 23.6±1.1 ms vs. fructose +2 DG: 23.7±1.0 ms; p<0.05). The presence of the fructose transporter, GLUT5 (Slc2a5) was demonstrated in ventricular cardiomyocytes using real time RT-PCR and this was confirmed by conventional RT-PCR. CONCLUSION: This is the first demonstration of an acute influence of fructose on cardiomyocyte excitation-contraction coupling. The findings indicate cardiomyocyte capacity to transport and functionally utilize exogenously supplied fructose. This study provides the impetus for future research directed towards characterizing myocardial fructose metabolism and understanding how long term high fructose intake may contribute to modulating cardiac function

Oxygen-regulated gene expression in murine cumulus cells

Oxygen is an important component of the environment of the cumulus–oocyte complex (COC), both in vivo within the ovarian follicle and during in vitro oocyte maturation (IVM). Cumulus cells have a key role in supporting oocyte development, and cumulus cell function and gene expression are known to be altered when the environment of the COC is perturbed. Oxygen-regulated gene expression is mediated through the actions of the transcription factors, the hypoxia-inducible factors (HIFs). In the present study, the effect of oxygen on cumulus cell gene expression was examined following in vitro maturation of the murine COC at 2%, 5% or 20% oxygen. Increased expression of HIF-responsive genes, including glucose transporter-1, lactate dehydrogenase A and BCL2/adenovirus E1B interacting protein 3, was observed in cumulus cells matured at 2% or 5%, compared with 20% oxygen. Stabilisation of HIF1α protein in cumulus cells exposed to low oxygen was confirmed by western blot and HIF-mediated transcriptional activity was demonstrated using a transgenic mouse expressing green fluorescent protein under the control of a promoter containing hypoxia response elements. These results indicate that oxygen concentration influences cumulus cell gene expression and support a role for HIF1α in mediating the cumulus cell response to varying oxygen.Karen L. Kind, Kimberley K. Y. Tam, Kelly M. Banwell, Ashley D. Gauld, Darryl L. Russell, Anne M. Macpherson, Hannah M. Brown, Laura A. Frank, Daniel J. Peet and Jeremy G. Thompso

Fructose and the heart: myocardial remodelling and functional responses

© 2011 Dr. Kimberley M. MellorContext: Large population studies have demonstrated that high dietary sugar is associated with increased risk for type 2 diabetes and cardiovascular disease independent of body mass index. Specifically, fructose intake has been linked with the onset of insulin resistance and evidence is emerging that dietary fructose induces a specific cardiac pathology. Metabolism of fructose bypasses the phosphofructokinase regulatory step of glycolysis and high throughput may lead to distinct cellular disturbances. But whether fructose can have direct effects on cardiomyocytes is unknown. The heart may be especially vulnerable to the indirect (i.e. systemic insulin resistance) and direct (i.e. cardiomyocyte metabolic dysregulation via phosphofructokinase bypass) effects of fructose and requires investigation. Aims: This thesis aimed to investigate the cardiac-specific effects of high dietary fructose, specifically assessing whole heart morphology and signalling, and cardiomyocyte performance. Evaluation of cardiomyocyte capacity for fructose transport and utilisation was undertaken to assess the potential for high fructose intake to have direct effects on the heart. Specific questions of cardiac pathophysiological importance were addressed: 1. Do cardiomyocytes have the capacity to transport and utilise fructose? [Chp3]2. How does dietary fructose affect cardiac morphology and cell survival signalling? [Chp4]3. How does dietary fructose affect cardiomyocyte contractile function and Ca2+ handling? [Chp5]4. Does cardiac angiotensin II (AngII) upregulation interact with dietary fructose-induced cardiac signalling alterations? [Chp6] Methods: Detailed in vitro studies manipulating glucose/fructose substrate were used to functionally demonstrate that rodent cardiomyocytes have the capacity to utilise fructose. Dietary fructose-induced cardiac pathology was evaluated in mice with histological, biochemical, molecular, and cellular techniques. Assessment of dietary fructose cardiac effects in the presence of an underlying predisposition for renin-angiotensin system upregulation utilised the cardiac-specific angiotensinogen overexpressing mouse model. Results: The major overall findings of this thesis are: 1. The fructose-specific transporter, GLUT5, is expressed in adult rat ventricular cardiomyocytes, and functional cardiomyocyte fructose utilisation is evident. Fructose reversed the 2-deoxyglucose(2DG)-induced slowing of the Ca2+ transient time to peak: 2DG: 29.0±2.1ms vs. glucose: 23.6±1.1ms vs. fructose + 2DG: 23.7±1.0ms; p<0.05). 2. A high fructose diet induces a specific cardiac pathology associated with systemic insulin resistance. This cardiac pathology is characterised by: • myocardial autophagy activation (46% increase in LC3BII:I ratio; 47% increase in p62) but no evidence of apoptosis induction • cardiac remodelling (28% increased collagen deposition with no change in heart size

Autophagic predisposition in the insulin resistant diabetic heart

Existence of a diabetic cardiopathology, independent of vascular abnormalities, has been well reported. Diffuse interstitial fibrosis throughout the diabetic myocardium (even in the absence of an acute coronary event) suggests widespread cardiomyocyte attrition and cytokine activity. In addition to apoptotic and necrotic events, there is now good evidence that significant cardiomyocyte loss in the diabetic heart is driven by a different, non-apoptotic type of programmed cell death: autophagy. Although considered to be beneficial and pro-survival as a short term strategy to deal with acute stress, when chronically elevated or constitutive, excess autophagic activity has potential to be lethal. The insulin resistant myocardium exhibits various pro-autophagic characteristics: suppression of the PI3K(I)-Akt signaling pathway, oxidative stress and metabolic dysregulation, rendering the diabetic heart vulnerable to autophagic demise. There is compelling new evidence that in the diabetic myocardium cardiomyocyte attrition can be linked to autophagic upregulation

Sex, sex steroids, and diabetic cardiomyopathy: making the case for experimental focus

More than three decades ago, the Framingham study revealed that cardiovascular risk is elevated for all diabetics and that this jeopardy is substantially accentuated for women in particular. Numerous studies have subsequently documented worsened cardiac outcomes for women. Given that estrogen and insulin exert major regulatory effects through common intracellular signaling pathways prominent in maintenance of cardiomyocyte function, a sex-hormone:diabetic-disease interaction is plausible. Underlying aspects of female cardiovascular pathophysiology that exaggerate cardiovascular diabetic risk may be identified, including increased vulnerability to coronary microvascular disease, age-dependent impairment of insulinsensitivity, and differential susceptibility to hyperglycemia. Since Framingham, considerable progress has been made in the development of experimental models of diabetic disease states, including a diversity of genetic rodent models. Ample evidence indicates that animal models of both type 1 and 2 diabetes variably recapitulate aspects of diabetic cardiomyopathy including diastolic and systolic dysfunction, and cardiac structural pathology including fibrosis, loss of compliance, and in some instances ventricular hypertrophy. Perplexingly, little of this work has explored the relevance and mechanisms of sexual dimorphism in diabetic cardiomyopathy. Only a small number of experimental studies have addressed this question, yet the prospects for gaining important mechanistic insights from further experimental enquiry are considerable. The case for experimental interrogation of sex differences, and of sex steroid influences in the aetiology of diabetic cardiomyopathy, is particularly compelling-providing incentive for future investigation with ultimate therapeutic potential

Cellular Mechanisms Mediating Exercise-Induced Protection against Cardiotoxic Anthracycline Cancer Therapy

Anthracyclines such as doxorubicin are widely used chemotherapy drugs. A common side effect of anthracycline therapy is cardiotoxicity, which can compromise heart function and lead to dilated cardiomyopathy and heart failure. Dexrazoxane and heart failure medications (i.e., beta blockers and drugs targeting the renin–angiotensin system) are prescribed for the primary prevention of cancer therapy-related cardiotoxicity and for the management of cardiac dysfunction and symptoms if they arise during chemotherapy. However, there is a clear need for new therapies to combat the cardiotoxic effects of cancer drugs. Exercise is a cardioprotective stimulus that has recently been shown to improve heart function and prevent functional disability in breast cancer patients undergoing anthracycline chemotherapy. Evidence from preclinical studies supports the use of exercise training to prevent or attenuate the damaging effects of anthracyclines on the cardiovascular system. In this review, we summarise findings from experimental models which provide insight into cellular mechanisms by which exercise may protect the heart from anthracycline-mediated damage, and identify knowledge gaps that require further investigation. Improved understanding of the mechanisms by which exercise protects the heart from anthracyclines may lead to the development of novel therapies to treat cancer therapy-related cardiotoxicity