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

Changes in Intracellular Na+ following Enhancement of Late Na+ Current in Virtual Human Ventricular Myocytes

The slowly inactivating or late Na+ current, INa-L, can contribute to the initiation of both atrial and ventricular rhythm disturbances in the human heart. However, the cellular and molecular mechanisms that underlie these pro-arrhythmic influences are not fully understood. At present, the major working hypothesis is that the Na+ influx corresponding to I(Na-L)significantly increases intracellular Na+, [Na]; and the resulting reduction in the electrochemical driving force for Na+ reduces and (may reverse) Na+/Ca2+ exchange. These changes increase intracellular Ca2+, [Ca2+]; which may further enhance I(Na-L)due to calmodulindependent phosphorylation of the Na+ channels. This paper is based on mathematical simulations using the O'Hara et al (2011) model of baseline or healthy human ventricular action potential waveforms(s) and its [Ca2(+)]; homeostasis mechanisms. Somewhat surprisingly, our results reveal only very small changes (<= 1.5 mM) in [Na] even when INa-L is increased 5-fold and steady-state stimulation rate is approximately 2 times the normal human heart rate (i.e. 2 Hz). Previous work done using well-established models of the rabbit and human ventricular action potential in heart failure settings also reported little or no change in [Na] when I(Na-L)was increased. Based on our simulations, the major short-term effect of markedly augmenting I(Na-L)is a significant prolongation of the action potential and an associated increase in the likelihood of reactivation of the L-type Ca2+ current, Ica-L. Furthermore, this action potential prolongation does not contribute to [Na]; increase.This work was supported by (i) the "VI Plan Nacional de Investigacion Cientifica, Desarrollo e Innovacion Tecnologica" from the Ministerio de Economia y Competitividad of Spain (grant number TIN2012-37546-C03-01) and the European Commission (European Regional Development Funds-ERDF-FEDER), (ii) by the Direccion General de Politica Cientifica de la Generalitat Valenciana (grant number GV/2013/119), and by (iii), Programa Prometeo (PROMETEO/2016/088) de la Conselleria d'Educacio Formacio I Ocupacio, Generalitat Valenciana. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.K Cardona; Trénor Gomis, BA.; W Giles (2016). Changes in Intracellular Na+ following Enhancement of Late Na+ Current in Virtual Human Ventricular Myocytes. PLoS ONE. 11(11). https://doi.org/10.1371/journal.pone.0167060S111

Na+ overload during ischemia and reperfusion in rat hearts: Comparison of the Na+/H+ exchange blockers EIPA, cariporide and eniporide

Intracellular myocardial Na+ overload during ischemia is an important cause of reperfusion injury via reversed Na+/Ca2+ exchange. Prevention of this Na+ overload can be accomplished by blocking the different Na+ influx routes. In this study the effect of ischemic inhibition of the Na+/H+ exchanger (NHE) on [Na+](i), pH(i) and post-ischemic contractile recovery was tested, using three different NHE-blockers: EIPA, cariporide and eniporide. pH(i) and [Na+](i) were measured using simultaneous P-31 and Na-23 NMR spectroscopy, respectively, in paced (5 Hz) isolated, Langendorff perfused rat hearts while contractility was assessed by an intraventricular balloon. NHE-blockers (3 muM) were administered during 5 min prior to 30 min of global ischemia followed by 30 min drug-free reperfusion. NHE blockade markedly reduced ischemic Na+ overload; after 30 min of ischemia [Na+](i) had increased to 293+/-26, 212+/-6, 157+/-5 and 146+/-6% of baseline values in untreated and EIPA (

Assessment of myocardial viability by intracellular Na-23 magnetic resonance Imaging

Background - Because of rapid changes in myocardial intracellular Na+ (Na-i(+)) during ischemia and reperfusion (R), Na-23 magnetic resonance imaging (MRI) appears to be an ideal diagnostic modality for early detection of myocardial ischemia and viability. So far, cardiac Na-23 MRI data are limited and mostly concerned with imaging of total Na+. For proper interpretation, imaging of both Na-i(+) and extracellular Na+ is essential. In this study, we tested whether Na-i(+) imaging can be used to assess viability after low-flow (LF) ischemia. Methods and Results - Isolated rat hearts were subjected to LF (1%, 2%, or 3% of control coronary flow) and R. A shift reagent was used to separate Na-i(+) and extracellular Na+ resonances. Acquisition-weighted Na-23 chemical shift imaging (CSI) was alternated with Na-23 MR spectroscopy. Already during control perfusion, Na-i(+) could be clearly seen on the images. Na-i(+) image intensity increased with increasing severity of ischemia. During R, Na-i(+) image intensity remained highest in 1% LF hearts. Not only did we find very good correlations between Na-i(+) image intensity at end-R and end-diastolic pressure ( R = 0.85, P <0.001) and recovery of the rate-pressure product ( R = - 0.88, P <0.001) at end-R, but most interestingly, also Na-i(+) image intensity at end-LF was well correlated with end-diastolic pressure ( R = 0.78, P <0.01) and with recovery of the rate-pressure product ( R = - 0.81, P <0.01) at end-R. Furthermore, Na-i(+) image intensity at end-LF was well correlated with creatine kinase release during R ( R = 0.79, P <0.05) as well as with infarct size ( R = 0.77, P <0.05). Conclusions - These data indicate that Na-23 CSI is a promising tool for the assessment of myocardial viability

Postischemic Na+-K+-ATPase reactivation is delayed in the absence of glycolytic ATP in isolated rat hearts

Normalization of intracellular sodium (Na-i(+)) after postischemic reperfusion depends on reactivation of the sarcolemmal Na+-K+-ATPase. To evaluate the requirement of glycolytic ATP for Na+-K+-ATPase function during postischemic reperfusion, 5-s time-resolution Na-23 NMR was performed in isolated perfused rat hearts. During 20 min of ischemia, Na-i(+) increased approximately twofold. In glucose-reperfused hearts with or without prior preischemic glycogen depletion, Na-i(+) decreased immediately upon postischemic reperfusion. In glycogen-depleted pyruvate-reperfused hearts, however, the decrease of Na-i(+) was delayed by similar to 25 s, and application of the pyruvate dehydrogenase (PDH) activator dichloroacetate (DA) did not shorten this delay. After 30 min of reperfusion, Na-i(+) had almost normalized in all groups and contractile recovery was highest in the DA-treated hearts. In conclusion, some degree of functional coupling of glycolytic ATP and Na+-K+-ATPase activity exists, but glycolysis is not essential for recovery of Na-i(+) homeostasis and contractility after prolonged reperfusion. Furthermore, the delayed Na+-K+-ATPase reactivation observed in pyruvate-reperfused hearts is not due to inhibition of PDH