Whole-heart Studies Research Articles

Cardiovascular risk of energy drinks: Caffeine and taurine facilitate ventricular arrhythmias in a sensitive whole-heart model.

Several case reports have suggested an increased risk of sudden cardiac death due to energy drinks. Therefore, the purpose of this study was to assess acute electrophysiologic effects of caffeine and taurine, two of the main ingredients of energy drinks, in an experimental whole-heart model. Twenty-five rabbit hearts were excised, retrogradely perfused, and assigned to two groups. Hearts were perfused with caffeine (2, 10, and 50 µM) or taurine (2, 10, and 50 µM) after generating baseline data. Eight monophasic action potentials and electrocardiography recordings showed a significant abbreviation of action potential duration (APD90 ), QT interval, and effective refractory periods (ERP) after caffeine treatment. With taurine, cardiac repolarization duration and ERP were significantly shortened. A ventricular vulnerability was assessed by a predefined pacing protocol. With caffeine, we observed a trend towards more ventricular arrhythmias in a dose-dependent manner. After treatment with taurine, significantly more episodes of ventricular arrhythmias occurred. In this experimental whole-heart study, treatment with caffeine and taurine provoked ventricular arrhythmias. The underlying mechanism was an abbreviation of cardiac repolarizations and effective refractory periods that may facilitate re-entry and thereby provokes arrhythmias. These findings help to understand the potentially hazardous and fatal outcomes after intoxication with energy drinks.

Propofol abolishes torsade de pointes in different models of acquired long QT syndrome

There is conflicting evidence regarding the impact of propofol on cardiac repolarization and the risk of torsade de pointes (TdP). The purpose of this study was to elucidate the risk of propofol-induced TdP and to investigate the impact of propofol in drug-induced long QT syndrome. 35 rabbit hearts were perfused employing a Langendorff-setup. 10 hearts were perfused with increasing concentrations of propofol (50, 75, 100 µM). Propofol abbreviated action potential duration (APD90) in a concentration-dependent manner without altering spatial dispersion of repolarization (SDR). Consequently, no proarrhythmic effects of propofol were observed. In 12 further hearts, erythromycin was employed to induce prolongation of cardiac repolarization. Erythromycin led to an amplification of SDR and triggered 36 episodes of TdP. Additional infusion of propofol abbreviated repolarization and reduced SDR. No episodes of TdP were observed with propofol. Similarly, ondansetron prolonged cardiac repolarization in another 13 hearts. SDR was increased and 36 episodes of TdP occurred. With additional propofol infusion, repolarization was abbreviated, SDR reduced and triggered activity abolished. In this experimental whole-heart study, propofol abbreviated repolarization without triggering TdP. On the contrary, propofol reversed prolongation of repolarization caused by erythromycin or ondansetron, reduced SDR and thereby eliminated drug-induced TdP.

Concomitant Treatment with Proton Pump Inhibitors and Cephalosporins Does Not Enhance QT-Associated Proarrhythmia in Isolated Rabbit Hearts.

Recent results from data mining analyses and reports of adverse drug events suggest a QT-prolonging drug-drug interaction resulting from the combination of distinct proton pump inhibitors and cephalosporins. Therefore, this study aimed at investigating the effect of the suspected QT-prolonging combinations of lansoprazole + ceftriaxone and esomeprazole + cefazolin, respectively. 26 hearts of New Zealand White rabbits were retrogradely perfused and paced at different cycle lengths. After generating baseline data, the hearts were assigned to two groups: In group 1, hearts were treated with 5µM lansoprazole. Thereafter, 200µM ceftriaxone was infused additionally. Group 2 was perfused with 10µM esomeprazole followed by 250µM cefazolin. In group 1, lansoprazole did not significantly alter QT intervals and APD90. Additional treatment with ceftriaxone significantly shortened QT interval, APD90 and slightly reduced dispersion of repolarization compared to sole lansoprazole infusion. In group 2, esomeprazole led to a significant shortening of the QT interval without altering APD90 or dispersion. Additional treatment with the antibiotic cefazolin further shortened QT interval, APD90 and reduced the dispersion of repolarization. Incidence of ventricular arrhythmias was not significantly altered in both groups. This is the first experimental whole-heart study that investigated the impact of a concomitant treatment with proton pump inhibitors and cephalosporins. In contrast to previous reports, the combination of both agents did not cause QT prolongation but instead shortened QT interval and action potential duration. As a consequence, no triggered activity occurred in the presence of a stable dispersion of repolarization.

Increased sodium/calcium exchanger activity enhances beta-adrenergic–mediated increase in heart rate: Whole-heart study in a homozygous sodium/calcium exchanger overexpressor mouse model

The cardiac sodium/calcium (Na+/Ca2+) exchanger (NCX) contributes to diastolic depolarization in cardiac pacemaker cells. Increased NCX activity has been found in heart failure and atrial fibrillation. The influence of increased NCX activity on resting heart rate, beta-adrenergic-mediated increase in heart rate, and cardiac conduction properties is unknown. The purpose of this study was to investigate the influence of NCX overexpression in a homozygous transgenic whole-heart mouse model (NCX-OE) on sinus and AV nodal function. Langendorff-perfused, beating whole hearts of NCX-OE and the corresponding wild-type (WT) were studied ± isoproterenol (ISO; 0.2 μM). Epicardial ECG, AV nodal Wenckebach cycle length (AVN-WCL), and retrograde AVN-WCL were obtained. At baseline, basal heart rate was unaltered between NCX-OE and WT (WT: cycle length [CL] 177.6 ± 40.0 ms, no. of hearts [n] = 20; NCX-OE: CL 185.9 ± 30.5 ms, n = 18; P = .21). In the presence of ISO, NCX-OE exhibited a significantly higher heart rate compared to WT (WT: CL 133.4 ± 13.4 ms, n = 20; NCX-OE: CL 117.7 ± 14.2 ms, n = 18; P <.001). ISO led to a significant shortening of the anterograde and retrograde AVN-WCL without differences between NCX-OE and WT. This study is the first to demonstrate that increased NCX activity enhances beta-adrenergic increase of heart rate. Mechanistically, increased NCX inward mode activity may promote acceleration of diastolic depolarization in sinus nodal pacemaker cells, thus enhancing chronotropy in NCX-OE. These findings suggest a novel potential therapeutic target for heart rate control in the presence of increased NCX activity, such as heart failure.

WHOLEheart study participant acceptance of wholegrain foods

This qualitative study explored the concept of acceptance of wholegrain foods in an adult population in the UK. Data was generated via focus groups with volunteers from a randomised controlled wholegrain based dietary intervention study (the WHOLEheart study). WHOLEheart volunteers, who did not habitually eat wholegrain foods, were randomised to one of three experimental regimes: (1) incorporating 60g/day whole grains into the diet for 16weeks; (2) incorporating 60g/day whole grains into the diet for 8weeks, doubling to 120g/day for the following 8weeks; (3) a control group. Focus groups to examine factors relating to whole grain acceptability were held one month post-intervention. For participants incorporating whole grains into their diet, acceptance was dependent upon: (a) ‘trial acceptance’, relating to the taste, preparation and perceived impact of the wholegrain foods on wellbeing, and (b) ‘dietary acceptance’ which involved the compatibility and substitutability of whole grains with existing ingredients and meal patterns. Barriers to sustained intake included family taste preferences, cooking skills, price and availability of wholegrain foods. Although LDL lowering benefits of eating whole grains provided the impetus for the WHOLEheart study, participants’ self-reported benefits of eating wholegrain foods included perceived naturalness, high fibre content, superior taste, improved satiety and increased energy levels provided a stronger rationale for eating whole grains.

Flash Photolysis of Caged Compounds during Simultaneous Imaging of Calcium and Voltage in the Whole Heart using Light-Emitting-Diodes

Flash photolysis of caged-compounds (i.e. photoactivatable compounds) allows one to interact with tissue by controlling the release of bioactive components, both spatially and temporally. The use of ultraviolet light-emitting-diodes (UV LEDs) for uncaging such compounds in cells has recently generated excitement, since LEDs offer high-speed control and are low cost.Traditional methods for simultaneous imaging of intracellular calcium transients (CaT) and membrane voltage (Vm) are technically challenging and require the use of two cameras. We developed a single-camera (Cascade 128+; Photometrics) approach that images rhod-2 (CaT) and the new near-infrared di-4-ANBDQPQ (Vm), without cross-talk. The technique relies on the isosbestic-point of di-4-ANBDQPQ, a custom-built multi-band optical filter (Chroma Technology) and visible LEDs (Luminus Devices). As the excitation wavelengths of these dyes are significantly longer than those needed to photoactivate caged-compounds, we combined them with a UV-sensitive calcium buffer (NP-EGTA; Invitrogen) to locally modulate free calcium in Langendorff-perfused rat hearts (N=3, Wistar-rat female).We demonstrate local uncaging of calcium by photo-inhibition of NP-EGTA with a powerful 365nm UV LED (Nichia), while simultaneously imaging CaT and Vm (128x128 pixels; 511 frames-per-second). CaT (shape and kinetics) were dramatically altered, in keeping with reported effects of EGTA-AM in whole-heart studies. The kinetics of CaT recovery changed from biphasic (in the presence of functional EGTA) to monophasic (after photo-inhibition of the buffer), and CaT duration increased.In summary: Caged-compounds can be used in multi-cellular preparations like the whole-heart to investigate the effects of spatiotemporal perturbations of bioactive compound concentrations, such as calcium buffers, and this approach can be combined with simultaneous measurement of CaT and Vm.Supported by NIH grant EB001963; Human Frontier Science Program grant RGP0027/09; The Clarendon Fund Scholarship; and The British Heart Foundation.

Adaptive control of cardiac contraction to changes in loading: from theory of sarcomere dynamics to whole-heart function

The heart accommodates to rapid changes in demands. This review elucidates the adaptive control of cardiac function by loading conditions, and integrates the sarcomeric control of contraction (SCC) with isolated trabeculae and in vivo whole-heart studies. The SCC includes two feedback mechanisms: (1) cooperativity that regulates cross-bridge (XB) recruitment and the force-length relationship, and (2) mechanical feedback, whereby the filament-sliding velocity determines the XB-weakening rate and the force-velocity relationship. An isolated rat trabeculae study tested the suggested mechanisms during sarcomeric lengthening. The observations indicate that lengthening decreases the XB-weakening rate in a velocity-dependent manner, congruent with the suggested hypothesis and in contrast to alternative theories. A whole-heart level study in sheep reveals the existence of a preload-independent linear relationship between the external work (EW) and pressure-time integral during transient vena cava occlusions, for any given afterload, and not just at isovolumic contractions. The slope of this relationship decreases as the afterload increases. These findings highlight the mechanisms underlying the pressure (Frank's phenomenon) and EW (Starling's phenomenon) generation and the roles that the preload and afterload play. The theoretical, isolated fibers and whole-heart studies provide complementary information that strengthens our understanding of cardiac function from the top-down and bottom-up.

Non-Genomic Effects of 17beta-Estradiol on Cardiomyocytes

Pre-menopausal women have a reduced risk from ischemic heart disease compared to men; this protection is lost after ovariectomy or menopause. Estrogen has been found to confer protection in whole-heart studies in animal models. ATP-sensitive K+ (KATP) channels act as metabolic sensors, opening when cellular energy levels fall and are thought to play a role in cardioprotection. We investigated whether the cardiac KATP currents play a role in the cardioprotective effects of 17β-estradiol in isolated cardiomyocytes. Ventricular myocytes were isolated from adult male Wistar rats by enzymatic digestion. The ability of myocytes to recover contractile function after simulated ischaemia/reperfusion injury was calculated by video microscopy. Contracting myocytes (1Hz) were continuously superfused with Tyrode solution, then subjected to 7 minutes of similated ischaemia by perfusion with a metabolic inhibition solution, comprising substrate-free Tyrode with cyanide (2mM) and idoacetic acid (1mM), followed by 10 minute reperfusion with Tyrode to simulate reperfusion. A 5 minute pre-treatment of 17β-estradiol (500nM) significantly increased contractile recovery; control 30% n=100, 17β-estradiol 68% n=185, DPN (ERβ agonist) 53% n= 111. Using the whole-cell patch-clamp mode we investigated the effect of estrogen on KATP channel activity. P1075 (10μM) elicited a KATP current which was completely blocked by bath application of estradiol (500nM). Both the classical estrogen receptors; ERα and ERβ have been implicated in several rapid effects of 17β-estradiol. DPN is a specific agonist of ERβ and application of DPN (500nM) following opening of KATP by P1075 (10μM) also resulted in 100% decrease in KATP current. We show that rapid estrogen signalling has a cardioprotective effect in isolated ventricular myocytes. Application of estradiol resulted in complete loss of KATP current recorded in cardiomyocytes; therefore questioning the role of KATP in estrogen-dependent cardioprotection. This work was supported by funding by The British Heart Foundation.

Markers of cardiovascular risk are not changed by increased whole-grain intake: the WHOLEheart study, a randomised, controlled dietary intervention

Recommendations for whole-grain (WG) intake are based on observational studies showing that higher WG consumption is associated with reduced CVD risk. No large-scale, randomised, controlled dietary intervention studies have investigated the effects on CVD risk markers of substituting WG in place of refined grains in the diets of non-WG consumers. A total of 316 participants (aged 18-65 years; BMI>25 kg/m2) consuming < 30 g WG/d were randomly assigned to three groups: control (no dietary change), intervention 1 (60 g WG/d for 16 weeks) and intervention 2 (60 g WG/d for 8 weeks followed by 120 g WG/d for 8 weeks). Markers of CVD risk, measured at 0 (baseline), 8 and 16 weeks, were: BMI, percentage body fat, waist circumference; fasting plasma lipid profile, glucose and insulin; and indicators of inflammatory, coagulation, and endothelial function. Differences between study groups were compared using a random intercepts model with time and WG intake as factors. Although reported WG intake was significantly increased among intervention groups, and demonstrated good participant compliance, there were no significant differences in any markers of CVD risk between groups. A period of 4 months may be insufficient to change the lifelong disease trajectory associated with CVD. The lack of impact of increasing WG consumption on CVD risk markers implies that public health messages may need to be clarified to consider the source of WG and/or other diet and lifestyle factors linked to the benefits of whole-grain consumption seen in observational studies.

Cgp-37157 Abrogates The Adverse Effect Of Ouabain On Mitochondrial Energetics

Cardiac glycosides have been used to treat heart failure for more than 200 years and their major effect is to inhibit the Na+/K+ pump. Inhibition of the pump on the sarcolemma of cardiac myocytes elevates intracellular Na+ ([Na+]i), resulting in a positive inotropic effect by increasing Ca2+ load. However, our previous work demonstrated that elevated [Na+]i impairs mitochondrial energetics by blunting mitochondrial Ca2+ ([Ca2+]m) accumulation. Moreover, we showed that CGP-37157, an inhibitor of [Ca2+]m efflux, restored [Ca2+]m accumulation and improved mitochondrial energetics. Here, we investigated the effects of ouabain with or without CGP-37157 on [Na+]i and NADH production in isolated cardiomyocytes and examined the effects on hemodynamics and Oxygen consumption (mVO2) in whole hearts. Application of ouabain to isolated myocytes elevated [Na+]i in a dose-dependent way. During 1 Hz stimulation, the NADH/NAD+ redox potential in ouabain treated myocytes was decreased significantly, whereas NADH levels were well maintained in the presence of CGP-37157. In whole-heart studies, ouabain increased LVDP, +dP/dt, and -dP/dt, and addition of CGP-37157 further increased +dP/dt and -dP/dt. When isoproterenol was employed to increase cardiac work, LVDP was not increased, but +dP/dt and -dP/dt were increased by 57% and 52%, respectively, in hearts without concomitant CGP-37157 treatment. In isoproterenol-treated hearts also exposed to CGP-37157, LVDP increased by 30%, and +dP/dt and -dP/dt were increased by 75% and 53%, respectively. Whole heart mVO2 increased by 18% after ouabain treatment and by 25% after isoproterenol administration compared to baseline. With concomitant CGP-37157 treatment, ouabain increased mVO2 by 32% and isoproterenol increased mVO2 by 53%. Our findings revealed an adverse effect of the glycoside on mitochondrial energetics and indicate that CGP-37157 can prevent this impairment. In addition, inotropic responses to both ouabain and isoproterenol were enhanced in the presence of CGP-37157.

Cardiovascular abnormalities in transgenic mice with reduced brown fat: an animal model of human obesity.

A new model of murine obesity has recently been developed through transgenic ablation of brown adipose tissue that manifests typical metabolic complications of obesity, including insulin resistance and non-insulin-dependent diabetes mellitus. The cardiovascular phenotype has not been defined. Transthoracic echocardiography, aortic catheterization, isolated whole-heart studies, and morphometric histology defined cardiac structure and function in 30 transgenic mice with reduced brown fat and 30 matched wild-type controls. Obesity was indicated by a 77% increase in body weight and was accompanied by elevated systemic pressures (mean aortic blood pressure 85+/-1 versus 66+/-2 mm Hg; P<0.01), left ventricular dilation and hypertrophy (mass/body weight 4.0+/-0.2 versus 2.7+/-0.3 mg/g; P<0.01), and high cardiac output (cardiac index 3.2+/-0.4 versus 2.4+/-0.1 mL x kg(-1) x min(-1); P<0.01). Baseline functional parameters assessed in vitro were not different, but after imposition of zero-flow ischemia, significant relaxation impairment developed in obese mice. Although morphometrically determined myocyte diameters were similar, the percentage of interstitial fibrosis was significantly increased in transgenic mice compared with wild-type controls (7.5+/-2% versus 4. 2+/-0.2%; P<0.01). Transgenic ablation of brown adipose tissue is associated not only with obesity but also with systemic hypertension, left ventricular hypertrophy with eccentric remodeling and fibrosis, and high cardiac output, a unique constellation of findings strikingly similar to that seen in human obesity. Mice with reduced brown fat may serve as a new model for the cardiovascular morbid complications associated with obesity in humans.

Geometric changes allow normal ejection fraction despite depressed myocardial shortening in hypertensive left ventricular hypertrophy

This study of hypertensive left ventricular hypertrophy 1) assessed myocardial shortening in both the circumferential and long-axis planes, and 2) investigated the relation between geometry and systolic function. In hypertensive left ventricular hypertrophy, whole-heart studies have suggested normal systolic function on the basis of ejection fraction-systolic stress relations. By contrast, isolated muscle data show that contractility is depressed. It occurred to use that this discrepancy could be related to geometric factors (relative wall thickness). We studied 43 patients with hypertensive left ventricular hypertrophy and normal ejection fraction (mean +/- SD 69 +/- 13%) and 50 clinically normal subjects. By echocardiography, percent myocardial shortening was measured in two orthogonal planes; circumferential shortening was measured at the endocardium and at the midwall, and long-axis shortening was derived from mitral annular motion (apical four-chamber view). Circumferential shortening was related to end-systolic circumferential stress and long-axis shortening to meridional stress. Endocardial circumferential shortening was higher than normal (42 +/- 10% vs. 37 +/- 5%, p < 0.01) and midwall circumferential shortening lower than normal in the left ventricular hypertrophy group (18 +/- 3% vs. 21 +/- 3%, p < 0.01). Differences between endocardial and midwall circumferential shortening are directly related to differences in relative wall thickness. Long-axis shortening was also depressed in the left ventricular hypertrophy group (18 +/- 6% in the left ventricular hypertrophy group, 21 +/- 5% in control subjects, p < 0.05). Midwall circumferential shortening and end-systolic circumferential stress relations in the normal group showed the expected inverse relation; those for approximately 33% of the left ventricular hypertrophy group were > 2 SD of normal relations, indicating depressed myocardial function. There was no significant relation between long-axis shortening and meridional stress, indicating that factors other than afterload influence shortening in this plane. High relative wall thickness allows preserved ejection fraction and normal circumferential shortening at the endocardium despite depressed myocardial shortening in two orthogonal planes.

Changes in the subcellular distribution of free carnitine and its acyl derivatives in diabetic rat hearts following treatment with L-carnitine.

Carnitine deficiency has been demonstrated in diabetic hearts, and it is also well known that L-carnitine administration has a beneficial effect on cardiac function. Carnitine treatment would be expected to reduce the accumulation of long-chain acylcarnitine. However, many reports have shown that myocardial long-chain acylcarnitine levels were increased following treatment with L-carnitine in whole-heart studies. Since acylcarnitine exists in both the mitochondrial and cytosolic compartments, it is difficult to investigate changes in subcellular distribution by studying whole-heart preparations. The present study investigated the myocardial subcellular distribution of carnitine and its acyl derivatives in diabetic rats with or without L-carnitine treatment. Approximately 90% of total cellular carnitine was located in the cytosol in the diabetic hearts. Both mitochondrial and cytosolic levels of free carnitine and short-chain acylcarnitine were significantly decreased in the diabetic heart. However, the mitochondrial level of long-chain acylcarnitine was significantly increased. L-carnitine treatment reduced the mitochondrial level of long-chain acylcarnitine, but the cytosolic level of long-chain acylcarnitine was significantly increased. These results show that L-carnitine treatment prevents the accumulation of long-chain acylcarnitine in the mitochondrial space and then reduces the detergent effect of long-chain acylcarnitine on the mitochondrial membrane. We suggest that it is a possible mechanism of the beneficial effect of L-carnitine treatment on the diabetic heart.

Cardiac basal and activation metabolism.

Cardiac basal metabolism is the rate of energy expenditure of the quiescent myocardium. It is species dependent and increases with pre-load. It has small contributions from membrane-bound cation pumps. The contribution of protein metabolism remains open to question. Calculations show that mitochondrial proton pumping may account for a large fraction of the cardiac basal metabolism. Nevertheless this component remains essentially ill-understood. Cardiac activation metabolism is the supra-basal rate of energy expenditure associated with those processes that activate contraction. In isolated muscle preparations it is typically measured as the rate of heat production or oxygen consumption of a muscle, pre-shortened to a length where active force production is negligible, although it is also estimated by pharmacological intervention. In whole-heart studies it is indexed by the supra-basal rate of oxygen consumption of the empty, beating but non-working heart. Activation metabolism underwrites electrical excitation (the ECG) and excitation-contraction coupling (the cycling of calcium ions). It is increased by agents that increase contractility; it probably increases with pre-load, via the phenomenon of length-dependent activation. The basal and activation components each account for one-quarter to one-third of the total energy expenditure of the heart under normal conditions.