The electrophysiological and mechanical effects of gap junction uncoupling in cardiac muscle

The aim of this study was to study the electrophysiological and mechanical effects of the gap junction uncoupler 1-heptanol in the left ventricle (epicardial surface) of the artificially perfused Langendorff rabbit heart. Specifically, electrical restitution and the dispersion of repolarisation were studied. Methods. Using a single monophasic action potential (MAP) electrode, in the healthy and failing (coronary ligated) heart, the effect of 1-heptanol was studied on rate dependent changes in action potential duration. Dispersion of repolarisation was measured sequentially. A 32 MAP electrode array was developed to simultaneously record dispersion of repolarisation from the epicardial surface of the left ventricle of healthy hearts. Restitution was studied using an extrastimulus protocol that involved electrically stimulating the heart with 16 S1 stimuli (350ms intervals), and an extrastimulus S2. S1-S2 interval was increased progressively from 70 to 600ms. S1-S2 changes of 5ms were made between 70 and 150ms, 10ms between 150 and 350ms, and 50ms between 350 and 600ms. Protocols were run at 37°C, initially in Tyrode's solution, then after addition of 0.3mM 1-heptanol. Results. The single catheter study showed that failure significantly (P<0.05) prolongs MAP duration between cycle lengths of 250ms and 650ms. No base to apex changes, changes in dispersion of repolarisation or ventricular fibrillation thresholds were observed. 1-Heptanol, at cycle lengths above 350ms, significantly (P<0.05) decreased MAP duration in failing and healthy hearts. 1-Heptanol however did not alter the dispersion of repolarisation or ventricular fibrillation threshold in healthy and failing hearts. The last SI MAP in the 16 beat train and the S2 MAP obtained using the 32 electrode array were analysed at 90% repolarisation (MAPD90). S2 MAPD90 increased with S I-S2 interval up to -180ms but decreased at longer intervals. 0.3mM l-heptanol exacerbated this negative slope in the restitution curve from (mean±SEM) -0.031±O.004 in Tyrode's compared to -O.063±O.005 in O.3mM 1-heptanol (PO.05) in the presence of O.3mM l-heptanol. Conduction delay was increased from (mean±SEM) 44.2±0.82ms to 49.2±O.87ms (P The possibility of an effect on the single cell being the mechanism behind the exacerbation of the negative slope by l-heptanol was investigated in a single cell study. The effect of l-heptanol on single cell fractional shortening and Ca2+ handling was examined. At 1 and 3Hz l-heptanol decreases fractional cell shortening from (mean±SEM): at 1Hz 9.3±0.8% in Krebs to 5.7±O.7% (O.03mM 1-heptanol), 5.6±O.9% (O.lmM l-heptanol) and 3.2±0.8% (O.3mM lheptanol) (PThe effect on the L-type Ca2+ channel was investigated by the use of nifedipine firstly in single cells then in the whole heart. Nifedipine decreased single cell fractional shortening at 1Hz from (mean±SEM) 8.9±O.7% in Krebs to S.8±O.7% (O.lJ,tM nifedipine), 3.4±O.9% (O.l5J,tM nifedipine, P<O.OOI)and O.8±O.2% (O.2JlM nifedipine, P<O.OOl). SR Ca2+ release was also reduced from (mean±SEM) O.75±O.017% in Krebs to O.72±O.024% (O.IJlM nifedipine), O.73±0.024% (O.l5JlM nifedipine), O.71±O.016% (O.2J,tM nifedipine) (P<O.OS). In the whole heart nifedipine did not induce a negative slope in the restitution curve indicating no role of the L-type Ca2+ channel in this phenomenon. Carbenoxolone, a novel specific gap junction uncoupler failed to induce a negative slope in the electrical restitution curve of the whole heart but did increase dispersion of repolarisation (P>0.05) and caused a significant conduction slowing from (mean±SEM) 45.50±2.I2ms in Tyrode to 55.11±2.82ms in carbenoxolone (P<0.05). Carbenoxolone has an inconsistent effect on single cell fractional shortening and Ca2+ handling. Conclusions. The biphasic relationship and the increased dispersion of repolarisation in the presence of 0.3mM I-heptanol may have implications for the development of alternans and/or arrhythmias (Gilmour and Chialvo, 2000). The cause of the negative slope is as yet unknown. but it is likely that it is an effect on the single cell rather than gap junction uncoupling

Exercise training reverses myocardial dysfunction induced by CaMKIIδC overexpression by restoring Ca2+-homeostasis

Several conditions of heart disease, including heart failure and diabetic cardiomyopathy, are associated with upregulation of cytosolic Ca2+/calmodulin-dependent protein kinase II (CaMKIIδC) activity. In the heart, CaMKIIδC isoform targets several proteins involved in intracellular Ca2+ homeostasis. We hypothesized that high-intensity endurance training activates mechanisms that enable a rescue of dysfunctional cardiomyocyte Ca2+ handling and thereby ameliorate cardiac dysfunction despite continuous and chronic elevated levels of CaMKIIδC. CaMKIIδC transgenic (TG) and wild-type (WT) mice performed aerobic interval exercise training over 6 wk. Cardiac function was measured by echocardiography in vivo, and cardiomyocyte shortening and intracellular Ca2+ handling were measured in vitro. TG mice had reduced global cardiac function, cardiomyocyte shortening (47% reduced compared with WT, P &#60; 0.01), and impaired Ca2+ homeostasis. Despite no change in the chronic elevated levels of CaMKIIδC, exercise improved global cardiac function, restored cardiomyocyte shortening, and reestablished Ca2+ homeostasis to values not different from WT. The key features to explain restored Ca2+ homeostasis after exercise training were increased L-type Ca2+ current density and flux by 79 and 85%, respectively (P &#60; 0.01), increased sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a) function by 50% (P &#60; 0.01), and reduced diastolic SR Ca2+ leak by 73% (P &#60; 0.01), compared with sedentary TG mice. In conclusion, exercise training improves global cardiac function as well as cardiomyocyte function in the presence of a maintained high CaMKII activity. The main mechanisms of exercise-induced improvements in TG CaMKIIδC mice are mediated via increased L-type Ca2+ channel currents and improved SR Ca2+ handling by restoration of SERCA2a function in addition to reduced diastolic SR Ca2+ leak

Chronic myocardial infarction promotes atrial action potential alternans, afterdepolarisations and fibrillation

Aims: Atrial fibrillation (AF) is increased in patients with heart failure resulting from myocardial infarction (MI). We aimed to determine the effects of chronic ventricular MI in rabbits on the susceptibility to AF, and underlying atrial electrophysiological and Ca2+-handling mechanisms. Methods and results: In Langendorff-perfused rabbit hearts, under beta-adrenergic-stimulation with isoproterenol (1 µM; ISO), 8 weeks MI decreased AF threshold, indicating increased AF-susceptibility. This was associated with increased atrial action potential duration-alternans at 90% repolarisation, by 147%, and no significant change in mean APD or atrial global conduction velocity (n=6-13 non-MI hearts, 5-12 MI). In atrial isolated myocytes, also under beta-stimulation, L-type Ca2+ current (ICaL) density and intracellular Ca2+-transient amplitude were decreased by MI, by 35% and 41%, respectively, and the frequency of spontaneous depolarisations (SDs) was substantially increased. MI increased atrial myocyte size and capacity, and markedly decreased transverse-tubule density. In non-MI hearts perfused with ISO, the ICaL-blocker nifedipine, at a concentration (0.02 µM) causing an equivalent ICaL-reduction (35%) to that from the MI, did not affect AF-susceptibility, and decreased APD. Conclusion: chronic MI in rabbits remodels atrial structure, electrophysiology and intracellular Ca2+-handling. Increased susceptibility to AF by MI, under beta-adrenergic-stimulation, may result from associated production of atrial APD-alternans and SDs, since steady-state APD and global conduction velocity were unchanged under these conditions, and may be unrelated to the associated reduction in whole-cell ICaL. Future studies may clarify potential contributions of local conduction changes, and cellular and sub-cellular mechanisms of alternans, to the increased AF-susceptibility

Moderate but not severe hypothermia causes pro-arrhythmic changes in cardiac electrophysiology

Aims: Treatment of arrhythmias evoked by hypothermia/rewarming remains challenging, and the underlying mechanisms are unclear. This in vitro experimental study assessed cardiac electrophysiology in isolated rabbit hearts at temperatures occurring in therapeutic and accidental hypothermia. Methods and results: Detailed ECG, surface electrogram, and panoramic optical mapping were performed in isolated rabbit hearts cooled to moderate (31°C) and severe (17°C) hypothermia. Ventricular activation was unchanged at 31°C while action potential duration (APD) was significantly prolonged (176.9 ± 4.2 ms vs. 241.0 ± 2.9 ms, P &lt; 0.05), as was ventricular repolarization. At 17°C, there were proportionally similar delays in both activation and repolarization. These changes were reflected in the QRS and QT intervals of ECG recordings. Ventricular fibrillation threshold was significantly reduced at 31°C (16.3 ± 3.1 vs. 35 ± 3.5 mA, P &lt; 0.05) but increased at 17°C (64.2 ± 9.9, P &lt; 0.05). At 31°C, transverse conduction was relatively unchanged by cooling compared to longitudinal conduction, but at 17°C both transverse and longitudinal conduction were proportionately reduced to a similar extent. The gap junction uncoupler heptanol had a larger relative effect on transverse than longitudinal conduction and was able to restore the transverse/longitudinal conduction ratio, returning ventricular fibrillation threshold to baseline values (16.3 ± 3.1 vs. 36.3 ± 4.3 mA, P &lt; 0.05) at 31°C. Rewarming to 37°C restored the majority of the electrophysiological parameters. Conclusions: Moderate hypothermia does not significantly change ventricular conduction time but prolongs repolarization and is pro-arrhythmic. Further cooling to severe hypothermia causes parallel changes in ventricular activation and repolarization, changes which are anti-arrhythmic. Therefore, relative changes in QRS and QT intervals (QR/QTc) emerge as an ECG-biomarker of pro-arrhythmic activity. Risk for ventricular fibrillation appears to be linked to the relatively low temperature sensitivity of ventricular transmural conduction, a conclusion supported by the anti-arrhythmic effect of heptanol at 31°C

Deleterious effects of phosphate on vascular and endothelial function via disruption to the nitric oxide pathway

Background: Hyperphosphataemia is an independent risk factor for accelerated cardiovascular disease in chronic kidney disease (CKD), although the mechanism for this is poorly understood. We investigated the effects of sustained exposure to a high-phosphate environment on endothelial function in cellular and preclinical models, as well as in human subjects. Methods: Resistance vessels from rats and humans (± CKD) were incubated in a normal (1.18 mM) or high (2.5 mM) phosphate concentration solution and cells were cultured in normal- (0.5 mM) or high-phosphate (3 mM) concentration media. A single-blind crossover study was performed in healthy volunteers, receiving phosphate supplements or a phosphate binder (lanthanum), and endothelial function measured was by flow-mediated dilatation. Results: Endothelium-dependent vasodilatation was impaired when resistance vessels were exposed to high phosphate; this could be reversed in the presence of a phosphodiesterase-5-inhibitor. Vessels from patients with CKD relaxed normally when incubated in normal-phosphate conditions, suggesting that the detrimental effects of phosphate may be reversible. Exposure to high-phosphate disrupted the whole nitric oxide pathway with reduced nitric oxide and cyclic guanosine monophosphate production and total and phospho endothelial nitric oxide synthase expression. In humans, endothelial function was reduced by chronic phosphate loading independent of serum phosphate, but was associated with higher urinary phosphate excretion and serum fibroblast growth factor 23. Conclusions: These directly detrimental effects of phosphate, independent of other factors in the uraemic environment, may explain the increased cardiovascular risk associated with phosphate in CKD

Parental separation and the formation of economic preferences

We estimate the effect of parental separation on the risk and trust attitudes of German adolescents using a large household survey dataset, which allows us to match respondents to their siblings and parents. Our results indicate that adolescents from separated families are less trusting but have the same risk tolerance as adolescents from non-separated families, even after conditioning on the attitudes of parents and other controls. This trust deficit persists into early adulthood. Moreover, for both trust and risk, we find that separation attenuates the transmission of preferences from father to child. Additional analyses point to reduced parental involvement and greater family conflict as potential mechanisms