In silico analysis of the dynamic regulation of cardiac electrophysiology by Kv 11.1 ion-channel trafficking

Cardiac electrophysiology is regulated by continuous trafficking and internalization of ion channels occurring over minutes to hours. Kv 11.1 (also known as hERG) underlies the rapidly activating delayed-rectifier K+ current (IKr ), which plays a major role in cardiac ventricular repolarization. Experimental characterization of the distinct temporal effects of genetic and acquired modulators on channel trafficking and gating is challenging. Computer models are instrumental in elucidating these effects, but no currently available model incorporates ion-channel trafficking. Here, we present a novel computational model that reproduces the experimentally observed production, forward trafficking, internalization, recycling and degradation of Kv 11.1 channels, as well as their modulation by temperature, pentamidine, dofetilide and extracellular K+ . The acute effects of these modulators on channel gating were also incorporated and integrated with the trafficking model in the O'Hara-Rudy human ventricular cardiomyocyte model. Supraphysiological dofetilide concentrations substantially increased Kv 11.1 membrane levels while also producing a significant channel block. However, clinically relevant concentrations did not affect trafficking. Similarly, severe hypokalaemia reduced Kv 11.1 membrane levels based on long-term culture data, but had limited effect based on short-term data. By contrast, clinically relevant elevations in temperature acutely increased IKr due to faster kinetics, while after 24 h, IKr was decreased due to reduced Kv 11.1 membrane levels. The opposite was true for lower temperatures. Taken together, our model reveals a complex temporal regulation of cardiac electrophysiology by temperature, hypokalaemia, and dofetilide through competing effects on channel gating and trafficking, and provides a framework for future studies assessing the role of impaired trafficking in cardiac arrhythmias. KEY POINTS: Kv 11.1 channels underlying the rapidly activating delayed-rectifier K+ current are important for ventricular repolarization and are continuously shuttled from the cytoplasm to the plasma membrane and back over minutes to hours. Kv 11.1 gating and trafficking are modulated by temperature, drugs and extracellular K+ concentration but experimental characterization of their combined effects is challenging. Computer models may facilitate these analyses, but no currently available model incorporates ion-channel trafficking. We introduce a new two-state ion-channel trafficking model able to reproduce a wide range of experimental data, along with the effects of modulators of Kv 11.1 channel functioning and trafficking. The model reveals complex dynamic regulation of ventricular repolarization by temperature, extracellular K+ concentration and dofetilide through opposing acute (millisecond) effects on Kv 11.1 gating and long-term (hours) modulation of Kv 11.1 trafficking. This in silico trafficking framework provides a tool to investigate the roles of acute and long-term processes on arrhythmia promotion and maintenance.

Late complications of an atrial septal occluder provoked by anticoagulant therapy

Late complications of an atrial septal occluder device (ASO) are rare but may be serious. We report a case with extensive hemopericardium five years after ASO implantation most likely triggered by anticoagulant therapy. Although not surgically confirmed, indirect clues for erosion of the atrial wall by the device were the exclusion of other etiologies, lack of recurrence after pericardial drainage and withdrawal of anticoagulants. In addition, multimodality imaging using echocardiography, computed tomography, and cardiac magnetic resonance imaging were helpful to elucidate this unusual cause. Initiation of anticoagulant treatment in patients with an ASO should be carefully balanced and may warrant more frequent echocardiographic follow-up. .

Enhanced Ca(2+) release and Na/Ca exchange activity in hypertrophied canine ventricular myocytes: potential link between contractile adaptation and arrhythmogenesis

BACKGROUND

Ventricular arrhythmias are a major cause of sudden death in patients with heart failure and hypertrophy. The dog with chronic complete atrioventricular block (CAVB) has biventricular hypertrophy and ventricular arrhythmias and is a useful model to study underlying cellular mechanisms. We investigated whether changes in Ca(2+) homeostasis are part of the contractile adaptation to CAVB and might contribute to arrhythmogenesis.

METHODS AND RESULTS

In enzymatically isolated myocytes, cell shortening, Ca(2+) release from the sarcoplasmic reticulum (SR), and SR Ca(2+) content were enhanced at low stimulation frequencies. Ca(2+) influx through L-type Ca(2+) channels was unchanged, but Ca(2+) influx via the Na/Ca exchanger was increased and contributed to Ca(2+) loading of the SR. Inward Na/Ca exchange currents were also larger. Changes in Ca(2+) fluxes were less pronounced in the right versus left ventricle.

CONCLUSIONS

Enhanced Na/Ca exchange activity may improve contractile adaptation to CAVB but at the same time facilitate arrhythmias by (1) increasing the propensity to Ca(2+) overload, (2) providing more inward current leading to (nonhomogeneous) action potential prolongation, and (3) enhancing (arrhythmogenic) currents during spontaneous Ca(2+) release.

Downregulation of delayed rectifier K(+) currents in dogs with chronic complete atrioventricular block and acquired torsades de pointes

BACKGROUND

Acquired QT prolongation enhances the susceptibility to torsades de pointes (TdP). Clinical and experimental studies indicate ventricular action potential prolongation, increased regional dispersion of repolarization, and early afterdepolarizations as underlying factors. We examined whether K(+)-current alterations contribute to these proarrhythmic responses in an animal model of TdP: the dog with chronic complete atrioventricular block (AVB) and biventricular hypertrophy.

METHODS AND RESULTS

The whole-cell K(+) currents I(TO1), I(K1), I(Kr), and I(Ks) were recorded in left (LV) and right (RV) ventricular midmyocardial cells from dogs with 9+/-1 weeks of AVB and controls with sinus rhythm. I(TO1) density and kinetics and I(K1) outward current were not different between chronic AVB and control cells. I(Kr) had a similar voltage dependence of activation and time course of deactivation in chronic AVB and control. I(Kr) density was similar in LV myocytes but smaller in RV myocytes (-45%) of chronic AVB versus control. For I(Ks), voltage-dependence of activation and time course of deactivation were similar in chronic AVB and control. However, I(Ks) densities of LV (-50%) and RV (-55%) cells were significantly lower in chronic AVB than control.

CONCLUSIONS

Significant downregulation of delayed rectifier K(+) current occurs in both ventricles of the dog with chronic AVB. Acquired TdP in this animal model with biventricular hypertrophy is thus related to intrinsic repolarization defects.

Repolarizing K+ currents ITO1 and IKs are larger in right than left canine ventricular midmyocardium

BACKGROUND

The ventricular action potential exhibits regional heterogeneity in configuration and duration (APD). Across the left ventricular (LV) free wall, this is explained by differences in repolarizing K+ currents. However, the ionic basis of electrical nonuniformity in the right ventricle (RV) versus the LV is poorly investigated. We examined transient outward (ITO1), delayed (IKs and IKr), and inward rectifier K+ currents (IK1) in relation to action potential characteristics of RV and LV midmyocardial (M) cells of the same adult canine hearts.

METHODS AND RESULTS

Single RV and LV M cells were used for microelectrode recordings and whole-cell voltage clamping. Action potentials showed deeper notches, shorter APDs at 50% and 95% of repolarization, and less prolongation on slowing of the pacing rate in RV than LV. ITO1 density was significantly larger in RV than LV, whereas steady-state inactivation and rate of recovery were similar. IKs tail currents, measured at -25 mV and insensitive to almokalant (2 micromol/L), were considerably larger in RV than LV. IKr, measured as almokalant-sensitive tail currents at -50 mV, and IK1 were not different in the 2 ventricles.

CONCLUSIONS

Differences in K+ currents may well explain the interventricular heterogeneity of action potentials in M layers of the canine heart. These results contribute to a further phenotyping of the ventricular action potential under physiological conditions.

Cellular basis of biventricular hypertrophy and arrhythmogenesis in dogs with chronic complete atrioventricular block and acquired torsade de pointes

BACKGROUND

In the dog with chronic complete atrioventricular block (AVB), torsade de pointes arrhythmias (TdP) can be induced reproducibly by class III antiarrhythmic agents. In vivo studies reveal important electrophysiological alterations of the heart at 5 weeks of AVB, resulting in increased proarrhythmia. Autopsy studies indicate the presence of biventricular hypertrophy. In this study, the cellular basis of proarrhythmia and hypertrophy in chronic AVB was investigated.

METHODS AND RESULTS

From chronic-AVB dogs with increased heart weights and TdP, left midmyocardial and right ventricular myocytes were isolated by enzymatic dispersion. These myocytes were significantly larger than sinus rhythm (SR) controls. In chronic AVB, the action potential spike-and-dome configuration was preserved. However, the action potential duration (APD) at 95% and 50% of repolarization of the left midmyocardium was significantly larger in chronic AVB than in SR, with little change in the right ventricle, causing enhanced interventricular dispersion of repolarization at slow pacing rates. Treatment with the class III agent almokalant increased the APD to a much larger extent in chronic-AVB than in SR myocytes and resulted in a higher incidence of early afterdepolarizations (EADs). EADs had their takeoff potential between -35 and 0 mV. There was no evidence that spontaneous sarcoplasmic reticulum Ca2+ release underlies these EADs.

CONCLUSIONS

In the dog, chronic AVB leads to hypertrophy of both right and left ventricular myocytes. The repolarization abnormalities predisposing for class III-dependent TdP in vivo are the results of cellular electrophysiological remodeling.

Similarities between early and delayed afterdepolarizations induced by isoproterenol in canine ventricular myocytes

OBJECTIVES

This study aims at clarifying the role of cellular Ca2+ overload and spontaneous sarcoplasmic reticulum (SR) Ca2+ release in the generation of early afterdepolarizations (EAD) by isoproterenol. The involvement of a Ca(2+)-activated membrane current in isoproterenol-induced EAD is investigated.

METHODS

Membrane potential and contraction (an indicator of SR Ca2+ release) were recorded in canine left ventricular myocytes at pacing cycle lengths (CL) of 300-4000 ms. Threshold concentration for EAD was 20-50 mmol/l isoproterenol. Ni2+ (2.0-5.0 mmol/l) was used at normal and high (5.4 mmol/l) [Ca2+]o to examine the role of Ca2+ current and/or Na(+)-Ca2+ exchange (1Na-Ca) in EAD.

RESULTS

In all cells delayed afterdepolarizations (DAD) appeared during isoproterenol. In most (approximately equal to 70%) cells EAD were also generated, which were fast-pacing dependent, occurring only at CL of 400-1000 ms. EAD were always initiated by a delay in repolarization. Early aftercontractions preceded the EAD upstrokes, often occurring without them. They coincided with the initial delays in repolarization. During treatment with isoproterenol, Ni2+ and high [Ca2+]o, EAD and DAD were suppressed despite the continued presence of early and delayed aftercontractions.

CONCLUSIONS

Our data indicate that beta-adrenergic EAD share a common ionic mechanism with DAD in terms of cellular Ca2+ overload and spontaneous SR Ca2+ release. beta-Adrenergic EAD consist of two phases: (1) a conditional phase coinciding with the onset of an early aftercontraction, often followed by (2) an EAD upstroke. A Ca2(+)-activated membrane current, probably I Na-Ca, is necessary at least for the initiation of these EAD.

Development of ST-segment elevation and Q- and R- wave changes in acute myocardial infarction and the influence of thrombolytic therapy

Sequential electrocardiograms for admission to 36 hours in 358 patient s with acute myocardial infarction (AMI) from the Pro-urokinase In Myocardial Infarction trial were assessed. The electrocardiogram was also examined at discharge in 69 of 358 patients. Patients underwent acute angiography, after which angioplasty was performed in most patients with impaired flow. The sum of the ST-segment deviation and Q- and R- wave voltages, and the QRS score were calculated and used for further evaluation. Development of Q waves, lost of R waves, and QRS score were completed within the first 9 hours after onset of AMI and remained stable thereafter. Reperfused patients had earlier stabilization and less severe electrocardiographic (ECG) abnormalities than nonreperfused patients. ST-segment elevation had already stabilized after 5 hours, was unchanged at 36 hours, and had significantly decreased at discharge. No significant ECG and clinical outcome differences were found between the Thrombolysis In Myocardial Infarction trial (TIMI) 2 and TIMI 3 patients. A 23.3% gain in ECG-estimated infarct size was found in the reperfusion group compared with a 12.0% gain in the nonreperfused group (p = 0.08). In summary, as early as 9 hours after onset of AMI, QRS changes were already complete. Thereafter, QRS morphology was stable. Thus, a QRS-based estimation of infarct size can be made as early as 9 hours after AMI. A similar ECG outcome for patients with TIMI 2 and 3 flow was found, which was significantly different from patients with TIMI 0 to 1 flow.

Interstitial collagen is increased in the non-infarcted human myocardium after myocardial infarction

In this study we report the changes of interstitial collagen in the human non-infarcted interventricular septum after a myocardial infarction as well as in hypertrophic human hearts with or without hypertension. The collagen amount was determined with the Sirius Red morphometry technique, which enabled us to perform these studies on routinely processed, paraffin embedded sections. The collagen amount was significantly increased in the septum of infarct patients as compared to non-infarcted controls (P < 0.001). The collagen amount in the septum of the hypertensive hypertrophy group (HH) was significantly increased as compared to the non-hypertensive hypertrophy group (NHH) (P < 0.01). The collagen content in the NHH was not significantly different from the controls in the infarct group, while the collagen amount in the HH showed no significant difference to the collagen content in the infarct group. The results indicate that collagen deposition is increased in the non-infarcted myocardium after a myocardial infarction, as well as in the hypertensive hypertrophied myocardium. The data suggest that the appearance of excessive collagen is not mediated by cardiac hypertrophy per se, but that the underlying cause, infarction or hypertension, is the significant factor.

Fatty acid-binding proteinuria diagnoses myocardial infarction in the rat

Cytoplasmic heart-type fatty acid-binding protein (H-FABPc) is a low molecular weight protein with abundant presence in the myocardium. Upon ischemia it is released from the heart and can subsequently be detected in plasma and urine. In this study, the value of measurement of H-FABPc excreted into urine for the diagnosis of myocardial infarction (MI) is investigated in the rat. To this end, firstly the kinetic behaviour of H-FABPc in plasma was examined and its release into urine quantified. After injection of purified H-FABPc in normal animals, a net recovery in urine of 14-29% was found. The kinetic behaviour of H-FABPc in plasma was characterized by a total clearance of 0.33 ml/min and a half-life value of total elimination of about 270 min. Knowing these plasma characteristics of H-FABPc, a comparison was made between the cumulative amounts of H-FABPc released in the 24-hour urine of MI rats and of sham-operated animals. In MI rats, with a mean morphometric MI size of 43%, the mean total amount of H-FABPc excreted into urine was 79 micrograms, while in sham-operated rats this was 23 micrograms. This difference between both groups is significant (p < 0.001). It is concluded that urinary H-FABPc can be used as a noninvasive marker for MI in the rat.