Effects of hypertrophy on regional action potential characteristics in the rat left ventricle: a cellular basis for T-wave inversion?

Spatial uniformity of action potentials indicates base-to-apex depolarization and repolarization of rainbow trout (Oncorhynchus mykiss) ventricle

The spatial pattern of electrical activation is crucial for a full understanding of fish heart function. However, it remains unclear whether there is regional variation in action potential (AP) morphologies and underlying ion currents. Because the direction of depolarization and spatial differences in the durations of ventricular APs set limits to potential patterns of ventricular repolarization, we determined AP morphologies, underlying ion currents, and ion channel expression in 4 different regions (spongy myocardium; and apex, base, and middle of the compact myocardium), and correlated them with in vivo electrocardiogram (ECG) in rainbow trout (Oncorhynchus mykiss). ECG recorded from 3 leads indicated that the depolarization and repolarization of AP propagate from base-to-apex, and the main depolarization axis of the ventricle is between +90° and +120°. AP shape was uniform across the whole ventricle, and little regional differences were found in density of repolarizing K+ currents or depolarizing Ca2+ and Na+ currents and the underlying transcripts of ion channels, providing compelling evidence for the suggested excitation pattern. The spatial uniformity of AP durations and base-to-apex propagation of activation with a relatively slow velocity of propagation indicates no special ventricular conduction pathway in the trout ventricle like the His-Purkinje system of mammalian hearts. The sequence of repolarization is solely determined by activation time without being affected by regional differences in AP duration.

Action potential responses to changes in stimulation frequency and isoproterenol in rat ventricular myocytes

PURPOSE

Current understanding of ventricular action potential adaptation to physiological stress is generally based on protocols using non-physiological rates and conditions isolating rate effects from escalating adrenergic stimulation. To permit refined understanding, ventricular action potentials were assessed across physiological pacing frequencies in the presence and absence of adrenergic stimuli. Isolated and combined effects were analyzed to assess their ability to replicate in-vivo responses.

METHODS

Steady-state action potentials from ventricular myocytes isolated from male Wistar rats (3 months; N = 8 animals) were recorded at 37°C with steady-state pacing at 1, 2, 4, 6, 8 and 10 Hz using whole-cell patch-clamp. Action potential repolarization to 25, 50, 75, 90 and 100% of full repolarization (APD25-100 ) was compared before and after 5 nM, 100 nM and 1 µM isoproterenol doses.

RESULTS

A Repeated measures ANOVA found APD50-90  shortened with 5 nM isoproterenol infusion by 6-25% (but comparable across doses) (p ≤ 0.03). Pacing frequencies emulating a normal rat heart rate (6 Hz) prolonged APD50 23% compared with 1 Hz pacing. Frequencies emulating exercise or stress (10 Hz) shortened APD90 (29%).

CONCLUSION

These results demonstrate modest action potential shortening in response to adrenergic stimulation and elevations in pacing beyond physiological resting rates. Our findings indicate changes in action potential plateau and late repolarization predominantly underlie simulated exercise responses in the rat heart. This work provides novel action potential reference data and will help model cardiac responses to physiological stimuli in the rat heart via computational techniques.

Left Ventricular Hypertrophy Increases Susceptibility to Bupivacaine-induced Cardiotoxicity through Overexpression of Transient Receptor Potential Canonical Channels in Rats

BACKGROUND

Local anesthetics, particularly potent long acting ones such as bupivacaine, can cause cardiotoxicity by inhibiting sodium ion channels; however, the impact of left ventricular hypertrophy on the cardiotoxicity and the underlying mechanisms remain undetermined. Transient receptor potential canonical (TRPC) channels are upregulated in left ventricular hypertrophy. Some transient receptor potential channel subtypes have been reported to pass relatively large cations, including protonated local anesthetics; this is known as the "pore phenomenon." The authors hypothesized that bupivacaine-induced cardiotoxicity is more severe in left ventricular hypertrophy due to upregulated TRPC channels.

METHODS

The authors used a modified transverse aortic constriction model as a left ventricular hypertrophy. Cardiotoxicity caused by bupivacaine was compared between sham and aortic constriction male rats, and the underlying mechanisms were investigated by recording sodium ion channel currents and immunocytochemistry of TRPC protein in cardiomyocytes.

RESULTS

The time to cardiac arrest by bupivacaine was shorter in aortic constriction rats (n =11) than in sham rats (n = 12) (mean ± SD, 1,302 ± 324 s vs. 1,034 ± 211 s; P = 0.030), regardless of its lower plasma concentration. The half-maximal inhibitory concentrations of bupivacaine toward sodium ion currents were 4.5 and 4.3 μM, which decreased to 3.9 and 2.6 μM in sham and aortic constriction rats, respectively, upon coapplication of 1-oleoyl-2-acetyl-sn-glycerol, a TRPC3 channel activator. In both groups, sodium ion currents were unaffected by QX-314, a positively charged lidocaine derivative, that hardly permeates the cell membrane, but was significantly decreased with QX-314 and 1-oleoyl-2-acetyl-sn-glycerol coapplication (sham: 79 ± 10% of control; P = 0.004; aortic constriction: 47± 27% of control; P = 0.020; n = 5 cells per group). Effects of 1-oleoyl-2-acetyl-sn-glycerol were antagonized by a specific TRPC3 channel inhibitor.

CONCLUSIONS

Left ventricular hypertrophy exacerbated bupivacaine-induced cardiotoxicity, which could be a consequence of the "pore phenomenon" of TRPC3 channels upregulated in left ventricular hypertrophy.

EDITOR’S PERSPECTIVE

The rationale and emergence of electroconductive biomaterial scaffolds in cardiac tissue engineering

The human heart possesses minimal regenerative potential, which can often lead to chronic heart failure following myocardial infarction. Despite the successes of assistive support devices and pharmacological therapies, only a whole heart transplantation can sufficiently address heart failure. Engineered scaffolds, implantable patches, and injectable hydrogels are among the most promising solutions to restore cardiac function and coax regeneration; however, current biomaterials have yet to achieve ideal tissue regeneration and adequate integration due a mismatch of material physicochemical properties. Conductive fillers such as graphene, carbon nanotubes, metallic nanoparticles, and MXenes and conjugated polymers such as polyaniline, polypyrrole, and poly(3,4-ethylendioxythiophene) can possibly achieve optimal electrical conductivities for cardiac applications with appropriate suitability for tissue engineering approaches. Many studies have focused on the use of these materials in multiple fields, with promising effects on the regeneration of electrically active biological tissues such as orthopedic, neural, and cardiac tissue. In this review, we critically discuss the role of heart electrophysiology and the rationale toward the use of electroconductive biomaterials for cardiac tissue engineering. We present the emerging applications of these smart materials to create supportive platforms and discuss the crucial role that electrical stimulation has been shown to exert in maturation of cardiac progenitor cells.

Experimental model of acute myocardial infarction for evaluation of prevention and rehabilitation strategies in cardiovascular diseases – a pilot study

Introduction: Acute myocardial infarction (AMI) is an important acute disease of myocardial tissue, that occurs as a result of an imbalance between coronary blood supply and myocardial demand. Isoproterenol (ISO) is a synthetic catecholamine, a beta-adrenergic agonist that produces extensive biochemical, functional, and histological alterations in the heart, characteristic for AMI. The present study has been designed to identify the best dose of ISO that induces electrocardiogram (ECG) alterations, enzymatic reaction, and histopathological changes characteristic of AMI. Material and method: AMI was induced to Wistar-Bratislava white male rats, using three different subcutaneous doses of ISO (85 mg/kg bw, 100 mg/kg bw, and 150 mg/kg bw). ISO was administrated twice, with the second dose at 24h after the initial one. The ECGs were recorded at 24 hours after the last dose of ISO. Blood samples were collected for measurement of creatine kinase (CK), and CK-MB serum levels, and the hearts were excised and prepared for histopathologic examination. Results and discussions: All doses of ISO induced alterations in the ECG patterns such as increased heart rate and prolongation of QT and QTc intervals. Depression of the ST segment coupled with marked T wave inversion were observed at the doses of 100 mg/kg bw and 150 mg/kg bw of ISO. All doses of ISO induced an elevation of CK and CK-MB with highest levels observed for the dose of 150 mg/kg bw. Histopathologic examination revealed subendocardial AMI lesions for all doses tested. Conclusions: ISO in doses of 100 mg/kg and 150 mg/kg is useful for induction of infarct-like lesion on ECG, increased levels of myocardial necrosis enzymes and morphological changes characteristic for AMI.

Curcumin Nanoparticles Protect against Isoproterenol Induced Myocardial Infarction by Alleviating Myocardial Tissue Oxidative Stress, Electrocardiogram, and Biological Changes

Curcumin from Curcuma longa is a nutraceutical compound reported to possess strong antioxidant activity that makes it a candidate for use in counteracting oxidative stress-induced damage. The effect of pre-treatment with curcumin nanoparticles (nC) compared to conventional curcumin (Cs) on blood pressure, electrocardiogram, and biological changes on isoproterenol (ISO)-induced myocardial infarction (MI) in rats had been investigated. The Cs doses of 150 and 200 mg/kg bw and all nC doses (100, 150 and 200 mg/kg bw) significantly reduced heart rate before ISO administration and prevented QRS complex enlargement after MI induction (p < 0.026). All doses of Cs and nC prevented prolongation of the QT and QT corrected (QTc) intervals, with better results for higher doses (p < 0.048). The nC solution had more significant results than Cs in all metabolic parameters assessed (lactate dehydrogenase, glycaemia, aspartate transaminase, and alanine transaminase, p < 0.009). nC was more efficient than Cs in limiting myocardial oxidative stress and enhancing antioxidative capacity (p < 0.004). Compared to Cs, nC better prevented myocardial damage extension, reduced interstitial oedema, and inflammation. Curcumin nanoparticles as compared to conventional curcumin exert better antioxidative effects. Moreover, nC better prevent cardiomyocytes damage, and electrocardiogram alterations, in the case of ISO-induced MI in rats.

Regional effects of streptozotocin-induced diabetes on shortening and calcium transport in epicardial and endocardial myocytes from rat left ventricle

In the heart, the left ventricle pumps blood at higher pressure than the right ventricle. Within the left ventricle, the electromechanical properties of ventricular cardiac myocytes vary transmurally and this may be related to the gradients of stress and strain experienced in vivo across the ventricular wall. Diabetes is also associated with alterations in hemodynamic function. The aim of this study was to investigate shortening and Ca²⁺ transport in epicardial (EPI) and endocardial (ENDO) left ventricular myocytes in the streptozotocin (STZ)‐induced diabetic rat. Shortening, intracellular Ca²⁺ and L‐type Ca²⁺ current (I_Ca,L) were measured by video detection, fura‐2 microfluorimetry, and whole‐cell patch clamp techniques, respectively. Time to peak (TPK) shortening was prolonged to similar extents in ENDO and EPI myocytes from STZ‐treated rats compared to ENDO and EPI myocytes from controls. Time to half (THALF) relaxation of shortening was prolonged in ENDO myocytes from STZ‐treated rats compared to ENDO controls. TPK Ca²⁺ transient was prolonged in ENDO myocytes from STZ‐treated rats compared to ENDO controls. THALF decay of the Ca²⁺ transient was prolonged in ENDO myocytes from STZ‐treated rats compared to ENDO controls. Sarcoplasmic reticulum (SR) fractional release of Ca²⁺ was reduced in EPI myocytes from STZ‐treated rats compared to EPI controls. I_Ca,L activation, inactivation, and recovery from inactivation were not significantly altered in EPI and ENDO myocytes from STZ‐treated rats or controls. Regional differences in Ca²⁺ transport may partly underlie differences in ventricular myocyte shortening across the wall of the healthy and the STZ‐treated rat left ventricle.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Electronegative LDL-mediated cardiac electrical remodeling in a rat model of chronic kidney disease

The mechanisms underlying chronic kidney disease (CKD)–associated higher risks for life-threatening ventricular tachyarrhythmias remain poorly understood. In rats subjected to unilateral nephrectomy (UNx), we examined cardiac electrophysiological remodeling and relevant mechanisms predisposing to ventricular arrhythmias. Adult male Sprague-Dawley rats underwent UNx (n = 6) or sham (n = 6) operations. Eight weeks later, the UNx group had higher serum blood urea nitrogen and creatinine levels and a longer electrocardiographic QTc interval than did the sham group. Patch-clamp studies revealed epicardial (EPI)-predominant prolongation of the action potential duration (APD) at 50% and 90% repolarization in UNx EPI cardiomyocytes compared to sham EPI cardiomyocytes. A significant reduction of the transient outward potassium current (I_to) in EPI but not in endocardial (ENDO) cardiomyocytes of UNx rats led to a decreased transmural gradient of I_to. The reduction of I_to currents in UNx EPI cardiomyocytes was secondary to downregulation of KChIP2 but not Kv4.2, Kv4.3, and Kv1.4 protein expression. Incubation of plasma electronegative low-density lipoprotein (LDL) from UNx rats with normal EPI and ENDO cardiomyocytes recapitulated the electrophysiological phenotype of UNx rats. In conclusion, CKD disrupts the physiological transmural gradient of I_to via downregulation of KChIP2 proteins in the EPI region, which may promote susceptibility to ventricular tachyarrhythmias. Electronegative LDL may underlie downregulation of KChIP2 in CKD.

Chronic enalapril treatment increases transient outward potassium current in cardiomyocytes isolated from right ventricle of spontaneously hypertensive rats

It has been well established that chronic pressure overload resulting from hypertension leads to ventricular hypertrophy and electrophysiological remodeling. The transient outward potassium current (I _to) reduction described in hypertensive animals delays ventricular repolarization, leading to complex ventricular arrhythmias and sudden death. Antihypertensive drugs, as angiotensin-converting enzyme inhibitors (ACEi), can restore I _to and reduce the incidence of arrhythmic events. The purpose of this study was to evaluate the differential effects of long-term treatment with ACEi or direct-acting smooth muscle relaxant on the I _to of left and right ventricle myocytes of spontaneously hypertensive rats (SHR). Animals were divided into four groups: normotensive Wistar-Kyoto rats (WKY), hypertensive (SHR), SHR treated for 6 weeks with enalapril 10 mg/kg/day (SHRE), or hydralazine 20 mg/kg/day (SHRH). Systolic blood pressure (SBP) and hypertrophy index (heart weight/body weight (HW/BW)) were determined at the end of treatment period. Cell membrane capacitance (C _m) and I _to were assessed in cardiomyocytes isolated from left and right ventricles. The SHR exhibited significantly increased SBP and HW/BW when compared to the WKY. The treated groups, SHRE and SHRH, restored normal SBP but not HW/BW. The SHR group exhibited a diminished I _to in the left but not the right ventricle. Both the treated groups restored I _to in the left ventricle. However, in the right ventricle, only enalapril treatment modified I _to. The SHRE group exhibited a significant increase in I _to compared to all the other groups. These findings suggest that enalapril may increase I _to by a pressure overload independent mechanism.

Efficacy of female rat models in translational cardiovascular aging research

Cardiovascular disease is the leading cause of death in women in the United States. Aging is a primary risk factor for the development of cardiovascular disease as well as cardiovascular-related morbidity and mortality. Aging is a universal process that all humans undergo; however, research in aging is limited by cost and time constraints. Therefore, most research in aging has been done in primates and rodents; however it is unknown how well the effects of aging in rat models translate into humans. To compound the complication of aging gender has also been indicated as a risk factor for various cardiovascular diseases. This review addresses the systemic pathophysiology of the cardiovascular system associated with aging and gender for aging research with regard to the applicability of rat derived data for translational application to human aging.

Effect of fosinopril on the transient outward potassium current of hypertrophied left ventricular myocardium in the spontaneously hypertensive rat

To investigate fosinopril's effect on the transient outward potassium current (Ito) of differing degrees of hypertrophied myocytes in the spontaneously hypertensive rat (SHR). Ten- and 24-week-old SHRs were used as models for cardiac hypertrophy. Hypertrophied ventricular myocytes were exposed to 1, 10, and 100 μmol/L fosinopril; the whole-cell patch-clamp technique was used to study the effects on the transient outward potassium current. Ito current density was decreased in SHR myocytes relative to controls (14.17 ± 0.31 and 11.62 ± 0.08 pA/pF in 10- and 24-week-old SHR versus 16.73 ± 0.15 pA/pF, p < 0.01). Higher concentrations of fosinopril (10 and 100 μmol/L) increased Ito peak current density in 10-week-old SHR myocytes compared with controls (14.92 ± 0.14 and 15.27 ± 0.13 pA/pF versus 14.17 ± 0.31 pA/pF, p < 0.01). Fosinopril increased Ito peak current density in 24-week-old SHR myocytes at all doses (12.70 ± 0.07, 13.74 ± 0.10, and 14.53 ± 0.13 versus 11.62 ± 0.08 pA/pF for controls, p < 0.01). Fosinopril had a greater Ito elevation potential on hypertrophied myocytes in 24-week-old compared with 10-week-old SHR for each dose (1.08 ± 0.09 versus 0.37 ± 0.26 pA/pF, p < 0.01; 2.13 ± 0.05 versus 0.75 ± 0.35 pA/pF, p < 0.01; 2.92 ± 0.07 versus 1.10 ± 0.40 pA/pF, p < 0.01). Fosinopril increased Ito current density in hypertrophied myocytes. This effect was more pronounced in myocytes with a greater degree of hypertrophy.

Altered spatial calcium regulation enhances electrical heterogeneity in the failing canine left ventricle: implications for electrical instability

Myocytes across the left ventricular (LV) wall of the mammalian heart are known to exhibit heterogeneity of electrophysiological properties; however, the transmural variation of cellular electrophysiology and Ca2+ homeostasis in the failing LV is incompletely understood. We studied action potentials (APs), the L-type calcium (Ca2+) current ( ICa,L), and intracellular Ca2+ transients ([Ca2+]i) of subendocardial (Endo), midmyocardial (Mid), and subepicardial (Epi) tissue layers in the canine normal and tachycardia pacing-induced failing left ventricles. Heart failure (HF) was associated with significant prolongation of the AP duration in Mid myocytes. There were no differences in ICa,L density in normal Endo, Mid, and Epi myocytes, whereas in the failing heart, ICa,L density was downregulated by 45% and 26% (at +10 mV) in Endo and Mid myocytes, respectively. The rates of sarcoplasmic reticulum (SR) Ca2+ release and decay of the [Ca2+]i were slowed, and the amplitude of the [Ca2+]i was depressed in Endo and Epi myocytes isolated from failing, compared with normal, hearts. Experiments in sodium (Na+)-free solutions showed that Epi and Mid myocytes of the failing ventricle exhibit a greater reliance on the Na+-Ca2+ exchanger to remove cytosolic Ca2+ than myocytes isolated from normal hearts. Simulation studies in Endo, Mid, and Epi canine myocytes demonstrate the importance of L-type current density and SR Ca2+ uptake in modulating the potentially arrhythmogenic repolarization in HF. In conclusion, these results demonstrate that spatially heterogeneous decreases in ICa,L and defective cytosolic Ca2+ removal contribute to the altered [Ca2+]i and AP profiles across the canine failing LV. These distinct electrophysiological features in myocytes from a failing heart contribute to a characteristic electrogram arising from increased dispersion of refractoriness across the LV, which may result in significant arrhythmogenic sequellae.

Simvastatin modulates remodeling of Kv4.3 expression in rat hypertrophied cardiomyocytes

Objectives: Hypertrophy has been shown to be associated with arrhythmias which can be caused by abnormal remodeling of the Kv4-family of transient potassium channels. Inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase (statins) have recently been shown to exert pleiotropic protective effects in cardiovascular diseases, including anti-arrhythmias. It is hypothesized that remodeling of Kv4.3 occurs in rat hypertrophied cardiomyocytes and is regulated by simvastatin.

Methods: Male Sprague-Dawley rats and neonatal rat ventricular myocytes (NRVMs) underwent abdominal aortic banding (AAB) for 7 weeks and angiotensin II (AngII) treatment, respectively, to induce cardiac hypertrophy. Kv4.3 expression by NRVMs and myocardium (subepicardial and subendocardial) in the left ventricle was measured. The transient outward potassium current (I_to) of NRVMs was recorded using a whole-cell patch-clamp method.

Results: Expression of the Kv4.3 transcript and protein was significantly reduced in myocardium (subepicardial and subendocardial) in the left ventricle and in NRVMs. Simvastatin partially prevented the reduction of Kv4.3 expression in NRVMs and subepicardial myocardium but not in the subendocardial myocardium. Hypertrophied NRVMs exhibited a significant reduction in the I_to current and this effect was partially reversed by simvastatin.

Conclusions: Simvastatin alleviated the reduction of Kv4.3 expression, I_to currents in hypertrophied NRVMs and alleviated the reduced Kv4.3 expression in subepicardial myocardium from the hypertrophied left ventricle. It can be speculated that among the pleiotropic effects of simvastatin, the anti-arrhythmia effect is partly mediated by its effect on Kv4.3.

Pathophysiology of the cardiac late Na current and its potential as a drug target

A pathological increase in the late component of the cardiac Na(+) current, I(NaL), has been linked to disease manifestation in inherited and acquired cardiac diseases including the long QT variant 3 (LQT3) syndrome and heart failure. Disruption in I(NaL) leads to action potential prolongation, disruption of normal cellular repolarization, development of arrhythmia triggers, and propensity to ventricular arrhythmia. Attempts to treat arrhythmogenic sequelae from inherited and acquired syndromes pharmacologically with common Na(+) channel blockers (e.g. flecainide, lidocaine, and amiodarone) have been largely unsuccessful. This is due to drug toxicity and the failure of most current drugs to discriminate between the peak current component, chiefly responsible for single cell excitability and propagation in coupled tissue, and the late component (I(NaL)) of the Na(+) current. Although small in magnitude as compared to the peak Na(+) current (~1-3%), I(NaL) alters action potential properties and increases Na(+) loading in cardiac cells. With the increasing recognition that multiple cardiac pathological conditions share phenotypic manifestations of I(NaL) upregulation, there has been renewed interest in specific pharmacological inhibition of I(Na). The novel antianginal agent ranolazine, which shows a marked selectivity for late versus peak Na(+) current, may represent a novel drug archetype for targeted reduction of I(NaL). This article aims to review common pathophysiological mechanisms leading to enhanced I(NaL) in LQT3 and heart failure as prototypical disease conditions. Also reviewed are promising therapeutic strategies tailored to alter the molecular mechanisms underlying I(Na) mediated arrhythmia triggers.

Assessment of repolarization heterogeneity for prediction of mortality in cardiovascular disease: peak to the end of the T wave interval and nondipolar repolarization components

BACKGROUND

In the canine wedge preparation, the interval from the peak to the end of the T wave (TpTe) reflects transwedge heterogeneities. Increase of ventricular dispersion of action potential durations has been repeatedly shown to be arrhythmogenic; thus, prolonged TpTe intervals were assumed to reflect increased risk. However, despite attempted extrapolation to clinical electrocardiograms, the appropriateness of this assumption has not been investigated in a large population. In another animal model, nondipolar components of the descending T-wave limb (TWRd) have been shown to correlate with TpTe interval. Although total T-wave nondipolar components (TWRt), believed to reflect heterogeneities during total repolarization, were shown associated with worse outcome of cardiac patients, this has not been investigated for TWRd.

METHODS AND RESULTS

Male cardiovascular patients (n = 813) had digital 12-lead electrocardiograms recorded between 1984 and 1991 and were followed until 2000. Using commercial and previously validated technology, QT intervals, TpTe intervals, TWRd, and TWRt were calculated, heart rate corrected, and compared between survivors and nonsurvivors. Their predictive power was also compared with established markers of mortality risk. In contrast to former reports, TpTe(c) intervals were significantly shorter in nonsurvivors (98.76 ± 20.63 milliseconds vs 103.14 ± 20.87 milliseconds, P = .016) and not predictive of outcome. Although TWRd(c) was significantly higher in nonsurvivors (0.007% ± 0.02% vs 0.005% ± 0.08%, P = .03), it was also not predictive of outcome. Only increased TWRt(c), increased heart rate, and increased age were predictive of death.

CONCLUSIONS

The findings challenge the concept that prolongation of TpTe corresponds to higher risk of death from any cause in every population. Further investigations are needed to confirm that clinically measured TpTe reflects transmural repolarization heterogeneity in all clinical populations and indeed is a useful risk marker.

How different two almost identical action potentials can be: a model study on cardiac repolarization

Spatial heterogeneity in the properties of ion channels generates spatial dispersion of ventricular repolarization, which is modulated by gap junctional coupling. However, it is possible to simulate conditions in which local differences in excitation properties are electrophysiologically silent and only play a role in pathological states. We use a numerical procedure on the Luo-Rudy phase 1 model of the ventricular action potential (AP1) in order to find a modified set of model parameters which generates an action potential profile (AP2) almost identical to AP1. We show that, although the two waveforms elicited from resting conditions as a single AP are very similar and belong to membranes sharing similar passive electrical properties, the modified membrane generating AP2 is a weaker current source than the one generating AP1, has different sensitivity to up/down-regulation of ion channels and to extracellular potassium, and a different electrical restitution profile. We study electrotonic interaction of AP1- and AP2- type membranes in cell pairs and in cable conduction, and find differences in source-sink properties which are masked in physiological conditions and become manifest during intercellular uncoupling or partial block of ion channels, leading to unidirectional block and spatial repolarization gradients. We provide contour plot representations that summarize differences and similarities. The present report characterizes an inverse problem in cardiac cells, and strengthen the recently emergent notion that a comprehensive characterization and validation of cell models and their components are necessary in order to correctly understand simulation results at higher levels of complexity.

The role of transient outward K+ current in electrical remodelling induced by voluntary exercise in female rat hearts

Regular exercise can lead to electrical remodelling of the heart. The cellular mechanisms associated with these changes are not well understood, and are difficult to study in human tissue but are important given that exercise is recommended to the general population. We have investigated the role played by the transient outward K+ current (I(to)) in the changes in electrical activity seen in response to voluntary exercise training in rats. Female rats undertook 6 weeks of voluntary wheel running exercise (TRN) or were sedentary controls (SED). Monophasic action potentials (MAPs) were recorded from the surface of whole hearts. Whole cell patch clamp recordings of I(to); mRNA and protein levels of selected targets in sub-epicardial (EPI) and sub-endocardial myocardium of SED and TRN hearts were compared. In TRN rats, heart weight:body weight was significantly increased and epicardial MAPs significantly prolonged. I(to) density was reduced in TRN EPI myocytes, such that the transmural gradient of I(to) was significantly reduced (P < 0.05). Computer modelling of these changes in I(to) predicted the observed changes in action potential profile. However, transmural gradients in mRNA and protein expression for Kv4.2 or mRNA levels of the Kv4.2 regulators; KChIP2 and Irx-5 were not significantly altered by voluntary exercise. We conclude that voluntary exercise electrical remodelling is caused, at least in part, by a decrease in EPI I(to), possibly because of fewer functional channels in the membrane, which results in a fall in the transmural action potential duration gradient.

Myocardial remodelling induced by repeated low doses of isoproterenol

In the present work, the effect of isoproterenol on the electrical properties of the rat heart and on the cytoarchitecture of the surviving cardiomyocytes was studied. Myocardial remodelling was induced by the daily administration of 5 mg/kg isoproterenol (Iso) for 7 days. Administration resulted in a significant increase (52%) in the ratio of left ventricular weight to body weight. ECG voltage criteria confirmed the presence of left ventricular hypertrophy. QT interval prolongation by 23% and 58% was found in Iso rats and in the corresponding isolated hearts, respectively. Spontaneously beating Iso hearts had a higher incidence of dysrhythmias. The surviving cardiomyocytes showed an irregular shape with cytoplasmic processes rich in ribosomes and rough endoplasmic reticulum. In these regions, myofibril disorganization and mitochondrial fission were observed. A greatly increased incidence of caveolae was seen in the plasma membrane and in the mouth of t-tubules. The membranes of t-tubules showed vesiculation, especially near the dyads. Repeated administration of isoproterenol led to hypertrophy, characterized by the existence of myocytes with simultaneous signs of both mature and postnatally developing cardiomyocytes. Structural microheterogeneities at the level of individual cells may represent one of the factors leading to electrical imbalance in the myocardial tissue remodelled by isoproterenol.

Altered expression of Na+ transporters at the mRNA level in rat normal and hypertrophic myocardium

Intracellular Na(+) ([Na(+)](i)) regulation plays a crucial role in the structural, mechanical, and electrical properties of myocardium. It is assumed that the [Na(+)](i) handling system may differ not only between normal and diseased hearts but also regionally within a heart. To gain new insight concerning disease- and region-dependent differences in the [Na(+)](i)-regulatory system, we investigated mRNA expression of Na+ transporters, the principal determinants of [Na(+)](i). Nonischemic pressure-overloaded hypertrophy was created by suprarenal abdominal aortic constriction of 50% for 7 weeks. mRNA abundances of Na(+)-Ca(2+) exchanger (NCX1), Na(+)-H(+) exchanger (NHE1), Na(+)-K(+)-2Cl(-) exchanger (NKCC1) and Na(+), K(+)-ATPase multigene family(alpha(1), alpha(2), alpha(3), and beta(1) isoforms) were measured by the real-time quantitative polymerase chain reaction method. mRNA abundance of all transporters mediating Na(+) influx (NCX1, NHE1, and NKCC1) was significantly upregulated as compared to normal. In contrast, Na(+)-efflux-mediating transporter (Na(+), K(+)-ATPase) mRNA expression was unaltered between normal and hypertrophic hearts. Losartan, an angiotensin II AT1 receptor antagonist, significantly attenuated upregulation of Na(+)-influx-mediating transporters induced by aortic constriction. The onset of Na(+)-influx-mediating transporter upregulation occurred within 5 days following constriction. In normal and hypertrophied hearts, mRNA of all Na(+)-influx-mediating transporters was expressed in order of abundance as: apex > septum approximately free wall of left ventricles. A transmural gradient in expression was also evident in normal hearts (midcardium > endo- and epicardium), which was attenuated under hypertrophic development. Myocardial hypertrophy is associated with significant changes in the spatial distribution and expression levels of Na(+) transporters. The upregulation of Na influx transporters during hypertrophy may contribute to the remodeling process, modulate contractility and promote arrhythmias.

2-hydroxyoleic acid affects cardiomyocyte [Ca2+]i transient and contractility in a region-dependent manner

Monounsaturated fatty acids such as oleic acid are cardioprotective, modify the physicochemical properties of cardiomyocyte membranes, and affect the electrical stability of these cells by regulating the conductance of ion channels. We have designed a nonhydrolysable oleic acid derivative, 2-hydroxyoleic acid (2-OHOA), which regulates membrane lipid structure and cell signaling, resulting in beneficial cardiovascular effects. We previously demonstrated that 2-OHOA induces PKA activation and PKCalpha translocation to the membrane; both pathways are thought to regulate transient outward K(+) current (I(to)) depending on the stimulus and the species used. This study was designed to investigate the effect of 2-OHOA on isolated cardiomyocytes. We examined the dose- and time-dependent effect of 2-OHOA on cytosolic Ca(2+) concentration ([Ca(2+)](i)) transient and contraction of myocytes isolated from different parts of the rat ventricular myocardium. Although this drug had no effect on [Ca(2+)](i) transient and cell shortening in myocytes isolated from the septum, it increased (up to 95%) [Ca(2+)](i) transient and cell shortening in subpopulations of myocytes from the right and left ventricles. The pattern of the effects of 2-OHOA was similar to that observed following the application of the I(to) blocker 4-aminopyridine, suggesting that the drug may act on this channel. Unlike the effect of 2-OHOA on [Ca(2+)](i) transient and cell shortening, PKCalpha translocation to membranes was not region specific. Thus 2-OHOA-induced effects on [Ca(2+)](i) transients and cell shortening are likely related to reductions in I(to) function, but PKCalpha translocation does not seem to play a role. The present results indicate that 2-OHOA selectively increases myocyte inotropic responsiveness, which could underlie its beneficial cardiovascular effects.

Temporal variability of repolarization in rat ventricular myocytes paced with time-varying frequencies

Adaptation of action potential duration (APD) to pacing cycle length (CL) has been previously characterized in isolated cardiomyocytes for sudden changes in constant CL and for pre-/postmature stimuli following constant pacing trains. However, random fluctuations characterize both physiological sinus rhythm (up to 10% of mean CL) and intrinsic beat-to-beat APD at constant pacing rate. We analysed the beat-to-beat sensitivity of each APD to the preceding CL during constant-sudden, random or linearly changing pacing trains in single patch clamped rat left ventricular myocytes, in the absence of the autonomic and electrotonic effects that modulate rate dependency in the intact heart. Beat-to-beat variability of APD at -60 mV (APD(-60 mV)), quantified as S.D. over 10-beat sequences, increased with corresponding mean APD. When measured as coefficient of variability (CV), APD(-60 mV) variability was inversely proportional to pacing frequency (from 1.2% at 5 Hz to 3.2% at 0.2 Hz). It was increased, at a basic CL (BCL) of 250 ms, by 55% by the L-type calcium current (I(CaL)) blocker nifedipine, and decreased by 23% by the transient-outward potassium current (I(to)) blocker 4-aminopyridine. Variability of APD at BCL of 250 ms prevented the detection of random changes of CL smaller than approximately 5%. Ten per cent random changes in CL were detected as a 40% increase in CV of APD and tended to correlate with it (r = 0.43). Block of I(CaL) depressed this correlation (r = 0.23), whereas block of I(to) significantly increased it (r = 0.67); this was similar with linearly changing CL ramps (ranging +/-10% and +/-20% of 250 ms). We conclude that beat-to-beat APD variability, a major determinant of the propensity for development of arrhythmia in the heart, is present in isolated myocytes, where it is dependent on mean APD and pacing rate. Action potential duration shows a beat-to-beat positive correlation with preceding randomly/linearly changing CL, which can be pharmacologically modulated.

QT peak prolongation is not associated with left ventricular hypertrophy in teenage professional football players

OBJECTIVE

QT peak prolongation is associated with left ventricular hypertrophy (LVH) in patients with hypertension. This study tests the hypothesis that QT peak prolongation correlates with LV mass index in apparently healthy young football players.

METHODS

QT peak and other ECG criteria for LVH were assessed in 117 male professional footballers (mean age 16.4 years +/- SD 0.76). Their left ventricular mass index (LVMI) was assessed by transthoracic echocardiography. Heart rate-corrected QT peak (QTpc) interval was measured in lead I using Bazett's formula. Spearman (2-tailed) test and UNIANOVA was used to assess if there were correlations between QT peak and the various echocardiographic and ECG indices of LVH.

RESULTS

Echocardiographic LVH, defined as LVMI > or = 134 g/m(2), was seen in 79 (70.5%) subjects. ECG-defined LVH was present in 54 (50 %) players by Sokolow-Lyon criteria, in 19 (16 %) players by Romhilt Score, in 5 (4 %) players by Cornell voltage criteria, and in 7 (6 %) players by Cornell product >2436 mm ms. There was no correlation between QT peak (QTpc) and LVMI on echocardiography (Spearman r = 0.058, 2-tailed P = 0.54). In addition, there was no relation between LVH and QTpc of lead I using any of the following ECG criteria: Sokolow-Lyon (P = 0.6), Romhilt (P = 0.3), Cornell voltage (P = 0.8), or Cornell product (P = 0.6).

CONCLUSION

QT peak interval, which is associated with pathological LVH in hypertensive patients and is a measure of risk of cardiac death, does not correlate with LVH characterized by myocyte hypertrophy in young apparently healthy professional footballers.

Enhanced activity of ventricular Na+-HCO3- cotransport in pressure overload hypertrophy

The Na(+)-HCO(3)(-) cotransporter (NBC) plays a key role in intracellular pH (pH(i)) regulation in normal ventricular muscle. However, the state of NBC in nonischemic hypertrophied hearts is unresolved. In this study, we examined functional and molecular properties of NBC in adult rat ventricular myocytes. The cells were enzymatically isolated from both normal and hypertrophied hearts. Ventricular hypertrophy was induced by pressure overload created by suprarenal abdominal aortic constriction of 50% for 7 wk. pH(i) was measured in single cells using the fluorescent pH indicator 2',7'-bis(2-carboxyethyl)5-(6)carboxyfluorescein. Real-time PCR analysis was used to quantitatively assess expression of NBC-encoding mRNA, including SLC4A4 (encoding electrogenic NBC, NBCe1) and SLC4A7 (electroneutral NBC, NBCn1). Our results demonstrate that: 1) mRNA levels of both the electrogenic NBCe1 (SLC4A4) and electroneutral NBCn1 (SLC4A7) forms of NBC were increased by aortic constriction, 2) the onset of NBC upregulation occurred within 3 days after constriction, 3) normal and hypertrophied ventricles displayed regional differences in NBC expression, 4) acid extrusion via NBC (J(NBC)) was increased significantly in hypertrophied myocytes, 5) although acid extrusion via Na(+)/H(+) exchange was also increased in hypertrophied myocytes, the relative enhancement of J(NBC) was larger, 6) membrane depolarization markedly increased J(NBC) in hypertrophied myocytes, and 7) losartan, an ANG II AT(1) receptor antagonist, significantly attenuated the upregulation of both NBCs induced by 3 wk of aortic constriction. Enhanced NBC activity during hypertrophic development provides a mechanism for intracellular Na(+) overload, which may render the ventricles more vulnerable to Ca(2+) overload during ischemia-reperfusion.

Das Herz bei arterieller Hypertonie

The term hypertensive heart disease covers the entities of left ventricular hypertrophy, microangiopathy and endothelial dysfunction resulting in diastolic and systolic dysfunction, arrhythmias and increased cardiovascular risk. From the pathophysiological point of view, this is caused by the hypertrophy of cardiac myocytes, interstitial fibrosis and media hypertrophy of the arterioles. Microangiopathy can be diagnosed as the earliest sign of hypertensive heart disease, with diastolic dysfunction also being found as an early change. In further persisting arterial hypertension left ventricular hypertrophy develops (often asymmetric) and later a systolic dysfunction. Clinically, the patients suffer from angina pectoris, dyspnea and rhythm disorders. Left ventricular hypertrophy is associated with an increased risk of malignant ventricular arrhythmias. Thus, the main therapeutic principle should be antihypertensive therapy with the goal of regression of hypertrophy leading to decreased mortality risk.

Evaluation of Exercise-Induced T Wave Changes in Patients with Idiopathic Dilated Cardiomyopathy Before and After Beta-Blocker Therapy

INTRODUCTION

Ventricular repolarization abnormalities are thought to contribute to lethal ventricular arrhythmias in patients with idiopathic dilated cardiomyopathy (DCM). The purpose of this study was to evaluate exercise-induced T wave changes in DCM patients before and after beta-blocker therapy to investigate repolarization abnormalities.

METHODS AND RESULTS

Treadmill exercise testing was performed in 20 DCM patients and 50 normal subjects. T wave amplitude (TA: baseline to T wave apex; mV) and recovery time (RT: QRS onset to the maximum dV/dt point of the T wave; msec) were measured before and 1 minute after peak exercise. TA was averaged in the right and left precordial leads (TA(V1-3), TA(V4-6)). RT was normalized to the maximum QT interval in the 12-lead ECG and expressed as the %RT (%RT). %RT was also averaged in the precordial leads (%RT(V1-3), %RT(V4-6)). After exercise, TA increased and %RT decreased in both groups. In DCM patients, TA(V1-3) was greater and TA(V4-6) was less than in normal subjects before and after exercise. There was no difference in %RT(V1-3) between the groups, but %RT(V4-6) was greater in DCM patients both before and after exercise. DCM patients repeated the same evaluation after 6 months of oral beta-blocker therapy. Compared with measurements before beta-blocker therapy, TA(V1-3) and %RT(V1-3) did not change. However, TA(V4-6) increased and %RT(V4-6) decreased significantly both before and after exercise.

CONCLUSION

DCM patients showed small TA and large %RT in the left precordial leads at rest as well as after exercise. Chronic beta-blocker therapy in DCM patients normalized these ventricular repolarization abnormalities.

Late sodium current in the pathophysiology of cardiovascular disease: consequences of sodium-calcium overload

Late sodium current in cardiac cells is very small compared with the fast component, but as it flows throughout the action potential it may make a substantial contribution to sodium loading during each cardiac cycle. Late sodium current may contribute to triggering arrhythmia in two ways: by causing repolarisation failure (early after depolarisations); and by triggering late after depolarisations attributable to calcium oscillations in sodium-calcium overload conditions. Reduction of late sodium current would therefore be expected to have therapeutic benefits, particularly in disease states such as ischaemia in which sodium-calcium overload is a major feature.

Dynamical and cellular electrophysiological mechanisms of ECG changes during ischaemia

The interpretation of normal and pathological electrocardiographic (ECG) patterns in terms of the underlying cellular and tissue electrophysiology is rudimentary, as the existing theories rely on geometrical aspects. We relate effects of sub-endocardial ischaemia on the ST-segment depression in ECG to patterns of transmural action potential propagation in a one-dimensional virtual ventricular wall. Our computational study exposes two electrophysiological mechanisms of ST depression: dynamic-predominantly positive spatial gradients in the membrane potential during abnormal repolarization of the wall, produced by action potential duration changes in the ischaemic region; and static-a negative spatial gradient of the resting membrane potential between the normal and ischaemic regions. Hyperkalaemia is the major contributor to both these mechanisms at the cellular level. These results complement simulations of the effects of cardiac geometry on the ECG, and dissect spatio-temporal and cellular electrophysiological mechanisms of ST depression seen in sub-endocardial ischaemia.

Prolonged recovery time in the left precordial leads reflects increased left ventricular mass in the hypertensive patients

Hypertensive left ventricular hypertrophy (LVH) is considered to be a risk for arrhythmogenicity, but the quantification of the changes in T-wave morphology, as the reflection of repolarization abnormality, has not been fully established. The purpose of this study was to quantify the T-wave changes in the hypertensive patients and to investigate the relationship with the increased left ventricular mass. Standard 12-lead electrocardiogram and echocardiogram were recorded in 90 hypertensive patients. Activation time (AT), activation recovery interval (ARI), and recovery time (RT) were measured in the precordial lead and QT interval in the 12 leads. To compare the left precordial T-wave changes among patients, measurements of ARI and RT in the right precordial negative T wave were excluded. Each parameter excluding AT was corrected with Bazett formula, and then the dispersion was calculated. Left ventricular mass index was determined echocardiographically to select non-LVH group (n=31) and LVH group (n=59). In both groups, AT, ARI, and RT in the left precordial leads were larger compared with those in the right precordial leads. Dispersion of AT was not different between the 2 groups. However, the dispersion of ARI and RT in LVH group was significantly greater than that in non-LVH group. There were correlations between left ventricular mass index and the dispersion of RT (r=0.66, P<.001), ARI (r=0.61, P<.001), and 12-lead QT (r=0.42, P<.001). In patients with LVH, significant prolongation of RT in the left precordial leads was observed, suggesting that this RT change resulted from the nonuniformity of epicardial action potential duration.

Ascending aortic stenosis selectively increases action potential-induced Ca2+ influx in epicardial myocytes of the rat left ventricle

A decrease of the transient outward potassium current (Ito) has been observed in cardiac hypertrophy and contributes to the altered shape of the action potential (AP) of hypertrophied ventricular myocytes. Since the shape and duration of the ventricular AP are important determinants of the Ca2+ influx during the AP (QCa), we investigated the effect of ascending aortic stenosis (AS) on QCa in endo- and epicardial myocytes of the left ventricular free wall using the AP voltage-clamp technique. In sham-operated animals, QCa was significantly larger in endocardial compared to epicardial myocytes (803 +/- 65 fC pF(-1), n = 27 vs. 167 +/- 32 fC pF(-1), n = 38, P < 0.001). Ascending aortic stenosis significantly increased QCa in epicardial myocytes (368 +/- 54 fC pF(-1), n = 42, P < 0.05), but did not alter QCa in endocardial myocytes (696 +/- 65 fC pF(-1), n = 26). Peak and current-voltage relation of the AP-induced Ca2+ current were unaffected by AS. However, the time course of the current-voltage relation was significantly prolonged in epicardial myocytes of AS animals. Model calculations revealed that the increase in QCa can be ascribed to a prolonged opening of the activation gate, whereas an increase in inactivation prevents an excessive increase in QCa. In conclusion, AS significantly increased AP-induced Ca2+ influx in epicardial but not in endocardial myocytes of the rat left ventricle.

Intracellular Ca2+ concentration and rate adaptation of the cardiac action potential

Influx of Ca(2+) ions through the cardiac plasma membrane contributes to the shaping of the action potential plateau and acts as trigger for the release of Ca(2+) ions from the sarcoplasmic reticulum and the initiation of the contractile process. The increased intracellular Ca(2+) concentration feeds back on the channels and transporters in the plasma membrane and modulates the electrical activity. This interaction and its change with rate of pacing is the topic of this review, which is subdivided in three parts. In part I a description is given of different channels and transporters that carry Ca(2+) ions, or are activated-modulated by intracellular Ca(2+) ions. In part II an analysis is given of the changes in action potential duration and shape when stimuli are applied in the relative refractory period (electrical restitution) and when rate is suddenly increased and kept at the higher level until steady-state is obtained. A description of experimental findings in each case is followed by a discussion of possible mechanisms. Part III deals with physiopathological aspects of Ca(2+) handling and discusses recent information on hypertrophy, heart failure and atrial fibrillation.

Vulnerability to ventricular arrhythmias [corrected] and heterogeneity of action potential duration in normal rats

In normal rats, we analysed the arrhythmogenic role of intrinsic action potential duration (APD) heterogeneity. In each animal, ventricular arrhythmic events (VAEs) occurring spontaneously and during the exposure to an acute social challenge were telemetrically recorded. Action potentials were recorded from isolated left ventricular myocytes, at a pacing rate of 5 Hz (patch clamp: current-clamp mode). APDs were measured at -20 mV, -30 mV, -40 mV, -50 mV and -60 mV. The difference between the shortest and the longest APD was also computed, as an index of individual APD heterogeneity. Animals predisposed to stress-induced arrhythmias showed higher values of APD and APD heterogeneity as compared with the remaining rats. We concluded that, in the normal heart, a large intrinsic APD heterogeneity resulting from specific electrophysiological properties of ventricular myocytes is not in itself arrhythmogenic, but can predispose towards arrhythmia development under certain conditions, such as autonomic activation.

Regional distribution of hyperpolarization-activated current (If) and hyperpolarization-activated cyclic nucleotide-gated channel mRNA expression in ventricular cells from control and hypertrophied rat hearts

Hyperpolarization-activated inward current (If) and changes in the messenger RNA (mRNA) expression levels of hyperpolarization-activated cyclic nucleotide-gated channel (HCN)2 and HCN4 encoding If channels of the rat heart were studied in control and hypertrophied myocytes isolated from three ventricular regions: the septum (S), the left ventricular free wall (LV) and the right ventricular free wall (RV). Electrophysiological experiments were conducted by ruptured and perforated-patch clamp techniques and quantification of mRNA levels was carried out by quantitative reverse transcriptase polymerase chain reaction. The occurrence, density and maximal specific conductance of If were found to be significantly higher in hypertrophied ventricular myocytes isolated from S and LV than in those isolated from RV or sham-operated rats. Half-maximal activation potential, the slope of the activation curve and the threshold for activation were similar in ventricular myocytes from sham and aortic stenosed rats in the three regions studied. Isoproterenol 1 micromol l-1 increased current size by shifting current activation to more positive potentials in both sham and hypertrophied myocytes. When we studied the mRNA levels of If channel isoforms present in the ventricle, we found a significant increase of HCN2 and HCN4 mRNA levels in hypertrophied myocytes from S and LV but not in RV. We conclude that the occurrence, density and conductance of If is higher in hypertrophied than in control ventricular myocytes, S being the region where all these changes were most evident. These findings are associated with a higher expression of HCN2 and HCN4 mRNA levels in the two regions that developed hypertrophy.

Regional differences in effects of exercise training on contractile and biochemical properties of rat cardiac myocytes

Myocardial function is enhanced by endurance exercise training, but the cellular mechanisms underlying this improved function remain unclear. A number of studies have shown that the characteristics of cardiac myocytes vary across the width of the ventricular wall. We have previously shown that endurance exercise training alters the Ca2+ sensitivity of tension as well as contractile protein isoform expression in rat cardiac myocytes. We tested the hypothesis that these effects of training are not uniform across the ventricular wall but are more pronounced in the subendocardial (Endo) region of the myocardium. Female Sprague-Dawley rats were divided into sedentary control (C) and exercise trained (T) groups. T rats underwent 11 wk of progressive treadmill exercise. Myocytes were isolated from the Endo region of the myocardium and from the subepicardial (Epi) region of both T and C hearts. We found an increase in the Ca2+ sensitivity of tension in T cells compared with C cells, but this difference was larger in the Endo cells than in the Epi cells. In addition, we found a training-induced increase in atrial myosin light chain 1 (aMLC1) expression that was larger in the Endo compared with Epi samples. We conclude that effects of exercise training on myocyte contractile and biochemical properties are greater in myocytes from the Endo region of the myocardium than those from the Epi region. In addition, these results provide evidence that the increase in aMLC1 expression may be responsible for some of the training-induced increase in myocyte Ca2+ sensitivity of tension.

A topographical study of mechanical and electrical properties of single myocytes isolated from normal guinea-pig ventricular muscle

Major regional differences in the electrical properties of myocytes from ventricular muscle have been described previously, on the basis of samples taken from a maximum of three regions in each heart. In order to define the topographical basis for such differences, we studied the electrical and mechanical properties of single myocytes isolated from 20 regions throughout the ventricles in the normal guinea-pig heart. Single myocytes were isolated using an enzymatic dispersion method, and were studied under conditions that were close to physiological. Cell capacitance and action potentials were recorded using the switch-clamp technique, and cell length and evoked shortening were measured using a photodiode array system. In the left ventricular free wall, mid-myocardial cells were longer and had greater capacitative surface area than surface myocytes. There were transmural but not longitudinal differences in APD90 (action potential duration to 90% repolarization), with the longest APD90 in subendocardial and the shortest in subepicardial myocytes. We found a septum-left ventricular free wall-right ventricular free wall gradient, with the longest APD90 in the septum and the shortest in the right ventricular free wall. The regional distribution of APD90 was closely mirrored by relaxation time. Peak cell shortening was greater in subendocardial myocytes than in subepicardial myocytes in the left ventricular free wall, and in myocytes from the left side of the septum compared with the right. We concluded that the regional distribution of APD is closely and inversely related to the sequence of ventricular depolarization, and that the regional variations in cell shortening amplitude are related principally to reported regional variations in wall stress.

The long QT interval is not only inherited but is also linked to cardiac hypertrophy

This review focuses on the molecular determinants of the duration of the QT interval as measured on by electrocardiography in normal subjects and during cardiac hypertrophy and failure. (a) In control conditions, on a single cell, the shape and duration of the action potential is the result of a balance between different ion currents which in turn were determined by the number of functional channels. On multicellular preparations the QT duration also represents the repolarization time; nevertheless it is modified by the transmural gradients. On body-surface electrocardiography the duration of the QT interval depends also of an additional factor: the spatial three-dimensional projection of the electrical waves vectors, which makes any determination of the epicardial dispersion by measuring QT interval dispersion questionable. (b) The enhanced action potential duration is well documented in cardiac hypertrophy and heart failure and is usually caused by a reduction in outward current densities in most of the species except mice. Among these currents I(tO) is the most frequently altered, especially in humans. Such an altered current density is caused by a diminished expression of the genes encoding either the ion channel subunits or regulatory proteins, such as KChIP2. In addition, hypertrophy modifies or even reverses the transmural gradient. In human and rats hypertensive cardiopathy is associated with a prolongation of the QT interval duration. The reduction in I(tO) is likely to be adaptive; it participates in the slowing of the cardiac cycle and reflects the fetal genetic reprogramming. Recent data also suggest that a reduction in the transient outward K(+) current density triggers protein synthesis through an activation of the calcineurin pathways. Thus a prolongation of the QT interval is not only inherited or drug-induced; it is also an essential component of the adaptive process in chronic mechanical overload. It is fundamentally incorrect to measure QT dispersion on a surface electrocardiography, but the mean QT interval may provide information concerning the progression of the disease, just as, and with the same restrictions, in the case of the quantification of V(max).

Myocardial infarction in rat eliminates regional heterogeneity of AP profiles, I(to) K(+) currents, and [Ca(2+)](i) transients

Transient outward K(+) current density (I(to)) has been shown to vary between different regions of the normal myocardium and to be reduced in heart disease. In this study, we measured regional changes in action potential duration (APD), I(to), and intracellular Ca(2+) concentration ([Ca(2+)](i)) transients of ventricular myocytes derived from the right ventricular free wall (RVW) and interventricular septum (SEP) 8 wk after myocardial infarction (MI). At +40 mV, I(to) density in sham-operated hearts was significantly higher (P < 0.01) in the RVW (15.0 +/- 0.8 pA/pF, n = 47) compared with the SEP (7.0 +/- 1.1 pA/pF, n = 18). After MI, I(to) density was not reduced in SEP myocytes but was reduced (P < 0.01) in RVW myocytes (8.7 +/- 1.0 pA/pF, n = 26) to levels indistinguishable from post-MI SEP myocytes. These changes in I(to) density correlated with Kv4.2 (but not Kv4.3) protein expression. By contrast, Kv1.4 expression was significantly higher in the RVW compared with the SEP and increased significantly after MI in RVW. APD measured at 50% or 90% repolarization was prolonged, whereas peak [Ca(2+)](i) transients amplitude was higher in the SEP compared with the RVW in sham myocytes. These regional differences in APD and [Ca(2+)](i) transients were eliminated by MI. Our results demonstrate that the significant regional differences in I(to) density, APD, and [Ca(2+)](i) between RVW and SEP are linked to a variation in Kv4.2 expression, which largely disappears after MI.