Different effects of gadolinium on I(KR), I(KS) and I(K1) in guinea-pig isolated ventricular myocytes

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

Gadolinium as an Inhibitor of Ionic Currents in Isolated Rat Ventricular Cardiomyocytes

The whole-cell patch-clamp technique was used to examine the effect of gadolinium Gd³⁺ (a non-specific blocker of mechanically gated current I_MGCh, a component of late current I_L) on ionic currents in insolated rat ventricular cardiomyocytes alone and in combination with the blockers of L-type calcium currents (I_CaL) nifedipine (10 μM) or verapamil (1 μM). In K⁺_in/K⁺_out or Cs⁺_in/Cs⁺_out media, blockade of I_CaL produced no effect on I_L at negative potentials, but inhibited I_L at positive ones. In K⁺_in/K⁺_out medium, Gd³⁺ (5 μM) decreased the net persistent current (I_np) at -45 mV from 198.6±6.4 to 96.7±9.5 pA over 15 min. Gd³⁺ alone or in combination with I_CaL blockers shifted the reversal potential of I_L to more negative values. At negative potentials, Gd³⁺ decreased I_K1 and inward current including I_MGCh. At positive potentials, Gd³⁺ alone or in combination with I_CaL blockers decreased I_L. When applied for 15 min in Cs⁺_in/Cs⁺_out medium at -45 mV, Gd³⁺ produced no effect on net current and inward and outward components of I_L. Thus, Gd³⁺ can be viewed as a specific blocker of I_MGCh only in Cs⁺ medium.

Collagen regulates transforming growth factor-β receptors of HL-1 cardiomyocytes through activation of stretch and integrin signaling

The extracellular matrix (ECM) and transforming growth factor-β (TGF)-β are important in cardiac fibrosis, however, the effects of the ECM on TGF‑β signaling remain to be fully elucidated. The aims of the present study were to evaluate the role of collagen in TGF‑β signaling and examine the underlying mechanisms. In the present study, western blot analysis was used to examine TGF‑β signaling in HL‑1 cells treated with and without (control) type I collagen (10 µg/ml), which was co‑administered with either an anti‑β1 integrin antibody (10 µg/ml) or a stretch‑activated channel inhibitor (gadolinium; 50 µM). Cell proliferation and adhesion assays were used to investigate the roles of integrin, mechanical stretch and mitogen‑activated protein kinases (MAPKs) on cell proliferation and adhesion. The type I collagen (10 µg/ml)‑treated HL‑1 cells were incubated with or without anti‑β1 integrin antibody (10 µg/ml), gadolinium (50 µM) or inhibitors of p38 (SB203580; 3 µM), extracellular signal‑regulated kinase (ERK; PD98059; 50 µM) and c‑Jun N‑terminal kinase (JNK; SP600125; 50 µM). Compared with the control cells, the collagen‑treated HL‑1 cells had lower expression levels of type I and type II TGF‑β receptors (TGFβRI and TGFβRII), with an increase in phosphorylated focal adhesion kinase (FAK), p38 and ERK1/2, and a decrease in JNK. Incubation with the anti‑β1 integrin antibody reversed the collagen‑induced downregulation of the expression of TGFβRII and phosphorylated FAK. Gadolinium downregulated the expression levels of TGFβRI and small mothers against decapentaplegic (Smad)2/3, and decreased the levels of phosphorylated p38, ERK1/2 and JNK. In addition, gadolinium reversed the collagen‑induced activation of p38 and ERK1/2. In the presence of gadolinium and anti‑β1 integrin antibody, collagen regulated the expression levels of TGFβRI, TGFβRII and Smad2/3, but did not alter the phosphorylation of p38, ERK1/2 or JNK. In addition, collagen increased cell proliferation and adhesion, and this collagen‑induced cell proliferation was inhibited by the anti‑β1 integrin antibody and ERK inhibitor. Taken together, the data obtained suggested that collagen differentially regulated the expression levels of TGFβRI and TGFβRII, and modulated the phosphorylation of MAPKs through integrin‑ or stretch‑dependent and ‑independent signaling pathways.

Influence of mechanical stress on fibroblast-myocyte interactions in mammalian heart

Cardiac fibroblasts are an essential component of cardiac tissue. These cells not only produce the extracellular matrix, but also are electrically and mechanically coupled with cardiomyocytes. In this way, fibroblasts can influence the electrical activity of cardiomyocytes. Cardiac fibroblasts cannot generate action potentials, but their membrane potential is controlled by mechanical stretch or compression of the surrounding myocardium which in turn affects their interaction with myocytes and the way myocytes respond to mechanical stress. This review discusses the electrical properties of cardiac fibroblasts, the present evidence of fibroblast-myocyte coupling and the way in which these cells respond to mechanical stress. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

Long-chain acylcarnitines regulate the hERG channel

Background and purpose

In some pathological conditions carnitine concentration is high while in othersitis low.In bothcases,cardiac arrhythmiascan occur and lead to sudden cardiac death. It has been proposed that in ischaemia, acylcarnitine (acyl-CAR), but not carnitine, is involved in arrhythmiasthrough modulation of ionic currents. We studied the effects of acyl-CARs on hERG, K_IR2.1 and K_v7.1/minKchannels (channels responsible for I_KR, I_K1 and I_KS respectively).

Experimental approach

HEK293 cells stably expressing hERG, K_IR2.1 or Kv7.1/minK were studied using the patch clamp technique. Free carnitine (CAR) and acyl-CAR derivatives from medium- (C8 and C10) and long-chain (C16 and C18∶1) fatty acids were applied intra- and extracellularly at different concentrations. Forstudies onhERG, C16 and C18∶1 free fatty acid were also used.

Key results

Extracellular long-chain (LCAC), but not medium-chain, acyl-CAR,induced an increase of I_hERG amplitude associated with a dose-dependent speeding of deactivation kinetics. They had no effect on K_IR2.1 or Kv7.1/minK currents.Computer simulations of these effects wereconsistent with changes in action potential profile.

Conclusions and applications

Extracellular LCAC tonically regulates I_hERG amplitude and kineticsunder physiological conditions. This modulation maycontribute tothe changes in action potential duration thatprecede cardiac arrhythmias in ischaemia, diabetes and primary systemic carnitine deficiency.

A step towards characterisation of electrophysiological profile of torsadogenic drugs

INTRODUCTION

In a previous study, two electrophysiological patterns for torsadogenic drugs were characterised in the model of isolated canine Purkinje fibres from their respective effects on action potential. This study was designed to elucidate the possible mechanisms underlying these two electrophysiological profiles.

METHODS

Effects of representative torsadogenic agents and non torsadogenic drugs on I(Kr), I(Ks), I(K1), I(Na) and I(CaL) were studied in transfected HEK 293 cells using the path-clamp method as well as in conscious beagle dogs and cynomolgus monkeys by telemetry.

RESULTS

Patch-clamp studies confirmed that torsadogenic molecules could be discriminated into at least two subgroups. The first subgroup can be defined as apparently pure I(Kr) blockers. The second subgroup can be defined as I(Kr) blockers with ancillary properties on sodium and/or calcium channels which counterbalance the I(Kr) prolongation component. This discrimination is transposable to the telemetered cynomolgus monkey model in terms of QT prolongation but not to the telemetered beagle dog model. This latter inter-species difference could be related to the sympathetic/parasympathetic balance and could involve reserve repolarisation dependent mechanisms.

DISCUSSION

The confirmation that torsadogenic drugs might have at least two different electrophysiological profiles should be taken into consideration in preclinical safety pharmacology studies because it increases the value of the cynomolgus monkey model in two particular situations: firstly when an NCE causes sympathetic activation and secondly, when an NCE exhibits a pure I(Kr) blocker pattern independently of its potency to block HERG channels.

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.

Cardiovascular Safety of Gadoterate Meglumine (Gd-DOTA)

OBJECTIVES

Gadolinium complexes are not considered to be a drug class at high risk for prolonging cardiac repolarization, which can lead to potentially life-threatening arrhythmias such as torsade de pointes. However, only limited robust data are available on these compounds despite their extensive use as contrast enhancers in magnetic resonance imaging. We present an overview of recent cardiovascular safety data obtained on gadoterate meglumine (Gd-DOTA).

MATERIALS AND METHODS

Cardiovascular safety was evaluated by "state-of-the-art" nonclinical ex vitro (dog Purkinje fibers) and in vivo studies in both normal (dogs) and sensitized animal models (rabbits) and in patients with various diseases in a specific clinical trial.

RESULTS

In all of these studies, Gd-DOTA did not show any direct deleterious effect on cardiac electrophysiology and especially on ventricular repolarization.

CONCLUSION

These results confirmed the good safety profile of Gd-DOTA derived from postmarketing evaluations. Nonspecific gadolinium complexes used for magnetic resonance contrast enhancement do not constitute a class-at-risk for drug-related arrhythmias.

Effects of mechanosensitive ion channels on ventricular electrophysiology: experimental and theoretical models

The heart is an electrically driven mechanical pump, somewhat like an electric motor. Interestingly, like an electric motor in 'dynamo mode', the heart can also convert mechanical stimuli into electrical signals. This feedback from cardiac mechanics to electrical activity involves mechanosensitive ion channels, whose properties and pathophysiological relevance are reviewed in the context of experimental and theoretical modelling of ventricular beat-by-beat electromechanical function.

Trapidil enhances the slowly activating delayed rectifier potassium current and suppresses the transient inward current induced by catecholamine in Guinea pig ventricular myocytes

We examined the electrophysiological effects of trapidil on the ionic currents influencing the repolarization and on the transient inward current (ITi) that can cause triggered arrhythmia using the whole-cell patch-clamp technique in guinea pig ventricular myocytes. Trapidil shortened the action potential duration (APD) and increased the delayed rectifier potassium current (IK) in a concentration-dependent manner. The effect of trapidil on the rapidly and slowly activating components of IK (IKr and IKs, respectively) was studied by the envelope of tails test. Trapidil failed to affect IKr and selectively enhanced IKs. Trapidil increased the amplitude of the L-type Ca2+ current (ICa,L), with an acceleration of its inactivation, whereas isoproterenol, a beta-adrenoceptor agonist, increased the amplitude of the ICa,L in a different manner. Isoproterenol activated ITi; however, trapidil not only failed to facilitate ITi but also suppressed isoproterenol-induced ITi. The inhibitory effect of trapidil on isoproterenol-induced ITi is at least partly via a reduction of Ca2+ overload through an acceleration of ICa,L inactivation and/or a sarcoplasmic reticulum (SR) Ca channel modulation. These results suggest that trapidil does not prolong the QT interval and has an antiarrhythmic effect on arrhythmias elicited by triggered activity secondary to Ca2+ overload at much higher concentrations than clinical concentration.

Inward-rectifier K+ current in guinea-pig ventricular myocytes exposed to hyperosmotic solutions

Superfusion of heart cells with hyperosmotic solution causes cell shrinkage and inhibition of membrane ionic currents, including delayed-rectifer K+ currents. To determine whether osmotic shrinkage also inhibits inwardly-rectifying K+ current (I(K1)), guinea-pig ventricular myocytes in the perforated-patch or ruptured-patch configuration were superfused with a Tyrode's solution whose osmolarity (T) relative to isosmotic (1T) solution was increased to 1.3-2.2T by addition of sucrose. Hyperosmotic superfusate caused a rapid shrinkage that was accompanied by a negative shift in the reversal potential of Ba(2+)-sensitive I(K1), an increase in the amplitude of outward I(K1), and a steepening of the slope of the inward I(K1)-voltage (V) relation. The magnitude of these effects increased with external osmolarity. To evaluate the underlying changes in chord conductance (G(K1)) and rectification, G(K1)-V data were fitted with Boltzmann functions to determine maximal G(K1) (G(K1)max) and voltage at one-half G(K1)max (V(0.5)). Superfusion with hyperosmotic sucrose solutions led to significant increases in G(K1)max (e.g., 28 +/- 2% with 1.8T), and significant negative shifts in V(0.5) (e.g., -6.7 +/- 0.6 mV with 1.8T). Data from myocytes investigated under hyperosmotic conditions that do not induce shrinkage indicate that G(K1)max and V(0.5) were insensitive to hyperosmotic stress per se but sensitive to elevation of intracellular K+. We conclude that the effects of hyperosmotic sucrose solutions on I(K1) are related to shrinkage-induced concentrating of intracellular K+.

Modulation of cardiac Na(+) current by gadolinium, a blocker of stretch-induced arrhythmias

Gd(3+) blocks stretch-activated channels and suppresses stretch-induced arrhythmias. We used whole cell voltage clamp to examine whether effects on Na(+) channels might contribute to the antiarrhythmic efficacy of Gd(3+). Gd(3+) inhibited Na(+) current (I(Na)) in rabbit ventricle (IC(50) = 48 microM at -35 mV, holding potential -120 mV), and block increased at more negative test potentials. Gd(3+) made the threshold for I(Na) more positive and reduced the maximum conductance. Gd(3+) (50 microM) shifted the midpoints for activation and inactivation of I(Na) 7.9 and 5.7 mV positive but did not alter the slope factor for either relationship. Activation and inactivation kinetics were slowed in a manner that could not be explained solely by altered surface potential. Paradoxically, Gd(3+) increased I(Na) under certain conditions. With membrane potential held at -75 mV, Gd(3+) still shifted threshold for activation positive, but I(Na) increased positive to -40 mV, causing the current-voltage curves to cross over. When availability initially was low, increased availability induced by Gd(3+) dominated the response at test potentials positive to -40 mV. The results indicate that Gd(3+) has complex effects on cardiac Na(+) channels. Independent of holding potential, Gd(3+) is a potent I(Na) blocker near threshold potential, and inhibition of I(Na) by Gd(3+) is likely to contribute to suppression of stretch-induced arrhythmias.

Diastolic (dys-)function and electrophysiology

Cross-talk between cardiac electrical and mechanical function is a bidirectional process: The origin and spread of electric excitation govern cardiac contraction and relaxation, while the mechanic environment provides feedback information to the heart's electric behavior. The latter tends to be unduly disregarded by the medical community. This article reviews experimental findings on the effects of diastolic mechanics on cardiac electrophysiology, and describes physiological correlates, clinical manifestations, and therapeutic utility of cardiac mechanic stimulation in humans.

Gadolinium inhibits Na(+)-Ca(2+) exchanger current in guinea-pig isolated ventricular myocytes

The trivalent cation, gadolinium (Gd(3+)) is commonly used to inhibit stretch-activated channels. In this report, we show that Gd(3+) also inhibits ionic current (I(NaCa)), carried by the Na(+)-Ca(2+) exchanger protein. Under selective recording conditions, Gd(3+) inhibited both outward and inward I(NaCa) from guinea-pig isolated ventricular myocytes in a dose-dependent manner, with half-maximal inhibition concentrations (IC(50)) of 30.0+/-4.0 microM at +60 mV (Hill-coefficient, h=1.04+/-0.13) and 20.0+/-2.7 microM at -100 mV (h=1.13+/-0.16), respectively (P>0.05, n=5 - 9). Thus, inhibition was not voltage-dependent. The time from Gd(3+) application to steady-state effect was slow compared to the divalent blocker Ni(2+). The slow time course appeared to reflect gradual Gd(3+) accumulation at its binding site on the exchanger, rather than a use-dependent blocking mechanism. This study indicates that for experiments in which Gd(3+) is used, its inhibitory effect on I(NaCa) should be taken into account.

Gadolinium decreases stretch-induced vulnerability to atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is frequently associated with atrial dilatation caused by pressure or volume overload. Stretch-activated channels (SACs) have been found in myocardial cells and may promote AF in dilated atria. To prove this hypothesis, we investigated the effect of the SAC blocker gadolinium (Gd(3+)) on AF propensity in the isolated rabbit heart during atrial stretch.

METHODS AND RESULTS

In 16 isolated Langendorff-perfused rabbit hearts, the interatrial septum was perforated to equalize biatrial pressures. Caval and pulmonary veins were occluded. Intra-atrial pressure (IAP) was increased in steps of 2 to 3 cm H(2)O by increasing the pulmonary outflow fluid column. Vulnerability to AF was evaluated by 15-second burst pacing at each IAP level. At baseline, IAP needed to be raised to 8.8+/-0.2 cm H(2)O (mean+/-SEM) to induce AF. A dose-dependent decrease in AF vulnerability was observed after Gd(3+) 12.5, 25, and 50 micromol/L was added. AF threshold increased to 19.0+/-0.5 cm H(2)O with Gd(3+) 50 micromol/L (P<0.001 versus baseline). Spontaneous runs of AF occurred in 5 hearts on a rise of IAP to 13.8+/-3.3 cm H(2)O at baseline but never during Gd(3+). Atrial effective refractory period shortened progressively from 78+/-3 ms at 0.5 cm H(2)O to 52+/-3 ms at 20 cm H(2)O (P<0.05). Gd(3+) 50 micromol/L had no significant effect on effective refractory period.

CONCLUSIONS

Acute atrial stretch significantly enhances the vulnerability to AF. Gd(3+) reduces the stretch-induced vulnerability to AF in a dose-dependent manner. Block of SAC might represent a novel antiarrhythmic approach to AF under conditions of elevated atrial pressure or volume.

Cardiac mechano-electric feedback in man: clinical relevance

Clinical conditions associated with sudden cardiac death due to arrhythmia are frequently accompanied by abnormalities of mechanical loading and wall stretch. These arrhythmias may result from several mechanisms including secondary depolarisations during or following the action potential or from a combination of conduction slowing and action potential shortening. Mechanical perturbations have been shown to reproduce these electrophysiological effects experimentally. However the effect of mechanical intervention is complex depending on the timing and intensity of the stimulus and the interplay between effects mediated via stretch activated channels and calcium cycling. Studies in patients during cardiac catheterisation or cardiac surgery using monophasic action potentials have shown alteration in the time course and shape of action potential repolarisation in response to changes in ventricular loading. Although stretch in experimental preparations has been shown to be arrhythmogenic, particularly in pathological conditions, the role of mechanically induced electrophysiological changes in important clinical ventricular arrhythmias remains to be established.