Altered pharmacologic responsiveness of reduced L-type calcium currents in myocytes surviving in the infarcted heart

Models of the cardiac L-type calcium current: A quantitative review

The L-type calcium current (I CaL ) plays a critical role in cardiac electrophysiology, and models ofI CaL are vital tools to predict arrhythmogenicity of drugs and mutations. Five decades of measuring and modelingI CaL have resulted in several competing theories (encoded in mathematical equations). However, the introduction of new models has not typically been accompanied by a data-driven critical comparison with previous work, so that it is unclear which model is best suited for any particular application. In this review, we describe and compare 73 published mammalianI CaL models and use simulated experiments to show that there is a large variability in their predictions, which is not substantially diminished when grouping by species or other categories. We provide model code for 60 models, list major data sources, and discuss experimental and modeling work that will be required to reduce this huge list of competing theories and ultimately develop a community consensus model ofI CaL . This article is categorized under: Cardiovascular Diseases > Computational Models Cardiovascular Diseases > Molecular and Cellular Physiology.

Reduced effects of BAY K 8644 on L-type Ca2+ current in failing human cardiac myocytes are related to abnormal adrenergic regulation

Abnormal L-type Ca(2+) channel (LTCC, also named Cav1.2) density and regulation are important contributors to depressed contractility in failing hearts. The LTCC agonist BAY K 8644 (BAY K) has reduced inotropic effects on failing myocardium. We hypothesized that BAY K effects on the LTCC current (I(CaL)) in failing myocytes would be reduced because of increased basal activity. Since support of the failing heart with a left ventricular assist device (LVAD) improves contractility and adrenergic responses, we further hypothesized that BAY K effects on I(CaL) would be restored in LVAD-supported failing hearts. We tested our hypotheses in human ventricular myocytes (HVMs) isolated from nonfailing (NF), failing (F), and LVAD-supported failing hearts. We found that 1) BAY K had smaller effects on I(CaL) in F HVMs compared with NF HVMs; 2) BAY K had diminished effects on I(CaL) in NF HVM pretreated with isoproterenol (Iso) or dibutyryl cyclic AMP (DBcAMP); 3) BAY K effects on I(CaL) in F HVMs pretreated with acetylcholine (ACh) were normalized; 4) Iso had no effect on NF HVMs pretreated with BAY K; 5) BAY K effects on I(CaL) in LVAD HVMs were similar to those in NF HVMs; 6) BAY K effects were reduced in LVAD HVMs pretreated with Iso or DBcAMP; 7) Iso had no effect on I(CaL) in LVAD HVMs pretreated with BAY K. Collectively, these results suggest that the decreased BAY K effects on LTCC in F HVMs are caused by increased basal channel activity, which should contribute to abnormal contractility reserve.

Arrhythmogenic Ion-Channel Remodeling in the Heart: Heart Failure, Myocardial Infarction, and Atrial Fibrillation

Rhythmic and effective cardiac contraction depends on appropriately timed generation and spread of cardiac electrical activity. The basic cellular unit of such activity is the action potential, which is shaped by specialized proteins (channels and transporters) that control the movement of ions across cardiac cell membranes in a highly regulated fashion. Cardiac disease modifies the operation of ion channels and transporters in a way that promotes the occurrence of cardiac rhythm disturbances, a process called “arrhythmogenic remodeling.” Arrhythmogenic remodeling involves alterations in ion channel and transporter expression, regulation and association with important protein partners, and has important pathophysiological implications that contribute in major ways to cardiac morbidity and mortality. We review the changes in ion channel and transporter properties associated with three important clinical and experimental paradigms: congestive heart failure, myocardial infarction, and atrial fibrillation. We pay particular attention to K+, Na+, and Ca2+channels; Ca2+transporters; connexins; and hyperpolarization-activated nonselective cation channels and discuss the mechanisms through which changes in ion handling processes lead to cardiac arrhythmias. We highlight areas of future investigation, as well as important opportunities for improved therapeutic approaches that are being opened by an improved understanding of the mechanisms of arrhythmogenic remodeling.

Remodeled cardiac calcium channels

Cardiac calcium channels play a pivotal role in the proper functioning of cardiac cells. In response to various pathologic stimuli, they become remodeled, changing how they function, as they adapt to their new environment. Specific features of remodeled channels depend upon the particular disease state. This review will summarize what is known about remodeled cardiac calcium channels in three disease states: hypertrophy, heart failure and atrial fibrillation. In addition, it will review the recent advances made in our understanding of the function of the various molecular building blocks that contribute to the proper functioning of the cardiac calcium channel.

Molecular Basis of Arrhythmias

The characterization of single gene disorders has provided important insights into the molecular pathogenesis of cardiac arrhythmias. Primary electricalal diseases including long-QT syndrome, short-QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia have been associated with mutations in a variety of ion channel subunit genes that promote arrhythmogenesis. Pathological remodeling of ionic currents and network properties of the heart critical for normal electrical propagation plays a critical role in the initiation and maintenance of acquired arrhythmias. This review focuses on the molecular and cellular basis of electrical activity in the heart under normal and pathophysiological conditions to provide insights into the fundamental mechanisms of inherited and acquired cardiac arrhythmias. Improved understanding of the basic biology of cardiac arrhythmias holds the promise of identifying new molecular targets for the treatment of cardiac arrhythmias.

Remodeling in cells from different regions of the reentrant circuit during ventricular tachycardia

BACKGROUND

Anisotropic reentrant excitation occurs in the remodeled substrate of the epicardial border zone (EBZ) of the 5-day infarcted canine heart. Reentry is stabilized because of the formation of functional lines of block. We hypothesized that regional differences of ionic currents in cells of the EBZ form these lines of block. Therefore, we first mapped reentrant circuits of sustained tachycardias, then dispersed cells (infarct zone cells, IZs) from the central common pathway of the circuit (IZc) as well as from the other side of the line of block (outer pathway, IZo) for study.

METHODS AND RESULTS

We mapped reentrant circuits in the EBZ of infarcted hearts during sustained ventricular tachycardias (>30 seconds, n=17 episodes, cycle lengths=218+/-7.9 ms). INa density was reduced in both IZc and IZo, and the kinetic properties of IZc INa were markedly altered versus IZo. Structural remodeling of the sodium channel protein Nav1.5 occurred in IZs, with cell surface localization differing from normal cells. Both IZc and IZo have similar but reduced ICaL, whereas IZc showed changes in Ca2+ current kinetics with an acceleration of current decay. Computer simulations of the 2D EBZ showed that incorporating only differences between INa in IZc and IZo prevented stability of the reentrant circuit. Incorporating only differences between ICaL in the IZc and IZo cells also prevented stability of the circuit. However, incorporating both INa and ICaL current differences stabilized the simulated reentrant circuit, and lines of block formed between the 2 distinct regions.

CONCLUSIONS

Despite differences in INa and ICaL properties in cells of the center and outer pathways of a reentrant circuit, the resulting changes in effective refractory periods tend to stabilize reentry in this remodeled substrate.

Diverse phenotypes of outward currents in cells that have survived in the 5-day-infarcted heart

We have shown reduced density and altered kinetics in slowly activating K+ currents (I(Ks)) in epicardial border zone (EBZ) cells (IZs) of the 5-day-infarcted canine heart (Jiang M, Cabo, C, Yao J-A, Boyden PA, and Tseng G-N. Cardiovasc Res 48: 34-43, 2000). beta-Adrenergic stimulation with isoproterenol increases I(Ks) in normal cells (NZs). In this study, we used a voltage-clamp protocol with an external solution to isolate I(Ks) from contaminating currents to determine the effects of 1 muM isoproterenol on I(Ks) in IZs and NZs. Under our recording conditions, 10 microM azimilide-sensitive currents were stimulated with isoproterenol to compare responsiveness of I(Ks) to isoproterenol in the two cell groups. I(Ks) tail density was reduced 67% in IZs (group I, n = 26) compared with NZs (n = 24, P < 0.05). Isoproterenol-stimulated azimilide-sensitive tail currents were increased 1.72 +/- 0.2-fold in NZs and 2.2 +/- 0.3-fold in IZs (P > 0.05). In 33% of IZs (group II, n = 13), native currents showed no tail currents; however, isoproterenol-stimulated azimilide-sensitive currents were voltage dependent, fast activating, and large in amplitude compared with group I IZs, similar to "lone" KCNQ1 currents. Using short clamp pulses, we also found an increase in sustained currents sensitive to tetraethylammonium chloride (TEA) and no change in C-9356-sensitive currents in IZs with little or no transient outward current. In some IZs where I(Ks) is downregulated, the effect of isoproterenol on I(Ks) was similar to that on I(Ks) in NZs. In others, the existence of lone KCNQ1-type currents, which are sensitive to beta-adrenergic stimulation, is consistent with our findings of an increased KCNQ1-to-KCNE1 mRNA ratio (Jiang et al.). Accompanying altered I(Ks) in IZs are an enhanced TEA-sensitive current and a normal C-9356-sensitive current.

Dynamic remodeling of K+ and Ca2+ currents in cells that survived in the epicardial border zone of canine healed infarcted heart

Action potentials (APs) of the epicardial border zone (EBZ) cells from the day 5 infarcted heart continue to be altered by day 14 postocclusion, namely, they shortened. However, by 2 mo, EBZ APs appear "normal," yet conduction of wave fronts remains abnormal. We hypothesize that the changes in transmembrane APs are due to a change in the distribution of ion channels in either density or function. Thus we focused on the changes in Ca2+ and K+ currents in cells isolated from the 14-day (IZ14d) and 2-mo (IZ2m) EBZ and compared them with those occurring in cells from the same hearts but remote (Rem) from the EBZ. Whole cell voltage-clamp techniques were used to measure and compare Ca2+ and K+ currents in cells from the different groups. Ca2+ current densities remain reduced in cells of the 14-day and 2-mo infarcted heart and the kinetic changes previously identified in the 5-day heart begin to, but do not recover to, cells from noninfarcted epicardium (NZ) values. Importantly, I(Ca,L) in both the EBZ and Rem regions still show a slowed recovery from inactivation. Furthermore, during the remodeling process, there is an increased expression of T-type Ca2+ currents, but only regionally, and only within a specific time window postmyocardial infarction (MI). Regional heterogeneity in beta-adrenergic responsiveness of I(Ca,L) exists between EBZ and remote cells of the 14-day hearts, but this regional heterogeneity is gone in the healed infarcted heart. In IZ14d, the transient outward K+ current (Ito) begins to reemerge and is accompanied by an upregulated tetraethylammonium-sensitive outward current. By 2-mo postocclusion, Ito and sustained outward K+ current have completed the reverse remodeling process. During the healing process post-MI, canine epicardial cells downregulate the fast Ito but compensate by upregulating a K+ current that in normal cells is minimally functional. For recovering I(Ca,L) of the 14-day and 2-mo EBZ cells, voltage-dependent processes appear to be reset, such that I(Ca,L) "window" current occurs at hyperpolarized potentials. Thus dynamic changes in both Ca2+ and K+ currents contribute to the altered AP observed in 14-day fibers and may account for return of APs of 2 mo EBZ fibers.

Electrical remodeling of the epicardial border zone in the canine infarcted heart: a computational analysis

The density and kinetics of several ionic currents of cells isolated from the epicardial border zone of the infarcted heart (IZs) are markedly different from cells from the noninfarcted canine epicardium (NZs). To understand how these changes in channel function affect the action potential of the IZ cell as well as its response to antiarrhythmic agents, we developed a new ionic model of the action potential of a cell that survives in the infarct (IZ) and one of a normal epicardial cell (NZ) using formulations based on experimental measurements. The difference in action potential duration (APD) between NZ and IZ cells during steady-state stimulation (basic cycle length = 250 ms) was 6 ms (156 ms in NZ and 162 ms in IZ). However, because IZs exhibit postrepolarization refractoriness, the difference in the effective refractory period (ERP), calculated using a propagation model of a single fiber of 100 cells, was 43 ms (156 ms in NZ and 199 ms in IZ). Either an increase in L-type Ca(2+) current (to simulate the effects of BAY Y5959) or a decrease of both or either delayed rectifier currents (e.g., to simulate the effects of azimilide, sotalol, and chromanol) had significant effects on NZ ERP. In contrast, the effects of these agents in IZs were minor, in agreement with measurements in the in situ canine infarcted heart. Therefore 1) because IZs exhibit postrepolarization refractoriness, conclusions drawn from APD measurements cannot be extrapolated directly to ERPs; 2) ionic currents that are the major determinants of APD and the ERP in NZs are less important in IZs; and 3) differential effects of either BAY Y5959 or azimilide in NZs versus IZs are predicted to decrease ERP dispersion and in so doing prevent initiation of arrhythmias in a substrate of inhomogeneous APD/ERPs.

Protein tyrosine kinases and L-type Ca2+ currents in cells that have survived in epicardial border zone of canine infarcted heart

Previously a reduction was shown in the density of the L-type Ca currents in cells that have survived in the epicardial border zone of the 5-day infarcted canine heart (IZ). A hyporesponsiveness of I(CaL) to beta-adrenergic stimulation in IZs versus cells from the noninfarcted heart (NZs) was also shown. To determine the role of protein tyrosine kinase (PTK) activity in this altered adrenergic response as well as in the reduced basal current function in IZs, the effects of genistein and T23, specific inhibitors of PTK, on basal I(CaL) in the absence and presence of isoproterenol (5 nM ) were studied using whole-cell patch-clamp techniques. Genistein reduction of I(CaL) was similar in NZs and IZs and was not mimicked by daidzein, an inactive analogue of genistein. Submaximal isoproterenol produced a small response in both cell types that was potentiated in the presence of genistein. T23 also reduced I(CaL) in both NZs and IZs; however, submaximal isoproterenol was not potentiated in its presence. In sum, basal I(CaL) is sensitive to genistein and T23, suggesting that persistent PTK activity contributes to I(CaL) in both NZs and IZs. With genistein but not with T23, there is an enhanced sensitivity of I(CaL) to isoproterenol in both NZs and IZs but peak I(CaL) is not fully restored in IZs. Thus, dysregulation of PTK activity cannot account for the reduced basal Ca currents or hyporesponsiveness of I(CaL) to isoproterenol in the cells that have survived in the infarcted heart.

Abnormalities in Ca(i)handling in myocytes that survive in the infarcted heart are not just due to alterations in repolarization

Studies from our laboratory have defined alterations in Ca(i)handling in the non-dialyzed subepicardial cells that have survived in the 5 day infarcted heart (IZs). To determine whether changes in the action potential profile contributed to the observed Ca(i)changes we have used a combined voltage clamp/epifluorescent technique to determine and compare changes in fura 2 ratios in IZs compared to those of epicardial cells from the non-infarcted canine hearts (NZs). We found that Ca(i)changes in voltage clamped IZs persisted. In NZs, Ca(i)transients showed the expected voltage dependence while IZs did not. To determine whether altered NaCa exchanger activity contributed to the observed changes in Ca(i)in IZs, we measured NaCa exchanger Ca(2+)fluxes (reverse and forward mode) and ionic currents in both cell types and under different Na(i)loads (10 and 20 m m). We found that there were no significant differences in resting, peak or magnitude of fura 2 ratio changes or in outward current densities between NZs and IZs even under the different Na(i)loads. Thus, we suggest that chronic up- or downregulation of the NaCa exchanger protein does not underlie observed Ca(i)changes in IZs. Additionally, Ca(2+)released with paced voltage steps represented 79% of that released by caffeine in NZs while, in IZs, caffeine releasable Ca(2+)was equivalent to that released with step depolarization. Thus, abnormalities in Ca(i)handling in IZs appear not to arise secondarily to changes in action potential configuration nor do they appear to be due to disease-induced alteations in NaCa exchanger function.

Effects of Bay Y5959 on Ca2+ currents and intracellular Ca2+ in cells that have survived in the epicardial border of the infarcted canine heart

We determined and compared the effects of the dihydropyridine agonist, Bay Y5959, on the amplitude of L-type Ca2+ currents and intracellular Ca2+ transients in epicardial cells from noninfarcted hearts (NZs) and surviving cells from the epicardial border zone of 5-day infarcted canine hearts (IZs). We determined the effects of Bay Y5959 on the L-type Ca2+ current by using single cells and a whole-cell voltage-clamp approach. To elucidate the effects of Bay Y5959 on the amplitude and time course of the spatially averaged intracellular Ca2+ transient (Ca(i)T), myocytes from the two cell groups were loaded and studied by using the Ca2+-sensitive indicator fura-2/AM. Bay Y5959 increased the amplitude of the L-type Ca2+ current in both cell groups, but peak amplitude in NZs was always greater than that in IZs. Bay Y5959 also increased Ca(i)T amplitude in both NZs and IZs and significantly accelerated the Ca(i)T time course in IZs, particularly at the faster pacing-cycle length. We suggest that the Bay Y5959 effect to restore L-type Ca2+ currents in IZs contributes to its observed antiarrhythmic effects during the reentrant ventricular tachycardias that are known to occur in the epicardial border zone of the infarcted heart.

The search for novel antiarrhythmic strategies. Sicilian Gambit

The past fifty years of antiarrhythmic drug development have seen limited success in prolonging life and reducing morbidity. It is likely that arrhythmias are in most instances final common pathways through which changes in the cardiac substrate and in trigger mechanisms are expressed. We propose that the development and administration of therapies for the arrhythmias themselves, while offering a panacea for a disease entity that has evolved and is being overtly manifested, is also an admission of failure to identify and prevent evolution of the substrate and triggers such that arrhythmias can occur. We suggest that while strategies for treatment and prevention of recurrence of arrhythmias still warrant exploration, greater hope for the future lies in identifying means for earlier diagnosis of the arrhythmogenic substrate and triggers, and in developing therapies that are "upstream" to the arrhythmia and prevent their initial expression. Means to achieve this end are suggested, using specific arrhythmias as examples. Similarly, to increase the likelihood that clinical studies of new therapies can be successfully concluded and interpreted, we suggest new approaches to patient selection, risk stratification, trial endpoints, outcome events and trial methodologies.

Ionic remodeling underlying action potential changes in a canine model of atrial fibrillation

Rapid electrical activation, as occurs during atrial fibrillation (AF), is known to cause reductions in atrial refractoriness and in adaptation to heart rate of the atrial refractory period, which promote the maintenance of AF, but the underlying ionic mechanisms are unknown. In order to determine the cellular and ionic changes caused by chronic atrial tachycardia, we studied right atrial myocytes from dogs subjected to 1, 7, or 42 days of atrial pacing at 400/min and compared them with myocytes from sham-operated dogs (pacemaker inserted but not activated). Rapid pacing led to progressive increases in the duration of AF induced by bursts of 10-Hz stimuli (from 3 +/- 2 seconds in sham-operated dogs to 3060 +/- 707 seconds in dogs after 42 days of pacing, P < .001) and reduced atrial refractoriness and adaptation to rate of the atrial refractory period. Voltage-clamp studies showed that chronic rapid pacing did not alter inward rectifier K+ current, rapid or slow components of the delayed rectifier current, the ultrarapid delayed rectifier current, T-type Ca2+ current, or Ca(2+)-dependent Cl- current. In contrast, the densities of transient outward current (Ito) and L-type Ca2+ current (ICa) were progressively reduced as the duration of rapid pacing increased, without concomitant changes in kinetics or voltage dependence. In keeping with in vivo changes in refractoriness, action potential duration (APD) and APD adaptation to rate were decreased by rapid pacing. The response of the action potential and ionic currents flowing during the action potential (as exposed by action-potential voltage clamp) to nifedipine in normal canine cells and in cells from rapidly paced dogs suggested that the APD changes in paced dogs were largely due to reductions in ICa. We conclude that sustained atrial tachycardia reduces Ito and ICa, that the reduced ICa decreases APD and APD adaptation to rate, and that these cellular changes likely account for the alterations in atrial refractoriness associated with enhanced ability to maintain AF in the model.

Regional gradation of L-type calcium currents in the feline heart with a healed myocardial infarct

INTRODUCTION

Abnormal action potentials in myocytes adjacent to > 2-month-old feline LV myocardial infarcts (MI) may reflect alterations in Ca2+ currents (Ica).

METHODS AND RESULTS

We compared ICa, at 36 degrees C, in subendocardial myocytes isolated from areas adjacent to MI and to ICa in cells from remote areas (> 4 mm away; REM) and control cells from similar regions in normal hearts. Control (CON) myocytes had membrane capacitance of 234 +/- 10 pF (n = 81 cells) compared to 305 +/- 14 pF in REM (71 cells; P < 0.05 from CON) and 237 +/- 11 pF (n = 55 cells) in MI (not different from CON). From Vh = -40 mV; peak ICa elicited by test potentials (-35 to +70 mV) were significantly larger in CON (-1746 +/- 123 pA) and REM (-1795 +/- 142 pA) compared to MI (-1352 +/- 129 pA) (P < 0.05). Peak ICa density was significantly reduced in REM (-6.0 +/- 0.4 pA/pF) or MI (-5.7 +/- 0.4 pA/pF, P < 0.05) compared to CON (-7.5 +/- 0.4 pA/pF). Double exponential ICa decay was similar among groups. Half-inactivation potential (V0.5) was significantly shifted (hyperpolarizing direction) for MI (-29.1 +/- 2.6 mV) and REM (-24.6 +/- 1.2 mV) myocytes compared to -20.3 +/- 1.0 mV in CON. MI slope factor (k; 9.0 +/- 0.5) was significantly different from CON (6.8 +/- 0.3) and REM (7.3 +/- 0.4). No differences in time course of recovery from inactivation were noted. Five millimolar Ba2+o produced significant increases in ICa in CON and REM but an attenuated response in MI. Bay K8644 (1 microM) produced similar ICa increase in all groups. ICa increase due to isoproterenol (1 microM) in MI and REM was half that in CON, but there were no differences in increased ICa responses among groups following phenylephrine (10 microM).

CONCLUSION

Reduced ICa density in REM reflects cell hypertrophy, whereas altered ICa of MI may reflect altered channel structure and/or function.