Disease Model of GATA4 Mutation Reveals Transcription Factor Cooperativity in Human Cardiogenesis

Mutation of highly conserved residues in transcription factors may affect protein-protein or protein-DNA interactions, leading to gene network dysregulation and human disease. Human mutations in GATA4, a cardiogenic transcription factor, cause cardiac septal defects and cardiomyopathy. Here, iPS-derived cardiomyocytes from subjects with a heterozygous GATA4-G296S missense mutation showed impaired contractility, calcium handling, and metabolic activity. In human cardiomyocytes, GATA4 broadly co-occupied cardiac enhancers with TBX5, another transcription factor that causes septal defects when mutated. The GATA4-G296S mutation disrupted TBX5 recruitment, particularly to cardiac super-enhancers, concomitant with dysregulation of genes related to the phenotypic abnormalities, including cardiac septation. Conversely, the GATA4-G296S mutation led to failure of GATA4 and TBX5-mediated repression at non-cardiac genes and enhanced open chromatin states at endothelial/endocardial promoters. These results reveal how disease-causing missense mutations can disrupt transcriptional cooperativity, leading to aberrant chromatin states and cellular dysfunction, including those related to morphogenetic defects.

Expandable Cardiovascular Progenitor Cells Reprogrammed from Fibroblasts

Stem cell-based approaches to cardiac regeneration are increasingly viable strategies for treating heart failure. Generating abundant and functional autologous cells for transplantation in such a setting, however, remains a significant challenge. Here, we isolated a cell population with extensive proliferation capacity and restricted cardiovascular differentiation potentials during cardiac transdifferentiation of mouse fibroblasts. These induced expandable cardiovascular progenitor cells (ieCPCs) proliferated extensively for more than 18 passages in chemically defined conditions, with 10(5) starting fibroblasts robustly producing 10(16) ieCPCs. ieCPCs expressed cardiac signature genes and readily differentiated into functional cardiomyocytes (CMs), endothelial cells (ECs), and smooth muscle cells (SMCs) in vitro, even after long-term expansion. When transplanted into mouse hearts following myocardial infarction, ieCPCs spontaneously differentiated into CMs, ECs, and SMCs and improved cardiac function for up to 12 weeks after transplantation. Thus, ieCPCs are a powerful system to study cardiovascular specification and provide strategies for regenerative medicine in the heart.

KMT2D regulates specific programs in heart development via histone H3 lysine 4 di-methylation

KMT2D, which encodes a histone H3K4 methyltransferase, has been implicated in human congenital heart disease in the context of Kabuki syndrome. However, its role in heart development is not understood. Here, we demonstrate a requirement for KMT2D in cardiac precursors and cardiomyocytes during cardiogenesis in mice. Gene expression analysis revealed downregulation of ion transport and cell cycle genes, leading to altered calcium handling and cell cycle defects. We further determined that myocardial Kmt2d deletion led to decreased H3K4me1 and H3K4me2 at enhancers and promoters. Finally, we identified KMT2D-bound regions in cardiomyocytes, of which a subset was associated with decreased gene expression and decreased H3K4me2 in mutant hearts. This subset included genes related to ion transport, hypoxia-reoxygenation and cell cycle regulation, suggesting that KMT2D is important for these processes. Our findings indicate that KMT2D is essential for regulating cardiac gene expression during heart development primarily via H3K4 di-methylation.

Highlighted article: Cardiac-specific depletion of the H3K4 methyltransferase KMT2D causes dysregulation of genes associated with cell cycle regulation, ion homeostasis and hypoxia signaling.

Conversion of human fibroblasts into functional cardiomyocytes by small molecules

Reprogramming noncardiac cells into functional cardiomyocytes without any genetic manipulation could open up new avenues for cardiac regenerative therapies. Cao et al. identified a combination of nine small molecules that could epigenetically activate human fibroblasts, efficiently reprogramming them into chemically induced cardiomyocytes (ciCMs). The ciCMs contracted uniformly and resembled human cardiomyocytes. This method may be adapted for reprogramming multiple cell types and have important implications in regenerative medicine.

Science , this issue p. 1216

Conversion of human fibroblasts into functional cardiomyocytes by small molecules

Reprogramming somatic fibroblasts into alternative lineages would provide a promising source of cells for regenerative therapy. However, transdifferentiating human cells into specific homogeneous, functional cell types is challenging. Here we show that cardiomyocyte-like cells can be generated by treating human fibroblasts with a combination of nine compounds that we term 9C. The chemically induced cardiomyocyte-like cells uniformly contracted and resembled human cardiomyocytes in their transcriptome, epigenetic, and electrophysiological properties. 9C treatment of human fibroblasts resulted in a more open-chromatin conformation at key heart developmental genes, enabling their promoters and enhancers to bind effectors of major cardiogenic signals. When transplanted into infarcted mouse hearts, 9C-treated fibroblasts were efficiently converted to chemically induced cardiomyocyte-like cells. This pharmacological approach to lineage-specific reprogramming may have many important therapeutic implications after further optimization to generate mature cardiac cells.

Miniaturized iPS-Cell-Derived Cardiac Muscles for Physiologically Relevant Drug Response Analyses

Tissue engineering approaches have the potential to increase the physiologic relevance of human iPS-derived cells, such as cardiomyocytes (iPS-CM). However, forming Engineered Heart Muscle (EHM) typically requires >1 million cells per tissue. Existing miniaturization strategies involve complex approaches not amenable to mass production, limiting the ability to use EHM for iPS-based disease modeling and drug screening. Micro-scale cardiospheres are easily produced, but do not facilitate assembly of elongated muscle or direct force measurements. Here we describe an approach that combines features of EHM and cardiospheres: Micro-Heart Muscle (μHM) arrays, in which elongated muscle fibers are formed in an easily fabricated template, with as few as 2,000 iPS-CM per individual tissue. Within μHM, iPS-CM exhibit uniaxial contractility and alignment, robust sarcomere assembly, and reduced variability and hypersensitivity in drug responsiveness, compared to monolayers with the same cellular composition. μHM mounted onto standard force measurement apparatus exhibited a robust Frank-Starling response to external stretch, and a dose-dependent inotropic response to the β-adrenergic agonist isoproterenol. Based on the ease of fabrication, the potential for mass production and the small number of cells required to form μHM, this system provides a potentially powerful tool to study cardiomyocyte maturation, disease and cardiotoxicology in vitro.

CRISPR Interference Efficiently Induces Specific and Reversible Gene Silencing in Human iPSCs

Developing technologies for efficient and scalable disruption of gene expression will provide powerful tools for studying gene function, developmental pathways, and disease mechanisms. Here, we develop clustered regularly interspaced short palindromic repeat interference (CRISPRi) to repress gene expression in human induced pluripotent stem cells (iPSCs). CRISPRi, in which a doxycycline-inducible deactivated Cas9 is fused to a KRAB repression domain, can specifically and reversibly inhibit gene expression in iPSCs and iPSC-derived cardiac progenitors, cardiomyocytes, and T lymphocytes. This gene repression system is tunable and has the potential to silence single alleles. Compared with CRISPR nuclease (CRISPRn), CRISPRi gene repression is more efficient and homogenous across cell populations. The CRISPRi system in iPSCs provides a powerful platform to perform genome-scale screens in a wide range of iPSC-derived cell types, dissect developmental pathways, and model disease.

Automated Video-Based Analysis of Contractility and Calcium Flux in Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes Cultured over Different Spatial Scales

Contractile motion is the simplest metric of cardiomyocyte health in vitro, but unbiased quantification is challenging. We describe a rapid automated method, requiring only standard video microscopy, to analyze the contractility of human-induced pluripotent stem cell-derived cardiomyocytes (iPS-CM). New algorithms for generating and filtering motion vectors combined with a newly developed isogenic iPSC line harboring genetically encoded calcium indicator, GCaMP6f, allow simultaneous user-independent measurement and analysis of the coupling between calcium flux and contractility. The relative performance of these algorithms, in terms of improving signal to noise, was tested. Applying these algorithms allowed analysis of contractility in iPS-CM cultured over multiple spatial scales from single cells to three-dimensional constructs. This open source software was validated with analysis of isoproterenol response in these cells, and can be applied in future studies comparing the drug responsiveness of iPS-CM cultured in different microenvironments in the context of tissue engineering.

Calcium transients closely reflect prolonged action potentials in iPSC models of inherited cardiac arrhythmia

Long-QT syndrome mutations can cause syncope and sudden death by prolonging the cardiac action potential (AP). Ion channels affected by mutations are various, and the influences of cellular calcium cycling on LQTS cardiac events are unknown. To better understand LQTS arrhythmias, we performed current-clamp and intracellular calcium ([Ca(2+)]i) measurements on cardiomyocytes differentiated from patient-derived induced pluripotent stem cells (iPS-CM). In myocytes carrying an LQT2 mutation (HERG-A422T), APs and [Ca(2+)]i transients were prolonged in parallel. APs were abbreviated by nifedipine exposure and further lengthened upon releasing intracellularly stored Ca(2+). Validating this model, control iPS-CM treated with HERG-blocking drugs recapitulated the LQT2 phenotype. In LQT3 iPS-CM, expressing NaV1.5-N406K, APs and [Ca(2+)]i transients were markedly prolonged. AP prolongation was sensitive to tetrodotoxin and to inhibiting Na(+)-Ca(2+) exchange. These results suggest that LQTS mutations act partly on cytosolic Ca(2+) cycling, potentially providing a basis for functionally targeted interventions regardless of the specific mutation site.

Direct reprogramming of human fibroblasts toward a cardiomyocyte-like state

Direct reprogramming of adult somatic cells into alternative cell types has been shown for several lineages. We previously showed that GATA4, MEF2C, and TBX5 (GMT) directly reprogrammed nonmyocyte mouse heart cells into induced cardiomyocyte-like cells (iCMs) in vitro and in vivo. However, GMT alone appears insufficient in human fibroblasts, at least in vitro. Here, we show that GMT plus ESRRG and MESP1 induced global cardiac gene-expression and phenotypic shifts in human fibroblasts derived from embryonic stem cells, fetal heart, and neonatal skin. Adding Myocardin and ZFPM2 enhanced reprogramming, including sarcomere formation, calcium transients, and action potentials, although the efficiency remained low. Human iCM reprogramming was epigenetically stable. Furthermore, we found that transforming growth factor β signaling was important for, and improved the efficiency of, human iCM reprogramming. These findings demonstrate that human fibroblasts can be directly reprogrammed toward the cardiac lineage, and lay the foundation for future refinements in vitro and in vivo.

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Human fibroblasts can be directly induced toward a CM-like state by defined factors

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Reprogramming of fibroblasts toward a CM state is epigenetically stable

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Human and mouse in vitro iCMs display a comparable gene-expression shift

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TGF-β signaling is important for human iCM reprogramming

Ion channel pharmacology under flow: automation via well-plate microfluidics

Automated patch clamping addresses the need for high-throughput screening of chemical entities that alter ion channel function. As a result, there is considerable utility in the pharmaceutical screening arena for novel platforms that can produce relevant data both rapidly and consistently. Here we present results that were obtained with an innovative microfluidic automated patch clamp system utilizing a well-plate that eliminates the necessity of internal robotic liquid handling. Continuous recording from cell ensembles, rapid solution switching, and a bench-top footprint enable a number of assay formats previously inaccessible to automated systems. An electro-pneumatic interface was employed to drive the laminar flow of solutions in a microfluidic network that delivered cells in suspension to ensemble recording sites. Whole-cell voltage clamp was applied to linear arrays of 20 cells in parallel utilizing a 64-channel voltage clamp amplifier. A number of unique assays requiring sequential compound applications separated by a second or less, such as rapid determination of the agonist EC(50) for a ligand-gated ion channel or the kinetics of desensitization recovery, are enabled by the system. In addition, the system was validated via electrophysiological characterizations of both voltage-gated and ligand-gated ion channel targets: hK(V)2.1 and human Ether-à-go-go-related gene potassium channels, hNa(V)1.7 and 1.8 sodium channels, and (α1) hGABA(A) and (α1) human nicotinic acetylcholine receptor receptors. Our results show that the voltage dependence, kinetics, and interactions of these channels with pharmacological agents were matched to reference data. The results from these IonFlux™ experiments demonstrate that the system provides high-throughput automated electrophysiology with enhanced reliability and consistency, in a user-friendly format.

Mechanisms underlying the effects of the pyrethroid tefluthrin on action potential duration in isolated rat ventricular myocytes

Due to increased global use, acute exposures to pyrethroid insecticides in humans are of clinical concern. Pyrethroids have a primary mode of action that involves interference with the inactivation of Na+ currents (I(Na)) in excitable cells, which may include cardiac myocytes. To investigate the possible cardiac toxicity of these agents, we have examined the effects of a type-1 pyrethroid, tefluthrin, on isolated rat ventricular myocytes. Under whole-cell current-clamp, tefluthrin prolonged the mean action potential duration at 90% repolarization (APD90) by 216 +/- 34% in 19 myocytes isolated from 14 hearts. About one-third of this prolongation was apparently due to persistent I(Na), with the balance associated with spontaneous cytosolic Ca2+ waves, and Na+-Ca2+ exchange. In some action potentials, tefluthrin also activated early after-depolarizations (EADs). Using a selected EAD-containing action potential clamp, we observed that EADs could evoke a Cd2+-sensitive membrane current (I(EAD)) that triggered secondary sarcoplasmic reticulum (SR) Ca2+ release. The notion that EADs could stimulate Ca2+ current was strengthened by the persistence of I(EAD) in myocytes exposed to extracellular Li+ and Sr2+ ions, used to minimize Na+-Ca2+ exchange and SR Ca2+ release, respectively. Tefluthrin inhibited I(EAD) by approximately 10%. Together, our results support an arrhythmogenic model whereby tefluthrin exposure stimulated Na+ influx, provoking cellular Ca2+ overload by reverse Na+-Ca2+ exchange. During Ca2+ waves, forward Na+-Ca2+ exchange prolonged the action potential markedly and kindled EADs by permitting the reactivation of Ca2+ current. Similar mechanisms may be involved in pyrethroid toxicity in vivo, and also in type 3 long QT syndrome, wherein Na+ channel mutations prolong I(Na).

Differential effects of extracellular cesium on early afterdepolarizations in ventricular myocytes and arrhythmogenesis in isolated hearts of rats and guinea pigs

CsCl has been shown to be arrhythmogenic in-vivo and to cause early afterdepolarizations (EADs) in isolated cardiac preparations, but the underlying electrophysiological mechanisms are ill-defined. To elucidate these actions further, the effects of extracellular solutions containing 3 mM CsCl and either 2 mM KCl (Cs2K solution) or 5 mM KCl (Cs5K solution) on membrane potential and ionic currents in rat and guinea-pig ventricular myocytes were compared. Cs2K solution rapidly and reversibly inhibited outward I(K1), and reduced other K(+) currents by about 20%. Current-clamped myocytes were rapidly hyperpolarized by this solution and action potentials were prolonged, but EADs were not observed. In contrast, EADs were triggered by E-4031, H(2)O(2), and the pyrethroid tefluthrin. Membrane-potential changes reversed after replacing Cs2K with Cs5K solution, with the recovery of 50% of outward I(K1). These results suggest that Cs2K solution inhibited I(K1) and caused a late prolongation of the action-potential duration, but the affected membrane potentials were too negative to elicit EAD mechanisms. In isolated hearts perfused with modified Tyrode's, Cs2K, and Cs5K solutions, bradycardia and arrhythmias were evoked by both CsCl-containing solutions. A comparison of such results with the effects of these solutions on myocytes suggests that I(K1) inhibition and EADs in ventricular myocytes are unlikely to be involved in arrhythmogenesis under our conditions.

Effects of Na+/Ca2+ exchange induced by SR Ca2+ release on action potentials and afterdepolarizations in guinea pig ventricular myocytes

In cardiac cells, evoked Ca2+ releases or spontaneous Ca2+ waves activate the inward Na+/Ca2+ exchange current (INaCa), which may modulate membrane excitability and arrhythmogenesis. In this study, we examined changes in membrane potential due to INaCa elicited by sarcoplasmic reticulum (SR) Ca2+ release in guinea pig ventricular myocytes using whole cell current clamp, fluorescence, and confocal microscopy. Inhibition of INaCa by Na+-free, Li+-containing Tyrode solution reversibly abbreviated the action potential duration at 90% repolarization (APD90) by 50% and caused SR Ca2+ overload. APD90 was similarly abbreviated in myocytes exposed to the Na+/Ca2+ exchange inhibitor KB-R7943 (5 microM) or after inhibition of SR Ca2+ release with ryanodine (20 microM). In the absence of extracellular Na+, spontaneous SR Ca2+ releases caused minimal changes in resting membrane potential. After the myocytes were returned to Na+-containing solution, the potentiated intracellular Ca2+ concentration ([Ca2+]i) transients dramatically prolonged APD90 and [Ca2+]i oscillations caused delayed and early afterdepolarizations (DADs and EADs). Laser-flash photolysis of caged Ca2+ mimicked the effects of spontaneous [Ca2+]i oscillations, confirming that APD prolongation, DADs, and EADs could be ascribed to intracellular Ca2+ release. These results suggest that Na+/Ca2+ exchange is a major physiological determinant of APD and that INaCa activation by spontaneous SR Ca2+ release/oscillations, depending on the timing, can account for both DADs and EADs during SR Ca2+ overload.

Tefluthrin modulates a novel anionic background conductance (I(AB)) in guinea-pig ventricular myocytes

This report describes for the first time a novel anionic background current (I(AB)) identified in guinea-pig isolated ventricular myocytes. It also shows that I(AB) has both novel and differential pharmacology from other (cardiac) chloride currents. Using the whole-cell patch-clamp technique and external anion substitution, I(AB) was found to be outwardly rectifying and highly permeable to NO(-)(3), with a relative permeability sequence of NO(-)(3) > I(-) > Cl(-). I(AB) was not blocked by 50 microM DIDS, by hypertonic external solution, or by the nonselective protein kinase inhibitor H7-DHC. Exposure to the pyrethroid agent tefluthrin (10 microM) increased the current density of I(AB) significantly at positive voltages (P < 0.05), but had no significant effect on other cardiac chloride currents. We conclude that I(AB) possesses a distinct pharmacology and does not fall into the three major classes of cardiac chloride conductance commonly reported.

Actions of pyrethroid insecticides on sodium currents, action potentials, and contractile rhythm in isolated mammalian ventricular myocytes and perfused hearts

Pyrethroid insecticides are known to modify neuronal sodium channels, inducing persistent, steady-state sodium current at depolarized membrane potentials. Cardiac myocytes are also rich in sodium channels but comparatively little is known about the effect of pyrethroids on the heart, or on the cardiac sodium channel isoform. In the present study therefore, we determined the actions of type I and type II pyrethroids against rat and guinea pig ventricular myocytes under current and voltage clamp, and on isolated perfused rat hearts. In myocytes, tefluthrin (type I) and fenpropathrin and alpha-cypermethrin (type II) prolonged action potentials and evoked afterdepolarizations. The time course of sodium current (I(Na)) was also prolonged by these compounds. Pyrethroids delayed I(Na) inactivation, when measured under selective conditions as current sensitive to 30 microM tetrodotoxin, by increasing the proportion of slowly inactivating current at the expense of fast inactivating current. Further experiments, focusing on fenpropathrin, revealed that its effects on I(Na) inactivation time course were dose-dependent, and the Na(+) "window-current" was increased in its presence. In unstimulated, isolated hearts perfused with the same pyrethroids, the variability in contraction amplitude increased due to variations in the intervals between heartbeats. These potentially arrhythmogenic changes are consistent with the effects observed at the cellular level. The type I pyrethroid tetramethrin had little effect in any of the preparations. These findings suggest that some pyrethroids possess considerable mammalian cardiac arrhythmogenic potential, the manifestation of which in vivo may depend on the route of exposure.

Signature currents: a patch-clamp method for determining the selectivity of ion-channel blockers in isolated cardiac myocytes

BACKGROUND

We describe a simple method using membrane potential ramps for rapidly determining the ion-channel selectivity of drugs that affect action-potential duration in isolated cardiac myocytes. The method allows the simultaneous assay of compounds on a number of ionic currents in a single cardiac cell.

METHODS

Trains of membrane potential ramps were applied from -90 to +70 mV at 0.33 Hz to obtain a consistent "signature current," in which the major individual currents involved in the cardiac action potential could be easily identified. Confirmatory experiments were performed using known inhibitors of these currents.

RESULTS

The identities of the currents in the signature were established by varying the concentrations of extracellular cations and by adding known ion channel blockers to superfusion solutions. Inhibition of each current had a characteristic and reproducible effect on the overall signature current.

CONCLUSIONS

The consistent current signature in the presence and absence of blockers suggests that this method could be used for tertiary electrophysiological evaluation of compounds, eg, in a drug discovery program focusing on antiarrhythmic agents. The ability to assay for secondary effects of novel compounds against multiple currents in the target cell type is convenient and avoids the artefacts associated with using artificial expression systems.

Loading of calcium and strontium into the sarcoplasmic reticulum in rat ventricular muscle

Previous work suggests that strontium ions (Sr(2+)) are less effective than calcium ions (Ca(2+)) at supporting excitation-contraction (EC) coupling in cardiac muscle. We therefore tested whether this was due to differences in the uptake and release of Ca(2+)and Sr(2+)by the sarcoplasmic reticulum (SR) of rat ventricular trabeculae and myocytes at 22-24 degrees C. In permeabilized trabeculae, isometric contractions activated by exposure to Ca(2+)- and Sr(2+)-containing solutions produced similar maximal force, but were four times more sensitive to Ca(2+)than to Sr(2+). The rate of loading and maximal SR capacity for caffeine-releasable Ca(2+)and Sr(2+)were similar. In isolated, voltage-clamped ventricular myocytes, the SR content was measured as Na(+)-Ca(2+)exchange current during caffeine-induced SR cation releases. The SR Ca(2+)load reached a steady maximum during a train of voltage clamp depolarizations. A similar maximal Sr(2+)load was not observed, suggesting that the SR capacity for Sr(2+)exceeds that for Ca(2+). Therefore, the relative inability of Sr(2+)to support cardiac EC coupling appears not to be due to failure of the SR to sequester Sr(2+). Instead, increases in cytosolic [Sr(2+)] seem to poorly activate Sr(2+)release from the SR.

A novel anionic conductance affects action potential duration in isolated rat ventricular myocytes

Effects of extracellular anions were studied in electrophysiological experiments on freshly isolated rat ventricular myocytes. Under current-clamp, action potential duration (APD) was prolonged by reducing the extracellular Cl(-) concentration and shortened by replacement of extracellular Cl(-) with I(-). Under voltage-clamp, membrane potential steps or ramps evoked an anionic background current (I(AB)) carried by either Cl(-), Br(-), I(-) or NO(3)(-). Activation of I(AB) was Ca(2+)- and cyclic AMP-independent, and was unaffected by cell shrinkage. I(AB) was insensitive to stilbene and fenamate anion transport blockers at concentrations that inhibit Ca(2+)-, cyclic AMP- and swelling-activated Cl(-) currents in ventricular cells of other mammals. These results suggest that I(AB) may be carried by a novel class of Cl(-) channel. Correlation of anion substitution experiments on membrane current and action potentials revealed that I(AB) could play a major role in controlling rat ventricular APD. These findings have important implications for those studying cardiac Cl(-) channels as potential targets for novel antiarrythmic agents.

Calcium-induced release of strontium ions from the sarcoplasmic reticulum of rat cardiac ventricular myocytes

1. The effects of strontium ions, Sr2+, on Ca(2+)-dependent feedback mechanisms during excitation-contraction coupling were examined in voltage-clamped rat ventricular myocytes in which intracellular [Ca2+] and [Sr2+] were monitored with the fluorescent indicator, indo-1. 2. Voltage clamp depolarizations and caffeine applications during superfusion in Ca(2+)-free, Sr(2+)-containing solutions were employed to exchange intracellular Ca2+ with Sr2+. Myocytes were loaded with Sr2+ by applying voltage clamp depolarizations during superfusion in Na(+)-free, Sr(2+)-containing solutions. 3. Caffeine applications produced large fluorescence transients in Sr(2+)-loaded cells. Thus, Sr2+ could be sequestered and released from the sarcoplasmic reticulum. 4. Ca2+ influx, but not Sr2+ influx, via sarcolemmal Ca2+ channels evoked ryanodine-sensitive fluorescence transients in Sr(2+)-loaded cells. These results demonstrated that Ca2+ influx-induced Sr2+ release (CISR) from the sarcoplasmic reticulum occurred in these experiments, even though Sr2+ influx-induced Sr2+ release was not observed. 5. The amplitude of the Ca2+ influx-induced fluorescence transient was 17 +/- 1% of the caffeine-induced transient (n = 5 cells), an indication that fractional utilization of Sr2+ sequestered in the sarcoplasmic reticulum during CISR was low. 6. With increased Sr2+ loading, the amplitude of Ca2+ influx- and caffeine-induced fluorescence transients increased, but fractional utilization of sarcoplasmic reticulum divalent cation stores was independent of the degree of Sr2+ loading. These data suggest that Ca2+ influx directly activated the release of divalent cations from the sarcoplasmic reticulum, but mechanisms promoting positive feedback of Sr2+ release were minimal during CISR. 7. By comparison, in Ca(2+)-loaded myocytes, Ca2+ influx-induced Ca2+ release (CICR) utilized a greater fraction of caffeine-releasable stores than CISR. Fractional utilization of Ca2+ stores during CICR increased with the degree of Ca2+ loading. 8. Taken together, these results suggest that Ca(2+)-dependent feedback mechanisms play a major role in determining the extent of sarcoplasmic reticulum Ca2+ release during cardiac excitation-contraction coupling under a wide range of conditions.

Control of sarcoplasmic reticulum calcium release during calcium loading in isolated rat ventricular myocytes

1. Isolated rat ventricular myocytes were whole-cell voltage clamped using electrodes containing fluorescent Ca2+ indicators. Cytosolic [Ca2+] ([Ca2+]i) was estimated with calcium green-2 in combination with carboxy SNARF-1 to remove movement artifacts, or with indo-1. 2. Sarcoplasmic reticulum (SR) Ca2+ was depleted using 20 mM caffeine in Na(+)-containing superfusion solution, and cells were Ca2+ loaded by voltage clamp depolarizations applied during superfusion with Na(+)-free 2 mM Ca2+ solution. Ca2+ currents (ICa) and fluorescence transients elicited by these depolarizations were measured, and the releasable Ca2+ content of the Sr was estimated from the amplitude of fluorescence transients elicited by the rapid application of 20 mM caffeine. 3. Depolarization-induced [Ca2+]i transients increased in amplitude and duration during superfusion with Na(+)-free 2 mM Ca2+ solution, independent of changes in peak ICa. Caffeine application confirmed that the SR Ca2+ content increased during this manoeuvre. 4. With increased Ca2+ loading, the fraction of releasable SR Ca2+ involved in depolarization-induced transients increased, and the gradation in [Ca2+]i transient amplitude produced by beat-to-beat variation of voltage clamp pulse duration (10-100 ms) was progressively lost. This duration dependence of [Ca2+]i transients was maintained during Ca2+ loading when the Ca2+ buffering capacity of the electrode solution was increased with 100 microM BAPTA, 150 microM EGTA, or 60 microM indo-1. 5. These data suggest that Ca2+ released from the SR during a stimulated [Ca2+]i transient promotes further SR Ca2+ release to a degree which is smoothly graded with SR Ca2+ content. The effects of exogenous Ca2+ buffers suggest that this positive feedback is mediated, at least in part, by [Ca2+]i.

A method for recording intracellular [Ca2+] transients in cardiac myocytes using calcium green-2

Calcium green-2 (Ca green) is a non-ratiometric fluorescent Ca2+ indicator with an affinity for Ca2+ (dissociation constant Kd = 3 microM) that is lower than more commonly used indicators such as fura-2 and fluo-3. This low Ca2+ affinity, coupled with a high quantum yield, allows cells to be loaded with low concentrations of Ca green, avoiding problems of cytosolic Ca2+ buffering and a low signal-to-noise ratio. This communication presents a method for monitoring intracellular [Ca2+] changes in isolated rat ventricular myocytes loaded with Ca green and the fluorescent pH indicator carboxy SNARF-1 (SNARF). SNARF provides a Ca(2+)-insensitive signal with which Ca green fluorescence can be corrected for cell motion and dye-loading artifacts.

Effects of gadolinium on length-dependent force in guinea-pig papillary muscle

The degree to which stretch-activated channels operate during physiological length changes in multicellular heart preparations, or how much the channels could contribute to length-dependent activation, is not known. We studied the relationship between muscle length and contractile force in guinea-pig papillary muscles superfused with gadolinium chloride (10 microM), a stretch-activated channel blocker, and compared the effects to those with nifedipine (0.25 microM), a calcium channel blocker. Gadolinium reduced contractile force statistically significantly more at the longer muscle lengths than at the short muscle lengths. This did not apply with nifedipine, although a marginally greater effect at longer lengths was perceptible. The results can only partly be explained by gadolinium having a non-specific action via the calcium channel, or Na(+)-Ca2+ exchange, and are consistent with the possibility that stretch-activated channels contribute to length-dependent activation in cardiac muscle, and thus to 'Starling's Law of the Heart'.

Influences of stimulation frequency and temperature on interval-force relationships in guinea-pig papillary muscles

Relationships between contractile force and the preceding and pre-preceding stimulation intervals were studied in papillary muscles by interposing variable test intervals during steady-state pacing. The strength of test contractions increased exponentially to a maximum as the preceding (test) interval was lengthened. Contractility decreased as an exponential function of pre-preceding interval. At 37 degrees C, the half times for these processes were unaffected by increasing the steady-state frequency from 1 to 3 Hz. At 27 degrees C, the force increase with preceding interval was accelerated and the decay with pre-preceding interval was retarded as the stimulation frequency was increased from 0.33 to 2 Hz. The time-courses of force increase and decay were similar to each other during stimulation at an optimum frequency characteristic for the temperature. Cooling from 37 to 27 degrees C prolonged the half times for force increase and decay by factors of 4.5 and 3 respectively. The slope of the linear relationship between the force of the contraction pre-preceded by the test interval and the immediately subsequent contraction (recirculation fraction) was also halved. These results suggest that high stimulation frequency and low temperature uncouples cellular processes underlying the interval dependence of cardiac contractility. The temperature sensitivities are consistent with these processes being enzymatic. The reduced recirculation fraction provides a mechanism for the lowered threshold frequency for sustained mechanical alternans at 27 degrees C.

Effects of nifedipine and low [Ca2+] on mechanical restitution during hypothermia in guinea pig papillary muscles

Increasing the frequency of steady state stimulation increases the rate of mechanical restitution in hypothermic (27 degrees C) guinea pig papillary muscles. In this paper we have investigated the influences of the calcium antagonist nifedipine and of reduced extracellular calcium concentration on this phenomenon. We found that nifedipine abolished the frequency dependent increase in the restitution rate, which was also sensitive to extracellular [Ca2+]. These findings suggest that the level of intracellular [Ca2+] can influence the rate of restitution. It is implied that this effect is mediated via ICa, the inward calcium current, which makes a larger than normal contribution to direct contractile activation in hypothermic myocardium.

Mechanical restitution during alternans in guinea pig papillary muscles

OBJECTIVE

The aim was to investigate alternate acceleration and retardation of mechanical restitution as a possible mechanism for mechanical alternans in isolated myocardium.

METHODS

Mechanical alternans was induced in papillary muscles from the right ventricles of 11 guinea pigs (200-300 g) by rapid pacing under hypothermic conditions (T = 27 degrees C). Mechanical restitution curves were constructed by measuring the force responses to stimuli applied following variable test intervals during steady state pacing. Curves were obtained under control conditions (steady state stimulation interval 3 s), and for the beats following the large and small contractions during mechanical alternans. Monoexponentials were fitted to the restitution curves.

RESULTS

The mean rate constant for restitution following the large beat in alternans was found to be slightly but significantly smaller than that following the small. Both rate constants obtained during alternans were significantly larger than the control rate constant (restitution was faster in alternans). In addition, as the alternation widened, the restitution curve of the beat following the small contraction developed a higher plateau than that following the large.

CONCLUSIONS

The results confirm that the small beat in alternans is followed by faster restitution than the large. This alone is insufficient to explain the observed extent of alternans. The restitution curve for the beat following the small contraction must also rise to a higher plateau. Both the amount of calcium available for intracellular release and the rate at which it is made available vary from beat to beat.