Paradoxical twitch potentiation after rest in cardiac muscle: increased fractional release of SR calcium

Methods for assessing cardiac myofilament calcium sensitivity

Myofilament calcium (Ca2+) sensitivity is one of several mechanisms by which force production of cardiac muscle is modulated to meet the ever-changing demands placed on the heart. Compromised Ca2+ sensitivity is associated with pathologies, which makes it a parameter of interest for researchers. Ca2+ Sensitivity is the ratio of the association and dissociation rates between troponin C (TnC) and Ca2+. As it is not currently possible to measure these rates in tissue preparations directly, methods have been developed to infer myofilament sensitivity, typically using some combination of force and Ca2+ measurements. The current gold-standard approach constructs a steady-state force-Ca2+ relation by exposing permeabilised muscle samples to a range of Ca2+ concentrations and uses the half-maximal concentration as a proxy for sensitivity. While a valuable method for steady-state investigations, the permeabilisation process makes the method unsuitable when examining dynamic, i.e., twitch-to-twitch, changes in myofilament sensitivity. The ability of the heart to transiently adapt to changes in load is an important consideration when evaluating the impact of disease states. Alternative methods have been proffered, including force-Ca2+ phase loops, potassium contracture, hybrid experimental-modelling and conformation-based fluorophore approaches. This review provides an overview of the mechanisms underlying myofilament Ca2+ sensitivity, summarises existing methods, and explores, with modelling, whether any of them are suited to investigating dynamic changes in sensitivity. We conclude that a method that equips researchers to investigate the transient change of myofilament Ca2+ sensitivity is still needed. We propose that such a method will involve simultaneous measurements of cytosolic Ca2+ and TnC activation in actively twitching muscle and a biophysical model to interpret these data.

Loss of Mitochondrial Ca2+ Uniporter Limits Inotropic Reserve and Provides Trigger and Substrate for Arrhythmias in Barth Syndrome Cardiomyopathy

BACKGROUND

Barth syndrome (BTHS) is caused by mutations of the gene encoding tafazzin, which catalyzes maturation of mitochondrial cardiolipin and often manifests with systolic dysfunction during early infancy. Beyond the first months of life, BTHS cardiomyopathy typically transitions to a phenotype of diastolic dysfunction with preserved ejection fraction, blunted contractile reserve during exercise, and arrhythmic vulnerability. Previous studies traced BTHS cardiomyopathy to mitochondrial formation of reactive oxygen species (ROS). Because mitochondrial function and ROS formation are regulated by excitation-contraction coupling, integrated analysis of mechano-energetic coupling is required to delineate the pathomechanisms of BTHS cardiomyopathy.

METHODS

We analyzed cardiac function and structure in a mouse model with global knockdown of tafazzin (Taz-KD) compared with wild-type littermates. Respiratory chain assembly and function, ROS emission, and Ca²⁺ uptake were determined in isolated mitochondria. Excitation-contraction coupling was integrated with mitochondrial redox state, ROS, and Ca²⁺ uptake in isolated, unloaded or preloaded cardiac myocytes, and cardiac hemodynamics analyzed in vivo.

RESULTS

Taz-KD mice develop heart failure with preserved ejection fraction (>50%) and age-dependent progression of diastolic dysfunction in the absence of fibrosis. Increased myofilament Ca²⁺ affinity and slowed cross-bridge cycling caused diastolic dysfunction, in part, compensated by accelerated diastolic Ca²⁺ decay through preactivated sarcoplasmic reticulum Ca²⁺-ATPase. Taz deficiency provoked heart-specific loss of mitochondrial Ca²⁺ uniporter protein that prevented Ca²⁺-induced activation of the Krebs cycle during β-adrenergic stimulation, oxidizing pyridine nucleotides and triggering arrhythmias in cardiac myocytes. In vivo, Taz-KD mice displayed prolonged QRS duration as a substrate for arrhythmias, and a lack of inotropic response to β-adrenergic stimulation. Cellular arrhythmias and QRS prolongation, but not the defective inotropic reserve, were restored by inhibiting Ca²⁺ export through the mitochondrial Na⁺/Ca²⁺ exchanger. All alterations occurred in the absence of excess mitochondrial ROS in vitro or in vivo.

CONCLUSIONS

Downregulation of mitochondrial Ca²⁺ uniporter, increased myofilament Ca²⁺ affinity, and preactivated sarcoplasmic reticulum Ca²⁺-ATPase provoke mechano-energetic uncoupling that explains diastolic dysfunction and the lack of inotropic reserve in BTHS cardiomyopathy. Furthermore, defective mitochondrial Ca²⁺ uptake provides a trigger and a substrate for ventricular arrhythmias. These insights can guide the ongoing search for a cure of this orphaned disease.

The oral Ca/calmodulin-dependent kinase II inhibitor RA608 improves contractile function and prevents arrhythmias in heart failure

Aims

Excessive activation of Ca/calmodulin‐dependent kinase II (CaMKII) is of critical importance in heart failure (HF) and atrial fibrillation. Unfortunately, lack of selectivity, specificity, and bioavailability have slowed down development of inhibitors for clinical use. We investigated a novel CaMKIIδ/CaMKIIɣ‐selective, ATP‐competitive, orally available CaMKII inhibitor (RA608) on right atrial biopsies of 119 patients undergoing heart surgery. Furthermore, we evaluated its oral efficacy to prevent deterioration of HF in mice after transverse aortic constriction (TAC).

Methods and results

In human atrial cardiomyocytes and trabeculae, respectively, RA608 significantly reduced sarcoplasmic reticulum Ca leak, reduced diastolic tension, and increased sarcoplasmic reticulum Ca content. Patch‐clamp recordings confirmed the safety of RA608 in human cardiomyocytes. C57BL6/J mice were subjected to TAC, and left ventricular function was monitored by echocardiography. Two weeks after TAC, RA608 was administered by oral gavage for 7 days. Oral RA608 treatment prevented deterioration of ejection fraction. At 3 weeks after TAC, ejection fraction was 46.1 ± 3.7% (RA608) vs. 34.9 ± 2.6% (vehicle), n = 9 vs. n = 12, P < 0.05, ANOVA, which correlated with significantly less CaMKII autophosphorylation at threonine 287. Moreover, a single oral dose significantly reduced inducibility of atrial and ventricular arrhythmias in CaMKIIδ transgenic mice 4 h after administration. Atrial fibrillation was induced in 6/6 mice for vehicle vs. 1/7 for RA608, P < 0.05, 'n − 1'  χ² test. Ventricular tachycardia was induced in 6/7 for vehicle vs. 2/7 for RA608, P < 0.05, 'n − 1'  χ² test.

Conclusions

RA608 is the first orally administrable CaMKII inhibitor with potent efficacy in human myocytes. Moreover, oral administration potently inhibits arrhythmogenesis and attenuates HF development in mice in vivo.

Sources of Ca2+ for contraction of the heart tube of Tenebrio molitor (Coleoptera: Tenebrionidae)

Insect and vertebrate hearts share the ability to generate spontaneously their rhythmic electrical activity, which triggers the fluid-propelling mechanical activity. Although insects have been used as models in studies on the impact of genetic alterations on cardiac function, there is surprisingly little information on the generation of the inotropic activity in their hearts. The main goal of this study was to investigate the sources of Ca²⁺ for contraction in Tenebrio molitor hearts perfused in situ, in which inotropic activity was assessed by the systolic variation of the cardiac luminal diameter. Increasing the pacing rate from 1.0 to 2.5 Hz depressed contraction amplitude and accelerated relaxation. To avoid inotropic interference of variations in spontaneous rate, which have been shown to occur in insect heart during maneuvers that affect Ca²⁺ cycling, experiments were performed under electrical pacing at near-physiological rates. Raising the extracellular Ca²⁺ concentration from 0.5 to 8 mM increased contraction amplitude in a manner sensitive to L-type Ca²⁺ channel blockade by D600. Inotropic depression was observed after treatment with caffeine or thapsigargin, which impair Ca²⁺ accumulation by the sarcoplasmic reticulum (SR). D600, but not inhibition of the sarcolemmal Na⁺/Ca²⁺ exchanger by KB-R7943, further depressed inotropic activity in thapsigargin-treated hearts. From these results, it is possible to conclude that in T. molitor heart, as in vertebrates: (a) inotropic and lusitropic activities are modulated by the heart rate; and (b) Ca²⁺ availability for contraction depends on both Ca²⁺ influx via L-type channels and Ca²⁺ release from the SR.

Synchronization of Intracellular Ca2+ Release in Multicellular Cardiac Preparations

In myocardial tissue, Ca²⁺ release from the sarcoplasmic reticulum (SR) that occurs via the ryanodine receptor (RyR2) channel complex. Ca²⁺ release through RyR2 can be either stimulated by an action potential (AP) or spontaneous. The latter is often associated with triggered afterdepolarizations, which in turn may lead to sustained arrhythmias. It is believed that some synchronization mechanism exists for afterdepolarizations and APs in neighboring myocytes, possibly a similarly timed recovery of RyR2 from refractoriness, which enables RyR2s to reach the threshold for spontaneous Ca²⁺ release simultaneously. To investigate this synchronization mechanism in absence of genetic factors that predispose arrhythmia, we examined the generation of triggered activity in multicellular cardiac preparations. In myocardial trabeculae from the rat, we demonstrated that in the presence of both isoproterenol and caffeine, neighboring myocytes within the cardiac trabeculae were able to synchronize their diastolic spontaneous SR Ca²⁺ release. Using confocal Ca²⁺ imaging, we could visualize Ca²⁺ waves in the multicellular preparation, while these waves were not always present in every myocyte within the trabeculae, we observed that, over time, the Ca²⁺ waves can synchronize in multiple myocytes. This synchronized activity was sufficiently strong that it could trigger a synchronized, propagated contraction in the whole trabecula encompassing even previously quiescent myocytes. The detection of Ca²⁺ dynamics in individual myocytes in their in situ setting at the multicellular level exposed a synchronization mechanism that could induce local triggered activity in the heart in the absence of global Ca²⁺ dysregulation.

Sarcoplasmic Reticulum Structure and Functional Properties that Promote Long-Lasting Calcium Sparks

Calcium (Ca) sparks are the fundamental sarcoplasmic reticulum (SR) Ca release events in cardiac myocytes, and they have a typical duration of 20-40 ms. However, when a fraction of ryanodine receptors (RyRs) are blocked by tetracaine or ruthenium red, Ca sparks lasting hundreds of milliseconds have been observed experimentally. The fundamental mechanism underlying these extremely prolonged Ca sparks is not understood. In this study, we use a physiologically detailed mathematical model of subcellular Ca cycling to examine how Ca spark duration is influenced by the number of functional RyRs in a junctional cluster (which is reduced by tetracaine or ruthenium red) and other SR Ca handling properties. One RyR cluster contains a few to several hundred RyRs, and we use a four-state Markov RyR gating model. Each RyR opens stochastically and is regulated by cytosolic and luminal Ca. We varied the number of functional RyRs in the single cluster, diffusion within the SR network, diffusion between network and junctional SR, cytosolic Ca diffusion, SERCA uptake activity, and RyR open probability. For long-lasting Ca release events, opening events within the cluster must occur continuously because the typical open time of the RyR is only a few milliseconds. We found the following: 1) if the number of RyRs is too small, it is difficult to maintain consecutive openings and stochastic attrition terminates the release; 2) if the number of RyRs is too large, the depletion of Ca from the junctional SR terminates the release; and 3) very long release events require relatively small-sized RyR clusters (reducing flux as seen experimentally with tetracaine) and sufficiently rapid intra-SR Ca diffusion, such that local junctional intra-SR [Ca] can be maintained by intra-SR diffusion and overall SR Ca reuptake.

Failing Left Ventricles Have an Enhanced Post-Stimulation Potentiation Despite Their Impaired Force Frequency Relationship

The left ventricular contractile force (LV dP/dtmax) of patients with left ventricular systolic dysfunction does not increase effectively with an increase in heart rate. In other words, their force-frequency relationship (FFR) is impaired. However, it is unknown whether a longer coupling interval subsequent to tachycardia causes a stronger contraction (poststimulation potentiation, PSP) in a rate-dependent manner.In 16 patients with idiopathic dilated cardiomyopathy (DCM) (48 ± 2 years old, LVEF 30 ± 10%) and 6 control patients (58 ± 4 years old, LVEF 70 ± 7%), FFR was assessed by right atrial pacing using a micro-manometer-tipped catheter. At each pacing rate, the increase of LV dP/dtmax over basal LV dP/dt (ΔFFR) and the increase of LV dP/dtmax of the first beat after pacing cessation over LV dP/dtmax during pacing (ΔPSP) were evaluated.Patients with DCM had smaller LV dP/dtmax at baseline (872 ± 251 versus 1370 ± 123 mmHg/second, P = 0.0002) and developed smaller ΔFFR (eg, at 120/minute, 77 ± 143 versus 331 ± 131 mmHg/second, P = 0.0011). In contrast, they showed a rate-dependent increase of LV dP/dtmax of PSP and had greater ΔPSP (eg, at 120/minute, 294 ± 173 versus -152 ± 131 mmHg/second, P < 0.0001).Failing left ventricles develop little contractile force during tachycardia despite their rate-dependent enhancement in post-stimulation potentiation, suggesting that refractoriness of contractile force underlies impaired FFR.

Impact of detubulation on force and kinetics of cardiac muscle contraction

T-tubule uncoupling from the plasma membrane leads to myocardial contractile abnormalities.

Action potential–driven Ca²⁺ currents from the transverse tubules (t-tubules) trigger synchronous Ca²⁺ release from the sarcoplasmic reticulum of cardiomyocytes. Loss of t-tubules has been reported in cardiac diseases, including heart failure, but the effect of uncoupling t-tubules from the sarcolemma on cardiac muscle mechanics remains largely unknown. We dissected intact rat right ventricular trabeculae and compared force, sarcomere length, and intracellular Ca²⁺ in control trabeculae with trabeculae in which the t-tubules were uncoupled from the plasma membrane by formamide-induced osmotic shock (detubulation). We verified disconnection of a consistent fraction of t-tubules from the sarcolemma by two-photon fluorescence imaging of FM4-64–labeled membranes and by the absence of tubular action potential, which was recorded by random access multiphoton microscopy in combination with a voltage-sensitive dye (Di-4-AN(F)EPPTEA). Detubulation reduced the amplitude and prolonged the duration of Ca²⁺ transients, leading to slower kinetics of force generation and relaxation and reduced twitch tension (1 Hz, 30°C, 1.5 mM [Ca²⁺]_o). No mechanical changes were observed in rat left atrial trabeculae after formamide shock, consistent with the lack of t-tubules in rodent atrial myocytes. Detubulation diminished the rate-dependent increase of Ca²⁺-transient amplitude and twitch force. However, maximal twitch tension at high [Ca²⁺]_o or in post-rest potentiated beats was unaffected, although contraction kinetics were slower. The ryanodine receptor (RyR)2 Ca-sensitizing agent caffeine (200 µM), which increases the velocity of transverse Ca²⁺ release propagation in detubulated cardiomyocytes, rescued the depressed contractile force and the slower twitch kinetics of detubulated trabeculae, with negligible effects in controls. We conclude that partial loss of t-tubules leads to myocardial contractile abnormalities that can be rescued by enhancing and accelerating the propagation of Ca²⁺-induced Ca²⁺ release to orphan RyR2 clusters.

Decrease in sarcoplasmic reticulum calcium content, not myofilament function, contributes to muscle twitch force decline in isolated cardiac trabeculae

We set out to determine the factors responsible for twitch force decline in isolated intact rat cardiac trabeculae. The contractile force of trabeculae declined over extended periods of isometric twitch contractions. The force-frequency relationship within the frequency range of 4-8 Hz, at 37 °C, became more positive and the frequency optimum shifted to higher rates with this decline in baseline twitch tensions. The post-rest potentiation (37 °C), a phenomenon highly dependent on calcium handling mechanisms, became more pronounced with decrease in twitch tensions. We show that the main abnormality during muscle run-down was not due to a deficit in the myofilaments; maximal tension achieved using a K(+) contracture protocol was either unaffected or only slightly decreased. Conversely, the sarcoplasmic reticulum (SR) calcium content, as assessed by rapid cooling contractures (from 27 to 0 °C), decreased, and had a close association with the declining twitch tensions (R(2) ~ 0.76). SR Ca(2+)-ATPase, relative to Na(+)/Ca(2+) exchanger activity, was not altered as there was no significant change in paired rapid cooling contracture ratios. Furthermore, confocal microscopy detected no abnormalities in the overall structure of the cardiomyocytes and t-tubules in the cardiac trabeculae (~23 °C). Overall, the data indicates that the primary mechanism responsible for force run-down in multi-cellular cardiac preparations is a decline in the SR calcium content and not the maximal tension generation capability of the myofilaments.

Cardiac sarcoplasmic reticulum calcium leak: basis and roles in cardiac dysfunction

Synchronized SR calcium (Ca) release is critical to normal cardiac myocyte excitation-contraction coupling, and ideally this release shuts off completely between heartbeats. However, other SR Ca release events are referred to collectively as SR Ca leak (which includes Ca sparks and waves as well as smaller events not detectable as Ca sparks). Much, but not all, of the SR Ca leak occurs via ryanodine receptors and can be exacerbated in pathological states such as heart failure. The extent of SR Ca leak is important because it can (a) reduce SR Ca available for release, causing systolic dysfunction; (b) elevate diastolic [Ca]i, contributing to diastolic dysfunction; (c) cause triggered arrhythmias; and (d) be energetically costly because of extra ATP used to repump Ca. This review addresses quantitative aspects and manifestations of SR Ca leak and its measurement, and how leak is modulated by Ca, associated proteins, and posttranslational modifications in health and disease.

Cardiac electrophysiology in mice: a matter of size

Over the last decade, mouse models have become a popular instrument for studying cardiac arrhythmias. This review assesses in which respects a mouse heart is a miniature human heart, a suitable model for studying mechanisms of cardiac arrhythmias in humans and in which respects human and murine hearts differ. Section I considers the issue of scaling of mammalian cardiac (electro) physiology to body mass. Then, we summarize differences between mice and humans in cardiac activation (section II) and the currents underlying the action potential in the murine working myocardium (section III). Changes in cardiac electrophysiology in mouse models of heart disease are briefly outlined in section IV, while section V discusses technical considerations pertaining to recording cardiac electrical activity in mice. Finally, section VI offers general considerations on the influence of cardiac size on the mechanisms of tachy-arrhythmias.

Cardiac remodeling in rats with renal failure shows interventricular differences

Chronic renal failure (CRF) is associated with an increased incidence of cardiovascular diseases. Intensive research revealed a number of alterations in the heart during CRF; however, possible interventricular differences in CRF-induced cardiac remodeling have so far not been addressed. CRF was induced by two-stage surgical 5/6 nephrectomy (NX) in male Wistar rats. Cellular hypertrophy was quantified using immunohistological morphometric analysis. Contraction force and membrane potential were recorded in left and right ventricle papillary muscles with an isometric force transducer and high-resistance glass microelectrodes. Hypertrophy was present in the left ventricle (LV) of NX animals, but not in the right ventricle (RV) of NX animals, as documented by both ventricle/body weight ratios and cellular morphometric analysis of the cross-sectional area of myocytes. The contraction force was reduced in the LV of NX animals but increased in the RV of NX animals compared with sham-operated rats. Rest potentiation of contraction force was relatively more pronounced in the LV of NX rats. Fifty percent substitution of extracellular sodium with lithium significantly increased the contraction force only in the LV of NX animals. Action potential durations were shortened in both ventricles of CRF animals. Cardiac structural and contractile remodeling in CRF shows significant interventricular differences. CRF induces hypertrophy of the LV but not of the RV. LV hypertrophy was associated with a reduction of contraction force, whereas in the RV, the contraction force was enhanced. Partial recovery of contractile function of the LV by rest potentiation or lithium substitution indicates a role of the Na(+)/Ca(2+) exchanger in this phenomenon.

Local β-adrenergic stimulation overcomes source-sink mismatch to generate focal arrhythmia

RATIONALE

β-Adrenergic receptor stimulation produces sarcoplasmic reticulum Ca(2+) overload and delayed afterdepolarizations in isolated ventricular myocytes. How delayed afterdepolarizations are synchronized to overcome the source-sink mismatch and produce focal arrhythmia in the intact heart remains unknown.

OBJECTIVE

To determine whether local β-adrenergic receptor stimulation produces spatiotemporal synchronization of delayed afterdepolarizations and to examine the effects of tissue geometry and cell-cell coupling on the induction of focal arrhythmia.

METHODS AND RESULTS

Simultaneous optical mapping of transmembrane potential and Ca(2+) transients was performed in normal rabbit hearts during subepicardial injections (50 μL) of norepinephrine (NE) or control (normal Tyrode's solution). Local NE produced premature ventricular complexes (PVCs) from the injection site that were dose-dependent (low-dose [30-60 μmol/L], 0.45±0.62 PVCs per injection; high-dose [125-250 μmol/L], 1.33±1.46 PVCs per injection; P<0.0001) and were inhibited by propranolol. NE-induced PVCs exhibited abnormal voltage-Ca(2+) delay at the initiation site and were inhibited by either sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase inhibition or reduced perfusate [Ca(2+)], which indicates a Ca(2+)-mediated mechanism. NE-induced PVCs were more common at right ventricular than at left ventricular sites (1.48±1.50 versus 0.55±0.89, P<0.01), and this was unchanged after chemical ablation of endocardial Purkinje fibers, which suggests that source-sink interactions may contribute to the greater propensity to right ventricular PVCs. Partial gap junction uncoupling with carbenoxolone (25 μmol/L) increased focal activity (2.18±1.43 versus 1.33±1.46 PVCs per injection, P<0.05), which further supports source-sink balance as a critical mediator of Ca(2+)-induced PVCs.

CONCLUSIONS

These data provide the first experimental demonstration that localized β-adrenergic receptor stimulation produces spatiotemporal synchronization of sarcoplasmic reticulum Ca(2+) overload and release in the intact heart and highlight the critical nature of source-sink balance in initiating focal arrhythmias.

Molecular characterization and functional properties of cardiomyocytes derived from human inducible pluripotent stem cells

In view of the therapeutic potential of cardiomyocytes derived from induced pluripotent stem (iPS) cells (iPS-derived cardiomyocytes), in the present study we investigated in iPS-derived cardiomyocytes, the functional properties related to [Ca(2+) ](i) handling and contraction, the contribution of the sarcoplasmic reticulum (SR) Ca(2+) release to contraction and the b-adrenergic inotropic responsiveness. The two iPS clones investigated here were generated through infection of human foreskin fibroblasts (HFF) with retroviruses containing the four human genes: OCT4, Sox2, Klf4 and C-Myc. Our major findings showed that iPS-derived cardiomyocytes: (i) express cardiac specific RNA and proteins; (ii) exhibit negative force-frequency relations and mild (compared to adult) post-rest potentiation; (iii) respond to ryanodine and caffeine, albeit less than adult cardiomyocytes, and express the SR-Ca(2+) handling proteins ryanodine receptor and calsequestrin. Hence, this study demonstrates that in our cardiomyocytes clones differentiated from HFF-derived iPS, the functional properties related to excitation-contraction coupling, resemble in part those of adult cardiomyocytes.

Increased myofilament Ca2+-sensitivity and arrhythmia susceptibility

Increased myofilament Ca(2+) sensitivity is a common attribute of many inherited and acquired cardiomyopathies that are associated with cardiac arrhythmias. Accumulating evidence supports the concept that increased myofilament Ca(2+) sensitivity is an independent risk factor for arrhythmias. This review describes and discusses potential underlying molecular and cellular mechanisms how myofilament Ca(2+) sensitivity affects cardiac excitation and leads to the generation of arrhythmias. Emphasized are downstream effects of increased myofilament Ca(2+) sensitivity: altered Ca(2+) buffering/handling, impaired energy metabolism and increased mechanical stretch, and how they may contribute to arrhythmogenesis.

Effects of bupivacaine, levobupivacaine and ropivacaine on myocardial relaxation

PURPOSE

Ropivacaine and levobupivacaine were developed to reduce the risk of occasional toxicity reported with bupivacaine. While the effects of long-acting local anesthetics (LAAs) on myocardial contractility (inotropy) are well described, their effects on relaxation (lusitropy) remain largely unknown. The present study aimed to compare the effects of LAAs on rat myocardium.

METHODS

Left ventricular papillary muscles of male Wistar rats were used to compare the inotropic and lusitropic responses of increasing concentrations of LAAs (10(-8) to 10(-3) M) under isometric and isotonic conditions. Data are mean % (SD) of baseline value.

RESULTS

Long-acting local anesthetics induced a significant impairment of relaxation in isotonic and isometric conditions. As compared to ropivacaine, bupivacaine and levobupivacaine induced greater negative lusitropic effects in isotony [at 10(-3) M, maximum unloaded shortening velocity ((max)Vr) = 27 +/- 11 vs 13 +/- 6 and 8 +/- 5%] and isometry (at 10(-3) M, time-to-half-relaxation: 106 +/- 10 vs 127 +/- 17 and 133 +/- 17%). When the comparison was made with equipotent concentrations, the negative lusitropic effects induced with levobupivacaine were significantly greater than those of bupivacaine and ropivacaine in isometric and isotonic conditions (at 10(-3) M, (max)Vr = 7 +/- 4 vs 13 +/- 6 and 17 +/- 4 %). As previously described, LAAs also induced concentration-dependent negative inotropic effects that were greater for levobupivacaine compared to equivalent or equipotent concentrations of bupivacaine and ropivacaine.

CONCLUSIONS

Long-acting local anesthetics induce marked negative inotropic and lusitropic effects. Among LAAs, levobupivacaine exerts the greater depressant effects. Impairment of calcium handling and sarcoplasmic reticulum could explain the differential responses to local anesthetics.

Effects of interleukin-2 on the mechanical restitution and post-rest contraction in rat ventricular papillary muscle

To determine whether application of interleukin-2 (IL-2) alters function of sarcoplasmic reticulum (SR), we measured mechanical restitution and post-rest potentiation (PRP) in isolated rat papillary muscles. Mechanical restitution curves were constructed by interpolating extrasystoles at different test intervals following a train of steady state beats. In control group, the maximal PRP was reached after 60-120s of rest and the maximal potentiation factor was 2.36 +/- 0.23. IL-2 at 200 U/ml decreased the steady-state force of contraction to 56.4 +/- 7.2% of pre-drug control. But the time constant of recovery of steady-state force was not altered after IL-2. IL-2 decreased PRP at all intervals, shifted the potentiation curve parallel to lower values. But the potentiation was enhanced when compared with pre-rest control value in the presence of IL-2. In papillary muscle treated with IL-2, the onset of maximal PRP was delayed and the potentiation factor after 300s was 4.72 +/- 0.58 times that at the steady-state. Recirculation fraction of calcium calculated from the decay of PRP was 0.78 +/- 0.09 in control and 0.59 +/- 0.08 after IL-2 treatment. We conclude that IL-2 decreases the function of SR, which suggests that an impaired function of SR may contribute to the negative inotropic effect of IL-2.

Young MLP deficient mice show diastolic dysfunction before the onset of dilated cardiomyopathy

Targeted deletion of cytoskeletal muscle LIM protein (MLP) in mice consistently leads to dilated cardiomyopathy (DCM) after one or more months. However, next to nothing is known at present about the mechanisms of this process. We investigated whether diastolic performance including passive mechanics and systolic behavior are altered in 2-week-old MLP knockout (MLPKO) mice, in which heart size, fractional shortening and ejection fraction are still normal. Right ventricular trabeculae were isolated from 2-week-old MLPKO and wildtype mice and placed in an apparatus that allowed force measurements and sarcomere length measurements using laser diffraction. During a twitch from the unloaded state at 1 Hz, MLPKO muscles relengthened to slack length more slowly than controls, although the corresponding force relaxation time was unchanged. Active developed stress at a diastolic sarcomere length of 2.00 microm was preserved in MLPKO trabeculae over a wide range of pacing frequencies. Force relaxation under the same conditions was consistently prolonged compared with wildtype controls, whereas time to peak and maximum rate of force generation were not significantly altered. Ca2+ content of the sarcoplasmic reticulum (SR) and the quantities of Ca2+ handling proteins were similar in both genotypes. In summary, young MLPKO mice revealed substantial alterations in passive myocardial properties and relaxation time, but not in most systolic characteristics. These results indicate that the progression to heart failure in the MLPKO model may be driven by diastolic myocardial dysfunction and abnormal passive properties rather than systolic dysfunction.

Cardiac alternans do not rely on diastolic sarcoplasmic reticulum calcium content fluctuations

Cardiac alternans are thought to be a precursor to life-threatening arrhythmias. Previous studies suggested that alterations in sarcoplasmic reticulum (SR) Ca2+ content are either causative or not associated with myocyte Ca2+ alternans. However, those studies used indirect measures of SR Ca2+. Here we used direct continuous measurement of intra-SR free [Ca2+] ([Ca2+]SR) (using Fluo5N) during frequency-dependent Ca2+ alternans in rabbit ventricular myocytes. We tested the hypothesis that alternating [Ca2+]SR is required for Ca2+ alternans. Amplitudes of [Ca2+]SR depletions alternated in phase with cytosolic Ca2+ transients and contractions. Some cells showed clear alternation in diastolic [Ca2+]SR during alternans, with higher [Ca2+]SR before the larger SR Ca2+ releases. However, the extent of SR Ca2+ release during the small beats was smaller than expected for the modest decrease in [Ca2+]SR. In other cells, clear Ca2+ alternans was observed without alternations in diastolic [Ca2+]SR. Additionally, alternating cells were observed, in which diastolic [Ca2+]SR fluctuations occurred interspersed by depletions in which the amplitude was unrelated to the preceding diastolic [Ca2+]SR. In all forms of alternans, the SR Ca2+ release rate was higher during large depletions than during small depletions. Although [Ca2+]SR exerts major influence on SR Ca2+ release, alternations in [Ca2+](SR) are not required for Ca2+ alternans to occur. Rather, it seems likely that some other factor, such as ryanodine receptor availability after a prior beat (eg, recovery from inactivation), is of greater importance in initiating frequency-induced Ca2+ alternans. However, once such a weak SR Ca2+ release occurs, it can result in increased [Ca2+]SR and further enhance SR Ca2+ release at the next beat. In this way, diastolic [Ca2+]SR alternans can enhance frequency-induced Ca2+ alternans, even if they initiate by other means.

The effect of direct autonomic nerve stimulation on left ventricular force in the isolated innervated Langendorff perfused rabbit heart

The relative contribution of the chronotropic effects of stimulating sympathetic and vagus nerves on cardiac inotropic changes in the isolated Langendorff perfused rabbit heart with intact dual autonomic nerves was studied. The force-frequency relationship was investigated, in addition to sympathetic nerve stimulation (SS) at 2 Hz (low), 5 Hz (med) and 10 Hz (high), and left and right vagus nerve stimulation (VS) studied at 2 Hz (low), 5 Hz (med) and 7 Hz (high) with and without right ventricular pacing. It was shown that a biphasic force-frequency relationship is present with a positive relationship at low heart rates and a negative force-frequency relationship at higher heart rates. There was a trend for left- and right-VS to decrease left ventricular pressure with a decrease in heart rate, whilst SS had the opposing effects in a frequency-dependent manner. When heart rate was kept constant, there was no effect from left- or right-VS, while SS increased left ventricular pressure in a frequency-dependent manner. Together these results suggest that SS, left- and right-VS alter left ventricular force by two different mechanisms. Left- and right-VS decrease left ventricular pressure predominantly via chronotropic effects whilst SS increases force predominantly by direct changes in contractility.

Oscillatory transient inward currents in ventricular myocytes of healthy versus myopathic Syrian hamster

The present experiments were performed in order to study abnormal action potential configuration and ion channel activity in ventricular myocytes obtained from 23 male myopathic Syrian hamsters (Biobreeders strain 14.6, 32-52 weeks old) compared with 10 age-matched healthy control hamsters (Biobreeders F1B) by means of whole-cell patch-clamp techniques. The results show that the myopathic myocytes had a longer action potential duration, a reduced transient outward K(+) current on depolarization and a smaller transient inward current on repolarization after prolonged depolarizing pulses (> 500 msec). However, the L-type Ca(2+) current and the inwardly rectifing K(+) current were not significantly different from those of healthy myocytes. The oscillatory transient inward currents could be diminished by treatment with ryanodine (0.01-1 micromol/L), a sarcoplasmic reticulum (SR) Ca(2+) release channel blocker, or with Na(+)-free superfusate. We conclude that the hereditary myopathic hamsters are less likely to develop delayed after depolarization-related transient inward currents and triggered arrhythmias owing to a smaller SR Ca(2+) content.

Electromechanical properties of Purkinje fiber strands isolated from human ventricular endocardium

BACKGROUND

Abnormalities in the regulation of intracellular Ca2+ were observed in cardiac cells obtained from failing human hearts. However, the electromechanical properties and pharmacologic responses of human ventricular Purkinje fibers have not been well characterized.

METHODS

Strands of free-running Purkinje fibers and/or trabecular muscle fibers with a diameter of around 1.5 mm were removed from the endocardial surface of ventricles obtained from 16 transplant recipient hearts. Action potential (AP) was detected by conventional microelectrode techniques and twitch force by a force-displacement transducer.

RESULTS

The human Purkinje fiber strands as revealed by histologic examination were composed of Purkinje cells and the surrounding ventricular muscle cells. In well-polarized Purkinje fibers (mean +/- SE of maximum diastolic potential [MDP] = -85 +/- 1 mV) showing fast-response AP (Phase 0 Vmax >100 V/sec), the cardiotonic agents isoproterenol and strophanthidin (1 to 2 micromol/liter) accelerated the slope of diastolic depolarization and induced delayed afterdepolarization but not spontaneous APs. Steady-state contraction and the post-rest potentiation of contraction (PRPC) were similar in both Purkinje fibers and ventricular muscles, but inotropic agents induced tachyarrhythmia only in Purkinje fibers. In partially depolarized Purkinje fibers (MDP <-70 mV) with slow-response AP, isoproterenol and/or strophanthidin readily induced automatic and triggered rhythms.

CONCLUSIONS

Accumulation of excessive cytosolic Ca2+ in the presence of cardiotonic agents could lead to tachyarrhythmias in Purkinje fibers, but rarely in ventricular muscles of failing human hearts.

A mathematical treatment of integrated Ca dynamics within the ventricular myocyte

We have developed a detailed mathematical model for Ca2+ handling and ionic currents in the rabbit ventricular myocyte. The objective was to develop a model that: 1), accurately reflects Ca-dependent Ca release; 2), uses realistic parameters, particularly those that concern Ca transport from the cytosol; 3), comes to steady state; 4), simulates basic excitation-contraction coupling phenomena; and 5), runs on a normal desktop computer. The model includes the following novel features: 1), the addition of a subsarcolemmal compartment to the other two commonly formulated cytosolic compartments (junctional and bulk) because ion channels in the membrane sense ion concentrations that differ from bulk; 2), the use of realistic cytosolic Ca buffering parameters; 3), a reversible sarcoplasmic reticulum (SR) Ca pump; 4), a scheme for Na-Ca exchange transport that is [Na]i dependent and allosterically regulated by [Ca]i; and 5), a practical model of SR Ca release including both inactivation/adaptation and SR Ca load dependence. The data describe normal electrical activity and Ca handling characteristics of the cardiac myocyte and the SR Ca load dependence of these processes. The model includes a realistic balance of Ca removal mechanisms (e.g., SR Ca pump versus Na-Ca exchange), and the phenomena of rest decay and frequency-dependent inotropy. A particular emphasis is placed upon reproducing the nonlinear dependence of gain and fractional SR Ca release upon SR Ca load. We conclude that this model is more robust than many previously existing models and reproduces many experimental results using parameters based largely on experimental measurements in myocytes.

Action potential duration determines sarcoplasmic reticulum Ca2+ reloading in mammalian ventricular myocytes

After sarcoplasmic reticulum (SR) Ca2+ depletion in intact ventricular myocytes, electrical activity promotes SR Ca2+ reloading and recovery of twitch amplitude. In ferret, recovery of twitch and caffeine-induced contracture required fewer twitches than in rabbit or rat. In rat, there was no difference in action potential duration at 90% repolarization (APD90) at steady state (SS) versus at the first post-depletion (PD) twitch. The SS APD90 was similar in ferret and rabbit (but longer than in rat). However, compared to SS, the PD APD90 was lengthened in ferret, but shortened in rabbit. When rabbit myocytes were subjected to AP-clamp patterns during SR Ca2+ reloading (ferret- or rabbit-type APs), reloading was much faster using the ferret AP templates. We conclude that the faster SR Ca2+ refilling in ferret is due to the increased Ca2+ influx during the longer PD AP. The PD versus SS APD90 difference was suppressed by thapsigargin in ferret (indicating Ca2+ dependence). In rabbit, the PD AP shortening depended on the preceding diastolic interval (rather than Ca2+), because rest produced the same AP shortening, and SS APD90 increased as a function of frequency (in contrast to ferret). Transient outward current (Ito) was larger and recovered from inactivation much faster in ferret than in rabbit. Moreover, slow Ito recovery (tau approximately 3 s) in rabbit was a much larger fraction of Ito. Our data and a computational model (including two Ito components) suggest that in rabbit the slowly recovering Ito is responsible for short post-rest and PD APs, for the unusual frequency dependence of APD90, and ultimately for the slower post-depletion SR Ca2+ reloading.

HYPERHOMOCYSTEINAEMIA-INDUCED ATRIAL REMODELLING IN HYPERTENSIVE RATS

Studies from our laboratory and others indicate that, in addition to its purported atherothrombotic effects, hyperhomocysteinaemia (Hhe) is a powerful stimulus for ventricular remodelling, dysfunction and clinical heart failure. Because changes in atrial structure and function can impact on cardiac function in progressive ventricular remodelling and dysfunction, we conducted experiments to examine structural and functional remodelling of the atria in the hyperhomocysteinaemic hypertensive rat, a previously described model of ventricular diastolic dysfunction. Atrial muscle preparations were isolated from hearts of spontaneously hypertensive rats that were fed control, intermediate Hhe-inducing or severe Hhe-inducing diet for 10 weeks. Atrial developed tension, +dT/dt(max) and -dT/dt(max) were found to increase in parallel with levels of Hhe and ventricular diastolic dysfunction. Post-rest developed tension and the maximum developed tension observed in the presence of isoproterenol were also increased significantly in both Hhe groups compared with control. These results indicate that Hhe increases both basal and maximal contractile function in atrial muscle. Atrial structural remodelling was characterized by increased interstitial fibrosis in both Hhe groups. These data suggest that Hhe-associated changes in atrial structure and function may act to maintain ventricular filling in Hhe-induced diastolic dysfunction.

On the mechanisms of post-rest adaptation in the isolated electrically driven left atria of rats

We studied the role of the resting period (1, 2, 4, 8, 16 min; n = 6-7), external Ca2+ (0.2, 0.4, 0.6 g/l; n + 5-6), stimulation frequency (1, 2, 3 Hz; n = 6), 4-aminopyridine (4-AP, 2 mM; n = 5); theophylline (1 mM; n = 6), ouabain (5 microM; n = 6), and verapamil (1 microM; n = 6) on post-rest adaptation in the isolated left atria of rats driven electrically by a 2x threshold intensity for 2 ms. Resting periods resulted in three-phasic adaptive changes in contractility during the post-rest stimulation before normalization: P1, hypercontractile phase, an initial twitch potentiation; P2, post-rest hypocontractile decay reached after 8 to 12 single twitches; and P3, a late reactive hypercontractile phase marked less than that of P1 and gradually declining to the pre-resting level. P1 and P2 were augmented along with increasing the resting period from 1 min to 16 min, whereas t1 (time between P1 and P2) shortened and P2 and t2 (time between P2 and P3) were not affected. P1 and P3 to become more apparent after shifting the stimulation frequency from 1 Hz to 3 Hz, accompanied by a shortening of t1 and t2 (p < 0.05) and an insignificant reversal of P2. An increase in Ca2+ concentration by 2- or 3-fold at 2 Hz reduced P1 was and antagonized P2, while leaving other parameters almost unaffected. The reduction of P1 by Ca2+ became more prominent at 3 Hz. Exposure to 4-AP depressed P1 and P3 at 1 Hz, which was reversed by increasing the stimulation frequency--P3 tended to diminish, whereas t1 and t2 were shortened. Theophylline reduced P1 antagonized P2, and shortened t1 and t2 significantly, and a combination of theophylline and 4-AP augmented the effects. Ouabain increased P1 and P2 in a frequency-dependent manner; prolonged t2 at 1 Hz, but shortened t2 at higher frequencies. Verapamil inhibition of Ca2+ channels augmented P1 and t1 and reduced P2 and P3, and the effects on all three parameters were augmented by combined 4-AP/verapamil. We concluded that the post-rest adaptive changes in contractility are a consequence of phasic changes in sarcoplasmic Ca2+ concentration and that such changes reflect an imbalance between the release from and uptake into the sarcoplasmic reticulum of Ca2+ and transsarcolemmal Ca2+ loss.

Positive inotropic effect of ceramide in adult ventricular myocytes: mechanisms dissociated from its reduction in Ca2+ influx

Ceramide, a sphingolipid metabolite produced by activation of sphingomyelinase, has been previously shown to reduce L-type Ca2+ channel current (ICa,L) in adult rat ventricular myocytes; however, its effect on contractile function is unknown. In this study, we investigated the effects of ceramide on excitation-contraction coupling in adult ventricular myocytes and on left ventricular (LV) function in isolated hearts. Surprisingly, in patch-clamped myocytes, ceramide increased contraction concomitant with reductions in ICa,L. In intact myocytes, ceramide increased cell shortening (CS) concurrently with enhancing maximum rates of shortening and relaxation and the duration of contraction. Ceramide also increased the amplitudes of postrest potentiated (PRP) contraction. In fura-PE3-loaded myocytes, ceramide increased systolic Ca2+ and the magnitude and maximum rates of the rising and declining phases of Ca2+ transients. Ceramide-elicited decreases in magnitudes of PRP relative to steady-state contraction and the Ca2+ transient suggest an increased fractional Ca2+ release from the sarcoplasmic reticulum (SR). However, ceramide slightly reduced the caffeine-induced Ca2+ transient and had no significant effect on the amplitude of the PRP-elicited Ca2+ transient. Additionally, the ceramide-induced upward shift in the relationship of contraction and the Ca2+ transient and increase in the Ca2+ responsiveness of CS suggest an increase in myofilament Ca2+ sensitivity. In isolated hearts, ceramide increased LV developed pressure and maximum rates of contraction and relaxation at balloon volumes of 30-50 microl. In summary, regardless of decreasing ICa,L, ceramide elicits distinct positive inotropic and lusitropic effects, resulting probably from enhanced SR Ca2+ release and uptake, and increased Ca2+ sensitivity of ventricular myocytes.

Role of the sarcoplasmic reticulum in altered action potential and contraction of myopathic human and hamster ventricle

1. The present experiments were performed in order to study the role of the sarcoplasmic reticulum (SR) in the altered action potential and contraction of ventricular myocardium obtained from myopathic Syrian hamster and explanted human hearts (n = 8). The hamsters included age-matched healthy hamsters (F1B; n = 18), young myopathic hamsters (Bio 14.6; n = 8; aged 17-27 weeks) and older myopathic hamsters (n = 10; aged 39-43 weeks). 2. Action potentials were recorded by means of a microelectrode technique and force was recorded using a transducer. Post-rest potentiation of contraction (PRPC), a measure of the SR Ca2+-pumping activity, was determined after different rest intervals (2-60 s). Furthermore, cyclopiazonic acid (10 micro mol/L), a specific blocker of SR Ca2+-ATPase, was used to unmask abnormalities in the function of the SR. 3. The relationship between PRPC and rest interval was similar in younger healthy and myopathic hamsters, but the curve of the older myopathic muscle was obviously shifted downwards. Cyclopiazonic acid decreased predominantly the ascending part of the curve in both the healthy and myopathic hamster myocardium and could induce spontaneous action potentials during drug exposure or after washout. 4. In human myopathic myocardium, the curve of the PRPC-rest interval peaked at longer intervals (40-60 s) compared with that of the hamsters (10-20 s). Cyclopiazonic acid markedly depressed the relationship and increased the diastolic force (contracture) at high driving frequency, but did not induce action potentials during the rest interval. 5. We conclude that an impaired function of the SR contributes to the progressive deterioration of ventricular function in dilated cardiomyopathy and that the electromechanical behaviour of the ventricular myocardium of patients affected by dilated cardiomyopathy shows similarity and differences with the myopathic Syrian hamster model.

Role of creatine kinase in cardiac excitation-contraction coupling: studies in creatine kinase-deficient mice

To understand the role of creatine kinase (CK) in cardiac excitation-contraction coupling, CK-deficient mice (CK-/-) were studied in vitro and in vivo. In skinned fibers, the kinetics of caffeine-induced release of Ca2+ was markedly slowed in CK-/- mice with a partial restoration when glycolytic substrates were added. These abnormalities were almost compensated for at the cellular level: the responses of Ca2+ transient and cell shortening to an increased pacing rate from 1 Hz to 4 Hz were normal with a normal post-rest potentiation of shortening. However, the post-rest potentiation of the Ca2+ transient was absent and the cellular contractile response to isoprenaline was decreased in CK-/- mice. In vivo, echocardiographically determined cardiac function was normal at rest but the response to isoprenaline was blunted in CK-/- mice. Previously described compensatory pathways (glycolytic pathway and closer sarcoplasmic reticulum-mitochondria interactions) allow a quasi-normal SR function in isolated cells and a normal basal in vivo ventricular function, but are not sufficient to cope with a large and rapid increase in energy demand produced by beta-adrenergic stimulation. This shows the specific role of CK in excitation-contraction coupling in cardiac muscle that cannot be compensated for by other pathways.

Rest-dependence of twitch amplitude and sarcoplasmic reticulum calcium content in the developing rat myocardium

Post-rest contractile response was studied in isolated ventricular muscle from rats aged 1 to 90 days. Amplitude of rapid cooling contractures (RCC) was taken as an index of the sarcoplasmic reticulum (SR) Ca2+ content. We observed that: (a) developed tension (per cross-section area) increased with age; (b) time to peak twitch force and relaxation half-time decreased from 87+/-6 to 56+/-2 ms and from 68+/-6 to 36+/-1 ms, respectively, from the neonatal period to adulthood; (c) post-rest twitch potentiation was observed at all ages, with greater relative potentiation in younger preparations, although relative potentiation of [Ca2+]i transient amplitude was similar in young and adult isolated ventricular myocytes; (d) rest did not significantly affect the amplitude of RCC in muscle or caffeine-evoked [Ca2+]i transients in myocytes at any studied age; (e) favoring Ca2+ efflux via Na+-Ca2+ exchange (NCX) during rest reversed twitch potentiation and caused a similar decrease in RCC amplitude ( approximately 40%) at all ages; (f) stimulation of Ca2+ influx via NCX during rest increased RCC amplitude ( approximately 40%) only in immature preparations. However, when this procedure was repeated after partial SR Ca2+ depletion, increase in RCC amplitude was not significantly age-dependent. We conclude that post-rest twitch potentiation is already present early after birth and does not require rest-dependent changes in SR Ca2+ content at any studied age. Our results suggest that NCX is close to equilibrium during rest in both adult and developing rat myocardium, and does not seem to mediate diastolic net Ca2+ fluxes which may affect the SR Ca2+ content.

Ca2+-dependent regulation of cardiac L-type Ca2+ channels: is a unifying mechanism at hand?

Ca2+ entry (I(Ca)) through cardiac L-type Ca2+ channels (LTCC) drives critical cellular processes ranging from contraction to gene expression, and, when disordered, is implicated in arrhythmias and hypertrophy. LTCC activation occurs by cell membrane depolarization, but LTCCs are also regulated by auxiliary proteins, phosphorylation, and intracellular CA2+([Ca2+]i). LTCC regulation by [Ca2+]i is especially intriguing because increased [Ca2+]i signals dual and conflicting commands for I(Ca)inactivation and facilitation. A recent explosion of work has shed new light on the mechanisms and molecular identity of domains necessary for [Ca2+]i-dependent regulation of LTCC.

Differences in Ca(2+)-handling and sarcoplasmic reticulum Ca(2+)-content in isolated rat and rabbit myocardium

We made novel measurements of the influence of rest intervals and stimulation frequency on twitch contractions and on sarcoplasmic reticulum (SR) Ca(2+)-content (using rapid cooling contractures, RCCs) in isolated ventricular muscle strips from rat and rabbit hearts at a physiological temperature of 37 degrees C. In addition, the frequency-dependent relative contribution of SR Ca(2+)-uptake and Na(+)/Ca(2+)-exchange for cytosolic Ca(2+)-removal was assessed by paired RCCs. With increasing rest intervals (1-240 s) post-rest twitch force and RCC amplitude decreased monotonically in rabbit myocardium (after 240 s by 45+/-10% and 61+/-11%, respectively P<0. 05, n=14). In contrast, rat myocardium (n=11) exhibited a parallel increase in post-rest twitch force (by 67+/-16% at 240 s P<0.05) and RCC amplitude (by 20+/-14%P<0.05). In rabbit myocardium (n=11), increasing stimulation frequency from 0.25 to 3 Hz increased twitch force by 295+/-50% (P<0.05) and RCC amplitude by 305+/-80% (P<0.05). In contrast, in rat myocardium (n=6), twitch force declined by 43+/-7% (P<0.05), while RCC amplitude decreased only insignificantly (by 16+/-7%). The SR Ca(2+)-uptake relative to Na(+)/Ca(2+)-exchange (based on paired RCCs) increased progressively with frequency in rabbit, but not in rat myocardium (;66+/-2% at all frequencies). We conclude that increased SR Ca(2+)-load contributes to the positive force-frequency relationship in rabbits and post-rest potentiation of twitch force in rats. Decreased SR Ca(2+)-load contributes to post-rest decay of twitch force in rabbits, but may play only a minor role in the negative force-frequency relationship in rats. SR Ca(2+)-release channel refractoriness may contribute importantly to the negative force-frequency relationship in rat and recovery from refractoriness may contribute to post-rest potentiation.

Ca(2+) handling in isolated human atrial myocardium

Physiologically, human atrial and ventricular myocardium are coupled by an identical beating rate and rhythm. However, contractile behavior in atrial myocardium may be different from that in ventricular myocardium, and little is known about intracellular Ca(2+) handling in human atrium under physiological conditions. We used rapid cooling contractures (RCCs) to assess sarcoplasmic reticulum (SR) Ca(2+) content and the photoprotein aequorin to assess intracellular Ca(2+) transients in atrial and ventricular muscle strips isolated from nonfailing human hearts. In atrial myocardium (n = 19), isometric twitch force frequency dependently (0. 25-3 Hz) increased by 78 +/- 25% (at 3 Hz; P < 0.05). In parallel, aequorin light signals increased by 111 +/- 57% (P < 0.05) and RCC amplitudes by 49 +/- 13% (P < 0.05). Similar results were obtained in ventricular myocardium (n = 13). SR Ca(2+) uptake (relative to Na(+)/Ca(2+) exchange) frequency dependently increased in atrial and ventricular myocardium (P < 0.05). With increasing rest intervals (1-240 s), atrial myocardium (n = 7) exhibited a parallel decrease in postrest twitch force (at 240 s by 68 +/- 5%, P < 0.05) and RCCs (by 49 +/- 10%, P < 0.05). In contrast, postrest twitch force and RCCs significantly increased in ventricular myocardium (n = 6). We conclude that in human atrial and ventricular myocardium the positive force-frequency relation results from increased SR Ca(2+) turnover. In contrast, rest intervals in atrial myocardium are associated with depressed contractility and intracellular Ca(2+) handling, which may be due to rest-dependent SR Ca(2+) loss (Ca(2+) leak) and subsequent Ca(2+) extrusion via Na(+)/Ca(2+) exchange. Therefore, the influence of rate and rhythm on mechanical performance is not uniform in atrial and ventricular myocardium.

KB-R7943 block of Ca(2+) influx via Na(+)/Ca(2+) exchange does not alter twitches or glycoside inotropy but prevents Ca(2+) overload in rat ventricular myocytes

BACKGROUND

The Na(+)/Ca(2+) exchange (NCX) extrudes Ca(2+) from cardiac myocytes, but it can also mediate Ca(2+) influx, load the sarcoplasmic reticulum with Ca(2+), and trigger Ca(2+) release from the sarcoplasmic reticulum. In ischemia/reperfusion or digitalis toxicity, increased levels of intracellular [Na(+)] ([Na(+)](i)) may raise levels of intracellular [Ca(2+)] ([Ca(2+)](i)) via NCX, leading to cell injury and arrhythmia.

METHODS AND RESULTS

We used KB-R7943 (KBR) to selectively block Ca(2+) influx via NCX to study the role of NCX-mediated Ca(2+) influx in intact rat ventricular myocytes. Removing extracellular Na(+) caused [Ca(2+)](i) to rise, due to Ca(2+) influx via NCX, and this was blocked by 90% with 5 micromol/L KBR. However, KBR did not alter [Ca(2+)](i) decline due to NCX. Thus, we used 5 micromol/L KBR to selectively block Ca(2+) entry but not efflux via NCX. Under control conditions, 5 micromol/L KBR did not alter steady-state twitches, Ca(2+) transients, Ca(2+) load in the sarcoplasmic reticulum, or rest potentiation, but it did prolong the late low plateau of the rat action potential. When Na(+)/K(+) ATPase was inhibited by strophanthidin, KBR reduced diastolic [Ca(2+)](i) and abolished the spontaneous Ca(2+) oscillations, but it did not prevent inotropy.

CONCLUSIONS

In rat ventricular myocytes, Ca(2+) influx via NCX is not important for normal excitation-contraction coupling. Furthermore, the inhibition of Ca(2+) efflux alone (as [Na(+)](i) rises) may be sufficient to cause glycoside inotropy. In contrast, Ca(2+) overload and spontaneous activity at high [Na(+)](i) was blocked by KBR, suggesting that net Ca(2+) influx (not merely reduced efflux) via NCX is involved in potentially arrhythmogenic Ca(2+) overload.

Phospholamban: a major determinant of the cardiac force-frequency relationship

The cardiac force-frequency relationship has been known for over a century, yet its mechanisms have eluded thorough understanding. We investigated the hypothesis that phospholamban, a potent regulator of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), determines the cardiac force-frequency relationship. Isolated left ventricular papillary muscles from wild-type (WT) and phospholamban knockout (KO) mice were stimulated at 2 to 6 Hz. The force-frequency relationship was positive in WT but negative in KO muscles, i.e., it was inverted by ablation of phospholamban (P < 0.01, n = 6 mice). From 2 to 6 Hz, relaxation accelerated considerably (by 10 ms) in WT muscles but only minimally (by 2 ms) in KO muscles (WT vs. KO: P < 0. 0001, n = 6). To show that the lack of frequency potentiation in KO muscles was not explained by the almost maximal basal contractility, twitch duration was prolonged in six KO muscles with the SERCA inhibitor cyclopiazonic acid to WT values. Relaxation still failed to accelerate with increased frequency. In conclusion, our results clearly identify phospholamban as a major determinant of the cardiac force-frequency relationship.

Functional role of ionic regulation of Na+/Ca2+ exchange assessed in transgenic mouse hearts

Na+/Ca2+ exchange is the primary mechanism mediating Ca2+ efflux from cardiac myocytes during diastole and, thus, can prominently influence contractile force. In addition to transporting Na+ and Ca2+, the exchanger is also regulated by these ions. Although structure-function studies have identified protein regions of the exchanger subserving these regulatory processes, their physiological importance is unknown. In this study, we examined the electrophysiological and mechanical consequences of cardiospecific overexpression of the canine cardiac exchanger NCX1.1 and a deletion mutant of NCX1.1 (Delta680-685), devoid of intracellular Na+ (Na+i)- and Ca2+ (Ca2+i)- dependent regulatory properties, in transgenic mice. Using the giant excised patch-clamp technique, normal ionic regulation was observed in membrane patches from cardiomyocytes isolated from control and transgenic mice overexpressing NCX1.1. In contrast, ionic regulation was nearly abolished in mice overexpressing Delta680-685, indicating that the native regulatory processes could be overwhelmed by expression of the transgene. To address the physiological consequences of ionic regulation of the Na+/Ca2+ exchanger, we examined postrest force development in papillary muscles from NCX1.1 and Delta680-685 transgenic mice. Postrest potentiation was found to be substantially greater in Delta680-685 than in NCX1.1 transgenic mice, supporting the notion that ionic regulation of Na+/Ca2+ exchange plays a significant functional role in cardiac contractile properties.

Contractile properties of atrial and ventricular myocardium of the heart of rainbow trout oncorhynchus mykiss: effects of thermal acclimation

Atrial and ventricular myocardium perform different tasks in the pumping work of the vertebrate heart, which are reflected in their contractile properties. Although atrial contraction is assumed to have an important role in the function of fish heart, the contractile properties of atrial and ventricular myocardium have not been directly compared in any fish species. The objective of this study was to clarify any contractile differences in the heart of teleost fish and, in particular, to elucidate the contribution of myofibrillar ATPase and intracellular Ca(2+) stores to the characteristics of atrial and ventricular contraction. Experiments were conducted on thermally acclimated rainbow trout Oncorhynchus mykiss to determine whether the effects of temperature adaptation are the same in atrial and ventricular tissue. It was shown that the rate of isometric contraction is much faster in atrial than in ventricular tissue of the fish heart and that acclimation to cold increases the rate of contraction in both cardiac compartments. The rapid contraction kinetics of the atrial tissue were associated with higher myofibrillar ATPase activity and faster Ca(2+) uptake rate of the sarcoplasmic reticulum (SR) compared with ventricular tissue. Similarly, the faster kinetics of contraction following cold acclimation could be attributed to enhancement of the myofibrillar and/or SR function. The atrio-ventricular and temperature-induced differences were also expressed in the recovery of force from inactivation, i.e. in the mechanical restitution. The refractory period and the rate constant of force restitution were shorter in atrial than in ventricular muscle tissue. Similar differences also existed between the tissues of cold-acclimated (CA, 4 degrees C) and warm-acclimated (WA, 17 degrees C) fish. The fast recovery of force from inactivation in the heart of the CA trout was, at least in part, due to more active SR. Furthermore, it was shown that the force of atrial contraction in the CA trout is sensitive to ryanodine (10 (μ)mol l(−)(1)), a Ca(2+)-release channel blocker of SR, at physiological body temperature (4 degrees C) and at a physiological pacing rate (0.6 Hz). This finding indicates that the Ca(2+) stores of SR contribute to activation of cardiac contraction in the fish heart, and that the SR of fish heart is able to retain its Ca(2+) load at low body temperatures, i.e. the Ca(2+)release channels of SR are not leaky in the cold. The present data show that in the atrial tissue of CA trout, the SR directly contributes to the cytosolic Ca(2+) and that in the atrium and ventricle of CA trout, the SR significantly accelerates the recovery of contractility from inactivation. The fast recovery from inactivation allows relatively high heart rates and therefore adequate cardiac outputs at low environmental temperatures for the cold-active rainbow trout.

Ca2+ handling and sarcoplasmic reticulum Ca2+ content in isolated failing and nonfailing human myocardium

Disturbed sarcoplasmic reticulum (SR) Ca2+ content may underlie the altered force-frequency and postrest contractile behavior in failing human myocardium. We used rapid cooling contractures (RCCs) to assess SR Ca2+ content in ventricular muscle strips isolated from nonfailing and end-stage failing human hearts. With an increase in rest intervals (1 to 240 s; 37 degrees C), nonfailing human myocardium (n=7) exhibited a parallel increase in postrest twitch force (at 240 s by 121+/-44%; P<0.05) and RCC amplitude (by 69+/-53%; P<0.05). In contrast, in failing myocardium (n=30), postrest twitch force decreased at long rest intervals and RCC amplitude declined monotonically with rest (by 25+/-9% and 53+/-9%, respectively; P<0.05). With an increase in stimulation frequencies (0.25 to 3 Hz), twitch force increased continuously in nonfailing human myocardium (n=7) by 71+/-17% (at 3 Hz; P<0.05) and RCC amplitude increased in parallel by 247+/-55% (P<0.05). In contrast, in failing myocardium (n=26), twitch force declined by 29+/-7% (P<0. 05) and RCC amplitude increased only slightly by 36+/-14% (P<0.05). Paired RCCs were evoked to investigate the relative contribution of SR Ca2+ uptake and Na+/Ca2+ exchange to cytosolic Ca2+ removal during relaxation. SR Ca2+ uptake (relative to the Na+/Ca2+ exchange) increased significantly in nonfailing but not in failing human myocardium as stimulation rates increased. We conclude that the negative force-frequency relation in failing human myocardium is due to an inability of SR Ca2+ content to increase sufficiently at high frequencies and thus cannot overcome the frequency-dependent refractoriness of SR Ca2+ release. The rest-dependent decay in twitch force in failing myocardium is due to rest-dependent decline in SR Ca2+ content. These alterations could be secondary to depressed SR Ca2+-ATPase combined with enhanced cytosolic Ca2+ extrusion via Na+/Ca2+ exchange.

Positive force- and [Ca2+]i-frequency relationships in rat ventricular trabeculae at physiological frequencies

The isometric force-frequency relationship of isolated rat ventricular trabeculae (diameter <250 micrometer) was examined at 24, 30, and 37 degreesC at stimulation frequencies (0.1-12 Hz) encompassing the physiological range. Some muscles were microinjected with fura PE3 to monitor the diastolic and systolic intracellular concentration of Ca2+ ([Ca2+]i). At a near-physiological external Ca2+ concentration ([Ca2+]o) of 1 mM, a positive force-frequency relationship was demonstrated at all temperatures. The force-frequency relationship became negative at high frequencies (e. g., >6 Hz at 30 degreesC) at 1 mM [Ca2+]o or at low frequencies at 8 mM [Ca2+]o. The twitch and Ca2+ transient became shorter as stimulation frequency increased; these changes were related to changes in systolic, rather than diastolic, [Ca2+]i and were not blocked by inhibitors of Ca2+/calmodulin-dependent protein kinase II. The positive force-frequency relationship of rat trabeculae was caused by a frequency-dependent loading of the sarcoplasmic reticulum (SR) with Ca2+. We suggest that at high frequencies, or under conditions of Ca2+ overload, this loading saturates. Processes that tend to decrease SR Ca2+ release will then predominate, resulting in a negative force-frequency relationship.

Factors that control sarcoplasmic reticulum calcium release in intact ventricular myocytes

Much has been discovered studying sarcoplasmic reticulum (SR) Ca release channels in SR vesicles and lipid bilayers. We have focused on how SR Ca release is regulated in intact mammalian ventricular myocytes, using fluorescent Ca indicators, voltage clamp, and confocal microscopy. Three major factors appear to contribute to the probability of spontaneous localized SR Ca release events (or Ca "sparks") in resting myocytes: (1) cytosolic [Ca], (2) SR Ca content, and (3) time after previous activity (i.e., recovery from adapted or inactivated state). These same three factors function during excitation-contraction (E-C) coupling and can explain rest potentiation of twitches, increased fractional SR Ca release at higher SR Ca loads, and Ca overload. Since SR Ca release is sensitive to both ICa and SR Ca load, we have controlled (and measured) these parameters. At constant SR Ca load and ICa in intact cells we have found that SR Ca release is increased by Ca-calmodulin-dependent protein kinase (CaMKII) and FK506 (which may interfere with the interaction between the Ca release channel and the FK binding protein) and is reduced by the Ca channel agonist Bay K 8644, CaMKII inhibitors, and during ventricular hypertrophy. Thus the regulation of the SR Ca release channel in the intact cell is an important factor in cellular cardiac function.

Subcellular properties of [Ca2+]i transients in phospholamban-deficient mouse ventricular cells

The regulatory protein phospholamban exerts a physiological inhibitory effect on the sarcoplasmic reticulum (SR) Ca2+ pump that is relieved with phosphorylation. We have studied the subcellular properties of intracellular Ca2+ ([Ca2+]i) transients in ventricular myocytes isolated from wild-type (WT) and phospholamban-deficient (PLB-KO) mice. In PLB-KO myocytes, steady-state twitch [Ca2+]i transients revealed an accelerated relaxation and the occurrence of highly localized failures of Ca2+ release. The acceleration of SR Ca2+ uptake caused an increase in SR Ca2+ load with the frequent occurrence of spontaneous [Ca2+]i waves and Ca2+ sparks. [Ca2+]i waves in PLB-KO cells showed a marked decrease in spatial width and more frequently appeared to abort. Local Ca2+ release events (Ca2+ sparks) were larger and more variable in amplitude and [Ca2+]i declined faster in PLB-KO myocytes. Increased local buffering and reduction in the refractoriness of SR Ca2+ release caused by the increased SR pump rate led to an overall enhancement of local [Ca2+]i gradients and inhomogeneities in the [Ca2+]i distribution during spontaneous Ca2+ release, [Ca2+]i waves, and excitation-contraction coupling.

Effects of left ventricular hypertrophy on force and Ca2+ handling in isolated rat myocardium

To study the effect of left ventricular (LV) hypertrophy on force and Ca2+ handling in isolated rat myocardium, LV hypertrophy was induced in rats by banding of the abdominal aorta. After 16 wk, arterial pressure was assessed by catheterization. LV trabeculae were isolated and loaded with indo 1 salt by iontophoretic injection. Isometric force and intracellular free Ca2+ concentration ([Ca2+]i) were measured at stimulation frequencies between 0.25 and 3 Hz and rest intervals between 2 and 240 s. Sarcoplasmic reticulum (SR) Ca2+ content was also investigated using rapid cooling contractures (RCC). Systolic and diastolic pressure as well as heart weight-to-body weight ratios were significantly elevated in banded compared with control animals (167 vs. 117 mmHg, 108 vs. 83 mmHg, and 4.6 vs. 4.0 mg/g, respectively). At high frequencies, twitch relaxation and [Ca2+]i decline rates were significantly slower in banded compared with control rats, and diastolic [Ca2+]i was higher in the banded rat muscles (at 3 Hz, force half-time = 83 vs. 68 ms; time constant of [Ca2+]i decline = 208 vs. 118 ms; and diastolic [Ca2+]i = 505 vs. 353 nM). These differences could not be ascribed to altered Na+/Ca2+ exchange, since twitch relaxation and Ca2+ handling were not different between groups in the presence of caffeine (or cyclopiazonic acid plus ryanodine), where relaxation depends primarily on Na+/Ca2+ exchange. After long rest intervals (> or = 120 s), control rats showed a significant rest potentiation of force and Ca2+ transients, whereas banded rats did not. In addition, RCC amplitudes increased with rest in control but were unaltered in banded rats. In summary, pressure-overload hypertrophy was associated with slower twitch relaxation and [Ca2+]i decline but also with blunted rest potentiation of twitches and SR Ca2+ content of LV trabeculae. The decrease in SR Ca(2+)-ATPase function in banded rats may contribute to the observed diastolic dysfunction associated with pressure-overload hypertrophy.