Increase in intracellular sodium ion activity during stimulation in mammalian cardiac muscle

Heart rate changes and myocardial sodium

Historic studies with sodium ion (Na+ ) micropipettes and first-generation fluorescent probes suggested that an increase in heart rate results in higher intracellular Na+ -levels. Using a dual fluorescence indicator approach, we simultaneously assessed the dynamic changes in intracellular Na+ and calcium (Ca2+ ) with measures of force development in isolated excitable myocardial strip preparations from rat and human left ventricular myocardium at different stimulation rates and modeled the Na+ -effects on the sodium-calcium exchanger (NCX). To gain further insight into the effects of heart rate on intracellular Na+ -regulation and sodium/potassium ATPase (NKA) function, Na+ , and potassium ion (K+ ) levels were assessed in the coronary effluent (CE) of paced human subjects. Increasing the stimulation rate from 60/min to 180/min led to a transient Na+ -peak followed by a lower Na+ -level, whereas the return to 60/min had the opposite effect leading to a transient Na+ -trough followed by a higher Na+ -level. The presence of the Na+ -peak and trough suggests a delayed regulation of NKA activity in response to changes in heart rate. This was clinically confirmed in the pacing study where CE-K+ levels were raised above steady-state levels with rapid pacing and reduced after pacing cessation. Despite an initial Na+ peak that is due to a delayed increase in NKA activity, an increase in heart rate was associated with lower, and not higher, Na+ -levels in the myocardium. The dynamic changes in Na+ unveil the adaptive role of NKA to maintain Na+ and K+ -gradients that preserve membrane potential and cellular Ca2+ -hemostasis.

With a grain of salt: Sodium elevation and metabolic remodelling in heart failure

Elevated intracellular Na (Na_i) and metabolic impairment are interrelated pathophysiological features of the failing heart (HF). There have been a number of studies showing that myocardial sodium elevation subtly affects mitochondrial function. During contraction, mitochondrial calcium (Ca_mito) stimulates a variety of TCA cycle enzymes, thereby providing reducing equivalents to maintain ATP supply. Na_i elevation has been shown to impact Ca_mito; however, whether metabolic remodelling in HF is caused by increased Na_i has only been recently demonstrated. This novel insight may help to elucidate the contribution of metabolic remodelling in the pathophysiology of HF, the lack of efficacy of current HF therapies and a rationale for the development of future metabolism-targeting treatments. Here we review the relationship between Na pump inhibition, elevated Na_i, and altered metabolic profile in the context of HF and their link to metabolic (in)flexibility and mitochondrial reprogramming.

Mitochondrial Ca2+, redox environment and ROS emission in heart failure: Two sides of the same coin?

Heart failure (HF) is a progressive, debilitating condition characterized, in part, by altered ionic equilibria, increased ROS production and impaired cellular energy metabolism, contributing to variable profiles of systolic and diastolic dysfunction with significant functional limitations and risk of premature death. We summarize current knowledge concerning changes of intracellular Na+ and Ca2+ control mechanisms during the disease progression and their consequences on mitochondrial Ca2+ homeostasis and the shift in redox balance. Absent existing biological data, our computational modeling studies advance a new 'in silico' analysis to reconcile existing opposing views, based on different experimental HF models, regarding variations in mitochondrial Ca2+ concentration that participate in triggering and perpetuating oxidative stress in the failing heart and their impact on cardiac energetics. In agreement with our hypothesis and the literature, model simulations demonstrate the possibility that the heart's redox status together with cytoplasmic Na+ concentrations act as regulators of mitochondrial Ca2+ levels in HF and of the bioenergetics response that will ultimately drive ATP supply and oxidative stress. The resulting model predictions propose future directions to study the evolution of HF as well as other types of heart disease, and to develop novel testable mechanistic hypotheses that may lead to improved therapeutics.

The Control of Diastolic Calcium in the Heart: Basic Mechanisms and Functional Implications

Normal cardiac function requires that intracellular Ca²⁺ concentration be reduced to low levels in diastole so that the ventricle can relax and refill with blood. Heart failure is often associated with impaired cardiac relaxation. Little, however, is known about how diastolic intracellular Ca²⁺ concentration is regulated. This article first discusses the reasons for this ignorance before reviewing the basic mechanisms that control diastolic intracellular Ca²⁺ concentration. It then considers how the control of systolic and diastolic intracellular Ca²⁺ concentration is intimately connected. Finally, it discusses the changes that occur in heart failure and how these may result in heart failure with preserved versus reduced ejection fraction.

Electrical excitability of roach (Rutilus rutilus) ventricular myocytes: effects of extracellular K+, temperature, and pacing frequency

Exercise, capture, and handling stress in fish can elevate extracellular K+ concentration ([K+]o) with potential impact on heart function in a temperature- and frequency-dependent manner. To this end, the effects of [K+]o on the excitability of ventricular myocytes of winter-acclimatized roach ( Rutilus rutilus) (4 ± 0.5°C) were examined at different test temperatures and varying pacing rates. Frequencies corresponding to in vivo heart rates at 4°C (0.37 Hz), 14°C (1.16 Hz), and 24°C (1.96 Hz) had no significant effect on the excitability of ventricular myocytes. Acute increase of temperature from 4 to 14°C did not affect excitability, but a further rise to 24 markedly decreased excitability: stimulus current and critical depolarization needed to elicit an action potential (AP) were ~25 and 14% higher, respectively, at 24°C than at 4°C and 14°C ( P < 0.05). This depression could be due to temperature-related mismatch between inward Na+ and outward K+ currents. In contrast, an increase of [K+]o from 3 to 5.4 or 8 mM at 24°C reduced the stimulus current needed to trigger AP. However, other aspects of excitability were strongly depressed by high [K+]o: maximum rate of AP upstroke and AP duration were drastically (89 and 50%, respectively) reduced at 8 mM [K+]o in comparison with 3 mM ( P < 0.05). As an extreme case, some myocytes completely failed to elicit all-or-none AP at 8 mM [K+]o at 24°C. Also, amplitude and overshoot of AP were reduced by elevation of [K+]o ( P < 0.05). Although high [K+]o antagonizes the negative effects of high temperature on excitation threshold, the precipitous depression of the rate of AP upstroke and complete loss of excitability in some myocytes suggest that the combination of high temperature and high [K+]o will severely impair ventricular excitability in roach.

Is there a causal link between intracellular Na elevation and metabolic remodelling in cardiac hypertrophy?

Alterations in excitation–contraction coupling and elevated intracellular sodium (Na_i) are hallmarks of pathological cardiac remodelling that underline contractile dysfunction. In addition, changes in cardiac metabolism are observed in cardiac hypertrophy and heart failure (HF) that lead to a mismatch in ATP supply and demand, contributing to poor prognosis. A link between Na_i and altered metabolism has been proposed but is not well understood. Many mitochondrial enzymes are stimulated by mitochondrial calcium (Ca_mito) during contraction, thereby sustaining production of reducing equivalents to maintain ATP supply. This stimulation is thought to be perturbed when cytosolic Na_i is high due to increased Ca_mito efflux, potentially compromising ATP_mito production and leading to metabolic dysregulation. Increased Na_i has been previously shown to affect Ca_mito; however, whether Na_i elevation plays a causative role in energetic mismatching in the hypertrophied and failing heart remains unknown. In this review, we discuss the relationship between elevated Na_i, NaK ATPase dysregulation and the metabolic phenotype in the contexts of pathological hypertrophy and HF and their link to metabolic flexibility, capacity (reserve) and efficiency that are governed by intracellular ion homeostasis. The development of non-invasive analytical techniques using nuclear magnetic resonance able to probe metabolism in situ in the functioning heart will enable a better understanding of the underlying mechanisms of Na_i overload in cardiac pathophysiology. They will lead to novel insights that help to explain the metabolic contribution towards these diseases, the incomplete rescue observed with current therapies and a rationale for future energy-targeted therapies.

Electrophysiological Determination of Submembrane Na(+) Concentration in Cardiac Myocytes

In the heart, Na(+) is a key modulator of the action potential, Ca(2+) homeostasis, energetics, and contractility. Because Na(+) currents and cotransport fluxes depend on the Na(+) concentration in the submembrane region, it is necessary to accurately estimate the submembrane Na(+) concentration ([Na(+)]sm). Current methods using Na(+)-sensitive fluorescent indicators or Na(+) -sensitive electrodes cannot measure [Na(+)]sm. However, electrophysiology methods are ideal for measuring [Na(+)]sm. In this article, we develop patch-clamp protocols and experimental conditions to determine the upper bound of [Na(+)]sm at the peak of action potential and its lower bound at the resting state. During the cardiac cycle, the value of [Na(+)]sm is constrained within these bounds. We conducted experiments in rabbit ventricular myocytes at body temperature and found that 1) at a low pacing frequency of 0.5 Hz, the upper and lower bounds converge at 9 mM, constraining the [Na(+)]sm value to ∼9 mM; 2) at 2 Hz pacing frequency, [Na(+)]sm is bounded between 9 mM at resting state and 11.5 mM; and 3) the cells can maintain [Na(+)]sm to the above values, despite changes in the pipette Na(+) concentration, showing autoregulation of Na(+) in beating cardiomyocytes.

The calcium-frequency response in the rat ventricular myocyte: an experimental and modelling study

KEY POINTS

In the majority of species, including humans, increased heart rate increases cardiac contractility. This change is known as the force-frequency response (FFR). The majority of mammals have a positive force-frequency relationship (FFR). In rat the FFR is controversial. We derive a species- and temperature-specific data-driven model of the rat ventricular myocyte. As a measure of the FFR, we test the effects of changes in frequency and extracellular calcium on the calcium-frequency response (CFR) in our model and three altered models. The results show a biphasic peak calcium-frequency response, due to biphasic behaviour of the ryanodine receptor and the combined effect of the rapid calmodulin buffer and the frequency-dependent increase in diastolic calcium. Alterations to the model reveal that inclusion of Ca(2+) /calmodulin-dependent protein kinase II (CAMKII)-mediated L-type channel and transient outward K(+) current activity enhances the positive magnitude calcium-frequency response, and the absence of CAMKII-mediated increase in activity of the sarco/endoplasmic reticulum Ca(2+) -ATPase induces a negative magnitude calcium-frequency response.

ABSTRACT

An increase in heart rate affects the strength of cardiac contraction by altering the Ca(2+) transient as a response to physiological demands. This is described by the force-frequency response (FFR), a change in developed force with pacing frequency. The majority of mammals, including humans, have a positive FFR, and cardiac contraction strength increases with heart rate. However, the rat and mouse are exceptions, with the majority of studies reporting a negative FFR, while others report either a biphasic or a positive FFR. Understanding the differences in the FFR between humans and rats is fundamental to interpreting rat-based experimental findings in the context of human physiology. We have developed a novel model of rat ventricular electrophysiology and calcium dynamics, derived predominantly from experimental data recorded under physiological conditions. As a measure of FFR, we tested the effects of changes in stimulation frequency and extracellular calcium concentration on the simulated Ca(2+) transient characteristics and showed a biphasic peak calcium-frequency relationship, consistent with recent observations of a shift from negative to positive FFR when approaching the rat physiological frequency range. We tested the hypotheses that (1) inhibition of Ca(2+) /calmodulin-dependent protein kinase II (CAMKII)-mediated increase in sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) activity, (2) CAMKII modulation of SERCA, L-type channel and transient outward K(+) current activity and (3) Na(+) /K(+) pump dynamics play a significant role in the rat FFR. The results reveal a major role for CAMKII modulation of SERCA in the peak Ca(2+) -frequency response, driven most significantly by the cytosolic calcium buffering system and changes in diastolic Ca(2+) .

IK1-enhanced human-induced pluripotent stem cell-derived cardiomyocytes: an improved cardiomyocyte model to investigate inherited arrhythmia syndromes

Currently available induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) do not ideally model cellular mechanisms of human arrhythmic disease due to lack of a mature action potential (AP) phenotype. In this study, we create and characterize iPS-CMs with an electrically mature AP induced by potassium inward rectifier (IK1) enhancement. The advantages of IK1-enhanced iPS-CMs include the absence of spontaneous beating, stable resting membrane potentials at approximately -80 mV and capability for electrical pacing. Compared with unenhanced, IK1-enhanced iPS-CMs calcium transient amplitudes were larger (P < 0.05) with a typical staircase pattern. IK1-enhanced iPS-CMs demonstrated a twofold increase in cell size and membrane capacitance and increased DNA synthesis compared with control iPS-CMs (P < 0.05). Furthermore, IK1-enhanced iPS-CMs expressing the F97C-CAV3 long QT9 mutation compared with wild-type CAV3 demonstrated an increase in AP duration and late sodium current. IK1-enhanced iPS-CMs represent a more mature cardiomyocyte model to study arrhythmia mechanisms.

Distinguishing between overdrive excited and suppressed ventricular beats in guinea pig ventricular myocardium

Rapid ventricular pacing rates induces two types of beats following pacing cessation: recovery cycle length (RCL) prolongation (overdrive suppression) and RCL shortening (overdrive excitation). The goals of this study were to compare common experimental protocols for studying triggered activity in whole-heart preparations and differentiate between recovery beats using a new methodology. Post-pacing recovery beat cycle length (RCL) and QRS were normalized to pre-paced R-R and QRS intervals and analyzed using a K-means clustering algorithm. Control hearts only produced suppressed beats: RCL ratio increased with rapid pacing (25 ± 4.0%, n = 10) without changing QRS duration. Rapid pacing during hypercalcemia + hypothermia (5.5 mM and 34°C) produced significantly earlier excited beats (53 ± 14%, n = 5) with wider QRS durations (58 ± 6.3%, n = 5) than suppressed beats. Digoxin + hypothermia (0.75 μM) produced the most excited beats with significantly earlier RCL (44 ± 3.2%, n = 6) and wider QRS (60 ± 3.1%, n = 6) ratios relative to suppressed beats. Increasing pacing further shortened RCL (30 ± 7.8%, n = 6). In a prospective study, TTX (100 nM) increased RCL ratio (15 ± 6.0%, n = 10) without changing the QRS duration of excited beats. The algorithm was compared to a cross-correlation analysis with 93% sensitivity and 94% specificity. This ECG based algorithm distinguishes between triggered and automatic activity.

The arrhythmogenic consequences of increasing late INa in the cardiomyocyte

This review presents the roles of cardiac sodium channel NaV1.5 late current (late INa) in generation of arrhythmic activity. The assumption of the authors is that proper Na(+) channel function is necessary to the maintenance of the transmembrane electrochemical gradient of Na(+) and regulation of cardiac electrical activity. Myocyte Na(+) channels' openings during the brief action potential upstroke contribute to peak INa and initiate excitation-contraction coupling. Openings of Na(+) channels outside the upstroke contribute to late INa, a depolarizing current that persists throughout the action potential plateau. The small, physiological late INa does not appear to be critical for normal electrical or contractile function in the heart. Late INa does, however, reduce the net repolarizing current, prolongs action potential duration, and increases cellular Na(+) loading. An increase of late INa, due to acquired conditions (e.g. heart failure) or inherited Na(+) channelopathies, facilitates the formation of early and delayed afterpolarizations and triggered arrhythmias, spontaneous diastolic depolarization, and cellular Ca(2+) loading. These in turn increase the spatial and temporal dispersion of repolarization time and may lead to reentrant arrhythmias.

Regulation of ion gradients across myocardial ischemic border zones: a biophysical modelling analysis

The myocardial ischemic border zone is associated with the initiation and sustenance of arrhythmias. The profile of ionic concentrations across the border zone play a significant role in determining cellular electrophysiology and conductivity, yet their spatial-temporal evolution and regulation are not well understood. To investigate the changes in ion concentrations that regulate cellular electrophysiology, a mathematical model of ion movement in the intra and extracellular space in the presence of ionic, potential and material property heterogeneities was developed. The model simulates the spatial and temporal evolution of concentrations of potassium, sodium, chloride, calcium, hydrogen and bicarbonate ions and carbon dioxide across an ischemic border zone. Ischemia was simulated by sodium-potassium pump inhibition, potassium channel activation and respiratory and metabolic acidosis. The model predicted significant disparities in the width of the border zone for each ionic species, with intracellular sodium and extracellular potassium having discordant gradients, facilitating multiple gradients in cellular properties across the border zone. Extracellular potassium was found to have the largest border zone and this was attributed to the voltage dependence of the potassium channels. The model also predicted the efflux of [Formula: see text] from the ischemic region due to electrogenic drift and diffusion within the intra and extracellular space, respectively, which contributed to [Formula: see text] depletion in the ischemic region.

No rest for the weary: diastolic calcium homeostasis in the normal and failing myocardium

Following contraction of the heart, efficient relaxation (diastole) is essential for refilling the ventricles with blood. This review describes how ventricular relaxation is controlled by Ca(2+) homeostasis in cardiac muscle cells and how alterations in Ca(2+) cycling affect diastolic function in the normal and failing heart. These discussions illustrate that the diastolic phase is not simply a period of rest but rather involves highly regulated and dynamic Ca(2+) fluxes.

Toward an integrative computational model of the Guinea pig cardiac myocyte

The local control theory of excitation-contraction (EC) coupling asserts that regulation of calcium (Ca²⁺) release occurs at the nanodomain level, where openings of single L-type Ca²⁺ channels (LCCs) trigger openings of small clusters of ryanodine receptors (RyRs) co-localized within the dyad. A consequence of local control is that the whole-cell Ca²⁺ transient is a smooth continuous function of influx of Ca²⁺ through LCCs. While this so-called graded release property has been known for some time, its functional importance to the integrated behavior of the cardiac ventricular myocyte has not been fully appreciated. We previously formulated a biophysically based model, in which LCCs and RyRs interact via a coarse-grained representation of the dyadic space. The model captures key features of local control using a low-dimensional system of ordinary differential equations. Voltage-dependent gain and graded Ca²⁺ release are emergent properties of this model by virtue of the fact that model formulation is closely based on the sub-cellular basis of local control. In this current work, we have incorporated this graded release model into a prior model of guinea pig ventricular myocyte electrophysiology, metabolism, and isometric force production. The resulting integrative model predicts the experimentally observed causal relationship between action potential (AP) shape and timing of Ca²⁺ and force transients, a relationship that is not explained by models lacking the graded release property. Model results suggest that even relatively subtle changes in AP morphology that may result, for example, from remodeling of membrane transporter expression in disease or spatial variation in cell properties, may have major impact on the temporal waveform of Ca²⁺ transients, thus influencing tissue level electromechanical function.

Tachybradycardia in the isolated canine right atrium induced by chronic sympathetic stimulation and pacemaker current inhibition

The mechanisms of sinoatrial node (SAN) dysfunction in patients with chronically elevated sympathetic tone and reduced pacemaker current (I(f); such as heart failure) are poorly understood. We simultaneously mapped membrane potential and intracellular Ca(2+) in the Langendorff-perfused canine right atrium (RA). Blockade of either I(f) (ZD-7288) or sarcoplasmic reticulum Ca(2+) release (ryanodine) alone decreased heart rate by 8% (n = 3) and 16% (n = 3), respectively. Combined treatment of ZD-7288 and ryanodine consistently resulted in prolonged (> or =3 s) sinus pauses (PSPs) (n = 4). However, the middle SAN remained as the leading pacemaking site after these treatments. Prolonged exposure with isoproterenol (0.01 micromol/l) followed by ZD-7288 completely suppressed SAN but triggered recurrent ectopic atrial tachycardia. Cessation of tachycardia was followed by PSPs in five of eight RAs. Isoproterenol initially increased heart rate by 75% from baseline with late diastolic intracellular Ca(2+) elevation (LDCAE) from the superior SAN. However, after a prolonged isoproterenol infusion, LDCAE disappeared in the superior SAN, the leading pacemaker shifted to the inferior SAN, and the rate reduced to 52% above baseline. Caffeine (2 ml, 20 mmol/l) injection after a prolonged isoproterenol infusion produced LDCAE in the SAN and accelerated the SAN rate, ruling out sarcoplasmic reticulum Ca(2+) depletion as a cause of Ca(2+) clock malfunction. We conclude that in an isolated canine RA preparation, chronically elevated sympathetic tone results in abnormal pacemaking hierarchy in the RA, including suppression of the superior SAN and enhanced pacemaking from ectopic sites. Combined malfunction of both membrane and Ca(2+) clocks underlies the mechanisms of PSPs.

Purkinje cell calcium dysregulation is the cellular mechanism that underlies catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

Inherited arrhythmias can be caused by mutations in the cardiac ryanodine receptor (RyR2). The cellular source of these arrhythmias is unknown. Isolated RyR2(R4496C) mouse ventricular myocytes display arrhythmogenic activity related to spontaneous Ca(2+) release during diastole. On the other hand, recent whole-heart epicardial and endocardial optical mapping data demonstrate that ventricular arrhythmias in the RyR2(R4496C) mouse model of catecholaminergic polymorphic ventricular tachycardia (CPVT) originate in the His-Purkinje system, suggesting that Purkinje cells, and not ventricular myocytes, may be the cellular source of arrhythmogenic activity. The relative effect of the RyR2(R4496C) mutation on calcium homeostasis in ventricular myocytes versus Purkinje cells is unknown.

OBJECTIVE

This study sought to determine which cardiac cell type is more severely affected, in terms of calcium handling, by expression of the RyR2(R4496C) mutant channel: the ventricular myocytes or the Purkinje cells.

METHODS AND RESULTS

To discriminate Purkinje cells from ventricular myocytes, we crossed the RyR2(R4496C) mouse model of CPVT with the Cx40(EGFP/+) transgenic mouse. This genetic cross yields Purkinje cells that express eGFP, and therefore fluoresce green when excited by the appropriate wavelength; ventricular myocytes, which do not express connexin 40, are not green. Intracellular calcium was measured in each cell type using calcium-sensitive probes. Purkinje cells of the RyR2(R4496C) mouse model of CPVT show an approximately 2x greater rate (P < .05) and approximately 2x to 3x greater amplitude (P < .000001) of spontaneous calcium release events than ventricular myocytes isolated from the same heart.

CONCLUSION

These results demonstrate that focally activated arrhythmias originate in the specialized electrical conducting cells of the His-Purkinje system in the RyR2(R4496C) mouse model of CPVT.

The fateful encounter of mitochondria with calcium: how did it happen?

A number of findings in the 1950s had offered indirect indications that mitochondria could accumulate Ca2+. In 1961, the phenomenon was directly demonstrated using isolated mitochondria: the uptake process was driven by respiratory chain activity or by the hydrolysis of added ATP. It could be accompanied by the simultaneous uptake of inorganic phosphate, in which case precipitates of hydroxyapatite were formed in the matrix, buffering its free Ca2+ concentration. The properties of the uptake process were established in the 1960s and 1970s: the uptake of Ca2+ occurred electrophoretically on a carrier that has not yet been molecularly identified, and was released from mitochondria via a Na+/Ca2+ antiporter. A H+/Ca2+ release exchanger was also found to operate in some mitochondrial types. The permeability transition pore was later also found to mediate the efflux of Ca2+ from mitochondria. In the mitochondrial matrix two TCA cycle dehydrogenases and pyruvate dehydrogenase phosphate phosphatase were found to be regulated in the matrix by the cycling of Ca2+ across the inner membrane. In conditions of cytoplasmic Ca2+ overload mitochondria could store for a time large amounts of precipitated Ca2+-phosphate, thus permitting cells to survive situations of Ca2+ emergency. The uptake process was found to have very low affinity for Ca2+: since the bulk concentration of Ca2+ in the cytoplasm is in the low to mid-nM range, it became increasingly difficult to postulate a role of mitochondria in the regulation of cytoplsmic Ca2+. A number of findings had nevertheless shown that energy linked Ca2+ transport occurred efficiently in mitochondria of various tissues in situ. The paradox was only solved in the 1990s, when it was found that the concentration of Ca2+ in the cytoplasm is not uniform: perimitochondrial micropools are created by the agonist-promoted discharge of Ca2+ from vicinal stores in which the concentration of Ca2+ is high enough to activate the low affinity mitochondrial uniporter. Mitochondria thus regained center stage as important regulators of cytoplasmic Ca2+ (not only of their own internal Ca2+). Their Ca2+ uptake systems was found to react very rapidly to cytoplasmic Ca2+ demands, even in the 150-200 msec time scale of processes like the contraction and relaxation of heart. An important recent development in the area of mitochondrial Ca2+ transport is its involvement in the disease process. Ca2+ signaling defects are now gaining increasing importance in the pathogenesis of diseases, e.g., neurodegenerative diseases. Since mitochondria have now regained a central role in the regulation of cytoplasmic Ca2+, dysfunctions of their Ca2+ controlling systems have expectedly been found to be involved in the pathogenesis of numerous disease processes.

Role of maximum rate of depolarization in predicting action potential duration during ventricular fibrillation

During ventricular fibrillation (VF) only 39% of the variation in action potential duration (APD) is accounted for by the previous diastolic interval [DI((n-1))], i.e., restitution, and the previous APD [APD((n-1))], i.e., memory. We tested the hypothesis that a characteristic of the AP upstroke, the maximum rate of depolarization (V(max)), also helps account for its APD. A floating microelectrode was used to make transmembrane recordings at 16,000 samples/s from the anterior left ventricular wall during four 20-s episodes of VF in each of six pigs. V(max), time from V(max) to 60% repolarization (APD(60)), and DI were calculated throughout all episodes. Stepwise linear regression was used to determine how well each APD(60) (APD(60n)) was predicted by V(max) of that AP, the four previous DIs (n-1, n - 2, n - 3, n - 4), and the three previous APD(60)s (n-1, n - 2, n - 3). V(max) entered in the regression equation significantly more often (86% of VF episodes) than either APD((n-1)) (47% of episodes) or DI((n-1)) (58% of episodes). When these three variables entered first or second, their coefficients were almost always positive, consistent with a longer APD associated with 1) a larger V(max), 2) a longer APD((n-1)), and 3) a longer DI((n-1)). R(2) of the regression for all entered variables was 0.51 +/- 0.01 (mean +/- SD). During the first 20 s of VF in swine, V(max) is a more important determinant of APD than the previous DI (restitution) or the previous APD (memory). All variables together account for only one-half of APD variation during VF.

Multiple alterations in Ca2+ handling determine the negative staircase in a cellular heart failure model

BACKGROUND

The flat or negative force frequency relationship (FFR) is a hallmark of the failing heart. Either decreases in SERCA2a expression, increases in Na(+)/Ca(2+) exchanger (NCX) expression or elevated Na(+)(i) have been independently proposed as mediators of the negative FFR.

METHODS AND RESULTS

To determine whether each one of these mechanisms is sufficient to account for the negative FFR of the failing heart or on the contrary, various mechanisms, acting in concert are required. SERCA2a was pharmacologically inhibited with thapsigargin (TG) or cyclopiazonic acid (CPA) or by using siRNA technology; Na(+)(i) was increased with either ouabain (Oua) or monensin and NCX protein was overexpressed by gene transfer (Ad.NCX), to mimic in nonfailing cat myocytes the phenotype of the failing heart and examine their effect on the FFR. The positive FFR of healthy myocytes remained unaffected after either SERCA2a inhibition, Na(+)(i) elevation, or NCX overexpression. However, the combination of TG + Oua, Oua + Ad.NCX, or TG + Ad.NCX, converted the positive FFR to negative. Moreover, the FFR became negative at lower frequencies, when the 3 interventions were combined.

CONCLUSIONS

Ca(2+) handling has to be altered at several levels to explain the negative FFR of the failing heart. These anomalies in Ca(2+) homeostasis acting in synergy have additive effects.

Calcium antagonist verapamil and reperfusion injury of the heart

OBJECTIVE

An experimental study to examine the effect of verapamil, given into a coronary artery, on reperfusion injury.

DESIGN

The study was randomized but not blinded.

SETTING

This study was conducted in the animal laboratory of the Department of Anesthesiology and Critical Care in an academic institution.

PARTICIPANTS

The study was performed in an anesthetized open-chest pig model.

INTERVENTIONS

Left anterior coronary artery (LAD) occlusion for 15 minutes followed by 90 minutes of reperfusion. Verapamil or saline was given into the LAD artery either at the time the coronary artery was occluded (ie, during acute severe ischemia or during the reperfusion period).

MEASUREMENTS

LAD artery blood flow, regional myocardial function, and metabolism were assessed by the end-systolic pressure length relationship, regional systolic shortening, postsystolic shortening, regional myocardial oxygen consumption, and local cardiac vein lactate.

CONCLUSIONS

Verapamil given during ischemia resulted in a shorter period of regional myocardial stunning when compared with saline or verapamil during reperfusion. The difference in the verapamil strategies (ie, verapamil administered during ischemia versus verapamil during the reperfusion period) can probably be explained by a difference in the effective dose of the drug present in the heart at the time reperfusion started rather than the period of administration per se.

An ionically based mapping model with memory for cardiac restitution

Many features of the sequence of action potentials produced by repeated stimulation of a patch of cardiac muscle can be modeled by a 1D mapping, but not the full behavior included in the restitution portrait. Specifically, recent experiments have found that (i) the dynamic and S1-S2 restitution curves are different (rate dependence) and (ii) the approach to steady state, which requires many action potentials (accommodation), occurs along a curve distinct from either restitution curve. Neither behavior can be produced by a 1D mapping. To address these shortcomings, ad hoc 2D mappings, where the second variable is a "memory" variable, have been proposed; these models exhibit qualitative features of the relevant behavior, but a quantitative fit is not possible. In this paper we introduce a new 2D mapping and determine a set of parameters for it that gives a quantitatively accurate description of the full restitution portrait measured from a bullfrog ventricle. The mapping can be derived as an asymptotic limit of an idealized ionic model in which a generalized concentration acts as a memory variable. This ionic basis clarifies how the present model differs from previous models. The ionic basis also provides the foundation for more extensive cardiac modeling: e.g., constructing a PDE model that may be used to study the effect of memory on propagation. The fitting procedure for the mapping is straightforward and can easily be applied to obtain a mathematical model for data from other experiments, including experiments on different species.

Comparison of the effects of mepivacaine and lidocaine on rat myocardium

BACKGROUND AND OBJECTIVE

To compare the inotropic and lusitropic effect of lidocaine and mepivacaine on rat papillary muscle.

METHODS

Effects of lidocaine and mepivacaine (10-8-10-3 M) were studied in rat left ventricular papillary muscles in vitro at a calcium concentration of 1 mmol, under low (isotony) and high (isometric) loads.

RESULTS

Lidocaine induced a significant negative inotropic effect in isotonic and isometric conditions whereas mepivacaine did not. Mepivacaine only induced a negative inotropic effect when added as a bolus for the highest concentration and this effect was significantly more pronounced with lidocaine than with mepivacaine (active force at 10-3 M: 63 +/- 10 vs. 84 +/- 10% of baseline, P < 0.05). Increasing calcium concentration resulted in a greater positive inotropic effect in the control (199 +/- 11% of baseline) and mepivacaine groups (197 +/- 22% of baseline) when compared to the lidocaine group (163 +/- 19% of baseline, P < 0.05 vs. lidocaine and control groups), suggesting an impairment on intracellular Ca2+ handling by lidocaine. A negative lusitropic effect under low load was observed only for mepivacaine and suggested an impairment of sarcoplasmic reticulum function. Lidocaine and mepivacaine did not modify post-rest potentiation but significantly depressed the force-frequency relationship.

CONCLUSIONS

The negative inotropic and lusitropic effects induced by lidocaine were more important than that of mepivacaine and may involve an impairment of intracellular Ca2+ handling.

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

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

Elevated cytosolic Na+ decreases mitochondrial Ca2+ uptake during excitation-contraction coupling and impairs energetic adaptation in cardiac myocytes

Mitochondrial Ca2+ ([Ca2+]m) regulates oxidative phosphorylation and thus contributes to energy supply and demand matching in cardiac myocytes. Mitochondria take up Ca2+ via the Ca2+ uniporter (MCU) and extrude it through the mitochondrial Na+/Ca2+ exchanger (mNCE). It is controversial whether mitochondria take up Ca2+ rapidly, on a beat-to-beat basis, or slowly, by temporally integrating cytosolic Ca2+ ([Ca2+]c) transients. Furthermore, although mitochondrial Ca2+ efflux is governed by mNCE, it is unknown whether elevated intracellular Na+ ([Na+]i) affects mitochondrial Ca2+ uptake and bioenergetics. To monitor [Ca2+]m, mitochondria of guinea pig cardiac myocytes were loaded with rhod-2-acetoxymethyl ester (rhod-2 AM), and [Ca2+]c was monitored with indo-1 after dialyzing rhod-2 out of the cytoplasm. [Ca2+]c transients, elicited by voltage-clamp depolarizations, were accompanied by fast [Ca2+]m transients, whose amplitude (delta) correlated linearly with delta[Ca2+]c. Under beta-adrenergic stimulation, [Ca2+]m decay was approximately 2.5-fold slower than that of [Ca2+]c, leading to diastolic accumulation of [Ca2+]m when amplitude or frequency of delta[Ca2+]c increased. The MCU blocker Ru360 reduced delta[Ca2+]m and increased delta[Ca2+]c, whereas the mNCE inhibitor CGP-37157 potentiated diastolic [Ca2+]m accumulation. Elevating [Na+]i from 5 to 15 mmol/L accelerated mitochondrial Ca2+ decay, thus decreasing systolic and diastolic [Ca2+]m. In response to gradual or abrupt changes of workload, reduced nicotinamide-adenine dinucleotide (NADH) levels were maintained at 5 mmol/L [Na+]i, but at 15 mmol/L, the NADH pool was partially oxidized. The results indicate that (1) mitochondria take up Ca2+ rapidly and contribute to fast buffering during a [Ca2+]c transient; and (2) elevated [Na+]i impairs mitochondrial Ca2+ uptake, with consequent effects on energy supply and demand matching. The latter effect may have implications for cardiac diseases with elevated [Na+]i.

Mechanisms underlying overdrive suppression and overdrive excitation in guinea pig sino-atrial node

The hypothesis that the pause that follows overdrive of the sino-atrial node (SAN) might be the net result of overdrive excitation and overdrive suppression was tested by studying rate and force patterns induced by overdrive in isolated guinea pig SAN superfused in vitro. In Tyrode solution, the pause is short and changes but little with longer or faster drives. In high [K(+)](o) solution, longer overdrives increase force percent-wise more than in Tyrode solution, shorten the pause and are followed by greater rate and force. When the SAN (quiescent in high [K(+)](o)) is driven at 6/min, faster overdrives are followed by stronger, slowly decreasing contractions. Alternating 10 s drives with 10 s pauses have little effect on force and rate in Tyrode solution, but progressively increase force and rate in high [K(+)](o). Cesium has effects similar to high [K(+)](o). High [Ca(2+)](o) increases force and in high [K(+)](o) increases the rate as well as it shortens the pause, whereas Ni(2+) decreases force as well as rate and lengthens the pause. Barium dissociates the effects on force and rate. Lidocaine and tetrodotoxin decrease rate and force, and increase the pause duration. In overdrive excitation, the increase in rate is associated with an enhancement of diastolic voltage oscillations. It is concluded that in SAN the prevalence of Ca(2+) load leads to overdrive excitation whereas the prevalence of Na(+) load leads to overdrive suppression. In Tyrode solution, the pause after drive appears to be the net result of these two different mechanisms.

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.

Force-frequency relationship in intact mammalian ventricular myocardium: physiological and pathophysiological relevance

The force-frequency relationship (FFR) is an important intrinsic regulatory mechanism of cardiac contractility. The FFR in most mammalian ventricular myocardium is positive; that is, an increase in contractile force in association with an increase in the amplitude of Ca(2+) transients is induced by elevation of the stimulation frequency, which reflects the cardiac contractile reserve. The relationship is different depending on the range of frequency and species of animal. In some species, including rat and mouse, a 'primary-phase' negative FFR is induced over the low-frequency range up to approximately 0.5-1 Hz (rat) and 1-2 Hz (mouse). Even in these species, the FFR over the frequency range close to the physiological heart rate is positive and qualitatively similar to that in larger mammalian species, although the positive FFR is less prominent. The integrated dynamic balance of the intracellular Ca(2+) concentration ([Ca(2+)](i)) is the primary cellular mechanism responsible for the FFR and is determined by sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+) flux through the sarcolemma via L-type Ca(2+) channels and the Na(+)-Ca(2+) exchanger. Intracellular Na(+) concentration is also an important factor in [Ca(2+)](i) regulation. In isolated rabbit papillary muscle, over a lower frequency range (<0.5 Hz), an increase in duration rather than amplitude of Ca(2+) transients appears to be responsible for the increase in contractile force, while over an intermediate frequency range (0.5-2.0 Hz), the amplitude of Ca(2+) transients correlates well with the increase in contractile force. Over a higher frequency range (>2.5 Hz), the contractile force is dissociated from the amplitude of Ca(2+) transients probably due to complex cellular mechanisms, including oxygen limitation in the central fibers of isolated muscle preparations, while the amplitude of Ca(2+) transients increases further with increasing frequency ('secondary-phase' negative FFR). Calmodulin (CaM) may contribute to a positive FFR and the frequency-dependent acceleration of relaxation, although the role of calmodulin has not yet been established unequivocally. In failing ventricular myocardium, the positive FFR disappears or is inverted and becomes negative. The activation and overexpression of cardiac sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a) is able to reverse these abnormalities. Frequency-dependent alterations of systolic and diastolic force in association with those of Ca(2+) transients and diastolic [Ca(2+)](i) levels are excellent indicators for analysis of cardiac excitation-contraction coupling, and for evaluating the severity of cardiac contractile dysfunction, cardiac reserve capacity and the effectiveness of therapeutic agents in congestive heart failure.

Pacing Staircase Phenomenon in the Heart: From Bodwitch to the XXI Century

The frequency of pacing is a fundamental physiological modulator of myocardial function. When the pacing rate increases there is normally an increase in contractility (a positive force-frequency relationship). However in small rodents, fish and end-stage failing myocardium, the force-frequency response has been found to be flat or even negative. The positive staircase is understood to be related with the increase in the intracellular Ca(2+) transient, mainly due to an enhanced sarcoplasmic reticulum Ca(2+) content at higher stimulation frequencies, resulting from an increase in Ca(2+) influx per unit time and reduced Ca(2+) efflux between beats. However, additional mechanisms, such as increased activity of Ca(2+)/calmodulin-dependent protein kinase or enhanced myofilament responsiveness to Ca(2+) may also play a role. Although an increase in contraction frequency has been shown to be associated with an increase in intracellular Na(+), several studies have shown a temporal dissociation between the increase in Na(i)(+) and the increase in force evoked by changes in pacing frequency. The way in which the Na(+)/Ca(2+) exchanger contributes to contraction frequency inotropy is still not well understood. The aim of this review is to examine the contribution of the fundamental components of cardiac excitation-contraction coupling to frequency inotropy in healthy and failing hearts.

Calcium overload and cardiac function

The changes in cardiac function caused by calcium overload are reviewed. Intracellular Ca(2+) may increase in different structures [e.g. sarcoplasmic reticulum (SR), cytoplasm and mitochondria] to an excessive level which induces electrical and mechanical abnormalities in cardiac tissues. The electrical manifestations of Ca(2+) overload include arrhythmias caused by oscillatory (V(os)) and non-oscillatory (V(ex)) potentials. The mechanical manifestations include a decrease in force of contraction, contracture and aftercontractions. The underlying mechanisms involve a role of Na(+) in electrical abnormalities as a charge carrier in the Na(+)-Ca(2+) exchange and a role of Ca(2+) in mechanical toxicity. Ca(2+) overload may be induced by an increase in [Na(+)](i) through the inhibition of the Na(+)-K(+) pump (e.g. toxic concentrations of digitalis) or by an increase in Ca(2+) load (e.g. catecholamines). The Ca(2+) overload is enhanced by fast rates. Purkinje fibers are more susceptible to Ca(2+) overload than myocardial fibers, possibly because of their greater Na(+) load. If the SR is predominantly Ca(2+) overloaded, V(os) and fast discharge are induced through an oscillatory release of Ca(2+) in diastole from the SR; if the cytoplasm is Ca(2+) overloaded, the non-oscillatory V(ex) tail is induced at negative potentials. The decrease in contractile force by Ca(2+) overload appears to be associated with a decrease in high energy phosphates, since it is enhanced by metabolic inhibitors and reduced by metabolic substrates. The ionic currents I(os) and I(ex) underlie V(os) and V(ex), respectively, both being due to an electrogenic extrusion of Ca(2+) through the Na(+)-Ca(2+) exchange. I(os) is an oscillatory current due to an oscillatory release of Ca(2+) in early diastole from the Ca(2+)-overloaded SR, and I(ex) is a non-oscillatory current due to the extrusion of Ca(2+) from the Ca(2+)-overloaded cytoplasm. I(os) and I(ex) can be present singly or simultaneously. An increase in [Ca(2+)](i) appears to be involved in the short- and long-term compensatory mechanisms that tend to maintain cardiac output in physiological and pathological conditions. Eventually, [Ca(2+)](i) may increase to overload levels and contribute to cardiac failure. Experimental evidence suggests that clinical concentrations of digitalis increase force in Ca(2+)-overloaded cardiac cells by decreasing the inhibition of the Na(+)-K(+) pump by Ca(2+), thereby leading to a reduction in Ca(2+) overload and to an increase in force of contraction.

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

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

Na(+)-Ca2+ exchange function underlying contraction frequency inotropy in the cat myocardium

In most mammalian species, an increase in stimulation frequency (ISF) produces an increase in contractility (treppe phenomenon), which results from larger Ca2+ transients at higher frequencies, due to an increase in sarcoplasmic reticulum Ca2+ load and release. The present study attempts to elucidate the contribution of the Na(+)-Ca2+ exchanger (NCX) to this phenomenon. Isolated cat ventricular myocytes, loaded with [Ca2+]i- and [Na+]i-sensitive probes, were used to determine whether the contribution of the NCX to the positive inotropic effect of ISF is due to an increase in Ca2+ influx (reverse mode) and/or a decrease in Ca2+ efflux (forward mode) via the NCX, due to frequency-induced [Na+]i elevation, or whether it was due to the reduced time for the NCX to extrude Ca2+. The results showed that the positive intropic effect produced by ISF was temporally dissociated from the increase in [Na+]i and was not modified by KB-R7943 (1 or 5 microM), a specific blocker of the reverse mode of the NCX. Whereas the ISF from 10 to 30 beats min(-1) (bpm) did not affect the forward mode of the NCX (assessed by the time to half-relaxation of the caffeine-induced Ca2+ transient), the ISF to 50 bpm produced a significant reduction of the activity of the forward mode of the NCX, which occurred in association with an increase in [Na+]i (from 4.33+/-0.40 to 7.25+/-0.50 mM). However, both changes became significant well after the maximal positive inotropic effect had been reached. In contrast, the positive inotropic effect produced by ISF from 10 to 50 bpm was associated with an increase in diastolic [Ca2+]i, which occurred in spite of a significant increase in the relaxation rate and at a time at which no increases in [Na+]i were detected. The contribution of the NCX to stimulus frequency inotropy would therefore depend on a decrease in NCX-mediated Ca2+ efflux due to the reduced diastolic interval between beats and not on [Na+]i-dependent mechanisms.

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.

Mechanisms underlying the frequency dependence of contraction and [Ca(2+)](i) transients in mouse ventricular myocytes

In most mammalian species force of contraction of cardiac muscle increases with increasing rate of stimulation, i.e. a positive force-frequency relationship. In single mouse ventricular cells, both positive and negative relationships have been described and little is known about the underlying mechanisms. We studied enzymatically isolated single ventricular mouse myocytes, at 30 degrees C. During field stimulation, amplitude of unloaded cell shortening increased with increasing frequency of stimulation (0.04 +/- 0.01 Delta L/L(0) at 1 Hz to 0.07 +/- 0.01 Delta L/L(0) at 4 Hz, n = 12, P < 0.05). During whole cell voltage clamp with 50 microM [K5-fluo-3](pip), both peak and baseline [Ca(2+)](i) increased at higher stimulation frequencies, but the net Delta[Ca(2+)](i) increased only modestly from 1.59 +/- 0.08 Delta F/F(0) at 1 Hz, to 1.71 +/- 0.11 Delta F/F(0) at 4 Hz (n = 17, P < 0.05). When a 1 s pause was interposed during stimulation at 2 and 4 Hz, [Ca(2+)](i) transients were significantly larger (at 4 Hz, peak F/F(0) increased by 78 +/- 2 %, n = 5). SR Ca(2+) content assessed during caffeine application, significantly increased from 91 +/- 24 micromol l(-1) at 1 Hz to 173 +/- 20 micromol l(-1) at 4 Hz (n = 5, P < 0.05). Peak I(Ca,L) decreased at higher frequencies (by 28 +/- 6 % at 2 Hz, and 45 +/- 8 % at 4 Hz), due to slow recovery from inactivation. This loss of I(Ca,L) resulted in reduced fractional release. Thus, in mouse ventricular myocytes the [Ca(2+)](i)-frequency response depends on a balance between the increase in SR content and the loss of trigger I(Ca,L). Small changes in this balance may contribute to variability in frequency-dependent behaviour. In addition, there may be a regulation of the contractile response downstream of [Ca(2+)](i).

Effect of adrenergic stimulation on Rb(+) uptake in normal and ischemic areas of isolated pig hearts: (87)Rb MRI study

The role of adrenergic stimulation in Rb(+) uptake in normal and ischemic areas of pig hearts was investigated using an agonist (dobutamine) and an antagonist (S-propranolol). The left anterior descending artery (LAD) in isolated pig hearts was cannulated to maintain adequate perfusion of the LAD bed (2.2-2.8 ml/min/g). Rb(+) loading was initiated by switching perfusion to Rb(+)-containing Krebs-Henseleit (KH) buffer in the presence of 0.1 microM dobutamine, 0.5 microM propranolol, or no drug (control), and the LAD flow was reduced to 10% of the baseline for 120 min. The flow through the LAD was then restored to normal and the drugs were removed. In all groups the rate of Rb(+) uptake obtained from serial (87)Rb images was severely depressed in the ischemic anterior wall relative to that in the normal posterior wall. Dobutamine increased the rate of Rb(+) uptake in the posterior wall (6.3% +/- 1.4% vs. 2.5% +/- 0.25% per minute in control) and did not affect the rate in the anterior wall (0.86% +/- 0.40% vs. 1.14% +/- 0.19% per minute in control). Propranolol did not affect Rb(+) uptake in either of these areas. We conclude that the stimulation of Rb(+) uptake by dobutamine is related to activation of Na(+)/K(+) ATPase and passive Na(+) influx in normal tissue. The lack of any effect of propranolol implies a low level of endogenous norepinephrine during low-flow (LF) ischemia.

Effects of regional hypoxia and acidosis on Rb(+) uptake and energetics in isolated pig hearts: (87)Rb MRI and (31)P MR spectroscopic study

The study compared the effects of regional hypoxia and acidosis on Rb(+) uptake and energetics in isolated pig hearts perfused by the Langendorff method. The left anterior descending artery (LAD) was cannulated and the LAD bed was perfused with the same specific flow as the whole heart. Following equilibration with normal Krebs-Henseleit buffer (KHB, pO(2) 568 mm Hg, pH 7.42) the perfusate was switched to one that contained Rb(+) (Rb-KHB). Simultaneously, perfusion through the LAD was carried out with hypoxic (pO(2)=31 mm Hg), an acidemic (pH 7.12) or normal (pO(2)=550 mm Hg) Rb-KHB for 120 min. (87)Rb images of the entire heart or localized (31)P spectra from the left ventricular anterior wall were acquired. Hypoxia decreased the maximal (87)Rb image intensity and Rb(+) flux in the anterior wall to 79+/-9% and 85+/-7%, respectively, of that in the posterior wall. Extracellular acidosis did not affect (87)Rb image intensity and reduced Rb(+) flux (83+/-10%). During hypoxia phosphocreatine and ATP decreased to 36+/-10 and 50+/-15% of baseline, respectively and intracellular pH (pHi) decreased to 6.90+/-0.05. Extracellular acidosis did not affect the phosphocreatine or ATP levels but reduced pHi (7.06+/-0.18 vs. 7.26+/-0.06 in control). We suggest that intracellular acidosis plays a role in the inhibition of Rb(+) uptake during hypoxia.

Usefulness of Doppler tissue imaging for the assessment of right and left ventricular myocardial function in patients with dual-chamber pacing

The aim of the study was to evaluate by Doppler tissue imaging (DTI) the combined effects of atrio-ventricular (AV) delay and heart rate (HR) changes on global and segmental right (RV) and left (LV) ventricular diastolic function in 15 patients with dual-chamber pacemakers paced in the DDD mode. RV and LV inflow velocities and regional systolic and diastolic pulsed-wave (PW) DTI parameters were analyzed at four different pacing modes: (1) HR 70 beats/min, AV delay 125 ms; (2) HR 70 beats/min, AV delay 188 ms; (3) HR 89 beats/min, AV delay 125 ms; (4) HR 89 beats/min, AV delay 188 ms. For each pacing mode selected, RV diastolic filling velocities always prevailed over LV ones. As for RV and LV adaptation to the four different stimulation protocols, a higher paced rate and a prolonged AV delay caused across both the AV valves a decrease of E wave and of E/A ratios. The intersegmental comparison of PW-DTI parameters outlined that RV free wall exhibited significantly higher peak systolic (Sm) and early-diastolic (Em) wall velocities, and longer systolic ejection time. Considering separately RV and LV segmental physiology at the four programmed pacing modes, an increase in HR determined a progressive shortening of systolic ejection times in all the segments analyzed. Moreover, in each region the Em/Am ratio decreased with higher HR and longer AV delay. Conversely, Em encountered a progressive reduction in RV free wall, while remaining quite unchanged in all the LV regions. Both ventricles shared a similar pattern of global and regional adaptation to programmed HR and AV delay modifications, consisting in a progressive greater contribution of late diastole to ventricular filling at higher HR and more prolonged AV delay. However, at a regional level the right ventricle exhibited higher systolic and diastolic wall velocities than all left ventricular regions.

Noninvasive assessment of cardiac ischemic injury using (87)Rb and (23)Na MR imaging, (31)P MR, and optical spectroscopy

The aim of the study was to compare and analyze different noninvasive indices of cell damage in the isolated pig heart model of regional ischemia. We used (23)Na and (87)Rb MR imaging to evaluate Na(+)/K(+) balance, (31)P MR spectroscopy to measure energetics, and optical spectroscopy to assess oxymyoglobin (MbO(2)). Hearts were subjected to 120-min occlusion of the left anterior descending artery and were then reperfused for 120 min. Reperfusion resulted in an increase in (23)Na (37 +/- 18% of the posterior wall) and decrease in (87)Rb (55 +/- 15%) image intensities, partial recovery of PCr, ATP, the total phosphates, and MbO(2) in the anterior wall. The above changes are consistent with the irreversible cell damage in the anterior wall, confirmed by lack of staining with triphenyltetrazolium chloride. Changes in Na(+) and Rb(+) in the infarct area inversely correlated and their ratio is a more sensitive index of cell injury than either of them alone.

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.

Overdrive suppression of antegrade conduction over the accessory pathway

In a patient with Wolff-Parkinson-White syndrome whose accessory pathway was primarily capable of bidirectional conduction, antegrade conduction over the accessory pathway was transiently inhibited after rapid atrial or ventricular pacing or after spontaneous termination of atrioventricular reentrant tachycardia. Pacing rate and duration of tachycardia were related to the duration of the suppression of preexcitation, while the coupling interval of the first sinus beat to the last driven or tachycardia beat was irrelevant to the phenomenon. Thus, overdrive suppression of conduction may be the most likely mechanism of this phenomenon.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Effects of intracellular calcium elevation on action potential and L-type calcium current of normal and chronically infarcted rat ventricles

The present work investigated the effects of raising [Ca+2]i levels on action potential (AP) and L-type calcium current (I(Ca.L)) of normal and chronically infarcted rat ventricles. Experiments were performed by conventional electrophysiology and whole-cell patch-clamp techniques. In the former, APs were recorded in ventricular strips subjected to different pacing rates or elevation of [Ca+2]o levels. In the latter, I(Ca.L) was studied in isolated myocytes in the absence of an intracellular Ca+2 chelator. The acceleration of heart rate (6 to 240 beats/min) reduced AP duration measured at 20%, 50%, and 90% repolarization (APD20, APD50, and APD90) in the infarcted group, and increased APD20 and APD50 in the control group. Rising [Ca+]o (1.25 to 5.0 mmol/L) induced a decrease of APD20 and APD50 in both groups. Voltage clamp revealed a smaller I(Ca.L) density at approximately -17 mV in myocytes from infarcted ventricles (-1.86 +/- 0.37 vs -3.98 +/- 0.65 pA/pF, P < .05), and the appearance of a non-K+ outward current coupled to I(Ca.L). The results suggest the participation of a Ca+2-activated outward current in the repolarization of normal and infarcted rat ventricles.

Dynamics of action potential head-tail interaction during reentry in cardiac tissue: ionic mechanisms

In a sufficiently short reentry pathway, the excitation wave front (head) propagates into tissue that is partially refractory (tail) from the previous action potential (AP). We incorporate a detailed mathematical model of the ventricular myocyte into a one-dimensional closed pathway to investigate the effects of head-tail interaction and ion accumulation on the dynamics of reentry. The results were the following: 1) a high degree of head-tail interaction produces oscillations in several AP properties; 2) Ca(2+)-transient oscillations are in phase with AP duration oscillations and are often of greater magnitude; 3) as the wave front propagates around the pathway, AP properties undergo periodic spatial oscillations that produce complicated temporal oscillations at a single site; 4) depending on the degree of head-tail interaction, intracellular [Na(+)] accumulation during reentry either stabilizes or destabilizes reentry; and 5) elevated extracellular [K(+)] destabilizes reentry by prolonging the tail of postrepolarization refractoriness.

Na+-K+ pump inhibition caused by chronic amiodarone in guinea pig myocardium

Although it is known that amiodarone inhibits myocardial Na+-K+ pump activity, the potency and the time course of this inhibition are unknown. The aim of this study was to investigate these aspects with reference to digoxin, using guinea pigs treated with either intraperitoneal amiodarone (20mg/kg per week, up to 12 weeks, n = 26) or the same amount of vehicle as a control (n = 24). ECG recording and microelectrode experiments were conducted every 2 weeks. QT interval corrected by heart rate and action potential duration were prolonged as a function of the time of exposure to amiodarone. Hyperpolarization observed immediately after the overdrive (1.0Hz) termination or K+-replenishment following K+-depletion in the presence of 0.1mM Ba2+ was compared in the amiodarone-treated and untreated groups, as an index of the Na+-K+ pump activity. The resting membrane potential recovery from overdrive-induced depolarization was slower and the amplitude of K+-induced hyperpolarization was smaller in the amiodarone-treated group than in the untreated group. These changes were evident as the chronic amiodarone treatment progressed, although the changes in these parameters were greater in the case of acute application of 50 microM digoxin. In conclusion, this study indicates that treatment with amiodarone for longer than several weeks moderately inhibits the myocardial Na+-K+ pump.

Oscillatory zones and their role in normal and abnormal sheep Purkinje fiber automaticity

The mechanisms by which low [K(+)](o) induces spontaneous activity was studied in sheep Purkinje fibers. Purkinje strands were superfused in vitro and membrane potentials were recorded by means of a microelectrode technique. The results show that low [K(+)](o) increases the slope and amplitude of early diastolic depolarization, sharpens the transition between early and late diastolic depolarizations, induces an after-potential and large pre-potentials through a negative shift of an oscillatory zone. Pre-potentials occur progressively sooner during diastole and merge with the after-potential to induce uninterrupted spontaneous discharge. During recovery, when the rate slows, after- and pre-potentials separate once more, the slower discharge decreasing the after-potentials but not the pre-potentials. Low [K(+)](o) has little effect on the plateau, but markedly slows phase 3 repolarization and may altogether prevent it. At depolarized levels, voltage oscillations, slow responses, sinusoidal fluctuations or quiescence may be present depending on voltage. During the recovery, a train of either sub-threshold oscillations or spontaneous action potentials appear towards the end of phase 3 repolarization. The cessation of the action potentials unmasks large sub-threshold oscillations, that occur in the oscillatory zone. Drive, high [Ca(2+)](o) and norepinephrine increase slope and amplitude of early diastolic depolarization as low [K(+)](o) does. In low [K(+)](o), Cs(+) prevents spontaneous discharge at polarized levels, but not the decrease in resting potential nor the onset of slow responses at depolarized levels. Cs(+) blocks the train of oscillations and of action potentials occurring during recovery. We conclude that low [K(+)](o) steepens early diastolic depolarization and increases its amplitude through an after-potential that results from an increased Ca(2+) load; allows the attainment of the threshold through Cs(+)-sensitive voltage oscillations which develop when the oscillatory zone is entered either by diastolic depolarization or by phase 3 repolarization; and causes voltage oscillations also at depolarized levels, but through a Cs(+)-insensitive different mechanism.