Action potential duration and force-frequency relationship in isolated rabbit, guinea pig and rat cardiac muscle

The Role of Ca2+ Sparks in Force Frequency Relationships in Guinea Pig Ventricular Myocytes

Calcium sparks are the elementary Ca2+ release events in excitation-contraction coupling that underlie the Ca2+ transient. The frequency-dependent contractile force generated by cardiac myocytes depends upon the characteristics of the Ca2+ transients. A stochastic computational local control model of a guinea pig ventricular cardiomyocyte was developed, to gain insight into mechanisms of force-frequency relationship (FFR). This required the creation of a new three-state RyR2 model that reproduced the adaptive behavior of RyR2, in which the RyR2 channels transition into a different state when exposed to prolonged elevated subspace [Ca2+]. The model simulations agree with previous experimental and modeling studies on interval-force relations. Unlike previous common pool models, this local control model displayed stable action potential trains at 7 Hz. The duration and the amplitude of the [Ca2+]myo transients increase in pacing rates consistent with the experiments. The [Ca2+]myo transient reaches its peak value at 4 Hz and decreases afterward, consistent with experimental force-frequency curves. The model predicts, in agreement with previous modeling studies of Jafri and co-workers, diastolic sarcoplasmic reticulum, [Ca2+]sr, and RyR2 adaptation increase with the increased stimulation frequency, producing rising, rather than falling, amplitude of the myoplasmic [Ca2+] transients. However, the local control model also suggests that the reduction of the L-type Ca2+ current, with an increase in pacing frequency due to Ca2+-dependent inactivation, also plays a role in the negative slope of the FFR. In the simulations, the peak Ca2+ transient in the FFR correlated with the highest numbers of SR Ca2+ sparks: the larger average amplitudes of those sparks, and the longer duration of the Ca2+ sparks.

Canine Myocytes Represent a Good Model for Human Ventricular Cells Regarding Their Electrophysiological Properties

Due to the limited availability of healthy human ventricular tissues, the most suitable animal model has to be applied for electrophysiological and pharmacological studies. This can be best identified by studying the properties of ion currents shaping the action potential in the frequently used laboratory animals, such as dogs, rabbits, guinea pigs, or rats, and comparing them to those of human cardiomyocytes. The authors of this article with the experience of three decades of electrophysiological studies, performed in mammalian and human ventricular tissues and isolated cardiomyocytes, summarize their results obtained regarding the major canine and human cardiac ion currents. Accordingly, L-type Ca²⁺ current (I_Ca), late Na⁺ current (I_Na-late), rapid and slow components of the delayed rectifier K⁺ current (I_Kr and I_Ks, respectively), inward rectifier K⁺ current (I_K1), transient outward K⁺ current (I_to1), and Na⁺/Ca²⁺ exchange current (I_NCX) were characterized and compared. Importantly, many of these measurements were performed using the action potential voltage clamp technique allowing for visualization of the actual current profiles flowing during the ventricular action potential. Densities and shapes of these ion currents, as well as the action potential configuration, were similar in human and canine ventricular cells, except for the density of I_K1 and the recovery kinetics of I_to. I_K1 displayed a largely four-fold larger density in canine than human myocytes, and I_to recovery from inactivation displayed a somewhat different time course in the two species. On the basis of these results, it is concluded that canine ventricular cells represent a reasonably good model for human myocytes for electrophysiological studies, however, it must be borne in mind that due to their stronger I_K1, the repolarization reserve is more pronounced in canine cells, and moderate differences in the frequency-dependent repolarization patterns can also be anticipated.

Frequency-dependent effects of omecamtiv mecarbil on cell shortening of isolated canine ventricular cardiomyocytes

Omecamtiv mecarbil (OM) is a myosin activator agent developed for the treatment of heart failure. OM was reported to increase left ventricular ejection fraction and systolic ejection time, but little is known about the effect of heart rate on the action of OM. The present study, therefore, was designed to investigate the effects of OM on unloaded cell shortening and intracellular Ca²⁺ ([Ca²⁺]_i) transients as a function of the pacing frequency. Isolated cardiomyocytes were stimulated at various frequencies under steady-state conditions. Cell length was monitored by an optical edge detector and changes in [Ca²⁺]_i were followed using the Ca²⁺-sensitive dye Fura-2. At the pacing frequency of 1 Hz, OM (1-10 μM) significantly decreased both diastolic and systolic cell length, however, fractional shortening was augmented only by 1 μM OM. Time to peak tension and time of 90% relaxation were progressively increased by OM. At the frequency of 2 Hz, diastolic cell length was reduced by 10 μM OM to a larger extent than systolic cell length, resulting in a significantly decreased fractional shortening under these conditions. OM had no effect on the parameters of the [Ca²⁺]_i transient at any pacing frequency. The results suggest that supratherapeutic concentrations of OM may decrease rather than increase the force of cardiac contraction especially in tachycardic patients.

I-Wire Heart-on-a-Chip I: Three-dimensional cardiac tissue constructs for physiology and pharmacology

Engineered 3D cardiac tissue constructs (ECTCs) can replicate complex cardiac physiology under normal and pathological conditions. Currently, most measurements of ECTC contractility are either made isometrically, with fixed length and without control of the applied force, or auxotonically against a variable force, with the length changing during the contraction. The "I-Wire" platform addresses the unmet need to control the force applied to ECTCs while interrogating their passive and active mechanical and electrical characteristics. A six-well plate with inserted PDMS casting molds containing neonatal rat cardiomyocytes cultured with fibrin for 13-15days is mounted on the motorized mechanical stage of an inverted microscope equipped with a fast sCMOS camera. A calibrated flexible probe provides strain load of the ECTC via lateral displacement, and the microscope detects the deflections of both the probe and the ECTC. The ECTCs exhibited longitudinally aligned cardiomyocytes with well-developed sarcomeric structure, recapitulated the Frank-Starling force-tension relationship, and demonstrated expected transmembrane action potentials, electrical and mechanical restitutions, and responses to both β-adrenergic stimulation and blebbistatin. The I-Wire platform enables creation and mechanical and electrical characterization of ECTCs, and hence can be valuable in the study of cardiac diseases, drug screening, drug development, and the qualification of cells for tissue-engineered regenerative medicine.

STATEMENT OF SIGNIFICANCE

There is a growing interest in creating engineered heart tissue constructs for basic cardiac research, applied research in cardiac pharmacology, and repair of damaged hearts. We address an unmet need to characterize fully the performance of these tissues with our simple "I-Wire" assay that allows application of controlled forces to three-dimensional cardiac fiber constructs and measurement of both the electrical and mechanical properties of the construct. The advantage of I-Wire over other approaches is that the constructs being measured are truly three-dimensional, rather than a single layer of cells grown within a microfluidic device. We anticipate that the I-Wire will be extremely useful for the evaluation of myocardial constructs created using cardiomyocytes derived from human induced pluripotent stem cells.

Geometrical considerations in cardiac electrophysiology and arrhythmogenesis

The rate of repolarization (RRepol) and so the duration of the cardiac action potential are determined by the balance of inward and outward currents across the cardiac membrane (net ionic current). Plotting action potential duration (APD) as a function of the RRepol reveals an inverse non-linear relationship, arising from the geometric association between these two factors. From the RRepol-APD relationship, it can be observed that a longer action potential will exhibit a greater propensity to shorten, or prolong, for a given change in the RRepol (i.e. net ionic current), when compared with one that is initially shorter. This observation has recently been used to explain why so many interventions that prolong the action potential exert a greater effect at slow rates (reverse rate-dependence). In this article, we will discuss the broader implications of this simple principle and examine how common experimental observations on the electrical behaviour of the myocardium may be explained in terms of the RRepol-APD relationship. An argument is made, with supporting published evidence, that the non-linear relationship between the RRepol and APD is a fundamental, and largely overlooked, property of the myocardium. The RRepol-APD relationship appears to explain why interventions and disease with seemingly disparate mechanisms of action have similar electrophysiological consequences. Furthermore, the RRepol-APD relationship predicts that prolongation of the action potential, by slowing repolarization, will promote conditions of dynamic electrical instability, exacerbating several electrophysiological phenomena associated with arrhythmogenesis, namely, the rate dependence of dispersion of repolarization, APD restitution, and electrical alternans.

Species-dependent adaptation of the cardiac Na+/K+ pump kinetics to the intracellular Na+ concentration

The Na(+)/K(+) ATPase (NKA) plays a critical role in maintaining ionic homeostasis and dynamic function in cardiac myocytes, within both the in vivo cell and in silico models. Physiological conditions differ significantly between mammalian species. However, most existing formulations of NKA used to simulate cardiac function in computational models are derived from a broad range of experimental sources spanning many animal species. The resultant inability of these models to discern species-specific features is a significant obstacle to achieving a detailed quantitative and comparative understanding of physiological behaviour in different biological contexts. Here we present a framework for characterising the steady-state NKA current using a biophysical mechanistic model specifically designed to provide a mechanistic explanation of the NKA flux supported by self-consistent species-specific data. We thus compared NKA kinetics specific to guinea- pig and rat ventricular myocytes. We observe that the apparent binding affinity for sodium in the rat is significantly lower, whereas the overall pump cycle rate is doubled, in comparison to the guinea pig. This sensitivity of NKA to its regulatory substrates compensates for the differences in Na(+) concentrations between the cell types. NKA is thereby maintained within its dynamic range over a wide range of pacing frequencies in these two species, despite significant disparities in sodium concentration. Hence, by replacing a conventional generic NKA model with our rat-specific NKA formula into a whole-cell simulation, we have, for the first time, been able to accurately reproduce the action potential duration and the steady-state sodium concentration as functions of pacing frequency.

Acute effects of nonexcitatory electrical stimulation during systole in isolated cardiac myocytes and perfused heart

Application of electrical field to the heart during the refractory period of the beat has been shown to increase the force of contraction both in animal models and in heart failure patients (cardiac contractility modulation, or CCM). A direct increase in intracellular calcium during CCM has been suggested to be the mechanism behind the positive inotropic effect of CCM. We studied the effect of CCM on isolated rabbit cardiomyocytes and perfused whole rat hearts. The effect of CCM was observed in single cells via fluorescent measurements of intracellular calcium concentration ([Ca²⁺]_i) and cell length (L). Cells were paced once per second throughout these recordings, and CCM stimulation was delivered via biphasic electric fields of 20 ms duration applied during the refractory period. CCM increased the peak amplitude of both [Ca²⁺]_i and L for the first beat during CCM compared to control, but then [Ca²⁺]_i and L decayed to levels lower than the control. During CCM, all contractions had a faster time to peak for both [Ca²⁺]_i and L; after stopping CCM the rise times returned to control levels. In the whole rat heart, the positive inotropic effect of CCM stimulation on left ventricular pressure was completely abolished in the presence of metoprolol, a beta‐1 adrenergic blocker. In summary, the CCM‐induced changes in intracellular calcium handling by cardiomyocytes did not explain the sustained positive inotropic effect in the whole heart and the β‐adrenergic pathway may be involved in the CCM mechanism of action.

Cardiac contractility modulation (CCM) is a heart failure therapy which delivers electrical pulses to the heart during refractory period. While there are some promising reports on the therapy's safety and effectiveness in humans, the underlining mechanism remains unknown. We studied the effect of CCM pulses in isolated rabbit cardiomyocytes and isolated rat heart in the presence of beta adrenergic blocker and recorded intracellular calcium transients and contractions. We concluded that the CCM‐induced changes in intracellular calcium handling by cardiomyocytes did not explain the sustained positive iotropic efect in the whole heart and beta‐adrenergic pathway may be involved in the CCM mechanism of action.

Noninvasive evaluation of left ventricular force-frequency relationships by measuring carotid arterial wave intensity during exercise stress

Background and purpose

Estimation of the contractility of the left ventricle during exercise is important in drawing up a protocol of cardiac rehabilitation. It has been demonstrated that color Doppler- and echo tracking-derived carotid arterial wave intensity is a sensitive index of global left ventricular (LV) contractility. We assessed the feasibility of measuring carotid arterial wave intensity and determining force−frequency (contractility−heart rate) relations (FFRs) during exercise totally noninvasively.

Methods

We measured carotid arterial wave intensity with a combined color Doppler and echo tracking system in 25 healthy young male volunteers (age 20.8 ± 1.2 years) at rest and during exercise. FFRs were constructed by plotting the maximum value of wave intensity (WD₁) against heart rate (HR).

Results

We first confirmed that HR increased linearly with an increase in work load in each subject (r² = 0.95 ± 0.04). WD₁ increased linearly with an increase in HR. The goodness-of-fit of the regression line of WD₁ on HR in each subject was very high (r² = 0.48−0.94, p < 0.0001, respectively). The slope of the WD₁-HR relation ranged 0.30−2.20 [m/s³ (beat/min)].

Conclusions

Global LV FFRs can be generated in healthy young volunteers with an entirely noninvasive combination of exercise and wave intensity. These data should show the potential usefulness of the FFR in the context of cardiac rehabilitation.

Population of computational rabbit-specific ventricular action potential models for investigating sources of variability in cellular repolarisation

Variability is observed at all levels of cardiac electrophysiology. Yet, the underlying causes and importance of this variability are generally unknown, and difficult to investigate with current experimental techniques. The aim of the present study was to generate populations of computational ventricular action potential models that reproduce experimentally observed intercellular variability of repolarisation (represented by action potential duration) and to identify its potential causes. A systematic exploration of the effects of simultaneously varying the magnitude of six transmembrane current conductances (transient outward, rapid and slow delayed rectifier K⁺, inward rectifying K⁺, L-type Ca²⁺, and Na⁺/K⁺ pump currents) in two rabbit-specific ventricular action potential models (Shannon et al. and Mahajan et al.) at multiple cycle lengths (400, 600, 1,000 ms) was performed. This was accomplished with distributed computing software specialised for multi-dimensional parameter sweeps and grid execution. An initial population of 15,625 parameter sets was generated for both models at each cycle length. Action potential durations of these populations were compared to experimentally derived ranges for rabbit ventricular myocytes. 1,352 parameter sets for the Shannon model and 779 parameter sets for the Mahajan model yielded action potential duration within the experimental range, demonstrating that a wide array of ionic conductance values can be used to simulate a physiological rabbit ventricular action potential. Furthermore, by using clutter-based dimension reordering, a technique that allows visualisation of multi-dimensional spaces in two dimensions, the interaction of current conductances and their relative importance to the ventricular action potential at different cycle lengths were revealed. Overall, this work represents an important step towards a better understanding of the role that variability in current conductances may play in experimentally observed intercellular variability of rabbit ventricular action potential repolarisation.

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.

Systematic characterization of the ionic basis of rabbit cellular electrophysiology using two ventricular models

Several mathematical models of rabbit ventricular action potential (AP) have been proposed to investigate mechanisms of arrhythmias and excitation-contraction coupling. Our study aims at systematically characterizing how ionic current properties modulate the main cellular biomarkers of arrhythmic risk using two widely-used rabbit ventricular models, and comparing simulation results using the two models with experimental data available for rabbit. A sensitivity analysis of AP properties, Ca²⁺ and Na⁺ dynamics, and their rate dependence to variations (±15% and ±30%) in the main transmembrane current conductances and kinetics was performed using the Shannon et al. (2004) and the Mahajan et al. (2008a,b) AP rabbit models. The effects of severe transmembrane current blocks (up to 100%) on steady-state AP and calcium transients, and AP duration (APD) restitution curves were also simulated using both models. Our simulations show that, in both virtual rabbit cardiomyocytes, APD is significantly modified by most repolarization currents, AP triangulation is regulated mostly by the inward rectifier K⁺ current (I(K1)) whereas APD rate adaptation as well as [Na⁺](i) rate dependence is influenced by the Na⁺/K⁺ pump current (I(NaK)). In addition, steady-state [Ca²⁺](i) levels, APD restitution properties and [Ca²⁺](i) rate dependence are strongly dependent on I(NaK), the L-Type Ca²⁺ current (I(CaL)) and the Na⁺/Ca²⁺ exchanger current (I(NaCa)), although the relative role of these currents is markedly model dependent. Furthermore, our results show that simulations using both models agree with many experimentally-reported electrophysiological characteristics. However, our study shows that the Shannon et al. model mimics rabbit electrophysiology more accurately at normal pacing rates, whereas Mahajan et al. model behaves more appropriately at faster rates. Our results reinforce the usefulness of sensitivity analysis for further understanding of cellular electrophysiology and validation of cardiac AP models.

Human and rabbit inter-species comparison of ionic mechanisms of arrhythmic risk: A simulation study

Experimental studies of pro-arrhythmic mechanisms are scarcely performed in humans due to the limited availability of human cardiomyocytes. Subsequently, extrapolation of animal experimental research to humans is widely extended. Our aim is to systematically compare the ionic mechanisms of the main cellular biomarkers of arrhythmic risk between human and rabbit using computer simulations. For this purpose four stimulation protocols were applied to the Mahajan et al. rabbit ventricular action potential (AP) model for control conditions and for ± 15 and ± 30% variations in the ionic current conductances of the main repolarization currents to quantify cellular biomarkers. Sensitivity of every simulated biomarker to every parameter modification was compared to that obtained for human in our previous work. Our results show that the ionic mechanisms involved in AP triangulation, systolic intracellular calcium concentration and AP duration (APD) accommodation to abrupt changes of pacing rate are very similar in both species. Unfortunately, significant differences were found in the ionic mechanisms related to APD, restitution properties and rate dependence of intracellular calcium and sodium concentrations. In conclusion, extrapolation of experimental research in rabbit to humans is limited by the existence of species dependent ionic mechanisms. In addition, this analysis is very useful for understanding and improvement of mathematical models.

Pinacidil-primed ATP-sensitive potassium channels mediate feedback control of mechanical power output in isolated myocardium of rats and guinea pigs

We tested the hypothesis, that ATP-sensitive potassium (K(ATP)) channels limit cardiac energy demand by a feedback control of mean power output at increased cardiac rates. We analysed the interrelationships between rising energy demand of adult rat and guinea pig left ventricular papillary muscle and down-regulatory electromechanical effects mediated by K(ATP) channels. Using the K(ATP)-opener pinacidil the stimulation frequency was increased stepwise and the mechanical parameters and action potentials were recorded. Power output was derived from force-length area or force-time integral calculations, respectively. Simultaneously oxygen availability in the preparations was estimated by flavoprotein fluorescence measurements. ADP/ATP ratios were determined by HPLC. We found highly linear relationships between isotonic power output and the effects of pinacidil on isotonic shortening in both rat (r(2)=0.993) and guinea pig muscles (r(2)=0.997). These effects were solely observed for the descending limb of shortening-frequency relationships. In addition, a highly linear correlation between total force-time integral-derived power and pinacidil effects on action potential duration (APD(50), r(2)=0.92) was revealed. Power output became constant and frequency-independent in the presence of pinacidil at higher frequencies. In contrast, the K(ATP)-blocker glibenclamide produced a lengthening of APD(50) and increased force transiently at higher power levels. Pinacidil prevented core hypoxia and a change in ADP/ATP ratio during high frequency stimulation. We conclude, that pinacidil-primed cardiac K(ATP) channels homeostatically control power output during periods of high energy demand. This effect is associated with a reduced development of hypoxic areas inside the heart muscle by adapting cardiac function to a limited energy supply.

A modified rabbit model of reperfused myocardial infarction for cardiac MR imaging research

We sought to obtain a rabbit myocardial infarction (MI) model for research with cardiac magnetic resonance imaging (cMRI) by overcoming a few technical difficulties. A novel endotracheal method was developed for intubation and ventilation. Fourteen rabbits were divided into group-1 (n = 8) with open-chest occlusion of left circumflex coronary artery and closed-chest reperfusion, and group-2 (n = 6) of non-ischemic control; and received ECG-triggered cMRI with delayed contrast enhancement (DE-cMRI) at a 1.5 T clinical scanner. The MI areas in group-1 were morphometrically compared between DE-cMRI and histochemically stained specimens. Left ventricular (LV) functions were compared between two groups.The success rate of intubation and reperfused MI was 8/8 and 6/8, respectively. Global and regional LV functions significantly decreased in group-1 as evidenced by significant hypokinesis of lateral LV-wall and wall thickening (P \ 0.001). Mean MI-area was 19.41 +/- 21.92% on DE-cMRI and 19.10 +/- 22.61% with histochemical staining (r = 0.985). Global MI-volume was 17.92 +/- 7.42% on DE-cMRI and 16.62 +/- 7.16% with histochemistry (r = 0.994). The usefulness of this model was successfully tested for assessing a new contrast agent. The present rabbit MI model may offer a practical platform for more translational research using clinical MRI-facilities.

Na(+)/Ca(2+) exchanger inhibition exerts a positive inotropic effect in the rat heart, but fails to influence the contractility of the rabbit heart

BACKGROUND AND PURPOSE

The Na(+)/Ca(2+) exchanger (NCX) may play a key role in myocardial contractility. The operation of the NCX is affected by the action potential (AP) configuration and the intracellular Na(+) concentration. This study examined the effect of selective NCX inhibition by 0.1, 0.3 and 1.0 microM SEA0400 on the myocardial contractility in the setting of different AP configurations and different intracellular Na(+) concentrations in rabbit and rat hearts.

EXPERIMENTAL APPROACH

The concentration-dependent effects of SEA0400 on I(Na/Ca) were studied in rat and rabbit ventricular cardiomyocytes using a patch clamp technique. Starling curves were constructed for isolated, Langendorff-perfused rat and rabbit hearts. The cardiac sarcolemmal NCX protein densities of both species were compared by immunohistochemistry.

KEY RESULTS

SEA0400 inhibited I(Na/Ca) with similar efficacy in the two species; there was no difference between the inhibitions of the forward or reverse mode of the NCX in either species. SEA0400 increased the systolic and the developed pressure in the rat heart in a concentration-dependent manner, for example, 1.0 microM SEA0400 increased the maximum systolic pressures by 12% relative to the control, whereas it failed to alter the contractility in the rabbit heart. No interspecies difference was found in the cardiac sarcolemmal NCX protein densities.

CONCLUSIONS AND IMPLICATIONS

NCX inhibition exerted a positive inotropic effect in the rat heart, but it did not influence the contractility of the rabbit heart. This implies that the AP configuration and the intracellular Na(+) concentration may play an important role in the contractility response to NCX inhibition.

A computational model integrating electrophysiology, contraction, and mitochondrial bioenergetics in the ventricular myocyte

An intricate network of reactions is involved in matching energy supply with demand in the heart. This complexity arises because energy production both modulates and is modulated by the electrophysiological and contractile activity of the cardiac myocyte. Here, we present an integrated mathematical model of the cardiac cell that links excitation-contraction coupling with mitochondrial energy generation. The dynamics of the model are described by a system of 50 ordinary differential equations. The formulation explicitly incorporates cytoplasmic ATP-consuming processes associated with force generation and ion transport, as well as the creatine kinase reaction. Changes in the electrical and contractile activity of the myocyte are coupled to mitochondrial energetics through the ATP, Ca2+, and Na+ concentrations in the myoplasmic and mitochondrial matrix compartments. The pseudo steady-state relationship between force and oxygen consumption at various stimulus frequencies and external Ca2+ concentrations is reproduced in both model simulations and direct experiments in cardiac trabeculae under normoxic conditions, recapitulating the linearity between cardiac work and respiration in the heart. Importantly, the model can also reproduce the rapid time-dependent changes in mitochondrial NADH and Ca2+ in response to abrupt changes in workload. The steady-state and dynamic responses of the model were conferred by ADP-dependent stimulation of mitochondrial oxidative phosphorylation and Ca2+ -dependent regulation of Krebs cycle dehydrogenases, illustrating how the model can be used as a tool for investigating mechanisms underlying metabolic control in the heart.

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.

Electrophysiologic characterization of dronedarone in guinea pig ventricular cells

The electrophysiological properties of dronedarone (SR33589), a noniodinated amiodarone-like agent, were studied on action potential (AP) and contraction of papillary muscle and on membrane ionic currents, Ca2+ transient, and shortening of ventricular cells of the guinea pig heart. In multicellular preparations, dronedarone (3, 10, and 30 microM) decreased maximum rate of rise of AP (dV/dt max) with a concentration- and frequency-dependent relationship; resting potential was not modified and AP amplitude was decreased only at 30 microM. The effects of dronedarone on AP durations (APDs) at different percentages of repolarization were not significantly changed, except for a slight decrease in APD30 and APD50 at the highest concentration. In isolated ventricular myocytes, dronedarone inhibited rapidly activating delayed-rectifier K+ current (I(Kr)) (median inhibitory concentration [IC50] </= 3 microM voltage-independent); slowly activating delayed-rectifier K+ current (I(Ks)) (IC50 approximately/= 10 microM voltage-dependent and time-, frequency-, or use-independent); and inward rectifier potassium current (I(K1)) (IC50 >/= 30 microM). Dronedarone blocked L-type Ca2+ current (I(Ca(L))) (IC50 = 0.18 +/- 0.018 microM at a stimulation frequency of 0.033 Hz) in a use- and frequency-dependent manner. Simultaneously to these electrophysiological effects, dronedarone reduced contraction amplitudes of papillary muscle and decreased Ca2+ transient and shortening of ventricular myocytes. The results show that dronedarone is a multichannel blocker because it decreases dV/dt max (I(Na)), I(Ca(L)), I(Kr), I(Ks), and I(K1). These effects are accompanied by a reduction in free intracellular calcium and contraction amplitudes. Dronedarone does not significantly change APD whatever the stimulation frequency. Our data demonstrate that the acute electrophysiological characteristics of dronedarone, despite absence of iodine in its molecular structure, are very similar to those of amiodarone in cardiac ventricle.

Adrenergic-mediated effects of cocaine on the myocardial force-frequency relationship

We studied the role of sarcolemmal alpha- and beta-adrenoceptors activation in the effects of cocaine on the positive force staircase in isolated guinea pig atria. The preparations were superfused with Tyrode's solution at 31 degrees C while attached to a force transducer to measure peak tension developed (PTD), maximum velocity of development of tension (Vmax T) and time to peak tension (TPT). The positive force staircase was not affected by propranolol or phentolamine, but it was abolished by nifedipine. Cocaine 1 mg/l (2.9 microM) enhanced PTD and Vmax T, while TPT remained unchanged. On the other hand, cocaine did not modify the increase in PTD induced by the increase in frequency of stimulation, but significantly reduced the magnitude of the increase in Vmax T. The cocaine-induced attenuation of the increase in Vmax T in response to changes in the frequency of stimulation was abolished by both propranolol and phentolamine. It is concluded that the effect of cocaine on the force-frequency relationship required background activation of alpha- and beta-adrenergic receptors.

Age and gender differences in excitation-contraction coupling of the rat ventricle

1. The objective of this study was to determine potential post-pubertal gender-specific differences in the contractility of papillary muscles, the electrophysiological properties and Ca2+ transients of freshly dissociated ventricular myocytes from the rat heart. 2. The contractions of rat papillary muscles from 2- to 14-month-old male and female rats were studied under isometric and isotonic conditions (29 degrees C). While the hearts of young (2-4 months) male and female rats displayed a similar contractile profile, papillary muscles of female rats aged 6 months and older exhibited smaller isometric and isotonic contractions, smaller maximal rates of tension and shortening development and decline (+/-DT/dt and +/-DL/dt) velocities during both the onset and relaxation phases, and shorter contractions than age-matched males. 3. To explore the possible cellular basis accounting for these differences, action potentials and macroscopic currents were recorded from freshly dissociated myocytes using the whole-cell patch clamp technique (35 C). Action potentials from male and female myocytes of 3- and 9-month-old rats did not vary as a function of age or gender. Consistent with these results, the magnitude (expressed in pA pF-1), voltage-dependence and kinetics of the inward rectifier (IK1), transient outward (Ito) and sustained (IK) K+ currents displayed little, if any dependence on age or gender. 4. L-type Ca2+ current (ICa(L)) measured in caesium-loaded myocytes (35 C) from male and female rats of 3, 6 and 9 months of age exhibited similar characteristics. In contrast, while Ca2+ transients measured with indo-1 were similar between 3-month-old male and female rat myocytes, Ca2+ transients of 10-month-old female myocytes were significantly reduced and showed a diminished rate of relaxation in comparison with those recorded in male rats of similar age. 5. These results suggest that important gender-related changes in excitation-contraction coupling occur following puberty, probably due to differences in Ca2+ handling capabilities at the level of the sarcoplasmic reticulum.

Isolated myocyte contractile function is normal in postinfarct remodeled rat heart with systolic dysfunction

BACKGROUND

Postinfarction ventricular remodeling is associated with lengthening and contractile dysfunction of the remote noninfarcted myocardium. Mechanisms underlying this phenomenon remain unclear.

METHODS AND RESULTS

We studied serial changes in global left ventricular (LV) structure and function in infarcted (1, 2, 4, and 6 weeks after myocardial infarction) and sham-operated rat hearts and correlated them with structural and functional changes in myocytes isolated from the remote LV myocardium in the same hearts. Rats with myocardial infarction developed significant remodeling. The heart weight-to-body weight ratios were increased. LV volumes at filling pressure of 10 mm Hg were higher (305+/-28 versus 215+/-12 microL, P<.01). This was accompanied by global LV dysfunction (in vivo LV end-diastolic pressure, 4+/-1 versus 23+/-1.6 mm Hg; Langendorff LV developed pressure, 105+/-4 versus 62+/-9 mm Hg, P<.001 for both). Myocytes isolated from these hearts showed significant structural remodeling (LV myocytes, 24% longer and 15% wider; right ventricular myocytes, 38% longer and 31% wider, all P<.05). LV myocyte length correlated with changes in LV volume (r=.79) and function (LV developed pressure, r=-.81). However, LV myocytes from the same hearts showed normal contractile function and intracellular Ca2+ transients at baseline and during inotropic stimulation with increasing extracellular Ca2+ (1 to 6 mmol/L). The shortening-frequency relationship was also similar in myocytes from sham and myocardial infarction rats.

CONCLUSIONS

Postinfarct LV remodeling occurs predominantly by myocyte lengthening rather than by myocyte slippage. However, contractile function of the unloaded myocytes from the remote noninfarcted LV myocardium of the remodeled heart is normal. Therefore, myocyte contractile abnormalities may not contribute to global dysfunction of the remodeled heart. Reduced myocyte mass and nonmyocyte factors like increased wall stress, altered LV geometry, and changes in the myocardial interstitium may be more important in the genesis of postinfarct LV dysfunction in this model.