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

A random cycle length approach for assessment of myocardial contraction in isolated rabbit myocardium

It is well known that the strength of cardiac contraction is dependent on the cycle length, evidenced by the force-frequency relationship (FFR) and the existence of postrest potentiation (PRP). Because the contractile strength of the steady-state FFR and force-interval relationship involve instant intrinsic responses to cycle length as well as slower acting components such as posttranslational modification-based mechanisms, it remains unclear how cycle length intrinsically affects cardiac contraction and relaxation. To dissect the impact of cycle length changes from slower acting signaling components associated with persisting changes in cycle length, we developed a novel technique/protocol to study cycle length-dependent effects on cardiac function; twitch contractions of right ventricular rabbit trabeculae at different cycle lengths were randomized around a steady-state frequency. Patterns of cycle lengths that resulted in changes in force and/or relaxation times can now be identified and analyzed. Using this novel protocol, taking under 10 min to complete, we found that the duration of the cycle length before a twitch contraction ("primary" cycle length) positively correlated with force. In sharp contrast, the cycle length one ("secondary") or two ("tertiary") beats before the analyzed twitch correlated negatively with force. Using this protocol, we can quantify the intrinsic effect of cycle length on contractile strength while avoiding rundown and lengthiness that are often complications of FFR and PRP assessments. The data show that the history of up to three cycle lengths before a contraction influences myocardial contractility and that primary cycle length affects cardiac twitch dynamics in the opposite direction from secondary/tertiary cycle lengths.

Effects of therapy using the Celacade system on structural and functional cardiac remodelling in rats following myocardial infarction

BACKGROUND

Immune modulation by the Celacade system (Vasogen Inc, Canada) decreases mortality and hospitalization in human heart failure.

OBJECTIVES

To study the effects of Celacade in rats on acute cytokine expression after coronary artery ligation, cardiac dimensions following myocardial infarction (MI), and systolic and diastolic function of cardiac muscle in MI.

METHODS

Celacade treatment was administered 14 days before coronary artery ligation and monthly after the surgery. Cytokine expression in cardiac tissue was measured on days 1 and 7 by ELISA in sham rats and in rats with MI (with or without Celacade treatment). Echocardiograms were obtained serially for 16 weeks. Force and sarcomere length (SL) were measured by strain gauge and laser diffraction in isolated right ventricle trabeculas at 16 weeks. The inotropic effect of pacing on force was quantified as F5 Hz/0.5 Hz. Diastolic dysfunction was quantified as the root mean square of spontaneous SL fluctuations.

RESULTS

Celacade inhibited transforming growth factor beta-1 production in the infarct area on day 7 (191.6+/-22.6 pg/mg versus 275.4+/-30.1 pg/mg; P<0.05), but did not attenuate cardiac dilation in MI. Celacade restored positive inotropism of pacing in MI (F5 Hz/0.5 Hz in Celacade, 219.1+/-46.7%; MI, 148.1+/-27.1% [P<0.05 compared with 211.4+/-37.9% in sham]). Celacade reduced diastolic dysfunction in MI (root mean square of spontaneous SL fluctuations: 121+/-15% and 143+/-19% with Celacade versus 184+/-19% and 190+/-26% without Celacade at 26 degrees C and 36 degrees C, respectively) compared with sham (100%; P<0.05).

CONCLUSIONS

Celacade reduces the increase of transforming growth factor beta-1 expression during the acute stage of MI in rats, but does not prevent chronic cardiac dilation. Celacade restores the positive inotropic effect of increased pacing rate in trabeculas from rat right ventricles with large MIs and reduces diastolic dysfunction.

Force frequency relationship of the human ventricle increases during early postnatal development

Understanding developmental changes in contractility is critical to improving therapies for young cardiac patients. Isometric developed force was measured in human ventricular muscle strips from two age groups: newborns (<2 wk) and infants (3-14 mo) undergoing repair for congenital heart defects. Muscle strips were paced at several cycle lengths (CLs) to determine the force frequency response (FFR). Changes in Na/Ca exchanger (NCX), sarcoplasmic reticulum Ca-ATPase (SERCA), and phospholamban (PLB) were characterized. At CL 2000 ms, developed force was similar in the two groups. Decreasing CL increased developed force in the infant group to 131 +/- 8% (CL 1000 ms) and 157 +/- 18% (CL 500 ms) demonstrating a positive FFR. The FFR in the newborn group was flat. NCX mRNA and protein levels were significantly larger in the newborn than infant group whereas SERCA levels were unchanged. PLB mRNA levels and PLB/SERCA ratio increased with age. Immunostaining for NCX in isolated newborn cells showed peripheral staining. In infant cells, NCX was also found in T-tubules. SERCA staining was regular and striated in both groups. This study shows for the first time that the newborn human ventricle has a flat FFR, which increases with age and may be caused by developmental changes in calcium handling.

Cardiac force-frequency relationship and frequency-dependent acceleration of relaxation are impaired in LPS-treated rats

Introduction

Frequency-dependent acceleration of relaxation (FDAR) ensures appropriate ventricular filling at high heart rates and results from accelerated sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) activity independent of calcium removal from the cell. Because lipopolysaccharide (LPS) challenge may induce aberrations in calcium trafficking and protein phosphorylation, we tested whether LPS would abolish FDAR in rats.

Methods

Following LPS injection, changes in force-frequency relationship and FDAR were studied in cardiomyocytes, isolated hearts and in vivo by echocardiography. Calcium uptake and phosphatase activities were studied in sarcoplasmic reticulum (SR) vesicle preparations. Western blots of phospholamban and calcium/calmodulin-dependent protein kinase II, and serine/threonine phosphatase activity were studied in heart preparations.

Results

In cardiomyocytes and isolated heart preparations, reductions in time constant of relaxation (τ) and time to 50% relaxation at increasing rate of pacing were blunted in LPS-treated rats compared with controls. Early diastolic velocity of the mitral annulus (Ea), a relaxation parameter which correlates in vivo with τ, was reduced in LPS rats compared with control rats. LPS impaired SR calcium uptake, reduced phospholamban phosphorylation and increased serine/threonine protein phosphatase activity. In vivo inhibition of phosphatase activity partially restored FDAR, reduced phosphatase activity and prevented phospholamban dephosphorylation in LPS rat hearts.

Conclusions

LPS impaired phospholamban phosphorylation, cardiac force-frequency relationship and FDAR. Disruption of frequency-dependent acceleration of LV relaxation, which normally participates in optimal heart cavity filling, may be detrimental in sepsis, which is typically associated with elevated heart rates and preload dependency.

Contribution of frequency-augmented inward Ca2+ current to myocardial contractility

The sarcoplasmic reticular Ca2+ pump (SERCA) is thought to be the primary determinant of heart rate-dependent increases in myocardial contractile [Ca2+]i and force (force–frequency relationship (FFR)), an important mechanism to increase cardiac output. This report demonstrates a rate-dependent role for inward Ca2+ current (ICa) in the human and rat FFR. Human action potential plateau height increased linearly with contractility when heart rate increased in vivo, as measured by monophasic action potential catheter and echocardiography. Rat rate-dependent developed force and cytosolic [Ca2+]i transients were quantified in isolated left ventricular papillary muscles, and ICa and action potential duration in cardiomyocytes. ICa and SERCA measurements better reflected [Ca2+]i and force transients than SERCA activity alone. These data support a direct and (or) indirect contribution to myocardial contractility by ICa at heart rates from approximately 1 to 3–4 Hz (60 to 180–240 bpm) in tandem with SERCA to sustain the typical ‘bell shape’ of the FFR across species.

Cardiac Ca2+ signaling and Ca2+ sensitizers

The role of Ca2+ in cardiac excitation-contraction (E-C) coupling has been established by simultaneous measurements of contractility and Ca2+ transients by means of aequorin in intact myocardium and Ca2+ sensitive fluorescent dyes in single myocytes. The E-C coupling process can be classified into 3 processes: upstream (Ca2+ mobilization), central (Ca2+ binding to troponin C) and downstream mechanism (thin filament regulation and crossbridge cycling). These mechanisms are regulated differentially by various inotropic interventions. Positive force-frequency relationship and effects of beta-adrenoceptor stimulation, phosphodiesterase 3 inhibitors and digitalis are essentially exerted via upstream mechanism. Alpha-adrenoceptor stimulation, endothelin-1, angiotensin II, and clinically available Ca2+ sensitizers, such as levosimendan and pimobendan, act by a combination of the upstream and central/downstream mechanism. The Frank-Starling mechanism and effects of Ca2+ sensitizers such as EMD 57033 and Org 30029 are primarily induced via the central/downstream mechanism. Whereas the upstream and central mechanisms are markedly suppressed in failing myocytes and under acidotic conditions, Ca2+ sensitizers such as EMD 57033 and Org 30029 can induce cardiotonic effects under such conditions. Ca2+ sensitizers have high therapeutic potential for the treatment of contractile dysfunction in congestive heart failure and ischemic heart diseases, because they have energetic advantages and less risk of Ca2+ overload and can maintain effectiveness under pathological conditions.

A simulation study on the activation of cardiac CaMKII delta-isoform and its regulation by phosphatases

Although the highly conserved Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is known to play an essential role in cardiac myocytes, its involvement in the frequency-dependent acceleration of relaxation is still controversial. To investigate the functional significance of CaMKII autophosphorylation and its regulation by protein phosphatases (PPs) in heart, we developed a new mathematical model for the CaMKIIdelta isoform. Due to better availability of experimental data, the model was first adjusted to the kinetics of the neuronal CaMKIIalpha isoform and then converted to a CaMKIIdelta model by fitting to kinetic data of the delta isoform. Both models satisfactorily reproduced experimental data of the CaMKII-calmodulin interaction, the autophosphorylation rate, and the frequency dependence of activation. The level of autophosphorylated CaMKII cumulatively increased upon starting the Ca(2+) stimulation at 3 Hz in the delta model. Variations in PP concentration remarkably affected the frequency-dependent activation of CaMKIIdelta, suggesting that cellular PP activity plays a key role in adjusting CaMKII activation in heart. The inhibitory effect of PP was stronger for CaMKIIalpha compared to CaMKIIdelta. Simulation results revealed a potential involvement of CaMKIIdelta autophosphorylation in the frequency-dependent acceleration of relaxation at physiological heart rates and PP concentrations.

Differential effects of phospholamban and Ca2+/calmodulin-dependent kinase II on [Ca2+]i transients in cardiac myocytes at physiological stimulation frequencies

In cardiac myocytes, the activity of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is hypothesized to regulate Ca(2+) release from and Ca(2+) uptake into the sarcoplasmic reticulum via the phosphorylation of the ryanodine receptor 2 and phospholamban (PLN), respectively. We tested the role of CaMKII and PLN on the frequency adaptation of cytosolic Ca(2+) concentration ([Ca(2+)](i)) transients in nearly 500 isolated cardiac myocytes from transgenic mice chronically expressing a specific CaMKII inhibitor, interbred into wild-type or PLN null backgrounds under physiologically relevant pacing conditions (frequencies from 0.2 to 10 Hz and at 37 degrees C). When compared with that of mice lacking PLN only, the combined chronic CaMKII inhibition and PLN ablation decreased the maximum Ca(2+) release rate by more than 50% at 10 Hz. Although PLN ablation increased the rate of Ca(2+) uptake at all frequencies, its combination with CaMKII inhibition did not prevent a frequency-dependent reduction of the amplitude and the duration of the [Ca(2+)](i) transient. High stimulation frequencies in the physiological range diminished the effects of PLN ablation on the decay time constant and on the maximum decay rate of the [Ca(2+)](i) transient, indicating that the PLN-mediated feedback on [Ca(2+)](i) removal is limited by high stimulation frequencies. Taken together, our results suggest that in isolated mouse ventricular cardiac myocytes, the combined chronic CaMKII inhibition and PLN ablation slowed Ca(2+) release at physiological frequencies: the frequency-dependent decay of the amplitude and shortening of the [Ca(2+)](i) transient occurs independent of chronic CaMKII inhibition and PLN ablation, and the PLN-mediated regulation of Ca(2+) uptake is diminished at higher stimulation frequencies within the physiological range.

Transformation of adult rat cardiac myocytes in primary culture

We characterized the morphological, electrical and mechanical alterations of cardiomyocytes in long-term cell culture. Morphometric parameters, sarcomere length, T-tubule density, cell capacitance, L-type calcium current (I(Ca,L)), inward rectifier potassium current (I(K1)), cytosolic calcium transients, action potential and contractile parameters of adult rat ventricular myocytes were determined on each day of 5 days in culture. We also analysed the health of the myocytes using an apoptotic/necrotic viability assay. The data show that myocytes undergo profound morphological and functional changes during culture. We observed a progressive reduction in the cell area (from 2502 +/- 70 microm(2) on day 0 to 1432 +/- 50 microm(2) on day 5), T-tubule density, systolic shortening (from 0.11 +/- 0.02 to 0.05 +/- 0.01 microm) and amplitude of calcium transients (from 1.54 +/- 0.19 to 0.67 +/- 0.19) over 5 days of culture. The negative force-frequency relationship, characteristic of rat myocardium, was maintained during the first 2 days but diminished thereafter. Cell capacitance (from 156 +/- 8 to 105 +/- 11 pF) and membrane currents were also reduced (I(Ca,L), from 3.98 +/- 0.39 to 2.12 +/- 0.37 pA pF; and I(K1), from 34.34p +/- 2.31 to 18.00 +/- 5.97 pA pF(-1)). We observed progressive depolarization of the resting membrane potential during culture (from 77.3 +/- 2.5 to 34.2 +/- 5.9 mV) and, consequently, action potential morphology was profoundly altered as well. The results of the viability assays indicate that these alterations could not be attributed to either apoptosis or necrosis but are rather an adaptation to the culture conditions over time.

Mechanisms underlying adaptation of action potential duration by pacing rate in rat myocytes

Heart rate is an essential determinant of cardiac performance. In rat ventricular myocytes, a sudden increase in rate yields to a prolongation of the action potential duration (APD). The mechanism underlying this prolongation is controversial: it has been proposed that the longer APD is due to either: (1) a decrease in K+ currents only or (2) an increase in Ca2+ current only. The aim of this study was to quantitatively investigate the contribution of Ca2+ and K+ currents in the adaptation of APD to pacing rate. Simulation using a mathematical model of ventricular rat cardiac cell model [Pandit, S.V., Clark, R.B., Giles, W.R., Demir, S.S., 2001. A mathematical model of action potential heterogeneity in adult rat left ventricular myocytes. Biophys. J. 81, 3029-3051] predicted a role in the prolongation of APD for K+ currents only. In patch clamp experiments, increasing the pacing rate leads to a significant increase in APD in both control and detubulated myocytes, although it was more marked in control than detubulated myocytes. Supporting the model prediction, we observed that increasing stimulation frequency leads to a decrease in K+ currents in voltage clamped rat ventricular myocytes (square and action potential waveforms), and to a similar extent in both cell types. We have also observed that frequency-dependent facilitation of Ca2+ current occurred in control cells but not in detubulated cells (square and action potential waveforms). From these experiments, we calculated that the relative contribution of Ca2+ and K+ currents to the longer APD following an increase in pacing rate is approximately 65% and approximately 35%, respectively. Therefore, in contrast to the model prediction, Ca2+ current has a significant role in the adaptation of APD to pacing rate. Finally, we have introduced a simplistic modification to the Pandit's model to account for the frequency-dependent facilitation of Ca2+ current.

Positive inotropic effect of coenzyme Q10, omega-3 fatty acids and propionyl-L-carnitine on papillary muscle force-frequency responses of BIO TO-2 cardiomyopathic Syrian hamsters

The inability of heart muscle to generate ventricular pressure to adequately propel blood through the cardiovascular system is a primary defect associated with congestive heart failure (CHF). Force-frequency relationship (FFR) is one of the main cardiac defects associated with congestive heart failure. Thus FFR is a convenient methodological tool for evaluating the severity of muscle contractile dysfunction and the effectiveness of therapeutic agents. Papillary muscle isolated from BIO TO-2 cardiomyopathic Syrian hamsters (CMSHs), show a depressed FFR and represents an animal model of human idiopathic dilated cardiomyopathy. In the present study we investigated the effect of CoQ10, omega-3 fatty acids, propionyl-L-carnitine (PLC) and a combination of these 3 agents (formulation HS12607) on FFR in 8 month old BIO TO-2 CMSHs. Papillary muscles isolated from the anesthetized animals were placed in an incubation bath and attached to an isometric force transducer. A digital computer with an analog/digital interface allowed control of both muscle developed force and electrical stimulus parameters. Force-frequency response was evaluated, at Lmax, with increasing frequencies: 0.06, 0.12, 0.25, 0.5, 1, 2 and 4 Hz. HS12607-treatment produced a positive inotropic effect resulting in a significant enhancement (p < 0.05) of the peak force at the highest frequencies (1-4 Hz). In the range of frequency of 1-4 Hz also CoQ10 and omega-3 significantly (p < 0.05) attenuated the fractional decline in developed force. The significant improvement (p < 0.05) of the timing parameter peak rate of tension rise (+ T') and peak rate of tension fall (-T') indicating a faster rate of muscle contraction and relaxation respectively, found in CoQ10, omega-3 and PLC-treated CMSHs, may be due to the positive effects of these substances on sarcoplasmic reticulum functions. These findings suggest that naturally occurring CoQ10, omega-3 and PLC, particularly when administered together in a coformulation, might be a valid adjuvant to conventional therapy in dilated cardiomyopathy especially when considering that they are natural substances, devoid of side effects.

Divergent biophysical defects caused by mutant sodium channels in dilated cardiomyopathy with arrhythmia

Mutations in SCN5A encoding the principal Na+ channel alpha-subunit expressed in human heart (Na(V)1.5) have recently been linked to an inherited form of dilated cardiomyopathy with atrial and ventricular arrhythmia. We compared the biophysical properties of 2 novel Na(V)1.5 mutations associated with this syndrome (D2/S4--R814W; D4/S3--D1595H) with the wild-type (WT) channel using heterologous expression in cultured tsA201 cells and whole-cell patch-clamp recording. Expression levels were similar among WT and mutant channels, and neither mutation affected persistent sodium current. R814W channels exhibited prominent and novel defects in the kinetics and voltage dependence of activation characterized by slower rise times and a hyperpolarized conductance-voltage relationship resulting in an increased "window current." This mutant also displayed enhanced slow inactivation and greater use-dependent reduction in peak current at fast pulsing frequencies. By contrast, D1595H channels exhibited impaired fast inactivation characterized by slower entry into the inactivated state and a hyperpolarized steady-state inactivation curve. Our findings illustrate the divergent biophysical defects caused by 2 different SCN5A mutations associated with familial dilated cardiomyopathy. Retrospective review of the published clinical data suggested that cardiomyopathy was not common in the family with D1595H, but rather sinus bradycardia was the predominant clinical finding. However, for R814W, we speculate that an increased window current coupled with enhanced slow inactivation and rate-dependent loss of channel availability provided a unique substrate predisposing myocytes to disordered Na+ and Ca2+ homeostasis leading to myocardial dysfunction.

A simple technique for isolating healthy heart cells from mouse models

Single heart cells of mouse models provide powerful tools for heart research. However, their isolation is not easy, and it imposes a significant bottleneck on their use in cellular studies of the heart. Aiming to overcome this problem, this report introduces a novel technique that reproducibly isolates healthy heart cells from mouse models. Using simple devices that ensure easy handling and the rapid aortic cannulation of a small mouse heart, cell isolation was done under physiological conditions without using the "KB" medium or 2,3-butanedione monoxime (BDM). The isolated cells consistently had a healthy appearance and a high viability of 75 +/- 5% (mean +/- SD) in Tyrode solution containing 1.8 mM Ca2+. After 8 h of storage at 37 degrees C, they still had a viability of 45 +/- 12%. The cells showed normal contraction properties when field-stimulated, and they generated normal action potentials and membrane currents under the whole-cell clamp condition. The beta-adrenergic signal transduction of the cells was also normal when it was examined with the isoproterenol enhancement of the L-type Ca2+ current.

Determinants of frequency-dependent contraction and relaxation of mammalian myocardium

An increase in heart rate is the primary mechanism that up-regulates cardiac output during conditions such as exercise and stress. When the heart rate increases, cardiac output increases due to (1) an increased number of beats per time period, and (2) the fact that myocardium generates a higher level of force. In this review, we focus on the underlying mechanisms that are at the basis of frequency-dependent activation of the heart. In addition to increased force development, the kinetics of both cardiac activation and relaxation are faster. This is crucial, as in between successive beats there is less time, and cardiac output can only be maintained if the ventricle can fill adequately. We will discuss the cellular mechanisms that are involved in the regulation of rate-dependent changes in kinetics, with a focus on changes that occur in regulation of the intracellular calcium transient, and the changes in the myofilament responsiveness that occur when the heart rate changes.

High [Na+]i in cardiomyocytes from rainbow trout

Intracellular Na(+)-concentration, [Na(+)](i) modulates excitation-contraction coupling of cardiac myocytes via the Na(+)/Ca(2+) exchanger (NCX). In cardiomyocytes from rainbow trout (Oncorhyncus mykiss), whole cell patch-clamp studies have shown that Ca(2+) influx via reverse-mode NCX contributes significantly to contraction when [Na(+)](i) is 16 mM but not 10 mM. However, physiological [Na(+)](i) has never been measured. We recorded [Na(+)](i) using the fluorescent indicator sodium-binding benzofuran isophthalate in freshly isolated atrial and ventricular myocytes from rainbow trout. We examined [Na(+)](i) at rest and during increases in contraction frequency across three temperatures that span those trout experience in nature (7, 14, and 21 degrees C). Surprisingly, we found that [Na(+)](i) was not different between atrial and ventricular cells. Furthermore, acute temperature changes did not affect [Na(+)](i) in resting cells. Thus, we report a resting in vivo [Na(+)](i) of 13.4 mM for rainbow trout cardiomyocytes. [Na(+)](i) increased from rest with increases in contraction frequency by 3.2, 4.7, and 6.5% at 0.2, 0.5, and 0.8 Hz, respectively. This corresponds to an increase of 0.4, 0.6, and 0.9 mM at 0.2, 0.5, and 0.8 Hz, respectively. Acute temperature change did not significantly affect the contraction-induced increase in [Na(+)](i). Our results provide the first measurement of [Na(+)](i) in rainbow trout cardiomyocytes. This surprisingly high [Na(+)](i) is likely to result in physiologically significant Ca(2+) influx via reverse-mode NCX during excitation-contraction coupling. We calculate that this Ca(2+)-source will decrease with the action potential duration as temperature and contraction frequency increases.

The seasonal peculiarities of force-frequency relationships in active ground squirrel Spermophilus undulatus ventricle

The plasticity of calcium homeostasis is of crucial importance for the unique ability of the hibernators' heart to function under conditions of body temperature changing from 37 degrees C to near freezing point. However, the precise mechanism of calcium homeostasis regulation in these animals is largely unknown. Force-frequency relationship, as an indicator of participation of various sources of calcium (external and intracellular) in the activation of contraction, and post-rest potentiation as an index of the capacity of sarcoplasmic reticulum (intracellular calcium source) to store and release Ca(2+), were studied to analyse the role of different calcium-transporting systems in seasonal and temperature-induced changes in isometric twitch force of ground squirrel papillary muscles. The obtained results revealed significant functional differences during the annual cycle, which are indicative of an increased role of the sarcoplasmic reticulum in regulation of contractility in animals in transition to the hibernation period. Also, how myocardium during the hibernation period copes functionally with acute decreases in temperature was investigated.

Impact of varying pulse frequency and duration on muscle torque production and fatigue

Neuromuscular electrical stimulation (NMES) involves the use of electrical current to facilitate contraction of skeletal muscle. However, little is known concerning the effects of varying stimulation parameters on muscle function in humans. The purpose of this study was to determine the extent to which varying pulse duration and frequency altered torque production and fatigability of human skeletal muscle in vivo. Ten subjects underwent NMES-elicited contractions of varying pulse frequencies and durations as well as fatigue tests using stimulation trains of equal total charge, yet differing parametric settings at a constant voltage. Total charge was a strong predictor of torque production, and pulse trains with equal total charge elicited identical torque output. Despite similar torque output, higher- frequency trains caused greater fatigue. These data demonstrate the ability to predictably control torque output by simultaneously controlling pulse frequency and duration and suggest the need to minimize stimulation frequency to control fatigue.

Cardiac remodeling and dysfunction in nephrotic syndrome

There is an increased incidence of heart disease in patients with chronic nephrotic syndrome (NS), which may be attributable to the malnutrition and activated inflammatory state accompanying the sustained proteinuria. In this study, we evaluated renal function, cardiac morphometry, contractile function, and myocardial gene expression in the established puromycin aminonucleoside nephrosis rat model of NS. Two weeks after aminonucleoside injection, there was massive proteinuria, decreased creatinine clearance, and a negative sodium balance. Skeletal and cardiac muscle atrophy was present and was accompanied by impaired left ventricular (LV) hemodynamic function along with decreased contractile properties of isolated LV muscle strips. The expression of selected cytokines and proteins involved in calcium handling in myocardial tissue was evaluated by real time polymerase chain reaction. This revealed that the expression of interleukin-1beta, tumor necrosis factor-alpha, and phospholamban were elevated, whereas that of cardiac sarco(endo)plasmic reticulum calcium pump protein was decreased. We suggest that protein wasting and systemic inflammatory activation during NS contribute to cardiac remodeling and dysfunction.

The value added by measuring myocardial contractility 'in vivo' in safety pharmacological profiling of drug candidates

INTRODUCTION

The objective of this study was to define the normal LVdP/dt (an index of myocardial contractility)-heart rate relationship in telemetered conscious dogs, primates and mini-pigs in our laboratory and to use these data as the basis for an additional parameter useful in drug safety evaluation.

METHODS

Trained dogs, Rhesus monkeys, Cynomolgus monkeys and mini-pigs (Goettinger) were equipped with radiotelemetry transmitters (ITS). Aortic pressure (AP), left ventricular pressure (LVP), a lead II ECG and body temperature could be continuously monitored. The contractility index LVdP/dtmax was derived from the LVP signal. Notocord HEM 4.1 software was used for data acquisition. For each species an LVdP/dt-heart rate relationship was evaluated using spontaneous heart rates (HR) throughout the observation period. A validation compound with positive inotropic effects (pimobendan) was then used to investigate the LVdP/dt-heart rate relationship.

RESULTS

There was a clear LVdP/dt-HR relationship in the animals tested. The inotropic agent pimobendan demonstrated the expected shift in this relationship.

DISCUSSION

Contractility of the myocardium is regulated by autonomic input activating primarily myocardial beta1-adrenoceptors, but it is also affected by the "force-frequency" relationship. Compounds can therefore either directly or indirectly affect the contractility of the heart. The chronotropic effects are routinely measured in preclinical studies; however, the inotropic effects are not routinely analysed in cardiovascular safety studies. Our experience strongly recommends including this evaluation for drug candidate selection. The evaluation of LVdP/dtmax, as an index of myocardial contractile state must, however, take into account its HR-dependency.

Effects of complete heart block on myocardial function, morphology, and energy metabolism in the rat

AIMS

Severe sustained bradycardia may cause acute and possibly chronic congestive heart failure (CHF). The aim of this study was to investigate acute and chronic effects of complete heart block (CHB) on cardiac function, morphology, and creatine (Cr) metabolism.

METHODS AND RESULTS

CHB was induced in male Sprague-Dawley rats (approximately 250 g, n = 11) by means of electrocautery applied to the region of AV node and were compared with controls (n = 15). The rats were investigated at 1, 3, and 12 weeks after CHB induction with transthoracic echocardiography. Invasive haemodynamic assessment of left and right ventricular pressures was performed at 12 weeks. After the sacrifice, the hearts were freeze-clamped for analysis of myocardial Cr, and high energy phosphometabolites. The efficacy of operative procedure was 54%. The peri-operative mortality rate was 20%. Heart rate (HR) decreased by approximately 50% (P < 0.01) while stroke volume (SV) increased 2.5 times (P < 0.01) in the CHB rats. Cardiac index remained unchanged. The rats with CHB grew normally and were in no apparent distress. Filling pressures in left and right ventricles were normal. The CHB rats developed marked cardiomegaly with biventricular dilatation and eccentric left ventricular hypertrophy (P < 0.01). There was no change in the myocardial content of Cr and high energy phosphometabolites.

CONCLUSION

Rats with CHB are compensating for reduction in HR with increased SV without haemodynamic and biochemical characteristics of CHF. This model may be useful to study the effects of CHB and bradycardia on myocardial structure, function, electrophysiology, and metabolism as well as for studies of cell therapy for reparation of AV conductance.

2-APB Induces Instability in Rat Left Atrial Mechanical Activity

Atrial contractile abnormalities are common clinical disorders but few pharmacological models can reliably produce such abnormalities in isolated atrial muscle. Since sarcoplasmic reticulum (SR) calcium leak may underlie these contractile irregularities, we investigated whether 2-aminoethoxydiphenyl borate (2-APB), a calcium leak-inducer, affects mechanical function in isolated, superfused rat left atria. Exposing left atria paced at 3 Hz to >10 microM 2-APB produced sporadic mechanical events that occurred in the absence of pacing stimulus. Prolonging atrial diastole in the presence of 2-APB produced spontaneous mechanical activity (SMA) defined as numerous mechanical events occurring in the absence of pacing stimulus. SMA depends on atrial sodium and chloride gradients as decreasing superfusate concentration of either ion suppressed SMA. Increasing superfusate potassium to produce an EK of approximately -74mV reversed SMA, revealing possible membrane potential sensitivity. Mechanical function decreased with time in left atria treated with 2-APB and low sodium or the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) compared with atria exposed to low sodium or DIDS alone, suggesting 2-APB may decrease left atrial SR activator calcium. Thus, 2-APB produces instability in regular left atrial mechanical activity that may require forward-mode sodium-calcium exchange and chloride channel activities. This data identify a new model for studying atrial contractile abnormalities.

Direct myocardial effects of levosimendan in humans with left ventricular dysfunction: alteration of force-frequency and relaxation-frequency relationships

BACKGROUND

Enthusiasm for the development of Ca2+ sensitizers as inotropic agents for heart failure has been tempered by reports of impaired relaxation. Levosimendan, which increases myofilament Ca2+ sensitivity via Ca2+-dependent binding to troponin C, exerts positive inotropic and lusitropic effects in failing human myocardium in vitro. We sought to determine the direct effects of levosimendan on failing human myocardium in vivo, and in particular whether levosimendan exerts heart rate-dependent effects on systolic or diastolic function.

METHODS AND RESULTS

Ten patients with left ventricular dysfunction caused by nonischemic dilated cardiomyopathy (mean left ventricular ejection fraction, 27+/-2%) were instrumented with an infusion catheter in the left main coronary artery, a high-fidelity micromanometer-tipped catheter in the left ventricle, and a bipolar pacing wire in the right atrium. Inotropic (peak +dP/dt) and lusitropic (Tau) responses were assessed during continuous intracoronary drug infusion in sinus rhythm followed by atrial pacing at 20, 40, and 60 beats per minute above the sinus rate. Under control conditions (intracoronary 5% dextrose in water), atrial-pacing tachycardia decreased Tau by 13% (P<0.05), but did not increase +dP/dt. Intracoronary levosimendan (3.75 and 12.5 microg/min for 15 minutes each) increased +dP/dt dose-dependently and decreased Tau over a range of heart rates, but did not alter the slope of the force-frequency or relaxation-frequency relationship.

CONCLUSIONS

Myocardial calcium sensitization with levosimendan exerts mild inotropic and lusitropic effects in humans with left ventricular dysfunction, but does not alter the force-frequency or relaxation-frequency relationship.

Novel means to monitor cardiac performance: the impact of the force-frequency and force-interval relationships on recirculation fraction and potentiation ratio

Insights into intracellular calcium regulation and contractile state can be accomplished by changing pacing rate. Steady-state increases in heart rate (HR) (force-frequency relationship, FFR), and introduction of extrasystoles (ES) (force-interval relationship, FIR) have been used to investigate this relationship. This study focused on the recirculation fraction (RF) and potentiation ratio (PR), obtained from the recovery of the FFR and FIR. These parameters may provide insight on intracellular Ca(2+) regulation. Left ventricular (LV) pressures and HR were assessed in anesthetized canines (n = 7). Intrinsic data were collected prior to and following HR increases to 150, 180, and 200 bpm, as well as following delivery of an ES at 280 ms. The RF was calculated as the slope of dP/dt(max(n + 1)) vs. dP/dt(max(n)), where n = beat number. The PR was calculated by normalizing dP/dt(max) from the first beat following the ES (or the last paced beat) to the steady-state dP/dt(max). The RF due to an ES was not significantly different than that from a HR of 200 bpm. The PR from an ES was not significantly different than from a HR of 150 bpm. The impact of an ES delivered at an interval of 280 ms produces a PR similar to that from a HR of 150 bpm; yet, it recovers similarly to the termination of pacing at 200 bpm, eliciting a similar RF value. The method of measuring RF by an ES versus an increased HR may provide a safer and more feasible approach to collecting diagnostic information.

Frequency-dependent acceleration of relaxation involves decreased myofilament calcium sensitivity

The force-frequency relationship is an intrinsic modulator of cardiac contractility and relaxation. Force of contraction increases with frequency, while simultaneously a frequency-dependent acceleration of relaxation occurs. While frequency dependency of calcium handling and sarcoplasmic reticulum calcium load have been well described, it remains unknown whether frequency-dependent changes in myofilament calcium sensitivity occur. We hypothesized that an increase in heart rate that results in acceleration of relaxation is accompanied by a proportional decrease in myofilament calcium sensitivity. To test our hypothesis, ultrathin right ventricular trabeculae were isolated from New Zealand White rabbit hearts and iontophorically loaded with the calcium indicator bis-fura 2. Twitch and intracellular calcium handling parameters were measured and showed a robust increase in twitch force, acceleration of relaxation, and rise in both diastolic and systolic intracellular calcium concentration with increased frequency. Steady-state force-intracellular calcium concentration relationships were measured at frequencies 1, 2, 3, and 4 Hz at 37 degrees C using potassium-induced contractures. EC(50) significantly and gradually increased with frequency, from 475 +/- 64 nM at 1 Hz to 1,004 +/- 142 nM at 4 Hz (P < 0.05) and correlated with the corresponding changes in half relaxation time. No significant changes in maximal active force development or in the myofilament cooperativity coefficient were found. Myofilament protein phosphorylation was assessed using Pro-Q Diamond staining on protein gels of trabeculae frozen at either 1 or 4 Hz, revealing troponin I and myosin light chain-2 phosphorylation associated with the myofilament desensitization. We conclude that myofilament calcium sensitivity is substantially and significantly decreased at higher frequencies, playing a prominent role in frequency-dependent acceleration of relaxation.

Effects of halothane, sevoflurane and desflurane on the force-frequency relation in the dog heart in vivo

PURPOSE

Frequency potentiation is the increase in force of contraction induced by an increased heart rate (HR). This positive staircase phenomenon has been attributed to changes in Ca2+ entry and loading of intracellular Ca2+ stores. Volatile anesthetics interfere with Ca2+ homeostasis of cardiomyocytes. We hypothesized that frequency potentiation is altered by volatile anesthetics and investigated the influence of halothane (H), sevoflurane (S) and desflurane (D) on the positive staircase phenomenon in dogsin vivo.

METHODS

Dogs were chronically instrumented for measurement of left ventricular (LV) pressure and cardiac output. Heart rate was increased by atrial pacing from 120 to 220 beats·min^-1 and the LV maximal rate of pressure increase (dP/ dt_max) was determined as an index of myocardial performance. Measurements were performed in conscious dogs and during anesthesia with 1.0 minimal alveolar concentrations of each of the three inhaled anesthetics.

RESULTS

Increasing HR from 120 to 220 beats·min^-1 increased dP/dt_max from 3394 ± 786 (mean ± SD) to 3798 ± 810 mmHg sec-1 in conscious dogs. All anesthetics reduced dP/dt_max during baseline (at 120 beatss·min^-1: H, 1745 ± 340 mmHgs·sec^-1; S, 1882 ± 418; D, 1928 ± 454, allP < 0.05vs awake) but did not influence the frequency potentiation of dP/dtmax (at 220 beatss·min^-1: H, 1981 ± 587 mmHgs·sec^-1; S, 2187 ± 787; D, 2307 ± 691). The slope of the regression line correlating dP/dt_max and HR was not different between awake and anesetized dogs. Increasing HR did not influence cardiac output in awake or anesthetized dogs.

CONCLUSION

These results indicate that volatile anesthetics do not alter the force-frequency relation in dogs in vivo.

Acute heart failure: inotropic agents and their clinical uses

Inotropic agents are indispensable for the improvement of cardiac contractile dysfunction in acute or decompensated heart failure. Clinically available agents, including sympathomimetic amines (dopamine, dobutamine, noradrenaline) and selective phosphodiesterase-3 inhibitors (amrinone, milrinone, olprinone and enoximone) act via cAMP/protein kinase A (PKA)-mediated facilitation of intracellular Ca2+ mobilisation. Phosphodiesterase-3 inhibitors also have a vasodilatory action, which plays a role in improving haemodynamic parameters in certain patients, and are termed inodilators. The available inotropic agents suffer from risks of Ca2+ overload leading to arrhythmias, myocardial cell injury and ultimately, cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and cellular metabolism. Furthermore, they lose their effectiveness under pathophysiological conditions, such as acidosis, stunned myocardium and heart failure. Pimobendan and levosimendan (that act by a combination of an increase in Ca2+ sensitivity and phosphodiesterase-3 inhibition) appear to be more beneficial among existing agents. Novel Ca2+ sensitisers that are under basic research warrant clinical trials to replace available inotropic agents.

Myocardial dysfunction and neurohumoral activation without remodeling in left ventricle of monocrotaline-induced pulmonary hypertensive rats

In monocrotaline (MCT)-induced pulmonary hypertension (PH), only the right ventricle (RV) endures overload, but both ventricles are exposed to enhanced neuroendocrine stimulation. To assess whether in long-standing PH the left ventricular (LV) myocardium molecular/contractile phenotype can be disturbed, we evaluated myocardial function, histology, and gene expression of autocrine/paracrine systems in rats with severe PH 6 wk after subcutaneous injection of 60 mg/kg MCT. The overloaded RV underwent myocardial hypertrophy ( P < 0.001) and fibrosis ( P = 0.014) as well as increased expression of angiotensin-converting enzyme (ACE) (8-fold; P < 0.001), endothelin-1 (ET-1) (6-fold; P < 0.001), and type B natriuretic peptide (BNP) (15-fold; P < 0.001). Despite the similar upregulation of ET-1 (8-fold; P < 0.001) and overexpression of ACE (4-fold; P < 0.001) without BNP elevation, the nonoverloaded LV myocardium was neither hypertrophic nor fibrotic. LV indexes of contractility ( P < 0.001) and relaxation ( P = 0.03) were abnormal, however, and LV muscle strips from MCT-treated compared with sham rats presented negative ( P = 0.003) force-frequency relationships (FFR). Despite higher ET-1 production, BQ-123 (ETA antagonist) did not alter LV MCT-treated muscle strip contractility distinctly ( P = 0.005) from the negative inotropic effect exerted on shams. Chronic daily therapy with 250 mg/kg bosentan (dual endothelin receptor antagonist) after MCT injection not only attenuated RV hypertrophy and local neuroendocrine activation but also completely reverted FFR of LV muscle strips to positive values. In conclusion, the LV myocardium is altered in advanced MCT-induced PH, undergoing neuroendocrine activation and contractile dysfunction in the absence of hypertrophy or fibrosis. Neuroendocrine mediators, particularly ET-1, may participate in this functional deterioration.

Signal transduction and Ca2+ signaling in intact myocardium

The experimental procedures to simultaneously detect contractile activity and Ca(2+) transients by means of the Ca(2+) sensitive bioluminescent protein aequorin in multicellular preparations, and the fluorescent dye indo-1 in single myocytes, provide powerful tools to differentiate the regulatory mechanisms of intrinsic and external inotropic interventions in intact cardiac muscle. The regulatory process of cardiac excitation-contraction coupling is classified into three categories; upstream (Ca(2+) mobilization), central (Ca(2+) binding to troponin C), and/or downstream (thin filament regulation of troponin C property or crossbridge cycling and crossbridge cycling activity itself) mechanisms. While a marked increase in contractile activity by the Frank-Starling mechanism is associated with only a small alteration in Ca(2+) transients (downstream mechanism), the force-frequency relationship is primarily due to a frequency-dependent increase of Ca(2+) transients (upstream mechanism) in mammalian ventricular myocardium. The characteristics of regulation induced by beta- and alpha-adrenoceptor stimulation are very different between the two mechanisms: the former is associated with a pronounced facilitation of an upstream mechanism, whereas the latter is primarily due to modulation of central and/or downstream mechanisms. alpha-Adrenoceptor-mediated contractile regulation is mimicked by endothelin ET(A)- and angiotensin II AT(1)-receptor stimulation. Acidosis markedly suppresses the regulation induced by Ca(2+) mobilizers, but certain Ca(2+) sensitizers are able to induce the positive inotropic effect with central and/or downstream mechanisms even under pathophysiological conditions.

Brain death leads to abnormal contractile properties of the human donor right ventricle

OBJECTIVES

Experimental and clinical data suggest that brain death predominantly affects the right ventricle. We aimed to investigate right ventricle function after brain death and during clinical transplantation with load-independent methods.

METHODS

Patients with and without brain death were enrolled. A total of 33 consecutive heart donors (5 live, "domino" donors) and 10 patients undergoing coronary surgery (coronary artery bypass graft controls) were studied with pressure-volume loops in the right ventricle. Contractile reserve was measured with dopamine stimulation.

RESULTS

Brain-dead donors had a higher mean cardiac index than coronary artery bypass graft controls (3.3 vs 2.8 L/min/m2), but impaired load-independent indices. Despite increased right ventricle stroke volume, the ejection fraction and slope of the end-systolic pressure-volume relationship were significantly reduced in brain-dead donors compared with controls. Diastolic abnormalities were also manifest as increased end-diastolic volume index and prolonged Tau (P < .05). Dopamine improved cardiac output, but without influencing end-systolic pressure-volume relationship, or Tau, and at the expense of further increased right ventricle end-diastolic volume. Before explantation, a significantly higher diastolic volume was also seen in hearts that developed postoperative dysfunction compared with organs without this complication (114.4 vs 77.2 mL/m2, P = .02).

CONCLUSIONS

Brain death leads to right ventricle dysfunction, which may go undetected with conventional techniques. Right ventricle dilatation could represent an early marker of failure. Refinement of selection criteria to include load-independent indices of performance may be desirable to help expand the donor pool.

ATP splitting by half the cross-bridges can explain the twitch energetics of mouse papillary muscle

The aim of this study was to quantify the fraction of cross-bridges that cycle during a cardiac twitch. Measurements of the energetics of contracting left ventricular mouse papillary muscle were made in vitro (27 degrees C) using the myothermic technique. Enthalpy output was partitioned into force-dependent and force-independent components using 2,3-butanedione monoxime (BDM) to selectively inhibit cross-bridge cycling. For isometric contractions and a contraction frequency of 2 Hz the net enthalpy output was 5.7 +/- 0.8 mJ g(-1) twitch(-1) and initial enthalpy output was 2.3 +/- 0.3 mJ g(-1) twitch(-1) (n = 11). Assuming that low concentrations of BDM did not affect Ca2+ cycling, force-independent enthalpy output was 18.6 +/- 1.9% (n = 7) of the initial enthalpy output. Enthalpy output decreased with increased contraction frequency but was independent of shortening velocity. On the basis of these values, it was calculated that the twitch energetics were consistent with ATP splitting by half the cross-bridges and the pumping of one Ca2+ into the sarcoplasmic reticulum for every three cross-bridge cycles. The simplest interpretation is that half the cross-bridges completed one ATP-splitting cycle in each twitch. The lack of influence of shortening velocity on energy cost supports the idea that the amount of energy to be used is determined early in a twitch and is not greatly influenced by events that occur during the contraction.

Unique excitation–contraction characteristics of mouse myocardium as revealed by SEA0400, a specific inhibitor of Na+–Ca2+ exchanger

The functional role of the sodium-calcium exchanger in mouse ventricular myocardium was evaluated with a newly developed specific inhibitor, SEA0400. Contractile force and action potential configuration were measured in isolated ventricular tissue preparations, and cell shortening and Ca2+ transients were measured in indo-1-loaded isolated ventricular cardiomyocytes. SEA0400 increased the contractile force, cell shortening and Ca2+ transient amplitude, and shortened the late plateau phase of the action potential. alpha-adrenergic stimulation by phenylephrine produced a sustained decrease in contractile force, cell shortening and Ca2+ transient amplitude, which were all inhibited by SEA0400. Increasing the contraction frequency resulted in a decrease in contractile force in the absence of drugs (negative staircase phenomenon). This frequency-dependent decrease was attenuated by SEA0400 and enhanced by phenylephrine. Phenylephrine increased the Ca2+ sensitivity of contractile proteins in isolated ventricular cardiomyocytes, while SEA0400 had no effect. These results provide the first pharmacological evidence in the mouse ventricular myocardium that inward current generated by Ca2+ extrusion through the sodium-calcium exchanger during the Ca2+ transient contributes to the action potential late plateau, that alpha-adrenoceptor-mediated negative inotropy is produced by enhanced Ca2+ extrusion through the sodium-calcium exchanger, and that the negative staircase phenomenon can be explained by increased Ca2+ extrusion through the sodium-calcium exchanger at higher contraction frequencies.