Cardiovascular Research

Cardiovascular Research 1998 37(2):456-466; doi:10.1016/S0008-6363(97)00246-0
© 1998 by European Society of Cardiology

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Copyright © 1998, European Society of Cardiology

Characterization of excitation–contraction coupling in conscious dogs with pacing-induced heart failure

Till Neumann, Ursula Ravens¹ and Gerd Heusch^*

Dept. of Pathophysiology, Center of Internal Medicine, and Dept. of Pharmacology, University of Essen, School of Medicine, Hufelandstr. 55, D-45122 Essen, Germany

^* Corresponding author. Tel.: (+49-201) 723 4480; Fax: (+49-201) 723 4481.

Received 13 May 1997; accepted 15 September 1997

	Abstract

Top Abstract 1 Introduction 2 Methods 3 Results 4 Discussion References

 
Objective: In isolated cardiac preparations of non-failing hearts from different species, including man, there is a positive force–frequency relation which is reversed into a negative relation in preparations from failing hearts. Whether or not such relations between ventricular function and heart rate hold true in the in situ heart is not clear at present. Mechanical restitution and postextrasystolic potentiation might serve as alternative measures of excitation–contraction coupling. Methods: Eleven dogs were instrumented with a left ventricular micromanometer, ultrasonic crystals for the measurement of regional wall thickness, two hydraulic occluders around the descending aorta and the inferior caval vein, and left atrial and ventricular pacing leads with a subcutaneous pacemaker. Left ventricular dP/dt_max, as an isovolumic phase index, and systolic wall thickening, as an ejection phase index, were plotted versus heart rate, and heart rate was increased by left atrial pacing from rest to 200 min^–1 in increments of 25 min^–1. In a subset of dogs, left ventricular filling was controlled and the frequency range expanded by the bradycardic agent UL-FS 49. Measurements were performed in the presence and absence of autonomic blockade (hexamethonium, atropine). Mechanical restitution and postextrasystolic potentiation were determined as normalized dP/dt_max and systolic wall thickening, respectively, of the extra- and postextrasystolic beat versus defined variations of the extrasystolic time interval (250–550 ms). Following control studies, heart failure was induced by rapid left ventricular pacing at 250 min^–1 for 20 days±6 (SD) and measurements repeated. Isolated left ventricular trabeculae from non-failing and failing hearts were studied during stimulation at 0.2–4 Hz. Results: Only with filling control and in the absence of autonomic blockade, was there a slightly positive relation between dP/dt_max and heart rate in the control state. Otherwise, the relation of dP/dt_max to heart rate was flat both in the control state and in heart failure. The relation between systolic wall thickening and heart rate in the control state was negative, unless filling was controlled, and it was flat in heart failure. In contrast, the time constants of mechanical restitution and postextrasystolic potentiation were increased significantly with heart failure from 91±25 (SD) to 164±13 ms and from 107±18 to 156±4 ms, respectively, for dP/dt_max and from 76±22 to 162±10 ms and from 101±17 to 160±17 ms, respectively, for systolic wall thickening. These time constants were, however, insensitive to UL-FS 49 and autonomic blockade. There was a negative force–frequency relation in left ventricular trabeculae from non-failing hearts at higher calcium concentrations, where it was flat in trabeculae from failing hearts. Conclusion: Time constants of mechanical restitution and postextrasystolic potentiation are more sensitive than the steady state relation of ventricular function and heart rate to characterize the impairment of excitation–contraction coupling in heart failure.

KEYWORDS Excitation–contraction coupling; Force–frequency relation; Mechanical restitution; Postextrasystolic potentiation; Heart failure; Dog, conscious

	1 Introduction

Top Abstract 1 Introduction 2 Methods 3 Results 4 Discussion References

 
The relation between contractile force and stimulation frequency is supposed to reflect myocardial excitation–contraction coupling [1]. Indeed, a positive force–frequency relation, as first described by Bowditch in isolated frog hearts [2], has been confirmed in various isolated cardiac preparations of healthy hearts from different species, [3–10], including man [5, 11–14]. Conversely, lack of a positive or even a negative force–frequency relation was obtained in isolated preparations from failing hearts [6, 9, 15], again also in isolated human cardiac muscle preparations [11–14]. It remains unclear, however, to what extent such findings in isolated cardiac preparations can be extrapolated to ventricular function in situ and even to therapeutic recommendations for patients with heart failure [1, 16].

There is only a slightly positive relation between dP/dt_max and heart rate in conscious dogs [17–21]and rabbits [22]. A positive relation between dP/dt_max and heart rate was not confirmed in conscious dogs and pigs under control conditions [19, 23–25], but only during β-adrenergic stimulation [23, 24, 26]or with ryanodine, a blocker of sarcoplasmic calcium release [25]. In conscious dogs and pigs with heart failure, the relation between dP/dt_max and heart rate is flat [21, 23]. In patients without heart failure, dP/dt_max is slightly increased with increased heart rate [27–29]. The only available study in patients with heart failure also revealed a flat relation between dP/dt_max and heart rate [27].

In those studies also looking at ejection phase contractile indices, increased heart rate in the control state induced no increases in contractile function [17, 21, 26], and when associated with a decrease in preload even decreases in contractile function [24, 30–32]. There is only one study so far looking at ejection phase contractile indices during increased heart rate in heart failure, and this study in conscious dogs revealed a flat response of systolic segment shortening at increased heart rate in the control state and in heart failure [21].

Extrapolation of the in vitro data to the in situ setting is of concern for several reasons: (1) A positive force–frequency relation in a number of in vitro studies was found only in a range of stimulation frequencies below the physiological range of heart rate in man [5, 6, 8, 11, 12]. (2) Any change in heart rate will also impact on preload [33], and myocardial function in situ must therefore be assessed using both an isovolumic index such as dP/dt_max and an ejection index such as systolic wall thickening [33]. (3) The relation between contractile function and heart rate in situ does not simply reflect myocardial excitation–contraction coupling, but an interaction of myocardial properties with cardiac sympathetic innervation [34].

The present study therefore determined steady-state relations of both dP/dt_max and systolic wall thickening, respectively, versus heart rate in conscious dogs before and after induction of congestive heart failure by chronic rapid left ventricular pacing [35]. To account for the impact of preload and sympathetic innervation, measurements were taken in the absence and presence of filling control and autonomic blockade. Also, the frequency range under study was expanded by the use of the bradycardic agent UL-FS 49 [22, 36], which has no negative inotropic properties.

As alternative measures of excitation–contraction coupling, mechanical restitution and postextrasystolic potentiation were determined using the relations of dP/dt_max and systolic wall thickening, respectively, to defined variations of the stimulation interval, as previously described for dP/dt_max by Yue et al. [37]and Prabhu and Freeman [38, 39]. The present study therefore is the first to provide a comprehensive analysis of excitation–contraction coupling using both isovolumic and ejection phase indices of myocardial contractile function in the control state and in heart failure, accounting for ventricular filling and autonomic innervation, and comparing steady-state responses to increases in heart rate with instantaneous responses to variations in coupling intervals. Finally to compare directly in vitro to in situ data, steady-state force–frequency relations were obtained from isolated left ventricular trabeculae from non-failing and failing hearts.

	2 Methods

Top Abstract 1 Introduction 2 Methods 3 Results 4 Discussion References

 
The experimental protocols employed in this study were approved by the bioethical committee of the district of Düsseldorf, and they conform with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health.

2.1 Instrumentation
The experiments were performed in a total of 16 adult mongrel dogs (23–36 kg) of either sex. Before instrumentation, all dogs were accustomed to human handling and trained to lay quietly on a laboratory table.

Before surgical instrumentation, the dogs were premedicated with 3.0 mg/kg propofol and 0.02 mg/kg fentanyl intravenously. After tracheal intubation, anesthesia was maintained with 10–20 mg/kg/h propofol and 0.01–0.02 mg/kg/h fentanyl intravenously. The anesthesia was supplemented by ventilation with isoflurane (0.2–0.4%) in an oxygen/nitrous oxide mixture (30%/70%) using a respirator (Spiromat 656, Dräger Werke AG, Lübeck, FRG). During instrumentation, body temperature was measured with an esophageal probe and kept constant between 37°C and 38°C with a heating pad.

The chest was opened in the fifth intercostal space under sterile conditions. The pericardium was opened, and a micromanometer (model P-3.5S, Konigsberg Instruments, Pasadena, USA) was placed in the left ventricle through the apex together with a fluid-filled silicone catheter, used to calibrate the micromanometer in situ. One balloon cuff occluder was placed around the inferior caval vein and another around the descending part of the thoracic aorta. Piezoelectric crystals were implanted into the left ventricular free wall for measurement of regional wall thickness (Sonomicrometer 120, Triton Technologies, San Diego, USA). Pacing leads (model 5071, Medtronic, Düsseldorf, FRG) were sutured to the base of the left atrial appendage and the left ventricular free wall. All wires were exteriorized between the scapulae. The chest was closed in layers and evacuated. The dogs were placed on an antibiotic regimen (penicillin, streptomycin) for six days postoperatively. After instrumentation, the dogs were allowed to recover for seven to ten days before the control experiments. The position of all catheters and crystals was confirmed at autopsy. A pacemaker (model Legend II 8424 and 8426, type VVI, Medtronic, Düsseldorf, FRG) was implanted under local anesthesia (2% lidocaine) in a subcutaneous pouch after completion of the control studies.

2.2 Induction of heart failure
After completion of the control studies, heart failure was induced by rapid left ventricular pacing, as originally reported by Coleman et al. [35]. The pacing rate was set to 250 min^–1, and minimal voltage and pulse duration for capture were determined using a programming unit (model 9710, Medtronic, Düsseldorf, FRG). The presence of heart failure was determined from both clinical signs, such as ascites, pulmonary edema, exercise intolerance, cachexia, and hemodynamic parameters, such as the increases in baseline heart rate and left ventricular end-diastolic pressure and the decreases in left ventricular maximal pressure and dP/dt_max. The mean time to induce heart failure was 20 days±6 (SD), ranging from 14 to 31 days. Five dogs were euthanized prior to the final experiments when regarded too unstable for the studies or when technical problems precluded meaningful measurements.

2.3 Protocols
All experiments were performed in conscious, unsedated dogs lying comfortably on their right side. The responses to steady-state atrial pacing were first examined without any additional intervention. The measurements were repeated after reducing the spontaneous heart rate to less than 75 min^–1 with 1–3 mg/kg UL-FS 49 (Zatebradine, Karl Thomae, Biberach, FRG). UL-FS 49 has no direct inotropic effects in the dog [36]. The responses to steady-state atrial pacing were first determined without autonomic blockade and filling control, then without autonomic blockade but with filling control, finally with autonomic blockade and filling control. In heart failure, the responses to steady-state atrial pacing were determined under autonomic blockade without and with filling control. Due to the stability requirements and respective duration of the various protocols, not every dog completed each protocol. In particular, in dogs with heart failure experiments had to be completed in one or two successive sessions of 1–2 h duration.

Heart rate was controlled by left atrial pacing with a computer-controlled external pacemaker and was increased in increments of 25 min^–1. Systemic hemodynamics and regional myocardial dimensions were measured at each heart rate step in a steady state. For control of filling, end-diastolic wall thickness was used as the controlled variable. End-diastolic wall thickness was kept constant by a combination of controlled inflation of the inferior caval occluder at lower heart rates and left atrial and ventricular volume (500–1500 ml; 0.9% NaCl; 37°C) loading at higher heart rates. Autonomic blockade was achieved by administration of hexamethonium (30 mg/kg, i.v.) and atropine (1.5 mg, i.v.) and verified by lack of changes in heart rate in response to inferior caval vein and aortic occlusion.

2.3.1 Mechanical restitution and postextrasystolic potentiation
Baseline heart rate was held constant at 150 min^–1 (400 ms interval) with left atrial pacing. To determine left ventricular mechanical restitution, the interval between the last steady-state beat and the extrasystolic beat was decreased in decrements of 40 ms (10%) from 600 ms (150% of a regular interval) to 260 ms (65% of a regular interval). The interval between the extrasystolic beat and the postextrasystolic beat was kept constant at 400 ms (100%). Mechanical restitution and postextrasystolic potentiation were characterized by the relationships of left ventricular dP/dt_max (normalized to the last steady-state dP/dt_max and expressed in percent) of the extrasystolic and postextrasystolic beat, respectively, versus the interval between the immediately preceding steady-state beat and the extrasystolic beat (extrasystolic time interval), as previously described [37–39]. Data were fitted to a monoexponential function using the Marquardth–Levenberg algorithm (Sigma Plot, Jandel Scientific, San Rafael, USA); i.e.where dP/dt_max(ES) and dP/dt_max(SS) are defined as dP/dt_max of the extrasystolic (ES) and steady-state (SS) beat, respectively, CR_max as the plateau level, ESI as the extrasystolic time interval, t_0,mrc as the X-axis intercept and tc_mrc as the time constant of the curve,where dP/dt_max(PES) and dP/dt_max(SS) are defined as dP/dt_max of the postextrasystolic (PES) and steady-state (SS) beat, respectively, A as the amplitude, B as the plateau level, ESI as the extrasystolic time interval, t_0,pesp as the X-axis intercept and tc_pesp as the time constant of the curve. In the same way, relations of systolic wall thickening versus extrasystolic time interval were generated.

2.4 Force–frequency relations in isolated, left ventricular trabeculae
Following experimental studies in heart failure, the dogs were euthanized by bolus injection of 10 ml KCl solution (1 M) into the left atrial catheter. In vitro data were obtained from 7 dogs with heart failure. Data were also obtained from 5 additional control dogs anesthetized, thoracotomized, and euthanized in the same fashion on the dogs in heart failure. The heart was excised and placed immediately into ice-cold, pre-oxygenated Tyrode solution. Left ventricular thin trabeculae were excised. The tissue specimens were trimmed to strips of less than 1 mm in diameter, and care was taken to cut them in the direction of fiber orientation. The specimens were fixed to a holding device that also carried two platinum wires insulated, with exception of their tips, for electrical stimulation. Each holder was mounted in a 25 ml organ bath in which circulation of Tyrode solution was maintained by a jet stream of carbogen (95% O₂ and 5% CO₂) that also provided oxygenation of the solution. The composition of the Tyrode solution was (in mM): NaCl 126.7, KCl 5.4, MgCl₂ 1.05, CaCl₂ 2, NaH₂PO₄ 0.42, NaHCO₃ 22.0, glucose 5.0. The temperature was kept at 35±0.5°C. The free end of each muscle strip was tied to an isometric force transducer (Statham Instr., Oxnard, USA) with a silk thread. Contractions were monitored continuously on a chart recorder (Servomed, Hellige, Freiburg, FRG). Stimulation parameters were 2 ms duration, 10% above threshold (4–8 V), 0.5 Hz standard frequency of stimulation, unless otherwise indicated. Step changes in stimulation (SD9, Grass Instruments, Quincy, MA, USA) frequency were controlled by a personal computer using CORDAT II software [40]. All preparations were allowed to stabilize for at least 30 min at an extracellular Ca²⁺ concentration of 2 mM. A force–frequency response was obtained by increasing the stimulation frequency stepwise from 0.2 to 4 Hz (i.e. 0.2, 0.33, 0.5, 0.67, 0.83, 1.0, 1.25, 1.43, 2.0, 3.0, 4.0 Hz). In preliminary experiments, 1 min of regular stimulation was sufficient for a new steady state to develop. After stimulation at 4 Hz or upon development of mechanical alternans, the stimulation frequency was reduced again to 0.5 Hz. Thereafter, the extracellular Ca²⁺ concentration was increased to 4 and 8 mM.

2.5 Data analysis and statistics
Left ventricular pressure, left ventricular dP/dt and regional wall thickness were simultaneously recorded on an eight-channel recorder (model RS800, Gould Inc., Cleveland, USA) and an AT-type computer. Hemodynamic data were digitized and directly stored on hard disk using CORDAT II software [40]. End-diastole was defined as the time point when left ventricular dP/dt started its upstroke after crossing the zero line. Regional end-systole was defined as the point of maximal wall excursion within 20 ms prior to peak negative dP/dt. The measured variables were heart rate, left ventricular maximal (LVP_max) and end-diastolic (LVP_ed) pressure, left ventricular dP/dt_max (LV dP/dt_max), end-diastolic wall thickness (WT_ed), and systolic wall thickening, presented as the percent change from the end-diastolic thickness (WT%). End-diastolic wall thickness was normalized to 10 mm under control conditions [41].

Statistical analysis was performed with Sigma Stat software (Jandel Scientific, San Rafael, USA). For the analysis of responses to steady-state atrial pacing, systemic hemodynamic and regional wall functional data of fifteen sequential beats were averaged and analyzed by a one-way analysis of variance for repeated measures. When a significant overall effect was detected, multiple comparisons were performed and p-values adjusted using Bonferroni's method. Time constants and plateau values of mechanical restitution and postextrasystolic potentiation in the control state and in heart failure were compared by an one-way analysis of variance. Force–frequency relations of left ventricular trabeculae from non-failing and failing hearts were compared by unpaired t-tests. Data are reported as mean values±standard deviation, with exception of data for in vitro experiments which are reported as mean values±S.E.M. Differences are considered statistically significant at a p<0.05.

	3 Results

Top Abstract 1 Introduction 2 Methods 3 Results 4 Discussion References

 
3.1 Baseline hemodynamics
After 3–5 weeks of rapid ventricular pacing, dogs developed heart failure, characterized by clinical signs, such as ascites, pulmonary edema, exercise intolerance, cachexia, and hemodynamic variables, such as increases in baseline heart rate and left ventricular end-diastolic pressure and decreases in left ventricular maximal pressure, dP/dt_max and systolic wall thickening (Table 1).

View this table: [in this window] [in a new window]   Table 1 Baseline systemic hemodynamics and regional dimensions in conscious dogs in the normal state and in heart failure

 
3.2 Responses to steady-state atrial pacing
A typical tracing of left ventricular pressure, left ventricular dP/dt and regional wall thickness from a representative dog at a heart rate of 100, 150 and 200 min^–1 is given in Fig. 1. There was no increase in left ventricular dP/dt_max with increasing heart rate from 100 to 200 min^–1. Ventricular filling decreased, as reflected by the increase in end-diastolic wall thickness, and systolic wall thickening decreased (Fig. 2). With UL-FS 49 and at constant end-diastolic wall thickness, however, left ventricular dP/dt_max increased moderately, but significantly with increasing heart rate, from 75 to 200 min^–1, whereas systolic wall thickening remained unchanged (Fig. 3). In contrast, even with an expanded heart rate range and filling control, there was no increase in left ventricular dP/dt_max with increasing heart rate following autonomic blockade, and systolic wall thickening remained again unchanged (Fig. 4). In heart failure, as in the control state, there were no changes of left ventricular dP/dt_max and systolic wall thickening with increasing heart rate, in the presence of autonomic blockade, regardless of whether end-diastolic wall thickness increased (Fig. 5) or not (Fig. 6).

View larger version (26K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 1 Analog tracing of steady-state responses to increased atrial pacing rate from a representative dog with autonomic blockade and filling control in control and heart failure; showing left ventricular pressure [LVP], left ventricular dP/dt [dP/dt] and left ventricular wall thickness [WT].

 

View larger version (18K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 2 Left ventricular dP/dtmax (upper panel), systolic wall thickening (middle panel) and end-diastolic wall thickness (lower panel) as a function of heart rate in the control state; there is no significant change in dP/dtmax with increased heart rate; end-diastolic wall thickness increases significantly with increased heart rate and systolic wall thickening is reduced at the highest heart rate; data are means±SD; : p<0.05 vs. 100 min–1.

 

View larger version (18K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 3 Left ventricular dP/dtmax (upper panel), systolic wall thickening (middle panel) and end-diastolic wall thickness (lower panel) in the control state, with UL-FS 49 and filling control, as a function of heart rate; with end-diastolic wall thickness constant, dP/dtmax increases with heart rate and systolic wall thickening remains unchanged; n=4; data are means±SD; : p<0.05 vs. 75 min–1.

 

View larger version (18K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 4 Left ventricular dP/dtmax (upper panel), systolic wall thickening (middle panel) and end-diastolic wall thickness (lower panel) in the control state, with UL-FS 49, filling control and autonomic blockade, as a function of heart rate; there is no change in dP/dtmax or systolic wall thickening with increased heart rate; n=7; data are means±SD.

 

View larger version (18K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 5 Left ventricular dP/dtmax (upper panel), systolic wall thickening (middle panel) and end-diastolic wall thickness (lower panel) in heart failure, with autonomic blockade and UL-FS 49, as a function of heart rate; there is no change in dP/dtmax or systolic wall thickening with increased heart rate; data are means±SD; : p<0.05 vs. 75 min–1.

 

View larger version (18K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 6 Left ventricular dP/dtmax (upper panel), systolic wall thickening (middle panel) and end-diastolic wall thickness (lower panel) in heart failure, with UL-FS 49, filling control and autonomic blockade, as a function of heart rate; there is no change in dP/dtmax or systolic wall thickening with increased heart rate; n=5; data are means±SD.

 
3.3 Force–frequency relations in isolated left ventricular trabeculae
With increasing extracellular calcium concentration, the force–frequency relation in trabeculae from non-failing hearts became negative. The force–frequency relation in trabeculae from failing hearts was flat (Fig. 7).

View larger version (22K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 7 Contractile force (Fc) in isolated left ventricular trabeculae from non-failing (NF) and failing (F) hearts at three extracellular calcium concentrations. The force–frequency relations were mostly flat, with a more pronounced negative relationship in the non-failing trabeculae at higher calcium concentrations. Data are means±S.E.M., n=number of dogs. * p<0.05 in unpaired t-test.

 
3.4 Mechanical restitution and postextrasystolic potentiation
Fig. 8Fig. 9 show representative mechanical restitution and postextrasystolic potentiation curves for dP/dt_max and Fig. 10Fig. 11 for systolic wall thickening in the control state and in heart failure. The time constants of mechanical restitution and postextrasystolic potentiation consistently increased significantly from control to heart failure (Table 2Table 3). UL-FS 49 and autonomic blockade did not affect these time constants.

View larger version (19K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 8 Mechanical restitution curves for LV dP/dtmax from a representative animal in the control state and in heart failure. The time constant was increased with heart failure.

 

View larger version (21K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 9 Postextrasystolic potentiation curves for LV dP/dtmax from a representative animal in the control state and in heart failure. The time constant was increased with heart failure.

 

View larger version (19K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 10 Mechanical restitution curves for systolic wall thickening from a representative animal in the control state and in heart failure. The time constant was increased with heart failure.

 

View larger version (20K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 11 Postextrasystolic potentiation curves for systolic wall thickening from a representative animal in the control state and in heart failure. The time constant was increased with heart failure.

 

View this table: [in this window] [in a new window]   Table 2 Parameters of mechanical restitution for dP/dtmax and systolic wall thickening (WT%) in conscious dogs in the normal state and in heart failure

 

View this table: [in this window] [in a new window]   Table 3 Parameters of postextrasystolic potentiation for dP/dtmax and systolic wall thickening (WT%) in conscious dogs in the normal state and in heart failure

 

	4 Discussion

Top Abstract 1 Introduction 2 Methods 3 Results 4 Discussion References

 
The main message of the present study is methodological in nature. The in vitro findings of steady-state positive force–frequency relations in normal myocardium and negative force–frequency relations in failing myocardium cannot be extrapolated to left ventricular function in situ. Time constants of mechanical restitution and postextrasystolic potentiation for both an isovolumic phase index and an ejection phase index of ventricular function are better suited to characterize the impairment of excitation–contraction coupling in failing hearts in situ.

The present study utilized an established experimental model of heart failure, i.e. chronic ventricular pacing in conscious dogs [21, 35, 38, 39]. In ventricular trabeculae from non-failing hearts in the present study there was a flat force–frequency relation at lower calcium concentrations and a pronounced negative force–frequency relation at higher calcium concentrations. These findings conform with prior data on shortening-frequency relations in isolated cardiomyocytes [15]. In contrast, two other studies reported positive force–frequency relations in in vitro canine cardiac preparations [6, 7]. The reasons for this discrepancy are unclear at present; possibly non-failing canine myocardium differs from human myocardium in vitro. More importantly, however, the force–frequency relation in left ventricular trabeculae from failing hearts was flat or slightly negative, as previously reported also for isolated canine cardiomyocytes [15]. Thus, the present canine heart failure model mimics not only the characteristic clinical signs (ascites, pulmonary edema, exercise intolerance, cachexia) and hemodynamic alterations (increases in heart rate and left ventricular end-diastolic pressure, decreases in left ventricular maximal pressure and dP/dt_max) seen in patients with heart failure, but also the negative [11, 14]or flat [12, 13]force–frequency relations seen in isolated human cardiac muscle preparations from failing hearts.

The hemodynamic responses to steady-state atrial pacing in situ, however, were different from the reported force–frequency relations in vitro. The marked decrease in ventricular filling with increasing heart rate reduced systolic wall thickening and prevented an increase in dP/dt_max. Only with end-diastolic wall thickness artificially held constant was there a modest increase in dP/dt_max. Part of this increase in dP/dt_max may be the result of the volume loading-associated decrease in hematocrit and reflex sympathetic activation. The modest (10–20%) increases in dP/dt_max previously seen in humans without heart failure during heart rate increases by atrial pacing also occurred in the absence of significant decreases in filling [27–29]. When filling, however, was significantly decreased in patients during atrial pacing, stroke work was decreased [32], as also seen with systolic wall thickening in the present study. Interestingly, the modest increases in dP/dt_max during increased heart rate with filling control were prevented by autonomic blockade, stressing the importance of adrenergic activation for the positive force–frequency relation [34]. A flat response of dP/dt_max with right atrial pacing was also previously reported in conscious dogs with surgical cardiac denervation [42]. Studies on hemodynamic responses to tachycardia in humans with autonomic blockade are not available. It is, however, well conceivable that the modest positive relation between dP/dt_max and heart rate in humans without heart failure reflects an interaction between adrenergic activation and direct cardiomyocyte properties.

The flat relation of dP/dt_max and systolic wall thickening to heart rate in dogs with heart failure in the present study conforms with similarly flat relations in prior studies in conscious dogs [21], rabbits [23]and pigs [22], and — more importantly — with the flat relation in the single available study in patients with heart failure [27]. In contrast to the negative relation between systolic wall thickening and heart rate with decreasing ventricular filling in the control state (Fig. 2), the relation between systolic wall thickening and heart rate was flat in heart failure, whether end-diastolic wall thickness was constant (Fig. 6) or not (Fig. 5), confirming prior studies which indicated only a minor impact of preload on ventricular function in failing hearts [43–45]. As compared to the control state, the decrease in end-diastolic pressure with increasing heart rate was more pronounced and the increase in end-diastolic wall thickness less pronounced in heart failure, indicating an increased stiffness of the failing left ventricle [46].

In contrast to the steady-state relations of dP/dt_max and systolic wall thickening versus heart rate, the time constants of mechanical restitution and postextrasystolic potentiation were well suited to characterize the impairment of excitation–contraction coupling in heart failure in conscious dogs in situ, confirming prior studies for dP/dt_max [38, 39]. Also, the time constants of mechanical restitution and postextrasystolic potentiation reflected cardiomyocyte properties per se, as they were not different without and with autonomic blockade. Whether or not the time constants of mechanical restitution and postextrasystolic potentiation, respectively, reflect different steps of excitation–contraction coupling, as previously suggested by Yue et al. [37], was not the subject of the present study, and all time constants were increased to a similar extent in heart failure. The magnitude of the increase in time constants was comparable to that previously reported by Prabhu and Freeman in closed-chest dogs [38, 39], although the absolute values were somewhat lower in their study, possibly due to their use of a different index (single-beat elastance) and a different normalization (to the maximum vs. the steady state in our study). The model parameters calculated to characterize mechanical restitution (t₀, tc, CR_max) are interdependent. Not surprisingly, there were not only consistent increases in the time constants tc with heart failure, but also significant changes in the other parameters. The significant changes in the plateau parameters CR_max and B are probably due to our technical inability to collect enough data points with longer extrasystolic time intervals, as we used atrial pacing.

Our study is the first to compare steady-state relations of dP/dt_max and systolic wall thickening to heart rate with the time constants of mechanical restitution and postextrasystolic potentiation in failing hearts in situ. Clearly, extrapolation of the in vitro findings on a negative force–frequency relation in isolated preparations from failing hearts to left ventricular function in situ is not warranted. Moreover, since the only existing study in patients with heart failure also revealed a flat relation between dP/dt_max and heart rate [27], therapeutic recommendations that are based on a negative force–frequency relation in vitro [1, 16]are premature. In contrast to steady-state responses of dP/dt_max and systolic wall thickening to increased heart rate, time constants of mechanical restitution and postextrasystolic potentiation for both dP/dt_max (isovolumic phase index) and systolic wall thickening (ejection phase index) are not only robust against the impact of adrenergic activation but also sensitive to heart failure.

Time for primary review 28 days.

	Acknowledgements

 
This study was supported by IFORES grant 107401-0 of the Medical Faculty of the University of Essen.

	Notes

 
¹ Present address: Institut für Pharmakologie und Toxikologie, Universitätsklinikum Carl Gustav Carus, Technische Universität Dresden, Karl-Marx-Str. 3, 01109 Dresden, Germany.

	References

Top Abstract 1 Introduction 2 Methods 3 Results 4 Discussion References

 

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