Alterations of myocardial dynamic stiffness implicating abnormal crossbridge function in human mitral regurgitation heart failure.

Mitral regurgitation (MR) causes ventricular dilation, a blunted myocardial force-frequency relation, and increased crossbridge force-time integral (FTI). The mechanism of FTI increase was investigated using sinusoidal length perturbation analysis to compare crossbridge function in skinned left ventricular (LV) epicardial muscle strips from 5 MR and 5 nonfailing (NF) control hearts. Myocardial dynamic stiffness was modeled as 3 parallel viscoelastic processes. Two processes characterize intermediate crossbridge cycle transitions, B (work producing) and C (work absorbing) with Q(10)s of 4 to 5. No significant differences in moduli or kinetic constants of these processes were observed between MR and NF. The third process, A, characterizes a nonenzymatic (Q(10)=0.9) work-absorbing viscoelasticity, whose modulus increases sigmoidally with [Ca(2+)]. Effects of temperature, crossbridge inhibition, or variation in [MgATP] support associating the calcium-dependent portion of A with the structural "backbone" of the myosin crossbridge. Extension of the conventional sinusoidal length perturbation analysis allowed using the A modulus to index the lifetime of the prerigor, AMADP crossbridge. This index was 75% greater in MR than in NF (P=0.02), suggesting a mechanism for the previously observed increase in crossbridge FTI. Notably, the A-process modulus was inversely correlated (r(2)=0.84, P=0.03) with in vivo LV ejection fraction in MR patients. The longer prerigor dwell time in MR may be clinically relevant not only for its potential role as a compensatory mechanism (increased economy of tension maintenance and increased resistance to ventricular dilation) but also for a potentially deleterious effect (reduced elastance and ejection fraction).

A mechanistic analysis of reduced mechanical performance in human heart failure.

In failing human hearts (FHH) (NYHA IV) the cardiac output is inadequate to meet the metabolic needs of the peripheral systems. By means of thermo-mechanical analysis we have shown that epicardial strips from FHH (37 degrees C) have a depressed tension independent heat (TIH) and tension independent heat rate (dTIH / dt) liberation that correlates with depression in peak isometric force and the rate of relaxation. Furthermore, in response to a change in frequency of stimulation, FHH shows a severe blunting of the force-frequency relationship resulting in a decrease in myocardial reserve and in the frequency at which optimum force is obtained. We used ventricular ANF as an index of the severity of myocardial disease and demonstrated an inverse relationship between ANF mRNA and the sarcoplasmic reticulum (SR) calcium cycling proteins (SERCA 2, Phospholamban, Ryanodine Receptor) while these latter proteins all had a positive correlation with each other. At the same time there was an increase in sarcolemmal sodium calcium exchange protein. The decrease in SR pump proteins correlates with the decrease in myocardial reserve and optimum frequency of contraction. The latter mechanical changes are explainable in terms of a frequency dependent decrease in calcium concentration (aequorin light) in FHH.

A mechanistic analysis of the force-frequency relation in non-failing and progressively failing human myocardium.

This review focuses on the role of the myocardial force-frequency relation (FFR) in human ventricular performance and how changes in the FFR can reduce cardiac output and, ultimately, can contribute to altering the stability of the in-vivo cardiovascular system in a way that contributes to the progression of heart failure. Changes in the amplitude, shape, and position of the myocardial FFR occurring in various forms of heart failure are characterized in terms of maximal isometric twitch tension, slope of the ascending limb (myocardial reserve), and position of the peak of the FFR on the frequency axis (optimum stimulation frequency). All three of these parameters decline according to severity of myocardial disease in the following order: non-failing atrial septal defect, non-failing coronary artery disease, non-failing coronary artery disease with diabetes mellitus, failing mitral regurgitation, failing viral myocarditis, failing idiopathic dilated cardiomyopathy. Evidence is presented supporting a sarcoplasmic reticulum Ca-pump based mechanism for this progressive depression of the FFR. Intracellular calcium cycling and concentration and Ca-pump content all diminish in proportion to degree of depression of the FFR. Additional evidence from myocyte culture studies suggests a cause of diminished Ca-pump content is sustained, elevated levels of plasma norepinephrine. A hypothesis is presented to explain the mechanism of myocardial failure and its progression in terms of changes in the cardiovascular feedback control system that are triggered by reduced myocardial reserve. Sustained elevation of plasma norepinephrine levels depresses expression of sarcoplasmic reticulum Ca-pump protein causing depression of the FFR and this causes a compensatory further increase in norepinephrine levels and a further depression of Ca-pump protein.

Work production and work absorption in muscle strips from vertebrate cardiac and insect flight muscle fibers.

Stretch activation, which underlies the ability of all striated muscles to do oscillatory work, is a prominent feature of both insect flight and vertebrate cardiac muscle. We have examined and compared work-producing and work-absorbing processes in skinned fibers of Drosophila flight muscle, mouse papillary muscle, and human ventricular strips. Using small amplitude sinusoidal length perturbation analysis, we distinguished viscoelastic properties attributable to crossbridge processes from those attributable to other structures of the sarcomere. Work-producing and work-absorbing processes were identified in Ca(2+)-activated fibers by deconvolving complex stiffness data. An 'active' work-producing process ("B"), attributed to crossbridge action, was identified, as were two work-absorbing processes, one attributable to crossbridge action ("C") and the other primarily to viscoelastic properties of parallel passive structures ("A"). At maximal Ca(2+)-activation (pCa 5, 27 degrees C), maximum net power output (processes A, B and C combined) occurs at a frequency of: 1.3 +/- 0.1 Hz for human, 10.9 +/- 2.2 Hz for mouse, and 226 +/- 9 Hz for fly, comparable to the resting heart rate of the human (1 Hz, 37 degrees C) and mouse (10 Hz, 37 degrees C) and to the wing beat frequency of the fruit fly (200 Hz, 22 degrees C). Process B maximal work production per myosin head is 7-11 x 10(-21) J per perturbation cycle, equivalent to approximately 2 kT of energy. Process C maximal work absorption is about the same magnitude. The equivalence suggests the possibility that a thermal ratchet type mechanism operates during small amplitude length perturbations. We speculate that there may be a survival advantage in having a mechanical energy dissipater (i.e., the C process) at work in muscles if they can be injuriously stretched by the system in which they operate.

Role of cAMP in modulating relaxation kinetics and the force-frequency relation in mitral regurgitation heart failure.

The report is a discussion of previously published and newly analyzed results concerning the association between heart diseases and alterations in the force-frequency relation (FFR). The optimum stimulation frequency of the FFR is measured and compared in isolated left ventricular myocardium from non-failing hearts with atrial septal defect, coronary artery disease (without and with insulin dependent diabetes mellitus) and from failing hearts with mitral regurgitation, or idiopathic dilated cardiomyopathy. Specifically, we examine the role of altered control of the excitation-contraction coupling system in blunting the force-frequency relation. We use the percent slope of the FFR as a measure of changes in the frequency sensitivity of this control. Our finding of a linear, direct relation between optimum stimulation frequency and % slope across all disease types suggests both parameters are coupled to the same underlying mechanism. To investigate the possible role of altered control of the calcium pump in this mechanism, we analyzed the detailed relation between isometric twitch relaxation kinetics and stimulation frequency in mitral regurgitation myocardium (MR). In the presence of 0.5 microM forskolin the depressed slope and optimum frequency of the FFR and the prolonged half-time of twitch relaxation were all restored to values found in non-failing myocardium. We use the kinetics of isometric twitch relaxation as an index of changes in pumping rate that occur in response to changes in stimulation frequency or in intracellular cyclic adenosine monophosphate concentration. A mathematical model based on the Hill relations for calcium pump uptake rate and for isometric tension as a function of intracellular pCa is developed to simulate isometric twitch relaxation in MR and non-failing myocardium. The success of this model in simulating non-failing and failing twitch relaxation supports a proposed mechanism for the prolonged relaxation time and depressed FFR in MR involving depressed protein kinase-A activity (due to lowered cAMP or to a defect in the Ser16 site of phospholamban) as a mechanism of altered control of the calcium pump in MR heart disease.

Human heart failure: determinants of ventricular dysfunction.

Thin muscle strips were obtained from non-failing (NF) and failing (dilated cardiomyopathy (DCM)) hearts, using a new harvesting and dissection technique. The strips were used to carry out a myothermal and mechanical analysis so that contractile and excitation coupling phenomena in the NF and failing (DCM-F) preparations can be compared. Peak isometric force and rate of relaxation in DCM-F were reduced 46% (p < 0.02) while time to peak tension was increased 14% (p < 0.03). Initial, tension dependent, tension independent and the rate of tension independent heat liberation were reduced 62-70% in DCM-F (p < 0.03). The crossbridge force-time integral (FTIXBr) was calculated from these measurements and was shown to increase 40% while the amount and rate of calcium cycled per beat was reduced 70%. As a result of these changes in the contractile and excitation-contraction coupling systems in DCM-F, the force-frequency relationship was significantly blunted while the power output was markedly reduced. These fundamental alterations account for the substantial ventricular dysfunction found in the dilated cardiomyopathic failing heart.

Influence of isoproterenol and ouabain on excitation-contraction coupling, cross-bridge function, and energetics in failing human myocardium.

BACKGROUND: In patients with heart failure, long-term treatment with catecholamines and phosphodiesterase inhibitors, both of which increase cyclic AMP levels, may be associated with increased mortality, whereas mortality may not be increased with glycoside treatment. Differences in clinical benefit between cyclic AMP-dependent inotropic agents and cardiac glycosides may be related to differences of these drugs on calcium cycling and myocardial energetics. METHODS AND RESULTS: Isometric heat and force measurements were used to investigate the effects of isoproterenol and ouabain on myocardial performance, cross-bridge function, excitation-contraction coupling, and energetics in myocardium from end-stage failing human hearts. Isoproterenol (1 mumol/L) increased peak twitch tension by 55% and decreased time to peak tension and relaxation time by 30% and 26%, respectively (P < .005). Ouabain (0.38 +/- 0.11 mumol/L) increased peak twitch tension and relaxation time by 41% and 20%, respectively, and decreased time to peak tension by 12% (P < .05). With isoproterenol, the amount of excitation-contraction coupling-related heat evolution (tension-independent heat) increased by 246% (P < .05) and the economy of excitation-contraction coupling decreased by 61% (P < .05). Ouabain increased tension-independent heat by only 61% (P < .05) and did not significantly influence economy of excitation-contraction coupling. The effects of isoproterenol on excitation-contraction coupling resulted in a 21% (P < .005) decrease of overall contraction economy, which was not significantly changed with ouabain. Neither isoproterenol nor ouabain influenced energetics of cross-bridge cycling or recovery metabolism. CONCLUSIONS: Major differences between the effects of isoproterenol and ouabain in failing human myocardium are related to calcium cycling with secondary effects on myocardial energetics.

Maximal actomyosin ATPase activity and in vitro myosin motility are unaltered in human mitral regurgitation heart failure.

Myofibrillar but not actomyosin ATPase is depressed in failing myocardium from patients with dilated cardiomyopathy. Since there is a similar depression of myofibrillar ATPase in mitral regurgitation myocardium, we investigated whether or not the hydrolytic and mechanical performances of myosin are altered by comparing the maximal actomyosin ATPase activity and the in vitro myosin motility of myocardial myosin from patients with mitral regurgitation heart failure with that of patients with normal ventricular function. The results show that there is no significant difference (P > .05) between nonfailing and failing values for either the maximal actomyosin ATPase activity (0.3 s-1.head-1) or the myosin motility (1 micron/s). These observations suggest that changes, other than in the myosin heavy chain, contribute to the altered myocardial performance in mitral regurgitation myocardium.

Myocyte reorganization in hypertrophied and failing hearts.

In hypertrophied and failing hearts there are major changes in the overall contractile performance. We present a review of our previous work relating the alterations in myocardial force, work, power and relaxation, that lead to changes in overall ventricular performance, to changes in the actin-myosin cross-bridge cycle characteristics along with the degree of activation and inactivation (calcium cycling). Tissues from hypertrophied rabbit and failing human (volume overload, dilated cardiomyopathy) heart were used in these studies. Myocardial peak twitch tension (mN.mm-2) was reduced in dilated cardiomyopathy (human) (25.9 +/- 3.9 vs 13.9 +/- 2.0, 37 degrees C), volume overload (human) (44.0 +/- 11.7 vs 19.9 +/- 3.7, 21 degrees C) and pressure overload (rabbit) (46.1 +/- 2.6 vs 41.7 +/- 5.0, 21 degrees C). We used myothermal and mechanical data to analyse the average cross-bridge force time integral and the amount of calcium cycled per gram per beat. Tension-dependent Heat (mJ.g-1) (TDH) (cross-bridge cycling) and tension-independent heat (mJ.g-1) (TIH) were reduced in all of the experimental preparations (dilated cardiomyopathy, human, 37 degrees C: TDH, 3.39 +/- 0.59 vs 1.34 +/- 0.22; TIH 1.51 +/- 0.02 vs 0.16 +/- 0.03) (volume overload, human 21 degrees C: TDH, 7.23 +/- 2.22 vs 1.92 +/- 0.25; TIH, 0.75 +/- 0.19 vs 0.39 +/- 0.04) (pressure overload, rabbit, 21 degrees C: TDH, 6.60 +/- 0.75 vs 3.05 +/- 0.46; TIH, 1.00 +/- 0.17 vs 0.41 +/- 0.08).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of isoproterenol on contractile protein function, excitation-contraction coupling, and energy turnover of isolated nonfailing human myocardium.

Previous animal experiments indicated that the effects of catecholamines on myocardial function and subcellular systems vary considerably depending on the species and type of myocardium investigated. In the present study, we used isometric force and heat measurements to investigate the influence of isoproterenol on energetics of excitation-contraction coupling and contractile proteins in isolated nonfailing human myocardium. Isoproterenol, in an average concentration of 0.8 +/- 0.3 microM, resulted in a significant increase in peak twitch tension, maximum rate of tension rise and maximum rate of tension fall by 46% (P < 0.025), 126% (P < 0.001) and 137% (P < 0.005), respectively (37 degrees C, 60 beat/min). The amount and rate of excitation-contraction coupling-related heat evolution (tension-independent heat) increased by 116% (P < 0.03) and 176% (P < 0.02), respectively. Furthermore, the relationship of tension-independent heat to isometric tension or tension-time integral increased by 47% (P < 0.05) and 91% (P < 0.01), respectively. That is, the energy used in calcium cycling increased by a greater proportion than did mechanical output. Isoproterenol increased the rate of the acto-myosin crossbridge high-energy phosphate hydrolysis (tension-dependent heat rate) by 61% (P < 0.006) and decreased the force-time integral (consistent with decrease in the attachment time) of the individual crossbridge cycle by 21% (P < 0.025). Decreased crossbridge force-time integral with isoproterenol indicates decreased economy of isometric force production at the level of the contractile proteins. Increased energy turnover of excitation-contraction coupling processes and reduced force-time integral generation by the individual crossbridge cycle resulted in increased myocardial energy turnover as indicated by a 41% increase in the ratio of total activity related heat per tension-time integral (P < 0.02). The efficiency of the metabolic recovery process as assessed by the ratio of initial heat to total activity related heat, was similar with and without isoproterenol (0.52 +/- 0.05 v 0.49 +/- 0.03; P > 0.05). Thus, isoproterenol significantly influences excitation-contraction coupling processes and crossbridge cycling, thereby increasing myocardial energy turnover per unit of isometric force production in the human myocardium.

Myocardial force-frequency defect in mitral regurgitation heart failure is reversed by forskolin.

BACKGROUND: Postoperative ejection phase parameters and patient survival rates for mitral valve replacement surgery are considerably lower than for similar aortic valve surgery. While chordal transection probably is the major contributor to the lowered values, there is also evidence for decreased preoperative myocardial contractile reserve in mitral regurgitation patients. This study characterizes abnormalities in the force-frequency relation that may underlie impaired function of myocardium isolated from mitral regurgitation patients with New York Heart Association class II-III heart failure. METHODS AND RESULTS: Left ventricular epicardial myocardium was obtained by surgical biopsy during mitral valve replacement surgery in patients with mitral regurgitation heart failure (left ventricular ejection fraction, 0.64 +/- 0.05) and during coronary artery bypass surgery in patients with normal ventricular function. The steady-state twitch tension versus frequency relation was measured in myocardial strip preparations (37 degree C, 12 to 228 min-1) in the absence and presence of forskolin. Relative to normal function, peak isometric twitch tension in mitral regurgitation is depressed by 50% (P < .02) and 74% (P < .003) at contraction frequencies of 60 min-1 and 168 min-1, respectively. The slope of the tension-frequency curve is blunted and its peak is shifted to a lower frequency (mitral regurgitation: 134 min-1; normal function: 173 min-1; P < .02). The myosin heavy chain concentration did not differ between mitral regurgitation and normal function strips (53 +/- 4 versus 54 +/- 4 nmol/g blotted wt). Forskolin (0.5 mumol/L) completely reversed the tension depression, blunting, and the lowered peak frequency in the mitral regurgitation preparations. CONCLUSIONS: Preoperatively, myocardial tension generation in mitral regurgitation patients is severely depressed, and the force-frequency curve is blunted and has a negative slope in the exercise range of heart rates. The reversal of these defects by forskolin suggests that abnormal excitation-contraction coupling may underlie the decreased contractile reserve in mitral regurgitation patients.

Optimization of myocardial function.

Under normal conditions the cardiac output is designed to meet the metabolic needs of the organism. Thus, the demands imposed on the heart muscle can range from low values at rest to an order of magnitude greater values during exercise. The heart uses a number of strategies to meet the short- and long-term changes in demand. These strategies are of general biological interest and employ similar mechanisms to those responsible for the differences in muscle performance seen between muscle from various species and diverse muscle types within a given animal. This review deals with the heart's utilization of these strategies to meet a broad range of requirements. Tortoise (TM) and rat soleus (RS) muscles are slow, have high economy and develop low power. In contrast (FM) and rat extensor digitorum longus (REDL) are fast, have low economy and have a high power output. These differences are explainable in terms of the characteristics of the myosin head cross-bridge cycle (Cross-bridge tension-time integral: FM/FT = 0.024; REDL/RS = 0.16. Myosin ATPase activity: FM/TM = 15; RDEL/RS = 2.3) and excitation contraction coupling system (time to peak tension: FM/TM = 0.2; REDL/RS = 0.4). Heart muscle employs similar strategies (cross-bridge cycle; excitation contraction coupling) to meet short (catecholamine) and long (hypertrophy secondary to pressure overload or thyrotoxicosis) term changes in demand. In the presence of catecholamine power is increased while economy is decreased. This difference between control (C) and isoproterenol treated hearts (I) is explainable in terms of the contractile and excitation contraction coupling systems (Cross-bridge tension-time integral: I/C = 0.4. Tension independent heat: I/C = 2.0. Tension independent heat rate: I/C = 2.5). A persistent increase in the demand on the heart results in myocardial hypertrophy that is associated with intracellular reorganization. Hyperthyroidism (T) and pressure overload (PO) were used to produce myocardial hypertrophy. In T hearts the economy is decreased while the power is increased; in PO hearts oppositely directed changes occur. These alterations are attributable to changes in the performance of the contractile and excitation contraction coupling systems (Cross-bridge force-time integral: T/C/PO = 0.5/1.0/2.6. Tension independent heat: T/C/PO = 1.4/1.0/0.4. Tension independent heat rate: T/C/PO = 1.4/1.0/0.3). Thus it is clear that in meeting changes in demand, the heart uses strategies comparable to those seen between species and muscle types within a given muscle.

Myocardial adaptation to stress from the viewpoint of adaptation and development.

Myocardial adaptation to stress and development includes reorganization of subcellular systems. Using a myothermal method, changes in the contractile protein system were investigated across species (rat, rabbit, human myocardium) and in consequence of hemodynamic (volume overload human, pressure overload rabbit myocardium) or hormonal stresses (hypothyroid rat, hyperthyroid rabbit myocardium). Mechanical and myothermal measurements were performed in isometrically contracting right or left ventricular muscle strips and the force-time integral of the individual crossbridge cycle was calculated from heat and force data. Within species, crossbridge force-time integral increased by 85% from control human to volume overload human myocardium. Crossbridge force-time integral increased by 100% from control to hypothyroid rat myocardium. In rabbit myocardium, crossbridge force-time integral increased by 164% in pressure overload and decreased by 47% in hyperthyroid compared to control myocardium. Across species, crossbridge force-time integral was smallest in control rat myocardium (0.16 +/- 0.01 pNs) and increased in the order: control rat < hyperthyroid rabbit < hypothyroid rat, control rabbit < control human < pressure overload rabbit < volume overload human myocardium (0.96 +/- 0.01 pNs). Within and across species, crossbridge force-time integral was positively correlated with time to peak tension (r = 0.86; p < 0.05) and negatively correlated with maximum rate of tension rise (r = -0.85; p < 0.05) and maximum rate of tension fall (r = -0.78; p < 0.05). Furthermore, there were significant correlations between crossbridge force-time integral and total activity related heat (r = -0.81; p < 0.05) as well as total activity related heat per tension-time integral (r = -0.89; p < 0.005). Thus, the close relationship between crossbridge force-time integral and myocardial function within and across species demonstrates that alterations of crossbridge force-time integral reflect an important mechanism of subcellular adaptation to stress from a mechanical point of view. Moreover, alterations of the crossbridge force-time integral have pronounced effects on energy consumption in the different types of myocardium.

The reorganization of the human and rabbit heart in response to haemodynamic overload.

A myothermal/mechanical analysis on non-failing and failing human hearts and normal and pressure overloaded rabbit hearts is reported. Heat production is partitioned into tension-dependent and tension-independent components together with force measurements to provide information about calcium and cross-bridge cycling. In the non-failing human heart the cross-bridge force-time integral is 0.51 +/- 0.06 (ns). This value is increased to 0.97 +/- 0.09 (P less than 0.05 s) in failing hearts. In control as compared to pressure-overload rabbit hearts the cross-bridge force-time integral increases from 0.36 +/- 0.02 to 0.96 +/- 0.11 (P less than 0.05 s). The increase in force-time integral allows the heart muscle to develop force with greater economy (less high energy phosphate hydrolysis) but at the expense of velocity and power. The amount of calcium cycled following activation in non-failing human hearts is 32.2 +/- 8.17 nmoles.g-1.-beat-1. In the failing preparations calcium cycling is reduced to 16.7 +/- 1.72 nmoles.g-1.-beat-1. In pressure-overloaded hypertrophied, as compared with control rabbit hearts, the calcium cycled per beat is reduced from 43.0 +/- 7.3 to 17.6 +/- 3.4 nmoles.g-1. It is suggested that the alterations in cross-bridge cycling are more likely to be related to isoenzyme shifts in light chains or troponin T than to myosin isoforms. The calcium cycling changes are well correlated with changes in the sarcoplasmic reticular and sarcolemmal calcium transport proteins. The alterations in the contractile and excitation contractions coupling systems contribute to the functional changes observed in the failing human and pressure-overload rabbit hearts.

Cellular basis of negative inotropic effect of 2,3-butanedione monoxime in human myocardium.

2,3-Butanedione monoxime (BDM) exerts a marked negative inotropic effect and has been shown to have protective actions on human myocardial force production that may be of clinical use. To determine the underlying mechanisms, we studied the effects of BDM on chemically skinned and aequorin-loaded myopathic human myocardium from transplant recipients. Eighteen muscles were chemically skinned with saponin (250 micrograms/ml) and then subjected to activation-relaxation cycles, with and without 5 mM BDM. Contracture force vs. Ca2+ data were fitted to a modified Hill equation, and values for 50% maximal activation (pCa50) and maximal Ca(2+)-activated force (Fmax) were obtained. pCa50 was decreased by 0.2 pCa units, indicating myofilament Ca2+ desensitization, and Fmax was reduced by 48% in 5 mM BDM. A second group of intact muscles (n = 8) was loaded with aequorin to monitor intracellular calcium (Cai2+) transients (peak light) and twitch force in the presence of BDM (1-30 mM). Over a range of 1-20 mM, BDM depressed peak light by 3-49% while force was depressed by 10-82%. This was accompanied by an abbreviation of the duration of the twitch but not of the Cai2+ transient. At a concentration of 30 mM, BDM completely inhibited force generation, but an Cai2+ transient was still present. We conclude that in human myocardium, 5 mM BDM predominantly affects cross-bridge force production and Ca2+ sensitivity and has a less pronounced effect on Cai2+.

Alteration of contractile function and excitation-contraction coupling in dilated cardiomyopathy.

Myocardial failure in dilated cardiomyopathy may result from subcellular alterations in contractile protein function, excitation-contraction coupling processes, or recovery metabolism. We used isometric force and heat measurements to quantitatively investigate these subcellular systems in intact left ventricular muscle strips from nonfailing human hearts (n = 14) and from hearts with end-stage failing dilated cardiomyopathy (n = 13). In the failing myocardium, peak isometric twitch tension, maximum rate of tension rise, and maximum rate of relaxation were reduced by 46% (p = 0.013), 51% (p = 0.003), and 46% (p = 0.018), respectively (37 degrees C, 60 beats per minute). Tension-dependent heat, reflecting the number of crossbridge interactions during the isometric twitch, was reduced by 61% in the failing myocardium (p = 0.006). In terms of the individual crossbridge cycle, the average crossbridge force-time integral was increased by 33% (p = 0.04) in the failing myocardium. In the nonfailing myocardium, the crossbridge force-time integral was positively correlated with the patient's age (r = 0.86, p less than 0.02), whereas there was no significant correlation with age in the failing group. The amount and rate of excitation-contraction coupling-related heat evolution (tension-independent heat) were reduced by 69% (p = 0.24) and 71% (p = 0.028), respectively, in the failing myocardium, reflecting a considerable decrease in the amount of calcium released and in the rate of calcium removal. The efficiency of the metabolic recovery process, as assessed by the ratio of initial heat to total activity-related heat, was similar in failing and nonfailing myocardium (0.54 +/- 0.03 versus 0.50 +/- 0.02, p = 0.23).(ABSTRACT TRUNCATED AT 250 WORDS)

Altered myocardial force-frequency relation in human heart failure.

BACKGROUND: In congestive heart failure (idiopathic dilated cardiomyopathy), exercise is accompanied by a smaller-than-normal decrease in end-diastolic left ventricular volume, depressed peak rates of left ventricular pressure rise and fall, and depressed heart-rate-dependent potentiation of contractility (bowditch treppe). We studied contractile function of isolated left ventricular myocardium from New York Heart Association class IV-failing and nonfailing hearts at physiological temperature and heart rates in order to identify and quantitate abnormalities in myocardial function that underlie abnormal ventricular function. METHODS AND RESULTS: The isometric tension-generating ability of isolated left ventricular strips from nonfailing and failing human hearts was investigated at 37 degrees C and contraction frequencies ranging from 12 to 240 per minute (min-1). Strips were dissected using a new method of protection against cutting injury with 2,3-butanedione monoxime (BDM) as a cardioplegic agent. In nonfailing myocardium the twitch tension-frequency relation is bell-shaped developing 25 +/- 2 mN/mm2 at a contraction frequency of 72 min-1 and peaking at 44 +/- 3.7 mN/mm2 at a contraction frequency of 174 +/- 4 min-1. In failing myocardium the peak of the curve occurs at lower frequencies between 6 and 120 min-1 averaging 81 +/- 22 min-1, and it develops 48% (p less than 0.001) and 80% (p less than 0.001) less tension than in nonfailing myocardium at 72 and 174 min-1, respectively. Between 60 and 150 min-1 tension increases by 107% in nonfailing myocardium, but it does not change significantly in failing myocardium. Peak rates of rise and fall of isometric twitch tension vary in parallel with twitch tension as stimulation frequency rises in nonfailing myocardium but not in failing myocardium. CONCLUSIONS: The quantitative agreement between these results from isolated myocardium and those from catheterization laboratory measurements on intact humans suggest that alterations of myocardial origin, independent of systemic factors, may contribute to the above mentioned abnormalities in left ventricular function seen in dilated cardiomyopathy.

Contractile protein function in failing and nonfailing human myocardium.

Isometric heat and force measurements were used to relate mechanical performance to function of contractile proteins in muscle strips from failing and nonfailing human hearts (37 degrees C, 60 beats per minute). Compared to control myocardium, crossbridge behavior was altered in myocardium from hearts with end-stage failing dilated and ischemic cardiomyopathy, resulting in increased crossbridge force-time integral by 33% and 36%, respectively. Peak isometric twitch tension was reduced significantly by 46% in muscle strips from hearts with dilated cardiomyopathy. In myocardium from hearts with ischemic cardiomyopathy peak isometric twitch tension was comparable to values from nonfailing hearts. Including all three types of myocardium, there was a close correlation between the number of crossbridge interactions during the isometric twitch (tension-dependent heat) and peak twitch tension (r = 0.88; p less than 0.001). Compared to control, in failing myocardium from dilated cardiomyopathic hearts, tension-independent heat (calcium cycling) was significantly reduced. This indicates that in dilated cardiomyopathy reduced peak twitch tension results from decreased calcium activation of contractile proteins with reduced number of crossbridge interactions during the isometric twitch. In ischemic cardiomyopathy mechanisms different from those observed in dilated cardiomyopathy seem to be involved in the development of heart failure.

Contraction frequency dependence of twitch and diastolic tension in human dilated cardiomyopathy (tension-frequency relation in cardiomyopathy).

We studied isometric twitch tension and diastolic tension at 37 degrees C as a function of stimulation frequency (12-240 min-1) in very thin (.07-.5 mm2), parallel fibered strips of left-ventricular myocardium. Non-failing control tissue (C) was obtained from epicardial biopsies taken during myocardial revascularization surgery on patients with normal ventricular function. End-stage failing tissue was obtained from endocardial and epicardial biopsies from explanted hearts with idiopathic dilated cardiomyopathy (DCM). The methods and apparatus for biopsy and dissection of myocardium are described. Maximal peak twitch tension at optimal stimulation frequency of 163 +/- 5 min-1 was 41.8 +/- 10 mN/mm2 in non-failing myocardium and it was reduced by 70% (p less than .02) to 12.9 +/- 1.6 mN/mm2 at an optimal frequency of 72 +/- 17 min-1 in DCM. The peaks of the tension-frequency curves occurred at frequencies between 12 and 60 min-1 in most DCM strips (5/9), while in C most of the peaks (8/9) fell between 156 and 180 min-1. The peaks from four DCM hearts fell in an intermediate range of frequencies (96-144 min-1) which also included one non-failing peak at 132 min-1. Diastolic tension declined in both groups as stimulation frequency increased above 12 min-1 and it began increasing when stimulation frequency rose above optimal frequency by 19 +/- 5% and 110 +/- 50% in C and DCM, respectively. Total duration of the isometric twitch diminished with tachycardia remaining shorter than stimulation intervals up to 140 +/- 16 min-1 (3.1 +/- 1 times optimal frequency) in DCM and up to 161 +/- 14 min-1 (not different than optimal frequency) in C. Decline in peak twitch tension above optimal stimulation frequency was 4 to 6 times larger than the accompanying rise in diastolic tension in both groups. The premature decline in tension at lower than normal degrees of tachycardia in DCM does not arise from incomplete relaxation of the twitch response. The 70% deficit in tension generating ability of DCM may be a major contributor to heart failure. Moderate shift in the peak of the tension-frequency curves to lower frequencies (130 min-1) in C does not appear to predispose end-stage failure, but it may make the ventricle more susceptible to dilation.

Dynamic calcium requirements for activation of human ventricular muscle calculated from tension-independent heat.

The heat and tension generated by strips of human left ventricle taken from nonfailing hearts were measured at 30 C before and after partial inhibition of ATP splitting by the contractile proteins. We used 2, 3-butanedione monoxime (BDM) (4mM) as the chemical inhibition agent and alterations in solution calcium concentration and stimulus frequency to estimate the heat associated with calcium cycling for a wide range of activation levels. Tension-independent heat (TIH) was used to calculate the total calcium cycled per twitch by assuming that two-thirds of TIH was due to ATP splitting by the sarcoplasmic reticulum CA2+ ATPase with a coupling ratio of 2 Ca2+/ATP split and that one-third of TIH was due to ATP splitting by the sarcolemmal Na+ -K+ ATPase supporting the Na+ -Ca2+ exchanger (1 Ca2+/ATP). The enthalpy of creatine phosphate hydrolysis buffering ATP was taken as -34 KJ/mol. There was a highly positive correlation between TIH and mechanical activation during steady-state and nonsteady-state stimulation. The estimated total calcium turnover per twitch at 39% activation (0.3 Hz pacing rate and 2.5 mM Calcium) was approximately 0.17 nmol/g wet weight. This estimate is less than that calculated from biochemical data describing the cellular content and Ca2+ affinity of major Ca2+ buffers, but is similar to values calculated from recent electron probe microanalysis experiments.