Regional differences in the force-frequency relation of human left ventricular myocardium in mitral regurgitation: implications for ventricular shape.

Sphericalization of the left ventricular (LV) chamber shape in patients with mitral regurgitation (MR) contributes to increased LV wall stress and energy consumption. On the basis of previous observations, we hypothesized the existence of regional differences in the force-frequency relation (FFR) within the LV that may contribute to its shape. Accordingly, in the present study, we assessed regional variation in the FFR in patients undergoing surgery for chronic, nonischemic MR with class II-III heart failure symptoms and related our findings to the in vivo LV shape.

Molecular and phenotypic effects of heterozygous, homozygous, and compound heterozygote myosin heavy-chain mutations.

Autosomal dominant familial hypertrophic cardiomyopathy (FHC) has variable penetrance and phenotype. Heterozygous mutations in MYH7 encoding beta-myosin heavy chain are the most common causes of FHC, and we proposed that "enhanced" mutant actin-myosin function is the causative molecular abnormality. We have studied individuals from families in which members have two, one, or no mutant MYH7 alleles to examine for dose effects.

Reduced cross-bridge dependent stiffness of skinned myocardium from mice lacking cardiac myosin binding protein-C.

The role of cardiac myosin binding protein-C (MyBP-C) on myocardial stiffness was examined in skinned papillary muscles of wild-type (WT(+/+)) and homozygous truncated cardiac MyBP-C (MyBP-C(t/t) male mice. No MyBP-C was detected by gel electrophoresis or by Western blots in the MyBP-C(t/t) myocardium. Rigor-bridge dependent myofilament stiffness, i.

Thin-filament-based modulation of contractile performance in human heart failure.

Background: The contribution of the sarcomere's thin filament to the contractile dysfunction of human cardiomyopathy is not well understood.

Methods And Results: We have developed techniques to isolate and functionally characterize intact (native) thin filaments obtained from failing and nonfailing human ventricular tissue. By use of in vitro motility and force assays, native thin filaments from failing ventricular tissue exhibited a 19% increase in maximal velocity but a 27% decrease in maximal contractile force compared with nonfailing myocardium.

Reduced cross-bridge dependent stiffness of skinned myocardium from mice lacking cardiac myosin binding protein-C.

The role of cardiac myosin binding protein-C (MyBP-C) on myocardial stiffness was examined in skinned papillary muscles of wild-type (WT(+/+)) and homozygous truncated cardiac MyBP-C (MyBP-C(t/t)) male mice. No MyBP-C was detected by gel electrophoresis or by Western blots in the MyBP-C(t/t) myocardium. Rigor-bridge dependent myofilament stiffness, i.

Effect of cardiac myosin binding protein-C on mechanoenergetics in mouse myocardium.

We examined the effect of cardiac myosin binding protein-C (cMyBP-C) on contractile efficiency in isovolumically contracting left ventricle (LV) and on internal viscosity and oscillatory work production in skinned myocardial strips. A 6-week diet of 0.15% 6-n-propyl-2-thiouracil (PTU) was fed to wild-type (+/+(PTU)) and homozygous-truncated cMyBP-C (t/t(PTU)) mice starting at age approximately 8 weeks and leading to a myosin heavy chain (MHC) isoform profile of 10% alpha-MHC and 90% beta-MHC in both groups.

Role of cardiac myosin binding protein C in sustaining left ventricular systolic stiffening.

Despite advances in the molecular biology of cardiac myosin binding protein-C (cMyBP-C), little is understood about its precise role in muscle contraction, particularly in the intact heart. We tested the hypothesis that cMyBP-C is central to the time course and magnitude of left ventricular systolic elastance (chamber stiffening), and assessed mechanisms for this influence in intact hearts, trabeculae, and skinned fibers from wild-type (+/+) and homozygous truncated cMyBP-C (t/t) male mice. cMyBP-C protein was not detected by gel electrophoresis or Western blot in t/t myocardium.

Differential cross-bridge kinetics of FHC myosin mutations R403Q and R453C in heterozygous mouse myocardium.

The kinetic effects of the cardiac myosin point mutations R403Q and R453C, which underlie lethal forms of familial hypertrophic cardiomyopathy (FHC), were assessed using isolated myosin and skinned strips taken from heterozygous (R403Q/+ and R453C/+) male mouse hearts. Compared with wild-type (WT) mice, actin-activated ATPase was increased by 38% in R403Q/+ and reduced by 45% in R453C/+, maximal velocity of regulated thin filament (V(RTF)) in the in vitro motility assay was increased by 8% in R403Q/+ and was not different in R453C/+, myosin concentration at half-maximal V(RTF) was reduced by 30% in R403Q/+ and not different in R453C/+, and the characteristic frequency for oscillatory work production (b frequency), determined by sinusoidal analysis in the skinned strip at maximal calcium activation, was 27% lower in R403Q/+ and 18% higher in R453C/+. The calcium sensitivity for isometric tension in the skinned strip was not different in R403Q/+ (pCa(50) 5.

Analysis of myosin heavy chain functionality in the heart.

Comparison of mammalian cardiac alpha- and beta-myosin heavy chain isoforms reveals 93% identity. To date, genetic methodologies have effected only minor switches in the mammalian cardiac myosin isoforms. Using cardiac-specific transgenesis, we have now obtained major myosin isoform shifts and/or replacements.

Myosin from failing and non-failing human ventricles exhibit similar contractile properties.

In non-failing human myocardium, V1 myosin comprises a small amount (<10%) of the total myosin content, whereas end-stage failing hearts contain nearly 100% V3 myosin. It has been suggested that this shift in V1 myosin isoform content may contribute to the contractile deficit in human myocardial failure. To test this hypothesis, myosin was isolated from human failing and non-failing ventricles, and non-failing atria.

Molecular mechanics of mouse cardiac myosin isoforms.

Two myosin isoforms are expressed in myocardium, alphaalpha-homodimers (V(1)) and betabeta-homodimers (V(3)). V(1) exhibits higher velocities and myofibrillar ATPase activities compared with V(3). We also observed this for cardiac myosin from normal (V(1)) and propylthiouracil-treated (V(3)) mice.

Cardiac troponin T isoforms demonstrate similar effects on mechanical performance in a regulated contractile system.

Alteration of troponin T (TnT) isoform expression has been reported in human and animal models of myocardial failure. The two adult beef cardiac TnT isoforms (TnT(3) and TnT(4)) were isolated for comparative functional analysis. Thin filaments were reconstituted containing pure populations of the isoforms.

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.

Altered cross-bridge characteristics following haemodynamic overload in rabbit hearts expressing V3 myosin.

1. Our goal in this study was to evaluate the effect of haemodynamic overload on cross-bridge (XBr) kinetics in the rabbit heart independently of myosin heavy chain (MHC) isoforms, which are known to modulate kinetics in small mammals. We applied a myothermal-mechanical protocol to isometrically contracting papillary muscles from two rabbit heart populations: (1) surgically induced right ventricular pressure overload (PO), and (2) sustained treatment with propylthiouracil (PTU).

R403Q and L908V mutant beta-cardiac myosin from patients with familial hypertrophic cardiomyopathy exhibit enhanced mechanical performance at the single molecule level.

Familial hypertrophic cardiomyopathy (FHC) is a disease of the sarcomere. In the beta-myosin heavy chain gene, which codes for the mechanical enzyme myosin, greater than 40 point mutations have been found that are causal for this disease. We have studied the effect of two mutations, the R403Q and L908V, on myosin molecular mechanics.

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.

Kinetic differences at the single molecule level account for the functional diversity of rabbit cardiac myosin isoforms.

1. Cardiac V3 myosin generates slower actin filament velocities and higher average isometric forces (in an in vitro motility assay) when compared with the V1 isoform. 2.

Cross-bridge dynamics in the contracting heart.

The mechanical characteristics of the myosin motor is one of the key determinants of ventricular function. In small mammals there are two myosin isoforms, V1 and V3, with profoundly different performance characteristics. We used myothermal and mechanical analysis of intact papillary muscles from thryoxine (V1) and popylthiouracil (V3) treated rabbit hearts to assess the mechanical attributes of the myosin cross-bridge cycle.

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.

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.

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.

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.