Diastolic Left Ventricular Dysfunction : A Clinical Appraisal

Diastolic left ventricular (LV) distensibility is determined by the material properties of the LV wall and by LV geometry (i.e., LV shape, LV volume and LV wall thickness). These material properties are influenced both by the physical structure of the LV myocardium and by the dynamic process of myocardial relaxation. The material properties of the myocardium dictate the strain that follows a given stress, and determine position and shape of the myocardial stress-strain relationship. The material properties, together with the LV geometry, also determine position and shape of the diastolic LV pressure-volume relationship. Diastolic LV distensibility is best characterized by this diastolic LV pressure-volume relationship. The crucial role of diastolic LV distensibility in relation to the heart failure syndrome are discussed in chapter 2, 3 and 4 of this thesis. Chapter 2 of this thesis discusses diastolic left ventricular dysfunction, characterized by an upward shift of the left ventricular diastolic pressure-volume relationship. Chapter 2.1 describes the effects of myocardial ischemia, either pacing-induced or by coronary occlusion, on the diastolic properties of the same LV anterior wall segment in 12 patients with single-vessel proximal left anterior descending coronary artery stenosis at rest, immediately after 7 ± 1.2 minutes of pacing, and at the end of a 1- minute balloon occlusion of coronary angioplasty (CO). Shifts of the diastolic LV pressure-length relation, derived from simultaneous tip-micromanometer LV pressure recordings and digital subtraction LV angiograms, were used as an index of regional diastolic LV distensibility of the anterior wall segment. The diastolic LV Pressure(P)-Radial Length(RL) plot of the ischemic segment was shifted upward for portions of the plot that overlapped with the diastolic LV P-RL plot at rest. This upward shift at the end of CO was significantly smaller than that immediately after pacing. At the end of CO, a correlation was observed for the ischemic segment between percentage systolic shortening and upward shift of the diastolic LV pressure-radial length plot. The upward shift of the diastolic LV pressure-radial length plot, which was used as an index of decreased regional diastolic LV distensibility, was larger immediately after pacing than at the end of CO. Persistent systolic shortening of ischemic myocardium seems to be a prerequisite for a decrease in diastolic distensibility of the ischemic segment because of the higher percentage systolic shortening of the ischemic segment immediately after pacing, and because of the correlation at the end of CO between the upward shift of the diastolic LV pressure-radial length plot and percentage systolic shortening of the ischemic segment. Chapter 2.2 describes the different effects of low- flow ischemia, pacing-induced ischemia, and hypoxemic perfusion on LV performance in humans. During the initial phase of an ischemic insult, left ventricular (LV) performance depends on the complex interaction between oxygen deprivation, vascular turgor, and accumulation of metabolites. Summary 205 In experimental preparations, low-flow ischemia decreases systolic shortening and increases diastolic LV distensibility, whereas pacing- induced ischemia or hypoxic perfusion produces smaller decreases in systolic shortening but decreases LV diastolic distensibility. Therefore, the different effects of low-flow ischemia, pacing-induced ischemia, and hypoxemic perfusion on LV performance was studied in 20 patients with a significant stenosis in the left anterior descending coronary artery. Micromanometer-tip LV pressure recordings, LV angiography, and coronary sinus blood sampling were obtained at rest and during pacing-induced ischemia, low-flow ischemia due to balloon coronary occlusion, and hypoxemia induced by balloon coronary occlusion with hypoxemic perfusion distal to the occlusion. LV stroke work index was lower at the end of balloon coronary occlusion than during pacing-induced ischemia and was lower at the end of balloon coronary occlusion than at the end of hypoxemia induced by balloon coronary occlusion with hypoxemic perfusion distal to the occlusion. LV end-diastolic pressure rose from 16±5 mm Hg at rest to 23±6 mm Hg at the end of balloon coronary occlusion. However, LV end-diastolic pressure was lower at the end of balloon coronary occlusion than during pacing-induced ischemia and was lower at the end of balloon coronary occlusion than at the end of hypoxemia induced by balloon coronary occlusion with hypoxemic perfusion distal to the occlusion. LV end-diastolic volume index increased at the end of balloon coronary occlusion. Left ventricular end-diastolic volume index increased to values similar to those for balloon coronary occlusion during pacing-induced ischemia and at the end of hypoxemia induced by balloon coronary occlusion with hypoxemic perfusion distal to the occlusion. Higher values of LV end-diastolic pressure and unchanged values of LV end-diastolic volume index for pacing-induced ischemia and hypoxemia induced by balloon coronary occlusion with hypoxemic perfusion distal to the occlusion, compared with balloon coronary occlusion, suggested a lower end- diastolic LV distensibility during pacing-induced ischemia and during hypoxemia, as compared with low-flow ischemia. Upward shifts of individual diastolic LV pressure-volume curves during pacing-induced ischemia (9 of 11 patients) and at the end of hypoxemia induced by balloon coronary occlusion with hypoxemic perfusion distal to the occlusion (7 of 9 patients), compared with balloon coronary occlusion, were also consistent with lower LV diastolic distensibility during pacing-induced ischemia and during hypoxemia, compared with low-flow ischemia. Coronary sinus lactate, H+, and K+ levels increased after balloon deflation (balloon coronary occlusion and hypoxemia induced by balloon coronary occlusion with hypoxemic perfusion distal to the occlusion) and during pacing-induced ischemia. Thus, during low-flow ischemia, LV systolic performance was lower and LV diastolic distensibility larger than during pacing-induced ischemia or hypoxemia. The variable response of the human myocardium to different types of ischemia was probably related to the degree of vascular turgor and accumulation of tissue metabolites. Chapter 2.3 covers a pathophysiologic perspective of the comparative effects of ischemia and Chapter 6 206 hypoxemia on left ventricular diastolic function in humans. In Chapter 2.4 the presence of a deficient acceleration of left ventricular relaxation is reported during exercise after heart transplantation. The exercise-induced rise in left ventricular filling pressures after cardiac transplantation is considered to be the result of a blunted heart rate response, of elevated venous return, and of unfavorable passive late-diastolic properties of the cardiac allograft. In contrast to passive late-diastolic left ventricular properties, the effect of left ventricular relaxation on the exercise-induced rise in left ventricular filling pressures of the cardiac allograft has not yet been studied. In the present study, the response of left ventricular relaxation to exercise was investigated in transplant recipients and compared with left ventricular relaxation observed in normal control subjects exercised to the same heart rate. Moreover, the response of left ventricular relaxation of the cardiac allograft to beta-adrenoreceptor stimulation, to reduced left ventricular afterload, and to increased myocardial activator calcium was investigated by infusion of dobutamine and of nitroprusside and by postextrasystolic potentiation. Twenty-seven transplant recipients were studied 1 year (n = 17), 2 years (n = 7), 3 years (n = 2), and 4 years (n = 1) after transplantation. All patients were free of rejection and of significant graft atherosclerosis at the time of study. Tip-micromanometer left ventricular pressure recordings and cardiac hemodynamics were obtained at rest, during supine bicycle exercise stress testing, during dobutamine infusion at a heart rate matching the heart rate at peak exercise, during nitroprusside infusion, and after postextrasystolic potentiation. Tip-micromanometer left ventricular pressure recordings were also obtained in a normal control group at rest and during supine bicycle exercise stress testing to a heart rate, which matched the heart rate of the transplant recipient group at peak exercise. Left ventricular relaxation rate was measured by calculation of a time constant of left ventricular pressure decay (T) derived from an exponential curve fit to the digitized tip-micromanometer left ventricular pressure signal. In the transplant recipients, exercise abbreviated T and caused a rise of left ventricular minimum diastolic pressure. In normal control subjects, exercise induced a larger abbreviation of T and a smaller drop in left ventricular minimum diastolic pressure than was found in the transplant recipients. In the transplant recipients, the change in T from rest to exercise was variable, ranging from an abbreviation, as observed in normal controls, to a prolongation and was significantly correlated with the change in RR interval on the ECG and the change in left ventricular end-diastolic pressure. In a first subset of transplant recipients, dobutamine infusion resulted in a heart rate equal to the heart rate at peak exercise, a left ventricular end-diastolic pressure (lower than at peak exercise) and a T value, which was shorter than both the resting value and the value observed at peak exercise. In a second subset of transplant recipients, nitroprusside infusion and postextrasystolic potentiation resulted in a significant prolongation of T and a characteristic negative dP/dt upstroke pattern with downward convexity as previously observed in left ventricular hypertrophy. Exercise after cardiac transplantation resulted in a smaller acceleration Summary 207 of left ventricular relaxation than in a normal control group exercised to the same heart rate. These transplant recipients, who made the largest use of left ventricular preload reserve during exercise, showed least acceleration of left ventricular relaxation. This association between a rise of left ventricular end-diastolic pressure and slower left ventricular isovolumic relaxation was also evident in the individual transplant recipient from the slower isovolumic relaxation during exercise than during dobutamine infusion despite equal heart rates. After postextrasystolic potentation during nitroprusside infusion, a slow left ventricular relaxation with downward convexity of the dP/dt signal was observed in the cardiac allograft. This finding suggests depressed function of the sarcoplasmic reticulum in left ventricular myocardium after transplantation, which could be related either to decreased adrenergic tone or to preceding ischemic injury during organ retrieval or to hypertrophy caused by cyclosporine induced arterial hypertension. Chapter 3 of this thesis discusses diastolic left ventricular dysfunction, characterized by a lack of rightward shift of the left ventricular diastolic pressure-volume relationship. In chapter 3.1 the functional significance of a modified NOS gene expression for left ventricular (LV) contractile performance was investigated in patients with dilated nonischemic cardiomyopathy. Patients with heart failure have modified myocardial expression of nitric oxide synthase (NOS), as is evident from induction of calcium-insensitive NOS isoforms. In patients with dilated, nonischemic cardiomyopathy, invasive measures of LV contractile performance were derived from LV microtip pressure recordings and angiograms and correlated with intensity of gene expression of inducible (NOS2) and constitutive (NOS3) NOS isoforms in simultaneously procured LV endomyocardial biopsies. LV endomyocardial expression of NOS2 was linearly correlated with LV stroke volume, LV ejection fraction, and LV stroke work. In patients with elevated LV end-diastolic pressure, a closer correlation was observed between endomyocardial expression of NOS2 and LV stroke volume, LV ejection fraction, and LV stroke work. LV endomyocardial expression of NOS3 was linearly correlated with LV stroke volume and LV stroke work. To establish the role of nitric oxide (NO) as a mediator of the observed correlations, substance P (which causes endothelial release of NO) was infused intracoronarily. In patients with elevated LV end-diastolic pressure, an intracoronary infusion of substance P increased LV stroke volume and LV stroke work and shifted the LV end-diastolic pressurevolume relation to the right. It is concluded, that in patients with dilated cardiomyopathy, an increase in endomyocardial NOS2 or NOS3 gene expression augments LV stroke volume and LV stroke work because of a NO-mediated rightward shift of the diastolic LV pressure-volume relation and a concomitant increase in LV preload reserve. In Chapter 3.2, because nitric oxide (NO) reduces diastolic LV stiffness, diastolic LV stiffness and LV systolic performance are related to intensity of endomyocardial NO synthase (NOS) gene expression in dilated cardiomyopathy and in athlete's heart. In dilated cardiomyopathy and in athlete's heart, progressive LV dilatation is accompanied by rightward displacement of the diastolic LV pressure-volume relation. In dilated cardiomyopathy, an increase in diastolic LV Chapter 6 208 stiffness can limit this rightward displacement, thereby decreasing LV systolic performance. Microtip LV pressures, conductance-catheter or angiographic LV volumes, echocardiographic LV wall thicknesses and snap-frozen LV endomyocardial biopsies were obtained in 33 patients with dilated cardiomyopathy and in three professional cyclists referred for sustained ventricular tachycardia. Intensity of LV endomyocardial inducible NOS (NOS2) and constitutive NOS (NOS3) gene expression was determined using quantitative reverse transcription-polymerase chain reaction (RT-PCR). Dilated cardiomyopathy patients with higher diastolic LV stiffnessmodulus and lower LV stroke work had lower NOS2 and NOS3 gene expression at any given level of LV end-diastolic wall stress. The intensity of NOS2 and NOS3 gene expression observed in athlete's heart was similar to dilated cardiomyopathy with low LV diastolic stiffness-modulus and preserved LV stroke work. High LV endomyocardial NOS gene expression is observed in athlete's heart and in dilated cardiomyopathy with low diastolic LV stiffness and preserved LV stroke work. Favorable effects on the hemodynamic phenotype of high LV endomyocardial NOS gene expression could result from a NO-mediated decrease in diastolic LV stiffness and a concomitant rise in LV preload reserve. In Chapter 3.3 findings from recent experimental and clinical research are covered, which solved some of the controversies with respect to the myocardial contractile effects of NO. These controversies were: (1) does NO exert a contractile effect at baseline? (2) Is NO a positive or a negative inotrope? (3) Are the contractile effects of NO similar when NO is derived from NOdonors or from the different isoforms of NO synthases (NOS)? (4) Does NO exert the same effects in hypertrophied, failing or ischemic myocardium? Transgenic mice with cardioselective overexpression of NOS revealed NO to produce a small reduction in basal developed LV pressure and a LV relaxation-hastening effect mainly through myofilamentary desensitization. Similar findings had previously been reported during intracoronary infusions of NO-donors in isolated rodent hearts and in humans. The LV relaxation hastening effect was accompanied by increased diastolic LV distensibility, which augmented LV preload reserve, especially in heart failure patients. This beneficial effect on diastolic LV function always overrode the small NO-induced attenuation in LV developed pressure in terms of overall LV performance. In most experimental and clinical conditions, contractile effects of NO were similar when NO was derived from NOdonors or produced by the different isoforms of NOS. Because expression of inducible NOS (NOS2) is frequently accompanied by elevated oxidative stress, NO produced by NOS2 can lead to peroxynitrite-induced contractile impairment as observed in ischemic or septic myocardium. Finally, shifts in isoforms or in concentrations of myofilaments can affect NO-mediated myofilamentary desensitization and alter the myocardial contractile effects of NO in hypertrophied or failing myocardium. Chapter 4 of this thesis discusses diastolic left ventricular dysfunction, characterized by a steeper slope of the left ventricular diastolic pressure-volume relationship. The purpose of the study reported in chapter 4.1 was to investigate interactions between myocardial nitric oxide synthase (NOS) and myocardial fibrosis, both of which determine left ventricular (LV) preload reserve in patients with nonischemic dilated cardiomyopathy. In previous animal experiments, chronic inhibition of NOS induced myocardial fibrosis and limited Summary 209 LV preload reserve. Twenty-eight dilated cardiomyopathy patients underwent LV catheterization, balloon caval occlusions, intracoronary substance P infusion, and procurement of LV endomyocardial biopsies for determinations of collagen volume fraction, of gene expression of NOS2, NOS3, heme oxygenase, and TNF-alpha, and of NOS2 protein. Collagen volume fraction was unrelated to the intensity of NOS2, NOS3, heme oxygenase, or TNF-alpha gene expression or of NOS2 protein expression. Preload recruitable LV stroke work correlated directly with NOS2 gene expression and inversely with collagen volume fraction. High collagen volume fraction (>10%) reduced baseline LV stroke work and preload recruitable LV stroke work at each level of NOS2 gene expression. In dilated cardiomyopathy , myocardial fibrosis is unrelated to the intensity of myocardial gene expression of NOS, antioxidative enzymes (heme oxygenase), or cytokines (TNF-alpha) and blunts NOS2-related recruitment of LV preload reserve. Chapter 4.2 reports on heart failure patients, in which beneficial effects of NO on diastolic LV function always overrides a small NO-induced attenuation of LV developed pressure in terms of overall hemodynamic status, either at baseline or following ß-adrenergic stimulation. The absence of hemodynamic deterioration in transgenic mice over expressing either myocardial NOS2 or NOS3 confirms these clinical observations. In failing myocardium, NO’s correction of diastolic LV dysfunction reinforces NO’s energy sparing effects and the concerted action of NO on both diastolic LV dysfunction and deranged energetics could well be instrumental for preventing relentless deterioration of failing myocardium. Another beneficial effect of high endomyocardial NO activity on diastolic LV distensibility of the cardiomyopathic heart could result not only from NO-induced phosphorylation of troponin I and a concomitant reduction of diastolic crossbridge cycling but also from prevention of endomyocardial fibrosis. Chronic inhibition of NO synthesis has indeed been demonstrated to induce progressive myocardial fibrosis through a signaling cascade involving endothelin, angiotensin II, aldosterone and transforming growth factor ??. Chapter 5 discusses left ventricular dysfunction in relation to different changes in the left ventricular pressure-volume relationships. New, mainly noninvasive observations on diastolic LV dysfunction are confronted in this chapter with the old framework of diastolic LV pressurevolume relations to confirm their validity or to clarify their cause. Some questions recently emerged: 1. Is diastolic LV dysfunction always secondary to systolic LV dysfunction ? 2. Can transient elevations in LV loading induce diastolic LV dysfunction ? 3. Can diastolic LV dysfunction result from heightened active diastolic cardiac muscle tone?Visser, C.A. [Promotor]Paulus, W.J. [Copromotor

Stretch‐Induced Increase in Cardiac Contractility Is Independent of Myocyte Ca\u3csup\u3e2+\u3c/sup\u3e While Block of Stretch Channels by Streptomycin Improves Contractility After Ischemic Stunning

Stretching the cardiac left ventricle (LV) enhances contractility but its effect on myoplasmic [Ca2+] is controversial. We measured LV pressure (LVP) and [Ca2+] as a function of intra-LV stretch in guinea pig intact hearts before and after 15 min global stunning ± perfusion with streptomycin (STM), a stretch activated channel blocker. LV wall [Ca2+] was measured by indo-1 fluorescence and LVP by a saline-filled latex balloon inflated in 50 μL steps to stretch the LV. We implemented a mathematical model to interpret crossbridge dynamics and myofilament Ca2+ responsiveness from the instantaneous relationship between [Ca2+] and LVP ± stretching. We found that: (1) stretch enhanced LVP but not [Ca2+] before and after stunning in either control (CON) and STM groups, (2) after stunning [Ca2+] increased in both groups although higher in STM versus CON (56% vs. 39%), (3) STM-enhanced LVP after stunning compared to CON (98% vs. 76% of prestunning values), and (4) stretch-induced effects on LVP were independent of [Ca2+] before or after stunning in both groups. Mathematical modeling suggested: (1) cooperativity in cross-bridge kinetics and myofilament Ca2+ handling is reduced after stunning in the unstretched heart, (2) stunning results in depressed myofilament Ca2+ sensitivity in the presence of attached cross-bridges regardless of stretch, and (3) the initial mechanism responsible for increased contractility during stretch may be enhanced formation of cross-bridges. Thus stretch-induced enhancement of contractility is not due to increased [Ca2+], whereas enhanced contractility after stunning in STM versus CON hearts results from improved Ca2+ handling and/or enhanced actinomyosin cross-bridge cycling

A study of factors determining the crossbridge kinetics, work and power of cardiac muscle analysed with sinusoidal oscillation and other techniques

The pumping action of the heart is the driving force of the cardiovascular system that supplies all the organs and tissue with blood. The mechanical basis behind generation of sufficient pressure for the task lies in the ability of the cardiac cell to shorten. Shortening of the cardiac cells is accomplished by the interaction of two sets of proteins divided up into thick and thin filaments. Together these protein filaments form the backbone of the contractile protein matrix. The interaction between the myosin, of the thick filament, and actin, of the thin filament, also known as crossbridge cycling, is the basic process that can generate the force that is needed for the myocytes to shorten against a certain load. Regulation of the kinetics of crossbridge cycling is therefore a crucial element in myocardial contraction. This thesis focuses on studying how changes in muscle pH and [Pi], as well as variations in certain muscle protein isoforms, affect crossbridge kinetics. These studies were done using sinusoidal oscillation and other methods on rat and mouse cardiac muscle. Ischaemia of cardiac muscle is seen in a high percentage of heart failure cases. It is known that during acute ischaemic episodes, the intracellular pH within the cardiac muscle can drop as low as 6.2 and the intracellular [Pi] rises from a value of 1-3 mM to 20 mM or more. (In the present work, the intracellular space of the cardiac cells was brought under direct control by chemically skinning the rat myocardial muscle preparations to destroy the membranes.) The experiments in this thesis examined the effects on force and the frequency-dependence of dynamic stiffness over a range of pH and [Pi] when rapid, small length changes are imposed on the rat cardiac muscle trabeculae using a sinusoidal oscillation method. A primary finding of this study was that, at maximal Ca2+ activation, stiffness (at all frequencies) reduced progressively as pH was lowered, in agreement with previous studies. (Abstract shortened by ProQuest.)

Thick filament regulation of myocardial contraction

The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file.Vita."August 2006"Thesis (Ph. D.) University of Missouri-Columbia 2006.The ability of the heart to function as a pump is governed by mechanisms intrinsic to individual cardiac myocytes. The experiments in this dissertation were designed to examine the effects of sarcomere length and thick filament protein isoform expression on the contractile properties of single skinned cardiac myocytes. Myosin binding protein-C ablation (MyBP-C-\-) increases the rate of force development, loaded shortening velocity, and power output in mouse skinned cardiac myocytes, implying that MyBP-C regulates myocardial contractility by limiting crossbridge cycling. We also examined the effects of SL on mechanical properties in rat skinned cardiac myocytes containing either [alpha]-MyHC or [beta]-MyHC. Peak absolute and normalized loaded shortening velocity and power output was decreased at short SL in both [alpha]-MyHC and [beta]-MyHC myocytes. Matching myocyte force between long and short SL, however, sped loaded shortening velocity and increased power output in [alpha]-MyHC myocytes to values greater than at long SL, but this did not occur in [beta]-MyHC. Matching myocyte width between long and short SL sped loaded shortening velocity and increased power output to values greater than at long SL in both [alpha]-MyHC and [beta]-MyHC myocytes. It is concluded that there is an increase in crossbridge cycling at short SL as compared to long SL, but increased lattice spacing at short SL decreases actomyosin interactions. The data are presented in terms of a model whereby shortening SL induces a conformational change in MyBP-C that removes its constraint on the myosin heads, allowing them to cycle faster.Includes bibliographical reference

Modeling hypertrophic cardiomyopathy: Mechanistic insights and pharmacological intervention

Hypertrophic cardiomyopathy (HCM) is a prevalent and complex cardiovascular disease where cardiac dysfunction often associates with mutations in sarcomeric genes. Various models based on tissue explants, isolated cardiomyocytes, skinned myofibrils, and purified actin/myosin preparations have uncovered disease hallmarks, enabling the development of putative therapeutics, with some reaching clinical trials. Newly developed human pluripotent stem cell (hPSC)-based models could be complementary by overcoming some of the inconsistencies of earlier systems, whilst challenging and/or clarifying previous findings. In this article we compare recent progress in unveiling multiple HCM mechanisms in different models, highlighting similarities and discrepancies. We explore how insight is facilitating the design of new HCM therapeutics, including those that regulate metabolism, contraction and heart rhythm, providing a future perspective for treatment of HCM

Synergisitic role of ADP and Ca2+ in diastolic myocardial stiffness

Heart failure (HF) with diastolic dysfunction has been attributed to increased myocardial stiffness that limits proper filling of the ventricle. Altered cross-bridge interaction may significantly contribute to high diastolic stiffness, but this has not been shown thus far. Cross-bridge interactions are dependent on cytosolic [Ca2+] and the regeneration of ATP from ADP. Depletion of myocardial energy reserve is a hallmark of HF leading to ADP accumulation and disturbed Ca2+-handling. Here, we investigated if ADP elevation in concert with increased diastolic [Ca2+] promotes diastolic cross-bridge formation and force generation and thereby increases diastolic stiffness. ADP dose-dependently increased force production in the absence of Ca2+ in membrane-permeabilized cardiomyocytes from human hearts. Moreover, physiological levels of ADP increased actomyosin force generation in the presence of Ca2+ both in human and rat membrane-permeabilized cardiomyocytes. Diastolic stress measured at physiological lattice spacing and 37°C in the presence of pathologicallevels of ADP and diastolic [Ca2+] revealed a 76±1% contribution of cross-bridge interaction to total diastolic stress in rat membrane-permeabilized cardiomyocytes. Inhibition of creatine kinase (CK), which increases cytosolic ADP, in enzyme-isolated intact rat cardiomyocytes impaired diastolic re-lengthening associated with diastolic Ca2+- overload. In isolated Langendorff-perfused rat hearts, CK-inhibition increased ventricular stiffness only in the presence of diastolic [Ca2+]. We propose that elevations of intracellular ADP in specific types of cardiac disease, including those where myocardial energy reserve is limited, contribute to diastolic dysfunction by recruiting cross-bridges even at low Ca2+ and thereby increase myocardial stiffness

MULTISCALE MODELING OF CARDIAC GROWTH AND BAROREFLEX CONTROL

The heart functions within a complex system that adapts its function to any alteration in loading via several mechanisms. For example, the baroreflex is a short-term feedback loop that modulates the heart\u27s function on a beat-to-beat basis to control arterial pressure. On the other hand, cardiac growth is a long-term adaptive response that occurs over weeks or months in response to changes in left ventricular loading. Understanding the mechanisms that drive ventricular growth and biological remodeling is critical to improving patient care. Multiscale models of the cardiovascular system have emerged as effective tools for investigating G&R, offering the ability to evaluate the effects of molecular-level mechanisms on organ-level function. This dissertation presents MyoFE, a multiscale computer model that simulates the left ventricle (LV) pumping blood around a systemic circulation by bridging from molecular to organ-level mechanisms. The model integrates a baroreflex control of arterial pressure using feedback to regulate heart rate, intracellular Ca2+ dynamics, the molecular-level function of both the thick and thin myofilaments, and vascular tone. MyoFE is extended via a growth algorithm to simulate both concentric growth (wall thickening / thinning) and eccentric growth (chamber dilation / constriction). Specifically, concentric growth is controlled by the time-averaged total stress over the cardiac cycle, while eccentric growth responds to time-averaged intracellular myofiber passive stress. Our integrated model replicated clinical measures of left ventricular growth in two types of valvular diseases - aortic stenosis and mitral regurgitation - at two different levels of severity for each case. Furthermore, our results showed that incorporating the effects of baroreflex control of arterial pressure in simulations of left ventricular growth not only led to more realistic hemodynamics, but also impacted the magnitude of growth. Specifically, our results highlighted the role of regulating venous compliance (vasoconstriction) by the baroreflex immediately after the onset of valvular diseases, which has a significant role on the extent of LV growth in the long term

Alterations in myofilament properties in a rabbit coronary artery ligation model of left ventricular dysfunction

The work reported in this thesis examines alterations in the properties of the myofilaments in a rabbit coronary artery ligation model of heart failure. Heart failure is characterised by abnormalities of myocardial systolic and diastolic function. While much research in this field focuses on alterations in Ca+2 handling by the sarcoplasmic reticulum, fewer studies have purely examined alterations in the properties of the myofilaments, particularly so in this model of heart failure. This particular model has clinical relevance in that it produces a well defined region of infarcted tissue which induces ventricular remodelling in a similar way to that seen after infarction in the human. Treatment of ventricular trabeculae with the non-ionic detergent Triton X-100, results in the complete disruption of surface and intracellular membrane diffusion barriers, but leaves the myofilaments functionally intact. After this chemical 'skinning', the bathing solution essentially becomes an extension of the intracellular environment. Therefore, chemical interventions that produce an alteration in the mechanical activity of the preparation can be directly attributed to an effect on the myofilaments. The technique of applying small sinusoidal length changes to chemically-skinned trabeculae is ideal for examining both resting and dynamic properties of the myocardium as it allows both mechanical and chemical perturbations to be applied and studied simultaneously. Chapter 3 reports upon the alterations in the contractile activity of the myofilaments found in this model of heart failure using the technique of sinusoidal analysis. The results presented show that there is a significant decrease in the intrinsic cycling rate of the crossbridges (as indicated by a reduction in fmin, the frequency at which dynamic stiffness is lowest), without a decrease in maximal force generation or Ca+2-sensitivity of the muscle. Chapter 4 reports the alterations in the maximal work and power generating capacity of the myocardium found to be associated with heart failure using the sinusoidal analysis method. Chapter 5 reports the alterations in the relaxation properties of the myocardium in this model of heart failure using an 'EGTA'-jump protocol. Chapter 6 investigates the effects of the hypochlorite anion, a reactive oxygen species, on the mechanical functioning of the myocardium. The reason for investigating the effects of this compound is that increased oxidative stress is one detrimental factor to which the myocardium is subjected during the progression of heart failure. The oxidative stress results from an increase in the production of oxygen-derived free radicals and reactive oxygen species and/or a decrease in the antioxidant capacity of the myocardium. All the parameters mentioned so far were re-examined in tissues acutely exposed to the hypochlorite anion. The results show that exposure to the hypochlorite anion significantly reduced maximum Ca+2-activated force, fmin, positive and negative work/power generation and the relaxation rate of both sham and ligated trabeculae, whereas myocardial resting and dynamic stiffness was increased. The contractile proteins of the sham animals demonstrated an increased susceptibility to oxidant damage for all of these parameters just described. The relationship between the frequency of maximum positive and negative work/power generation and fmin was unchanged after hypochlorite anion exposure, though the frequencies at which all of these parameters were observed occurred at a lower frequency than pre-exposure. On examination of the results presented in Chapter 6 for the control animals, with the exception of a decrease in maximal force generation, the alterations in mechanical functioning that occur after free radical exposure closely resemble the changes seen in the mechanical functioning between sham and ligated animals described in the three preceding results chapters. The results presented in this chapter are consistent with the idea that pre-exposure to this reactive oxygen species has occurred in the intact myocardium in vivo in the ligated animals used in this study. (Abstract shortened by ProQuest.)

Effects of Protein Kinase C Phosphorylation of Cardiac Troponin I:An Experimental and Model-Based Study

This project was aimed at further elucidating the role of protein kinase C (PKC)-induced phosphorylation of troponin-I (cTnI) in cardiac contraction. We created a new transgenic mouse model (TG-E), expressing a mutant cardiac TnI constitutively pseudo-phosphorylated at the three PKC phosphorylation sites (S43, S45, T144 mutated to glutamate). 2D-DIGE (Difference in Gel Electrophoresis) gels indicated 7.2 ± 0.5% replacement, with no change in baseline level of actual phosphorylation of cTnI or other myofilamental proteins. Experiments were conducted in perfused isolated mouse hearts, isolated papillary muscles, and skinned fiber preparations. The mechanical measurements were complemented by biochemical and molecular biological measurements, and a mathematical model-based analysis for integrative interpretation. Compared to wild-type mice, TG-E mice exhibited negative inotropy in in vivo echocardiographic studies (9% decrease in fractional shortening), isolated hearts (14% decrease in peak developed pressure), papillary muscles (53% decrease in maximum developed force), and skinned fibers (14% decrease in maximally activated force, Fmax). Additionally, TG-E mice exhibited slowed relaxation in echocardiographic studies, isolated hearts and intact papillary muscles. The TG-E mice showed no differences in calcium sensitivity, cooperativity, steady-state force-ATPase relationship, and calcium transient (amplitude and relaxation). The four-state model of cardiac contraction was used for a model-based analysis of the data. The model was verified as a priori globally identifiable using a differential algebraic approach. The model-based analysis revealed that experimental observations in TG-E mice could be reproduced by two simultaneous perturbations: a decrease in the rate of crossbridge formation and an increase in calcium-independent persistence of the myofilament active state. In summary, a modest increase in PKC-induced cTnI phosphorylation can significantly regulate cardiac muscle contraction: (1) negative inotropy via decreased crossbridge formation and (2) negative lusitropy via persistence of myofilament active state. Based on our data and data from the literature we speculate that the effects of PKC-mediated cTnI phosphorylation are site-specific (S43/S45 vs. T144)