Contraction-relaxation Coupling Research Articles

Functional defects in hiPSCs-derived cardiomyocytes from patients with a PLEKHM2-mutation associated with dilated cardiomyopathy and left ventricular non-compaction

Dilated cardiomyopathy (DCM) is a primary myocardial disease, leading to heart failure and excessive risk of sudden cardiac death with rather poorly understood pathophysiology. In 2015, Parvari's group identified a recessive mutation in the autophagy regulator, PLEKHM2 gene, in a family with severe recessive DCM and left ventricular non-compaction (LVNC). Fibroblasts isolated from these patients exhibited abnormal subcellular distribution of endosomes, Golgi apparatus, lysosomes and had impaired autophagy flux. To better understand the effect of mutated PLEKHM2 on cardiac tissue, we generated and characterized induced pluripotent stem cells-derived cardiomyocytes (iPSC-CMs) from two patients and a healthy control from the same family. The patient iPSC-CMs showed low expression levels of genes encoding for contractile functional proteins (α and β-myosin heavy chains and 2v and 2a-myosin light chains), structural proteins integral to heart contraction (Troponin C, T and I) and proteins participating in Ca2+ pumping action (SERCA2 and Calsequestrin 2) compared to their levels in control iPSC-derived CMs. Furthermore, the sarcomeres of the patient iPSC-CMs were less oriented and aligned compared to control cells and generated slowly beating foci with lower intracellular calcium amplitude and abnormal calcium transient kinetics, measured by IonOptix system and MuscleMotion software. Autophagy in patient’s iPSC-CMs was impaired as determined from a decrease in the accumulation of autophagosomes in response to chloroquine and rapamycin treatment, compared to control iPSC-CMs. Impairment in autophagy together with the deficiency in the expression of NKX2.5, MHC, MLC, Troponins and CASQ2 genes, which are related to contraction-relaxation coupling and intracellular Ca2+ signaling, may contribute to the defective function of the patient CMs and possibly affect cell maturation and cardiac failure with time.

Contraction-relaxation coupling is unaltered by heart failure status in human myocardium

According to the Bowditch effect, increased cardiac contraction frequency leads to stronger contraction. To maintain end-organ perfusion, for each change in contractile rate, a compensatory parallel change in relaxation rate is necessary Heart failure (HF) was once thought to be primarily a disease of contractility, but it has become clear that it presents with relaxation dysfunction and insufficient ventricular filling before contractile pathology in more than half of HF patients. Whether contraction and relaxation rates are coupled in human myocardium is not well understood.

Feasibility of the contraction-relaxation coupling index in outcome prediction for patients with acute heart failure.

AimsContemporary heart failure (HF) classification based on left ventricular (LV) ejection fraction is limited for comprehensive assessment of LV function. We aimed to validate the feasibility of the contraction–relaxation coupling index (CRC) as a novel predictor for clinical outcomes in patients with acute HF.Methods and resultsA total of 3266 consecutive patients (median age: 74 years, 53% male) with acute HF were included. CRC was defined as the ratio of end‐diastolic elastance (LV end‐diastolic pressure/stroke volume) to end‐systolic elastance (LV end‐systolic pressure/end‐systolic volume). The risk for 1 year composite endpoint of all‐cause mortality or hospitalization for HF (primary outcome) was compared after group categorization using CRC tertiles (Tertile 1: CRC ≤ 0.17, Tertile 2: 0.17 < CRC ≤ 0.40, and Tertile 3: 0.40 < CRC). The median CRC was 0.3 and the median LVEF was 42%. After adjustment for clinical and echocardiographic covariates, CRC was an independent predictor for the primary outcome (hazard ratio [HR]: 1.74, 95% confidence interval [CI]: 1.47–2.07 in Tertile 3 and HR: 1.21, 95% CI: 1.02–1.44 in Tertile 2 when compared with Tertile 1; HR: 1.23, 95% CI: 1.14–1.33 per one‐standard deviation increment in CRC). The risk model with CRC showed better performance in outcome discrimination than the model with LVEF (c‐statistic 0.701 vs. 0.699, P for difference <0.001). Patients with higher CRC demonstrated better effectiveness of neurohormonal blockade for the primary outcome compared with those with lower CRC (HR: 0.38, 95% CI: 0.29–0.50 in Tertile 3 and HR: 0.67, 95% CI: 0.52–0.89 in Tertile 1).ConclusionsCRC provides an independent value for outcome prediction in patients with acute HF. CRC would be a sensitive indicator for prognostic risk stratification and for predicting treatment response to the neurohormonal blockade.

Abstract 14330: Contraction-Relaxation Coupling is Unaltered by Heart Failure Status in Human Myocardium

The two main phases of the mammalian cardiac cycle are contraction and relaxation; however, whether these phases are linked in human myocardium is not well understood. Prior investigations have shown that the structural and biochemical myocardial remodeling secondary to exercise training and/or myocardial infarction does not disrupt contraction-relaxation (CR) coupling in the canine model system. In addition, in a wide variety of murine transgenic models targeting EC coupling, the coupling between contraction and relaxation was unaltered. Given the vast differences between inbred animal models and the human population, we investigated whether contraction and relaxation are also coupled in human myocardium, and if so, whether that coupling is affected by heart failure disease status. We retrospectively analyzed data from contractility measurements taken under conditions closely mimicking those in vivo and calculated the correlation between the maximal speed of contraction and the maximal speed of relaxation in human ventricular multicellular preparations. We categorized our data based on heart failure status as follows: non-heart failure, heart failure with ischemic cardiomyopathy, or heart failure with non-ischemic cardiomyopathy. In every case isolated right ventricular trabeculae and/or left ventricular trabeculae (n=53 subjects LV and RV analyzed, 38 subjects only LV, 27 subjects only RV) were isolated from freshly explanted human hearts. Multicellular preparations were then electrically paced at different muscle lengths, frequencies, and with increasing β-adrenoceptor stimulation. In all conditions, contraction and relaxation were very tightly linearly correlated regardless of disease category (typical R 2 of &gt;0.98). While the mechanism governing this coupling is unknown, based on data form this study and past studies, we posit that CR-coupling is a fundamental myocardial property that resides in the structural arrangement of proteins at the level of the sarcomere at the basis of cross-bridge cycling.

Contraction-relaxation coupling is unaltered by exercise training and infarction in isolated canine myocardium.

The two main phases of the mammalian cardiac cycle are contraction and relaxation; however, whether there is a connection between them in humans is not well understood. Routine exercise has been shown to improve cardiac function, morphology, and molecular signatures. Likewise, the acute and chronic changes that occur in the heart in response to injury, disease, and stress are well characterized, albeit not fully understood. In this study, we investigated how exercise and myocardial injury affect contraction-relaxation coupling. We retrospectively analyzed the correlation between the maximal speed of contraction and the maximal speed of relaxation of canine myocardium after receiving surgically induced myocardial infarction, followed by either sedentary recovery or exercise training for 10-12 wk. We used isolated right ventricular trabeculae, which were electrically paced at different lengths, frequencies, and with increasing β-adrenoceptor stimulation. In all conditions, contraction and relaxation were linearly correlated, irrespective of injury or training history. Based on these results and the available literature, we posit that contraction-relaxation coupling is a fundamental myocardial property that resides in the structural arrangement of proteins at the level of the sarcomere and that this may be regulated by the actions of cardiac myosin binding protein C (cMyBP-C) on actin and myosin.

Rapid Transitions between the OFF and ON States of Myosin Contribute to Contraction-Relaxation Coupling in Cardiac Muscle

Rapid Transitions between the OFF and ON States of Myosin Contribute to Contraction-Relaxation Coupling in Cardiac Muscle

Abstract 614: Using Intact Trabeculae to Determine the Effect of Myosin-Modifying Drugs on Work, Power, and Mechanical Control of Relaxation

Background: Heart failure and especially heart failure with preserved ejection fraction are significant burdens on health. Intact cardiac trabeculae may reveal functional changes that cannot be seen in other ex vivo (skinned, single molecule) preparations. For example, using intact trabeculae, we have shown that relaxation is mechanically controlled by the lengthening strain rate at end systole, not afterload. Objective: We sought to evaluate the effect of myosin activator Omecamtiv Mercarbil (OM) and inhibitor Mavacamten (Mav) on physiologic function in intact trabeculae. Methods/Results: Afterload-clamp protocols were applied to intact cardiac trabeculae from Sprague Dawley rats to simulate physiologic work-loops and evaluate mechanical control of relaxation. Both OM and Mav reduced stroke work (force x length) by &gt;50% and power (force x velocity) by ~50% at doses reducing developed force by 50%. These were mediated by dose-dependent reductions in both force and shortening length. We have recently reported preliminary results that OM improves contraction-relaxation coupling and makes the relaxation rate more sensitive to strain rate in a dose-dependent manner. Mav does not lead to significant changes in contraction-relaxation coupling at any dose; Mav alters the sensitivity of relaxation rate to strain rate only at doses that reduce developed force by 50%. Summary/Perspective: Intact rat cardiac trabeculae reveal function mimicking physiology. OM and Mav show remarkable similarities in force, work, and power, which may be due to the high expression of alpha-myosin heavy chain. OM treatment not only modified contractility but increased sensitivity of the relaxation rate to strain rate in a dose dependent manner, which may explain why diastolic dysfunction is not more prevalent in clinical studies. Mav appears to only modify the attachment of crossbridges as expected.

Concomitant systolic and diastolic alterations during chronic hypertension in pig

The mechanical and cellular relationships between systole and diastole during left ventricular (LV) dysfunction remain to be established. LV contraction-relaxation coupling was examined during LV hypertrophy induced by chronic hypertension. Chronically instrumented pigs received angiotensin II infusion for4weeks to induce chronic hypertension (133 ± 7 mmHg vs 98 ± 5 mmHg for mean arterial pressure at Day 28 vs 0, respectively) and LV hypertrophy. LV function was investigated with the instrumentation and echocardiography for LV twist-untwist assessment before and after dobutamine infusion. The cellular mechanisms were investigated by exploring the intracellular Ca2+ handling. At Day 28, pigs exhibited LV hypertrophy with LV diastolic dysfunction (impaired LV isovolumic relaxation, increased LV end-diastolic pressure, decreased and delayed LV untwisting rate) and LV systolic dysfunction (impaired LV isovolumic contraction and twist) although LV ejection fraction was preserved. Isolated cardiomyocytes exhibited altered shortening and lengthening. Interestingly, contraction-relaxation coupling remained preserved both in vivo and in vitro during LV hypertrophy. LV systolic and diastolic dysfunctions were associated to post-translational remodeling and dysfunction of the type 2 cardiac ryanodine receptor/Ca2+ release channel (RyR2), i.e., PKA hyperphosphorylation of RyR2, depletion of calstabin 2 (FKBP12.6), RyR2 leak and hypersensitivity of RyR2 to cytosolic Ca2+ during both contraction and relaxation phases. In conclusion, LV contraction-relaxation coupling remained preserved during chronic hypertension despite LV systolic and diastolic dysfunctions. This implies that LV diastolic dysfunction is accompanied by LV systolic dysfunction. At the cellular level, this is linked to sarcoplasmic reticulum Ca2+ leak through PKA-mediated RyR2 hyperphosphorylation and depletion of its stabilizing partner.

Contraction and Relaxation Coupling Unaffected by Disease in Canine and Human Myocardium

In healthy rodent isolated ventricular myocardium, a strong positive correlation (R2&gt;0.95) has been observed between the rate of contraction versus rate of relaxation. Additionally, this correlation remains when muscles are stimulated to contract at different length, different frequency and various levels of beta‐adrenergic stimulation. However, there are drastic differences in EC coupling in small rodents versus larger mammals, including humans. Human and other large mammals, such as dogs, have a much wider range (~10 fold) of modulation of cardiac force and kinetics, thus it is unclear if a contraction‐relaxation coupling exists in larger mammals. In addition, it is unknown if such a relationship would be effected by disease. In this study, we investigated whether there is a correlation between maximal speed of contraction (dF/dt) and relaxation (‐dF/dt) in normal and exercise trained canines, with or without a myocardial infarction, as well as in normal human non‐failing, failing ischemic and failing non‐ischemic myocardium. Preliminary analysis on n=42 canines and N=54 humans suggests that there is a tight coupling between the maximal speed of contraction and relaxation in normal canine (R2=0.99) and non‐failing human myocardium (R2=0.99), with nearly identical slopes of this correlation. Additionally, within exercise or diseased hearts, there is no significant change with this correlation or slope of contraction‐relaxation coupling. We conclude that contraction‐relaxation coupling is a fundamental property that is unaffected by physiological contractile regulatory mechanisms.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Improvement of left ventricular filling by ivabradine during chronic hypertension: involvement of contraction-relaxation coupling.

Chronic hypertension is associated with left ventricular (LV) hypertrophy and LV diastolic dysfunction with impaired isovolumic relaxation and abnormal LV filling. Increased heart rate (HR) worsens these alterations. We investigated whether the I f channel blocker ivabradine exerts beneficial effects on LV filling dynamic. In this setting, we also evaluated the relationship between LV filling and isovolumic contraction as a consequence of contraction-relaxation coupling. Therefore, hypertension was induced by a continuous infusion of angiotensin II during 28 days in 10 chronically instrumented pigs. LV function was investigated after stopping angiotensin II infusion to offset the changes in loading conditions. In the normal heart, LV relaxation filling, LV early filling, LV peak early filling rate were positively correlated to HR. In contrast, these parameters were significantly reduced at day 28 vs. day 0 (18, 42, and 26 %, respectively) despite the increase in HR (108 ± 6 beats/min vs. 73 ± 2 beats/min, respectively). These abnormalities were corrected by acute administration of ivabradine (1 mg/kg, iv). Ivabradine still exerted these effects when HR was controlled at 150 beats/min by atrial pacing. Interestingly, LV relaxation filling, LV early filling and LV peak early filling were strongly correlated with both isovolumic contraction and relaxation. In conclusion, ivabradine improves LV filling during chronic hypertension. The mechanism involves LV contraction-relaxation coupling through normalization of isovolumic contraction and relaxation as well as HR-independent mechanisms.

Effects of Acute Respiratory and Metabolic Acidosis on Diaphragm Muscle Obtained from Rats

Acute respiratory acidosis is associated with alterations in diaphragm performance. The authors compared the effects of respiratory acidosis and metabolic acidosis in the rat diaphragm in vitro. Diaphragmatic strips were stimulated in vitro, and mechanical and energetic variables were measured, cross-bridge kinetics calculated, and the effects of fatigue evaluated. An extracellular pH of 7.00 was obtained by increasing carbon dioxide tension (from 25 to 104 mmHg) in the respiratory acidosis group (n = 12) or lowering bicarbonate concentration (from 24.5 to 5.5 mM) in the metabolic acidosis group (n = 12) and the results compared with a control group (n = 12, pH = 7.40) after 20-min exposure. Respiratory acidosis induced a significant decrease in maximum shortening velocity (-33%, P < 0.001), active isometric force (-36%, P < 0.001), and peak power output (-59%, P < 0.001), slowed relaxation, and decreased the number of cross-bridges (-35%, P < 0.001) but not the force per cross-bridge, and impaired recovery from fatigue. Respiratory acidosis impaired more relaxation than contraction, as shown by impairment in contraction-relaxation coupling under isotonic (-26%, P < 0.001) or isometric (-44%, P < 0.001) conditions. In contrast, no significant differences in diaphragmatic contraction, relaxation, or contraction-relaxation coupling were observed in the metabolic acidosis group. In rat diaphragm, acute (20 min) respiratory acidosis induced a marked decrease in the diaphragm contractility, which was not observed in metabolic acidosis.

Effect of caffeine on intrinsic mechanical properties of normal and malignant hyperthermia-susceptible muscle.

Malignant hyperthermia (MH) is a potentially lethal anesthesic complication. Pathological symptoms develop after exposure to triggering substances. It remains uncertain whether cellular alterations pre-exist. Mechanical properties of isolated muscle bundles were examined before and after exposure to a triggering substance. With prior written consent, muscle bundles of 12 MH-susceptible (MHS) and 56 MH-nonsusceptible (MHN) individuals were examined before and after exposure to incremental doses of caffeine. Mechanical properties (baseline tension, peak tension, time to peak tension, and relaxation time) were measured. Contraction and relaxation derivatives and contraction-relaxation coupling were calculated and analyzed. Mechanical properties were not different between the groups before caffeine application. Caffeine increased peak tension in both groups and baseline tension only in MHS muscle bundles; relaxation time/derivative and contraction-relaxation coupling were prolonged. Cellular changes seen in MH are not pre-existing. Exposure to triggering substance impairs relaxation in MHS muscle.

Ablation of Cardiac Myosin Binding Protein-C Accelerates Contractile Kinetics in the Absence of Hypertrophic Remodeling in Engineered Cardiac Tissue

Truncation mutations in cardiac myosin binding protein-C (cMyBP-C) are prevalent causes of hypertrophic cardiomyopathy (HCM). Mounting evidence suggest that truncated cMyBP-C causes HCM through haploinsufficiency. Mouse models in which cMyBP-C is ablated (KO), or truncated, show varying degrees of systolic and diastolic dysfunction and alterations in contractile kinetics. Phenotypic variation observed in these models may, however, be related to secondary adaptations and hypertrophic remodeling, rather than the direct effect of cMyBP-C depletion.In order to study the effects of cMyBP-C ablation on contractility in the absence of hypertrophic remodeling, we produced engineered cardiac tissue (ECT) from wild type (WT) and (KO) neonatal mouse cardiomyocytes.Expression levels of several hypertrophic markers were similar in WT and KO ECT, indicating a lack of hypertrophic remodeling. Contraction (50.0±1.1ms vs. 42.9±1.9ms; p=0.002) and relaxation (41.3±0.8ms vs. 29.2±1.7; p<0.001) times were significantly shorter in KO than WT ECT. This difference was more pronounced during relaxation, indicating a possible disconnect in contraction-relaxation coupling. In order to investigate this possibility, we studied the 1st order derivative (dFdt) of the force tracings. We found no significant difference in +dFdMax/-dFdtMax ratios (0.38±0.02 vs. 0.43±0.03; p=0.178), indicating that cMyBP-C ablation does not directly affect contraction-relaxation coupling. Elapsed times between +dFdtMax and peak developed force (FMax) (27.3±0.8ms vs. 21.7±1.5ms; p=0.002), as well as between FMax and -dFdtMax (36.5±1.6 vs. 18.4±2.1ms; p<0.001) was, however, significantly shorter.These findings indicate that cMyBP-C is involved in regulation of contractile velocity and that its ablation shortens the period of sustained force production. Our data indicates that previous reports of impaired relaxation and diastolic dysfunction due to cMyBP-C KO depletion in mice and humans may be caused by hypertrophic remodeling, rather than directly by cMyBP-C ablation/haploinsufficiency.

Comparative Therapeutic Effi cacy of Adenocin and Non-Glycoside Cardiotonic Drugs in Chronic Heart Failure at Rest and under Conditions of Heart Overload

Experiments were performed on the model of chronic heart failure. Functional capacity of myocardial structures under conditions of maximum pressure overload was within the upper limit of normal after treatment with Adenocin. The myocardial functional reserve and potential capacity index were shown to increase to normal under these conditions. Dobutamine, levosimendan, and milrinone increased functional capacity under conditions of maximum pressure overload. Treatment with adenocin restored diastolic function of the heart under conditions of maximum pressure overload. The end-diastolic pressure increased, but remained 1.7 times below the level observed in heart failure. After treatment with dobutamine and milrinone, the end-diastolic pressure (8th episode of ligation) did not differ from the level observed in heart failure, while after administration of levosimendan this parameter decreased by 31%. Contraction-relaxation coupling was completely restored under the influence of Adenocin in all episodes of ligation both before and after removal of the ligature. Nearly all animals with heart failure were resistant to 8 episodes of ligation after treatment with Adenocin (89 vs. 96% under normal conditions). Under these conditions, the survival rate of animals after administration of levosimendan, milrinone, and dobutamine was 65, 60, and 61%, respectively, (the mortality rate of animals with heart failure was 75%). Adenocin, a cardiotonic drug with cardioprotective properties, in contrast to other cardiotonic drugs, has a modulatory effect on the system of cell energy supply, restores myocardial reserves, and improves myocardial function under conditions of overload.

Myocardial contraction-relaxation coupling

Since the pioneering work of Henry Pickering Bowditch in the late 1800s to early 1900s, cardiac muscle contraction has remained an intensely studied topic for several reasons. The heart is located centrally in our body, and its pumping motion demands the attention of the observer. The contraction of the heart encompasses a complex interplay of mechanical, chemical, and electrical properties, and its function can thus be studied from any of these viewpoints. In addition, diseases of the heart are currently killing more people in the Westernized world than any other disease. When combined with the increasing emphasis of research to be clinically relevant, this contributes to the heart remaining a topic of continued basic and clinical investigation. Yet, there are significant aspects of cardiac muscle contraction that are still not well understood. A big complication of the study of cardiac muscle contraction is that there exists no equilibrium among many of the important governing parameters, which include pre- and afterload, intracellular ion concentrations, membrane potential, and velocity and direction of movement. Thus the classic approach of perturbing an equilibrium or a steady state to learn about the role of the perturbing factor in the system cannot be unambiguously interpreted, since each of the parameters that govern contraction are constantly changing, as well as constantly changing their interaction with each other. In this review, presented as the 54th Bowditch Lecture at Experimental Biology meeting in Anaheim in April 2010, I will revisit several governing factors of cardiac muscle relaxation by applying newly developed tools and protocols to isolated cardiac muscle tissue in which the dynamic interactions between the governing factors of contraction and relaxation can be studied.

Kinetics of cardiac muscle contraction and relaxation are linked and determined by properties of the cardiac sarcomere.

The regulation of myocardial contraction and relaxation kinetics is currently incompletely understood. When the amplitude of contraction is increased via the Frank-Starling mechanism, the kinetics of the contraction slow down, but when the amplitude of contraction is increased with either an increase in heart rate or via β-adrenergic stimulation, the kinetics speed up. It is also unknown how physiological mechanisms affect the kinetics of contraction versus those of relaxation. We investigated contraction-relaxation coupling in isolated trabeculae from the mouse and rat and stimulated them to contract at various temperatures, frequencies, preloads, and in the absence and presence of β-adrenergic stimulation. In each muscle at least 16 different conditions were assessed, and the correlation coefficient of the speed of contraction and relaxation was very close (generally >0.98). Moreover, in all but one of the analyzed murine strains, the ratio of the minimum rate of the derivative of force development (dF/dt) over maximum dF/dt was not significantly different. Only in trabeculae isolated from myosin-binding protein-C mutant mice was this ratio significantly lower (0.61 ± 0.07 vs. 0.84 ± 0.02 in 11 other strains of mice). Within each strain, this ratio was unaffected by modulation of length, frequency, or β-adrenergic stimulation. Rat trabeculae showed identical results; the balance between kinetics of contraction and relaxation was generally constant (0.85 ± 0.04). Because of the great variety in underlying excitation-contraction coupling in the assessed strains, we concluded that contraction-relation coupling is a property residing in the cardiac sarcomere.

Left Ventricular Contraction-Relaxation Coupling in Normal, Hypertrophic, and Failing Myocardium Quantified by Speckle-Tracking Global Strain and Strain Rate Imaging

The aim of this study was to noninvasively quantify global left ventricular (LV) contraction and relaxation, and to investigate their relationship in normal, hypertrophic, and failing myocardium. Fifty patients with hypertensive LV hypertrophy (LVH) (LVH group), 50 patients with dilated cardiomyopathy (DCM) (DCM group), and 50 normal subjects (control group) had echocardiographic evaluations. Global LV peak systolic strain (PSS) and peak relaxation rate (PRR) during early diastole were analyzed by speckle-tracking strain and strain rate imaging in the longitudinal and circumferential directions. Both global PSS and PRR were reduced in the LVH group in the longitudinal direction. In the circumferential direction, global PSS was maintained and global PRR was reduced in the LVH group. The reductions in both global PSS and PRR were more pronounced in both directions in the DCM group compared with the other 2 groups. Global PSS correlated strongest with global PRR among the clinical and echocardiographic variables, which exhibited the best fit with exponential regressions in both the longitudinal and circumferential directions in all subjects (longitudinal: y=0.15e(-0.10x), r2=0.75; circumferential: y=0.21e(-0.09x), r2=0.76, P<.01, respectively). Multiple regression analysis indicated that global PSS was the most powerful determinant of global PRR in both longitudinal and circumferential directions. Global LV function quantified using speckle-tracking echocardiography revealed strong coupling of LV contraction to relaxation sequentially from normal to failing myocardium, regardless of their heterogeneous pathophysiology. In addition, the extent of myocardial systolic shortening was the most powerful independent contributor of LV relaxation in both the longitudinal and circumferential directions. These results strongly indicate that LV myocardial systolic contraction directly regulates its relaxation.

Pressure Phase-plane Based Determination of the Onset of Left Ventricular Relaxation

Contraction-relaxation coupling is often characterized in terms of its effects on contraction or relaxation parameters, such as the time-constant of isovolumic relaxation (tau). While thermodynamics-based LV function characterization methods exist, landmark relaxation-onset determination studies used surgical methods. One classic, open-chest preparation study found that relaxation-onset occurs during early ejection, i.e. 34% of systolic time, TSYS, defined as the time from end-diastolic pressure to peak negative dP/dt. Because ventricular pumping is a steady state system, the laws of thermodynamics and nonlinear dynamics require that energy generation (during contraction) and energy utilization (during relaxation) must be balanced in a time-averaged (steady-state) sense. We calculated both energy generation and energy utilization, via novel pressure phase-plane (PPP) based parameters, including isovolumic stiffness analogs, in 29 subjects, 20 cardiac cycles per subject (580 beats). Results in control subjects show that relaxation-onset occurs near or prior to 34% of TSYS. In hearts with sever dysfunction including prolonged tau, relaxation-onset commences after 50% of TSYS (p<0.05). We conclude that PPP-based analysis can characterize relaxation-onset in vivo in thermodynamic and nonlinear dynamics terms without requiring an open-chest preparation, and may facilitate characterization of cellular mechanisms of relaxation-onset at the organ system level.

Contraction-relaxation coupling mechanism characterization in the thermodynamic phase plane: normal vs. impaired left ventricular ejection fraction

Using simultaneous pressure-volume measurements obtained during cardiac catheterization, we employ the thermodynamic phase-plane (TPP) method to characterize global contraction-relaxation coupling (CRC) between normal and impaired left ventricular (LV) ejection fraction (LVEF) groups. The cardiac cycle inscribes a closed loop in the TPP defined by the coordinates "potential" power [V(dP/dt), ergs/s] and "kinetic" power [P(dV/dt), ergs/s]. The TPP-derived indexes kappa and rho define the chamber's contractile and CRC attributes, respectively. Data from 33 subjects dichotomized as normal control (n = 22, >50% LVEF) and impaired LVEF (n = 11, <50% LVEF) were analyzed. The results were as follows: kappa = 3.0 +/- 1.1 and rho = -0.38 +/- 0.21 for controls and kappa = 5.4 +/- 1.6 and rho = -1.14 +/- 0.47 for the impaired LVEF group; kappa and rho are significantly higher for impaired LVEF than for control (P < 0.001 for both). As kappa increased, rho decreased (r = -0.69) for all subjects. Hence, ventricles with impaired LVEF are thermodynamically less efficient because they require more potential power per unit of delivered kinetic power than controls. We conclude that TPP-derived indexes of CRC facilitate assessment of chamber efficiency in thermodynamic terms and elucidate the dominant differentiating features in terms of CRC indexes.

Measurement of myofilament calcium sensitivity at physiological temperature in intact cardiac trabeculae

Cardiac contraction-relaxation coupling is determined by both the free intracellular calcium concentration ([Ca2+]i) and myofilament properties. We set out to develop a technique where we could assess these parameters (twitch and steady-state force [Ca2+]i) under near physiological conditions. Bis-fura-2 was iontophorically introduced into ultrathin rat trabeculae preparations to monitor the [Ca2+]i, and steady-state contractures were achieved by using a modified Krebs-Henseleit solution containing high K+. During K+ contractures, the very slow changes in [Ca2+]i and force development were in equilibrium and allowed for the construction of a steady-state, force-[Ca2+]i relationship. Twitch contractions before and after this myofilament calcium sensitivity assessment were unaltered, and this protocol could be repeated several times. For the first time, this novel protocol allows us to measure myofilament calcium sensitivity under physiological temperature. Not only do the data so obtained allow us to assess myofilament calcium sensitivity, the data also will allow us, in the same preparation under nearly identical conditions, to compare the dynamic to the steady-state, force-calcium relationship. To test whether the steady-state relationship between force and calcium in our novel protocol reproduces expected changes, we determined this relationship in the presence of isoproterenol and under acidosis and alkalosis. As expected, beta-adrenergic stimulation resulted in an increase of calcium amplitude and twitch force and a desensitization of the myofilaments as indicated by a rightward shift of the obtained steady-state, force-calcium relationship. An increase in pH shifted the curve leftward, whereas a decrease in pH resulted in the expected rightward shift.