Yuri Protsenko

Slow force response and auto-regulation of contractility in heterogeneous myocardium.

Classically, the slow force response (SFR) of myocardium refers to slowly developing changes in cardiac muscle contractility induced by external mechanical stimuli, e.g. sustained stretch. We present evidence for an intra-myocardial SFR (SFR(IM)), caused by the internal mechanical interactions of muscle segments in heterogeneous myocardium. Here we study isometric contractions of a pair of end-to-end connected functionally heterogeneous cardiac muscles (an in-series muscle duplex). Duplex elements can be either biological muscles (BM), virtual muscles (VM), or a hybrid combination of BM and VM. The VM implements an Ekaterinburg-Oxford mathematical model accounting for the ionic and myofilament mechanisms of excitation-contraction coupling in cardiomyocytes. SFR(IM) is expressed in gradual changes in the overall duplex force and in the individual contractility of each muscle, induced by cyclic auxotonic deformations of coupled muscles. The muscle that undergoes predominant cyclic shortening shows force enhancement upon return to its isometric state in isolation, whereas average cyclic lengthening may decrease the individual muscle contractility. The mechanical responses are accompanied with slow and opposite changes in the shape and duration of both the action potential and Ca²⁺ transient in the cardiomyocytes of interacting muscles. Using the mathematical model we found that the contractility changes in interacting muscles follow the alterations in the sarcoplasmic reticulum loading in cardiomyocytes which result from the length-dependent Ca²⁺ activation of myofilaments and intracellular mechano-electrical feedback. The SFR(IM) phenomena unravel an important mechanism of cardiac functional auto-regulation applicable to the heart in norm and pathology, especially to hearts with severe electrical and/or mechanical dyssynchrony.

Contribution of mechanical factors to arrhythmogenesis in calcium overloaded cardiomyocytes: Model predictions and experiments

It is well-known that Ca²⁺ overload in cardiomyocytes may underlie arrhythmias. However, the possible contribution of mechanical factors to rhythm disturbances in Ca²⁺ overloaded myocytes has not been sufficiently investigated. We used a mathematical model of the electrical and mechanical activity of cardiomyocytes to reveal an essential role of the mechanisms of cardiac mechano-electric feedback in arrhythmogenesis in Ca²⁺ overloaded myocardium. In the model, the following mechanical factors increased Ca²⁺ overload in contracting cardiomyocytes and promoted rhythm disturbances: i) a decrease in the mechanical load for afterloaded contractions; and ii) a decrease in the initial length of sarcomeres for isometric twitches. In exact accordance with the model predictions, in experiments on papillary muscles from the right ventricle of guinea pigs with Ca²⁺ overloaded cardiomyocytes (using 0.5-1 μM of ouabain), we found that emergence of rhythm disturbances and extrasystoles depends on the mechanical conditions of muscle contraction.

Sex differences in stretch-dependent effects on tension and Ca 2+ transient of rat trabeculae in monocrotaline pulmonary hypertension

We aim to compare the effects of stretch on isometric tension/Ca2+ transient in the right ventricular trabeculae of control (CONT) and hypertensive (MCT, monocrotaline application) adult male and female rats. The treatment with MCT resulted in RV hypertrophy in males only. Blunted active force-length relation and substantially prolonged twitch were found in MCT-males but not MCT-females (vs same-sex CONT). Ca2+ transient was prolonged in both MCT-treated groups but extremely so in the MCT-males. The gradual stretch resulted in a distinct “bump” on Ca2+ transient decline in CONT and MCT-treated groups. The integral magnitude of the “bump” was unaffected by the treatment with MCT in males or females but was larger in males vs females. The rate of “bump” development was significantly slower in MCT-males. In conclusion, the sex-specific differences in the stretch-dependent regulation of [Ca2+]i may underlie preservation of the Frank–Starling mechanism in female rat myocardium in monocrotaline-induced pulmonary hypertension.

The length-dependent activation of contraction is equally impaired in impuberal male and female rats in monocrotaline-induced right ventricular failure.

The length‐dependent activation of contraction is attenuated in the failing myocardium of adult male rats. This pathological change is not seen in adult female rats, possibly because of a protective effect of sex hormones. The present study evaluated length‐dependent changes in isometric twitch, Ca2+ transient (CaT) and action potential (AP) in the right ventricular myocardium of impuberal healthy male and female rats (control) and in rats treated with a single injection of 50 mg/kg monocrotaline (MCT). Compared with sex‐matched control rats, MCT‐treated male and female rats exhibited increased right ventricular weight (134% and 142% of control, respectively), decreased left ventricular weight (72% and 79%), twitch attenuation (48.8 ± 2.7% and 57.5 ± 1.2%) and prolongation (125 ± 3% and 127 ± 2%), CaT attenuation (37.8 ± 0.4% and 39.1 ± 1.1%) and prolongation (114 ± 1% and 116 ± 1%) and AP prolongation at 90% repolarization (195 ± 2% and 203 ± 1%). The MCT‐treated male rats exhibited a 50% lower integral magnitude and an approximately 25% larger time‐to‐peak ‘bump’ compared with control male rats. These parameters in MCT‐treated female rats tended to show similar changes to those seen in the control female rats, with no significant difference between the two groups. In all groups, integral magnitude and time‐to‐peak ‘bump’ increased with length. In conclusion, the length‐dependent activation of contraction was equally blunted in the failing right ventricular myocardium of impuberal male and female rats. This was related to changes in CaT and AP, which were similar between male and female rats. Therefore, puberty is necessary for manifestation of the protective effects of sex hormones on this remodelling.

Preload-induced changes in isometric tension and [Ca2+]i in rat myocardium

Preload-induced changes of active tension and [Ca2+]i are “dissociated” in mammalian myocardium. This study aimed to describe the distinct effects of preload at low and physiological [Ca2+]o. Rat RV papillary muscles were studied in isometric conditions at 25‡C and 0.33 Hz at 1 mM (hypo-Ca group) and 2.5 mM [Ca2+]o (normal-Ca group). [Ca2+]i was monitored with fura-2/AM. Increase of preload caused a rise of active tension in hypo-Ca and normal-Ca groups whereas peak fluorescence rose significantly only at low [Ca2+]o. End-diastolic tension, end-diastolic level of fluorescence, time-to-peak tension, but not time-to-peak of Ca2+ transient, progressively increased with preload. Mechanical relaxation decelerated with preload while Ca2+ transient decay time decreased in the initial phase and increased in the late phase, resulting in a prominent “bump” configuration. The “bump” was assessed as a ratio of its area to the fluorescence trace area. It was a new finding that the preload-induced rise of this ratio was twice as large in hypo-Ca. Our results indicate that preload-induced changes in active tension and [Ca2+]i are “dissociated” in rat myocardium, with relatively higher expression at low [Ca2+]o. Ca-dependence of Ca-TnC association/dissociation kinetics is thought to be a main contributor to these preload-induced effects.

Stress relaxation of heterogeneous myocardial tissue. Numerical experiments with 3D model

We present here the simple block model of a heterogeneous system describing the processes of stress relaxation in spatial and mechanically heterogeneous myocardial tissue. In numerical experiments by the model it is established that to obtain the same level of tension as in the case of a uniform model, less stiff and less viscous heterogeneous blocks are required. It is shown that in the model of heterogeneous myocardial tissue one observes not only stress relaxation but also strain relaxation—creep. Notably, step-by-step adjustment of the neighbor structural blocks in response to the whole model deformation takes place. Furthermore, stationary deformation properties of separated blocks become nonlinear.

Effects of subchronic lead intoxication of rats on the myocardium contractility

Outbred male rats were repeatedly injected IP with sub-lethal doses of lead acetate 3 times a week during 5 weeks. They developed an explicit, even if moderate, lead intoxication characterized by typical hematological and some other features. The next day after the last injection the heart of each animal was excised, and the trabecules and papillary muscles from the right ventricle were used for modeling in vitro isometric (with varying starting length of the preparation) regimes of the contraction-relaxation cycle with different preloads. Several well-established parameters of this model were found changed compared with the preparations taken from the hearts of healthy control rats. Background in vivo calcium treatment attenuated both systemic and cardiotoxic effects of lead to an extent. We show for the first time that subchronic intoxication with lead caused myocardial preparations in a wide range of lengths to respond by a decrease in the time and speed parameters of the isometric contraction while maintaining its amplitude and by a decrease in the passive stiffness of trabecules. The responses of the various heart structures are outlined, and the isomyosin ratio is shown to have shifted towards the slow isoform. Mechanistic and toxicological inferences from the results are discussed.

Quantitative analysis of connective tissue matrix structure during myocardial remodeling

A new approach is proposed to quantify the contribution of connective tissue matrix into the myocardial structure. Also we introduced a new morphometric criterion to estimate mean thickness of connective tissue fibers by histological section of isolated myocardial samples. The approach allows correlating structural and functional changes in cardiac tissue by myocardium remodeling during pathology, age-related changes.

Novel approach to studying relationship of 3D structure and mechanical properties of biological tissue

We present here a novel approach to studying the relationship of three-dimensional structural organization of myocardial tissue as an example of biological tissue and its mechanical properties. The approach consists in correlation of image series of the preparation internal structure obtained with a confocal laser scanning microscope LSM-710 and registration of tension-deformation properties of the preparation. The spatial structure images of the segment of rat right ventricular wall at different stretching levels have been obtained. It is shown that the proposed approach allows one to study in more detail the mechanisms of tension and strain development in living biological tissues.

Viscoelastic Models Describing Stress Relaxation and Creep in Soft Tissues

The review of our approach to the specific rheological properties modeling of myocardium has been given. We are working on the level of ‘fascicule’ as a tissue element, which consists of several cardiomyocytes surrounded by a connective tissue shell. Actually, these properties are characteristic of large majority of living soft tissues. In order to describe essentially nonlinear static ‘force-deformation’ curves and quasi-static hysteresis loops together with non-exponential stress relaxation and creep time courses we suggest a set of 2D graphs with different topology composed of classical linear Hooks springs and Newtonian damps. Each spring and dashpot represents a group of 3D tissue structural elements. The stress response functions of these models are found for the uniaxial step-wise, or pulse (column-like) external stretching. The inverse response functions of the longitudinal displacement for the external stress loading of the pulse shape time dependencies were also found. The values of elastic modules and viscous coefficients are estimated by comparison theoretical curves of relaxation, creep and recovery with the experimental data. The latter are obtained on rather different objects, passive muscle preparations (the stress relaxation response) and endothelium cells (the creep response). It has been stated that the proposed 2D models appear to be quite general to describe nonlinear relaxation and creep properties, which are lacking in the traditionally used uniaxial 1D models.