[Quantitative analysis of connective tissue matrix structure during myocardial remodeling].

A new approach is proposed to quantify a contribution of connective tissue matrix to the myocardial structure. Also we introduced a new morphometrical criterion to estimate mean thickness of connective tissue fibers by histological section of isolated myocardial samples. The approach allows of 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 the correlation of image series of the preparation internal structure obtained by confocal laser scanning microscope LSM-710 and the 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 provides the possibility to study the mechanisms of tension and strain development in living biological tissues in more detail.

[Viscoelastic properties of relaxed papillary muscle at physiological hypertrophy].

Viscoelastic properties of relaxed rat papillary muscles at physiological hypertrophy (intensive swimming for 5 weeks) have been obtained. It has been ascertained that viscoelastic properties of hypertrophied muscles are not significantly distinguished from those of control papillary muscles. A three-dimensional model of myocardial fascicle has been verified in compliance with experimental data of biomechanical tests of hypertrophied muscles. Elastic and viscous parameters of structural elements of the model negligibly differ from the parameters of the model of a control muscle. It is shown that physiological hypertrophy has a slight influence on viscoelastic properties of papillary muscles.

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

We present here a simple block model of heterogeneous system describing the processes of stress relaxation in spatial and mechanically heterogeneous myocardial tissue. In numerical experiments with this model it is established that in order to obtain the same level of tension as in the case with a uniform model the less stiff and less viscous heterogeneous blocks are required. It is shown that in the model of heterogeneous myocardial tissue not only stress relaxation but also strain relaxation -- creep is observed. 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.

[Modeling of steady-state and relaxation elastic properties of the papillary muscle at rest].

The biomechanical modeling of a papillary muscle preparation as an adequate object for studying the properties of the myocardial tissue under uniaxial stretching has been performed. The steady-state and relaxation tests of the papillary muscle of laboratory animals (rabbit and rat) have been conducted in normal conditions and after the maceration of intracellular structures with high ionic strength solution. It has been shown that the main contribution to the viscoelastic properties in the initial range of physiological deformations is made by the connective tissue skeleton, whereas under large physiological deformations, by intracellular structures.

[Mechanical properties of the passive myocardium: experiment and a mathematical model].

A three-dimensional mathematical model of the rheological properties of a morphofunctional unit of the myocardium has been constructed, which consists of transversal and longitudinal elastic elements and tilted viscoelastic elements hinged without friction. The parameters of the viscosity and elasticity of structural elements of the model do not depend on the deformation value. The model makes it possible to adequately describe the features of the viscoelastic behavior of isolated samples of the passive myocardium and of the myocardium under longitudinal strain. A good agreement between the calculated and experimental data for an intact preparation of the rat myocardium and a preparation with removed cardiomyocytes has been shown.

[Viscoelastic properties of isolated papillary muscle: contributions of connective tissue skeleton and intracellular matrix].

Peculiarities of viscoelastic behavior of rabbit papillary muscle in passive state are studied by transversal versus longitudinal deformation curves, stress-strain and hysteresis curves, and stress relaxation curves under ramp stretching. The papillary muscle was chosen because of mostly longitudinal orientation of fibers and its elongated shape, which both make it as an appropriate model for uniaxial tests. The problem of evaluation of connective tissue protein structures and intracellular matrix contribution into the properties under consideration is solved by using the maceration method to remove intracellular structures. The different contribution of intracellular and extracellular protein features into total properties of a papillary muscle leads to nonlinearity of myocardial viscoelastic properties, such as the increase of differential elastic module and relaxation time with deformation.