We study collective phenomena in highly heterogeneous cardiac cell culture and its models. A cardiac culture is a mixture of passive (fibroblasts), oscillatory (pacemakers), and excitable (myocytes) cells. There is also heterogeneity within each type of cell as well. Results of in vitro experiments are modelled by Luo-Rudy and FitzHugh-Nagumo systems. For oscillatory and excitable media, we focus on the transitions from fully incoherent behavior to partially coherent behavior and then to global synchronization as the coupling strength is increased. These regimes are characterized qualitatively by spatiotemporal diagrams and quantitatively by profiles of dependence of individual frequencies on coupling. We find that synchronization clusters are determined by concentric and spiral waves. These waves arising due to the heterogeneity of medium push covered cells to oscillate in synchrony. We are also interested in the influence of passive and excitable elements on the oscillatory characteristics of low- and high-dimensional ensembles of cardiac cells. The mixture of initially silent excitable and passive cells shows the transitions to oscillatory behavior. In the media of oscillatory and passive or excitable cells, the effect of oscillation death is observed.

Synchronization of heterogeneous systems that consist of oscillatory and passive elements are studied in cardiac myocytes/fibroblasts co-cultures. It is found that beating clusters of cardiac myocytes surrounded by fibroblasts will be formed. The beatings of the cardiac myocyte clusters are not correlated at early times, but get synchronized as the cultures mature. This synchronization can be understood by a Kuramoto model with a time-increasing coupling strength. Our findings show that the growth of the coupling strength between clusters is linear, while the overall wave dynamics of the system is controlled by the passive fibroblast in the system which presumably is growing exponentially.

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Total ischemia of myocardium has been simulated on isolated hearts of rats. Effects of a low-power HeNe laser (λ=632.8 nm) and a fiber optic photo-luminescent radiation source of red light ( λ of the spectral peak is equal to 630 nm) on isolated heart contractile function characteristics and on lipid peroxidation (LPO) level in myocardium tissues have been investigated. Two groups of the specimens have been irradiated with red light during the postischemia (reperfusion) period of time. The first group has been treated with laser light and the second one with the luminescent radiation. More rapid restoration of the speed of contraction, of the force of contraction, of the relaxation speed and of the heart rate with respect to the data of the control group has been observed in both experimental groups. The similar tendency was observed in the laser treated specimens. The effects of fibrillation of myocardium of isolated hearts irradiated by lowpower He-Ne laser light were observed. These effects could be caused by the local light fluence rate excess in the interference pattern of laser light diffracted on the heart muscle structures.

The aim of the study was to assess the effect of mechanical right atrium distension of the isolated rat heart on the heart rate and heart rate variability, and the velocity of excitation wave propagation in the left ventricular myocardium using multielectrode mapping with flexible arrays. Materials and Methods. Experimental studies have been performed on the isolated rat heart in compliance with the Langendorff technique. Electrical heart activity was recorded using a flexible multielectrode array system. Results. Characteristic electrophysiological parameter changes of the isolated heart with the right atrium distension were detected using multielectrode mapping with flexible arrays. The flexible array design allowed registration of electrical potentials from the left ventricular surface of the actively contracting rat heart perfused according to the Langendorff technique and assessment of interconnection in the work of different parts of the heart: the right atrium in which the sinus node regulating the heart rate is located and the left ventricle. Application of multiple electrodes arranged in a specific way in the array made it possible to analyze spatio-temporal characteristics of electrical activity on the heart surface and to establish both the increase of the sinus node excitation frequency and excitation wave propagation velocity in the left ventricle. Conclusion. The growth of heart rate variability may suggest the existence of additional mechano-induced processes generating electrical instability in the distended atrium. The effects detected in the left ventricle with the given method may be caused by triggering intracardial regulation mechanisms.