<title>Low-power visible light and hydrogen peroxide change intracellular calcium concentration in cardiac cells</title>

Abstract

Intracellular Ca concentration [Ca2 is ofparamount importance in the regulation of many cellular processes. The cytosolic-free calcium is buffered to approximately 100-150 nM Calcium oscillations arise either spontaneously'2 response to stimulation by extracellular signals such as hormones or neurotransmitters . Irradiation of endogenous or exogenous photosensitizers with visible or ulimviolet light is a way to cause an elevation in [Ca21, .This elevation was reported to be dependent on the concentration of photosensitizers, light dose and exiracellular cations . Prolonged mcubalion of the photosensitizer with the cell causes a persistent elevalion in [Ca211 following light irradiation, which is believed to be the early event in cell death in photodynamic therapy (PDT) .A short-time incubation with the photosensitizer followed by "°low-level light irradiation (LLL) causes only a iransient increase in the [Ca21,"'2 In previous studies we noticed that light at the same fluences which cause small [Ca?1 oscillation 13 induces acceleration in cell mitosis, while with higher doses of light cell proliferation was inhibited'4 j general, it seems that whereas high light intensity (over 20 JIcm2)causes irreversible damage to the cells, low level light (LLL) irradiation causes biostiinulation. Thus, there are numerous examples where LLL irradiation has been beneficial in dermatology, gynecology, otorhinolaryngology and other medical areas1549 In addition to the intensity dependent elevation of calcium in response to photostimulation, there is accumulating evidence concerning the generation of reactive oxygen species (ROS) in cell association with light absorbency2026 j the photohemolysis process of human eiythrocytes caused by PDT using bacteriocklorin a as sensitizer, generation of hydroxyl radical (.OH) and singlet oxygen ('02) was observed26 Singlet oxygen '02 was also found in Friend Erythroleukaemic cells after irradiation with HeNe laser without using photosensitizers 24 ROS are toxic to the cell 21 concenirations, it is believed that low concenirations have a positive biostimulalive effect ROS were repoited to stimulate cultured fibroblasts to proliferate 25 and were found to play an important role in the acquisition of fertilizing ability 24,27 Consequently, considerable attention has been paid to the generation and the role of ROS in cell development Consideiable attention was also given to the changes in [Ca21, in response to chemicals and irradiation. However there are not enough reports dealing with the relation between ROS production and [Ca211 changes, in particular the connection between ROS, [Q2J biostimnlation. We have compared free iniracellular 2+ concentration changes in cardiac myocytes after visible light irradiation and upon exposure to low concentration 11202. Hydrogen peroxide is relatively stable, has a higher oxithtion potential than 02 and, being uncharged, can therefore freely cross cell membranes. In addition, H202 in the presence of low concentmtion transition metal, present in almost any solution or in the cells, can produce another ROS, 'OH .Thus, H202 is a good candidate for studying the influence of ROS on cells in vitro. Intracellular 2+ homeostasis in cardiac myocytes is of great functional importance for diastolic resting state ofthe contractile elements, for excitation-contraction-coupling, and for the force of contraction modulated by variation in the magnitude of the transient [Ca211 .Asa result, any [Ca211 changes may influence the heartbeat The relation between ROS and 2+ homeostasis in heart cells is also Significant due to the role ofROS in myocardial ischemia-reperfusion injuiy 2932 These make cardiac cells an important model for the investigation of the LCa2I/ROS relation due to LLL stimulation