Low Energy Visible Light Induces Reactive Oxygen Species Generation and Stimulates an Increase of Intracellular Calcium Concentration in Cardiac Cells*

Low energy visible light (LEVL) irradiation has been shown to exert some beneficial effects on various cell cultures. For example, it increases the fertilizing capability of sperm cells, promotes cell proliferation, induces sprouting of neurons, and more. To learn about the mechanism of photobiostimulation, we studied the relationship between increased intracellular calcium ([Ca²⁺] _i) and reactive oxygen species production following LEVL illumination of cardiomyocytes. We found that visible light causes the production of ${\mathrm{O}}_2^{\overline{·}}$ and H₂O₂ and that exogenously added H₂O₂ (12 μm) can mimic the effect of LEVL (3.6 J/cm²) to induce a slow and transient increase in [Ca²⁺] _i. This [Ca²⁺] _i elevation can be reduced by verapamil, a voltage-dependent calcium channel inhibitor. The kinetics of [Ca²⁺] _i elevation and morphologic damage following light or addition of H₂O₂ were found to be dosedependent. For example, LEVL, 3.6 J/cm², which induced a transient increase in [Ca²⁺] _i , did not cause any cell damage, whereas visible light at 12 J/cm² induced a linear increase in [Ca²⁺] _i and damaged the cells. The linear increase in [Ca²⁺] _i resulting from high energy doses of light could be attenuated into a non-linear small rise in [Ca²⁺] _i by the presence of extracellular catalase during illumination. We suggest that the different kinetics of [Ca²⁺] _i elevation following various light irradiation or H₂O₂ treatment represents correspondingly different adaptation levels to oxidative stress. The adaptive response of the cells to LEVL represented by the transient increase in [Ca²⁺] _i can explain LEVL beneficial effects.