Fundamental Investigation on Kinetics of Low-Intensity Laser Therapy: A Hypothesis of Localized Tempeature Elevation in Cells

Abstract

We have investigated fundamental action kinetics of low-intensity laser irradiation to living tissue as therapeutic procedure. Various kinds of laser therapies using low-intensity irradiation were reported. Some of them had been received wide recognition and become to be clinical therapy. However fundamental kinetics of therapeutic mechanism was not well-understood. The kinetics should be extremely complex due to stimulation effect, that is, the effect is extremely large comparing to irradiation light dose. In the other hand, there is a possibility of simple kinetics because low-intensity light reveals wide spectrum on therapies.
The temperature elevation of tissue and/or cell during low-intensity irradiation is small enough to deny contribution of thermal kinetics on therapeutic effect. Mitochondria are the major light absorber except blood erythrocyte and melanophore in the living tissue against visible light. Mitochondria are major energy production organella and its function is sensitive in temperature due to complex enzyme reactions. We therefore have one hypothesis of which light absorptions of mitochondria induce localized temperature elevation even tissue temperature elevation is small enough. We measured absorption spectrum of extracted rat liver mitochondria in vitro. We assumed adiabatic condition surrounding a mitochondrion to estimate the maximum temperature elevation. We calculated sufficient temperature elevation to change mitochondria function by the irradiation condition of normal low-intensity laser therapy. Our hypothesis is not denied by this results as a part of the kinetics, however further experiment is necessary to give precise conclusion. Since this hypothesis can not explain the wavelength dependence of our series of low-intensity laser effects, the different kinetics was suggested for different wavelength.