Mechanisms of low-power laser light action on cellular level

The most frequently used mechanism of photon energy conversion in laser medicine is heating. Average heating of irradiated samples occurs with all methods of tissue destruction (cutting, vaporization, coagulation, ablation). At low light intensities the photochemical conversion of the energy absorbed by a photoacceptor prevails. This type of reaction is well known for specialized photoacceptors such as rhodopsin or chlorophyll. In medicine light absorption by non-specialized photoacceptor molecules (that is, molecules that can absorb light at certain wavelengths, but that are not integral to specialized light reception organs) is used rather extensively. The absorbing molecule can transfer the energy to another molecule, and this activated molecule can then cause chemical reactions in the surrounding tissue. This type of reaction is successfully used in photodynamic therapy (PDT) of tumors. Alternatively, the absorbing molecule in a light-activated form can take part in chemical reactions, as occurs in treatment of skin diseases with psoralens and UVA radiation (PUVA). Importantly, in both PDT and PUVA therapy the photoabsorbing molecules are artificially introduced into a tissue before irradiation. Irradiation of cells at certain wavelengths can also activate some of the native components. In this way specific biochemical reactions as well as whole cellular metabolism can be altered. This type of reaction is believed to form the basis for low-power laser effects [1-41. One should note that light therapy methods based on photochemical conversion of photoabsorbing molecules are not laser-specific methods [1]: conventional sources generating the appropriate wavelength can also be used (as is done in PUVA and UV therapy). Laser sources are just handy tools providing many practical advantages (e.g., efficient fiber optic coupling to irradiate interior body parts, high monochromaticity and easy wavelength tunability, simplicity in use and electrical safety in the case of semiconductor lasers). It was shown that coherent effects in the light-cells interaction occur at intensities 2x1015 W/m2 [1]. Bear in mind that typical intensities used in low-power laser therapy are in the range 1OlO2 W/m2. The successful use of light-emitting diodes in low-power laser therapy in the last years [5] proves that the coherence of light is of no importance in low-power laser clinical effects. However, one should not forget about coherent laser-light speckles which can cause local heating of inhomogeneous tissues [6]. This possible mechanism forms a basis of laser thermotherapy [7] and supposedly can be taken into account at certain laser light parameters in low-power laser therapy [8] when irradiating inhomogeneously absorbing tissues. For cell culture, the localized transient heating of absorbing chromophores is considered as one ofpossible primary mechanisms (Section 2.2). A photobiological reaction involves the absorption of a specific wavelength of light by the functioning photoreceptor (jhotoacceptor) molecule. To distinguish specialized photoreceptor molecules such as rhodopsin, phytochrome, bacteriorhodopsin and chlorophyll's from nonspecialized chromophores (molecules capable of absorbing the wavelength used for irradiation resulting in a photobiological response), below we will use the term photoacceptors to refer to the nonspecialized photoabsorbers. The photoacceptors take part in a metabolic reaction in a cell that is not connected with a light response. After absorbing the light of the wavelength used for irradiation this molecule assumes an electronically excited state from which primary molecular processes can lead to a measurable biological effect in certain circumstances. To work as a photoacceptor taking part in photobioregulation, this molecule must be part of a key structure that can regulate a metabolic pathway. Redox chains are example of this type of key structures which suit to these requirements. Possible photoacceptors (respiratory chain components) as well as chemical reactions occurring under illumination (primary reactions) or those following after the end of the irradiation (secondary reactions) are considered in Section 2: