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620 Chaves MEA, Araújo AR, Piancastelli ACC, Pinotti M Optical therapeutic window Water Hemoglobin Oxyhemoglobin Melanin Wavelength (nm) FIGURE 1: Optical therapeutic window achieve significant effects in cells, the use of low power light in animals and humans involves almost exclusively light in red and near infrared wave- lengths.81 Historical issues, mainly cost and availabili- ty may be related to this fact. As noted in Tables 1, 2 and 3 studies applied LASER or LED with doses around 0.09 to 90 J/cm2. The most significant biological effects were seen with predominant dose values, i.e. up to 5 J/cm2, which are within the Arndt-Schultz curve (Figure 2). According to Sommer et al, very low doses do not promote bio- logical effects, while higher doses result in inhibition of cellular functions.82 The energetic state of the cell, i.e. the physiological condition of the tissue in treatment is therefore critical to determine which dose to use. The mechanism of light action on the cellular level that supports its biological effects is based on photobiological reactions. A photobiological reaction involves the absorption of a specific wavelength of light by photoreceptor molecules.83 There is evidence that wavelengths in the spec- tral range from red to near infrared are absorbed by cytochrome c oxidase.83,84 In the study by Karu and Kolyakov action spectra of monochromatic light from Source: Schubert EF, et al. 2003. FIGURE 2: Arndt-Schultz Curve 580 to 860 nm were analyzed.85 The authors noted four active spectral regions, two in the red range (peaks from 613.5 to 623.5 nm and 667.5 to 683.7 nm) and two infrared (peaks from 750.7 to 772, 3 nm and 812.5 to 846.0 nm). In addition, they also observed the absorp- tion by cytochrome c oxidase in these four bands. The authors concluded that cytochrome c oxidase could absorb light in different spectral bands (red and near infrared), probably in the binuclear centers CuA and CuB (oxidized forms). Photobiological reactions can be classified into primary and secondary. Primary reactions derive from the interaction between photons and the pho- toreceptor, and they are observed in a few seconds or minutes after the irradiation of light. On the other hand, secondary reactions are effects that occur in response to primary reactions, in hours or even days after the irradiation procedure.84,86 The primary reactions of light action on pho- toreceptors are not yet clearly established, but there are some hypotheses. After the absorption of light in the irradiated wavelength, cytochrome c oxidase dis- plays an electronically excited status, from which it alters its redox status and causes the acceleration of electron transfer in the respiratory chain.87 Another hypothesis is that a part of the electronically excited status energy is converted into heat, causing a local- ized and transient heating in photoreceptors.88 A third assumption would be that when enabling the flow of electrons in the respiratory chain by light irradiation, an increase in the production of superoxide anion can be expected.89 A fourth reaction formula assumes that porphyrins and flavoproteins absorb photons and generate reactive species of singlet oxygen.90 It has also been proposed that light can reverse cytochrome c oxidase inhibition through nitric oxide and thereby increase the rate of respiration.91 The mechanism of secondary photobiological reactions is determined by transduction (energy trans- fer from one system to another) and photosignal amplification leading to photoresponse. This means that effects derived from primary reactions are ampli- fied and transmitted to other parts of the cell, result- ing in physiological effects such as alterations in cell membrane permeability with changes in intracellular calcium levels, increased cellular metabolism, DNA and RNA syntheses, fibroblast proliferation, activa- tion of T lymphocytes, macrophages and mast cells, increased synthesis of endorphins and decreased bradykinin.83 Secondary reactions are responsible for the con- nection between response to light action by photore- ceptors located inside the mitochondria and the effects located in the nucleus or different phenomena in other cell components. This process makes it possi- Source: Schindl A, et al. 2000. Cellular stimulation Cellular Inhibition Dose J/cm2 An Bras Dermatol. 2014;89(4):616-23. Relative absorbancePDF Image | Effects of low-power light therapy on wound healing
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