ORIGINAL ARTICLE GaAs 904-nm laser irradiation improves myofiber mass recovery during regeneration of skeletal muscle previously damaged by crotoxin Lucila H. Silva & Meiricris T. Silva & Rita M. Gutierrez & Talita C. Conte & Cláudio A. Toledo & Marcelo S. Aoki & Richard E. Liebano & Elen H. Miyabara Received: 7 March 2011 /Accepted: 10 November 2011 /Published online: 6 December 2011 # Springer-Verlag London Ltd 2011 Abstract This work investigated the effect of gallium arse- nide (GaAs) irradiation (power: 5 mW; intensity: 77.14 mW/cm 2 , spot: 0.07 cm 2 ) on regenerating skeletal muscles damaged by crotoxin (CTX). Male C57Bl6 mice were divided into six groups (n 0 5 each): control, treated only with laser at doses of 1.5 J or 3 J, CTX-injured and, CTX-injured and treated with laser at doses of 1.5 J or 3 J. The injured groups received a CTX injection into the tibialis anterior (TA) muscle. After 3 days, TA muscles were sub- mitted to GaAs irradiation at doses of 1.5 or 3 J (once a day, during 5 days) and were killed on the eighth day. Muscle histological sections were stained with hematoxylin and eosin (H&E) in order to determine the myofiber cross- sectional area (CSA), the previously injured muscle area (PIMA) and the area density of connective tissue. The gene expression of MyoD and myogenin was detected by real- time PCR. GaAs laser at a dose of 3 J, but not 1.5 J, significantly increased the CSA of regenerating myofibers and reduced the PIMA and the area density of intramuscular connective tissue of CTX-injured muscles. MyoD gene expression increased in the injured group treated with GaAs laser at a dose of 1.5 J. The CTX-injured, 3-J GaAs laser- treated, and the CTX-injured and treated with 3-J laser groups showed an increase in myogenin gene expression when compared to the control group. Our results suggest that GaAs laser treatment at a dose of 3 J improves skeletal muscle regeneration by accelerating the recovery of myo- fiber mass. Keywords Gallium arsenide . Low-level laser therapy . Skeletal muscle . Regeneration Introduction Skeletal muscles have an outstanding capacity to regenerate in response to injury. Such regeneration includes installation of an inflammatory process, proliferation and differentiation of satellite cells, remodeling of connective tissue, angiogen- esis, and functional recovery of injured muscles [1, 2]. Although the process of muscle regeneration is well known, some questions remain to be answered concerning the effects of physical agents frequently used in rehabilitation to improve muscle regeneration. In this context, low-power laser therapies have been used to accelerate the regenerative process of many tissues, such as skin [3, 4], bone [5, 6], peripheral and central nervous system [7, 8], and skeletal muscle [9–13]. In addition, the effects of this therapy in tissue regeneration essentially depend on the irradiation time and frequency, dose, and wavelength [9, 14]. Helium-neon (HeNe), gallium arsenide (GaAs), gallium- aluminum-arsenide (GaAlAs), and indium-gallium-aluminum- phosphide (InGaAlP) are the most commonly used low-level lasers in physical therapy treatments [15]. It has also been Lucila H. Silva and Meiricris T. Silva contributed equally to this work. L. H. Silva : M. T. Silva : T. C. Conte : E. H. Miyabara (*) Department of Anatomy, Biomedical Sciences Institute, University of São Paulo, Lineu Prestes Av., 2415, 05508-000 São Paulo, SP, Brazil e-mail: elenm@usp.br R. M. Gutierrez : C. A. Toledo : R. E. Liebano Neuroscience Research Nucleus, University of São Paulo City, São Paulo, SP, Brazil M. S. Aoki School of Arts, Sciences and Humanities, University of São Paulo, São Paulo, SP, Brazil Lasers Med Sci (2012) 27:993–1000 DOI 10.1007/s10103-011-1031-x