Fig. 1

Diagram illustrating four possible mechanisms of PBM therapy on human sperm. Photonic energy in low level light is absorbed by the enzyme cytochrome c oxidase (CcO) located in the mitochondrial respiratory chain. The activated enzyme leads to a proton gradient. Consequently, the levels of reactive oxygen species (ROS) and adenosine triphosphate (ATP) are increased. On the other hand, the application of low level light activates specific receptors of the opsin family coupled to G-proteins in sperm. Phosphatidylinositol 4,5-diphosphate (PIP₂) is catalyzed by hydrolysis to produce inositol 1,4,5-triphosphate (IP₃). All of these activities activate light-sensitive ion channels and increase the levels of calcium ions (Ca²⁺). Soluble adenylyl cyclase (sAC) is activated by Ca²⁺. The increased sAC activity activates 3’,5’-cyclic adenosine monophosphate (cAMP)/ protein kinase A (PKA) pathway, thereby promoting sperm motility. In addition, exposure to low level light induces nitric oxide (NO) production by nitric oxide synthase (NOS), which activates cyclic guanosine monophosphate (cGMP)-dependent protein kinases production from soluble guanylate cyclase (sGC). Sperm motility is promoted by activation of protein kinase G (PKG).