The effect of oxidative and reductive stress on semen parameters and functions of physiologically normal human spermatozoa

Both oxidative stress (OS) and reductive stress (RS) are the two extreme facets of redox imbalance that can have deleterious effects on sperm function. However, there is a lack of information on the physiological range of oxidation-reduction potential (ORP). The aim of this study was to investigate the effect of OS and RS on functions and associated molecular changes in normal spermatozoa in order to establish the physiological range of ORP. In the current study, total and progressive motility remained unchanged in spermatozoa exposed to ORP values 0.33 and 0.72 mV/10 6 sperm/mL. However, a significant (P < 0.05) decline in total and progressive motility were observed at ORP values 1.48, 2.75, -11.24, -9.76 and -9.35 mV/10 6 sperm/mL. Sperm vitality also decreased significantly (P < 0.0001) at 2.75, -11.24 and -9.76 mV/10 6 sperm/mL. Spermatozoa exposed to ORP levels 2.75 and -11.24 mV/10 6 sperm/mL showed a significant (P < 0.01) decrease in mitochondrial membrane potential. Intracellular reactive oxygen species (iROS) production increased (P < 0.05) in sper- matozoa exposed to ORP levels of 1.48 and 2.75 mV/10 6 sperm/mL, while iROS decreased (P < 0.05) at ORP levels -9.76 and -11.24 mV/10 6 sperm/mL. No significant change in sperm DNA fragmentation was noted in sperm exposed to OS/RS and the values were below the reference range (< 19.25%). Western blot analysis revealed decreased expression of CV-ATPA, CIII-UQCRC2 and CIV-MTCO1 proteins at 60 and 120 min (P < 0.05) in both OS and RS conditions. This is the first study to report physiological range of ORP (between -9.76 and 1.48 mV/10 6 sperm/mL) and to elucidate the role of altered expression of oxidative phosphorylation (OXPHOS) complexes proteins in mediating detrimental effects of oxidative and reductive conditions on sperm functions. A decreased expression of OXPHOS proteins and associated mitochondrial dysfunction contributes to decreased sperm motility and vitality under oxidative and reductive stress.