Physiological roles of relaxin in prefertilizing activities of spermatozoa

Relaxin was first introduced in 1926 by Frederick Hisaw. Previously, it was considered only having a role in pregnancy of mammals due to its important roles in pregnancy and parturition. In the last decade, the physiological role of relaxin in male reproduction has received attention, and it has become clear that relaxin can no longer be considered strictly as only a hormone of female reproduction. The accessory glands (especially the prostate gland) of the male reproductive system are the source of seminal relaxin, which is secreted into the seminal plasma and saturates the spermatozoa at the time of ejaculation. Several studies have reported that relaxin has important roles in improving motility in human spermatozoa. Investigations into the role of relaxin in other physiological sperm phenomena such as capacitation, acrosome reaction, and their mediating factors associated with successful fertilization have intensified. This review aims to provide up-to-date information about the physiological roles of relaxin in sperm motility, capacitation, acrosome reaction, and their mediating factors. Some studies demonstrated that relaxin increased the total motility and progressive motility. Several studies showed that relaxin not only increased sperm motility but also increased the rate of sperm capacitation and acrosome reaction. Though few studies revealed that relaxin improved the sperm prefertilizing activities through increasing the utilization of glucose and mediating the cholesterol efflux, Ca2+-influx, intracellular cAMP and protein tyrosine phosphorylation. Thus, the review concludes that the supplementation of relaxin into capacitating medium may have a beneficial role in prefertilizing activities of fresh and cryopreserved spermatozoa.