Hyperactivated Sperm Motility: Are Equine Sperm Different?

A recent study has suggested a central role for hyperactivated sperm motility in successful equine in vitro fertilization, as the highest apparent fertilization rates reported yet were obtained using exogenous stimulation of hyperactivated motility in sperm that had been incubated for capacitation. Hyperactivated motility has been studied intensively in other species, but little data are available in this area in the horse. Hyperactivated motility is associated with an influx of calcium from the extracellular space, and in other species this occurs because of an increase in intracellular pH during capacitation. Influx of calcium seems to trigger changes in flagellar function through calcium–calmodulin–calmodulin kinase-related effects on dynein function, but the exact mechanisms resulting in hyperactivated motility are still unclear. We have been exploring the physiology of hyperactivated motility in stallion spermatozoa and have found many of the mechanisms present in other species to be in place in the stallion. Further research is needed to determine why stallion spermatozoa seem to fail to undergo hyperactivation in response to capacitating environments that support this activity in other species.