Involvement of osmo-sensitive calcium influx in human sperm activation.

Mammalian spermatozoa must undergo capacitation and acrosome reaction before fertilization. To date, the precise mechanisms regulating these complex processes are not well understood but it is generally agreed that they involve an influx of calcium from the extracellular space through, as yet, poorly characterized plasma membrane pathways. Here we present evidence for a novel mechanism to increase intracellular calcium concentration via a calcium influx pathway activated by sperm cell swelling. Activation of this influx pathway by a mild hypo-osmotic shock and the ensuing calcium rise are a potent stimulus for sperm acrosome reaction. Furthermore, hypo-osmolarity-activated spermatozoa are fully competent for oocyte fertilization. During transit along male and, after ejaculation, female genital tracts spermatozoa are known to be exposed to extracellular fluids of widely different osmolarity; thus osmo-sensitive calcium influx could have a crucial regulatory role in the cellular events preceding fertilization.