No abstract

The mammalian sperm acrosome reaction (AR), an essential fertilization event, requires an influx of Ca2+. The Ca2+ increase occurring in the human sperm head during the AR initiated by progesterone, a putative in vivo AR initiator, was investigated using video-image analysis with fura-2, a fluorescent Ca2+ probe. Progesterone treatment of capacitated human sperm resulted in a wave-like increase in sperm head cytosolic [Ca2+]i that appears to increase fastest in a region near the equatorial segment and then spreads throughout the rest of the head. The progesterone-mediated Ca2+ increase in the sperm head was strongly inhibited and the wave eliminated by picrotoxin, a blocker of GABAA receptor/Cl- channels and an inhibitor of the progesterone-mediated Cl- efflux and progesterone-initiated AR of human sperm. These results are the first to detect a ligand-mediated Ca2+ wave in sperm and to suggest that Cl- efflux influences Ca2+ influx during the AR.

Previous studies have established that the mammalian sperm acrosome reaction (AR) is dependent upon an influx of extracellular Ca2+, but the involvement of a mobilizable store of intracellular Ca2+ has not been shown. In many other cells, the endoplasmic reticulum is the site of such a Ca(2+)-store. Here, we show that thapsigargin, a highly specific inhibitor of the endoplasmic reticulum Ca(2+)-ATPase Ca(2+)-pump (and thus a mobilizer of intracellular Ca2+) in other cells, can initiate the AR in capacitated human sperm. Thapsigargin at concentrations from 50-500 nM significantly increased the AR to the same extent when incubated with capacitated sperm for 1 min (assayed by indirect immunofluorescence). Transmission electron microscopy confirmed the occurrence of normal morphology in the AR initiated by thapsigargin. Thapsigargin (200 nM) did not initiate the AR in noncapacitated sperm. Initiation of the AR by thapsigargin apparently requires an influx of Ca2+ since 1 min preincubation with the calcium channel blockers La3+ (250 microM) or Ni2+ (250 microM) prior to addition of thapsigargin completely inhibits AR-initiation. Mobilization of an intracellular Ca(2+)-store by thapsigargin in capacitated human sperm may lead to an influx of extracellular Ca2+ and subsequently the AR. Putative sites for thapsigargin-sensitive intracellular Ca(2+)-stores in human sperm include the cytoplasmic droplet, the sperm nucleus and the acrosome.

No abstract

Induction of the acrosome reaction in boar sperm by the zona pellucida (ZP) was investigated. A modified cytochemical staining method for measuring the acrosome reaction in boar sperm gave equivalent results to those obtained with transmission electron microscopy. Isolated heat-solubilized ZP effectively induced the acrosome reaction in boar sperm at a concentration of 25 micrograms/ml. Electrophoretically purified ZP components were also tested for acrosome reaction-inducing activity; both the 55,000 and 90,000 components of the ZP were effective. The carbohydrate moiety of the 55,000 component was necessary for activity because the polypeptides derived by chemical deglycosylation of the two glycoproteins did not induce the acrosome reaction.