Spatiotemporal characterization of intracellular Ca2+ rise during the acrosome reaction of mammalian spermatozoa induced by zona pellucida

The mammalian sperm acrosome reaction (AR) is an essential event prior to sperm-egg fusion at fertilization, and it is primarily dependent on an increase in intracellular Ca2+ concentration ([Ca2+](i)). Spatiotemporal aspects of the [Ca2+](i) increase during the AR induced by solubilized zona pellucida (ZP) in hamster spermatozoa were precisely investigated with a Ca2+ imaging technique using confocal laser scanning microscopy with two fluorescent Ca2+ indicators. A rapid rise in [Ca2+](i) occurred immediately after the application of ZP solution through a micropipette. The rise was always initiated in the sperm head, even when the application was directed toward the tail. The elevated [Ca2+](i) was little attenuated during measurement for 30-40 s. Acrosomal exocytosis was detected as a sudden decrease of fluorescence in the acrosomal vesicle similar to 20 s after the onset of the [Ca2+](i) rise. High-resolution imaging revealed that the [Ca2+](i) rise in the sperm head began at the region around the equatorial segment and spread over the posterior region of the head within 0.6 s, whereas Ca2+ concentration in the acrosomal vesicle appeared to be unaltered. The [Ca2+](i) rise was completely abolished under Ca2+-free extracellular conditions, indicating that it is totally attributable to Ca2+ influx. Nifedipine, an inhibitor of L-type Ca2+ channels, did not affect the rising phase of the ZP-induced Ca2+ response, but accelerated the decline of the [Ca2+](i) rise and inhibited acrosomal exocytosis. The present study provides implicative information about the spatial organization of functional molecules involved in the signal transduction in mammalian AR. (C) 1999 Academic Press.