Spatiotemporal Characterization of Intracellular Ca²⁺ Rise during the Acrosome Reaction of Mammalian Spermatozoa Induced by Zona Pellucida

Authors: Shirakawa H.¹; Miyazaki S.^{1, 2}

Source: Developmental Biology, Volume 208, Number 1, April 1999 , pp. 70-78(9)

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Abstract:

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 sim 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. Copyright 1999 Academic Press.

Language: English

Document Type: Research article

Affiliations: 1: Department of Physiology, Tokyo Women's Medical University School of Medicine, 8-1 Kawada-cho, Shinjuku-ku, Tokyo, 162-8666, Japan 2: Department of Molecular Physiology, National Institute of Physiological Sciences, Okazaki, 444-8585, Japan

Publication date: 1999-04-01