Capacitation (modifications required for gamete fusion) is produced by incubating guinea-pig spermatozoa in vitro in a chemically defined medium. It is shown that during such incubation a net uptake of Ca2+ by the sperm occurs in two distinguishable phases. An initial loose association of Ca2+, possibly to surface sites, is unaffected by agents (Mg2+, inhibitors of mitochondiral function) that prevent or delay the exocytotic spermatozoal acrosome reaction. The time course of a secondary Ca2+ uptake parallels or slightly precedes the time course of the acrosome reaction. This parallelism is maintained during a variety of treatments that either expedite (local anaesthetics, ionophore A23187, Triton X-100) or delay (Mg2+, low external Ca2+) the acrosome reaction. We conclude that the secondary Ca2+ influx described herein apparently serves to link alterations of the spermatozoal membrane to subsequent contractile and secretory components of the capacitation sequence.

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