Requirement of monovalent cations in the acrosome reaction of guinea pig spermatozoa

The majority of the spermatozoa precapacitated in Ca2+‐free medium underwent the acrosome raction rapidly when they were transferred to Ca2+‐containing medium. The presence of Na+ and Ca2+ in the medium was essential for the acrosome reaction. The vast majority of spermatozoa failed to undergo the reaction in Ca2+ medium lacking monovalent ions, although they remained motile. At the concentration of 140 mM, Na+, K+, Rb+, and Cs+ all supported the reaction at the maximum level, but at 50 mM the latter three ions were not as effective as Na+. Li+ was least effective in supporting the reaction. Virtually no acrosome reactions took place when precapacitated spermatozoa were first exposed to Na+ medium (no Ca2+) and then to Ca2+ medium (no Na+). On the other hand, a considerably higher proportion of spermatozoa acrosome reacted when they were exposed to these media in the reverse order. The most efficient acrosome reactions took place when the medium contained both a monovalent ion (Na+) and Ca2+ simultaneously. Possible mechanisms by which monovalent and divalent cations participate in the acrosome reaction are discussed.