Strontium supports capacitation and the acrosome reaction in mouse sperm and rapidly activates mouse eggs

Extracellular Ca2+ is required for capacitation and fertilization in the mouse, but very little is known about the ability of other divalent cations to substitute for Ca2+. In this study, Sr2+, Ba2+, and Mg2+ were evaluated for their ability to support capacitation, the acrosome reaction, hyperactivated motility, and fertilization. Ba2+ proved to be ineffective, but Mg2+-containing medium was able to support capacitation to a greater extent than unsupplemented Ca2+-deficient media; despite this, Ca2+ was required for fertilization. In contrast, Sr2+ proved capable of substituting for Ca2+ in all events. Furthermore, Sr2+-induced responses were indistinguishable from the corresponding Ca2+-induced ones: Sperm capacitated at the same rate and underwent the acrosome reaction to the same extent. However, demonstration of sperm:egg fusion in Sr2+ required the use of zona-free eggs. This was due not to the inability of the sperm to penetrate the zona but to the very rapid activation and cortical granule release by eggs in response to Sr2+. When zona-intact eggs were used, the block to polyspermy had been mounted by the time sperm had penetrated the zona. A 15 min exposure to Sr2+ was sufficient to block sperm fusion, but a longer exposure was required to ensure the resumption of meiosis in eggs; such a response was surprising in that the eggs were freshly ovulated and not susceptible to activation by many different treatments. Thus Sr2+ can profoundly affect both gametes in the mouse: It substitutes completely for Ca2+ in sperm responses and rapidly activates eggs, possibly by displacing Ca2+ from intracellular stores into the cytoplasm, where the Ca2+ can then trigger the various events of activation.