Acrosome‐reacted guinea pig spermatozoa become fusion competent in the presence of extracellular potassium ions

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Abstract
Guinea pig spermatozoa are able to undergo capacitation and the acrosome reaction in a K+‐free (‐deficient) medium. However, they are unable to fuse with eggs unless they are exposed to a millimolar concentration of extracellular K+ during or after the acrosome reaction. Apparently, the plasma membrane over the equatorial segment gains the ability to fuse with eggs in the presence of K+ during and/or after the acrosome reaction. Once it becomes fusible, the membrane retains its fusibility even in a K+‐deficient medium. Rb+ is almost as effective as K+ in rendering the sperm membrane fusible. Li+ and Cs+ are less effective. The molecular mechanism by which K+ renders acrosome‐reacted spermatozoa fusion competent is unknown, but it may involve K+‐mediated efflux of H+ from the spermatozoa.