Acrosomal alkalization triggers Ca2+ release and acrosome reaction in mammalian spermatozoa

The sperm acrosome reaction (AR), an essential event for mammalian fertilization, involves Ca 2+ permeability changes leading to exocytosis of the acrosomal vesicle. The acrosome, an intracellular Ca 2+ store whose luminal pH is acidic, contains hydrolytic enzymes. It is known that acrosomal pH (pH acr ) increases during capacitation and this correlates with spontaneous AR. Some AR inducers increase intracellular Ca 2+ concentration ([Ca 2+ ] i ) through Ca 2+ release from internal stores, mainly the acrosome. Catsper, a sperm specific Ca 2+ channel, has been suggested to participate in the AR. Curiously, Mibefradil and NNC55‐0396, two CatSper blockers, themselves elevate [Ca 2+ ] i by unknown mechanisms. Here we show that these compounds, as other weak bases, can elevate pH acr , trigger Ca 2+ release from the acrosome, and induce the AR in both mouse and human sperm. To our surprise, μM concentrations of NNC55‐0396 induced AR even in nominally Ca 2+ free media. Our findings suggest that alkalization of the acrosome is critical step for Ca 2+ release from the acrosome that leads to the acrosome reaction.