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

The sperm acrosome reaction (AR), an essential event for mammalian fertilization, involves Ca2+ permeability changes leading to exocytosis of the acrosomal vesicle. The acrosome, an intracellular Ca2+ 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 Ca2+ concentration ([Ca2+](i)) through Ca2+ release from internal stores, mainly the acrosome. Catsper, a sperm specific Ca2+ channel, has been suggested to participate in the AR. Curiously, Mibefradil and NNC55-0396, two CatSper blockers, themselves elevate [Ca2+](i) by unknown mechanisms. Here we show that these compounds, as other weak bases, can elevate pH(acr), trigger Ca2+ release from the acrosome, and induce the AR in both mouse and human sperm. To our surprise, M concentrations of NNC55-0396 induced AR even in nominally Ca2+ free media. Our findings suggest that alkalization of the acrosome is critical step for Ca2+ release from the acrosome that leads to the acrosome reaction.