Increasing intracellular pH of sea urchin sperm with NH4Cl induces Ca2+ uptake and acrosome reaction in the absence of egg jelly

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Abstract
Intracellular alkalinization and Ca2+ uptake accompany the egg jelly‐induced acrosome reaction of sea urchin sperm. Here, sperm were incubated in NF4Cl‐containing seawater maintained at its physiological pH (8.0). Under these conditions, 20 mM NH4C1 induced intracellular alkalinization, Ca2+ uptake and acrosome reaction in the absence of egg jelly. The NH4Cl‐triggered acrosome reaction required external Ca2+. Our results indicate that internal alkalinization activates Ca2+ influx into sperm possibly through a pH sensitive Ca2+ channel. Thus egg jelly could first stimulate the sperm plasma membrane Na+/H+ exchanger inducing intracellular alkalinization which would then activate Ca2+ uptake and acrosome reactions.