High pH-induced acrosome reaction and Ca 2+ uptake in sea urchin sperm suspended in Na +-free seawater

Abstract

The egg jelly-induced acrosome reaction of sea urchin sperm requires the presence of Ca 2+ and Na + in seawater at its normal pH 8. Sperm suspended in seawater at pH 9 undergo the acrosome reaction in the absence of jelly. We have attempted to understand the role of external Na + in this reaction. Sperm were suspended in Na +-free seawater and the percentage of acrosome reaction and the amount of Ca 2+ uptake were determined as a function of external pH. High pH (9.0) in Na +-free medium without jelly triggered a high percentage (above 65%) of sperm acrosome reactions and a two to fourfold increase in Ca 2+ uptake. Both the percentage of acrosome reactions and the amount of Ca 2+ uptake were similar to those induced by either jelly or pH 9 in Na +-containing seawater. On the other hand, the absence of Na + in seawater inhibits jelly from inducing Ca 2+ uptake and acrosome reactions at pH 8.0 and even at pH 8.5. These results indicate that the Na + requirement for the acrosome reaction induced by jelly is lost when triggering is by high pH. In contrast, Ca 2+ was strictly required since sperm did not react in Ca 2+-free seawater at pH 9. We also found that like the jelly-induced acrosome reaction the high-pH-induced acrosome reaction and Ca 2+ uptake in complete and Na +-free seawater were inhibited by D600. This finding suggests that the same transport system for Ca 2+ uptake associated with the acrosome reaction operates at both triggering conditions, i.e., jelly or pH 9. Although D600 is not now considered a specific blocker, its effect has suggested the involvement of Ca 2+ channels in the acrosome reaction. This proposal is supported by our results with nisoldipine, a highly specific inhibitor of calcium channels. The drug inhibited both the sperm acrosome reaction and Ca 2+ uptake induced by jelly or pH 9 in complete seawater.