Activation of Ca2+ channels during the acrosome reaction of sea urchin sperm is inhibited by inhibitors of chymotrypsin-like proteases.

Probable participation of sperm protease in the acrosome reaction was investigated using several inhibitors and substrates. Among those examined, L-1-tosylamide-2-phenylethyl chloromethyl ketone (TPCK) and chymostatin, chymotrypsin inhibitors, p-nitrophenyl-p'-guanidinobenzoate (NPGB), a serine protease inhibitor, and N-benzoyl-L-tyrosine ethyl ester (BTEE), a chymotrypsin substrate, inhibited the egg jelly-induced acrosome reaction of Strongylocentrotus intermedius. TPCK and BTEE, however, did not inhibit the reaction caused by ionophores, A23187, or nigericin. To know the mechanism of inhibition by chymotrypsin inhibitors and substrates of the egg jelly-induced acrosome reaction, intracellular Ca2+ concentration [( Ca2+]i) and pH (pHi) were measured with fura-2 and 2',7'-bis (carboxy-ethyl)carboxyfluorescein (BCECF), respectively. Egg jelly caused increase of [Ca2+]i, which was depressed by BTEE. Egg jelly also caused a transient rise of pHi, which was not depressed by BTEE. In the presence of verapamil, the acrosome reaction by egg jelly was significantly inhibited concomitant with depressed increase of [Ca2+]i. The rise of pHi was not depressed by verapamil. Thus, modes of action of BTEE and of verapamil are similar to each other. Bringing these findings together, the authors present a view that a chymotrypsin-like protease of sea urchin sperm activates verapamil-sensitive Ca2+ channels, which take part in the acrosome reaction.