Acrosome reaction inactivation in sea urchin sperm

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Acrosome reaction inactivation (ARI) is a process that renders sperm irreversibly refractory to the egg jelly (the natural inducer of the acrosome reaction, AR). This process triggered by the egg jelly, is associated with an increase in [Ca 2+] i. However, we show here that a rise in [Ca 2+] i alone is not sufficient to induce ARI, since artificially increasing [Ca 2+] i with either an ionophore or rising external pH, does not trigger ARI. Contrary to the AR which strictly requires Ca 2+, ARI can be triggered almost equally well by Sr 2+. On the other hand, Mn 2+ inhibits ARI and, as we showed earlier, does not affect AR. These observations indicate that the mechanisms involved in ARI differ from those leading to AR. In addition, we report here that high external pH (a non-physiological inducer of AR) triggers the AR in previously inactivated sperm by opening the same Ca 2+ channels activated by the egg jelly. Considering that the opening of Ca 2+ channels is one of the earliest responses triggered by the egg jelly and that ARI requires the egg jelly receptor to be activated, we have concluded that ARI involves the uncoupling between the egg jelly receptor and Ca 2+ channels. Furthermore, intracellular pH (pH i) determinations, in the presence or absence of ionomycin to substitute for the uncoupled Ca 2+ channels, indicate that pH i regulation is also impaired in inactivated sperm. In conclusion, ARI is a manifestation of the uncoupling of the egg jelly receptor from the different ion transport systems required for the acrosome reaction.