Preloading of micromolar intracellular Ca2+ during capacitation of Sicyonia ingentis sperm, and the role of the pHi decrease during the acrosome reaction.

In studying the mechanism controlling the sperm acrosome reaction (AR) in the marine shrimp Sicyonia ingentis, intracellular Ca2+ and pH were measured using the fluorescent indicators Fura-2 and Fluo-3 for Ca2+, and SNARF-1 for pH. Capacitated sperm possessed an apparent resting Ca2+ concentration of 1-2 microM which remained constant upon induction of the AR with egg water. Uncapacitated sperm had extremely low Ca2+ levels and did not respond to egg water. These results suggest that, while in other species the Ca2+ is elevated to micromolar levels during initiation of the AR, S. ingentis sperm are preloaded with Ca2+ during capacitation and the trigger for the AR is downstream of the Ca2+ increase. The notion that Ca2+ influx is not involved at the actual time of the AR in capacitated S. ingentis sperm is supported by the inability of Ca2+ ionophore A23187 to induce the AR and the ineffectiveness of Ca2+ channel antagonists to block egg water-induced AR. Measurements of capacitated sperm pH showed a significant decrease during the first 10-15 min of the AR, which did not correlate temporally to either acrosomal exocytosis (at 5 min post-induction) or filament formation (after 45 min). Inhibition of egg protease activity required for induction of filament formation did not inhibit the pH drop, indicating that intracellular acidification is not the final trigger for filament formation, although it may be required prior to action of the protease.