INTRACELLULAR pH DECREASES DURING THE IN VITRO INDUCTION OF THE ACROSOME REACTION IN THE SPERM OF SICYONIA INGENTIS

¹ Department of Zoology, University of California, Davis, California 95616 and Bodega Marine Laboratory, Bodega Bay, California 94923

Activation of the sperm of many invertebrate and some vertebrate species to undergo an acrosome reaction is accompanied by an increase in intracellular pH (pH_i). In each of these instances the pH_i of the unactivated cell is relatively low (6.9-7.4). Unactivated sperm of the marine shrimp, Sicyonia ingentis, possess an elevated pH_i (8.5). Induction of the acrosome reaction (exocytosis of the acrosomal vesicle and generation of an acrosomal filament) is accompanied by a decrease in pH_i (7.8). Low external pH elicits acrosomal filament formation in sperm that have undergone acrosomal exocytosis, but does not induce exocytosis in unreacted sperm. The ionophore, nigericin, enhances the percent of sperm that form filaments in low pH seawater (pH < 8.0), but does not elicit filament formation at external pHs 8.0. Valinomycin induces filament formation in sperm that have undergone exocytosis over a wide range of external pHs (5.75-8.5). The ability of valinomycin to induce filament formation in the upper portion of this pH range (8.0) declines as the extracellular K⁺ concentration rises. These results demonstrate that the sperm of S. ingentis undergo a pH_i decrease as a result of the acrosome reaction and that the decrease is associated with acrosomal filament formation. In addition, they also suggest that an efflux of K⁺ ions is connected to the pH_i decrease.

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