| Abstract: | During fertilization in Limulus, the spermatozoa first attach to the egg and then undergo an acrosomal reaction. In this reaction, the acrosomal vesicle exocytoses, and a long, preformed acrosomal filament is extruded (and subsequently penetrates the egg chorion). The egg surface component that triggers the acrosome reaction has not yet been solubilized; therefore, previous studies have examined either spontaneous acrosome reactions or acrosome reactions that were triggered by eggs (or insoluble egg fragments), elevated extracellular Ca2+, or Ca2+ ionophores. In this study, we report a new method for initiating acrosome reactions in Limulus sperm. When the Limulus sperm motility-initiating peptide (SMI) is added to sperm in K+-free sea water, greater than 90% acrosome reactions are initiated within 5 min. However, less than 5% acrosome reactions occur either in K+-free sea water lacking SMI or when SMI is added to sperm in either normal sea water or K+- and Ca2+-free sea water. Experiments with K+ ionophores (nigericin and valinomycin), a K+ channel blocking agent (tetraethyl ammonium), an Na+ ionophore (monensin), and reagents that increase the intracellular pH (monensin, nigericin, and NH4Cl) indicate that changes in intracellular K+, Na+, or H+ do not mediate SMI-initiated acrosome reactions. The K+/Ca2+ ratio determines whether or not SMI will initiate acrosome reactions, with greater than 50% acrosome reactions being initiated when this ratio is below 0.3. In that K+ movement does not appear to be the critical event, possibly the K+/Ca2+ ratio either determines the rate of Ca2+ entry or controls the conformation of sperm surface molecules to allow SMI to initiate acrosome reactions in low K+. |
