title: The Limulus sperm motility-initiating peptide initiates acrosome reactions in sea water lacking potassium.
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+.
mesh_terms: D000177:Acrosome / Q000378:metabolism*; D000818:Animals; D005260:Female; D006737:Horseshoe Crabs / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D008297:Male; D011506:Proteins / Q000502:physiology*; D012623:Seawater; D012964:Sodium / Q000378:metabolism*; D013081:Sperm Motility; D013084:Sperm-Ovum Interactions; D013094:Spermatozoa / Q000378:metabolism*
chemical_list: D011506:Proteins; C029700:sperm forward motility protein, Bos taurus; D012964:Sodium